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

We studied the field dependencies of specific heat coefficient γ(H)=lim⁡T→0C(T,H)/T and thermal conductivity coefficient lim⁡T→0κ(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 Δ_S and large Δ_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 H dependence of the d-wave state. The impurity scattering effect makes this generic H-linear dependence sublinear approaching to the H behavior. Our calculations of lim⁡T→0κ(T,H)/T successfully fit the experimental data of Ba(Fe_1-xCo_x)₂ As₂ with different Co-doping x by systematically varying the gap size ratio R=|Δ_S|/|Δ_L|. We also resolve the dilemma of a substantial value of γ(H→0) but almost zero value of lim⁡T→0κ(T,H→0)/T.