Publikationen


Suche nach „[Stirner] [Thomas]“ hat 139 Publikationen gefunden
Suchergebnis als PDF
    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    J. Sun, Thomas Stirner

    Molecular dynamics simulation of optically trapped colloidal particles at an oil-water interface

    The Journal of Chemical Physics, vol. 121, no. 9, pp. 4292-4296

    2004

    DOI: 10.1063/1.1779569

    Abstract anzeigen

    Using molecular dynamics simulations, we calculate the net force on a colloidal particle trapped by an optical tweezer and confined within a particle monolayer which is in motion relative to the trapped particle. The calculations are compared with recent experimental data on polystyrene particles located at an oil-water interface. Good agreement between theory and experiment is obtained over the investigated range of lattice constants for an interaction mechanism between the polystyrene particles which is dominated by an effective dipole-dipole potential. The assumed interaction mechanism is consistent with the formation of surface charge dipoles at the particle-oil interface due to the dissociaton of the hydrophilic sulfate headgroups at the surface of the polystyrene particles. A possible physical mechanism for the formation of the surface charge dipoles, involving a diffuse cloud of fully hydrated counterions, is described, and the fraction of surface groups contributing to the formation of surface charge dipoles is estimated to be of the order of 10−1 for the present system.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Vortrag

    Thomas Stirner

    Computer simulation of colloidal monolayers (lecture)

    2003

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    N. Kirkman, W. Hagston, Thomas Stirner

    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.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    J. Sun, Thomas Stirner

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

    Physical Review E - statistical, nonlinear, and soft matter physics, vol. 67, no. 5 (Pt 1)

    2003

    DOI: 10.1103/PhysRevE.67.051107

    Abstract anzeigen

    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.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    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

    Vortrag

    P. Jackson, W. Duffy, S. Kelly, G. Bergmann, Thomas Stirner, M. O’Neill, J.H.C. Hogg, G. Clark

    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

    2002

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    Thomas Stirner

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

    The Journal of Chemical Physics, vol. 117, pp. 6715-6720

    2002

    DOI: 10.1063/1.1506144

    Abstract anzeigen

    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.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    J. Sun, Thomas Stirner, D. Wang

    A kinetic study of ozone and nitric oxides in dielectric barrier discharges for O2/NOx mixtures

    Plasma Science & Technology, vol. 4, pp. 1227-1238

    2002

    DOI: 10.1088/1009-0630/4/2/008

    Abstract anzeigen

    A simple model is described to simulate kinetic processes in dielectric barrier discharges for O2/NOx mixtures. A threshold of ozone production found experimentally is confirmed by the calculations of this modeling, and the underlying chemical reaction mechanisms are discussed. It is also found that the effects of diffusion processes in the period of the lifetime of O atoms are not important to microdischarge channels with a large radius, i.e. larger than 150 μm.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    J. Sun, Thomas Stirner

    A photonic crystal with dielectric insertion in macroporous silicon

    Physica B: Condensed Matter, vol. 322, no. 323-327

    2002

    DOI: 10.1016/S0921-4526(02)01202-4

    Abstract anzeigen

    A photonic crystal structure, designed with dielectric rods at the centres of the air holes in a triangular lattice in macroporous silicon, is investigated theoretically. The calculations show that this structure has several large photonic band gaps at successively higher photon energies. Potential applications of this structure, based on photonic and optoelectronic devices utilizing the higher-lying band gaps, such as optical filters, dual-mode and three-mode lasers, are discussed briefly.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

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

    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.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    Y. Semenov, H. Mino, S. Tozer, W. Hagston, Thomas Stirner, S. Adachi, J. Kossut, G. Karczewski, H. Yokoi, G. Kim, S. Takeyama

    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

    C. Ellis, L. May, N. Kirkman, S. Kelly, Thomas Stirner, J. Nicholls, M. O’Neill, J.H.C. Hogg

    CdTe nanocrystals: synthesis, optical characterisation and pseudopotential calculation of the bandgap

    Journal of Nanoscience and Nanotechnology, vol. 1, no. 4, pp. 451-455

    2001

    DOI: 10.1166/jnn.2001.053

    Abstract anzeigen

    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.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    W. Hagston, Thomas Stirner, 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

    Jizhong Sun, Thomas Stirner

    Molecular Dynamics Simulation of the Surface Pressure of Colloidal Monolayers

    Langmuir, vol. 17, no. 10, pp. 3103-3108

    2001

    DOI: 10.1021/la001574k

    Abstract anzeigen

    The compression of two-dimensional colloidal monolayers consisting of polystyrene particles at an oil−water interface is investigated theoretically. Expressions for the pair−particle interactions are derived both for point particles and for particles of radius R. The resulting expressions for the interaction energies are employed in a molecular dynamics simulation of the surface pressure of these monolayers. A comparison of the theoretical results with recent observations allows the determination of the dominant interaction mechanisms in the experimental system. Possible physical interpretations of the interaction mechanisms between the particles are discussed.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    W. Hagston, F. Long, Thomas Stirner

    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

    W. Hagston, Thomas Stirner, 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)

    W. Hagston, Thomas Stirner

    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

    W. Hagston, Thomas Stirner

    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

    Y. Semenov, H. Mino, S. Tozer, W. Hagston, Thomas Stirner, S. Adachi, J. Kossut, G. Karczewski, H. Yokoi, G. Kim, S. Takeyama

    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

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

    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