Thermally Induced Quasiparticle Branch Imbalance in Thin Film Superconductors

Thermoelectric effects in superconductors are quite different than in normal metals. One effect is the voltage difference generated between the normal electrons and the superelectrons by a temperature gradient in an electrically isolated thin film of superconductor, the thermally induced quasiparticle branch imbalance. The theoretical background of the effect is investigated, using a two fluid model for the quasiparticle-superelectron system, with attention being given to the question of thermal contact between the sample (thin Sn film) and the (sapphire) substrate. Sample fabrication, using liftoff photolithography to pattern normal metal (Ag) tunnel junction probes (which sample the potential of the quasiparticles) is discussed, as is the experimental apparatus, and the scheme for making measurements using a SQUID picovoltmeter. The thermopower of Sn films in the normal state was measured to be between .19 and 1.4 nV/K, varying with the thickness of the film, a value which is considerably smaller than the bulk value of 20 nV/K. An experimental upper bound was placed on the branch imbalance voltage of 10 pV, which gives an upper bound for the thermopower of the normal electrons in the superconductor of 2 nV/K, in agreement with the theory presented.