PHONON ANOMALIES ABOVE T(C) IN YBA2CU4O8 AND YBA2CU3O7-DELTA SUPERCONDUCTORS - AN EFFECT OF COUPLING TO SPIN EXCITATIONS

Anomalies in the temperature dependence well above T(c) of infrared and Raman-active phonon frequencies are reported for the superconductors YBa2Cu4O8 and YBa2Cu3O7-delta with various oxygen contents 0.16 less-than-or-equal-to delta less-than-or-equal-to 0.43, Au and Pr doping levels. Frequency softenings of oxygen-related phonon modes with decreasing temperature were found to start up to 50-60 K above T(c). They are especially pronounced for YBa2Cu4O8 (T(c) = 78 K) and oxygen-deficient YBa2Cu3O7-delta (delta almost-equal-to 0.4, T(c) almost-equal-to 60 K). These anomalies in the temperature behavior are similar to those found in the spin susceptibility obtained from NMR and quasi-elastic neutron-scattering experiments, indicating an effect of magnetic order above T(c) on the phonon excitations.