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Zeitschriftenartikel

  • H. Zhang
  • J. Sun
  • Y. Wang
  • M. Qin
  • Thomas Stirner

Molecular dynamics simulation analysis of the stress around an up-migrating helium bubble under tungsten surface and its bursting

In: Materials Today Communications vol. 31 pg. 103602.

  • (2022)

DOI: 10.1016/j.mtcomm.2022.103602

Tungsten has been considered as the most qualified candidate for the plasma-facing material of modern fusion devices up to date. However, in the future tokamaks such as ITER and CFETR, the helium species will be trapped inevitably in tungsten, and these alien species gradually agglomerate and coalesce into bubbles under appropriate conditions. The building-up of internal pressure in a bubble near the tungsten surface finally engenders the bubble bursting. The bubble rupture not only produces helium and tungsten impurities but also changes the topography of the tungsten surface; the latter further affects the sputtering. This paper uses molecular dynamics simulations to show how a helium bubble behaves in the course of its migration in tungsten towards the surface at different temperatures, and correspondingly, estimates the critical bursting pressure of bubbles when they come close to the surface. In the meantime, the paper also studies how the helium bubble at different depths affects its surrounding stress in tungsten at the atomic level. The study indicates that the helium bubble makes the stress of its close neighboring part of tungsten increase and that the internal pressure of the bubble is higher when it is deeper below the surface. The study also simulates how the surface morphology evolves after the bubble bursts. It is found that the bubble can burst easily when the temperature is over 1200 K and a considerably deep, cone-shaped pit forms after the bursting. During the bubble bursting, tungsten atoms are ejected out of the bulk. The velocity of these emitted atoms increases with temperature. The emitted atoms fall around the pit more locally during the bursting at 1200 K than at 2300 K. Further discussion is also made over the implications towards the formation of fuzz.
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Zeitschriftenartikel

  • A. Hamid
  • J. Sun
  • H. Zhang
  • Thomas Stirner

Molecular dynamics simulation analysis of helium cluster growth conditions under tungsten surfaces

In: Computational Materials Science vol. 186

  • (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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Zeitschriftenartikel

  • Thomas Stirner
  • David Scholz
  • J. Sun

Ab initio simulation of structure and surface energy of low-index surfaces of stoichiometric alpha-Fe2O3

In: Surface Science vol. 671 pg. 11-16.

  • (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

In: Computational Materials Science vol. 137 pg. 340-345.

  • (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.
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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

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 vol. 463 pg. 363-366.

  • (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

In: Physics of Plasmas vol. 20 pg. 3507 ff..

  • (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

  • Thomas Stirner
  • et al.

Ab initio Hartree-Fock simulation of r-plane sapphire

In: Physics Procedia vol. 32 pg. 635-639.

  • (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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Beitrag (Sammelband oder Tagungsband)

  • Thomas Stirner
  • J. Sun

Computer simulation of alpha-chromia and its surface

In: Chromium. Environmental, medical, and materials studies (Chemical engineering methods and technology) pg. 239-306.

New York

  • (2011)
  • Extern
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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)

In: Surface Science vol. 605 pg. 1298-1303.

  • (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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Zeitschriftenartikel

  • S. Liu
  • J. Sun
  • Thomas Stirner
  • et al.

A general model for chemical erosion of carbon materials due to low-energy H+ impact

In: Journal of Applied Physics vol. 108 pg. 073302.

  • (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
  • C. Sang
  • Thomas Stirner

Characteristics of plasma immersion ion implantation with a nanosecond rise-time pulse: particle-in-cell simulations

In: Journal of Physics D: Applied Physics vol. 43 pg. 275201.

  • (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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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)
  • Hochschulleitung und -einrichtungen
  • Maschinenbau und Mechatronik
  • TC Teisnach Optik
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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)

Peking, China

  • 23.-27.08.2010 (2010)
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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

In: Thin Solid Films vol. 517 pg. 5512-5515.

  • (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

  • Thomas Stirner
  • J. Sun

Non-equilibrium simulation of optically trapped colloidal particles at liquid interfaces

In: Computational Materials Science vol. 43 pg. 769-801.

  • (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

San Diego, CA, USA

  • 10.-14.08.2008 (2008)
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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

In: Journal of the American Chemical Society (JACS) vol. 129 pg. 7674-7679.

  • (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

Structure and electronic properties calculation of ultrathin α-Al2O3 films on (0001) α-Cr2O3 templates

In: Surface Science vol. 601 pg. 5050-5056.

  • (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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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

In: Physical Review B - condensed matter and materials physics vol. 75 pg. 035207.

  • (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

In: Liquid Crystals vol. 34 pg. 1357-1367.

  • (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

A low-temperature synthesis for organically soluble HgTe nanocrystals exhibiting near-infrared photoluminescence and quantum confinement

In: Journal of the American Chemical Society (JACS) vol. 128 pg. 7087-7090.

  • (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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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)

San Diego, CA, USA

  • 01.-05.05.2006 (2006)
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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

In: Applied Physics Letters vol. 87 pg. 061914.

