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 Δ${}_0$ $=$ 1.33 meV on the hole Fermi sheets and the extended $s$-wave gap Δ $=$ Δ${}_{\mathrm{e}}$(1$+$αcos2ϕ) with Δ${}_{\mathrm{e}}$ $=$ 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.

• Received 8 September 2011

DOI:https://doi.org/10.1103/PhysRevB.84.220507