E. Kaldis

Above-T c anomalies of the infrared-active phonons in RBa2Cu4O8 (R=Dy, Ho) and Y2Ba4Cu7O15−δ superconductors

We report a far-infrared reflectivity study of DyBa2Cu4O8, HoBa2Cu4O8 and Y2Ba4Cu7O15−δ superconductors in the temperature range 10–300 K. The assignment of thez-polarized infrared-acitve vibrations of the rare-earth-ion (R) was verified by the substitution ofR. Since the structure of the Y2Ba4Cu7O15−δ compound inz-direction is an ordered sequence of YBa2Cu3O7−δ and YBa2Cu4O8 blocks, the phonon spectrum of Y2Ba4Cu7O15−δ is a superposition of their respective phonon modes. The temperature dependence of the infrared-active phonons in RBa2Cu4O8 and Y2Ba4Cu7O15−δ was analyzed. A narrowing of the plane-oxygen phonon upon sample cooling was found to occur atTc, as it was reported for single-chain YBa2Cu3O7 materials; however a pronounced frequency softening of this mode for both RBa2Cu4O8 and Y2Ba4Cu7O15−δ was found to start well aboveTc, suggesting the interaction of infrared-active phonons with an excitation gap similar to that observed in spin-dependent measurements.