Temperature dependence of the Raman and infrared phonons of MBa2Cu3Osub(7-δ)-type superconductors

An anomalous temperature dependence of the approx. 340 cm-1 Raman phonons of YBa2Cu3Osub(7-δ) has been recently reported by Macfarlane et al. Here we show that this phenomenon, a phonon softening with decreasing T for T < Tsub(c), is common to many materials of the family (M = Y, Sm, Eu, Ho, Gd). We also show that the ir-active Davidov counterpart of this phonon, and another ir-active phonon at slightly lower frequency, also exhibit similar effects. These phonons are attributed to bending vibrations of the Cu2-02 and Cu3-03 bonds. All other ir and Raman phonons observed exhibit a normal temperature dependence (i.e., hardening with decreasing T even for T < Tsub(c)). The 310 cm-1 phonon of the tetragonal non-superconducting YBa2Cu306 also shows a normal behavior vs. T. (author)