Gate-modified giant Andreev backscattering in a superconductor-semiconductor junction

We investigated giant Andreev backscattering (GAB) in a superconductor (S)- semiconductor two-dimensional electron gas (2DEG) junction with a special g ate configuration: a split gate which produces a quantum point contact (QPC ) and an ordinary wide gate set between S and the QPC, which alters the pro perties of the diffusive transport in the 2DEG. Different combinations of t he two gates voltages offered us a many "samples" for one junction, which e nabled us to systematically study GAB. By applying a negative split gate vo ltage, the number of modes in the QPC was decreased and a crossover from th e reentrant conductance of the disordered 2DEG to enhanced (peaked) QPC con ductance was observed. The peak width at different degrees of diffusiveness clearly showed that coherent Andreev backscattering is essential for the e nhancement. We discuss the influence of the bias voltage and the magnetic f ield on the enhancement quantitatively.