Zeitschriftenartikel
  • A. Hamid
  • J. Sun
  • H. Zhang
  • Thomas Stirner
Molecular dynamics simulation analysis of helium cluster growth conditions under tungsten surfaces, vol. 186

In: Computational Materials Science

  • 2021

DOI: 10.1016/j.commatsci.2020.109994

Molecular dynamics simulations have been performed to study the effects of helium fluxes on helium cluster size underneath tungsten surfaces under bombardment of helium atoms with incident energy 30 100 eV at temperature 300 2100 K. The simulation results show that the helium cluster size depends on the magnitude of the helium flux: at a higher flux, the helium clusters on average form in smaller size in tungsten but with larger number; while the clusters form further away from the surface at a lower flux. The coalescence of He atoms and helium bubbles depends on the tungsten temperature: at elevated temperatures around 2000 K, the incident He atoms in tungsten slow down more rapidly than at 1000 K but the number of vacancies per He cluster is smaller. The incident energy has a strong effect on the retention of helium atoms: The helium retention rate increases with the incident energy, and the helium retention depends weakly on temperature in the low energy range of interest. It is also found that the surface orientation plays an important role not only in determining the depth distribution but also in determining the helium retention and cluster size: at the surface {1 1 0}, the retention rate of helium atoms is the lowest, and at the surface {1 1 1}, the clusters grow easily in the lateral direction. The present simulation results suggest that the {0 0 1} surface is favorable for fuzz growth. The results obtained in the present work provide insight to the reasons why the fuzz only grows within a certain parameter range at the atomic level.
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Vortrag
  • David Scholz
  • Thomas Stirner
Hartree-Fock simulation of hematite surfaces with a posteriori calculation of correlation energy. Preisträgervortrag zur Verleihung des Georg-Simon-Ohm-Preises der Deutschen Physikalischen Gesellschaft an David Scholz

In: DPG-Frühjahrstagung, Dresden (abgesagt wegen Covid-19, gehalten an THD)

  • 2020
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Zeitschriftenartikel
  • H. Zhang
  • J. Sun
  • Y. Wang
  • Thomas Stirner
  • A. Hamid
  • C. Sang
Study of lattice thermal conductivity of tungsten containing bubbles by molecular dynamics simulation, vol. 161

In: Fusion Engineering and Design

  • 2020

DOI: 10.1016/j.fusengdes.2020.112004

Exposed to high fluxes of helium/hydrogen isotope particles and heat, tungsten divertor plates will suffer damage thus degrading its performance such as its thermal conductivity. This paper presents a study on the effect of bubbles on the lattice thermal conductivity of tungsten at the atomic level using molecular dynamics simulations. The present study finds that empty bubbles in tungsten lead to a decrease in the lattice thermal conductivity of tungsten. Furthermore, He/D filled bubbles aggravate this decrease. The physical origin of this behavior is discussed. It is also found that the decrease in lattice thermal conductivity depends strongly on both the impurity density in the bubbles and the bubble size.
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Zeitschriftenartikel
  • Thomas Stirner
  • David Scholz
  • J. Sun
Convergence of surface energy calculations for various methods: (001), (012), (100) hematite and the applicability of the standard approach, vol. 32, pg. 185002 (5pp).

In: Journal of Physics: Condensed Matter

  • 2020

DOI: 10.1088/1361-648X/ab6f88

Three different methods for the calculation of the surface energy, namely the standard approach, the Boettger relation and the linear-fit method, are applied to the (0 0 1), (0 1 2) and (1 0 0) hematite surfaces. The standard approach was previously shown to suffer from a divergence problem, and the Boettger relation was shown to exhibit quantum size effects. While the linear-fit method, in general, leads to a good convergence behavior of the surface energy, the questions arise whether the relative order of the calculated surface energies depends on the chosen calculation method, and whether there is any merit at all in employing the standard approach. The present work investigates these questions with hematite as a benchmark material system. The simulations show that, for the surface facets and slab thicknesses studied here, the relative order of the surface energies is unaffected by the chosen calculation method. A regime is found where the three methods are in reasonably good agreement with respect to the obtained surface energies. Finally, a procedure is put forward to extract meaningful surface energy values from the standard approach.
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Zeitschriftenartikel
  • Thomas Stirner
  • Ali Hamid
  • J. Sun
  • H. Zhang
  • A. Jadon
Molecular dynamics simulations of helium clustering and bubble growth under tungsten surfaces, vol. 163, pg. 141-147.

In: Computational Materials Science

  • 2019

DOI: 10.1016/j.commatsci.2019.03.008

We study the surface response of W to helium bombardment using molecular dynamics simulations. Simulations have been performed for incident helium of energy 80 eV and surface temperature 2100 K. The saturation of He retention has been observed to be high, a result of the bubbles trapping helium atoms and preventing them from diffusing to the surface and further back into the plasma. On the other hand, we have observe near-surface “cluster rupture” leading to the expulsion of helium atoms towards the vacuum. We have found that bubbles typically grow in a relatively narrow band of He/V ratios (1–3). Besides, it was observed that tungsten atoms migrated from the top surface into the bulk. The coalescence of helium bubbles has also been observed.
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Zeitschriftenartikel
  • David Scholz
  • Thomas Stirner
Convergence of surface energy calculations for various methods: (0 0 1) hematite as benchmark, vol. 31, pg. 195901-19508.

In: Journal of Physics: Condensed Matter

  • 2019

DOI: 10.1088/1361-648X/ab069d

Different methods for calculating the surface energy from ab initio simulations are applied to the relaxed (0 0 1) surface of the metal oxide hematite (α-Fe2O3). The simulations are carried out with a rather moderate k-point grid with shrinking factors of (6 6 6) for all bulk and (6 6) for all slab simulations. Very good convergence is obtained if a linear fit of the slab energies with respect to the number of layers in the slab is performed. In comparison to the other methods employed, this procedure is ultimately the most accurate and reliable method for extracting convergent surface energies from (0 0 1) hematite slabs. Additionally, we propose a way to determine the least possible starting point for calculating the surface energy by the linear-fit method. Furthermore, we find the Boettger method to perform nearly equally well, if the bulk energy is extracted from the energy difference per layer between the slabs with 12 and 18 layers thickness. Both methods give a surface energy of 2.43 J m-2 with a deviation of less than ±0.005 J m-2. The standard approach, which uses a separate bulk simulation, instead shows a significant linear divergence with increasing number of layers in the slab. We also carried out bulk simulations with a surface-oriented bulk unit cell, but found it in our case not to improve the convergence of the standard approach.
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Zeitschriftenartikel
  • Thomas Stirner
  • David Scholz
  • J. Sun
Ab initio simulation of structure and surface energy of low-index surfaces of stoichiometric alpha-Fe2O3, vol. 671, pg. 11-16.

In: Surface Science

  • 2018

DOI: 10.1016/j.susc.2018.01.010

The structure and surface energy of a series of low-index surfaces of stoichiometric α-Fe2O3 (hematite) are investigated using the periodic Hartree–Fock approach with an a posteriori correction of the correlation energy. The simulations show that, amongst the modeled facets, (012) and (0001) are the most stable surfaces of hematite, which is consistent with the fact that the latter are the dominant growth faces exposed on natural α-Fe2O3. The Fe-terminated (0001) surface is shown to exhibit a large relaxation of the surface atoms. It is argued that this arises mainly due to the fact that the surface cations are located opposite empty cation sites in the filled-filled-unfilled cation sequence along the c-axis. In contrast, the (012) plane cuts the crystal through a plane of empty cation sites, thus giving rise to relatively small relaxations and surface energies. The small relaxations and concomitant exposure of five-coordinate cation sites may be important for the catalytic activity of hematite. The simulations also show that the relative stability of the investigated surfaces changes after a full lattice relaxation with the (0001) and (116) facets relaxing disproportionately large. Wherever possible, the simulations are compared with previous simulation data and experimental results. A Wulff–Gibbs construction is also presented.
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Zeitschriftenartikel
  • Thomas Stirner
  • David Scholz
  • J. Sun
Hartree-Fock simulation of the (0 0 0 1) surface of hematite with a posteriori calculation of the correlation energy, vol. 137, pg. 340-345.

In: Computational Materials Science

  • 2017

DOI: 10.1016/j.commatsci.2017.06.011

The results of Hartree-Fock simulations of the (0 0 0 1) surface of hematite including an a posteriori calculation of the correlation energy are presented. Structural as well as electronic and magnetic properties of the surface atoms are considered. Infrared and Raman bands of the hematite slab are also presented. The calculated surface relaxation is shown to be in good agreement with experimental data originating from a recent LEED analysis. The iron-oxygen bond at the single-iron-terminated surface is shown to be less ionic than in the bulk. Also, the magnetic dipole moment of the surface iron atom is smaller than in the bulk. Finally, the surface energy of the (0 0 0 1) facet is presented for hematite, and for comparison also for α-alumina and α-chromia for various hybrid functionals. Here it is shown that the effects of electron correlation play an important role for hematite and chromia due to their localized 3d electrons.
  • Maschinenbau und Mechatronik
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Zeitschriftenartikel
  • S. Liu
  • S. Dai
  • C. Sang
  • J. Sun
  • Thomas Stirner
  • D. Wang
Molecular dynamics simulation of the formation, growth and bursting of bubbles in tungsten exposed to high fluxes of low energy deuterium, vol. 463, pg. 363-366.

In: Journal of Nuclear Materials - PLASMA-SURFACE INTERACTIONS 21 — Proceedings of the 21st International Conference on Plasma-Surface Interactions in Controlled Fusion Devices Kanazawa, Japan, May 26-30, 2014

  • 2015

DOI: 10.1016/j.jnucmat.2014.12.060

Molecular dynamics simulations are carried out to investigate the formation, growth and bursting of bubbles in tungsten exposed to the irradiation of an extremely high deuterium flux. It is found that the bubbles form in the region near the location of the implanted ion distribution peaks, and that the effect of the substrate temperature on the bubble formation depth is negligible; it is also found that the percentage of deuterium that is found in D2 molecules increases as the bubble grows, and that the evolution of the bubble’s internal pressure is strongly associated with the properties of its surrounding structure. The simulations display the development of a dome-shaped structure at the tungsten surface during the bubble growth. The merging of two deuterium bubbles is also observed. The present simulations also show that the bubble bursts by generating a partially opened lid, which has already been observed in previous independent experiments.
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Zeitschriftenartikel
  • Y. Fan
  • Y. Zou
  • J. Sun
  • Thomas Stirner
  • D. Wang
Study of the effects of a transverse magnetic field on radio frequency argon discharges by two-dimensional particle-in-cell-Monte-Carlo collision simulations, vol. 20, pg. 3507 ff..

In: Physics of Plasmas

  • 2013

DOI: 10.1063/1.4826215

The influence of an applied magnetic field on plasma-related devices has a wide range of applications. Its effects on a plasma have been studied for years; however, there are still many issues that are not understoodwell. This paper reports a detailed kinetic study with thetwo-dimension-in-space and three-dimension-in-velocity particle-in-cellplus Monte Carlo collision method on the role of E×B drift in acapacitive argon discharge, similar to the experiment of You et al.[Thin Solid Films 519, 6981 (2011)]. The parameters chosen in the present study for the external magnetic field are in a range common tomany applications. Two basic configurations of the magnetic field areanalyzed in detail: the magnetic field direction parallel to theelectrode with or without a gradient. With an extensive parametricstudy, we give detailed influences of the drift on the collectivebehaviors of the plasma along a two-dimensional domain, which cannot berepresented by a 1 spatial and 3 velocity dimensions model. By analyzingthe results of the simulations, the occurring collisionless heating mechanism is explained well.
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Zeitschriftenartikel
  • J. Sun
  • Y. Fan
  • Y. Zou
  • Thomas Stirner
  • D. Wang
Investigation of the effects of a thin dielectric layer on low-pressure hydrogen capacitive discharges driven by combined radio frequency and pulse power sources, vol. 20, pg. 3508 ff..

In: Physics of Plasmas

  • 2013

DOI: 10.1063/1.4831775

Negative hydrogen ion sources, for instance for fusion devices, currently attract considerable attention. To generate the precursors—highly rovibrationally excited hydrogen molecules—for negative hydrogen ions effectively by electron excitation, a thin dielectric layer is introduced to cover the surface of the electrically grounded electrode of two parallel metal plates in a low-pressure hydrogen capacitive discharge driven by combined rf and pulse power sources. To understand the characteristics of such discharges, particle-in-cell simulations are conducted to study the effects that the single dielectric layer would bring onto the discharges. The simulation results show that the dielectric layer leads to a much higher plasma density and a much larger production rate of highly vibrationally excited hydrogen molecules compared to discharges without the dielectric layer on the electrode. Further investigation indicates that the nonlinear oscillation of the electrons induced by the nanosecond-pulse continues until it is finally damped down and does not show any dependence on the pulse plateau-time, which is in stark contrast to the case without the dielectric layer present. The physical reason for this phenomenon is explored and explained.
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Zeitschriftenartikel
  • Thomas Stirner
  • et al.
Ab initio Hartree-Fock simulation of r-plane sapphire, vol. 32, pg. 635-639.

In: Physics Procedia

  • 2012
R-plane sapphire is the preferred substrate material for silicon-based radiation-hard devices and RF integrated circuits. Ab initio periodic Hartree-Fock simulations of r-plane sapphire slabs are presented with a particular focus on the surface relaxation and the surface energy. The calculations show that there is a considerable relaxation of the four outermost atomic layers of r-plane sapphire. A comparison with c-, a- and m-plane sapphire shows that the surface energy of fully relaxed r-plane sapphire is considerably larger than the surface energy of c-plane sapphire, but similar to the surface energies of a- and m-plane sapphire. The implications of these findings for chemical mechanical polishing (CMP) and electrolytic in-process dressing (ELID) grinding of sapphire are discussed briefly.
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Zeitschriftenartikel
  • J. Sun
  • L. Li
  • Thomas Stirner
  • J. Bai
  • D. Wang
Particle simulation of the nonlinear oscillation of electrons induced by a nanosecond pulse in rf capacitive hydrogen discharges, vol. 19, pg. 033511.

