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Plenarvorträge - DPG-Tagungen

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Halbleiterphysik Montag<br />

HL 5.7 Mo 11:45 H14<br />

Quantum Dots as Quantum Bits — •Daniel Schröer 1 , Andreas<br />

K. Hüttel 1 , Stefan Ludwig 1 , Werner Wegscheider 2 , and<br />

Jörg Kotthaus 1 — 1 CeNS und Sektion Physik der LMU München —<br />

2 Institut für Angewandte und Experimentelle Physik der Universität Re-<br />

gensburg<br />

We create lateral quantum dots by depleting the two-dimensional electron<br />

system of an AlGaAs/GaAs heterostructure with lithographically<br />

defined Schottky gates. The advantage of this lateral approach is the<br />

flexibility in number and orientation of leads and gates. Multiple quantum<br />

dots can be coupled capacitively or via tunnel barriers. The coupling<br />

strengths and the number of free charge carriers per dot are tuned by<br />

means of gate voltages. A double quantum dot is often regarded as an<br />

artificial molecule, as the discrete states of both dots split into molecular<br />

bonding and antibonding states. These form a two-level system that is<br />

applicable as a qubit for quantum information processing. The complexity<br />

of the molecular states increases with the number of free electrons per<br />

dot. Hence, we aim to reduce the dot sizes and integrate suitable tools to<br />

read out the number of charge carriers (e.g. a quantum point contact).<br />

The molecular states are probed by analysing the charging diagram of<br />

the system. In addition we will attempt to couple more than two quantum<br />

dots to study the interaction of several qubits. First results will be<br />

presented.<br />

HL 5.8 Mo 12:00 H14<br />

Scattering Wavefunctions in Ballistic Rectifyers<br />

— •Oliver Bendix, Ragnar Fleischmann, and Theo Geisel —<br />

MPI für Strömungsforschung and Institut für Nichtlineare Dynamik, Universität<br />

Göttingen, Bunsenstraße 10, 37073 Göttingen<br />

Electron transport in mesoscopic systems in semiconductors with typical<br />

length scales shorter than the mean free path can be assumed to<br />

be ballistic. Recent experimental and theoretical work has revealed an<br />

unexpected rectification effect based on ballistic transport in mesoscopic<br />

cross-junctions with broken symmetry[1,2,3].<br />

In our simplified model[2] a crucial role is played by the energydependence<br />

of the number of current-carrying modes. In a more detailed<br />

description this corresponds to the energy-dependence of the scattering<br />

coefficients. Here we present numerical results for the scattering wavefunctions<br />

