Weak localization of disordered quasiparticles in the mixed superconducting state

Starting from a random-matrix model, we construct the low-energy effective- held theory for the noninteracting gas of quasiparticles of a disordered su perconductor in the mixed state. The theory is a nonlinear sigma model, wit h the order-parameter field being a supermatrix whose form is determined so lely on symmetry grounds. The weak-localization correction to the field-axi s thermal conductivity is computed for a dilute array of s-wave vortices ne ar the lower critical field H-c1. We propose that weak-localization effects , cut off at low temperatures by the Zeeman splitting, are responsible for the field dependence of the thermal conductivity seen in recent high-T-c ex periments by Aubin et al. [S0163-1829(99)08205-3].