Infrared conductivity of cuprate metals: Detailed fit using Luttinger-liquid theory

Measurements of infrared conductivity in the normal state of the cuprate layer metals show a characteristic behavior in the plane of the layers which is in essential agreement among many experiments. A simple parametrization of this behavior, proposed originally by Collins and Schlesinger, and exploited by Bontemps and her group, which gives an adequate fit over frequencies from a few hundred cm(-1) to > 5000 cm(-1), is that the phase angle of the complex conductivity is independent of frequency. This fit is shown to be a natural consequence of Luttinger-liquid theory with charge-spin separation, and determines the exponent of the singularity at the Fermi surface to be similar to 0.15 +/- 0.05.