THEORY OF PHONON THERMAL-CONDUCTIVITY FOR STRONG-COUPLING S-WAVE AND D-WAVE PAIRING IN HIGH-TC SUPERCONDUCTORS

First we calculate the decay rate of a phonon due to phonon-electron interaction by employing strong-coupling theory for s- and d-wave pairing states. The resulting phonon thermal conductivity K exhibits a characteristic bump below Tc. The upturn of K below Tc is rounded off for the d-wave state in comparison to the BCS state. It becomes steeper for the s-wave state as the coupling strenght is increased. We calculate also the in-plane Kab and out-of-plane Kc by taking into account the anisotropy of the phonon-electron interaction and momentum conservation below Tc is drastically reduced in comparison to that of Kab. It seems in the ab-plane. Then we find that the height of the bump of K c below Tc is drastically reduced in comparison to that of Kab. It seems that recent thermal conductivity data on single crystals of YBa2Cu3O7 and Bi-Sr-Ca-Cu-O can be fit somewhat better with the Kab for d-wave than for s-wave pairing. © 1990.