In: Physics of Plasmas

  • 2012

DOI: 10.1063/1.3695121

A particle-in-cell simulation was employed to investigate the nature and physical cause of the nonlinear oscillation of electrons induced by a nanosecond pulse in rf capacitive hydrogen discharges. It was found that the applied nanosecond pulse converted the plasma quickly from the bi-Maxwellian equilibrium formed in the rf capacitive discharge into another temporal bi-Maxwellian equilibrium. When the applied electric field collapses within a few nanoseconds, the electric field arising from the space charge serves as a restoring force to generate a swift oscillation of the electrons. The energy stored in the plasma is converted gradually into the chemical energy during the electron periodic movement. It is also found that the rise-, plateau-, and fall-times of the applied pulse affect the evolution of the electron energy distribution. The collective electron oscillation has a repetition frequency approximately equal to the electron plasma frequency, independent of pulse rise-, plateau-, and fall-times. This oscillation of electrons induced by a nanosecond pulse can be used to generate highly excited vibrational states of hydrogen molecules, which are a necessary precursor for negative hydrogen ions.
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Zeitschriftenartikel
  • J. Sun
  • S. Liu
  • Thomas Stirner
  • et al.
Dynamic Monte Carlo simulation of film-substrate interface mixing in the deposition of Co on Cu (001), vol. 605, pg. 1298-1303.

In: Surface Science

  • 2011
Dynamic Monte Carlo simulations are performed to investigate the interface mixing of Co atoms deposited on a Cu (001) substrate. A tight-binding potential was used to determine the input parameters (jump probabilities and energy barriers) for the Dynamic Monte Carlo model. The results show that more Co adatoms penetrate into the substrate as the temperature rises and/or as the deposition rate decreases, and that the intermixing between the layers becomes concomitantly more pronounced. Cu atoms migrating into the Co layer via exchange processes during the growth of consecutive Co layers are proposed to be responsible for the intermixing. Furthermore, an initial Co clustering followed by a layer-by-layer growth mode was observed in the simulations, with the surface concentration of Cu atoms depending on the fraction of migrating Cu atoms and decaying into the Co film following a power law. The fraction of Cu atoms migrating into the Co layer can be adjusted by varying the deposition rate and the substrate temperature.
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Beitrag (Sammelband oder Tagungsband)
  • Thomas Stirner
  • J. Sun
Computer simulation of alpha-chromia and its surface, pg. 239-306.

In: Chromium. Environmental, medical, and materials studies (Chemical engineering methods and technology)

  • 2011
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
  • Extern
Zeitschriftenartikel
  • S. Back
  • Thomas Stirner
  • Christian Schopf
Prozessstabilität beim Schleifen mit Ultraschall verbessern, pg. 35-36.

In: MM Industriemagazin-Maschinenmarkt

  • 2010
Zur Prozessverbesserung bei der Ultraschallbearbeitung optischer Flächen wurden eine Versuchsplanung und eine statistische Analyse der Messdaten durchgeführt. Somit können Wechselwirkungen zwischen den Parametern vorhergesagt und die Stabilität des Prozesses verbessert werden.
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  • Angewandte Wirtschaftswissenschaften
Zeitschriftenartikel
  • S. Liu
  • J. Sun
  • Thomas Stirner
  • et al.
A general model for chemical erosion of carbon materials due to low-energy H+ impact, vol. 108, pg. 073302.

In: Journal of Applied Physics

  • 2010
Modeling the chemical erosion of carbon materials due to low-energy H+ impact is of paramount importance for the prediction of the behavior of carbon-based plasma-facing components in nuclear fusion devices. In this paper a simple general model describing both energy and temperature dependence of carbon-based chemical erosion is presented. Enlightened by Hopf’s model {Hopf et al. , [J. Appl. Phys.94, 2373 (Year: 2003)}, the chemical erosion is separated into the contributions from three mechanisms: thermal chemical erosion, energetic chemical sputtering, and ion-enhanced chemical erosion. Using input from the Monte Carlo code TRIDYN, this model is able to reproduce experimental data well.
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Zeitschriftenartikel
  • J. Sun
  • S. Li
  • Thomas Stirner
  • et al.
Molecular dynamics simulation of energy exchanges during hydrogen collision with graphite sheets, vol. 107, pg. 113533.

In: Journal of Applied Physics

  • 2010

DOI: 10.1063/1.3428447

Experiments show that the energy of particles incident on divertor plates in fusion devices seldom exceeds 100 eV. Trim code and its variants are not suitable to predict the sputtering yield of carbon-based divertor plates for this energy range and, therefore, a dynamic model, taking into account the C–H bond formation and breaking, and the structure of carbon, is needed. In this paper, the molecular dynamics method is employed to investigate collision processes between incident hydrogen atoms and a graphene sheet. The simulation results demonstrate that the collision processes cannot be adequately described by a simple binary approximation. The energy transfer from the projectile to the graphite sheet exhibits a very complicated behavior when the kinetic energy of the incident hydrogen atom is below 30 eV, strongly depending on the impact position. When its kinetic energy is lower than 0.35 eV, the incident hydrogen is always reflected back from the single, perfect graphite sheet; when its kinetic energy is higher than 0.35 eV, then whether the incident particle penetrates the graphite sheet, is reflected back or is adsorbed depends on the impact position. In certain areas of the graphite sheet, either adsorption or reflection of an incident hydrogen atom can occur in two different energy ranges.
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Vortrag
  • Thomas Stirner
  • J. Sun
  • et al.
Ab initio Hartree-Fock simulation of r-plane sapphire. Posterpräsentation

In: 18th International Vacuum Congress (IVC-18)

  • 2010
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Beitrag (Sammelband oder Tagungsband)
  • Igor Makarenko
  • Christian Vogt
  • Rolf Rascher
  • Peter Sperber
  • Thomas Stirner
ELID supported grinding of thin sapphire wafers, pg. 76550S ff..
  • 2010
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
  • Maschinenbau und Mechatronik
  • Hochschulleitung und -einrichtungen
Zeitschriftenartikel
  • J. Sun
  • C. Sang
  • Thomas Stirner
Characteristics of plasma immersion ion implantation with a nanosecond rise-time pulse: particle-in-cell simulations, vol. 43, pg. 275201.

In: Journal of Physics D: Applied Physics

  • 2010

DOI: 10.1088/0022-3727/43/27/275201

Processes of plasma immersion ion implantation are analyszed numerically using a one-dimension-in-space and three-dimension-in-velocity particle-in-cell plus Monte Carlo collision (1D3V PIC–MCC) model. The behaviour of ions and electrons between the processed target and the source plasma is simulated after a nanosecond rise-time voltage pulse is applied to the target. The simulation results show that electron–neutral ionization collisions play a significant role in determining the magnitudes of the ion and electron densities when the pulse rise time is very short, and that the plasma density can be enhanced many times. The physical mechanism for this phenomenon is explained in terms of the formation of a reverse electric field inside the plasma chamber.
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Zeitschriftenartikel
  • J. Sun
  • Thomas Stirner
Ab initio supercell calculations of the (0001) alpha-Cr203 surface with a partially or totally Al-substituted external layer, vol. 517, pg. 5512-5515.

In: Thin Solid Films

  • 2009
Ab initio supercell calculations employing the periodic Hartree–Fock formalism are presented of the (0001) α-Cr2O3 surface with a partially or totally Al-substituted external layer. In the simulations a fraction of the Cr atoms at the surface of the chromia slab are replaced by Al atoms, and the Al surface coverage is varied between zero (pure chromia) and 100% (Al-terminated chromia). The surface Al atoms are found to relax inwards considerably, with the magnitude of the relaxation decreasing with increasing Al surface coverage. The calculations also reveal that the surface energy of the slab decreases with increasing Al coverage. Finally, the electronic properties at the surface of the Al-substituted (0001) α-Cr2O3 slabs are investigated. Here the calculations show that the substitution of Cr by Al gives rise to an increase in the covalency of the AlO bonds compared to slabs of pure alumina. In contrast, the influence of the surface Al atoms on the electrostatic potential in the (0001) plane of metal ions is relatively small. These findings support the utilisation of α-chromia substrates for the templated growth of α-alumina, which is consistent with recent experiments.
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Zeitschriftenartikel
  • M.-O. M. Piepenbrock
  • Thomas Stirner
  • M. O’Neill
  • S. Kelly
Dynamics of the size distribution of CdTe quantum dot ensembles during growth in liquid and crystalline phases, vol. 9, pg. 1057-1061.

In: Physical Chemistry Chemical Physics (PCCP)

  • 2008
We recently reported that the growth rate of colloidal CdTe nanoparticles decreases by orders of magnitude when the particles undergo a phase transition from liquid to crystal. The dynamics of nanoparticle growth are dominated by this factor rather than the size dependence of the chemical potential. Herein we discuss how the phase transition affects the size distribution and photoluminescence quantum efficiency of the nanoparticles. We suggest that the absorption linewidth is a better monitor of size distribution than the photoluminescence linewidth because the photoluminescence quantum efficiency, which affects the latter via energy transfer, varies substantially with reaction time. We find that the size distribution broadens in the early stages of growth possibly because of inhomogeneities in the phase transition radius or because particles nucleated at later times coalesce with nanocrystals. The quantum efficiency is enhanced when tellurium is depleted in the reaction solution, giving a cadmium-enriched surface. Batches with high initial tellurium and cadmium concentrations show a substantial amount of delayed nucleation, lower quantum efficiency and some anisotropic growth.
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Zeitschriftenartikel
  • Thomas Stirner
  • J. Sun
Non-equilibrium simulation of optically trapped colloidal particles at liquid interfaces, vol. 43, pg. 769-801.

In: Computational Materials Science

  • 2008
A theoretical study of the non-equilibrium behaviour of colloidal particle monolayers under the influence of the displacement of a single, optically trapped colloidal particle is presented. Two different types of monolayer (i.e., one relatively rigid and the other relatively soft) confined to a liquid–liquid interface are investigated using the Stokesian dynamics simulation technique. An effective dipole–dipole interaction is assumed to act between the colloidal particles and viscous drag forces acting on the particles are taken into account. The calculations reveal periodic oscillations of the net force on the trapped particle for the soft monolayer and highly non-linear, non-monotonic variations for the rigid monolayer. The highly non-linear variations of the net force for the rigid monolayer are concomitant with rapid, cooperative particle rearrangements and large oscillations in the global orientational order parameter of the monolayer. These findings, combined with the results of optical-tweezer experiments, should be useful for the rheology of liquid interfaces and the investigation of Pickering emulsions.
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Vortrag
  • M.-O. M. Piepenbrock
  • M. O’Neill
  • Thomas Stirner
  • S. Kelly
Growth dynamics of CdTe quantum dot ensembles in liquid and crystalline phases

In: SPIE Optics and Photonics /Nanoscience and Engineering

  • 2008
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • M. Carrasco-Or
  • D. Dong
  • R. Kelly
  • C. Ellis
  • M. O’Neill
  • Thomas Stirner
  • M.-O. M. Piepenbrock
Superlattices of organic/inorganic semiconductor nanostructures from liquid-crystal templates, vol. 75, pg. 035207.

In: Physical Review B - condensed matter and materials physics

  • 2007

DOI: 10.1103/PhysRevB.75.035207

A lyotropic liquid-crystal-templated superlattice of inorganic CdSe semiconductor nanostructures within an organic semiconductor polymer-network film was fabricated by a self-assembly process. The film is characterized with small-angle x-ray diffraction experiments and ultraviolet-visible absorption measurements. Empirical pseudopotential calculations of the effective CdSe nanocrystal band gap show that the experimental observations are consistent with the formation of CdSe nanostructures inside lyotropic liquid-crystal nanotubes. A potential application of these nanocomposite supramolecular arrays is demonstrated via a photoconductive device.
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Zeitschriftenartikel
  • A. Löbbert
  • S. Kitney
  • M. O’Neill
  • S. Kelly
  • Thomas Stirner
Terminal end-group efficiency for the nematic phase using model bicyclo[2.2.2]octanes, vol. 34, pg. 1357-1367.

In: Liquid Crystals

  • 2007
The synthesis is reported of new liquid crystals incorporating the 1,4-disubstituted bicyclo[2.2.2] octane ring and a series of substituents in a terminal position on the molecular core. The nature of the terminal substituent is varied from apolar with a small dipole moment to polar with a strong dipole moment. The angle of the dipole moment with respect to the molecular axis is also varied. An updated order of terminal group efficiency for substituents in a terminal position for the nematic phase is provided. The bicyclo[2.2.2] octane ring shields halogen substituents in a lateral position on phenyl rings attached to the bicyclooctane ring to a small degree and reduces the steric efects of these substituents, giving rise to high relative nematic - isotropic transition temperatures.
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Zeitschriftenartikel
  • M.-O. M. Piepenbrock
  • Thomas Stirner
  • M. O’Neill
  • S. Kelly
Growth dynamics of CdTe nanoparticles in liquid and crystalline phases, vol. 129, pg. 7674-7679.

In: Journal of the American Chemical Society (JACS)

  • 2007
Normally the size dependence of the chemical potential is used to explain the growth dynamics of semiconductor nanoparticles. Instead we show that very small CdTe nanoparticles continue to grow at high dilution, the growth rate is virtually independent of monomer concentration, nucleation continues after the growth of larger particles has saturated, and the growth rate has a much greater nonlinear dependence on particle size than predicted by theory. We suggest that nanoparticle growth is fast in the liquid phase and then saturates as the particles change phase from liquid to crystal at a threshold size which depends on the growth temperature and not the monomer concentration. The photoluminescence quantum efficiency becomes high when tellurium is depleted in the reaction solution giving a cadmium enriched surface.
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Zeitschriftenartikel
  • J. Sun
  • A. Matthews
  • Thomas Stirner
Molecular dynamics simulation of the (0001) α-Al2O3 and α-Cr2O3 surfaces, vol. 601, pg. 1358-1364.

