arXiv subjects

A. D. Wieck

Publications and source records attributed to A. D. Wieck.

137 records · Page 8Linked to original sources

arXiv2005

Coulomb-interaction induced incomplete shell filling in the hole system of InAs quantum dots

We have studied the hole charging spectra of self-assembled InAs quantum dots in perpendicular magnetic fields by capacitance-voltage spectroscopy. From the magnetic field dependence of the individual peaks we conclude that the s-like ground state is completely filled with two holes but that the fourfold degenerate p-shell is only half filled with two holes before the filling of the d-shell starts. The resulting six-hole ground state is highly polarized. This incomplete shell filling can be explained by the large influence of the Coulomb interaction in this system.

cond-mat.mes-hall↗

arXiv2004

Optical orientation of electron spins in GaAs quantum wells

We present a detailed experimental and theoretical analysis of the optical orientation of electron spins in GaAs/AlAs quantum wells. Using time and polarization resolved photoluminescence excitation spectroscopy, the initial degree of electron spin polarization is measured as a function of excitation energy for a sequence of quantum wells with well widths between 63 Ang and 198 Ang. The experimental results are compared with an accurate theory of excitonic absorption taking fully into account electron-hole Coulomb correlations and heavy-hole light-hole coupling. We find in wide quantum wells that the measured initial degree of polarization of the luminescence follows closely the spin polarization of the optically excited electrons calculated as a function of energy. This implies that the orientation of the electron spins is essentially preserved when the electrons relax from the optically excited high-energy states to quasi-thermal equilibrium of their momenta. Due to initial spin relaxation, the measured polarization in narrow quantum wells is reduced by a constant factor that does not depend on the excitation energy.

cond-mat.mes-hall↗

arXiv2004

Magnetotransport in C-doped AlGaAs heterostructures

High-quality C-doped p-type AlGaAs heterostructures with mobilities exceeding 150 000 cm$^2$/Vs are investigated by low-temperature magnetotransport experiments. We find features of the fractional quantum Hall effect as well as a highly resolved Shubnikov-de Haas oscillations at low magnetic fields. This allows us to determine the densities, effective masses and mobilities of the holes populating the spin-split subbands arising from the lack of inversion symmetry in these structures.

cond-mat.mes-hall↗

arXiv2004

Phonon Excitations of Composite Fermion Landau Levels

Phonon excitations of fractional quantum Hall states at filling factors nu = 1/3, 2/5, 4/7, 3/5, 4/3, and 5/3 are experimentally shown to be based on Landau level transitions of Composite Fermions. At filling factor nu = 2/3, however, a linear field dependence of the excitation energy in the high-field regime rather hints towards a spin transition excited by the phonons. We propose to explain this surprising observation by an only partially polarized 2/3-ground-state making the energetically lower lying spin transition also allowed for phonon excitations.

cond-mat.mes-hall↗

arXiv2003

Magnetization of a two-dimensional electron gas with a second filled subband

We have measured the magnetization of a dual-subband two-dimensional electron gas, confined in a GaAs/AlGaAs heterojunction. In contrast to two-dimensional electron gases with a single subband, we observe non-1/B-periodic, triangularly shaped oscillations of the magnetization with an amplitude significantly less than $1 μ_{\mathrm{B}}^*$ per electron. All three effects are explained by a field dependent self-consistent model, demonstrating the shape of the magnetization is dominated by oscillations in the confining potential. Additionally, at 1 K, we observe small oscillations at magnetic fields where Landau-levels of the two different subbands cross.

cond-mat.mes-hall↗

arXiv2003

Phase measurements using Two-Channel Fano Interference in a Semiconductor Quantum Dot

We investigate a lateral semiconductor quantum dot with a large number of electrons in the semi-open Fano regime. In transport measurements we observe three stable series of Fano resonances with similar lineshapes. We present a simple model explaining the temperature and V_{SD} dependence of the resonances. The Fano regime allows to investigate phase and coherence of the electronic wave function and astonishingly, we find no signs of decoherence in our system.

cond-mat.mes-hall↗

arXiv2003

Hopping Conductivity Beyond The Percolation Regime Probed By Shot-Noise Measurements

We have observed suppression of shot noise in the variable-range hopping regime for a two-dimensional electron gas whose localization length, xi, is controlled by a gate voltage. We have found that the suppression factor F (Fano factor) is approximately inversely proportional to the length of the system L (F=L0*/L) for a broad range of values of xi/L_0*, where L0* is the characteristic length. In the case xi/L0*<<1, we have identified L0* with the distance L0 between the hard hops along the sample length. On the other hand, when xi is of the order of L0*, we have observed that L0* does not agree with L0 calculated from the percolation model of hopping. We attribute this discrepancy to a breakdown of that model and to a reconstruction of the hopping paths.

cond-mat.mes-hall↗

arXiv2003

Observation of two relaxation mechanisms in transport between spin split edge states at high imbalance

Using a quasi-Corbino geometry to directly study electron transport between spin-split edge states, we find a pronounced hysteresis in the I-V curves, originating from slow relaxation processes. We attribute this long-time relaxation to the formation of a dynamic nuclear polarization near the sample edge. The determined characteristic relaxation times are 25 s and 200 s which points to the presence of two different relaxation mechanisms. The two time constants are ascribed to the formation of a local nuclear polarization due to flip-flop processes and the diffusion of nuclear spins.

cond-mat.mes-hall↗

arXiv2002

Flux-quantum-modulated Kondo conductance in a multielectron quantum dot

We investigate a lateral semiconductor quantum dot with a large number of electrons in the limit of strong coupling to the leads. A Kondo effect is observed and can be tuned in a perpendicular magnetic field. This Kondo effect does not exhibit Zeeman splitting. It shows a modulation with the periodicity of one flux quantum per dot area at low temperatures. The modulation leads to a novel, strikingly regular stripe pattern for a wide range in magnetic field and number of electrons.

cond-mat.mes-hall↗

arXiv2002

Fabrication of quantum point contacts by engraving GaAs/AlGaAs-heterostructures with a diamond tip

We use the all-diamond tip of an atomic force microscope for the direct engraving of high quality quantum point contacts in GaAs/AlGaAs-heterostructures. The processing time is shortened by two orders of magnitude compared to standard silicon tips. Together with a reduction of the line width to below 90 nm the depletion length of insulating lines is reduced by a factor of two with the diamond probes. The such fabricated defect free ballistic constrictions show well resolved conductance plateaus and the 0.7 anomaly in electronic transport measurements.

cond-mat.mes-hall↗

arXiv1999

Electron-Assisted Hopping in Two Dimensions

We have studied the non-ohmic effects in the conductivity of a two-dimensional system which undergoes the crossover from weak to strong localization with decreasing electron concentration. When the electrons are removed from equilibrium with phonons, the hopping conductivity depends only on the electron temperature. This indicates that the hopping transport in a system with a large localization length is assisted by electron-electron interactions rather than by the phonons.

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