Volovik Effect in the ±s-Wave State for the Iron-Based Superconductors

We studied the field dependencies of specific heat coefficient gamma(H) = lim(T -> 0)C(T, H)/T and thermal conductivity coefficient lim(T -> 0)kappa(T,H)/T of the +/- s-wave state in the mixed state. It is a generic feature of the two band s-wave state with the unequal sizes of gaps, small Delta(S) and large Delta(L), that Doppler shift of the quasiparticle excitations (Volovik effect) creates a finite density of states, on the extended states outside of vortex cores, proportional to H in contrast to the root H dependence of the d-wave state. The impurity scattering effect makes this generic H-linear dependence sublinear approaching to the root H behavior. Our calculations of lim(T -> 0)kappa(T,H)/T successfully fit the experimental data of Ba(Fe(1-x)Co(x))(2) As(2) with different Co-doping x by systematically varying the gap size ratio R = vertical bar Delta(S)vertical bar/vertical bar Delta(L)vertical bar. We also resolve the dilemma of a substantial value of gamma(H -> 0) but almost zero value of lim(T -> 0)kappa(T,H -> 0)/T.