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Semiconductor Physics Monday for even integer filling factors ν at fixed B. The behavior is consistent within a density-independent quantum scattering time of 7 · 10 −13 s. For the dHvA amplitude at ν = 1 we find a characteristic dependence on B and nS which we explain by exchange enhancement. The authors thank D. Heitmann and A. Schwarz for continuous support and the Deutsche Forschungsgemeinschaft for financial support via SFB 508 and GR 1640/1. HL 9.106 Mon 15:15 P3 Magneto-resistance studies on evenly curved Hall bars — •Olrik Schumacher, Miriam Stampe, Christian Heyn, and Wolfgang Hansen — Institut für Angewandte Physik, Jungiusstr. 11, 20355 Hamburg, Germany We present transport measurements on evenly curved two-dimensional electron systems in InGaAs-microtubes. The method of self-rolling strained semiconductor double layers enables us to build tubes with tuneable radii [1][2]. Using an optimized epitaxial layer design combined with a special lithographic procedure we fabricate GaAs/InGaAs-microtubes with Hall bars containing a two-dimensional electron system (2-DES). When brought into a magnetic field the field component perpendicular to the 2-DES plane is strongly modulated. Transport measurements on such curved Hall bars with current direction along the axis of the microtube will be shown. By rotating the curved Hall bar in the magnetic field one can tune the perpendicular field component such that there is a zero-crossing on the Hall bar. In that case, this field component has HL 10 Invited Talk Santos opposite sign at the edges of the bar and new electron trajectories like snake orbits are predicted [3]. We discuss ac- as well as dc-measurements in view of the signatures of such orbits. [1] V. Ya. Prinz et al., Physica E 6 (2000) 828-831 [2] O. Schumacher et al., Appl. Phys. Lett. 86 (2005) 143109 [3] J. E. Müller, Phys. Rev. Lett. 68 (1992) 385-388 HL 9.107 Mon 15:15 P3 Local properties of disordered fractional quantum hall systems — •Christian Mueller and Daniela Pfannkuche — Jungiusstrasse 9 20355 Hamburg We propose a new method to study the local properties of highly correlated FQH states when disorder is present. The method is based on the correlations between electrons and wavefunction vortices which characterizes different fractional quantum Hall states. We calculate the conditional electron-vortex correlation function gr0 x,y for finite size systems at different filling factors and disorder potentials. Monte-Carlotechniques are used to evaluate the multidimensional integrals involving few-electron-wavefunctions obtained from an exact diagonalization of the Hamiltonian. Usage of the method is demonstrated in a fully spin polarized ν = 1 state with up to 6 electrons. The system is disturbed by 3 several disorder potential types. Regions of different filling factors can de distinguished by comparison of the conditional correlation functions. Time: Tuesday 09:30–10:15 Room: HSZ 01 Invited Talk HL 10.1 Tue 09:30 HSZ 01 Coherent spin transport by acoustic fields in GaAs quantum wells — •Paulo Santos, James Stotz, Odilon Couto, Fernando Iikawa, Rudolph Hey, and Klaus Ploog — Paul-Drude-Institut, Berlin Spintronic applications require processes for the storage, manipulation, and coherent transport of spins. Here, we show that these tasks can be realized in undoped GaAs quantum wells (QWs) by mobile potential dots (dynamic quantum dots, DQDs) induced by surface acoustic waves (SAWs). The type-II piezoelectric potential induces spatially separated negative and positive DQDs, which store and transport photogenerated electrons and holes. The spatial separation prevents recombination and quenches excitonic spin relaxation mechanisms. Measurements of HL 11 Invited Talk Bauer the spin transport length ℓs on QWs grown along different orientations demonstrate that ℓs is limited by the Dyakonov-Perel mechanism. Here, the carriers experience an effective magnetic field Bint associated with spin-orbit coupling, which leads to spin precession angles that depend on the individual carrier trajectory and velocity. Variations in Bint can be minimized by confining all spins within a small DQD during transport. In fact, we demonstrate ℓs approaching 100 µm when the DQD dimensions are reduced below 1 µm. The spins precess coherently around Bint during transport. The spin precession frequency can be controlled by an external magnetic field, thus opening the way for the realization of spin control gates based on SAWs.