Thermoelectric generation of charge imbalance at a superconductor-normal metal interface

The thermoelectric voltage produced across a superconductor-normal metal-superconductor (SNS) sandwich by an applied heat current has been measured in Pb-Cu-PbBi and In-Al-Sn as a function of temperature. The observed divergence of the thermoelectric voltage near T_c is attributed to a charge imbalance region decaying into the superconductor from the NS interface over the quasiparticle diffusion length λ_Q^*. The charge imbalance is generated by thermoelectrically driven quasiparticle currents in the super-conductor. It contributes a voltage per unit heat power given by V_s/P = λ_Q*S/κA, where A is the sample cross-sectional area, and S and κ are the thermopower and the thermal conductivity of quasiparticles in the super-conductor. For Pb and In, we find the measured thermopower in the super-conducting state to be slowly varying with temperature near T_c and consistent in magnitude with normal state values. This result is in agreement with theoretical predictions of thermoelectric effects in superconductors but contrary to previous experimental results obtained by other methods.