Universal dependences of the conductivity of metallic disordered systems on temperature, magnetic field and frequency

The authors generalise the Mooij rule (1973) for disordered metals. They find that the temperature dependence of the conductivity sigma of a disordered metal as a function of temperature must change slope due to diffusion effects, and if interaction effects are included, sigma changes its slope three times. The crossover temperature (if it occurs at high temperatures) from positive to negative d sigma /dT due to diffusion effects varies as C⁵, where C is the average concentration of impurities or scattering centres. Another crossover temperature which separates electron correlation effects from diffusion effects is predicted. This explains the temperature dependence observed for Ge_1-xAu_x. It is also shown that the non-metallic behaviour of the AC conductivity is accounted for by diffusion effects and there is no need to invoke the concept of a pseudogap due to electron interaction. The negative magnetoresistance follows from the theory for diffusion effects in agreement with the perturbation theory of Kawabata (1980).