In: Surface Science

  • 2007
A simple, rigid pair-potential model is applied to investigate the dynamics of the (0 0 0 1) α-Al2O3 and α-Cr2O3 surfaces using the molecular dynamics technique. The simulations employ a two-stage equilibration process: in the first stage the simulation-cell size is determined via the constant-stress ensemble, and in the second stage the equilibration of the size-corrected simulation cell is continued in the canonical ensemble. The thermal expansion coefficients of bulk alumina and chromia are evaluated as a function of temperature. Furthermore, the surface relaxation and mean-square displacement of the atoms versus depth into the slab are calculated, and their behaviour in the surface region analysed in detail. The calculations show that even moderate temperatures (∼400 °C) give rise to displacements of the atoms at the surface which are similar to the lattice mismatch between α-alumina and chromia. This will help in the initial nucleation stage during thin film growth, and thus facilitate the deposition of α-Al2O3 on (0 0 0 1) α-Cr2O3 templates.
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • J. Sun
  • A. Matthews
  • Thomas Stirner
Structure and electronic properties calculation of ultrathin α-Al2O3 films on (0001) α-Cr2O3 templates, vol. 601, pg. 5050-5056.

In: Surface Science

  • 2007
Ab initio total energy Hartree–Fock calculations of ultrathin films of α-Al2O3 on (0 0 0 1) α-Cr2O3 templates are presented. The surface relaxation, the in-plane reconstruction and the surface and strain energies of the slabs are studied as a function of alumina film thickness. The surface Al layer is found to relax inwards considerably, with the magnitude of the inwards relaxation depending on the thickness of the ultrathin alumina film in a non-linear manner. The calculations also reveal that ultrathin films of alumina lower the surface energy of (0 0 0 1) α-chromia substrates. This indicates that the (0 0 0 1) α-chromia surface provides favourable conditions for the templated growth of α-alumina. However, increasing the alumina film thickness is found to give rise to a significant increase in strain energy. Finally, the electronic properties at the surface of the (0 0 0 1) α-Al2O3/α-Cr2O3 slabs are investigated. Here it is found that the alumina coating gives rise to an increase in the covalency of the bonds at the surface of the slabs. In contrast, the influence of an alumina layer on the electrostatic potential at the surface of the chromia slab is relatively minor, which should also be beneficial for the templated growth of α-alumina on (0 0 0 1) α-chromia substrates.
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Vortrag
  • J. Sun
  • Thomas Stirner
  • A. Matthews
Structure and surface energy of low-index surfaces of stoichiometric α-Al2O3 and α-Cr2O3. Posterpräsentation

In: International Conference on Metallurgical Coatings and Thin Films (ICMCTF 2006)

  • 2006
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • J. Sun
  • Thomas Stirner
  • A. Leyland
  • A. Matthews
  • W. Hagston
A simple transferable interatomic potential model for binary oxides applied to bulk α-Al2O3 and the (0001) α-Al2O3 surface, vol. 290, pg. 235-240.

In: Journal of Crystal Growth

  • 2006
A simple transferable potential for binary, highly ionic oxides is derived from a previous work. In this new potential the van der Waals terms involving cations and the cation–cation short-range repulsive interactions do not appear explicitly. The potential parameters of this new model are optimised for α-Al2O3α-Al2O3, MgO and CaO. Good agreement between theory and experiment is obtained for the structural parameters and lattice energies. The potential is then employed to study the relaxation of the Al-terminated (0 0 0 1) α-Al2O3α-Al2O3 surface as well as the dynamics of this surface at non-zero temperatures using the molecular dynamics method. Here it is shown that the new potential predicts a relaxation of the (0 0 0 1) α-Al2O3α-Al2O3 surface which is consistent with experiment and shell model calculations. Finally, the calculated dynamical features of the (0 0 0 1) α-Al2O3α-Al2O3 surface are explained in terms of a combination of the loss of translational symmetry and the resulting relaxation of the surface atoms.
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Zeitschriftenartikel
  • M.-O. M. Piepenbrock
  • Thomas Stirner
  • M. O’Neill
A low-temperature synthesis for organically soluble HgTe nanocrystals exhibiting near-infrared photoluminescence and quantum confinement, vol. 128, pg. 7087-7090.

In: Journal of the American Chemical Society (JACS)

  • 2006
A new low-temperature, one-pot method is introduced for the preparation of organically passivated HgTe nanocrystals, without the use of highly toxic precursors. The nanocrystals show bright photoluminescence in the infrared telecommunication windows about 1300 and 1550 nm with quantum efficiencies between 55 and 60%. They have a zinc blende structure with a mean particle diameter of 3.4 nm, thus exhibiting quantum confinement effects. Particle growth is self-limited by temperature quenching, so a narrow size distribution is obtained. The measured size of the particles agrees with calculations using the pseudopotential method.
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Zeitschriftenartikel
  • J. Sun
  • A. Matthews
  • Thomas Stirner
Structure and surface energy of low-index surfaces of stoichiometric α-Al2O3 and α-Cr2O3, vol. 201, pg. 4205-4208.

In: Surface & Coatings Technology

  • 2006
There is considerable interest in the low-temperature growth of the stable α-phase of alumina, α-Al2O3, due to its superior chemical and mechanical properties compared to the other forms of alumina. Conventional methods for achieving this phase (such as chemical vapour deposition) utilise high temperatures which limits the range of substrate materials that can be beneficially coated. Recently, it has been reported that α-Al2O3 can be grown on α-Cr2O3 templates at a much lower substrate temperature (around 400 °C) by the RF magnetron sputtering deposition technique. Previous studies showed that both α-Al2O3 and α-Cr2O3 (0001) free surfaces relax considerably in comparison with the corresponding bulk structures. From the experimental point of view, a knowledge of the surface structure and stability of both materials will be of great help in determining the facet of α-Cr2O3 which best facilitates the growth of α-Al2O3. Here we present the results of first-principles Hartree–Fock calculations on the surface energy of the low-index planes of both α-alumina and α-chromia. The suitability of possible α-Cr2O3 facets as templates for the growth of α-Al2O3 is discusssed.
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Vortrag
  • J. Sun
  • A. Matthews
  • Thomas Stirner
Calculation of native defect energies in α-Al2O3 and α-Cr2O3 using a modified Matsui potential. Posterpräsentation

In: International Conference on Metallurgical Coatings and Thin Films (ICMCTF 2006)

  • 2006
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • J. Sun
  • A. Matthews
  • Thomas Stirner
Calculation of native defect energies in α-Al2O3 and α-Cr2O3 using a modified Matsui potential, vol. 201, pg. 4201-4204.

In: Surface & Coatings Technology

  • 2006
Alumina and chromia are very important materials in the surface coatings industry, e.g. for corrosion protection and as catalyst supports. The type of defects and the associated formation energy in these materials are of direct relevance to the surface stability and reaction kinetics. In the present work, a modified Matsui potential is applied to calculate the native point defect energies in α-Al2O3 and α-Cr2O3 based on the Mott–Littleton theory. Particular attention is paid to the convergence of the defect energies with the number of atoms surrounding the defect. The results show that the relative values of the defect formation energies are such that the Schottky energy is smaller than either of the Frenkel energies, which is in agreement with experimental data and recent results of first-principles calculations. The implications of these findings for diffusion mechanisms and the associated reaction kinetics are discussed briefly.
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Zeitschriftenartikel
  • K. Woon
  • M. Aldred
  • P. Vlachos
  • G. Mehl
  • M. O’Neill
  • Thomas Stirner
Electronic charge transport in extended nematic liquid crystals, vol. 18, pg. 2311-2317.

In: Chemistry of Materials

  • 2006
We report a systematic study of charge transport in a range of low-molar-mass and extended (having at least six aromatic rings) nematic liquid crystals, some of which are reactive mesogens, with a high degree of shape anisotropy, i.e., the length-to-width (aspect) ratio is exceptionally high. We demonstrate that the hole mobility is independent of the macroscopic, but not microscopic, ordering of the nematic and isotropic phases of these nematic liquid crystals with a long, rigid, and extended aromatic molecular core, because no discontinuity is observed at the transition between these phases. A room-temperature mobility of up to 1.0 × 10-3 cm2 V-1 s-1 is obtained in the nematic phase, which is attributed to the short intermolecular distances between the highly polarizable but rigid long aromatic cores. We show that the intermolecular separation can be easily fine-tuned by changing the lateral and terminal aliphatic groups of these nematic liquid crystals. Hence, the charge mobility can be varied by up to 2 orders of magnitude without altering the core structure of the molecules, and this chemical fine control could be used to limit hole transport and so provide better charge balance in organic light-emitting diodes. X-ray diffraction is used to obtain the intermolecular separation and shows local lamellar order in the nematic phase.
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Zeitschriftenartikel
  • J. Wild
  • K. Bartle
  • M. O’Neill
  • N. Kirkman
  • P. Tuffin
  • Thomas Stirner
Synthesis and investigation of nematic liquid crystals with flexoelectric properties, vol. 17, pg. 6354-6360.

In: Chemistry of Materials

  • 2005

DOI: 10.1021/cm051682y

We describe the synthesis and characterization of a series of model nematic liquid crystals with transverse dipole moments used to study the flexoelectric effect in guest−host mixtures with commercial liquid crystal host. The flexoelectric coefficicent of the mixtures, containing only 10% by weight of the dopant, are up to 6 times higher than those of the pure hosts. The length, bend angle, and dipole moments of the molecules are systematically varied to investigate any correlations with the flexoelectric effect. We find that the flexoelectric coefficients increase with molecular length, are inversely correlated with the bend angle, and are independent of the dipole moment of the dopant. Although these findings seem to contradict predictions from dipolar flexoelectric theories, they can be reconciled by considering the properties of both the guest and host in the mixture. Thiophenes and dimesogens show particularly large flexoelectric effects. This work should inform the molecular design of new materials with enhanced flexoelectric properties.
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • G. Bergmann
  • P. Jackson
  • J.H.C. Hogg
  • Thomas Stirner
  • M. O’Neill
  • W. Duffy
  • S. Kelly
  • G. Clark
Photoinduced changes of surface order in coumarin side-chain polymer films used for liquid crystal photoalignment, vol. 87, pg. 061914.

In: Applied Physics Letters

  • 2005

DOI: 10.1063/1.2009070

Specular x-rayreflectivity probes morphological changes in a crosslinkable coumarin photoalignment polymer film resulting from ultraviolet irradiation. An ordered surface layer with density oscillations compatible with planar side-chain alignment is obtained before irradiation. The ordering is enhanced in the early stages of crosslinking. This is attributed to the photoinduced increase of mobility of the side-chains resulting from the creation of free volume by the crosslinking process. The expansion of the thin film confirms that free volume is created. The surface ordering decreases with prolonged ultraviolet irradiation because of increased material viscosity resulting from a high crosslinked density. The implications of surface ordering on liquid crystal photoalignment are discussed.
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • Thomas Stirner
  • J. Sun
Molecular dynamics simulation of the structural configuration of binary colloidal monolayers, vol. 21, pg. 6636-6641.

In: Langmuir

  • 2005

DOI: 10.1021/la050402q

Molecular dynamics simulations of binary colloidal monolayers, i.e., monolayers consisting of mixtures of two different particle sizes, are presented. In the simulations, the colloid particles are located at an oil-water interface and interact via an effective dipole-dipole potential. In particular, the influence of the particle ratio on the configurations of the binary monolayers is investigated for two different relative interaction strengths between the particles, and the pair correlation functions corresponding to the binary monolayers are calculated. The simulations show that the binary monolayers can only form two-dimensional crystals for certain particle ratios, for example, 2:1, 6:1, etc., while, for example, for a particle ratio of 7:1 the monolayers are found to be in a disordered, glassy state. The calculations also reveal that in analogy to the Wigner lattice the configurations are very sensitive to the relative interaction strength between the particles but not to the absolute magnitude of the interaction strength, even when particle size effects are taken into account. Consequently, it is argued that a comparison between the calculated configurations and actual binary particle monolayer systems could provide useful information on the relative interaction strength between large and small particles. Possible mechanisms giving rise to disparities in the interaction strength between large and small particles are described briefly.
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Vortrag
  • J. Sun
  • Thomas Stirner
Computer simulation of binary oxides. Invited Lecture

In: International Workshop on Hard Metallic Phases

  • 2004
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Vortrag
  • Thomas Stirner
Computer simulation of colloidal particle monolayers at liquid-liquid interfaces

In: Simulation and Modelling of Colloidal Systems (Meeting of the RSC Colloids and Interface Science Group, the RSC Statistical Mechanics and Thermodynamics Group and the IoP Complex Fluids Group)

  • 2004
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • Thomas Stirner
  • J. Sun
Molecular dynamics simulation of optically trapped colloidal particles at an oil-water interface, vol. 121, pg. 4292-4296.

In: The Journal of Chemical Physics

  • 2004

DOI: 10.1063/1.1779569

Using molecular dynamics simulations, we calculate the net force on a colloidal particle trapped by an optical tweezer and confined within a particle monolayer which is in motion relative to the trapped particle. The calculations are compared with recent experimental data on polystyrene particles located at an oil-water interface. Good agreement between theory and experiment is obtained over the investigated range of lattice constants for an interaction mechanism between the polystyrene particles which is dominated by an effective dipole-dipole potential. The assumed interaction mechanism is consistent with the formation of surface charge dipoles at the particle-oil interface due to the dissociaton of the hydrophilic sulfate headgroups at the surface of the polystyrene particles. A possible physical mechanism for the formation of the surface charge dipoles, involving a diffuse cloud of fully hydrated counterions, is described, and the fraction of surface groups contributing to the formation of surface charge dipoles is estimated to be of the order of 10−1 for the present system.
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • N. Kirkman
  • W. Hagston
  • Thomas Stirner
A new method for investigating the surface tension from molecular dynamics simulations applied to liquid droplets, vol. 30, pg. 126-130.

