Spin susceptibility in small Fermi energy systems: effects of nonmagnetic impurities

In small Fermi energy metals, disorder can deeply modify superconducting state properties leading to a strong suppression of the critical temperature Tc. In this paper, we show that also normal state properties can be seriously influenced by disorder when the Fermi energy EF is sufficiently small. We calculate the normal state spin susceptibility ? for a narrow band electron-phonon coupled metal as a function of the non-magnetic impurity scattering rate ?imp. We find that as soon as ?imp is comparable to EF, ? is strongly reduced with respect to its value in the clean limit. The effects of the electron-phonon interaction including the nonadiabatic corrections are discussed. Our results strongly suggest that the recent finding on irradiated MgB2 samples can be naturally explained in terms of small EF values associated with the ?-bands of the boron plane, sustaining therefore the hypothesis that MgB2 is a nonadiabatic metal.