Pseudogap and Charge Dynamics in CuO2 Planes in YBCO

We report on a study of the infrared conductivity of YBa2Cu3Ox and YBa2Cu4O8 crystals in different regimes of oxygen content and Zn doping. We find a correlation between the structure of the scattering rate spectra 1/τ*a(ω,T) in the CuO2 planes and the pseudogap that develops in the c-axis conductivity of underdoped samples for T<T*=140 endash 250 K. In the pseudogap state, the scattering rate is depressed at low frequencies and follows an ω2±0.3 law for ω<400 cm-1. When the c-axis pseudogap is suppressed, either by an increase of temperature above T* or by a substitution of Cu with Zn, the spectra of 1/τ*a(ω,T) revert to the nearly linear ω dependence of the optimally doped compounds. copyright 1996 The American Physical Society