Coherent transport in the diffusive regime in mesoscopic superconductor-semiconductor-superconductor (S/Sm/S) junctions of Al/n++-GaAs/Al

In this chapter we review our recent experimental work on phase correlated quasi-particle transport phenomena in mesoscopic Al/n(++)-GaAs/Al (S/N/S) junctions in the diffusive regime (short mean free path, l, compared to the coherence length in GaAs, xi(N)) The junction geometry is planar with a channel length L much greater than l. For T < T-c (Al) = 1.20 K the coupling between the superconducting Al electrodes through the strongly doped semiconductor channel is seen as weak perturbations of the de current-voltage (I-V) characteristics: 1) a minimum in dV/dI around V = 0 (zero-voltage conductance peak, ZVCP) and 2) dV/dI minima around V = 2 Delta / e and V = Delta / e, the sub-harmonic energy gap structure (SGS) there Delta is the energy gap in Al), and 3) excess current (EC) at higher voltage (V > 2 Delta / e). The strength of these features was studied as a function of temperature and channel length. In flux-sensitive AVGaAs/Al junctions (interferometer type junctions) we have observed phase modulated conductance (PMC). We find the characteristic decay length for the underlying transport phenomena which we believe are mediated by Andreev reflections at the GaAs/Al interfaces. The results show that the strength of these effects falls off over the phase breaking length, l(phi), in the GaAs, a decay length which we have determined independently by weak localization measurements. We conclude that also in the diffusive regime, interfererence effects stemming from interfacial Andreev scattering in S/N/S junctions give rise to the conductance perturbations well-known from cleaner junctions: the zero-voltage conductance peak (ZVCP), the sub-harmonic energy gap structure (SGS), the excess current (EC), and the phase modulated conductance (PMC). These effects decay over the phase breaking length, l(phi), a length scale which in most cases exceeds the coherence length in the channel (xi(N) = root (h) over bar / 2 pi k(B)T).