Collective Modes and Electronic Raman Scattering in the Cuprates

In spite of the considerable efforts to explain the experimental Raman spectra of cuprate superconductors, the A1g superconducting response is not yet completely understood. It has been shown that the theoretical description of the A1g Raman response was very sensitive to small changes in the Raman vertex harmonic representations, yielding peak positions varying between ∆ and 2∆ [1]. However, the data show peaks consistently slightly above ∆ for both YBCO and BSCCO. In this paper we present calculations suggesting that the A1g peak position is largely controlled by a collective spin fluctuation (SF) mode near 41 meV, consistent with inelastic neutron scattering (INS) observations [2,3]. We show that the A1g response is strongly modified by the SF term and is not sensitive to small changes in the Raman vertex. The experimental peak position is well reproduced by our model whereas the B1g and B2g response remain essentially unaffected by the SF mode.