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

Abstract

We studied the field dependencies of specific heat coefficient γ(H)=limT→0C(T,H)/T and thermal conductivity coefficient limT→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 ΔS and large ΔL, 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 limT→0κ(T,H)/T successfully fit the experimental data of Ba(Fe1−xCox)2 As2 with different Co-doping x by systematically varying the gap size ratio R=|ΔS|/|ΔL|. We also resolve the dilemma of a substantial value of γ(H→0) but almost zero value of limT→0κ(T,H→0)/T.

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