Quasiparticle recombination time in superconducting tin and normal electronic density of states at the fermi surface from tunnel junction experiments

Abstract

We have measured the temperature dependence of the effective quasiparticle recombination time in superconducting tin tunnel junctions by current and laser pulse excitation. The experimental times show satisfactory agreement with calculations based on the ray acoustic lifetime model of Eisenmenger et al. taking into account the film thickness dependence of the phonon reabsorption, 2A-phonon volume loss processes and phonon transmission from the junction into the substrate and liquid helium. On the basis of the BCS density of thermally excited quasiparticles and simplified rate equations for quasiparticle recombination, and from the analysis of measurements of decaying excess quasiparticle concentrations we obtain a mean value N o =(2.73_+0.03) 1022 eV-1 cm-3 for the electronic density of states at the Fermi Surface in thin, evaporated tin films. This value differs less than 5 % from that obtained from the experimental electronic heatcapacity coefficient of the bulk material.

Measurements of effective quasiparticle recombination times and of densities of electronic states at the Fermi level in superconducting Al- and Pb-films

“…tin [1] and aluminium [see Table 2] the density of electronic states N O (with both spins) in evaporated lead films of polycrystalline structure agree with the corresponding "bulk value".…”

“…In the present paper current pulses have been used to generate bN. By current injection additional, nonequilibrium quasiparticles are generated in single particle tunneling via breaking up Cooper-pairs (U>2A/e) in the tunneling volume VTu with the rate I o = 2 ieff/eVT~; /eft is an effective excitation current (see [1]). Experimentally bN/Nth is determined from the corresponding increase of the thermal tunneling current ith , i.e.…”

“…6N/Nth=(Si/ith, and Zexp, 0 can directly be observed from the time decay of 6i sufficiently long after switch-off of the injection pulse. Accurate values for x(0) and Z~xp,0 are obtained from a mean least squares fit of the nonexponential decay function [1] …”

“…We report here first temperature-dependent decay time measurements on low-ohmic (mf~), high-quality (i2A÷/i2A_>200) Pbtunnel junctions under vacuum conditions. In the preparation of the Pb-junctions we had to dispense with the sample configuration of low inductive crosstalk, used in the Al-experiments and reported in [1,25], since nearly all junctions prepared with an intermediate layer (SiO window) showed too high leakage currents. Proper junctions were obtained, however, with the configuration shown in the inset of Fig.…”

“…Using the relations for the BCS density Nth of thermally excited quasiparticles, the lifetime "Cex v from the rate equations for quasiparticle recombination [3] and the excess quasiparticle density 6N under a stationary injection rate I o we derived in [1] Accordingly, a measurement of N o requires absolute values of the temperature T, the energy gap A (T), the rate Io, the effective lifetime Zexp, 0 at (SN/Nth ~ 1 and the relative quasiparticle overinjection ,SN(t=O)/Nth =x(0) at the switch-off time t=0 of the injection pulse. These quantities can be measured with a single, symmetrical tunnel junction, i.e.…”