PSEUDOGAP AND CHARGE DYNAMICS IN CUO2 PLANES IN YBCO

We report on the infrared conductivity of YBa2Cu3Ox and YBa2Cu4O8 crys tals with different oxygen content and Zn doping. We find a correlatio n between the structure of the scattering rate spectra 1/tau(a)(omega ,T) in the CuO2 planes and the pseudogap that develops in the c-axis c onductivity of underdoped samples for T < T = 140-250 K. In the pseud ogap state, the scattering rate is depressed at low frequencies and fo llows an omega(2 +/- 0.3) law for omega < 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/tau(a)(omega, T ) revert to the nearly linear omega dependence of the optimally doped compounds.