Quasiparticle nonequilibrium dynamics in a superconducting Ta film

Abstract

Nonequilibrium quasiparticle dynamics in Ta are studied using a superconducting Ta film with an Al tunnel junction connected at each end. The quasiparticle system is driven out of the equilibrium by absorption of an x-ray photon. Millions of quasiparticles, created by each photon, diffuse in the Ta film. When the quasiparticles reach the Al junctions they lose energy by emitting phonons and are trapped in the Al film. By measuring the tunneling current, the number of excess quasiparticles can be calculated. In Ta, the diffusion constant of 8.2Ϯ0.2 cm 2 /s and quasiparticle lifetime of 83 Ϯ5 s at 0.21 K are derived from fitting the measured current pulses, and are compared with theoretical predictions.

Inelastic scattering of quasiparticles in a superconductor with magnetic impurities

“…12. The inset of this figure shows that, contrary to the predictions of BCS theory for the ideal superconductor ͑dashed lines͒, the QP lifetime rises exponentially with decreasing tempera- Values of QP lifetime obtained for a variety of different materials [20][21][22][23] at low temperatures are consistently, in some cases by orders of magnitude, shorter than can be explained simply by thermal recombination of nonequilibrium QPs ͑Ref. 15͒ and in addition are independent on temperature.…”

“…15͒ so that trap ϳ 1.8/ c ns. The observed lifetimes in Ta, at such low temperatures that thermal recombination is absent, are typically several tens of microseconds 11,[18][19][20][21][22][23] leading to an estimate for c of between 1 ϫ 10 −5 and 1 ϫ 10 −4 or a range of impurity concentration of 10-100 ppb. Thus even if the state were not strongly localized, for example, say ͑a 2 / a 0 ͒ 4 Ӎ 100, in order to produce the observed QP lifetimes in Ta at low temperature, it would require a concentration of only 1-10 ppm.…”

Enhancing the Energy Resolution of a Singles Photon STJ Spectrometer Using Diffusion Engineering

“…They diffuse out into the Au pad in the case of the single-tunnel devices, while for the diffusion-engineered case the out-diffusion is hampered by the narrow wire. The idea of backtunneling due to geometrical confinement originated from x-ray experiments with STJs [6]. If, due to a slow out-diffusion, quasiparticles dwell in the right electrode for a time longer than the tunneling time from the right to the left electrode, they will backtunnel.…”

“…If, due to a slow out-diffusion, quasiparticles dwell in the right electrode for a time longer than the tunneling time from the right to the left electrode, they will backtunnel. Since they cool during this time, reverse tunneling is suppressed [6]. Backtunneling is a process that transfers a negative charge in the same direction as the initial (left to right) electron tunneling process.…”

Diffusion-engineered single-photon spectrometer for UV/visible detection

“…2). The idea arose from experiments with X-ray devices, where the backtunneling was due to narrowed leads [5]. The signature was an extended current pulse, with a decay time constant of order 200 ms.…”