Electrical read-out of the local nuclear polarization in the quantum Hall effect

It is demonstrated that the now well-established `flip-flop' mechanism of spin exchange between electrons and nuclei in the quantum Hall effect can be reversed. We use a sample geometry which utilizes separately contacted edge states to establish a local nuclear spin polarization --close to the maximum value achievable-- by driving a current between electron states of different spin orientation. When the externally applied current is switched off, the sample exhibits an output voltage of up to a few tenths of a meV, which decays with a time constant typical for the nuclear spin relaxation. The surprizing fact that a sample with a local nuclear spin polarization can act as a source of energy and that this energy is well above the nuclear Zeeman splitting is explained by a simple model which takes into account the effect of a local Overhauser shift on the edge state reconstruction.Comment: Submitted to Phys. Rev. Let

Waveform sampling using an adiabatically driven electron ratchet in a two-dimensional electron system

We utilize a time-periodic ratchet-like potential modulation imposed onto a two-dimensional electron system inside a GaAs/Al$_x$Ga$_{1-x}$As heterostructure to evoke a net dc pumping current. The modulation is induced by two sets of interdigitated gates, interlacing off center, which can be independently addressed. When the transducers are driven by two identical but phase-shifted ac signals, a lateral dc pumping current $I(\phi)$ results, which strongly depends on both, the phase shift $\phi$ and the waveform $V(t)$ of the imposed gate voltages. We find that for different periodic signals, the phase dependence $I(\phi)$ closely resembles $V(t)$. A simple linear model of adiabatic pumping in two-dimensional electron systems is presented, which reproduces well our experimental findings.Comment: 3 figure

Magneto-capacitance probing of the many-particle states in InAs dots

We use frequency-dependent capacitance-voltage spectroscopy to measure the tunneling probability into self-assembled InAs quantum dots. Using an in-plane magnetic field of variable strength and orientation, we are able to obtain information on the quasi-particle wave functions in momentum space for 1 to 6 electrons per dot. For the lowest two energy states, we find a good agreement with Gaussian functions for a harmonic potential. The high energy orbitals exhibit signatures of anisotropic confinement and correlation effects.Comment: 3 pages, 3 figure

Asymmetry of charge relaxation times in quantum dots: The influence of degeneracy

Using time-resolved transconductance spectroscopy, we study the tunneling dynamics between a two-dimensional electron gas (2DEG) and self-assembled quantum dots (QDs), embedded in a field-effect transistor structure. We find that the tunneling of electrons from the 2DEG into the QDs is governed by a different time constant than the reverse process, i.e., tunneling from the QDs to the 2DEG. This asymmetry is a clear signature of Coulomb interaction and makes it possible to determine the degeneracy of the quantum dot orbitals even when the individual states cannot be resolved energetically because of inhomogeneous broadening. Our experimental data can be qualitatively explained within a master-equation approach

Equilibration between edge states in the fractional quantum Hall effect regime at high imbalances

We experimentally study equilibration between edge states, co-propagating at the edge of the fractional quantum Hall liquid, at high initial imbalances. We find an anomalous increase of the conductance between the fractional edge states at the filling factor $\nu=2/5$ in comparison with the expected one for the model of independent edge states. We conclude that the model of independent fractional edge states is not suitable to describe the experimental situation at $\nu=2/5$.Comment: 4 page

Evidence for the Luttigger liquid density of states in transport across the incompressible stripe at fractional filling factors

We experimentally investigate transport across the incompressible stripe at the sample edge in the fractional quantum Hall effect regime at bulk filling factors $\nu=2/3$ and $\nu=2/5$. We obtain the dependence of the equilibration length, that is a phenomenological characteristics of the transport, on the voltage imbalance and the temperature, at high voltage imbalances. These dependencies are found to be of the power-law form, which is a strong evidence for the Luttigger liquid density of states.Comment: 4 pages, to appear in EP