'A gap can hide another one'. An electronic Raman probe exploration of the superconducting phase of cuprates

We present here an electronic Raman scattering study of charge dynamics in superconducting cuprates. Selection rules allow us to probe different parts of the Fermi surface. Among them, we focus on the nodal regions, where the d-wave superconducting gap of cuprates vanishes, and the anti-nodal regions, where it reaches its maximum value. Systematic measurements of the evolution of nodal and anti-nodal Raman responses with doping show that a BCS-like description of the superconducting condensate was satisfactory for the optimally and over-doped regions of cuprate phase diagram, but absolutely not for under-doped one. Indeed, in this case our measurements have revealed the existence of two distinct energy scales, behaving in opposite ways as doping decreases from optimal doping. Temperature dependence as well as impurity substitution effects were measured and have shown that the energy scale associated with anti-nodal regions was a signature of the pseudogap, rather than the superconducting gap, as usually it is admitted. The superconducting gap can be detected only in the nodal regions, and its amplitude tracks the superconducting critical temperature T_c, as expected. Finally, the effect of magnetic and non-magnetic impurities on the fully symmetric A_1g response confirms the presence in the charge channel, of a mode related to a collective spin excitation, namely, the neutron resonance. (author)