Coupling to spin fluctuations from conductivity scattering rates

A recent analysis of optical conductivity data which has provided evidence for coupling of the charge carriers to the 41-meV spin resonance seen in th e superconducting state of optimally doped YBa2Cu3O6.95 (Y123) is extended to other systems. We find that the corresponding spin resonance is consider ably broader in Tl2Sr2CuO8+delta (Tl2201) and YBa2Cu4O8 (Y124) than it is i n Bi2Sr2CaCu2O8+delta (Bi2212) and there is no resonance in overdoped Tl220 1 with T-c=23 K. The effective charge-spin spectral density is temperature dependent and contains feedback effects that further stabilize superconduct ivity as T is reduced.