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

Abstract

We studied the field dependencies of specific heat coefficient $\gamma (H)=\underset{T\to 0}{lim}C(T,H)/T$ and thermal conductivity coefficient $\underset{T\to 0}{lim}\kappa (T,H)/T$ of the $\pm s$-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 $\sqrt{H}$ dependence of the $d$-wave state. The impurity scattering effect makes this generic $H$-linear dependence sublinear approaching to the $\sqrt{H}$ behavior. Our calculations of $\underset{T\to 0}{lim}\kappa (T,H)/T$ successfully fit the experimental data of $\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_2$ ${\mathrm{As}}_2$ with different Co-doping $x$ by systematically varying the gap size ratio $R=|{\Delta }_S|/|{\Delta }_L|$. We also resolve the dilemma of a substantial value of $\gamma (H\to 0)$ but almost zero value of $\underset{T\to 0}{lim}\kappa (T,H\to 0)/T$. DOI: http://dx.doi.org/10.1103/PhysRevLett.104.217001 © 2010 The American Physical Society