Symmetry dependence of phonon line shapes in superconductors with anisotropic gaps.

The temperature dependence below Tc of the lineshape of optical phonons of different symmetry as seen in Raman scattering is investigated for superconductors with anisotropic energy gaps. It is shown that the symmetry of the electron-phonon vertex produces non-trivial couplings to an anisotropic energy gap which leads to unique changes in the phonon lineshape for phonons of different symmetry. The phonon lineshape is calculated in detail for B1g and A1g phonons in a superconductor with dx2−y2 pairing symmetry. The role of satellites peaks generated by the electronphonon coupling are also addressed. The theory accounts for the substantial phonon narrowing of the B1g phonon, while narrowing of the A1g phonon which is indistinguishable from the normal state is shown, in agreement with recent measurements on Bi2Sr2CaCu2O8 and YBa2Cu3O7.