THE PEAK IN THE THERMAL-CONDUCTIVITY OF CU-O SUPERCONDUCTORS - ELECTRONIC OR PHONONIC ORIGIN

The thermal conductivity kappa of hole-doped Cu-0 plane high-T(c) pero vskites exhibits a dramatic increase below T(c) which results in a pro nounced peak near T(c)/2. The origin of this peak was initially though t to arise from an enhancement in the mean-free path of phonons as the charge carriers undergo condensation. Indeed, excellent fits to the d ata can be obtained with physically reasonable parameters using the co nventional theory of lattice conduction in superconductors. In contras t, a recently observed sharp decrease in the quasiparticle scattering rate of YBCO single crystals below T(c) has motivated proposals for an electronic origin of the thermal conductivity peak. We shall critical ly examine experimental evidence and highlight relative advantages and shortcomings of the two contrasting interpretations. Furthermore, we shall draw attention to recently available data on the relaxation time of out-of-equilibrium carriers in Cu-O superconductors obtained using pump-probe femtosecond laser studies and what new light they shed on the controversy.