L. W. Molenkamp

Hydrodynamic electron flow in high-mobility wires

Hydrodynamic electron flow is experimentally observed in the differential resistance of electrostatically defined wires in the two-dimensional electron gas in (Al,Ga)As heterostructures. In these experiments current heating is used to induce a controlled increase in the number of electron-electron collisions in the wire. The interplay between the partly diffusive wire-boundary scattering and the electron-electron scattering leads first to an increase and then to a decrease of the resistance of the wire with increasing current. These effects are the electronic analog of Knudsen and Poiseuille flow in gas transport, respectively. The electron flow is studied theoretically through a Boltzmann transport equation, which includes impurity, electron-electron, and boundary scattering. A solution is obtained for arbitrary scattering parameters. By calculation of flow profiles inside the wire it is demonstrated how normal flow evolves into Poiseuille flow. The boundary-scattering parameters for the gate-defined wires can be deduced from the magnitude of the Knudsen effect. Good agreement between experiment and theory is obtained.

SCALING OF THE COULOMB ENERGY DUE TO QUANTUM FLUCTUATIONS IN THE CHARGE ON A QUANTUM-DOT

The charging energy of a quantum dot is measured through the effect of its potential on the conductance of a second dot. This technique allows a measurement of the scaling of the dots charging ener ...

STOCHASTIC COULOMB-BLOCKADE IN A DOUBLE-QUANTUM DOT

The series resistance of two coupled quantum dots in a (Al,Ga)As heterostructure has been studied experimentally. At very low lattice temperatures conductance oscillations of irregular amplitude and spacing are observed. The irregularities decrease on raising the temperature. The observations are interpreted as resulting from the stochastic Coulomb blockade effect.

Ultralong minority‐carrier lifetimes in GaAs grown by low‐pressure organometallic vapor phase epitaxy

We have measured minority‐carrier lifetimes of up to 4.9 μs in GaAs layers that have been grown by low‐pressure organometallic vapor phase epitaxy. These lifetimes, representing a major improvement compared with previously obtained results, are governed by radiative recombination processes. Carbon incorporation during crystal growth at low arsine partial pressures is of prime importance in understanding the origin of these very long lifetimes.

Near-band-gap luminescence from a GaAs-AlGaAs interface

We report on low‐temperature photoluminescence from an ultrapure GaAs‐Al0.33Ga0.67As heterostructure grown with molecular beam epitaxy. A luminescence band centered at 1.509–1.510 eV is observed. In spectral shape and polarization this band distinguishes itself from the defect related lines of H. Kuenzel and K. Ploog [Appl. Phys. Lett. 37, 416 (1980)]. Furthermore, this band disappears when the top cladding Al0.33Ga0.67As layer is removed. The emission band is further characterized by measuring its dependence on excitation density and temperature as well as its temporal behavior. We discuss possible recombination mechanisms for this band. The most likely candidate is luminescence related to a defect pair situated at or near the interface of the heterojunction.

Experimental study of reduced shot noise in a diffusive mesoscopic conductor

We show that a diffusive narrow wire of a length much longer than the elastic scattering length and the phase-coherence length, but of the order of the energy relaxation length, exhibits shot noise with an intensity lower than the full shot-noise level. At 4.2 K, the reduction factor is shown to vary between 0.2 and 0.45, depending on the width of the wire. The reduction is consistent with recent theoretical predictions.

Valence-band effective-mass anisotropy and the optical spectra of (Al, Ga)As quantum wells

Abstract We present excitation and luminescence spectra of (Al, Ga)As quantum wells grown in three different crystal orientations ((001), (310) and (111B)). The features in these spectra are assigned using an eight-band k.p bandstructure calculation and effective-mass type of calculations of the excitonic binding energy, yielding detailed information concerning the GaAs valence-band anisotropy and exciton mixing phenomena.

Stochastic Coulomb blockade and scaling of charging energy in a double quantum dot system

We have studied the transport properties of a device consisting of two quantum dots, defined electrostatically in a (Al.Ga)As heterostructure. In the series conductance of the two dots, we observe irregularly spaced conductance peaks of fluctuating amplitude. This behaviour results from transport in the stochastic Coulomb blockade regime. In a second experiment, we measure the charging energy of one dot through the effect of its potential (which varies in a saw-tooth fashion with gate voltage) on the conductance of the other dot. We find that the charging energy scales quadrat-ically with the reflection probability of the tunnel barriers, in agreement with a recent theory.

Oscillating Transverse Voltage in a Channel with Quantum Point Contact Voltage Probes

We have observed a transverse voltage on passing a current through a narrow channel, electrostatically defined in a two-dimensional electron gas, at zero magnetic field. The channel is fitted with two opposite quantum point contact voltage probes, and the voltage occurs when these probes are differently adjusted, so that the transmission probabilities through the probes have a different energy dependence. The transverse voltage occurs only in the nonlinear response regime, and is even in the applied current; the driving force of the effect is the current-heating of the electrons in the channel. We observe strong oscillations in the transverse voltage as the number of occupied subbands in one of the voltage probes is varied by means of electrostatic or magnetic depopulation. Model calculations show that this novel effect is a manifestation of the oscillatory thermopower of a quantum point contact predicted by Streda. The effect can thus be used to obtain information on electron heating.