Abstract
Anomalies in the temperature dependence well above T c of infrared and Ramanactive phonon frequencies are reported for the superconductors YBa 2Cu 4O 8 and YBa 2Cu 3O 7-δ with various oxygen contents 0.16 ≤ δ ≤ 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 YBa 2Cu 4O 8 (T c = 78 K) and oxygen-deficient YBa 2Cu 3O 7-δ (δ ≈ 0.4, T c ≈ 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.
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