Two-band theory of specific heat and thermal conductivity in the mixed state of MgB_2

Abstract

We solve the coupled gap equations for the $\sigma$- and $\pi$-bands of MgB$_2$ in the vortex state and calculate the resulting field dependencies of the specific heat coefficient $\gamma$ and the thermal conductivity $\kappa $. The crucial parameters of the theory are the interband pairing interaction $\lambda_{\pi\sigma}$ and the ratio $s=\xi_{\sigma}/\xi_{\pi}$ of the coherence lengths. For reasonably small $\lambda_{\pi\sigma}$ and s, the small gap $\Delta_{\pi}$ decreases with increasing magnetic field $H$ much faster than the large gap $\Delta_{\sigma} $. This gives rise to the observed rapid increase of $\gamma_{\pi}$ and $\kappa_{\pi}$ for small fields while $\gamma_{\sigma}$ and $\kappa_{\sigma}$ exhibit conventional field dependencies. Inclusion of intraband impurity scattering yields fairly good agreement with experiments for applied fields along the c axis.