SYMMETRY-DEPENDENT PHONON INTERACTIONS IN YBA2CU4O8 SUPERCONDUCTORS -A RAMAN AND INFRARED SPECTROSCOPIC STUDY

Superconductivity induced renormalization of Raman- and infrared-activ e optical phonons is studied for undoped and T-c-reduced Pr- and Zn-su bstituted double-chain YBa2Cu4O8 materials. The magnitude of the phono n frequency and the linewidth variation upon cooling below T-c is foun d to be dependent on the mode symmetry; odd- end even-parity phonons r espond qualitatively different to the reduction of T-c. Assuming phono n coupling to the excitations participating in the formation of the su perconducting gap, these experimental facts provide evidence for the g ap anisotropy of YBa2Cu4O8.