Coexistence of isotropic and extended<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>s</mml:mi></mml:math>-wave order parameters in FeSe as revealed by low-temperature specific heat

Abstract

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 \ensuremath{\Delta}${}_{0}$ $=$ 1.33 meV on the hole Fermi sheets and the extended $s$-wave gap \ensuremath{\Delta} $=$ \ensuremath{\Delta}${}_{\mathrm{e}}$(1$+$\ensuremath{\alpha}cos2\ensuremath{\phi}) with \ensuremath{\Delta}${}_{\mathrm{e}}$ $=$ 1.13 meV and \ensuremath{\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 \ensuremath{\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.