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

J.-Y. Lin, Y. S. Hsieh, D. A. Chareev, A. N. Vasiliev, Y. Parsons, H. D. Yang
2011 Physical Review B  
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 Δ0=1.33 meV on the hole Fermi sheets and the extended s-wave gap Δ=Δe(1+αcos2_theta) with Δe=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
more » ... fficient 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.
doi:10.1103/physrevb.84.220507 fatcat:g7jxz6sewrclbo4omh3td6k3ju