Electronic Raman scattering in Bi₂Sr₂CaCu₂O_8+δ excited with c-axis polarized light

We report on the electronic Raman spectra excited and detected with photon polarization directions along the c-axis in Bi₂Sr₂CaCu₂O_8+δ single crystals. In the normal state, a resonant electronic continuum extends to 1.5 eV and gains significant intensity as the incoming photon energy increases. Below T_c, there is a low-frequency redistribution of the electronic continuum and the formation of a 2Δ peak in the superconducting state. The monotonic increase of 2Δ and 2Δ/k_BT_c with decreasing doping is unusual. In particular, this 2Δ feature, which has A_1g symmetry, is found at higher frequencies than those seen in the planar symmetries for all doping levels studied.