Electronic Raman-scattering study of low-energy excitations in single and double CuO2-layer Tl-Ba-(Ca)-Cu-O superconductors

The low-energy Raman continuum and the redistribution of the continuum to a peak (the open-quotes 2Δ peakclose quotes) in the superconducting state have been studied in Tl-Ba-(Ca)-Cu-O superconductors with a single CuO2 layer (Tl-2201) and a double CuO2 layer (Tl-2212). The 2Δ/kBTc ratios in A1g and B1g symmetries are larger for Tl-2212 than for Tl-2201. The B1g/A1g gap ratio is also larger in Tl-2212. The A1g intensities of the continuum and the 2Δ peak are significantly weaker than the B1g intensities in Tl-2201, but are comparable in Tl-2212. This shows that the Coulomb screening is much stronger in Tl-2201. The change from Tl-2201 to Tl-2212 of the normalized A1g 2Δ peak intensity is identical within experimental error to that of the normalized A1g continuum intensity. This suggests that the excitations forming the 2Δ peak and the continuum couple to light by the same mechanism. copyright 1997 The American Physical Society