In: Computational Materials Science

  • 2004

DOI: 10.1016/j.commatsci.2004.01.020

A new method for investigating surface tension, using molecular dynamics simulations, is proposed. The method uses the inter-particle forces to characterise the surface region. The average particle energy in this region is then employed to evaluate the surface tension. The main advantage of the proposed method is its applicability to any surface geometry and multi-component systems. For the purpose of illustration, and in view of its practical importance, the model is then applied to spherical droplets. The droplets contain 500, 1000, 2000 and 4000 particles respectively, acting under the influence of a Lennard-Jones pair potential. The effects of temperature and droplet size on the surface tension are investigated. Finally, the calculations are compared with experimental and theoretical results in the literature
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Beitrag (Sammelband oder Tagungsband)
  • S. Kelly
  • Thomas Stirner
  • M. O’Neill
Cadmium chalcogenide nanocrystals. Chapter 4, pg. 158-192.

In: The handbook of electroluminescent materials. null (Series in optics and optoelectronics)

  • 2004
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • Thomas Stirner
  • J. Sun
Molecular dynamics simulation of compression-induced solid-to-solid phase transitions in colloidal monolayers, vol. 67, pg. 051107.

In: Physical Review E - statistical, nonlinear, and soft matter physics

  • 2003

DOI: 10.1103/PhysRevE.67.051107

The compression of two-dimensional colloidal monolayers, consisting of polystyrene particles trapped at an oil-water interface and interacting via dipole-dipole potentials, is investigated by the molecular dynamics technique. In particular, the pair correlation function and global orientational order parameter of the monolayers are calculated as a function of the particle coverage. The simulation results exhibit a sequence of hexagonal-to-rhombohedral-to-hexagonal phase transitions of the monolayers under anisotropic compression. The influence of defects in the monolayers on the solid-to-solid phase transitions is also examined. The simulations show that the stability of the rhombohedral phase is relatively sensitive to lattice defects, while, under the same conditions, the hexagonal phase is very stable. Finally, the simulation results are compared with recent experimental observations, and the implications of the present computer simulations for diffusion mechanisms and protein folding studies are discussed briefly.
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Vortrag
  • Thomas Stirner
Computer simulation of colloidal monolayers (lecture)
  • 2003
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • Thomas Stirner
  • W. Hagston
  • N. Kirkman
Continuum modelling of hybrid-aligned nematic liquid-crystal cells: optical response and flexoelectricity-induced voltage shift, vol. 30, pg. 1115-1122.

In: Liquid Crystals

  • 2003

DOI: 10.1080/02678290310001594562

A continuum model is employed to study systematically the optical response of hybrid-aligned nematic (HAN) liquid crystal cells under the application of an external electric field. The influence of the flexoelectric effect is discussed for a large range of anchoring strengths at the homeotropic alignment layer. It is shown that the optical response of HAN cells is governed by a complicated interplay between the flexoelectric coefficient and homeotropic anchoring strength. In particular, the calculations reveal that, for weak homeotropic anchoring, the flexoelectric effect leads to a non-linear voltage shift of the optical transmittance as a function of flexoelectric coefficient, and gives rise to an asymmetry in the transmittance–voltage curve. Finally, a comparison of the continuum-model simulations with recent experimental observations indicates that both the flexoelectric coefficient and the anchoring strength of the nematic liquid crystal MBBA on a homeotropic polyimide alignment layer are significantly lower than previously reported.
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • Thomas Stirner
  • J. Sun
  • D. Wang
A kinetic study of ozone and nitric oxides in dielectric barrier discharges for O2/NOx mixtures, vol. 4, pg. 1227-1238.

In: Plasma Science & Technology

  • 2002

DOI: 10.1088/1009-0630/4/2/008

A simple model is described to simulate kinetic processes in dielectric barrier discharges for O2/NOx mixtures. A threshold of ozone production found experimentally is confirmed by the calculations of this modeling, and the underlying chemical reaction mechanisms are discussed. It is also found that the effects of diffusion processes in the period of the lifetime of O atoms are not important to microdischarge channels with a large radius, i.e. larger than 150 μm.
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Vortrag
  • N. Kirkman
  • W. Hagston
  • Thomas Stirner
A new method for investigating the surface tension from molecular dynamics simulations applied to liquid droplets

In: 12th International Workshop on Computational Materials Science (CMS2002)

  • 2002
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • Thomas Stirner
Interdiffusion in core-shell and quantum-dot-quantum-well nanocrystals, vol. 117, pg. 6715-6720.

In: The Journal of Chemical Physics

  • 2002

DOI: 10.1063/1.1506144

Theoretical calculations, utilizing the effective-mass theory, are presented of exciton energies in semiconductor core-shell and quantum-dot-quantum-well nanocrystals. For core-shell nanocrystals, the influence of diffusion on the transition energies is investigated. It is shown that the diffusion-induced blue shift of the transition energy is a nonmonotonic function of the nanocrystal radius, and that the Coulomb interaction energy of the exciton is a strong function of the diffusion time. The calculations also show that the intersublevel energy spacing is a nonmonotonic function of the ground-state interband transition energy. For quantum-dot-quantum-well nanocrystals, both the exciton transition energy and the overlap integral between the electron and hole wave function is calculated. It is shown that quantum-dot-quantum-well nanocrystals can be designed such that the overlap integral either increases or decreases with increasing amounts of diffusion.
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • Thomas Stirner
  • J. Sun
A photonic crystal with dielectric insertion in macroporous silicon, vol. 322

In: Physica B: Condensed Matter

  • 2002

DOI: 10.1016/S0921-4526(02)01202-4

A photonic crystal structure, designed with dielectric rods at the centres of the air holes in a triangular lattice in macroporous silicon, is investigated theoretically. The calculations show that this structure has several large photonic band gaps at successively higher photon energies. Potential applications of this structure, based on photonic and optoelectronic devices utilizing the higher-lying band gaps, such as optical filters, dual-mode and three-mode lasers, are discussed briefly.
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Vortrag
  • G. Bergmann
  • P. Jackson
  • J.H.C. Hogg
  • Thomas Stirner
  • W. Duffy
  • S. Kelly
  • G. Clark
  • M. O’Neill
In-situ study of the ultraviolet irradiation of side-chain coumarin polymers using specular x-ray reflectivity

In: Synchrotron Radiation in Polymer Science II Conference

  • 2002
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • Thomas Stirner
  • W. Hagston
  • F. Rasul
Quantum theory of infrared detectors based on intrasubband transitions in III-V quantum wells, vol. 89, pg. 1087-1100.

In: Journal of Applied Physics

  • 2001

DOI: 10.1063/1.1333032

One of the perceived drawbacks of multiquantum well (QW)infrared detectors based on intrasubband conduction band transitions in III–V materials (e.g., GaAs/AlxGa1−xAs) is that photons at normal incidence to the multiquantum well interfaces will either not be absorbed or, at best, will be absorbed very weakly. The present article reviews briefly certain key aspects of the experimental evidence and theoretical arguments that both support and challenge this view, prior to developing a completely different approach to the problem. It is shown that the questions at issue relate to basic concepts of quantum mechanics. On this basis it is argued that in QW structures of appropriate design normal incidence absorption should occur, thus enabling, in principle, two-dimensional infrared detectorimaging arrays to be fabricated in a simple manner.
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Zeitschriftenartikel
  • Thomas Stirner
  • W. Hagston
  • F. Long
Application of pseudopotential theory to magnetic semiconductor heterostructures, vol. 29, pg. 217-223.

In: Superlattices and Microstructures

  • 2001

DOI: 10.1006/spmi.2000.0925

The results of empirical pseudopotential calculations for the semiconductor compound Cd1 − xMnxTe are presented. The effective electron and hole masses obtained from the pseudopotential calculations are then employed in an envelope function approximation, using two different effective mass Hamiltonians to evaluate the transition energies of the excitonic ground state in CdTe– Cd1 − xMnxTe quantum wells of variable width. It is shown that in non-magnetic systems it is not possible to utilize exciton energies alone to either distinguish between different model Hamiltonians or to quantify the interface roughness. However, it is shown that the latter can be quantified in magnetic systems via the resulting Zeeman effect.
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • Thomas Stirner
  • W. Hagston
  • F. Rasul
Simplified treatment of scattering processes in quantum well structures, vol. 90, pg. 1082-1089.

In: Journal of Applied Physics

  • 2001

DOI: 10.1063/1.1383265

Arguments are developed which show that, to a good approximation, the essential physics and many of the quantitative details pertaining to the relative rates of carrier–carrier, carrier–longitudinal optical phonon, and carrier–photon scattering in different quantum well structures can be understood in terms of a simplified approach. The latter is based on fundamental concepts of quantum theory and involves the separation of the matrix element in the scattering rate calculation into two simpler parts: an “energy effect” term and a “wave-function effect” term. As an application of the method, the question of attaining lasing action in the far-infrared (terahertz) region of the spectrum is discussed briefly.
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Zeitschriftenartikel
  • J. Sun
  • Thomas Stirner
Molecular Dynamics Simulation of the Surface Pressure of Colloidal Monolayers, vol. 17, pg. 3103-3108.

In: Langmuir

  • 2001

DOI: 10.1021/la001574k

The compression of two-dimensional colloidal monolayers consisting of polystyrene particles at an oil−water interface is investigated theoretically. Expressions for the pair−particle interactions are derived both for point particles and for particles of radius R. The resulting expressions for the interaction energies are employed in a molecular dynamics simulation of the surface pressure of these monolayers. A comparison of the theoretical results with recent observations allows the determination of the dominant interaction mechanisms in the experimental system. Possible physical interpretations of the interaction mechanisms between the particles are discussed.
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • Thomas Stirner
  • W. Hagston
  • S. Takeyama
  • G. Karczewski
  • T. Wojtowicz
  • J. Kossut
Magnetic polaron bifurcation in asymmetric diluted magnetic semiconductor quantum wells, vol. 10, pg. 331-335.

In: Physica E: Low-dimensional Systems and Nanostructures

  • 2001

DOI: 10.1016/S1386-9477(01)00110-2

Calculations of exciton energies are presented in a asymmetric quantum well. The results of the calculations are compared to recent photoluminescence (PL) experiments. The analysis of the PL spectra in a magnetic field reveals the occurrence of a transition with an energy lower than the “free” exciton transition in the quantum well. This additional transition is attributed to the formation of a “structural-asymmetry induced” bifurcated magnetic polaron state. The magnetic-field dependence of the bifurcated polaron state is calculated and found to be consistent with the experimental results.
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Zeitschriftenartikel
  • S. Takeyama
  • W. Hagston
  • J. Kossut
  • Thomas Stirner
  • H. Mino
  • S. Adachi
  • H. Yokoi
  • Y. Semenov
  • S. Tozer
  • G. Kim
  • G. Karczewski
Photoexcited spin states in diluted magnetic semiconductor quantum structures, vol. 294-295, pg. 453-458.

In: Physica B: Condensed Matter

  • 2001

DOI: 10.1016/S0921-4526(00)00698-0

Exciton photoluminescence in a diluted magnetic asymmetric quantum well has given clear evidence of “asymmetric exciton magnetic polaron states” in magnetic fields. The theoretical calculation to interpret the observed magneto-optical data has clarified the importance of exciton localization associated with the interface disorder and the alloy potential fluctuation in addition to the exciton magnetic polaron formation in the asymmetric diluted magnetic barrier. The possibility of a “bifurcated” exciton states is discussed. The transient four-wave-mixing nonlinear spin selective optical measurements revealed the spin-structure of excitons and biexcitons in diluted magnetic quantum structures. The effects of the magnetic field on biexciton states have been investigated. The photoluminescence transition from the biexciton to exciton states has been investigated in magnetic fields, and the stability of the biexciton states in either bonding (spin singlet) or anti-bonding states (spin triplet) is discussed. We have proposed methods to deduce the interface magnetism in the diluted magnetic quantum structures from the high-field Zeeman splitting of exciton photoluminescence up to 60 T.
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Zeitschriftenartikel
  • Thomas Stirner
  • N. Kirkman
  • L. May
  • C. Ellis
  • J. Nicholls
  • S. Kelly
  • M. O’Neill
  • J.H.C. Hogg
CdTe nanocrystals: synthesis, optical characterisation and pseudopotential calculation of the bandgap, vol. 1, pg. 451-455.

In: Journal of Nanoscience and Nanotechnology

  • 2001

DOI: 10.1166/jnn.2001.053

CdTe nanocrystals were synthesized in aqueous solution using 1-thioglycerol and 2-mercaptoethanol as surface stabilizers. The nanocrystals were characterized by means of X-ray powder diffraction and UV-vis absorption measurements. The UV-vis absorption spectra exhibit two distinct transition lines. Comparison of the experimental measurements with the results of the empirical pseudopotential calculations of the CdTe nanocrystals showed that the lower energy absorption line can be assigned to the heavy-hole exciton transition, whereas the higher energy absorption line can be attributed to the light-hole exciton transition.
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Beitrag (Sammelband oder Tagungsband)
  • Thomas Stirner
  • W. Hagston
Dynamical aspects of exciton magnetic polaron formation in diluted magnetic semiconductor quantum wells, pg. 270-271.