(Work supported by the NanoQuit/BMBF, Germany.) Time: Tuesday 10:15–11:00 Room: HSZ 01 Invited Talk HL 11.1 Tue 10:15 HSZ 01 Spin accumulation dynamics in semiconductors close to ferromagnetic contacts — •Gerrit E.W. Bauer — TU Delft In recent experiments the spin accumulation electrically injected into semiconductors has been spatially imaged by Faraday or Kerr rotation spectroscopy [1,2]. Here I will review a theory for the spin accumulation that acquires a time-dependence through proximity of ferromagnets and applied magnetic fields [3]. It is shown that the experiments provide important insights into the nature of the spin injection and the semiconductor-ferromagnet interface. [1] J. Stephens, J. Berezovsky, J.P. McGuire, L.J. Sham, A.C. Gossard, D.D. Awschalom, Phys. Rev. Lett. 93, 097602 (2004)} [2] S.A. Crooker, M. Furis, X. Lou, C. Adelmann, D.L. Smith, C.J. Palmstrom, P.A. Crowell, Science 309, 2191 (2005)} [3] G.E.W. Bauer, Y. Tserkovnyak, A. Brataas, J. Ren, K. Xia, M. Zwierzycki, and P. J. Kelly, Phys. Rev. B 72,155304 (2005)} HL 12 Symposium Quantum optics in semiconductors I Time: Tuesday 11:15–13:15 Room: HSZ 01 Keynote Talk HL 12.1 Tue 11:15 HSZ 01 Single Photon Nanotechnology based on Semiconductor Quantum Dots — •A. J. Shields — Quantum Information Group, Toshiba Research Europe Ltd, 260 Cambridge Science Park, Milton Road, Cambridge CB4 0WE UK The potential application of quantum optics in areas such as secure optical communications and ultra-fine imaging has stimulated research on novel components for the generation and detection of single photons. We summarise here progress on quantum photonics based on integrating semiconductor quantum dots into conventional semiconductor devices. The electroluminescence of a single quantum dot in a p-i-n junction can be used to realise a light emitting diode (LED) for single photons.[1] We describe here recent work on incorporating cavities to enhance the emission efficiency,[2] control the polarisation of the emission,[3] as well as gating schemes to reduce the jitter in the emission time and exceed GHz repetition rates.[4] Engineering the dot nanostructure allows the emission wavelength to be tuned to the fibre optic transmission band at 1300nm.[5] We show also that quantum dots integrated inside resonant tunnelling diodes make efficient, low noise detectors of single photons.[6] [1] Yuan et al, Science 295, 102 (2002) [2] Bennett et al, Appl. Phys. Lett. 86, 181102 (2005)
Semiconductor Physics Tuesday [3] Unitt et al, Phys. Rev. B 72, 033318 (2005) [4] Bennett et al, Phys. Rev. B 72, 033316 (2005) [5] Ward et al, Appl. Phys. Lett. 86, 201111 (2005) [6] Blakesley et al, Phys. Rev. Lett. 94, 067401 (2005) Keynote Talk HL 12.2 Tue 11:45 HSZ 01 Size-tunable exchange interaction in InAs/GaAs quantum dots — •U. W. Pohl, A. Schliwa, R. Seguin, S. Rodt, K. Pötschke, and D. Bimberg — Technische Universität Berlin, Hardenbergstr. 36, 10623 Berlin A systematic variation of the exciton fine-structure splitting ranging from above 500 µeV to -80 µeV was found in self-organized InAs/GaAs quantum dots of decreasing, well-defined sizes [1]. This finding is of basic interest for novel applications in e.g. quantum cryptography and logics. The assignment of the dependency was enabled by the recently reported multi-modal size distribution of equally shaped dots in our ensembles [2], providing a major step towards controlling the exchange interaction of confined excitons and resulting properties. We will discuss the origin of such intriguing dot ensembles and show how the comprehensive knowledge of structural dot properties advances understanding and control of excitonic properties. Comparison with model calculations allow statements on the main effects governing the electronic states, yielding the degree of correlation and piezoelectricity as key parameters being responsible for exciton binding energy and fine-structure splitting, respectively. [1] R. Seguin et al., Size-dependent fine-structure splitting in self-organized InAs/GaAs quantum dots, Phys. Rev. Lett., in print. [2] U. W. Pohl et al., Evolution of a multimodal distribution of selforganized InAs/GaAs quantum dots, Phys. Rev. B, in print. Keynote Talk HL 12.3 Tue 12:15 HSZ 01 Quantum Optics of Excitons in Semiconductors — •Heinrich Stolz — Institut für Physik, Universität Rostock, The quantum optical properties of excitons,the bound states of the electron hole system at low and moderate densities in semiconductors are determined by the interplay between the fermionic nature of their constituents and the interaction with the bosonic photon field. In semiconductors with reduced dimensionality we have in addition to take the HL 13 Spin controlled transport I breakdown of the k-selection rule due to the modified symmetry and due to the always present disorder into account. This leads to a large variety of effects, which go beyond those in normal atomic quantum optics. In this talk, the properties connected with the spontaneous radiative decay of excitons will be considered in more detail. In a coherently driven state this leads to the phenomena of resonance fluorescence (RF), which in a 2d quantum well shows up as the driven specularly reflected light and the spontaneous radiation in arbitrary directions. In a real quantum well, the latter is superimposed by the disorder induced Rayleigh scattering. By doing optical homodyning of the resoannce fluorescence under quasi-monochromtic resonant excitation, the reflected light shows strong phase-dependend noise, in contrast to what is expected from the usual semiclassical picture. In the same range of excitation