  • (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.
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Zeitschriftenartikel

  • Thomas Stirner
  • J. Sun

Molecular dynamics simulation of the structural configuration of binary colloidal monolayers

In: Langmuir vol. 21 pg. 6636-6641.

  • (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.
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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

In: Chemistry of Materials vol. 17 pg. 6354-6360.

  • (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.
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Vortrag

  • J. Sun
  • Thomas Stirner

Computer simulation of binary oxides . Invited Lecture

In: International Workshop on Hard Metallic Phases

Linköping, Schweden

  • November 2004 (2004)
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Beitrag (Sammelband oder Tagungsband)

  • S. Kelly
  • Thomas Stirner
  • M. O’Neill

Cadmium chalcogenide nanocrystals . Chapter 4

In: The handbook of electroluminescent materials. null (Series in optics and optoelectronics) pg. 158-192.

Bristol, UK; Philadelphia, PA

  • (2004)
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Zeitschriftenartikel

  • N. Kirkman
  • W. Hagston
  • Thomas Stirner

A new method for investigating the surface tension from molecular dynamics simulations applied to liquid droplets

In: Computational Materials Science vol. 30 pg. 126-130.

  • (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
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Zeitschriftenartikel

  • Thomas Stirner
  • W. Hagston
  • N. Kirkman

Continuum modelling of hybrid-aligned nematic liquid-crystal cells: optical response and flexoelectricity-induced voltage shift

In: Liquid Crystals vol. 30 pg. 1115-1122.

  • (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.
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Vortrag

  • Thomas Stirner

Computer simulation of colloidal monolayers (lecture)

Epson UK Ltd. Cambridge, Großbritannien

  • Oktober 2003 (2003)
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Zeitschriftenartikel

  • Thomas Stirner
  • J. Sun

Molecular dynamics simulation of compression-induced solid-to-solid phase transitions in colloidal monolayers

In: Physical Review E - statistical, nonlinear, and soft matter physics vol. 67 pg. 051107.

  • (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.
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Zeitschriftenartikel

  • Thomas Stirner

Interdiffusion in core-shell and quantum-dot-quantum-well nanocrystals

In: The Journal of Chemical Physics vol. 117 pg. 6715-6720.

  • (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.
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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

Sheffield, Großbritannien

  • 04.-06.09.2002 (2002)
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Zeitschriftenartikel

  • Thomas Stirner
  • W. Hagston
  • S. Takeyama
  • G. Karczewski
  • T. Wojtowicz
  • J. Kossut

Magnetic polaron bifurcation in asymmetric diluted magnetic semiconductor quantum wells

In: Physica E: Low-dimensional Systems and Nanostructures vol. 10 pg. 331-335.

  • (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

  • Thomas Stirner
  • W. Hagston
  • F. Long

Application of pseudopotential theory to magnetic semiconductor heterostructures

In: Superlattices and Microstructures vol. 29 pg. 217-223.

  • (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.
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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

In: Journal of Nanoscience and Nanotechnology vol. 1 pg. 451-455.

  • (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.
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Zeitschriftenartikel

  • Thomas Stirner
  • W. Hagston
  • F. Rasul

Quantum theory of infrared detectors based on intrasubband transitions in III-V quantum wells

In: Journal of Applied Physics vol. 89 pg. 1087-1100.

  • (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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Vortrag

  • Thomas Stirner

Computer simulation of alumina (lecture)

Fakultät für Physik Osaka, Japan

  • September 2000 (2000)
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Zeitschriftenartikel

  • Thomas Stirner
  • W. Hagston
  • M. Farrow

Semimagnetic semiconductor quantum wells: magnetic polarons and paramagnetic effects

In: Journal of Physics: Condensed Matter vol. 12 pg. 701-708.

  • Proceedings of the 25th International Conference on the Physics of Semiconductors (ICPS25), Springer Proceedings in Physics, p. 270-271 (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.
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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

Oporto, Portugal

  • 30.07.-03.08.2000 (2000)
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Zeitschriftenartikel

  • Thomas Stirner
  • W. Hagston
  • J.H.C. Hogg
  • J. Nicholls
  • H. Howari
  • D. Sands

Excimer laser induced diffusion in magnetic semiconductor quantum wells

In: Journal of Applied Physics vol. 88 pg. 1373-1379.

  • (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.
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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)

Osaka, Japan

  • 17.-22.09.2000 (2000)
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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)

Sendai, Japan

  • 13.-15.09.2000 (2000)
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel

  • Thomas Stirner
  • W. Hagston
  • J. Miao

Faraday rotation and interface enhancement effects in diluted-magnetic-semiconductor superlattices

In: Physical Review B - condensed matter and materials physics vol. 59 pg. 5784-5790.

