Symmetry-dependent phonon interactions in YBa2Cu4O8 superconductors: a Raman and infrared spectroscopic study

Superconductivity induced renormalization of Raman- and infrared-active optical phonons is studied for undoped and T c -reduced Pr- and Zn-substituted double-chain YBa 2 Cu 4 O 8 materials. The magnitude of the phonon frequency and the linewidth variation upon cooling below T c is found to be dependent on the mode symmetry; odd- end even-parity phonons respond qualitatively different to the reduction of T c . Assuming phonon coupling to the excitations participating in the formation of the superconducting gap, these experimental facts provide evidence for the gap anisotropy of YBa 2 Cu 4 O 8 .