In: Proceedings of the 25th International Conference on the Physics of Semiconductors. Osaka, Japan, September 17 - 22, 2000 (Springer Proceedings in Physics)

  • 2000
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Vortrag
  • Thomas Stirner
Computer simulation of alumina (lecture)
  • 2000
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Vortrag
  • S. Takeyama
  • H. Mino
  • S. Adachi
  • H. Yokoi
  • Y. Semenov
  • S. Tozer
  • G. Kim
  • G. Karczewski
  • J. Kossut
  • W. Hagston
  • Thomas Stirner
Photoexcited spin states in diluted magnetic semiconductor quantum structures

In: 6th International Symposium on Research in High Magnetic Fields

  • 2000
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Vortrag
  • S. Takeyama
  • G. Karczewski
  • J. Kossut
  • T. Wojtowicz
  • Thomas Stirner
  • W. Hagston
Magnetic polaron bifurcation in asymmetric diluted magnetic semiconductor quantum wells. Invited Lecture

In: 1st International Conference on the Physics and Applications of Spin-Related Phenomena in Semiconductors (PASPS 2000)

  • 2000
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Vortrag
  • Thomas Stirner
  • W. Hagston
Dynamical aspects of exciton magnetic polaron formation in diluted magnetic semiconductor quantum wells. Posterpräsentation

In: 25th International Conference on the Physics of Semiconductors (ICPS25)

  • 2000
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • Thomas Stirner
  • W. Hagston
  • M. Farrow
Semimagnetic semiconductor quantum wells: magnetic polarons and paramagnetic effects, vol. 12, pg. 701-708.

In: Journal of Physics: Condensed Matter

  • 2000

DOI: 10.1088/0953-8984/12/5/316

Photoluminescence experiments were carried out for Cd1-y Mny Te-Cd1-x Mnx Te-Cd1-y Mny Te quantum wells with y >x . The magnetic field dependence of the exciton emission lines reveals paramagnetic effects which are markedly different from those of bulk CdMnTe. It is shown that these effects are consistent with a clustering of the Mn ions. Furthermore, at low magnetic fields an additional energy shift in the transition energy is observed. This energy shift can be explained by the formation of exciton magnetic polarons in the quantum wells. The polaron energy is calculated as a function of magnetic field strength and an estimate of the localization radius of the magnetic polaron is made.
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • Thomas Stirner
  • W. Hagston
  • J.H.C. Hogg
  • J. Nicholls
  • H. Howari
  • D. Sands
Excimer laser induced diffusion in magnetic semiconductor quantum wells, vol. 88, pg. 1373-1379.

In: Journal of Applied Physics

  • 2000

DOI: 10.1063/1.373826

Studies of pulsed laser annealing (PLA) of CdTe/CdMnTe quantum well structures are made in order to examine depth dependent effects in laser irradiated semiconductors. Since diffusion coefficients are strongly dependent on the temperature, depth resolution is achieved because the diffusion of Mn from the barriers into the quantum wells is depth dependent. Multiple quantum well(MQW) structures of CdTe/CdMnTe were annealed with single pulses from an XeCl laser at 308 nm. At a threshold of 90 mJ cm−2 two new emission bands are observed that are attributed to the diffusion of Mn from barrier layers to QWs. The diffusion associated with these bands, measured as the integrated product of the diffusion constant and time, is found to be 300 and 30 Å2. Calculations of the temperature, reached within the surface following PLA, using an analytical solution of the heat diffusion equation coupled with known high temperature diffusion coefficients predict the diffusion to decrease by one order of magnitude within one period at the top of the MQW stack. It is suggested that at the threshold surface melting occurs and that these emission bands arise from the QWs immediately beneath the melt front. The diffusion of Mn ions into the QWs is confirmed by magneto-optical data. A further emission band occurs at this same threshold with a Mn concentration above that of the concentration in the barrier layers of the MQW stack. This emission is attributed tentatively to the segregation of the Mn ion within the molten region following recrystallization.
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Vortrag
  • Thomas Stirner
Computer simulation of alumina (lecture)
  • 2000
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Vortrag
  • Thomas Stirner
  • S. Takeyama
  • J. Miao
  • W. Hagston
Asymmetric two-dimensional exciton magnetic polarons. Posterpräsentation

In: Autumn Meeting

  • 1999
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Vortrag
  • Thomas Stirner
Diluted Magnetic Semiconductors (lecture)
  • 1999
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • Thomas Stirner
  • W. Hagston
  • J. Miao
Faraday rotation and interface enhancement effects in diluted-magnetic-semiconductor superlattices, vol. 59, pg. 5784-5790.

In: Physical Review B - condensed matter and materials physics

  • 1999
A two-level system model is extended to analyze the Faraday rotation effect in CdTe/Cd1-xMnxTe superlattice structures and to explain experimental Faraday rotation measurements. Furthermore the interface paramagnetism enhancement effect on the Faraday rotation is investigated.
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • Thomas Stirner
  • W. Hagston
  • J. Miao
  • S. Takeyama
  • G. Karczewski
  • T. Wojtowicz
  • J. Kossut
Exciton magnetic polarons in asymmetric diluted magnetic semiconductor quantum wells, vol. 60, pg. 11545-11549.

In: Physical Review B - condensed matter and materials physics

  • 1999

DOI: 10.1103/PhysRevB.60.11545

Photoluminescence experiments were carried out for a 3.6-nm Cd0.78Mg0.22Te-CdTe-Cd0.86Mn0.14Te asymmetric quantum well and show the occurrence of two lines, both associated with excitons confined in the quantum-well region. The magnetic-field dependence of the transition energy revealed the formation of magnetic polarons associated only with the excitonic line at the lower energy. Calculations are presented that are consistent with the experimental observations and in particular show that the lower energy line has a component associated with alloy fluctuations at the well-barrier interface in addition to a component associated with polaron bifurcation.
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Zeitschriftenartikel
  • Thomas Stirner
  • P. Harrison
  • W. Hagston
  • J.H.C. Hogg
  • J. Fatah
Double crystal x-ray diffraction simulations of diffusion in semiconductor microstructures, vol. 83, pg. 4037-4041.

In: Journal of Applied Physics

  • 1998

DOI: 10.1063/1.367159

Diffusion in group IV, III-V and II-VI semiconductors is an interesting problem not only from a fundamental physics viewpoint but also in practical terms, since it could determine the useful lifetime of a device. Any attempt to control the amount of diffusion in a semiconductor device, whether it be a quantum well structure or not, requires an accurate determination of the diffusion coefficient. The present theoretical study shows that this could be achieved via x-ray diffraction studies in quantum well structures. It is demonstrated that the rocking curves of single quantum wells are not sensitive to diffusion. However the intensity of the first order satellite, which is characteristic of superlattice rocking curves, is strongly dependent upon diffusion and it is proposed that this technique could be used to measure the diffusion coefficient D.
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Zeitschriftenartikel
  • P. Chen
  • J. Nicholls
  • M. O’Neill
  • Thomas Stirner
  • J.H.C. Hogg
  • B. Lunn
  • D. Ashenford
Spectroscopic evidence for different laser gain mechanisms in optically pumped ZnCdS/ZnS quantum well structures, vol. 84, pg. 5621-5625.

In: Journal of Applied Physics

  • 1998

DOI: 10.1063/1.368609

The role of biexcitons has been studied in two optically pumped quantum well(QW) structures of Zn1−xCdxS/ZnS, one with 18% Cd concentration and the other with 3% in the wells. For the x=18%QW structure, high excitation photoluminescence and stimulated emission indicate that the laser gain mechanism involves biexcitons. For the x=3%QWs, even though biexcitons are clearly observed in the spontaneous emission, they are not responsible for laser gain in this structure. Instead exciton–exciton scattering may be the more likely mechanism responsible for laser gain close to threshold, while at higher densities an estimate of the carrier density indicates an electron–hole plasma as the likely source of optical gain. The different mechanisms in the two cases can, very likely, be attributed to one of differing degrees of localization both within the QW and at alloy fluctuations.
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Zeitschriftenartikel
  • Thomas Stirner
  • W. Hagston
  • J. Nicholls
  • M. O’Neill
  • S. Weston
  • J. Miao
Mn ion clustering in II-VI semimagnetic semiconductor heterostructures, vol. 58, pg. 7040-7045.

In: Physical Review B - condensed matter and materials physics

  • 1998

DOI: 10.1103/PhysRevB.58.7040

The results of photoluminescence excitation experiments in Cd1-xMnxTe and Zn1-xMnxSe diluted magnetic semiconductor quantum-well structures are shown to be consistent with the occurrence of a clustering of the magnetic Mn ions. Possible clustering mechanisms are discussed briefly together with alternative explanations of the data.
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Zeitschriftenartikel
  • W. Hagston
  • Thomas Stirner
  • P. Harrison
Effects of interface imperfections on the Zeeman splitting of excitons in diluted magnetic semiconductor quantum wells, vol. 75, pg. 349-361.

In: Philosophical Magazine Part B

  • 1997
Theoretical estimates are given of the influence that imperfect interfaces have on the Zeeman splittings of excitons in diluted magnetic semiconductor quantum well structures in which one or both of the barriers are magnetic. A first-principles theoretical treatment of the microscopic magnetic response of the interfaces has been utilized in the calculations. The results show how sensitive these splittings can be to even the smallest degree of imperfection at the interface region provided that both barriers are magnetic and the initial potential profile is symmetric with respect to the centre of the well. A comparison with experimental data on the CdTe/Cd1- x Mn x Te system shows that all such symmetric quantum well structures reported to date have imperfections at the interface region extending over at least one or two monolayers. The calculations also show that asymmetric quantum well structures, in which one of the barriers is non-magnetic, can be a far less sensitive probe of interface imperfections.
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Zeitschriftenartikel
  • J. Miao
  • Thomas Stirner
  • W. Hagston
The magnetic localization of free exciton magnetic polarons in diluted magnetic semiconductors, vol. 81, pg. 6297-6302.

In: Journal of Applied Physics

  • 1997

DOI: 10.1063/1.364364

A model of free exciton magnetic polarons (EMPs) in diluted magnetic semiconductors is proposed on the basis of the localization of the hole in the magnetic potential well created by its own exchange field. The corresponding calculations of the free EMP binding energies, as a function of Mn2+ ion concentration and temperature, are performed. The results are in good agreement with recent experimental measurements. It is found that free EMPs are stable up to relatively high temperatures (T>~30 K) over a wide range of Mn concentrations x without any other primary localization, e.g., alloy potential fluctuations. The dependence of the critical temperature of free EMP formation on the Mn concentration is determined for various diluted magnetic semiconductor materials, and is in good agreement with the experimental results for Cd1-xMnxTe. The suppression of free EMPs under the application of an external magnetic field is also considered. © 1997 American Institute of Physics.
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Zeitschriftenartikel
  • F. Long
  • W. Hagston
  • Thomas Stirner
  • P. Harrison
The structural dependence of the effective mass and Luttinger parameters in semiconductor quantum wells, vol. 82, pg. 3414-3421.

In: Journal of Applied Physics

  • 1997
A detailed comparison of the empirical pseudopotential method with single and multiple band calculations based on the envelope function and effective mass approximations are presented. It is shown that, in order to give agreement with the more rigorous microscopic approach of the pseudopotential method, structural dependent effective masses and Luttinger parameters must be invoked. The CdTe/Cd1−xMnxTe system has been employed as an example, and the first pseudopotential calculations of quantum wells and superlattices in this material are presented. It is shown that the electron, light- and heavy-hole effective masses tend towards twice their bulk values in the limit of narrow quantum wells.
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • Thomas Stirner
  • W. Hagston
  • J. Miao
Localized magnetic polarons in diluted magnetic semiconductors, vol. 82, pg. 5653-5657.

In: Journal of Applied Physics

  • 1997

DOI: 10.1063/1.366426

A theory of localized magnetic polarons in diluted magnetic semiconductors is developed. The theory involves below band gap states and alloy fluctuations utilizing the optimal fluctuation method. A comparison of the theoretical calculations with published experimental results is performed. The implications of the comparison are discussed and the need for extending the theory to include an additional impurity/native lattice defect potential described. The magnetic field and temperature dependence of the polaron energies are also evaluated.
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Vortrag
  • Thomas Stirner
Free and localized magnetic polarons in diluted magnetic semiconductors

In: National II-VI Semiconductor Interaction Meeting

  • 1997
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • Thomas Stirner
  • M. O’Neill
  • D. Ashenford
  • N. Cain
  • W. Hagston
  • J. Nicholls
Photoluminescence of CdTe/CdMnTe multiple quantum wells excited near the Mott transition, vol. 75, pg. 269-275.

In: Journal of Luminescence

  • 1997

DOI: 10.1016/S0022-2313(97)00135-X

Using pulses from a 6 ns dye laser, the photoluminescence from CdTe/CdMnTe quantum wells is investigated as a function of carrier density. The spectra broaden with density but many sharp features remain. A biexciton is identified with a transition energy of 3.8meV less than that of the free exciton. The Mott transition for the el-hl bandedge is approached at an excitation density of 2.2 × 1012 cm−2. Photoluminescence from the el-h3 exciton is observed as a sharp peak at high densities. Its transition energy does not change and its intensity increases linearly with carrier density.
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Zeitschriftenartikel
  • Thomas Stirner
  • W. Hagston
  • J. Miao
Spin-fluctuation effects in magnetic-polaron formation, vol. 75, pg. 897-908.

In: Philosophical Magazine Part B

  • 1997

DOI: 10.1080/13642819708205715

Magnetic-polaron energies in various systems (e.g. donor- or acceptor-bound magnetic polarons and free magnetic polarons) are calculated and the relative importance of the spin-fluctuation term is investigated. It is found that the spin fluctuations of the magnetic ions play a different role in the various kinds of magnetic-polaron complex. Futhermore, it is demonstrated how experimental measurements, for example the magnetic field or temperature dependence of the polaron energy, can be utilized to determine the spin state of the magnetic ions within the polaron. A clear picture of the relationship between the Mn2+ spin fluctuations and magnetic-polaron formation is thus revealed.
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Zeitschriftenartikel
  • Thomas Stirner
  • W. Hagston
  • J. Miao
Spontaneous symmetry breaking of exciton magnetic polarons in CdTe/Cd1-xMnxTe quantum wells, vol. 54, pg. 13938-13945.

