Critical scaling of the ac conductivity for a superconductor above T-c

We consider the effects of critical superconducting fluctuations on the sca ling of the linear ac conductivity, sigma(omega), Of a bulk superconductor slightly above T-c in zero applied magnetic field. The dynamic renormalizat ion group method is applied to the relaxational time-dependent Ginzburg-Lan dau model of superconductivity, with sigma(omega) calculated via the Kubo f ormula to O(epsilon(2)) in the epsilon = 4- d expansion. The critical dynam ics are governed by the relaxational XY-model renormalization-group fixed p oint. The scaling hypothesis sigma(omega) similar to xi(2-d+z)S(omega xi(z) ) proposed by Fisher, Fisher, and Huse is explicitly verified, with the dyn amic exponent z approximate to 2.015, the value expected for the d=3 relaxa tional XY model. The universal scaling function S(y) is computed and shown to deviate only slightly from its Gaussian form, calculated earlier. The pr esent theory is compared with experimental measurements of the ac conductiv ity of YBa2Cu3O7-sigma near T-c, and the implications of this theory for su ch experiments is discussed.