Transport in Nb-InAs structures: from phase coherence to the edge state regime

We investigated transport in Nb-InAs hybrid structures in perpendicular magnetic fields up to the quantum Hall regime. Due to the high contact quality of our samples, Andreev reflection dominates the transport properties in a range of experimental parameters. Our experiments were performed on periodic arrays of Nb filled stripes or antidots in an InAs-based 2DEG. According to geometry and field strength we observe the following effects: At low fields, up to a few flux quanta per unit cell, we find phase-coherent behavior, such as flux-periodic oscillations. At slightly higher fields, the Andreev reflection probability is determined by induced superconductivity in the 2DEG, which is gradually suppressed by an increasing magnetic field. In the arrays of Nb filled antidots we find that the commensurability peaks are suppressed since Andreev reflection restores regular motion in velocity space. Due to the high critical field of the Nb nanostructures, we can also enter the edge state regime, where we observe a pronounced increase of the amplitude of 1/B-periodic magnetoresistance oscillations. The latter can be traced to an enhanced backscattering of Andreev-reflected edge channels, which contain both electrons and holes.