W. Dietsche

NEW TYPE OF ELECTRON NUCLEAR-SPIN INTERACTION FROM RESISTIVELY DETECTED NMR IN THE FRACTIONAL QUANTUM HALL EFFECT REGIME

Two-dimensional electron gases in narrow GaAs quantum wells show huge longitudinal resistance (HLR) values at certain fractional filling factors. Applying an rf field with frequencies corresponding to the nuclear spin splittings of 69Ga, 71Ga, and 75As leads to a substantial decrease of the HLR establishing a novel type of resistively detected NMR. These resonances are split into four sublines each. Neither the number of sublines nor the size of the splitting can be explained by established interaction mechanisms.

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.

Electro-optic imaging of potential distributions in the quantum Hall regime

The potential distribution in Hall effect experiments on two-dimensional electron systems (2DES) is imaged with a scanning polarization optical microscope utilizing the internal electro-optic effect. Outside the quantum Hall regime (QHR), Hall potential profiles vary linearly between the two edges. In the QHR, the observed Hall potential profile is nonlinear. Its shape depends on magnetic field and current, indicating inhomogeneous current transport in the interior of the 2DES. In the plateau centre, the Hall potential drop is concentrated in the middle of the channel. The results are interpreted in terms of electron density inhomogeneities. These lead to the observed potential distributions because of the very strong dependence of the conductivity on the local electron density.

Imaging of the dissipation in quantum-Hall-effect experiments

The spatial distribution of heat dissipation in quantum-Hall-effect devices was imaged utilizing the fountain pressure effect of superfluid helium. This was done by observing visually the local thickness of a helium film covering the sample. We found that dissipation takes place only in two opposite corners at the current contacts. Unexpected asymmetries between the two corners were observed if the Hall currents exceeded a threshold value. A new and striking dissipative effect was found when current is passed from narrow to wide 2 DEG areas which also was asymmetric upon current reversal.

Critical tunneling currents in the regime of bilayer excitons

We have investigated the tunneling properties of an electron double quantum well system where the lowest Landau level of each quantum well is half filled. This system is expected to be a Bose condensate of excitons. Our four-terminal dc measurements reveal a nearly vanishing interlayer voltage and the existence of critical tunneling currents Icritical which depend on the strength of the condensate state.

Electron spin resonance on a two-dimensional electron gas in a single AlAs quantum well.

Direct electron spin resonance (ESR) on a high mobility two-dimensional electron gas in a single AlAs quantum well reveals an electronic g factor of 1.991 at 9.35 GHz and 1.989 at 34 GHz with a minimum linewidth of 7 G. The ESR amplitude and its temperature dependence suggest that the signal originates from the effective magnetic field caused by the spin-orbit interaction and a modulation of the electron wave vector caused by the microwave electric field. This contrasts markedly with conventional ESR that detects through the microwave magnetic field.

Potential drops across quantum Hall effect samples — in the bulk or near the edges?

The electrostatic potential distribution in two-dimensional electron gases has been imaged using the electro-optic effect. A potential drop over a length of about 100 μm has been found. This edge potential drop runs along one sample edge only if either the source or the drain contact are at substrate potential.

Interlayer tunneling in counterflow experiments on the excitonic condensate in quantum Hall bilayers.

The effect of tunneling on the transport properties of quantum Hall double layers in the regime of the excitonic condensate at a total filling factor one is studied in counterflow experiments. If the tunnel current I is smaller than a critical I{C}, tunneling is large and is effectively shorting the two layers. For I>I{C} tunneling becomes negligible. Surprisingly, the transition between the two tunneling regimes has only a minor impact on the features of the filling-factor one state as observed in magnetotransport, but at currents exceeding I{C} the resistance along the layers increases rapidly.

Two laterally arranged quantum dot systems with strong capacitive interdot coupling

A method has been developed to form two quantum dot systems in lateral arrangement in a two-dimensional electron system of a GaAs–AlGaAs heterostructure with strong capacitive interdot coupling. In the authors’ design, the interdot capacitance can reach more than one-third of the single-dot capacitance while tunneling between the dots is excluded. This has been achieved by a floating metallic electrode covering both quantum dots, a method already used in split-gate designs before. Here, however, they have reduced the capacitive coupling of this floating gate to other electrodes in the surroundings by an etching technique to obtain a large interdot coupling.

Molecular beam epitaxial growth and structural properties of DyBa2Cu3O7-y superconducting thin films

Abstract High T c films were prepared in situ with high reproducibility under MBE conditions. Pure ozone for oxidizing the metal constituents of the films let us reduce the total pressure during deposition below 10 −6 mbar which put our system far away from the stability line of Bormann and Nolting. The structure and morphology of the films were studied by RHEED, STM, AFM, and by X-ray techniques. It was found that all relevant properties of our films depended mainly on the epitaxy with the substrate. If there is a small lattice misfit as on (001) SrTiO 3 or on a Dy 2 O 3 buffer layer then the films are well ordered, also in-plane, and very good values of T c and J c of 92 K and 4×10 6 A/cm 2 , respectively, were obtained. On (001) MgO, where the misfit is worse, the films are textured in different ways and showed poor in-plane order. Very thin films with a thickness of about two average unit cells almost cover the SrTiO 3 but show large holes on MgO.