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Suche nach „[T.] [Piorek]“ hat 8 Publikationen gefunden
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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

    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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    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, T. Piorek, Thomas Stirner, J. Fatah

    Numerical simulation of antiferromagnetic spin-pairing effects in diluted magnetic semiconductors and enhanced paramagnetism at interfaces

    Physical Review B - condensed matter and materials physics, vol. 49, pp. 10341-10344

    1994

    DOI: 10.1103/PhysRevB.49.10341

    Abstract anzeigen

    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, T. Piorek, Thomas Stirner, J. Fatah, R. Roberts, D. Ortega, A. Chesworth

    Monte Carlo simulations of carrier transport and relaxation in superlattices

    Superlattices and Microstructures, vol. 15, no. 2, pp. 209-213

    1994

    DOI: 10.1006/spmi.1994.1042

    Abstract anzeigen

    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, T. Piorek, Thomas Stirner

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

    Superlattices and Microstructures, vol. 15, no. 2, pp. 199-202

    1994

    Abstract anzeigen

    The problem of the precise form (together with the associated boundary conditions) of a one-dimensional hamiltonian describing a particle of variable mass is addressed. It is shown that although hermiticity may be a necessary condition, it is not sufficient to specify uniquely the hamiltonian. In particular one form of the latter that is widely employed in the literature is shown to lead to a violation of the Heisenberg Uncertainty Principle. However imposition of the additional demand that any singular terms arising from the kinetic energy can be transformed away does lead to a unique specification of the hamiltonian. Finally the implications of the latter with regard to the standard probability interpretation of the wavefunction and for the observation of Bloch oscillations in quantum well structures is described.

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    Zeitschriftenartikel

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

    Effects of interfaces and interface roughness in diluted magnetic semiconductor microstructures

    Superlattices and Microstructures, vol. 16, no. 1, pp. 11-15

    1994

    DOI: 10.1006/spmi.1994.1100

    Abstract anzeigen

    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

    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

    Superlattices and Microstructures, vol. 14, no. 4, pp. 249-252

    1993

    DOI: 10.1006/spmi.1993.1133

    Abstract anzeigen

    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.