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 0=1.33 meV on the hole Fermi sheets and the extended s-wave gap =e(1+α*cos2theta) with e=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.
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