Temperature dependence of lower critical field Hc1(T) shows nodeless superconductivity in FeSe

TL;DR

This study examines the temperature dependence of the lower critical field in FeSe, revealing evidence for nodeless multiband superconductivity through experimental measurements and modeling.

Contribution

It provides experimental evidence and modeling analysis indicating nodeless multiband superconductivity in FeSe based on Hc1(T) data.

Findings

Hc1(T) shows a curvature change indicating anisotropic or multiband superconductivity.

The London penetration depth does not follow exponential behavior at low temperatures.

The data are well described by a two-band s-wave model or an anisotropic s-wave model.

Abstract

We investigate the temperature dependence of the lower critical field Hc1(T) of a high-quality FeSe single crystal under static magnetic fields H parallel to the c axis. The temperature dependence of the first vortex penetration field has been experimentally obtained by two independent methods and the corresponding Hc1(T) was deduced by taking into account demagnetization factors. A pronounced change in the Hc1(T) curvature is observed, which is attributed to anisotopic s-wave or multiband superconductivity. The London penetration depth {\lambda}ab(T) calculated from the lower critical field does not follow an exponential behavior at low temperatures, as it would be expected for a fully gapped clean s-wave superconductor. Using either a two-band model with s-wave-like gaps of magnitudes ?delta 1 = 0.41 meV and delta ?2 = 3.33 meV or a single anisotropic s-wave order parameter, the temperature dependence of the lower critical field Hc1(T) can be well described. These observations clearly show that the superconducting energy gap in FeSe is nodeless.