Superfluid density and microwave conductivity of FeSe superconductor: ultra-long-lived quasiparticles and extended s-wave energy gap

TL;DR

This study provides detailed measurements of FeSe's superconducting properties, revealing ultra-long quasiparticle mean free paths and evidence for an extended s-wave energy gap, highlighting its unconventional superconductivity features.

Contribution

It offers the first high-precision electrodynamic measurements of FeSe at microwave frequencies down to 0.1 K, demonstrating long-lived quasiparticles and multiband superconductivity evidence.

Findings

Quasiparticle scattering collapses rapidly in the superconducting state.

Quasiparticle mean free path exceeds 50 microns at low temperatures.

Strong evidence for a finite energy-gap minimum in FeSe.

Abstract

FeSe is an iron-based superconductor of immense current interest due to the large enhancements of Tc that occur when it is pressurized or grown as a single layer on an insulating substrate. Here we report precision measurements of its superconducting electrodynamics, at frequencies of 202 and 658 MHz and at temperatures down to 0.1 K. The quasiparticle conductivity reveals a rapid collapse in scattering on entering the superconducting state that is strongly reminiscent of unconventional superconductors such as cuprates, organics and the heavy fermion material CeCoIn5. At the lowest temperatures the quasiparticle mean free path exceeds 50 micron, a record for a compound superconductor. From the superfluid response we confirm the importance of multiband superconductivity and reveal strong evidence for a finite energy-gap minimum.