Ballistic Transport and Andreev Resonances in Nb-In Superconducting Contacts to InAs and LTG-GaAs

摘要

We have formed superconducting contacts in which Cooper pairs incident from athick In layer must move through a thin Nb layer to reach a semiconductor,either InAs or low temperature grown (LTG) GaAs. The effect of pair tunnelingthrough the Nb layer can be seen by varying the temperature through thecritical temperature of In. Several of the In/Nb-InAs devices display a peak inthe differential conductance near zero-bias voltage, which is strong evidenceof ballistic transport across the NS interface. The differential conductance ofthe In/Nb-(LTG) GaAs materials system displays conductance resonances ofMcMillan-Rowell type. These resonant levels exist within a band of conductingstates inside the energy gap, formed from excess As incorporation into the(LTG) GaAs during growth. Electrons propagating in this band of states multiplyreflect between the superconductor and a potential barrier in the GaAsconduction band to form the conductance resonances. A scattering state theoryof the differential conductance, including Andreev reflections from thecomposite In/Nb contact, accounts for most qualitative features in the data.