in ballistic rectifyers.<br />

HL 6 Photonische Kristalle I<br />

[1] A. M. Song et al., PRL 80 (1998) 3831<br />

[2] R. Fleischmann and T. Geisel, PRL 89 (2002) 16804<br />

[3] S. de Haan et al., PRL (in press)<br />

HL 5.9 Mo 12:15 H14<br />

Preparation of curved two dimensional electron systems in<br />

InGaAs-microtubes — •Stefan Mendach, Olrik Schumacher,<br />

Christian Heyn, Holger Welsch, and Wolfgang Hansen — Institut<br />

für Angewandte Physik, Universität Hamburg, Jungiusstrasse 11,<br />

D-20355 Hamburg, Germany<br />

We present a preparation method to realise transport measurements<br />

on evenly curved two-dimensional electron systems (2 DESs). By<br />

combining the method of self-rolling strained double-layers [1,2] with<br />

a special lithographic procedure we are able to roll-in and contact<br />

AlGaAs/GaAs/AlGaAs quantum well structures into tubes or curved<br />

lamellas, respectively. Applying a magnetic field to such structures<br />

results in a strong modulation with changing sign of the magnetic field<br />

components perpendicular to the curved 2 DES plane. Our preparation<br />

method allows transport measurements along or perpendicular to<br />

this modulation. We present and discuss our first magneto-transport<br />

measurements on such rolled 2 DESs.<br />

[1] V. Ya. Prinz et al., Physica E6 (2000) 828-831.<br />

[2] O.G. Schmidt, K. Eberl, Nature 410 (2001) 168.<br />

HL 5.10 Mo 12:30 H14<br />

Bosonization method for disordered and chaotic systems — •G.<br />

Schwiete 1 , K. B. Efetov 1,2 , and K. Takahashi 1 — 1 Theoretische<br />

Physik III, Ruhr-Universität Bochum, 44780 Bochum, Germany — 2 L.<br />

D. Landau Institute for Theoretical Physics, 117940 Moscow, Russia<br />

Using a supersymmetry formalism we derive a new supermatrix representation<br />

of the generating functional for the problem of electron motion<br />

in an external potential. Non-linear sigma models derived previously for<br />

disordered systems can be obtained from this model using a coarse graining<br />

procedure. As an example we treat smooth disorder in an infinite system.<br />

As a new application, we consider scattering on strong impurities<br />

for which the Born approximation cannot be used. Possible applications<br />

to quantum chaotic systems and the interacting disordered electron gas<br />

will be discussed.<br />

Zeit: Montag 12:45–13:30 Raum: H15<br />

HL 6.1 Mo 12:45 H15<br />

Space-dependent optical and electronic properties of semiconductor<br />

photonic-crystal structures — •B. Pasenow, M. Reichelt,<br />

T. Meier, T. Stroucken, P. Thomas, and S.W. Koch<br />

— Fachbereich Physik und Wissenschaftliches Zentrum für Materialwissenschaften,<br />

Philipps-Universität, Renthof 5, D-35032 Marburg<br />

The optical and electronic properties of a semiconductor heterostructure<br />

are modified in the vicinity of a structured dielectric environment,<br />

e.g., a photonic crystal [1-3]. In such structures the band gap and the<br />

Coulomb interaction potential are space dependent, which results in a<br />

spatial variation of the spectral position of the exciton resonance and its<br />

binding energy [2]. Therefore one can create spatially non-homogeneous<br />

carrier distributions by spectrally selective optical excitation [3]. Following<br />

the excitation, the carrier distributions show initially an interesting<br />

coherent wave packet dynamics, before they eventually approach an incoherent<br />

thermal quasi-equilibrium distribution.<br />

[1] T. Stroucken, R. Eichmann, L. Banyai, and S.W. Koch, J. Opt. Soc.<br />

Am. B 19, 2292 (2002).<br />

[2] R. Eichmann, B. Pasenow, T. Meier, T. Stroucken, P. Thomas, and<br />

S.W. Koch, Appl. Phys. Lett. 82, 355 (2003).<br />

[3] Semiconductor Optics in Photonic Crystal Structures, T. Meier and<br />

S.W. Koch, to appear in Photonic Crystals - Advances in Design, Fabrication<br />

and Characterization, Eds. K. Busch, S. Lölkes, R.B. Wehrspohn,<br />

and H. Föll, Wiley, 2004.<br />

HL 6.2 Mo 13:00 H15<br />

Strongly localized discrete solitons in photonic crystal defect<br />

chains — •Rumen Iliew, Christoph Etrich, Ulf Peschel, and<br />

Falk Lederer — Inst. für Festkörpertheorie und Theoretische Optik,<br />

Friedrich-Schiller-Univ. Jena, Max-Wien-Platz 1, 07743 Jena<br />

We found strongly localized solitons in a linear chain of coupled point<br />

defects embedded in a Kerr-type nonlinear photonic crystal.<br />

The dynamics of such a coupled-resonator optical waveguide (CROW)<br />

can be described in tight-binding approximation by a set of coupled<br />

modal equations [1] which were shown to allow for strongly localized soliton<br />

solutions [2]. To compare these results with the full system, for the<br />

first time, to our knowledge, the temporal dynamics of localized excitations<br />

is investigated in a two-dimensional (2D) nonlinear photonic crystal<br />

by employing 2D TM finite-difference time-domain simulations including<br />

Kerr-nonlinearity. Our model system is a 2D hexagonal photonic crystal<br />

with a TM photonic band gap in the linear regime where a chain<br />

of point-defects was embedded. Depending on the pump parameters we<br />

found strongly localized solitons in the nonlinear regime at frequencies<br />

below the miniband, as predicted by the tight-binding approach.<br />

We will present results of FDTD simulations and compare them against<br />

the results obtained from the modal equations.<br />

[1] D. N. Christodoulides and N. K. Efremidis, Opt. Lett. 27, 568 (2002).<br />

[2] F. Lederer, S. Darmanyan, and A. Kobyakov, Spatial Solitons,<br />

Springer Series in Optical Science 82, S. Trillo and W. E. Torruellas<br />

(eds.), (Springer-Verlag New York 2001), pp. 269–292.<br />

HL 6.3 Mo 13:15 H15<br />

Tuneable Opals — •A. von Rhein 1 , T. Kramer 2 , T. Roeder 2 ,<br />

K. Huber 2 , and S. Greulich-Weber 1 — 1 University of Paderborn,<br />

Fakulty of Science, Physics Department, D-33098 Paderbron, Germany<br />

— 2 University of Paderborn, Fakulty of Science, Chemistry Department,<br />

D-33098 Paderbron, Germany<br />

Photonic crystals already have been realized thru a wide range of methods.<br />

In particular nature has developed a selfassembling technique of creating<br />

photonic crytals. Nanospheres of about 200-400 nm diameter build

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