  • (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.
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Vortrag

  • Thomas Stirner
  • S. Takeyama
  • J. Miao
  • W. Hagston

Asymmetric two-dimensional exciton magnetic polarons . Posterpräsentation

In: Autumn Meeting

Physical Society of Japan Iwate, Japan

  • August 1999 (1999)
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Zeitschriftenartikel

  • Thomas Stirner
  • W. Hagston
  • J. Miao

Localized magnetic polarons in diluted magnetic semiconductors

In: Journal of Applied Physics vol. 82 pg. 5653-5657.

  • (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.
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Vortrag

  • Thomas Stirner

Free and localized magnetic polarons in diluted magnetic semiconductors

In: National II-VI Semiconductor Interaction Meeting

Norwich, Großbritannien

  • April 1997 (1997)
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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

In: Philosophical Magazine Part B vol. 75 pg. 349-361.

  • (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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Vortrag

  • Thomas Stirner

Optical monitoring of interface imperfections in DMS quantum wells

In: National II-VI Semiconductor Interaction Meeting

Durham, Großbritannien

  • März 1996 (1996)
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Zeitschriftenartikel

  • J. Miao
  • W. Hagston
  • Thomas Stirner

Towards a dynamical theory of magnetic polaron formation

In: Journal of Crystal Growth vol. 159 pg. 1032-1036.

  • (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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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)

Hamburg

  • 08.-13.09.1996 (1996)
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Zeitschriftenartikel

  • Thomas Stirner
  • P. Harrison
  • W. Hagston
  • T. Piorek

Optical properties of magnetic sawtooth superlattices

In: Physical Review B - condensed matter and materials physics vol. 53 pg. 11060-11063.

  • (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

  • 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

In: Physical Review B - condensed matter and materials physics vol. 52 pg. R5519-R5522.

  • (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

  • 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

In: Physical Review B - condensed matter and materials physics vol. 51 pg. 5477-5479.

  • (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

  • 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

In: Physical Review B - condensed matter and materials physics vol. 50 pg. 5392-5403.

  • (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

  • J. Goodwin
  • P. Harrison
  • W. Hagston
  • Thomas Stirner

Dynamical aspects of free exciton magnetic polaron formation in CdTe-Cd1-xMnxTe quantum wells

In: Advanced Materials for Optics and Electronics vol. 3 pg. 117-125.

  • (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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Zeitschriftenartikel

  • P. Harrison
  • W. Hagston
  • T. Piorek
  • Thomas Stirner
  • J. Fatah
  • R. Roberts

Effects of interfaces and interface roughness in diluted magnetic semiconductor microstructures

In: Superlattices and Microstructures vol. 16 pg. 11-15.

  • (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

  • S. Jackson
  • J. Nicholls
  • P. Harrison
  • W. Hagston
  • Thomas Stirner
  • S. Bardorf

Zeeman studies of CdTe-Cd1-xMnxTe multiquantum wells

In: Solid-State Electronics vol. 37 pg. 1129-1132.

  • (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
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)

Vancouver, Kanada

  • August 1994 (1994)
  • Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel

  • P. Harrison
  • W. Hagston
  • J. Goodwin
  • Thomas Stirner

Magnetic field effects in diluted magnetic semiconductors

In: Physical Review B - condensed matter and materials physics vol. 50 pg. 5255-5263.

  • (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

  • 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

In: Physical Review B - condensed matter and materials physics vol. 49 pg. 10341-10344.

  • (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

In: Physical Review B - condensed matter and materials physics vol. 50 pg. 5713-5716.

  • (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.
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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

In: Superlattices and Microstructures vol. 15 pg. 209-213.

  • (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

  • P. Harrison
  • W. Hagston
  • J. Goodwin
  • Thomas Stirner

Effect of interface disorder on the optical properties of CdTe-Cd1-xMnxTe microstructures

In: Surface Science vol. 313 pg. 417-426.

  • (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

  • P. Harrison
  • W. Hagston
  • T. Piorek
  • Thomas Stirner

Boundary conditions on current carrying states and the implications for observations of Bloch oscillations

In: Superlattices and Microstructures vol. 15 pg. 199-202.

  • (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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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

Stanford, CA, USA

  • 13.-15.09.1993 (1993)
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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

In: Journal de Physique IV vol. 3 pg. 397-400.

  • (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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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

Montpellier, Frankreich

  • 30.08.-02.09.1993 (1993)
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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

In: Journal de Physique IV vol. 3 pg. 401-404.

  • (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

  • Thomas Stirner
  • P. Harrison
  • W. Hagston

Violation of the Delta n=0 selection rule for optical transitions in magnetic sawtooth quantum wells

In: Solid State Communications vol. 86 pg. 815-818.

  • (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
  • 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

In: Superlattices and Microstructures vol. 14 pg. 249-252.

  • (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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Zeitschriftenartikel

  • P. Harrison
  • W. Hagston
  • Thomas Stirner

Excitons in diffused quantum wells

In: Physical Review B - condensed matter and materials physics vol. 47 pg. 16404-16409.

  • (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.
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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

Montpellier, Frankreich

  • 30.08.-02.09.1993 (1993)
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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)
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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

Sheffield, Großbritannien

  • 1992 (1992)
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