NORMAL-STATE OPTICAL CONDUCTIVITY OF YBA2CU3O6+X

The optical conductivity in the normal state of YBa2Cu3O6+x is calculated within the framework of the memory-function formalism. We assume that the carriers are constrained to the CuO2 planes and are scattered by spin fluctuations of the Cu spins. First, the calculation is done using the phenomenological form of the spin susceptibility proposed in the antiferromagnetic-Fermi-liquid theory. In a second step, we choose two different forms of the spin susceptibility to improve the high-frequency dependence of the optical conductivity. It is found that fitting the experimental data requires more than one form of the spin susceptibility. It is also found that the relaxation rate varies roughly linearly with the frequency and that the mass enhancement is weakly frequency dependent.