The pairing mechanism in HTSC investigated by electronic Raman scattering

By means of electronic Raman scattering we investigated the symmetry of the energy gap 2Δ(→k), its temperature dependence and its variation with doping of well characterized Bi₂Sr₂CaCu₂O_8+δ single crystals. The oxygen content δ was varied between the under- and the overdoped regime by subsequently annealing the same single crystal in Ar and in O₂, respectively. The symmetry analysis of the polarized electronic Raman scattering is consistent with a d_x²-y²-wave symmetry of the energy gap in both regimes. The gap ratio 2Δ_max/k_BT_c and its temperature dependence changes with doping similar to predictions of theories based on paramagnon coupling.