We present Raman scattering experiments in ${rm La_{2-x}Sr_xCuO_4}$ single crystals at various doping levels x and compare the results with theoretical predictions obtained assuming an interaction mediated by spin and charge fluctuations. The light-s
cattering selection rules allow us to disentangle their respective contributions. We find that the glue spectral function is spin-dominated at low doping while the contribution of charge fluctuations becomes dominant around optimal doping. This indicates that the fluctuations of a nearly ordered state with coexisting spin and charge order support the superconducting pairing.
We discuss how Raman spectra of high temperature superconducting cuprates are affected by nearly-critical spin and charge collective modes, which are coupled to charge carriers near a stripe quantum critical point. We find that specific fingerprints
of nearly-critical collective modes can be observed and that the selectivity of Raman spectroscopy in momentum space may be exploited to distinguish the spin and charge contribution. We apply our results to discuss the spectra of high-T_c superconducting cuprates finding that the collective modes should have masses with substantial temperature dependence in agreement with their nearly critical character. Moreover spin modes have larger masses and are more diffusive than charge modes indicating that in stripes the charge is nearly ordered, while spin modes are strongly overdamped and fluctuating with high frequency.
