Publikationen


Suche nach „[W.] [Hagston]“ hat 83 Publikationen gefunden
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    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

    Journal of Crystal Growth, vol. 290, no. 1, pp. 235-240

    2006

    Abstract anzeigen

    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

    N. Kirkman, W. Hagston, Thomas Stirner

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

    Computational Materials Science, vol. 30, no. 5, pp. 126-130

    2004

    DOI: 10.1016/j.commatsci.2004.01.020

    Abstract anzeigen

    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

    Zeitschriftenartikel

    Thomas Stirner, W. Hagston, N. Kirkman

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

    Liquid Crystals, vol. 30, no. 9, pp. 1115-1122

    2003

    DOI: 10.1080/02678290310001594562

    Abstract anzeigen

    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

    N. Kirkman, W. Hagston, Thomas Stirner

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

    12th International Workshop on Computational Materials Science (CMS2002), Villasimius, Italien

    2002

    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

    Physica E: Low-dimensional Systems and Nanostructures, vol. 10, no. 1-3, pp. 331-335

    2001

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

    Abstract anzeigen

    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

    Physica B: Condensed Matter, vol. 294-295, no. Januar, pp. 453-458

    2001

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

    Abstract anzeigen

    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.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    Thomas Stirner, W. Hagston, F. Rasul

    Simplified treatment of scattering processes in quantum well structures

    Journal of Applied Physics, vol. 90, pp. 1082-1089

    2001

    DOI: 10.1063/1.1383265

    Abstract anzeigen

    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.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    Thomas Stirner, W. Hagston, F. Long

    Application of pseudopotential theory to magnetic semiconductor heterostructures

    Superlattices and Microstructures, vol. 29, no. 3, pp. 217-223

    2001

    DOI: 10.1006/spmi.2000.0925

    Abstract anzeigen

    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

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

    Journal of Applied Physics, vol. 89, pp. 1087-1100

    2001

    DOI: 10.1063/1.1333032

    Abstract anzeigen

    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.

    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

    Proceedings of the 25th International Conference on the Physics of Semiconductors, Berlin; Heidelberg; New York; Barcelona; Hong Kong; London, vol. 87

    2000

    ISBN: 978-3-540-41778-1

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Vortrag

    Thomas Stirner, W. Hagston

    Dynamical aspects of exciton magnetic polaron formation in diluted magnetic semiconductor quantum wells

    Posterpräsentation

    25th International Conference on the Physics of Semiconductors (ICPS25), Osaka, Japan

    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

    6th International Symposium on Research in High Magnetic Fields, Oporto, Portugal

    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

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

    2000

    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

    Journal of Applied Physics, vol. 88, pp. 1373-1379

    2000

    DOI: 10.1063/1.373826

    Abstract anzeigen

    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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    Zeitschriftenartikel

    Thomas Stirner, W. Hagston, M. Farrow

    Semimagnetic semiconductor quantum wells: magnetic polarons and paramagnetic effects

    Journal of Physics: Condensed Matter, vol. 12, no. 5, pp. 701-708

    2000

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

    Abstract anzeigen

    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

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

    Asymmetric two-dimensional exciton magnetic polarons

    Posterpräsentation

    Autumn Meeting, Iwate, Japan

    1999

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    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

    Physical Review B - condensed matter and materials physics, vol. 60, no. 16, pp. 11545-11549

    1999

    DOI: 10.1103/PhysRevB.60.11545

    Abstract anzeigen

    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.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    Thomas Stirner, W. Hagston, J. Miao

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

    Physical Review B - condensed matter and materials physics, vol. 59, pp. 5784-5790

    1999

    Abstract anzeigen

    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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    Zeitschriftenartikel

    Thomas Stirner, W. Hagston, J. Nicholls, M. O’Neill, S. Weston, J. Miao

    Mn ion clustering in II-VI semimagnetic semiconductor heterostructures

    Physical Review B - condensed matter and materials physics, vol. 58, pp. 7040-7045

    1998

    DOI: 10.1103/PhysRevB.58.7040

    Abstract anzeigen

    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

    Thomas Stirner, P. Harrison, W. Hagston, J.H.C. Hogg, J. Fatah

    Double crystal x-ray diffraction simulations of diffusion in semiconductor microstructures

    Journal of Applied Physics, vol. 83, pp. 4037-4041

    1998

    DOI: 10.1063/1.367159

    Abstract anzeigen

    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.