Superconducting Gap Anisotropy vs Doping Level in High-Tc Cuprates.

Abstract

We report the results of electronic Raman scattering in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ (Bi2212) and ${\mathrm{Tl}}_{2}{\mathrm{Ba}}_{2}{\mathrm{CuO}}_{6+\ensuremath{\delta}}$ (Tl2201) high-${T}_{c}$ superconductors with variations in the oxygen content. Near optimal doping, both materials show gap anisotropy, with $2\ensuremath{\Delta}/{k}_{B}{T}_{c}$ 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 $2\ensuremath{\Delta}{/k}_{B}{T}_{c}$ ranging from 5.2 for Bi2212 ( ${T}_{c}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}57\mathrm{K}$) to 3.9 in Tl2201 ( ${T}_{c}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}37\mathrm{K}$). We compare the data with calculations using both isotropic s-wave and d-wave order parameters.