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Suche nach „[Stirner] [Thomas]“ hat 141 Publikationen gefunden
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    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Vortrag

    Thomas Stirner

    Magnetic polarons in diluted magnetic semiconductors

    Highlights of UK Research and R&D in Physics and Physics-based Technologies by Young Physicists, London, Großbritannien

    1996

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    Zeitschriftenartikel

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

    Magnetic field induced transitions in diluted magnetic semiconductor quantum wells

    Journal of Crystal Growth, vol. 159, no. 1-4, pp. 1037-1040

    1996

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

    Abstract anzeigen

    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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    Zeitschriftenartikel

    J. Miao, W. Hagston, Thomas Stirner

    Towards a dynamical theory of magnetic polaron formation

    Journal of Crystal Growth, vol. 159, no. 1-4, pp. 1032-1036

    1996

    Abstract anzeigen

    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, S. Ahmed, W. Hagston

    Anisotropic magnetic field effects in diluted magnetic semiconductor quantum wells

    Journal of Crystal Growth, vol. 159, no. 1-4, pp. 1027-1031

    1996

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

    Abstract anzeigen

    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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    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

    Proceedings of the 1996 European Quantum Electronics Conference, EQEC'96, Hamburg, 08.-13.09.1996

    1996

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Vortrag

    Thomas Stirner

    Optical monitoring of interface imperfections in DMS quantum wells

    National II-VI Semiconductor Interaction Meeting, Durham, Großbritannien

    1996

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    Zeitschriftenartikel

    Thomas Stirner, W. Hagston

    Influence of interface effects on the exciton magnetic-polaron energies in quantum wells and superlattices

    Journal of Applied Physics, vol. 80, no. 4, pp. 2339-2345

    1996

    DOI: 10.1063/1.363067

    Abstract anzeigen

    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.

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    Zeitschriftenartikel

    Thomas Stirner, W. Hagston, J. Miao

    Spontaneous symmetry breaking of exciton magnetic polarons in CdTe/Cd1-xMnxTe quantum wells

    Physical Review B - condensed matter and materials physics, vol. 54, pp. 13938-13945

    1996

    DOI: 10.1103/PhysRevB.54.13938

    Abstract anzeigen

    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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    Zeitschriftenartikel

    P. Harrison, W. Hagston, Thomas Stirner, J. Fatah

    Alloy nonrandomness in diluted magnetic semiconductors

    Journal of Applied Physics, vol. 79, pp. 1684-1688

    1996

    DOI: 10.1063/1.360954

    Abstract anzeigen

    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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    Zeitschriftenartikel

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

    Reply to "Comment on ‘Theoretical investigation of observed magnetic-polaron energies in quantum wells’"

    Physical Review B - condensed matter and materials physics, vol. 53, pp. 2143-2144

    1996

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

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

    Optical properties of magnetic sawtooth superlattices

    Physical Review B - condensed matter and materials physics, vol. 53, pp. 11060-11063

    1996

    DOI: 10.1103/PhysRevB.53.11060

    Abstract anzeigen

    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, P. Harrison, W. Hagston, M. O’Neill

    The symmetry of the relative motion of excitons in semiconductor heterostructures

    Superlattices and Microstructures, vol. 20, no. 1, pp. 45-47

    1996

    DOI: 10.1006/spmi.1996.0048

    Abstract anzeigen

    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

    J. Miao, W. Hagston, Thomas Stirner

    Towards a dynamical theory of magnetic polaron formation

    7th International Conference on II-VI Compounds and Devices, Edinburgh, Großbritannien

    1995

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Vortrag

    Thomas Stirner, S. Ahmed, W. Hagston

    Anisotropic magnetic field effects in diluted magnetic semiconductor quantum wells

    7th International Conference on II-VI Compounds and Devices, Edinburgh, Großbritannien

    1995

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Vortrag

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

    Magnetic field induced transitions in diluted magnetic semiconductor quantum wells

    7th International Conference on II-VI Compounds and Devices, Edinburgh, Großbritannien

    1995

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    P. Harrison, W. Hagston, Thomas Stirner

    Optical methods for determining diffusion in magnetic quantum well structures

    Journal of Applied Physics, vol. 77, no. 3, pp. 1314-1316

    1995

    DOI: 10.1063/1.358935

    Abstract anzeigen

    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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    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

    Physical Review B - condensed matter and materials physics, vol. 51, no. 8, pp. 5477-5479

    1995

    Abstract anzeigen

    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

    Physical Review B - condensed matter and materials physics, vol. 52, no. 7, pp. 4732-4735

    1995

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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

    Physical Review B - condensed matter and materials physics, vol. 52

    1995

    DOI: 10.1103/PhysRevB.52.R5519

    Abstract anzeigen

    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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    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

    22nd International Conference on the Physics of Semiconductors (ICPS22), Vancouver, Kanada

    1994