PHONON SUPPRESSION OF COHERENCE PEAK IN NUCLEAR-SPIN RELAXATION RATE OF SUPERCONDUCTORS

THE temperature dependence of the nuclear spin relaxation rate 1/T(1) peaks sharply (the 'Hebel-Slichter' or 'coherence' peak) just below the transition temperature T(c) of a superconductor 1. Because the observation 2 of this peak definitively confirmed BCS theory 3, its absence 4-6 in the high-T(c) oxide superconductors is the best evidence against a BCS picture. Here we show that, to the contrary, an extended form of BCS theory gives a natural explanation. Using the extension by Migdal 7 and Eliashberg 8 to retarded interactions (for phonon coupling, the attraction between electrons propagates at the speed of sound, slower than the Fermi velocity of electrons), we predict unexpectedly strong damping effects in all dynamical properties when the temperature T is close to T(c). The origin of the damping (which suppresses the coherence peak) is numerous electron-phonon decay channels open to excitations because of the high T(c) itself. This process still works even if another source of attraction beyond electron-phonon coupling causes the high T(c). Thus our observations remove a barrier inhibiting conventional descriptions of high-T(c) materials, but by no means force such as interpretation.