The transport properties of superconducting-normal interfaces
The resistive and thermoelectric properties of a series of SNS sandwiches, containing different concentrations of impurity in the superconductor, have been measured. The system used for this work was In/W/In with up to 10% lead added to the indium. A technique was developed in which the interfaces were prepared by melting indium onto tungsten slices in high vacuum. The samples produced in this way were of comparable quality to those used by previous workers. The samples with pure indium were found to obey a previously developed theory for the divergent temperature dependence of the resistance just below Tc. This theory was extended with more realistic boundary conditions and to include the proximity effect. It was then found to adequately explain the temperature dependence of the resistance over the whole range between 0.3Tc and Tc. The thermopower of these samples near Tc was also found to have the temperature dependence predicted by previous theory. The results imply that the thermopower of indium changes in a discontinuous and systematic way at Tc. This is at variance with what is expected theoretically and with previous experimental work on lead. The low temperature interface resistance was measured as a function of the concentration of lead in the indium. It was found that, for lead concentrations of up to 5%, the interface resistance was proportional to the residual resistivity of the indium as predicted by a theory of Pippard. The magnitude of this resistance was, however, not found to be in agreement with the theory. Above concentrations of 5% Pb, the low temperature resistance data was found to become irreproducible. A theory was constructed which adequately explained the temperature dependence of the resistance of the samples with up to 5% lead in the indium. The temperature dependence of the thermopower of these samples was found to be approximately as expected from theory in the region just below Tc. However, well below Tc, an unexplained divergence in the thermopower was found.