Negative temperature coefficient of resistivity in samples of Nb 3Ge irradiated by electrons or heavy ions

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Abstract
Nb3Ge samples have been irradiated at low temperature (T ∼ 20 K) by two drastically different irradiation projectiles : 2.5 MeV electrons and ∼ 100 MeV heavy ions (uranium fission fragments). The resistivity has been measured versus temperature before and after these irradiations. For both projectiles negative temperature coefficients of resistivity dρ/dT were measured and correlated with the residual resistivity p. These anomalous transport properties are discussed and compared to the results of the recent theories on the subject. It is qualitatively clear that the change of sign of the temperature coefficient of resistivity, occurring in a system where the electronic mean free path is of the order of the atomic distance, is indeed related to some localization process, helped by electron-phonon and electron-electron interactions. In other words, the deviation of the resistivity from the Boltzmann nearly free electron behaviour is due to some interference effect between impurity scattering and electron-phonon and/or electron-electron scattering. While the theory of defect induced tunnelling in strongly disordered metals developed by Belitz and Schirmacher seems to be the most appropriate for the explanation of the heavy ion irradiation results, although it includes electron-phonon interactions only, electron irradiation results reveal the necessity of including the interplay with Coulomb interactions as in the model of Alt'shuler and Aronov