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W. Wegscheider

Publications and source records attributed to W. Wegscheider.

At least 217 records · Page 12Linked to original sources
arXiv2004

Activated Transport in the individual Layers that form the $ν_T$=1 Exciton Condensate

We observe the total filling factor $ν_{T}$=1 quantum Hall state in a bilayer two-dimensional electron system with virtually no tunnelling. We find thermally activated transport in the balanced system with a monotonic increase of the activation energy with decreasing $d/\ell_B$ below 1.65. In the imbalanced system we find activated transport in each of the layers separately, yet the activation energies show a striking asymmetry around the balance point. This implies that the gap to charge-excitations in the {\em individual} layers is substantially different for positive and negative imbalance.

cond-mat.mes-hall↗
arXiv2004

Spin Effects in a Quantum Ring

Recent experiments are reviewed that explore the spin states of a ring-shaped many-electron quantum dot. Coulomb-blockade spectroscopy is used to access the spin degree of freedom. The Zeeman effect observed for states with successive electron number allows to select possible sequences of spin ground states of the ring. Spin-paired orbital levels can be identified by probing their response to magnetic fields normal to the plane of the ring and electric fields caused by suitable gate voltages. This narrows down the choice of ground-state spin sequences. A gate-controlled singlet--triplet transition is identified and the size of the exchange interaction matrix element is determined.

cond-mat.mes-hall↗
arXiv2004

Kondo Effect in a Many-Electron Quantum Ring

The Kondo effect is investigated in a many-electron quantum ring as a function of magnetic field. For fields applied perpendicular to the plane of the ring a modulation of the Kondo effect with the Aharonov-Bohm period is observed. This effect is discussed in terms of the energy spectrum of the ring and the parametrically changing tunnel coupling. In addition, we use gate voltages to modify the ground-state spin of the ring. The observed splitting of the Kondo-related zero-bias anomaly in this configuration is tuned with an in-plane magnetic field.

cond-mat.mes-hall↗
arXiv2004

Single-Electron Effects in a Coupled Dot-Ring System

Aharonov-Bohm oscillations are studied in the magnetoconductance of a micron-sized open quantum ring coupled capacitively to a Coulomb-blockaded quantum dot. As the plunger gate of the dot is modulated and tuned through a conductance resonance, the amplitude of the Aharonov-Bohm oscillations in the transconductance of the ring displays a minimum. We demonstrate that the effect is due to a single-electron screening effect, rather than to dephasing. Aharonov-Bohm oscillations in a quantum ring can thus be used for the detection of single charges.

cond-mat.mes-hall↗
arXiv2004

High-field magnetoresistance of Fe/GaAs/Fe tunnel junctions

We investigate transport through 6 to 10 nm thin epitaxial GaAs(001) barriers sandwiched between polycrystalline iron films. Apart from a pronounced tunneling magnetoresistance effect (TMR) at low magnetic fields we observe a distinct negative magnetoresistance (MR) at low and a positive MR at higher temperatures. We show that the negative MR contribution is only observed for the ferromagnetic iron contacts but is absent if iron is replaced by copper or gold electrodes. Possible explanations of the negative MR involve suppression of spin-flip scattering or Zeeman splitting of the tunneling barrier.

cond-mat.mes-hall↗
arXiv2004

Can an electric current orient spins in quantum wells?

A longstanding theoretical prediction is the orientation of spins by an electrical current flowing through low-dimensional carrier systems of sufficiently low crystallographic symmetry. Here we show by means of terahertz transmission experiments through two-dimensional hole systems a growing spin orientation with an increasing current at room temperature.

cond-mat.mes-hall↗
arXiv2004

Spin relaxation times of 2D holes from spin sensitive bleaching of inter-subband absorption

We present spin relaxation times of 2D holes obtained by means of spin sensitive bleaching of the absorption of infrared radiation in p-type GaAs/AlGaAs quantum wells (QWs). It is shown that the saturation of inter-subband absorption of circularly polarized radiation is mainly controlled by the spin relaxation time of the holes. The saturation behavior has been determined for different QW widths and in a wide temperature range with the result that the saturation intensity substantially decreases with narrowing of the QWs. Spin relaxation times are derived from the measured saturation intensities by making use of calculated (linear) absorption coefficients for direct inter-subband transitions. It is shown that spin relaxation is due to the D'yakonov-Perel' mechanism governed by hole-hole scattering. The problem of selection rules is addressed.

cond-mat.other↗
arXiv2003

Spontaneous supercurrent induced by ferromagnetic pi-junctions

We present magnetization measurements of mesoscopic superconducting niobium loops containing a ferromagnetic (PdNi) pi-junction. The loops are prepared on top of the active area of a micro Hall-sensor based on high mobility GaAs/AlGaAs heterostructures. We observe asymmetric switching of the loop between different magnetization states when reversing the sweep direction of the magnetic field. This provides evidence for a spontaneous current induced by the intrinsic phase shift of the pi-junction. In addition, the presence of the spontaneous current near zero applied field is directly revealed by an increase of the magnetic moment with decreasing temperature, which results in half integer flux quantization in the loop at low temperatures.

