"What really happens in the superconducting proximity effect? "

James N. Eckstein

Department of Physics and Fredrick Seitz Materials Research Laboratory
University of Illinois, Urbana

The superconducting proximity effect introduces particle correlations in otherwise normal metals that are joined to a superconductor via a transparent-enough junction. It causes changes to happen in the electrical response of the junction that can be divided into two parts. First, the transport in the normal metal parallel to the junction changes as a function of temperature. Second, the conductance between the two materials changes as a function of temperature. Measuring both of these parts simultaneously in a two terminal transport device is impossible. To address this experimental problem, we've introduced a new three terminal device design that, with simple modeling, allows these two aspects of the proximity effect to be separated. Using it, we've studied junctions between niobium and heavily doped (nGaAs ~ 10E19 cm-3) GaAs, which behaves as a low carrier concentration metal at least above 4.2 K. We find that the 2D sheet resistance of the normal layer first is aided by the proximity effect as the temperature is lowered below the Tc of the niobium, but at lower temperatures it actually increases. We also find a junction conductance enhancement as the temperature is lowered much greater than the factor of two obtained from simple Andreev reflection physics. At least some of our findings are consistent with a two fluid picture of the normal metal in which pairs don't become phase coherent until the temperature is more than a degree below the Tc of niobium. Other aspects of our data are not easily explained by this picture.

email: eckstein@uiuc.edu