The Peak in the Thermal Conductivity of Cu-O Superconductors: Electronic or Phononic Origin?

The thermal conductivity tcof hole-doped Cu-O plane high-T~, perovskites exhibits a dramatic increase below Tc which results in a pronounced peak near T~./2. The origin of this peak was initially thought o arise from an enhancement i the mean-free path of phonons as the charge carriers undergo condensation. I deed, excellent fits to the data can be obtained with physically reasonable parameters u ing the conventional theory of lattice conduction i superconductors. In contrast, a recently observed sharp decrease in the quasiparticle scattering rate of YBCO single crystals below Tc has motivated proposals for an electronic origin of the thermal conduc-tivity peak. We shall critically examine xperimental evidence and highlight relative advantages and shortcomings ofthe two contrasting interpretations. Furthermore, we shall draw attention to recently available data on the relaxation time of out-of-equilibrium carriers in Cu-O super-conductors obtained using pump-probe f mtosecond laser studies and what new light they shed on the controversy. KEY WORDS: High-T,. superconductors: thermal conductivity; scattering rate. 1