In: Physical Review B - condensed matter and materials physics

  • 1996

DOI: 10.1103/PhysRevB.54.13938

Theoretical calculations of exciton magnetic-polaron (EMP) energies in semimagnetic CdTe/Cd1-xMnxTe symmetric quantum wells have been performed. The influence of a spontaneous symmetry breaking of the carrier’s wave function (the so-called interface-bound magnetic polaron) on EMP energies is investigated for various magnetic (Mn) ion concentrations and valence-band offsets. It is demonstrated that the stability of interface-bound magnetic polarons in perfect quantum wells depends on the valence-band offset K, and that, for the accepted value of K in the CdTe/Cd1-xMnxTe system, interface-bound magnetic polarons can form only in samples with low-x values. It is also shown that interface-bound magnetic polarons are destabilized by the presence of interface disorder, or as a result of applying a magnetic field or increasing the lattice temperature. © 1996 The American Physical Society.
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Beitrag (Sammelband oder Tagungsband)
  • N. Cain
  • M. O’Neill
  • J. Nicholls
  • W. Hagston
  • D. Ashenford
  • Thomas Stirner
Photoluminescence of highly excited CdTe/CdMnTe quantum wells, pg. 7.
  • 1996
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • Thomas Stirner
  • P. Harrison
  • W. Hagston
  • T. Piorek
Optical properties of magnetic sawtooth superlattices, vol. 53, pg. 11060-11063.

In: Physical Review B - condensed matter and materials physics

  • 1996

DOI: 10.1103/PhysRevB.53.11060

Self-consistent calculations of the optical properties of magnetic sawtooth superlattices are presented. Sawtooth modulation of the graded gap superlattice as produced by linear variations of the magnetic Mn2+ ion in the Cd1-xMnxTe alloy is described. The Brillouin function response of the band gap together with the varying manganese concentration produces unusual potential profiles in an external magnetic field. The calculations show that the low-temperature magnetic field tunability of the exciton emission is greater in sawtooth superlattices than in an equivalent rectangular superlattice structure. A means of utilizing this for probing alloy nonrandomness is described. © 1996 The American Physical Society.
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Zeitschriftenartikel
  • Thomas Stirner
  • S. Ahmed
  • W. Hagston
Anisotropic magnetic field effects in diluted magnetic semiconductor quantum wells, vol. 159, pg. 1027-1031.

In: Journal of Crystal Growth

  • 1996

DOI: 10.1016/0022-0248(95)00658-3

A theoretical formalism has been developed for calculating the effects of an externally applied magnetic field in both the Faraday and Voigt configuration for diluted magnetic semiconductor structures. The analysis has been applied to the magnetic field dependence of exciton transition energies and of exciton magnetic-polaron energies. Marked anisotropies with respect to the Faraday and Voigt configurations are observed in both cases. The results are interpreted in terms of the different degrees of admixture of light- and heavy-hole states as a function of the field. A comparison with experimental observations enables conclusions to be drawn with regard to important questions such as the degree of interface roughness and the extent of exciton localization in these structures.
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Zeitschriftenartikel
  • T. Piorek
  • P. Harrison
  • W. Hagston
  • Thomas Stirner
Magnetic field induced transitions in diluted magnetic semiconductor quantum wells, vol. 159, pg. 1037-1040.

In: Journal of Crystal Growth

  • 1996

DOI: 10.1016/0022-0248(95)00693-1

The purpose of this paper is to present a detailed theoretical study of the behaviour of excitonic transitions which are forbidden in a zero magnetic field but whose oscillator strength increases with the applied field. The calculation utilizes a two parameter excitonic wave function which permits the relative motion term to assume the symmetry of either a prolate or an oblate spheroid should it be energetically favourable to do so. In addition the effect of the coulombic interaction of the electron and hole is treated in a self-consistent manner and the calculation includes enhanced paramagnetic effects at the interface. A detailed study is given of experimental observations [Ribayrol et al., Phys. Rev. B 51 (1995) 7882] in which the behaviour of the e1-hh3 transition is explained in terms of a magnetic field induced type-I–type-II transition in a diluted magnetic semiconductor quantum well system.
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Vortrag
  • N. Cain
  • M. O’Neill
  • J. Nicholls
  • W. Hagston
  • D. Ashenford
  • Thomas Stirner
Photoluminescence of highly excited CdTe/CdMnTe quantum wells

In: European Quantum Electronics Conference (EQEC)

  • 1996
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • Thomas Stirner
  • P. Harrison
  • W. Hagston
  • J. Goodwin
Reply to "Comment on ‘Theoretical investigation of observed magnetic-polaron energies in quantum wells’", vol. 53, pg. 2143-2144.

In: Physical Review B - condensed matter and materials physics

  • 1996
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • J. Miao
  • W. Hagston
  • Thomas Stirner
Towards a dynamical theory of magnetic polaron formation, vol. 159, pg. 1032-1036.

In: Journal of Crystal Growth

  • 1996
Calculations have been carried out of the magnetic polaron energies associated with a simple donor-type carrier as a function of its mass. Evaluation of the free energy surfaces show the important role of phonons in magnetic polaron formation. A long-range mechanism for the transfer of spin angular momentum out of the polaron complex is proposed, which can account for spin-diffusion associated with polaron formation.
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Zeitschriftenartikel
  • Thomas Stirner
  • W. Hagston
Influence of interface effects on the exciton magnetic-polaron energies in quantum wells and superlattices, vol. 80, pg. 2339-2345.

In: Journal of Applied Physics

  • 1996

DOI: 10.1063/1.363067

Calculations of exciton magnetic‐polaron (EMP) energies in semimagnetic quantum wellstructures and superlattices together with their temperature and magnetic field dependence have been performed. The influence on the EMP energies of the effects of enhanced paramagnetism at the interface and interface roughness are evaluated. It is shown that both these effects increase the theoretical EMP energy values and that interface disorder is the dominant factor in both cases. Furthermore, a more pronounced increase occurs for both effects in superlattices than in single quantum wells.Superlattices can also exhibit an additional type of disorder arising from variations in the barrier/well thickness. It is shown that this kind of disorder gives rise to an increase in the EMP energy values only when it leads to an overall narrowing of the well regions. A comparison with experimental values of the EMP energies in superlattices indicates the presence of small amounts of micro‐roughness in the reported structures on a scale that is similar to that required to account for magneto‐optical data.
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Vortrag
  • Thomas Stirner
Optical monitoring of interface imperfections in DMS quantum wells

In: National II-VI Semiconductor Interaction Meeting

  • 1996
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • P. Harrison
  • W. Hagston
  • Thomas Stirner
  • J. Fatah
Alloy nonrandomness in diluted magnetic semiconductors, vol. 79, pg. 1684-1688.

In: Journal of Applied Physics

  • 1996

DOI: 10.1063/1.360954

A simple model of alloy nonrandomness is introduced within a framework where the effective concentration of spin singlets as a function of the nominal concentration of magnetic ions in a nonrandom alloy can be obtained by transformations of the corresponding function in random alloys. The theory shows that a given system that is appreciably nonrandom can have a magnetic response almost identical with that of a random distribution. Possible ways of identifying alloy nonrandomness in diluted magnetic semiconductor structures are described.
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Vortrag
  • Thomas Stirner
Magnetic polarons in diluted magnetic semiconductors

In: Highlights of UK Research and R&D in Physics and Physics-based Technologies by Young Physicists

  • 1996
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • Thomas Stirner
  • P. Harrison
  • W. Hagston
  • M. O’Neill
The symmetry of the relative motion of excitons in semiconductor heterostructures, vol. 20, pg. 45-47.

In: Superlattices and Microstructures

  • 1996

DOI: 10.1006/spmi.1996.0048

A theoretical model of excitonic states in semiconductor heterostructures is presented. The approach employs the envelope function approximation, and involves a two parameter variational calculation in which the symmetry of the component of the wave function representing the relative motion is allowed to vary between the two- and three-dimensional limits. Detailed calculations are described for a variety of single quantum wells and superlattices. The results show that the excitons are neither 2D nor 3D like, but are intermediate in character. Furthermore, in the main, they assume the symmetry of a prolate spheroid. An exception to this occurs in the special case of an asymmetric double quantum well close to resonance, where two stable exciton states are found for the same one-particle states. One of these ‘twin’ exciton states is an oblate spheroid. The results illustrate the need for accurate determination of excitonic properties if the dynamical evaluation of exciton states, in for example, quantum well lasers, is to be readily determined.
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Vortrag
  • Thomas Stirner
  • S. Ahmed
  • W. Hagston
Anisotropic magnetic field effects in diluted magnetic semiconductor quantum wells

In: 7th International Conference on II-VI Compounds and Devices

  • 1995
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • Thomas Stirner
  • S. Weston
  • P. Harrison
  • S. Bardorf
  • S. Jackson
  • W. Hagston
  • J.H.C. Hogg
  • J. Nicholls
  • M. O’Neill
Experimental determination of the effect of alloy composition on the band alignment of the CdTe-Cd1-xMnxTe heterojunction, vol. 51, pg. 5477-5479.

In: Physical Review B - condensed matter and materials physics

  • 1995
A technique based on photoluminescence excitation measurements is described for the direct determination of the band alignment in a quantum well formed from nonmagnetic and semimagnetic materials. Results are presented for a series of CdTe-Cd1-xMnxTe quantum well samples. These show that the valence band offset, when expressed as a fraction of the total band discontinuity, is a function of the alloy concentration x in the barrier.
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Zeitschriftenartikel
  • J.H.C. Hogg
  • J. Nicholls
  • Thomas Stirner
  • P. Chen
  • W. Hagston
  • B. Lunn
  • D. Ashenford
Determination of the band offset of CdTe/Cd1-xMnxTe multiple quantum wells with very low x values, vol. 52, pg. 4732-4735.

In: Physical Review B - condensed matter and materials physics

  • 1995
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Vortrag
  • J. Miao
  • W. Hagston
  • Thomas Stirner
Towards a dynamical theory of magnetic polaron formation

In: 7th International Conference on II-VI Compounds and Devices

  • 1995
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • P. Harrison
  • W. Hagston
  • Thomas Stirner
Optical methods for determining diffusion in magnetic quantum well structures, vol. 77, pg. 1314-1316.

In: Journal of Applied Physics

  • 1995

DOI: 10.1063/1.358935

It is shown that the occurrence of magnetic polaron formation can have an appreciable influence on the values of the diffusion constants deduced from observations of the photoluminescence in diffused magnetic quantum‐well structures. Arguments are presented that show that photoluminescence excitation spectroscopy provides several independent, more accurate measurements from which the diffusion constants can be more reliably estimated. © 1995 American Institute of Physics.
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Vortrag
  • T. Piorek
  • P. Harrison
  • W. Hagston
  • Thomas Stirner
Magnetic field induced transitions in diluted magnetic semiconductor quantum wells

In: 7th International Conference on II-VI Compounds and Devices

  • 1995
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • P. Harrison
  • W. Hagston
  • Thomas Stirner
The magnitude and sign of the carrier‐magnetic-ion exchange-interaction term in the monolayer regions next to the interface between a magnetic and nonmagnetic semiconductor, vol. 52, pg. R5519-R5522.

In: Physical Review B - condensed matter and materials physics

  • 1995

DOI: 10.1103/PhysRevB.52.R5519

The magnitude and/or the sign of the carrier–magnetic-ion exchange interaction J could be different from that of bulk material in the monolayer regions next to an interface between a magnetic and a nonmagnetic semiconductor. A theoretical investigation of the influence this would have on the Zeeman splittings of both light and heavy holes is evaluated as a function of the well width. Comparison with experimental results for a range of quantum wells of varying width, and differing concentrations of the magnetic ion in the barrier, enables interesting conclusions to be drawn with respect to the variation in this parameter. In particular, the experimental data are shown to be consistent with a variation in J for the electron that is markedly different from that for the hole.
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Zeitschriftenartikel
  • W. Hagston
  • Thomas Stirner
  • P. Harrison
Origin of the Stokes’ shift in semiconductor quantum wells, vol. 3, pg. 95-101.

In: Advanced Materials for Optics and Electronics

  • 1994
Excitonic energy levels have been calculated in CdTe quantum wells with Cdl - xMn,Te barriers. It has been shown that the introduction of small-scale (relative to the exciton Bohr radius) interface disorder, which either preserves or breaks the inversion symmetry, can produce appreciable Stokes shifts between optical absorption and emission and that the associated iine-widths can remain narrow. A criterion for the existence of high-quality interfaces, based on optical spectra, is described.
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Zeitschriftenartikel
  • P. Harrison
  • W. Hagston
  • Thomas Stirner
Neutral donors and spin-flip Raman spectra in dilute-magnetic-semiconductor microstructures, vol. 49, pg. 8242-8248.

In: Physical Review B - condensed matter and materials physics

  • 1994

DOI: 10.1103/PhysRevB.49.8242

An approach is presented for calculating the binding energy of neutral donors in semiconductor heterostructures. The technique avoids the need for analytical solutions. A generalization of the method to include central cell corrections, effective mass, and dielectric mismatch at heterojunctions, together with band nonparabolicity, is discussed. The utility of the method is demonstrated via a study of the spin-flip Raman spectrum of double quantum wells. Finally it is shown how spin-flip Raman spectroscopy could be used, in conjunction with δ doping of donors, as a probe of the possible enhancements to the paramagnetism of a dilute magnetic semiconductor at a heterojunction with a nonmagnetic material.
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Zeitschriftenartikel
  • P. Harrison
  • W. Hagston
  • Thomas Stirner
  • J. Nicholls
Excitonic relaxation channels in double quantum wells, vol. 9, pg. 743-745.

