Universal thermal conductivity in the vortex state of cuprate superconductors

We formulate an effective low energy theory for the fermionic excitations in d-wave superconductors in the presence of periodic vortex lattices. These can be modeled by an effective free Dirac Hamiltonian with renormalized velocities and possibly a small mass term. In the presence of random nonmagnetic impurities this will result in universal (i.e. field and disorder strength independent) thermal and spin conductivities with values different from those occurring in the Meissner state.