Biomedical subjects

W Wegscheider

Publications and source records attributed to W Wegscheider.

At least 19 recordsLinked to original sources

PubMed2006

Double-dot quantum ratchet driven by an independently biased quantum point contact.

We study a double quantum dot (DQD) coupled to a strongly biased quantum point contact (QPC), each embedded in independent electric circuits. For weak interdot tunneling we observe a finite current flowing through the Coulomb blockaded DQD in response to a strong bias on the QPC. The direction of the current through the DQD is determined by the relative detuning of the energy levels of the two quantum dots. The results are interpreted in terms of a quantum ratchet phenomenon in a DQD energized by a nearby QPC.

Aluminum↗

PubMed2006

Dephasing in (Ga,Mn)As nanowires and rings.

To understand quantum mechanical transport in a ferromagnetic semiconductor, the knowledge of basic material properties such as the phase coherence length and corresponding dephasing mechanism are indispensable ingredients. The lack of observable quantum phenomena has prevented experimental access to these quantities so far. Here we report the observations of universal conductance fluctuations in ferromagnetic (Ga,Mn)As. The analysis of the length and temperature dependence of the fluctuations reveals a T(-1) dependence of the dephasing time.

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PubMed2006

Detection of the electron spin resonance of two-dimensional electrons at large wave vectors.

We have investigated the electron spin resonance at nonzero wave vector in GaAs single quantum wells by combining the virtues of high frequency surface acoustic wave generation to produce excitations with large wave numbers with a sensitive optical scheme to detect resonant absorption. The observed large deviations from the single particle Zeeman energy are attributed to the exchange interaction. The enhancement of the electronic g* factor is, however, substantially smaller compared with theoretical predictions for spin waves when adopting a bare Coulomb interaction potential.

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PubMed2006

Probing a Kondo-correlated quantum dot with spin spectroscopy.

We investigate the Kondo effect and spin blockade observed in a many-electron quantum dot and study the magnetic field dependence. At lower fields, a pronounced Kondo effect is found, which is replaced by the spin blockade at higher fields. In an intermediate regime, both effects are visible. We make use of this combined effect to gain information about the internal spin configuration of our quantum dot. We find that the data cannot be explained assuming regular filling of electronic orbitals. Instead, spin polarized filling seems to be probable.

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PubMed2005

Control and detection of singlet-triplet mixing in a random nuclear field.

We observed mixing between two-electron singlet and triplet states in a double quantum dot, caused by interactions with nuclear spins in the host semiconductor. This mixing was suppressed when we applied a small magnetic field or increased the interdot tunnel coupling and thereby the singlet-triplet splitting. Electron transport involving transitions between triplets and singlets in turn polarized the nuclei, resulting in marked bistabilities. We extract from the fluctuating nuclear field a limitation on the time-averaged spin coherence time T2* of 25 nanoseconds. Control of the electron-nuclear interaction will therefore be crucial for the coherent manipulation of individual electron spins.

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PubMed2005

Structure of a single sharp quantum Hall edge probed by momentum-resolved tunneling.

Momentum-resolved magnetotunneling spectroscopy is performed at a single sharp quantum Hall (QH) edge to probe the structure of integer QH edge modes. An epitaxially overgrown cleaved edge is shown to realize the sharp-edge limit with interchannel distances smaller than both the magnetic length and the Bohr radius where the Chklovskii soft-edge picture is no longer valid. The line shape of principal conductance peaks is explained, and an edge filling factor is determined from the peak position. A step in the dispersion is attributed to fluctuations in the QH ground energy.

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PubMed2004

Activated transport in the separate layers that form the nuT=1 exciton condensate.

We observe the total filling factor nuT=1 quantum Hall state in a bilayer two-dimensional electron system with virtually no tunneling. We find thermally activated transport in the balanced system with a monotonic increase of the activation energy with decreasing d/lB 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, implying a different excitation spectrum for the separate layers forming the condensed state.

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PubMed2004

Spatially resolved manipulation of single electrons in quantum dots using a scanned probe.

