Thermoelectric Effect in a Nonequilibrium Superconductor

Abstract

As Ginzburg¹ first noted, there exists in a superconductor the possibility of a simultaneous flow of a normal current of density ${\vec j_n} = {L_T}( - \vec \nabla T)$ and a supercurrent ${\vec j_s} = - {\vec j_n}$. Recently² calculations based on the two fluid model have predicted that this flow of normal current in a superconductor will give rise to a non-quantized contribution to the magnetic flux in a loop made up of two different superconductors. Experimental data^3–5 indicate the existence of such a magnetic flux with a value of as much as five orders of magnitude⁵ larger than predicted by theory. However, more recent work⁶ has suggested that this magnetic flux may be due to the temperature dependence of the penetration depth. This discrepancy, coupled with the opportunity to study quasiparticle transport and relaxation processes in a nonequilibrium superconductor, prompted us to make measurements of the pair and quasi-particle electrochemical potentials in a superconductor held in a temperature gradient. It has been predicted⁷ that in nonequilibrium situations in which the electron and hole branches of the quasi-particle excitation spectrum are unequally populated, the quasi-particles in a superconductor may be described by a different electrochemical potential than that which describes the pairs. This paper reports initial results showing experimental evidence for a pair-quasiparticle electrochemical potential difference in a superconductor in a temperature gradient.

Work performed under the auspices of the U.S. Energy Research and Development Administration.