Unravelling the glue and the competing order in superconducting cuprates

We present Raman scattering experiments in $\mathrm{L}{\mathrm{a}}_{2-\mathrm{x}}\mathrm{S}{\mathrm{r}}_{\mathrm{x}}\mathrm{C}\mathrm{u}{\mathrm{O}}_4$ 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-scattering 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.