Coexistence of isotropic s-wave and extended s-wave order parameters in FeSe as revealed by the low-temperature 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 Δ0=1.33 meV on the hole Fermi sheets and the extended s-wave gap Δ=Δe(1+α*cos2_theta) with Δe=1.13 meV and α=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 γ(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.

4 figures, 2 tables. References updated. Changed title