Thermal Conductivity of the Iron-Based Superconductor FeSe : Nodeless Gap with Strong Two-Band Character

TL;DR

This study measures the thermal conductivity of FeSe at ultra-low temperatures and high magnetic fields, revealing a nodeless, strongly two-band superconducting gap with a very small gap on one Fermi surface pocket.

Contribution

It provides the first detailed thermal conductivity measurements of FeSe, demonstrating a nodeless gap with strong two-band characteristics and a very small gap on one Fermi surface pocket.

Findings

FeSe has a nodeless superconducting gap.

The gap exhibits strong two-band behavior similar to MgB2.

One Fermi surface pocket has a very small superconducting gap.

Abstract

The thermal conductivity of the iron-based superconductor FeSe was measured at temperatures down to 50 mK in magnetic fields up to 17 T. In zero magnetic field, the electronic residual linear term in the T = 0 limit, \kappa_0/T, is vanishingly small. Application of a magnetic field H causes no increase in \kappa_0/T initially. Those two observations show that there are no zero-energy quasiparticles that carry heat and therefore no nodes in the superconducting gap of FeSe. The full field dependence of \kappa_0/T has the classic shape of a two-band superconductor, such as MgB2: it rises exponentially at very low field, with a characteristic field H* << Hc2, and then more slowly up to the upper critical field Hc2. This shows that the superconducting gap is very small on one of the pockets in the Fermi surface of FeSe.