Coexistence of isotropic s-wave and extended s-wave order parameters in FeSe as revealed by the low-temperature specific heat

TL;DR

This study reveals that FeSe superconductors exhibit coexisting isotropic s-wave and anisotropic extended s-wave order parameters, with detailed gap measurements and implications for quasiparticle excitations and magnetic field responses.

Contribution

It provides the first detailed experimental evidence of coexisting isotropic and extended s-wave gaps in FeSe using low-temperature specific heat data.

Findings

Coexistence of isotropic s-wave and extended s-wave gaps in FeSe.

No sign of accidental nodes despite anisotropic extended s-wave gap.

Non-linear magnetic field dependence of quasiparticle contribution to specific heat.

Abstract

The comprehensive low-temperature specific heat C(T) data identify both an isotropic s-wave and an extended s-wave order parameters coexisting in a superconducting single crystal FeSe with Tc=8.11 K. The isotropic gap {\Delta}0=1.33 meV on the hole Fermi sheets and the extended s-wave gap {\Delta}={\Delta}e(1+\alpha*cos2_theta) with {\Delta}e=1.13 meV and \alpha=0.78 on the electron Fermi sheets. The extended s-wave is rather anisotropic but the low energy quasiparticle excitations demonstrate no sign of the accidental nodes. The coefficient \gamma(H) manifesting the quasiparticle contribution to C is a non-linear function of the applied magnetic field H in the mixed state in accord with the anisotropic multi-order parameters.