The scanning metallic tip of a scanning force microscope was coupled capacitively to electrons confined in a lithographically defined gate-tunable quantum dot at a temperature of 300 mK. Single electrons were made to hop on or off the dot by moving the tip or by changing the tip bias voltage owing to the Coulomb-blockade effect. Spatial images of conductance resonances map the interaction potential between the tip and individual electronic quantum dot states. Under certain conditions this interaction is found to contain a tip-voltage induced and a tip-voltage-independent contribution.

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PubMed2004

Kondo effect in a many-electron quantum ring.

The Kondo effect is investigated in a many-electron quantum ring as a function of the 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.

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PubMed2004

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. 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. Phase lapses are found only in certain ranges of the magnetic field.

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PubMed2004

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.

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PubMed2004

Coulomb drag as a probe of the nature of compressible States in a magnetic field.

Magnetodrag reveals the nature of compressible states and the underlying interplay of disorder and interactions. At nu=3/2 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.

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PubMed2004

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.

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PubMed2004

Rectification in mesoscopic systems with broken symmetry: quasiclassical ballistic versus classical transport.

In suitably designed mesoscopic semiconductor structures, the phenomenon of ballistic rectification can be observed. A currently discussed microscopic model relates the observations to the interplay between fully quantized and quasiclassical current paths. We present measurements that contribute substantially to the clarification of the fascinating topic. In particular, we observe the opposite sign of the output voltage as compared to the prediction. Demonstrating the basic principle upon which the rectification is based--the asymmetry of the voltage drop in a quasiclassical wire--and extending the model to the classical transport regime, we can well explain our experiments as being caused by the interplay of quasiclassical ballistic and classical transport. Tunable ballistic rectifiers generating very large output signals and operating at room temperature raise the hope for future applications.

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PubMed2004

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

An electron-phonon cavity consisting of a quantum dot embedded in a freestanding GaAs/AlGaAs membrane is characterized using 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 discrete magnetic fields where higher electronic states with nonzero angular momentum are brought into resonance with the phonon energy.

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PubMed2003

Singlet-triplet transition tuned by asymmetric gate voltages in a quantum ring.

Wave-function 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.

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PubMed2003

Optical probing of a fractionally charged quasihole in an incompressible liquid.

In photoluminescence spectroscopy of a low-mobility two-dimensional electron gas subjected to a quantizing magnetic field, we observe an anomaly around nu=1 / 3 at a very low temperature (0.1 K) and an intermediate electron density (0.9 x 10(11) cm(-2)). The anomaly is explained as due to perturbation of the incompressible liquid at the Laughlin state due to close proximity of a localized charged exciton which creates a fractionally charged quasihole in the liquid. The anomaly of approximately 2 meV can be destroyed by applying a small thermal energy of approximately 0.2 meV that is enough to close the quasihole energy gap.

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PubMed2003

An uncertainty budget for trace analysis by isotope-dilution ICP-MS with proper consideration of correlation.

Isotope-dilution mass spectrometry (IDMS) is considered to be a method without significant correction factors. It is also believed that this method is well understood. But unfortunately a large number of different uncertainty budgets have been published that consider different correction factors. These differences lead to conflicting combined uncertainties especially in trace analysis. It is described how the known correction factors must be considered in the uncertainty budget of values determined by IDMS combined with ICP-MS (ICP-IDMS). The corrections applied are dead time, background, interference, mass discrimination, blank correction and air buoyancy.IDMS measurements consist always of a series of isotope abundance ratio measurements and can be done according to different measurement protocols. Because the measurement protocols of IDMS are often rather sophisticated, correlations of influence quantities are difficult to identify. Therefore the measurement protocol has to be carefully considered in the specification of the measurand and a strategy is presented to properly account for these correlations. This will be exemplified for the estimation of mass fractions of platinum group elements (PGEs) and Re in the geological reference material UB-N (from CRPG-CNRS, Nancy in France) with ICP-IDMS. The PGEs with more than one isotope and the element Re are measured with on-line cation-exchange chromatography coupled to a quadrupole ICP-MS. All contents are below 10 microg kg(-1). Only osmium is separated from the matrix by direct sparging of OsO(4) into the plasma. This leads to transient signals for all PGEs and Re. It is possible to estimate the combined uncertainties and keep them favourably small despite the low contents, the transient signals and the sophisticated correction model.

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