ABOVE-T(C) ANOMALIES OF THE INFRARED-ACTIVE PHONONS IN RBA2CU4O8 (R=DY, HO) AND Y2BA4CU7O15-DELTA SUPERCONDUCTORS

We report a far-infrared reflectivity study of DyBa2Cu4O8, HoBa2Cu4O8 and YBa4Cu7O15-delta superconductors in the temperature range 10-300 K . The assignment of the z-polarized infrared-acitve vibrations of the rare-earth-ion (R) was verified by the substitution of R. Since the st ructure of the Y2Ba4Cu7O15-delta compound in z-direction is an ordered sequence of YBa2Cu3O7-delta and YBa2Cu4O8 blocks, the phonon spectrum of Y2Ba4Cu7O15-delta is a superposition of their respective phonon mo des. The temperature dependence of the infrared-active phonons in RBa2 Cu4O8 and Y2Ba4Cu7O15-delta was analyzed. A narrowing of the plane-oxy gen phonon upon sample cooling was found to occur at T(c), as it was r eported for single-chain YBa2Cu3O7 materials; however a pronounced fre quency softening of this mode for both RBa2Cu4O8 and Y2Ba4Cu7O15-delta was found to start well above T(c), suggesting the interaction of inf rared-active phonons with an excitation gap similar to that observed i n spin-dependent measurements.