powers, the spectral shape of the RF changes dramatically by developing side wings similiar to the well known Mollow triplet in atomic RF. Both effects can be explained by the simple picture of an ensemble of two-level system. Applying in addition high spatial resolution, we are able to resolve RF from single exciton states and clarifiy their dependence on the disorder. Keynote Talk HL 12.4 Tue 12:45 HSZ 01 Microscopic Theory of Semiconductor Quantum Optics — •Mackillo Kira and Stephan W. Koch — Philipps-University Marburg Quantum optical properties of semiconductors are investigated on the basis of a microscopic theory that includes the quantized light field, Coulomb interacting fermionic electrons and holes, as well as phonons at the same consistent level. The theory is evaluated for a wide range of phenomena including excitonic photoluminescence, entanglement in incoherent emission, and squeezing in resonance fluorescence. The results provide the microscopic foundation of quantum-optical spectroscopy where one generates and detects quasi-particle states in semiconductors whose quantum-statistical properties are governed by that of the exciting light. References: [1] S. Chatterjee et al., Phys. Rev. Lett. 92, 067402 (2004). [2] W. Hoyer et al., Phys. Rev. Lett. 93, 067401(2004). [3] M. Kira and S.W. Koch, Phys. Rev. Lett. 93, 076402 (2004). [4] M. Kira and S.W. Koch, E. Phys. J. D 36, 143 (2005). Time: Tuesday 11:00–13:15 Room: BEY 118 HL 13.1 Tue 11:00 BEY 118 Dynamics of Spin-Flip Scattering due to the Bir-Aronov-Pikus Mechanism — •Hans Christian Schneider — FB Physik, TU Kaiserslautern After a brief review of existing theoretical results on electron-hole exchange scattering in semiconductors (Bir-Aronov-Pikus mechanism), numerical results on the dynamics of this scattering mechanism and its interplay with spin-conserving Coulomb scattering are presented. It is shown that for low electron densities a single spin relaxation-time can approximate the results of the full calculation rather well, even though the individual scattering rates are strongly energy-dependent. This spinrelaxation time is evaluated for a wide range of temperatures and densities and is shown to agree well with recent experiments. The results are also compared with widely-used simplified expressions for spin relaxationtimes that are only valid for extremely high and low p-doping concentrations. The interesting range of doping concentrations and temperatures around hole degeneracy, which cannot be described by simplified expressions, is discussed in detail. HL 13.2 Tue 11:15 BEY 118 Spin Relaxation Anisotropy in Semiconductor Quantum Wells — •Leonid Golub, Nikita Averkiev, Alexey Gurevich, Vadim Evtikhiev, Vladimir Kochereshko, Alexey Platonov, Alexey Shkolnik, and Yuri Dolgikh — A.F. Ioffe Physico-Technical Institute, Russian Academy of Sciences, 194021 St. Petersburg, Russia Spin relaxation of conduction electrons is an important field of research due to rapidly developing semiconductor spintronics. The Dyakonov- Perel spin relaxation process is the dominant spin decoherence mechanism in undoped GaAs-based heterostructures. This mechanism is caused by spin splitting of electron energy spectrum which originate either from the structure inversion asymmetry (Rashba term) or the bulk inversion symmetry (Dresselhaus term). The presence of both Rashba and Dresselhaus spin-splittings results in the electron spin relaxation times anisotropy. A set of triangular and rectangular GaAs (001) quantum wells has been MBE grown on semi-insulating GaAs substrates. Circularly polarized photoluminescence (PL) spectra under circularly polarized excitation have been studied at 77K in magnetic fields up to 0.5T. The magnitude of the optical orientation signal amounts to 10% and is virtually constant within the PL contour. We observe that when the magnetic field is directed along the [110] axis the Hanle curve is about 1.6 times wider than the depolarization curve for [1-10] direction. The observed anisotropy of the Hanle effect is caused by anisotropy of the electron spin relaxation. The analysis shows that the Rashba term is about 4 times stronger than the Dresselhaus term in the studied system. HL 13.3 Tue 11:30 BEY 118 Polarized Electric Current in Semiclassical Transport with Spin-Orbit Interaction — •P.G. Silvestrov 1 and E.G. Mishchenko 2 — 1 Theoretische Physik III, Ruhr-Universitt Bochum, 44780 Bochum, Germany — 2 Department of Physics, University of Utah, Salt Lake City, UT 84112, USA Semiclassical solutions of two-dimensional Schrödinger equation with spin-orbit interaction and smooth potential are considered. In the leading order, spin polarization is in-plane and follows the evolution of the electron momentum for a given subband. Out-of-plane spin polarization appears as a quantum correction, for which an explicit expression is obtained. We demonstrate how spin-polarized currents can be achieved with the help of a barrier or quantum point contact open for transmission only in the lower subband.
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