Resonance Raman Study of the Superconducting Gap and Low Energy Excitations inTl2Ba2CuO6+δSuperconductors

Low energy electronic Raman scattering from ${\mathrm{Tl}}_2{\mathrm{Ba}}_2{\mathrm{CuO}}_{6+\delta }$ single crystals with $T_c=25\mathrm{}\mathrm{and}85\mathrm{K}$ has been studied using excitation energies from 1.65 to 3.05 eV. The $B_{1g}$ $2\Delta$ peak position shows that the $2\Delta /k_B{T}_c$ value decreases as the doping level increases. The resonance profiles of the $2\Delta$ peak and the continuum show similar and strong resonances towards the 3.4 eV optical absorption peak in both crystals, which indicates that (1) the $2\Delta$ peak and the continuum are coupled to light scattering by the same mechanism, and (2) the light scattering mechanism for both features involves the 3.4 eV optical absorption process.