Low-Temperature Thermal Conductivity of Superconductors With Gap Nodes
  We report a detailed analytic and numerical study of electronic thermal conductivity in d-wave superconductors. We compare theory of the cross over at low temperatures from T-dependence to T^3-dependence for increasing temperature with recent experiments on YBCO in zero magnetic field for temperatures from 0.04K to 0.4K by Hill et al., Phys. Rev. Lett. 92, 027001 (2004). Transport theory, including impurity scattering and inelastic scattering within strong coupling superconductivity, can consistently fit the temperature dependence of the data in the lower half of the temperature regime. We discuss the conditions under which we expect power-law dependences over wide temperature intervals. 
Tomas Lofwander and Mikael Fogelstrom