S.G. den Hartog

Influence of low energy Ar‐sputtering on the electronic properties of InAs‐based quantum well structures

The influence of low energy (80–500 eV) Ar‐ion milling cleaning techniques on InAs based quantum well structures is investigated. It is found that both etching with a Kaufmann source and sputter‐etching with a rf‐plasma enhances the electron density and reduces the mobility. An anneal at 180 °C has little effect, and only recovers damage caused by low energy (80 eV) Kaufmann etching.

Reentrant behavior in the superconducting phase-dependent resistance of a disordered two-dimensional electron gas

We have investigated the bias-voltage dependence of the phase-dependent differential resistance of a disordered T-shaped two-dimensional electron gas coupled to two superconducting terminals. The resistance oscillations first increase upon lowering the energy. For bias voltages below the Thouless energy, the resistance oscillations are suppressed and disappear almost completely at zero bias voltage. We find a qualitative agreement with the calculated reentrant behavior of the resistance and discuss quantitative deviations.

Sample-specific conductance fluctuations modulated by the superconducting phase

Abstract We present an overview of sample-specific transport properties tuned by the superconducting phase difference between two superconductors connected to a disordered 2-dimensional electron gas (2DEG). We demonstrate a crossover from ensemble-averaged to sample-specific resistance oscillations of a T-shaped 2DEG interferometer by increasing the magnetic field. Multi-terminal resistances of a cross-shaped 2DEG interferometer are analyzed in terms of an extended Landauer–Buttiker transport formalism. We show that the extended reciprocity relations hold and that three-terminal resistances become negative.

Reentrant resistance and giant Andreev backscattering in a two-dimensional electron gas coupled to superconductors

We first present the bias-voltage dependence of the superconducting phase-dependent reduction in the differential resistance of a disordered T-shaped two-dimensional electron gas (2DEG) coupled to two superconductors. This reduction exhibits a reentrant behavior, since it first increases upon lowering the bias voltage, exhibits a maximum for a bias voltage of about the Thouless energy and disappears almost completely at zero bias voltage. Second, we have investigated the superconducting-phase-modulated reduction in the resistance of a ballistic quantum point contact (QPC) connected via a disordered 2DEG to superconductors. We show that this reduction is caused by coherent Andreev backscattering of holes through the QPC, which increases monotonically by reducing the bias voltage to zero.

Superconducting phase tuned sample-specific conductance fluctuations

We have studied sample-specific conductance fluctuations tuned by the phase difference between superconducting boundaries attached to a T-shaped two-dimensional electron gas. In low magnetic fields, oscillations due to phase-conjugated Andreev reflections were observed with an amplitude delta G(qp)similar or equal to 0.10e(2)/h. These oscillations were suppressed by a flux of approximately h/e through the interference region. For larger magnetic fields, superconducting-phase modulated sample-specific conductance fluctuations were found with an amplitude delta Gy(Delta phi)similar or equal to.0.005e(2)/h.

Giant Andreev backscattering and reentrant resistance in a 2- dimensional electron gas coupled to superconductors

Abstract We have investigated the superconducting-phase modulated reduction in the resistance of a ballistic quantum point contact (QPC) connected via a disordered 2-dimensional electron gas (2DEG) to superconductors. We show that this reduction is caused by coherent Andreev backscattering of holes through the QPC, which increases monotonically by reducing the bias voltage to zero. In contrast, the magnitude of the phase-dependent resistance of the disordered 2DEG displays a non-monotonic reentrant behavior versus bias voltage.

Comment on “Conductance Fluctuations in Mesoscopic Normal-Metal/Superconductor Samples”

Recently, Hecker et al. [Phys. Rev. Lett. 79, 1547 (1997)] experimentally studied magnetoconductance fluctuations in a mesoscopic Au wire connected to a superconducting Nb contact. They claimed to have observed an enhancement of the rms magnitude of these conductance fluctuations in the superconducting state (rms(Gns)) relative to that in the normal state (rms(Gn)) by a factor of 2.8. In this comment, we argue that the measured rms(Gns) is NOT significantly enhanced compared to rms(Gn) when we correct for the presence of an incoherent series resistance from the contacts, which is different when Nb is in the superconducting or normal state.

Electron transport in multi-terminal 2DEG-superconductor devices

We have studied multi-terminal transport in normal-superconductor devices. We will describe the transport in terms of an extended Landauer-Büttiker transport formalism, which will be illustrated by an experimental test of the reciprocity relations. Furthermore, we have found a negative three-terminal resistance and non-local resistance modulations.

Experimental determination of the quasiparticle decay length ξqp in a diffusive superconducting quantum well

We have experimentally investigated the electronic transport properties of an AlSb/InAs/AlSb quantum well, where part of the AlSb top layer has been replaced with a superconducting Nb strip. By doing a transmission experiment underneath the Nb strip and comparing the results with a model based on diffusive transport, with some of the injected electrons being Andreev reflected into holes, we can estimate the quasiparticle decay length, xi(qp) similar to 50 nm, in the Nb-InAs superconducting quantum well. This decay length corresponds to an interface transparency of T-SIN = 0.5 between the Nb and InAs. The observed voltage dependence cannot entirely be understood within the presented model.