Superconducting Gap Anisotropy vs Doping Level in High-TcCuprates

Abstract

We report the results of electronic Raman scattering in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ (Bi2212) and ${\mathrm{Tl}}_2{\mathrm{Ba}}_2{\mathrm{CuO}}_{6+\delta }$ (Tl2201) high-$T_c$ superconductors with variations in the oxygen content. Near optimal doping, both materials show gap anisotropy, with $2\Delta /k_B{T}_c$ values of 7.2 ( $B_{1g}$) vs 5.8 ( $A_{1g}$) in Tl2201 and 8.5 ( $B_{1g}$) vs 6.2 ( $A_{1g}$) in Bi2212. However, overdoped samples exhibit a symmetry independent gap with $2\Delta {/k}_B{T}_c$ ranging from 5.2 for Bi2212 ( $T_c=57\mathrm{}\mathrm{K}$) to 3.9 in Tl2201 ( $T_c=37\mathrm{}\mathrm{K}$). We compare the data with calculations using both isotropic s-wave and d-wave order parameters.