Electronic Raman scattering in superconductors as a probe of anisotropic   electron pairing
  A gauge invariant theory for electronic Raman scattering for superconductors with anisotropic pairing symmetry is analyzed in detail. It is shown that Raman scattering in anisotropic superconductors provides a wealth of polarization-dependent information which probes the detailed angular dependence of the superconducting ground state order parameter. The Raman spectra shows a unique polarization dependence for various anisotropic pair- state symmetries which affects the peak position of the spectra and generates symmetry dependent low frequency and temperature power-laws which can be used to uniquely identify the magnitude and symmetry of the energy gap. In particular, we calculate the collective modes and the subsequent symmetry--dependent Raman spectra for a $d_{x^{2}-y^{2}}$ superconductor and compare our results to the relevant data on the cuprate systems as well as theoretical predictions for $s$--wave, anisotropic $s$--wave and $s+id$ energy gaps. Favorable agreement is shown with the predictions for $d_{x^{2}-y^{2}}$ pairing and the experimental data on YBa$_{2}$Cu$_{3}$O$_{7}$, Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ and Tl$_{2}$Ba$_{2}$CuO$_{6}$. 
T. P. Devereaux and D. Einzel