Low-frequency conductivity in the average-atom approximation

The quantum-mechanical average-atom model is reviewed and applied to determine scattering phase shifts, mean-free paths, and relaxation times in warm-dense plasmas. Static conductivities sigma are based on an average-atom version of the Ziman formula. Applying linear response to the average-atom model leads to an average-atom version of the Kubo-Greenwood formula for the frequency-dependent conductivity sigma(omega). The free-free contribution to sigma(omega) is found to diverge as 1/omega(2) at low frequencies; however, considering effects of multiple scattering leads to a modified version of sigma(omega) that is finite and reduces to the Ziman formula at omega = 0. The resulting average-atom version of the Kubo-Greenwood formula satisfies the conductivity sum rule. The dielectric function epsilon(omega) and the complex index of refraction n(omega) + ik(omega) are inferred from sigma(omega) using dispersion relations. Applications to anomalous dispersion in laser-produced plasmas are discussed. (C) 2009 Elsevier B.V. All rights reserved.