Energy Gap Reduction in Superconducting Tin Films by Quasiparticle Injection

Abstract

In Sn-I-Sn-I-Pb tunneling structures the energy gap ~Sn of Sn is reduced by quasiparti­ cle injection via single-particle tunneling between the Sn films. ~Sn as function of the quasiparticle density is probed by the Pb contact and found in agreement with the theory of OWen and Scalapino. An instability of the energy gap of Sn is observed at the critical gap reduction ratio predicted by this theory for a first-ortler phase transition. Nonequilibrium quasiparticle distributions in superconductors can be produced by photonl - 3 and phonon4 irradiation or by quasiparticle 5 injection via tunneling. Under constant injection conditions the stationary quasiparticle energy distribution is determined by the energy distribution of the primary quasiparticle injection or excitation rates, by the energy dependence of relaxation and recombination probabilities, and by secon­ dary quasiparticle excitation and pair-breaking rates via phonon absorption. Since phonons are emitted in quasiparticle decay, the phonon escape probability from the superconducting film into the substrate and the intrinsic phonon decay also have a strong influence on the stationary quasi­ particle energy distribution. Whereas the gener­ al problem of the quasiparticle distribution can be solved numerically,6 two important simple models have been discussed in the past: For the limit of recombination lifetimes long compared to relaxation times, Owen and Scalapino7 pro­ posed a nonequilibrium quasiparticle distribution in which the excess number of quasiparticles is characterized by a chemical potential p* > 0 and their energy distribution by the unperturbed lat­ tice temperature T. Since most superconducting films show high phonon trappingS by pairbreaking, Parker9 proposed a in which an elevated temperature > T describes the number of quasi­ particles and their energy distribution. A signifi­ cant difference between the two models is that the /1* model predicts a first-order phase transi­ tion as the number of excess quasiparticles is in­ creased, whereas the T* model does not. Dif­ ferent experiments with optical excitation of qua­ siparticles 2,3.1o did not give clear evidence in fa­ vor of one of the two models. In this communication we report on experiments with quasiparticle injection via tunneling between two Sn films and probing the energy gap and the quasiparticle population with a Pb contact. In ac­ cord with the 11-* we find that the gap reduction as function of the quasiparticle density is stronger than in the thermal case and we observe an instability of the energy gap at the predicted critical gap reduction. The sample consists of two overlapping Sn films and one Pb film, width and thickness of each film being 1.4 mm and 1000 A, respectively (Fig. 1). Silicon single crystals are used as substrates which are cooled by direct contact to the liquid­ He bath on the backside. The front surface with the Sn-J-Sn-J-Pb structure can be kept under vac­ uum or also exposed to liquid He. By 15-min glow-discharge oxidation in O2 at 100 mTorr the tunneling resistance in the Sn-J-Sn junctions re­ sulted with the higher voltage asymptotic value of R,,?25 mn. For the Sn-J-Pb junctions typical values are Roo' 1 mn. The increased tunneling resistance in the Sn-J-Sn contact was necessary for obtaining a high stationary quasiparticle popu­ lation at injection currents below the critical cur­ rents of the film structure. This allows high bat­ tery voltages and primary quasiparticle injection energies at multiples of the energy gap with suc­ cessive relaxation-phonon emission and reabsorp­ tion by pairbreaking increaSing the effective rate of quasiparticle excitations. Using conventional electronic measuring tech