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

Abstract

The thermal conductivity κ of the iron-based superconductor FeSe was measured at temperatures down to 75 mK in magnetic fields up to 17 T. In a zero magnetic field, the electronic residual linear term in the T=0 K limit, κ0/T, is vanishingly small. The application of a magnetic field B causes an exponential increase in κ0/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 κ0/T has the classic two-step shape of a two-band superconductor: a first rise at very low field, with a characteristic field B⋆≪Bc2, and then a second rise up to the upper critical field Bc2. This shows that the superconducting gap is very small (but finite) on one of the pockets in the Fermi surface of FeSe. We estimate that the minimum value of the gap, Δmin, is an order of magnitude smaller than the maximum value, Δmax.

Source-reported materials — not catalogue approval

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