Electronic Raman-scattering study of low-energy excitations in single and double CuO_2-layer Tl-Ba-(Ca)-Cu-O superconductors

Low energy Raman continuum and the redistribution of the continuum to a peak (2Δ-peak) in the superconducting state have been studied in Tl-Ba-(Ca)-Cu-O superconductors with single (Tl-2201) and double (Tl-2212) CuO_2 layer. The 2Δ/k_BT_c ratios in A_1g and B_1g symmetries are larger for Tl-2212 than for Tl-2201. The B_1g/A_1g gap ratio is also larger in Tl-2212. The A_1g intensities of the continuum and the 2Δ-peak are significantly weaker than the B_1g 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 A_1g 2Δ-peak intensity is identical within experimental error to that of normalized A_1g continuum intensity. This suggests that the excitations forming the 2Δ-peak and the continuum couple to light by the same mechanism.

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