Effect of a magnetic field on the quasiparticle recombination in superconductors

TL;DR

This study investigates how magnetic fields influence quasiparticle recombination in superconductors, revealing that increasing magnetic fields slow down the process mainly due to gap reduction and spin effects.

Contribution

It provides experimental evidence that magnetic fields slow quasiparticle recombination primarily through gap reduction, with spin polarization effects minimized by spin-orbit scattering.

Findings

Magnetic fields significantly slow quasiparticle recombination.

Field-induced gap reduction is the main factor affecting recombination.

Spin polarization effects are suppressed by strong spin-orbit scattering.

Abstract

Quasiparticle recombination in a superconductor with an s-wave gap is typically dominated by a phonon bottleneck effect. We have studied how a magnetic field changes this recombination process in metallic thin-film superconductors, finding that the quasiparticle recombination process is significantly slowed as the field increases. While we observe this for all field orientations, we focus here on the results for a field applied parallel to the thin film surface, minimizing the influence of vortices. The magnetic field disrupts the time-reversal symmetry of the pairs, giving them a finite lifetime and decreasing the energy gap. The field could also polarize the quasiparticle spins, producing different populations of spin-up and spin-down quasiparticles. Both processes favor slower recombination; in our materials we conclude that strong spin-orbit scattering reduces the spin polarization, leaving the field-induced gap reduction as the dominant effect and accounting quantitatively for the observed recombination rate reduction.