ROLE OF PHONONS IN ELECTRONIC AND SPIN EXCITATIONS OF HIGH-T-C SUPERCONDUCTORS

By using large single crystals of YBa2Cu3Oy, precise temperature and d oping dependence of the c-axis infrared active phonons are investigate d. The effect of the superconducting gap opening on the phonon self-en ergy is observed for the optimally doped crystal but not for the overd oped crystal, which suggests that this effect becomes pronounced owing to a lower crystal symmetry due to oxygen deficiencies. The observed phonon anomalies for the in-plane oxygen mode above T-c in the underdo ped crystals might be associated with the c-axis conductivity suppress ion, and cannot be an evidence for a direct coupling of phonons to the spin system. An indirect contribution of the spin gap to the phonon s ystem is suggested by a spectral growth around 450 cm(-1).