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Suche nach „[Stirner] [Thomas]“ hat 143 Publikationen gefunden
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    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

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    Vortrag

    Thomas Stirner

    Computer simulation of colloidal particle monolayers at liquid-liquid interfaces

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

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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

    Synchrotron Radiation in Polymer Science II Conference, Sheffield, Großbritannien

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    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

    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

    J. Wild, K. Bartle, M. O’Neill, N. Kirkman, P. Tuffin, Thomas Stirner

    Synthesis and investigation of nematic liquid crystals with flexoelectric properties

    Chemistry of Materials, vol. 17, no. 25, pp. 6354-6360

    DOI: 10.1021/cm051682y

    Abstract anzeigen

    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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    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

    Journal of the American Chemical Society (JACS), vol. 128, no. 21, pp. 7087-7090

    Abstract anzeigen

    A new low-temperature, one-pot method is introduced for the preparation of organically passivated HgTe nanocrystals, without the use of highly toxic precursors. The nanocrystals show bright photoluminescence in the infrared telecommunication windows about 1300 and 1550 nm with quantum efficiencies between 55 and 60%. They have a zinc blende structure with a mean particle diameter of 3.4 nm, thus exhibiting quantum confinement effects. Particle growth is self-limited by temperature quenching, so a narrow size distribution is obtained. The measured size of the particles agrees with calculations using the pseudopotential method.

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    Zeitschriftenartikel

    K. Woon, M. Aldred, P. Vlachos, G. Mehl, M. O’Neill, Thomas Stirner

    Electronic charge transport in extended nematic liquid crystals

    Chemistry of Materials, vol. 18, no. 9, pp. 2311-2317

    Abstract anzeigen

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

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    Zeitschriftenartikel

    W. Hagston, S. Weston, M. O’Neill, J.H.C. Hogg, D. Ashenford, B. Lunn, Thomas Stirner, P. Harrison

    Characteristic Zeeman patterns in novel graded gap II-VI quantum well structures

    Journal of Vacuum Science & Technology B, vol. 12, pp. 1146-1149

    DOI: 10.1116/1.587069

    Abstract anzeigen

    Novel methods of growing graded gap structures are described. In order to provide a check that the structures are indeed graded in the correct manner, magnetic ions in the structure have been employed. The good agreement between the predicted and observed Zeeman splitting of the corresponding light‐ and heavy‐hole exciton transitions, together with the magnetic field dependence of the Zeeman splittings and intensity of a forbidden heavy‐hole exciton transition, is taken as direct evidence that graded gap structures having the correct composition have indeed been fabricated.

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    Zeitschriftenartikel

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

    Excitonic relaxation channels in double quantum wells

    Semiconductor Science and Technology, vol. 9, no. 5S, pp. 743-745

    Abstract anzeigen

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

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    Zeitschriftenartikel

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

    Zeeman studies of CdTe-Cd1-xMnxTe multiquantum wells

    Solid-State Electronics, vol. 37, no. 4-6, pp. 1129-1132

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

    Abstract anzeigen

    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

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

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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

    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

    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

    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

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    Beitrag (Sammelband oder Tagungsband)

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

    Cadmium chalcogenide nanocrystals

    Chapter 4

    The handbook of electroluminescent materials, Bristol, UK; Philadelphia, PA

    ISBN: 978-0750309233

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    Zeitschriftenartikel

    W. Hagston, J.H.C. Hogg, S. Jackson, J. Nicholls, B. Lunn, D. Ashenford, Thomas Stirner, P. Harrison

    Molecular-beam epitaxy investigation of interface disorder effects in magnetic II-VI quantum wells

    Journal of Vacuum Science & Technology B, vol. 11, no. 3, pp. 881-884

    Abstract anzeigen

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

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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

    Journal de Physique IV, vol. 3, no. C5, pp. 401-404

    DOI: 10.1051/jp4:1993585

    Abstract anzeigen

    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

    S. Weston, P. Harrison, W. Hagston, J. Nicholls, D. Ashenford, Thomas Stirner

    Optical properties of single magnetic quantum wells in an external magnetic field

    Journal de Physique IV, vol. 3, no. C5, pp. 397-400

    DOI: 10.1051/jp4:1993584

    Abstract anzeigen

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

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    Zeitschriftenartikel

    R. Roberts, P. Harrison, W. Hagston, Thomas Stirner

    Stark ladders in strongly coupled finite superlattices

    Journal de Physique IV, vol. 3, no. C5, pp. 203-206

    Abstract anzeigen

    It is shown that the nature and extent of wavefunction localisation of miniband states in an external electric field can be markedly different for finite superlattice structures, compared with infinite and is dependent on the strength of interaction between the initial electron (and hole) miniband states. The question of blue shifts and excitonic effects is described and possible device applications for strongly interacting superlattices structures are discussed.