SUPERCONDUCTIVITY-INDUCED PHONON SOFTENING IN YBA2CU3O7 OBSERVED BY INELASTIC NEUTRON-SCATTERING

Inelastic neutron scattering yielded the first information on superconductivity-induced frequency shifts for q not-equal 0 phonons in YBa2Cu3Ox. For branches starting from the Raman-active mode at 340 cm-1 (10.2 THz), the shifts were found to decrease only slowly with increasing momentum transfer (x=7.0) or even exhibit a maximum at finite q (x=6.92) when going along the [100] and [010] directions. They were independent of q in the [001] direction. The shifts were much smaller for the A(u) mode at 307 cm-1 and the E(u) mode at 343 cm-1 and for q not-equal 0 phonons related to these modes.