In: Semiconductor Science and Technology

  • 1994
The purpose of this paper is to give a theoretical interpretation of the experimental results of Goede et al. (Superlatt. Microstruct. 12 363 (1992)) describing the relaxation of heavy-hole excitons in asymmetric double quantum well systems based on the semimagnetic CdTe-CdMnTe system. In order to explain certain spectroscopic data, Goede et al. had to invoke excitonic tunnelling between the wells for systems with narrow (25 AA) inner barriers. In this work it is shown that the experimental results imply tunnelling via a 'crossed' excitonic state with the electron and hole localized in different wells. The experimental observations are interpreted in terms of a model involving phonon scattering and calculations of intersubband relaxation rates via confined phonon modes are shown to be in agreement with experimental observations.
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Zeitschriftenartikel
  • P. Harrison
  • W. Hagston
  • J. Goodwin
  • Thomas Stirner
Exciton magnetic polarons in quantum wells, vol. 75, pg. 3466-3471.

In: Journal of Applied Physics

  • 1994

DOI: 10.1063/1.356107

Theoretical calculations of free‐exciton magnetic polaron energies in semimagnetic quantum‐well structures have been performed. The dependence of the polaron energy on well width, magnetic field, and temperature has been calculated. Polaron energy calculations in diffused semimagnetic quantum wells show a strong dependence of the polaron energy on the amount of diffusion in the quantum well. Consequently, magnetic polaron energies could be used as a means of measuring the diffusion coefficient.
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Zeitschriftenartikel
  • P. Harrison
  • W. Hagston
  • T. Piorek
  • Thomas Stirner
  • J. Fatah
  • R. Roberts
Effects of interfaces and interface roughness in diluted magnetic semiconductor microstructures, vol. 16, pg. 11-15.

In: Superlattices and Microstructures

  • 1994

DOI: 10.1006/spmi.1994.1100

In this work a numerical model of anti-ferromagnetic spin-pairing in diluted magnetic semiconductors is used to determine the paramagnetic behaviour of both abrupt and rough interfaces. A detailed study of the paramagnetic behaviour of the individual monolayers adjacent to an abrupt interface with a non-magnetic material has not only predicted quantitatively the enhanced paramagnetism in the ultimate monolayer, it has also predicted a depressed paramagnetism in the penultimate monolayer. Furthermore, the results of the theoretical model have been compared with experimental observations in order to gain insight into intrinsic and structural properties of heterojunctions in diluted magnetic semiconductors.
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Zeitschriftenartikel
  • J. Goodwin
  • P. Harrison
  • W. Hagston
  • Thomas Stirner
Dynamical aspects of free exciton magnetic polaron formation in CdTe-Cd1-xMnxTe quantum wells, vol. 3, pg. 117-125.

In: Advanced Materials for Optics and Electronics

  • 1994

DOI: 10.1002/amo.860030117

Calculations of the energies of magnetic polarons formed by free excitons localised in non-magnetic CdTe wells next to magnetic Cd1 − xMnxTe barriers are presented. A comparison with recent time-resolved spectroscopy results allows insight to the physical aspects governing the dynamics of the formation of the polarons. It is shown that the experimentally measured energy shift is not the polaron energy itself but the difference between this and the change in the exciton binding energy. The latter is calculated within the envelope function approximation and by employing a variational technique. The polaron energy calculation uses a modified version of an approach described by Wolff. The results show that static polaron calculations are not generally reliable and that the exciton–magnetic polaron has to be viewed as a dynamically evolving complex. It is initially energetically favourable for the exciton–magnetic polaron complex to increase its spatial localisation, since the gains in polarisation energy exceed the loss in exciton binding energy. However, thermodynamic considerations suggest that in general saturation of the magnetic ions will not occur.
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Vortrag
  • Thomas Stirner
Alloy clustering and alloy randomness in DMS structures

In: National II-VI Semiconductor Interaction Meeting

  • 1994
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • P. Harrison
  • W. Hagston
  • B. Lunn
  • D. Ashenford
  • S. Jackson
  • J.H.C. Hogg
  • J. Nicholls
  • Thomas Stirner
Magneto-optical study of exciton binding energies, band offsets and the role of interface potentials in CdTe/Cd1-xMnxTe multiple quantum wells, vol. 50, pg. 5392-5403.

In: Physical Review B - condensed matter and materials physics

  • 1994

DOI: 10.1103/PhysRevB.50.5392

Magneto-optical studies on a series of CdTe/Cd1-xMnxTe multiple-quantum-well structures with x~=0.08 have identified the 1S and 2S states of both the light- and heavy-hole n=1 excitons. This has allowed changes of the exciton binding energies to be studied as a function of the depth of the confining potentials, which were tuned through the sp3-d exchange interaction in the barrier layers by application of a magnetic field. Calculations of these binding energies by a variational technique are in general agreement with the observations. The exchange-induced splitting of the heavy-hole exciton is found to be consistent with between 0.35 and 0.45 of the band offset being in the valence band, which accounts for the absence of any evidence that the valence-band structure changes from a type-I structure to a type-II structure above a certain value of a magnetic field. However, this offset is found to be too large to account for the exchange-induced splitting of the light-hole exciton, which appears to be anomalously large. Calculations have shown that this anomaly cannot be explained in terms of the diffusion of Mn ions from the barrier regions into the wells. An alternative explanation is given in terms of field-dependent interface potentials wherein the sp3-d exchange interaction is considered to be different in the interface regions of the multiple quantum wells to that in the barrier regions.
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Zeitschriftenartikel
  • P. Harrison
  • W. Hagston
  • T. Piorek
  • Thomas Stirner
Boundary conditions on current carrying states and the implications for observations of Bloch oscillations, vol. 15, pg. 199-202.

In: Superlattices and Microstructures

  • 1994
The problem of the precise form (together with the associated boundary conditions) of a one-dimensional hamiltonian describing a particle of variable mass is addressed. It is shown that although hermiticity may be a necessary condition, it is not sufficient to specify uniquely the hamiltonian. In particular one form of the latter that is widely employed in the literature is shown to lead to a violation of the Heisenberg Uncertainty Principle. However imposition of the additional demand that any singular terms arising from the kinetic energy can be transformed away does lead to a unique specification of the hamiltonian. Finally the implications of the latter with regard to the standard probability interpretation of the wavefunction and for the observation of Bloch oscillations in quantum well structures is described.
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Zeitschriftenartikel
  • P. Harrison
  • W. Hagston
  • T. Piorek
  • Thomas Stirner
  • J. Fatah
Numerical simulation of antiferromagnetic spin-pairing effects in diluted magnetic semiconductors and enhanced paramagnetism at interfaces, vol. 49, pg. 10341-10344.

In: Physical Review B - condensed matter and materials physics

  • 1994

DOI: 10.1103/PhysRevB.49.10341

A numerical simulation of the antiferromagnetic spin pairing of neighboring magnetic ions within a diluted magnetic semiconductor is presented. Utilizing a random distribution of the magnetic ions requires the inclusion of nearest-neighbor interactions only in order to give agreement with the low-field bulk-magnetization properties observed experimentally. However, the latter are shown to be a relatively insensitive measure of the degree of ordering in the alloy system, and the experimental results could equally well be accounted for by assuming appreciable alloy clustering. The simulations also show that there is an enhancement in the paramagnetism associated with the interface layers in a quantum-well structure.
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Zeitschriftenartikel
  • P. Harrison
  • W. Hagston
  • J. Goodwin
  • Thomas Stirner
Theoretical investigation of observed magnetic polaron energies in quantum wells, vol. 50, pg. 5713-5716.

In: Physical Review B - condensed matter and materials physics

  • 1994

DOI: 10.1103/PhysRevB.50.5713

Theoretical calculations of free-exciton magnetic-polaron energies in semimagnetic quantum-well structures have been performed. The dependence of the polaron energy on well width, magnetic field, and temperature has been calculated and compared with the experimentally determined values. In this manner conclusions have been drawn with regard to the structural characteristics of the quantum-well systems.
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • S. Jackson
  • J. Nicholls
  • P. Harrison
  • W. Hagston
  • Thomas Stirner
  • S. Bardorf
Zeeman studies of CdTe-Cd1-xMnxTe multiquantum wells, vol. 37, pg. 1129-1132.

In: Solid-State Electronics

  • 1994

DOI: 10.1016/0038-1101(94)90371-9

Various novel features of the magnetic field splitting associated with the photoluminescence excitation spectra (PLE) and the photoluminescence spectra (PL) for CdTe/Cd1 − xMnxTe quantum well structures are described. The unusual Zeeman splitting pattern of the heavy-hole barrier exciton state, together with that of the light-hole well exciton state, is shown to be consistent with the magnetic response of the monolayers adjacent to an interface being different from that of the bulk. In particular it is shown that for the carrier in the conduction band, the magnitude of the exchange integral with the magnetic ions in the first two monolayers is of approximately the same magnitude but of opposite sign to that occurring in the bulk.
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Beitrag (Sammelband oder Tagungsband)
  • J. Fatah
  • I. Karla
  • J.H.C. Hogg
  • P. Harrison
  • W. Hagston
  • Thomas Stirner
Defect induced diffusion mechanisms in ion implanted quantum well structures, pg. 2275-2278.
  • 1994
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • P. Harrison
  • W. Hagston
  • J. Goodwin
  • Thomas Stirner
  • O. Holbrook
Impurity bound magnetic polarons in diluted magnetic semiconductors, vol. 50, pg. 5264-5271.

In: Physical Review B - condensed matter and materials physics

  • 1994

DOI: 10.1103/PhysRevB.50.5264

Theoretical calculations of magnetic polaron energies resulting from charge carriers bound to donor or acceptor ion sites in diluted magnetic semiconductors have been performed. The dependence of the polaron energy on the magnetic field, the temperature, and the concentration of the magnetic ion has been evaluated. Similarly the effects of central cell corrections on polaron energies have been determined. The calculated values of the polaron energies are compared with experimental results and the implications of the comparison are discussed.
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Vortrag
  • J. Fatah
  • I. Karla
  • J.H.C. Hogg
  • P. Harrison
  • W. Hagston
  • Thomas Stirner
Defect induced diffusion mechanisms in ion implanted quantum well structures. Posterpräsentation

In: 22nd International Conference on the Physics of Semiconductors (ICPS22)

  • 1994
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • P. Harrison
  • W. Hagston
  • T. Piorek
  • Thomas Stirner
  • J. Fatah
  • R. Roberts
  • D. Ortega
  • A. Chesworth
Monte Carlo simulations of carrier transport and relaxation in superlattices, vol. 15, pg. 209-213.

In: Superlattices and Microstructures

  • 1994

DOI: 10.1006/spmi.1994.1042

Monte Carlo simulations of carrier transport through an undoped superlattice into an extended well are described. The results illustrate clearly the important role played by LO phonons in relaxing the energy and the desirability of designing structures which minimize the need for acoustic phonon participation. Attention is drawn to the fact that although, for the standard structures investigated, the LO phonon relaxation rate is relatively independent of the miniband and/or the quantum well eigenstates, the precise shape of the form-factor for an allowed LO transition could play an important role in deciding the relative number of 'slow' carriers in an actual system.
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Zeitschriftenartikel
  • Thomas Stirner
  • M. O’Neill
  • P. Harrison
  • W. Hagston
Exciton dynamics in multiquantum well CdTe-Cd1-xMnxTe systems, vol. 12, pg. 1150-1152.

In: Journal of Vacuum Science & Technology B

  • 1994

DOI: 10.1116/1.587070

The exciton dynamics associated with the transfer of energy from the light‐hole state to both the heavy‐hole state and to a donor bound exciton (D0X) are investigated utilizing time‐resolved spectroscopy, in a multiquantum well sample of CdTe–Cd1-xMnxTe containing 75 Å wells and 150 Å barriers. The results show that the relaxation time of the light‐hole to the heavy‐hole state occurs, at low temperature, in a time ≤40 ps whereas the combined drift/diffusion of the excitons in the plane of the quantum well and their subsequent trapping at donor states occurs in a time ∼80 ps. Theoretical calculations show that the light‐hole–heavy‐hole carrier relaxation time is consistent with confined (acoustic) phonon relaxation and that the equivalent in‐plane mobility is consistent with interface roughness scattering.
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Zeitschriftenartikel
  • P. Harrison
  • W. Hagston
  • J. Goodwin
  • Thomas Stirner
Effect of interface disorder on the optical properties of CdTe-Cd1-xMnxTe microstructures, vol. 313, pg. 417-426.

In: Surface Science

  • 1994

DOI: 10.1016/0039-6028(94)90061-2

High quality CdTe/Cd1−xMnxTe quantum well structures can be routinely grown in the authors' laboratory by molecular beam epitaxy (MBE). At temperatures of ∼ 2 K these structures have optical linewidths ≲ 3 meV for quantum well widths ≲ 20 Å. The present paper shows how information on the nature and extent of the interface roughness in such systems can be deduced from studies of the optical linewidths and Stokes' shifts.
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Zeitschriftenartikel
  • W. Hagston
  • S. Weston
  • M. O’Neill
  • J.H.C. Hogg
  • D. Ashenford
  • B. Lunn
  • Thomas Stirner
  • P. Harrison
Characteristic Zeeman patterns in novel graded gap II-VI quantum well structures, vol. 12, pg. 1146-1149.

In: Journal of Vacuum Science & Technology B

  • 1994

DOI: 10.1116/1.587069

Novel methods of growing graded gap structures are described. In order to provide a check that the structures are indeed graded in the correct manner, magnetic ions in the structure have been employed. The good agreement between the predicted and observed Zeeman splitting of the corresponding light‐ and heavy‐hole exciton transitions, together with the magnetic field dependence of the Zeeman splittings and intensity of a forbidden heavy‐hole exciton transition, is taken as direct evidence that graded gap structures having the correct composition have indeed been fabricated.
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Zeitschriftenartikel
  • P. Harrison
  • W. Hagston
  • J. Goodwin
  • Thomas Stirner
Magnetic field effects in diluted magnetic semiconductors, vol. 50, pg. 5255-5263.

