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 Tc $=$ 8.11 K. The isotropic gap {Δ}₀ $=$ 1.33 meV on the hole Fermi sheets and the extended s-wave gap {Δ} $=$ {Δ}ₑ(1$+${α}cos2{φ}) with {Δ}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 nonlinear function of the applied magnetic field H in the mixed state in accord with the anisotropic multiorder parameters.