cond-mat.supr-con↗
arXiv2003

Magneto-Gyrotropic Photogalvanic Effect in Semiconductor Quantum Wells

We investigate both experimentally and theoretically, the magneto-gyrotropic photogalvanic effect in zinc-blende based quantum wells with $C_{2v}$ point-group symmetry using optical excitation in the terahertz frequency range. The investigated frequencies cause intra-subband but no inter-band and inter-subband transitions. While at normal incidence the photocurrent vanishes at zero magnetic field, it is shown that an in-plane magnetic field generates photocurrents both for polarized and unpolarized excitation. In general the spin-galvanic effect, caused by circularly polarized light, and the magneto-gyrotropic effect, caused by unpolarized excitation, is superimposed. It is shown that in the case of two specific geometries both effects are separable.

cond-mat↗
arXiv2003

Coulomb Drag as a Probe of the Nature of Compressible States in a Magnetic Field

Magneto-drag reveals the nature of compressible states and the underlying interplay of disorder and interactions. At ν=3/2 a clear T^{4/3} dependence is observed, which signifies the metallic nature of the N=0 Landau level. In contrast, drag in higher Landau levels reveals an additional contribution, which anomalously grows with decreasing T before turning to zero following a thermal activation law. The anomalous drag is discussed in terms of electron-hole asymmetry arising from disorder and localization, and the crossover to normal drag at high fields as due to screening of disorder.

cond-mat.mes-hall↗
arXiv2003

Probing the Electrostatics of Integer Quantum Hall Edges with Momentum-Resolved Tunnel Spectroscopy

We present measurements of momentum-resolved magneto-tunneling from a perpendicular two-dimensional (2D) contact into integer quantum Hall (QH) edges at a sharp edge potential created by cleaved edge overgrowth. Resonances in the tunnel conductance correspond to coincidences of electronic states of the QH edge and the 2D contact in energy-momentum space. With this dispersion relation reflecting the potential distribution at the edge we can directly measure the band bending at our cleaved edge under the influence of an external voltage bias. At finite bias we observe significant deviations from the flat-band condition in agreement with self-consistent calculations of the edge potential.

cond-mat↗
arXiv2003

Transmission Phase Through Two Quantum Dots Embedded in a Four-Terminal Quantum Ring

We use the Aharonov-Bohm effect in a four-terminal ring based on a Ga[Al]As heterostructure for the measurement of the relative transmission phase. In each of the two interfering paths we induce a quantum dot. The number of electrons in the two dots can be controlled independently. The transmission phase is measured as electrons are added to or taken away from the individual quantum dots.

cond-mat.mes-hall↗
arXiv2003

Magnetic field dependent transmission phase of a double dot system in a quantum ring

The Aharonov-Bohm effect is measured in a four-terminal open ring geometry based on a Ga[Al]As heterostructure. Two quantum dots are embedded in the structure, one in each of the two interfering paths. The number of electrons in the two dots can be controlled independently. The transmission phase is measured as electrons are added to or taken away from the individual quantum dots. Although the measured phase shifts are in qualitative agreement with theoretical predictions, the phase evolution exhibits unexpected dependence on the magnetic field. For example, phase lapses are found only in certain ranges of magnetic field.

cond-mat.mes-hall↗
arXiv2003

Kelvin Probe Spectroscopy of a Two-Dimensional Electron Gas Below 300 mK

A scanning force microscope with a base temperature below 300 mK is used for measuring the local electron density of a two-dimensional electron gas embedded in an Ga[Al]As heterostructure. At different separations between AFM tip and sample, a dc-voltage is applied between the tip and the electron gas while simultaneously recording the frequency shift of the oscillating tip. Using a plate capacitor model the local electron density can be extracted from the data. The result coincides within 10% with the data obtained from transport measurements.

cond-mat.mes-hall↗
arXiv2003

Combined atomic force microscope and electron-beam lithography used for the fabrication of variable-coupling quantum dots

We have combined direct nanofabrication by local anodic oxidation with conventional electron-beam lithography to produce a parallel double quantum dot based on a GaAs/AlGaAs heterostructure. The combination of both nanolithography methods allows to fabricate robust in-plane gates and Cr/Au top gate electrodes on the same device for optimal controllability. This is illustrated by the tunability of the interdot coupling in our device. We describe our fabrication and alignment scheme in detail and demonstrate the tunability in low-temperature transport measurements.

cond-mat.mes-hall↗
arXiv2003

Singlet-Triplet Transition Tuned by Asymmetric Gate Voltages in a Quantum Ring

Wavefunction and interaction effects in the addition spectrum of a Coulomb blockaded many electron quantum ring are investigated as a function of asymmetrically applied gate voltages and magnetic field. Hartree and exchange contributions to the interaction are quantitatively evaluated at a crossing between states extended around the ring and states which are more localized in one arm of the ring. A gate tunable singlet-triplet transition of the two uppermost levels of this many electron ring is identified at zero magnetic field.

cond-mat.mes-hall↗
arXiv2003

Experimental Separation of Rashba and Dresselhaus Spin-Splittings in Semiconductor Quantum Wells

The relative strengths of Rashba and Dresselhaus terms describing the spin-orbit coupling in semiconductor quantum well (QW) structures are extracted from photocurrent measurements on n-type InAs QWs containing a two-dimensional electron gas (2DEG). This novel technique makes use of the angular distribution of the spin-galvanic effect at certain directions of spin orientation in the plane of a QW. The ratio of the relevant Rashba and Dresselhaus coefficients can be deduced directly from experiment and does not relay on theoretically obtained quantities. Thus our experiments open a new way to determine the different contributions to spin-orbit coupling.

cond-mat↗