Point-contact spectroscopy in metals

Point contacts between normal metals at low temperatures show very interesting non-linear phenomena in the current-voltage characteristics. The observed deviations from Ohm's law in metallic constrictions can be used for an energy-resolved spectral analysis of the interaction mechanisms of the conduction electrons with elementary excitations in a metal. A review is given on the theoretical and experimental work dealing with this spectroscopic method. A theoretical analysis, involving an iterative solution of the full non-linear Boltzmann equation for the transport problem of a current through a point-contact geometry, is able to show that the observed second-derivative signal is proportional to Eliashberg's form of the electron-phonon interaction alpha 2F( omega ), with a slight modification due to a transport efficiency function. Non-equilibrium effects of the phonon system are discussed in relation to the observed background signal on which the alpha 2F signal is thought to be superimposed. The theory can easily be extended to other scattering mechanisms than the electron-phonon interaction, and some experiments show that the interactions of conduction electrons with magnons and paramagnetic impurities can be detected using point contacts between ferromagnetic metals and between magnetically dilute alloys.

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