In: Physical Review B - condensed matter and materials physics

  • 1994

DOI: 10.1103/PhysRevB.50.5255

A general formalism for evaluating the effects of the large magnetic exchange interaction between single carriers and the magnetic ions in a diluted magnetic semiconductor, in the presence of an external magnetic field, is developed. In particular, a justification is provided for incorporating a modified Brillouin function into the description of the resulting magnetic effects.
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Zeitschriftenartikel
  • S. Weston
  • P. Harrison
  • W. Hagston
  • J. Nicholls
  • D. Ashenford
  • Thomas Stirner
Optical properties of single magnetic quantum wells in an external magnetic field, vol. 3, pg. 397-400.

In: Journal de Physique IV

  • 1993

DOI: 10.1051/jp4:1993584

Photoluminescence measurement performed on magnetic Cd1-xMnxTe quantum wells surrounded by Cd1-yMny Te barriers ([MATH]), grown by M. B. E. and subjected to an external magnetic field, are described. The observed excitation spectra are shown to be in agreement with calculations of exciton energies based on the envelope function approximation. Characteristic features of the samples are either the large ([MATH]18meV) Stokes' shift and/or the width of the P. L. lines ([MATH]11meV). These are to be contrasted with the values in comparable non-magnetic wells of CdTe where the Stokes' shift is typically less than 1 meV and the halfwidth of the P. L. lines is less than 1 or 2 meV. Arguments are presented which show that these observations, together with their magnetic field dependence, are consistent with the formation of magnetic polarons. The photoluminescence emission also contains a band attributed to excitons bound to donors. It is observed experimentally that this donor-bound exciton emission disappears with increasing magnetic field. A theoretical interpretation of the observed properties of this band is given in terms of excitons bound to donors which are situated at different positions in the well and barrier region.
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Zeitschriftenartikel
  • S. Weston
  • J. Nicholls
  • M. O’Neill
  • P. Harrison
  • W. Hagston
  • J.H.C. Hogg
  • B. Lunn
  • D. Ashenford
  • Thomas Stirner
Comparative studies of excitons in magnetic asymmetric double quantum well structures, vol. 3, pg. 401-404.

In: Journal de Physique IV

  • 1993

DOI: 10.1051/jp4:1993585

Four different asymmetric double quantum well structures in the dilute magnetic semiconductor system Cd1-xMnx Te have been investigated. Theoretical calculations are presented which show that the observed photoluminescence (PL) and photoluminescence excitation (PLE) energies are consistent with an anticrossing of the lowest two hole states in one of the double quantum well structures when subjected to an external magnetic field.
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Zeitschriftenartikel
  • R. Roberts
  • P. Harrison
  • W. Hagston
  • Thomas Stirner
Stark ladders in strongly coupled finite superlattices, vol. 3, pg. 203-206.

In: Journal de Physique IV

  • 1993
It is shown that the nature and extent of wavefunction localisation of miniband states in an external electric field can be markedly different for finite superlattice structures, compared with infinite and is dependent on the strength of interaction between the initial electron (and hole) miniband states. The question of blue shifts and excitonic effects is described and possible device applications for strongly interacting superlattices structures are discussed.
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Vortrag
  • S. Weston
  • M. O’Neill
  • J. Nicholls
  • J.H.C. Hogg
  • B. Lunn
  • D. Ashenford
  • K. Hieke
  • P. Harrison
  • W. Hagston
  • Thomas Stirner
Comparative studies of excitons in magnetic asymmetric double quantum well structures. Posterpräsentation

In: 3rd International Conference on Optics of Excitons in Confined Systems

  • 1993
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • Thomas Stirner
  • P. Harrison
  • W. Hagston
Violation of the Delta n=0 selection rule for optical transitions in magnetic sawtooth quantum wells, vol. 86, pg. 815-818.

In: Solid State Communications

  • 1993
Sawtooth quantum wells, in which the graded gap is produced by a linear variation in the concentration of a magnetic ion, offer a unique opportunity to investigate the additional optical transitions resulting from symmetry breaking. In the present theoretical work linear variations in the concentration of the magnetic Mn2+ ion in the Cd1−xMnxTe alloy permits a study of the variation of both the energy and oscillator strength of forbidden transitions as a function of the external magnetic field. The resulting variations in the symmetry of the system produces profound effects on the oscillator strengths of the Δn≠0 transitions.
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Zeitschriftenartikel
  • P. Harrison
  • W. Hagston
  • S. Bardorf
  • S. Jackson
  • K. Dhese
  • J.H.C. Hogg
  • V. Hewer
  • J. Nicholls
  • M. O’Neill
  • Thomas Stirner
Interface disorder and the inhomogeneous broadening of optical spectra in semiconductor quantum wells, vol. 13, pg. 431-435.

In: Superlattices and Microstructures

  • 1993
The photoluminescence linewidth of a CdTe/Cd1-xMnxTe structure, grown by molecular beam epitaxy, containing quantum wells of width 26, 37 and 45Å separated by large (250Å) barriers, was investigated. The high quality of the quantum well structure was confirmed by X-ray topography, X-ray rocking curves and narrow photoluminescence lines. It is shown that these results are consistent with small scale interface roughness resulting from clusters involving pairs or triplets of Mn2+ ions forming into small islands.
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Vortrag
  • Thomas Stirner
  • M. O’Neill
  • P. Harrison
  • W. Hagston
Exciton dynamics in multiquantum well CdTe-Cd1-xMnxTe systems

In: 13th North American Conference on Molecular Beam Epitaxy

  • 1993
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • P. Harrison
  • W. Hagston
  • J. Nicholls
  • M. O’Neill
  • T. Piorek
  • Thomas Stirner
  • S. Weston
Investigation of the effect of quantum well width on the binding energy of excitons to neutral donors, vol. 14, pg. 249-252.

In: Superlattices and Microstructures

  • 1993

DOI: 10.1006/spmi.1993.1133

Observations of bound exciton states in some bulk semiconductor materials has shown the validity of Haynes' rule, namely that the binding energy of the exciton to a donor is a multiple 1/ξ (say) of the binding energy of the electron to the donor (i.e., the neutral donor binding energy E D ), although in other bulk materials more general linear dependencies are required. Quantum well structures (QWS) typically exhibit such donor hound exciton complexes. There are several points of difference however, E D is a function of donor position, the donor distribution is unknown and interface roughness could also influence the donor bound exciton emission. Hence in order to determine whether a generalised form of Haynes' rule can be applied to excitons bound to donors in quantum wells of varying width requires a careful combination of theory with experiment. The binding energies of the donors at various positions in each well region must be calculated with due allowance made for the effects of interface roughness. A presumed distribution of the donor concentration is then made and, on the assumed validity of Haynes' rule, the lineshape of the bound exciton emission calculated. Comparison with the observed emission spectra will then give insight into whether Haynes' rule is satisfied as a function of position and well width in QWS. This analysis has been carried out for a series of single quantum wells, all of different widths and all grown in the same sample by the technique of molecular beam epitaxy. A careful analysis of all the data showed that Haynes' rule is not applicable--i.e., the observed exciton energies are not a constant multiple of the donor binding energy for QWS of different well width. PACS: 71.55.Gs; 78.66.Hf.
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Vortrag
  • R. Roberts
  • P. Harrison
  • W. Hagston
  • Thomas Stirner
Stark ladders in strongly coupled finite superlattices. Posterpräsentation

In: 3rd International Conference on Optics of Excitons in Confined Systems

  • 1993
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • Thomas Stirner
  • P. Harrison
  • W. Hagston
  • J. Goodwin
Band gap renormalization and observation of the type I-type II transition in quantum well systems, vol. 73, pg. 5081-5087.

In: Journal of Applied Physics

  • 1993
Quantum wells (or barriers) are of finite extent (≤100 Å) along the growth direction. Hence, for at least one or two monolayers adjacent to the interface, the corresponding one‐electron potential in the conduction (or valence) band must differ from that appropriate to the bulk material. The present article demonstrates the pronounced effect such short‐range ‘‘interface potentials’’ can have on the transition energies and oscillator strengths in a dilute magnetic semiconductor system undergoing a type I–type II transition in an external magnetic field.
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Vortrag
  • S. Weston
  • P. Harrison
  • W. Hagston
  • J. Nicholls
  • D. Ashenford
  • Thomas Stirner
Optical properties of single magnetic quantum wells in an external magnetic field

In: 3rd International Conference on Optics of Excitons in Confined Systems

  • 1993
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • W. Hagston
  • J.H.C. Hogg
  • S. Jackson
  • J. Nicholls
  • B. Lunn
  • D. Ashenford
  • Thomas Stirner
  • P. Harrison
Molecular-beam epitaxy investigation of interface disorder effects in magnetic II-VI quantum wells, vol. 11, pg. 881-884.

In: Journal of Vacuum Science & Technology B

  • 1993
The present article describes the results of a combined theoretical and experimental investigation into the quality of the interfaces of II–VI quantum well structures grown by molecular‐beam epitaxy (MBE). Detailed information is presented on the dilute magnetic semiconductor system CdTe/Cd1-xMnxTe in which the CdTe forms the well and the Cd1-xMnxTe forms the barrier. Structures are grown routinely in which both the photoluminescence (PL) and photoluminescence excitation (PLE) linewidths are narrow (∼1–2 meV). This is indicative of high quality material, a feature which is confirmed by the x‐ray data in which Pendellösung fringes can be seen. However, in spite of this, it is found that although the same growth conditions are nominally employed quantum well structures are obtained which show either (i) different discernible structures in the PL and the PLE spectra, or (ii) marked differences in the Stokes’ shift between the PLE and PL from one sample to the next. A related feature concerns observations of an asymmetrical magnetic field splitting of the heavy‐hole exciton states in the barriers. The results of theoretical calculations of the exciton energy levels and their associated linewidths are presented. A comparison of theory with the experimental observations shows that the above effects can be accounted for in terms of interface disorder and magnetic field dependent interface potentials. Thus in case (i) above, the relative intensity of the components in the PLE is different from that in the PL. This is consistent with large island growth in the plane of the well, large here meaning that the island diameters exceed those of the exciton diameter (∼140–150 Å). Observations of a Stokes’ shift, even though the absorption and emission lines are narrow, can be accounted for theoretically if the concept of smaller scale disorder is i- - ntroduced, i.e., island sizes that are small compared with the diameter of the exciton. Finally, results are presented which show a remarkable asymmetry in the Zeeman splitting of the heavy‐hole exciton lines. It is shown that this can be accounted for by introducing deep, short‐range interface potentials (∼1 or 2 monolayers). The latter are magnetic field dependent, the source of the asymmetry being attributed to band‐gap renormalization effects. The implications of all these results for MBE growth conditions are described.
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Vortrag
  • S. Jackson
  • J. Nicholls
  • P. Harrison
  • W. Hagston
  • Thomas Stirner
  • S. Bardorf
Zeeman studies of CdTe-Cd1-xMnxTe multiquantum wells. Posterpräsentation

In: 6th International Conference on Modulated Semiconductor Structures

  • 1993
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Vortrag
  • P. Harrison
  • W. Hagston
  • Thomas Stirner
  • J. Nicholls
Excitonic relaxation channels in double quantum wells

In: 8th International Conference on Hot Carriers in Semiconductors

  • 1993
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Vortrag
  • W. Hagston
  • Thomas Stirner
  • S. Weston
  • M. O’Neill
  • J.H.C. Hogg
  • B. Lunn
  • D. Ashenford
  • P. Harrison
Characteristic Zeeman patterns in novel graded gap II-VI quantum well structures

In: 13th North American Conference on Molecular Beam Epitaxy

  • 1993
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
  • P. Harrison
  • W. Hagston
  • Thomas Stirner
Excitons in diffused quantum wells, vol. 47, pg. 16404-16409.

In: Physical Review B - condensed matter and materials physics

  • 1993
The Schrödinger equation corresponding to the potential profile resulting from manganese diffusion in CdTe-Cd1-xMnxTe quantum wells has been solved using a numerical method. Calculation of the exciton binding energies has allowed predictions of the effect of diffusion on the resulting optical properties of such quantum-well structures. In addition to the magnetic-field-dependent potential profiles that accompany the diffusion process, it is shown that the ratio of the Zeeman splitting of the light- and heavy-hole excitonic transitions is strongly dependent on the extent of the manganese diffusion. Consequently, this ratio can be used as a measure of the diffusion coefficient.
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Beitrag (Sammelband oder Tagungsband)
  • Thomas Stirner
Origin of the Stokes’ shift in semiconductor quantum wells (lecture). Universität Würzburg/Fakultät für Physik und Astronomie, pg. 409 ff..
  • 1992
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Vortrag
  • W. Hagston
  • J.H.C. Hogg
  • S. Jackson
  • J. Nicholls
  • B. Lunn
  • D. Ashenford
  • Thomas Stirner
  • P. Harrison
Molecular-beam epitaxy investigation of interface disorder effects in magnetic II-VI quantum wells. Posterpräsentation

In: 12th North American Conference on Molecular Beam Epitaxy

  • 1992
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Vortrag
  • J. Goodwin
  • Thomas Stirner
  • P. Harrison
  • W. Hagston
Dynamical aspects of free exciton magnetic polaron formation in CdTe-Cd1-xMnxTe quantum wells. Posterpräsentation

In: European Workshop on II-VI Semiconductors

  • 1992
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Vortrag
  • W. Hagston
  • Thomas Stirner
  • P. Harrison
Origin of the Stokes’ shift in semiconductor quantum wells. Posterpräsentation

In: European Workshop on II-VI Semiconductors

  • 1992
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Vortrag
  • Thomas Stirner
  • P. Harrison
  • W. Hagston
  • J. Goodwin
Effect of interface disorder on Stokes’ shift and photoluminescence line width in CdTe-CdMnTe microstructures

In: IoP Meeting on Strained Layer Structures & Devices

  • 1992
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen