arXiv subjects

W. Wegscheider

Publications and source records attributed to W. Wegscheider.

At least 235 records · Page 13Linked to original sources

arXiv2003

Spin-galvanic effect due to optical spin orientation

Under oblique incidence of circularly polarized infrared radiation the spin-galvanic effect has been unambiguously observed in (001)-grown $n$-type GaAs quantum well (QW) structures in the absence of any external magnetic field. Resonant inter-subband transitions have been obtained making use of the tunability of the free-electron laser FELIX. It is shown that a helicity dependent photocurrent along one of the $<110>$ axes is predominantly contributed by the spin-galvanic effect while that along the perpendicular in-plane axis is mainly due to the circular photogalvanic effect. This strong non-equivalence of the [110] and [1$\bar{1}$0] directions is determined by the interplay between bulk and structural inversion asymmetries. A microscopic theory of the spin-galvanic effect for direct inter-subband optical transitions has been developed being in good agreement with experimental findings.

cond-mat↗

arXiv2003

Kondo effect in a few-electron quantum ring

A small quantum ring with less than 10 electrons was studied by transport spectroscopy. For strong coupling to the leads a Kondo effect is observed and used to characterize the spin structure of the system in a wide range of magnetic fields. At small magnetic fields Aharonov-Bohm oscillations influenced by Coulomb interaction appear. They exhibit phase jumps by $π$ at the Coulomb-blockade resonances. Inside Coulomb-blockade valleys the Aharonov-Bohm oscillations can also be studied due to the finite conductance caused by the Kondo effect. Astonishingly, the maxima of the oscillations show linear shifts with magnetic field and gate voltage.

cond-mat.mes-hall↗

arXiv2003

Single electron-phonon interaction in a suspended quantum dot phonon cavity

An electron-phonon cavity consisting of a quantum dot embedded in a free-standing GaAs/AlGaAs membrane is characterized in Coulomb blockade measurements at low temperatures. We find a complete suppression of single electron tunneling around zero bias leading to the formation of an energy gap in the transport spectrum. The observed effect is induced by the excitation of a localized phonon mode confined in the cavity. This phonon blockade of transport is lifted at magnetic fields where higher electronic states with nonzero angular momentum are brought into resonance with the phonon energy.

cond-mat.mes-hall↗

arXiv2003

Spin-sensitive Bleaching and Spin-Relaxation in QW's

Spin-sensitive saturation of absorption of infrared radiation has been investigated in p-type GaAs QWs. It is shown that the absorption saturation of circularly polarized radiation is mostly controlled by the spin relaxation time of the holes. The saturation behavior has been investigated for different QW widths and in dependence on the temperature with the result that the saturation intensity substantially decreases with narrowing of QWs. Spin relaxation times were experimentally obtained by making use of calculated (linear) absorption coefficients for inter-subband transitions.

cond-mat↗

arXiv2003

Resonant inversion of the circular photogalvanic effect in n-doped quantum wells

We show that the sign of the circular photogalvanic effect can be changed by tuning the radiation frequency of circularly polarized light. Here resonant inversion of the photogalvanic effect has been observed for direct inter-subband transition in n-type GaAs quantum well structures. This inversion of the photon helicity driven current is a direct consequence of the lifting of the spin degeneracy due to k-linear terms in the Hamiltonian in combination with energy and momentum conservation and optical selection rules.

cond-mat↗

arXiv2003

Spin-Galvanic Effect in Quantum Wells

It is shown that a homogeneous non-equlibrium spin-polarization in semiconductor heterostructures results in an electric current. The microscopic origin of the effect is an inherent asymmetry of spin-flip scattering in systems with lifted spin degeneracy caused by k-linear terms in the Hamiltonian.

cond-mat↗

arXiv2002

Aharonov-Bohm oscillations of a tunable quantum ring

With an atomic force microscope a ring geometry with self-aligned in-plane gates was directly written into a GaAs/AlGaAs-heterostructure. Transport measurements in the open regime show only one transmitting mode and Aharonov-Bohm oscillations with more than 50% modulation are observed in the conductance. The tuning via in-plane gates allows to study the Aharonov-Bohm effect in the whole range from the open ring to the Coulomb-blockade regime.

cond-mat.mes-hall↗

arXiv2001

Energy spectra of quantum rings

Ring geometries have fascinated experimental and theoretical physicists over many years. Open rings connected to leads allow the observation of the Aharonov-Bohm effect, a paradigm of quantum mechanical phase coherence. The phase coherence of transport through a quantum dot embedded in one arm of an open ring has been demonstrated. The energy spectrum of closed rings has only recently been analysed by optical experiments and is the basis for the prediction of persistent currents and related experiments. Here we report magnetotransport experiments on a ring-shaped semiconductor quantum dot in the Coulomb blockade regime. The measurements allow us to extract the discrete energy levels of a realistic ring, which are found to agree well with theoretical expectations. Such an agreement, so far only found for few-electron quantum dots, is here extended to a many-electron system. In a semiclassical language our results indicate that electron motion is governed by regular rather than chaotic motion, an unexplored regime in many-electron quantum dots.

cond-mat.mes-hall↗

arXiv2001

Electronic properties of antidot lattices fabricated by atomic force lithography

Antidot lattices were fabricated by atomic force lithography using local oxidation. High quality finite 20 x20 lattices are demonstrated with periods of 300 nm. The low temperature magnetoresistance shows well developed commensurability oscillations as well as a quenching of the Hall effect around zero magnetic field. In addition, we find B periodic oscillations superimposed on the classical commensurability peaks at temperatures as high as 1.7 K. These observations indicate the high electronic quality of our samples.

cond-mat.mes-hall↗

arXiv2001

Spin sensitive bleaching and monopolar spin orientation in quantum wells

Spin sensitive bleaching of the absorption of far-infrared radiation has been observed in $p$-type GaAs/AlGaAs quantum well structures. The absorption of circularly polarized radiation saturates at lower intensities than that of linearly polarized light due to monopolar spin orientation in the first heavy hole subband. Spin relaxation times of holes in $p$-type material in the range of tens of ps were derived from the intensity dependence of the absorption.

cond-mat↗

arXiv2000

Analysis of the temperature-dependent quantum point contact conductance in view of the metal-insulator transition in two dimensions

The temperature dependence of the conductance of a quantum point contact has been measured. The conductance as a function of the Fermi energy shows temperature-independent fixed points, located at roughly multiple integers of $e^{2}/h$. Around the first fixed point at e$^{2}$/h, the experimental data for different temperatures can been scaled onto a single curve. For pure thermal smearing of the conductance steps, a scaling parameter of one is expected. The measured scaling parameter, however, is significantly larger than 1. The deviations are interpreted as a signature of the potential landscape of the quantum point contact, and of the source-drain bias voltage. We relate our results phenomenologically to the metal-insulator transition in two dimensions.

cond-mat.mes-hall↗

arXiv2000

Determination of the complex microwave photoconductance of a single quantum dot

A small quantum dot containing approximately 20 electrons is realized in a two-dimensional electron system of an AlGaAs/GaAs heterostructure. Conventional transport and microwave spectroscopy reveal the dot's electronic structure. By applying a coherently coupled two-source technique, we are able to determine the complex microwave induced tunnel current. The amplitude of this photoconductance resolves photon-assisted tunneling (PAT) in the non-linear regime through the ground state and an excited state as well. The out-of-phase component (susceptance) allows to study charge relaxation within the quantum dot on a time scale comparable to the microwave beat period.

cond-mat.mes-hall↗

arXiv2000

Quantum Hall Ferromagnetism in a Two-Dimensional Electron System

Experiments on a nearly spin degenerate two-dimensional electron system reveals unusual hysteretic and relaxational transport in the fractional quantum Hall effect regime. The transition between the spin-polarized (with fill fraction $ν= 1/3$) and spin-unpolarized ($ν= 2/5$) states is accompanied by a complicated series of hysteresis loops reminiscent of a classical ferromagnet. In correlation with the hysteresis, magnetoresistance can either grow or decay logarithmically in time with remarkable persistence and does not saturate. In contrast to the established models of relaxation, the relaxation rate exhibits an anomalous divergence as temperature is reduced. These results indicate the presence of novel two-dimensional ferromagnetism with a complicated magnetic domain dynamic.

cond-mat.mes-hall↗

arXiv2000

Spin effects in the magneto-drag between double quantum wells

We report on the selectivity to spin in a drag measurement. This selectivity to spin causes deep minima in the magneto-drag at odd fillingfactors for matched electron densities at magnetic fields and temperatures at which the bare spin energy is only one tenth of the temperature. For mismatched densities the selectivity causes a novel 1/B-periodic oscillation, such that negative minima in the drag are observed whenever the majority spins at the Fermi energies of the two-dimensional electron gasses (2DEGs) are anti-parallel, and positive maxima whenever the majority spins at the Fermi energies are parallel.

cond-mat.mes-hall↗

arXiv2000

Transport properties of quantum dots with hard walls

Quantum dots are fabricated in a Ga[Al]As-heterostructure by local oxidation with an atomic force microscope. This technique, in combination with top gate voltages, allows us to generate steep walls at the confining edges and small lateral depletion lengths. The confinement is characterized by low-temperature magnetotransport measurements, from which the dots' energy spectrum is reconstructed. We find that in small dots, the addition spectrum can qualitatively be described within a Fock-Darwin model. For a quantitative analysis, however, a hard-wall confinement has to be considered. In large dots, the energy level spectrum deviates even qualitatively from a Fock-Darwin model. The maximum wall steepness achieved is of the order of 0.4 meV/nm.

cond-mat.mes-hall↗

arXiv2000

Magnetotransport measurements on freely suspended two-dimensional electron gases

We present magnetotransport measurements on freely suspended two-dimensional electron gases from AlGaAs/GaAs heterostructures. The technique to realize such devices relies on a specially MBE grown GaAs/AlGaAs-heterostructure including a sacrificial layer. We fabricated simple Hall-bars as well as quantum cavities and quantum dot systems. We find well-pronounced Shubnikov-de Haas oscillations and observe commensurability resonances, allowing characterization of the electron gas in these 100 nm thin membranes.

cond-mat.mes-hall↗

arXiv2000

Edge and bulk effects in the Terahertz-photoconductivity of an antidot superlattice

We investigate the Terahertz(THz)-response of a square antidot superlattice by means of photoconductivity measurements using a Fourier-transform-spectrometer. We detect, spectrally resolved, the cyclotron resonance and the fundamental magnetoplasmon mode of the periodic superlattice. In the dissipative transport regime both resonances are observed in the photoresponse. In the adiabatic transport regime, at integer filling factor $ν=2$, only the cyclotron resonance is observed. From this we infer that different mechanisms contribute to converting the absorption of THz-radiation into photoconductivity in the cyclotron and in the magnetoplasmon resonances, respectively.

cond-mat.mes-hall↗

arXiv2000

Signatures of spin pairing in a quantum dot in the Coulomb blockade regime

Coulomb blockade resonances are measured in a GaAs quantum dot in which both shape deformations and interactions are small. The parametric evolution of the Coulomb blockade peaks shows a pronounced pair correlation in both position and amplitude, which is interpreted as spin pairing. As a consequence, the nearest-neighbor distribution of peak spacings can be well approximated by a smeared bimodal Wigner surmise, provided that interactions which go beyond the constant interaction model are taken into account.

cond-mat.mes-hall↗