Dynamics and Energy Distribution of Non-Equilibrium Quasiparticles in Superconducting Tunnel Junctions

TL;DR

This paper provides a comprehensive theoretical and experimental analysis of non-equilibrium quasiparticle behavior in superconducting tunnel junctions, crucial for improving single-photon detection technologies.

Contribution

It introduces a detailed numerical model for quasiparticle tunneling currents and demonstrates that diffusion accurately describes quasiparticle dynamics without thermal equilibration.

Findings

Diffusion model effectively describes quasiparticle transport.

Quasiparticles do not equilibrate to lattice temperature during tunneling.

Physical timescales for quasiparticle dynamics are extracted from experimental data.

Abstract

We present a full theoretical and experimental study of the dynamics and energy distribution of non-equilibrium quasiparticles in superconducting tunnel junctions (STJs). STJs are often used for single-photon spectrometers, where the numbers of quasiparticles excited by a photon provide a measure of the photon energy. The magnitude and fluctuations of the signal current in STJ detectors are in large part determined by the quasiparticle dynamics and energy distribution during the detection process. We use this as motivation to study the transport and energy distribution of non-equilibrium quasiparticles excited by x-ray photons in a lateral, imaging junction configuration. We present a full numerical model for the tunneling current of the major physical processes which determine the signal. We find that a diffusion framework models the quasiparticle dynamics well and that excited quasiparticles do not equilibrate to the lattice temperature during the timescales for tunneling. We extract physical timescales from the measured data, make comparisons with existing theories, and comment on implications for superconducting mesoscopic systems and single-photon detectors.