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

The comprehensive low-temperature specific-heat C(T) data identify both an isotropic s-wave and extended s-wave order parameters coexisting in a superconducting single-crystal FeSe with T-c = 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 phi) 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 nonlinear function of the applied magnetic field H in the mixed state in accord with the anisotropic multiorder parameters.