Point-contact spectroscopy

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Abstract
Micro-contacts between metals at low temperatures reveal non-linear structures in the current-voltage characteristics. These deviations from Ohm's law allow an energy-resolved spectroscopy of the interaction of the conduction electrons with elementary excitations (e.g. phonons) in a metal. To explain the method, the important parameters (electron mean free path versus contact dimension) in point-contact spectroscopy will be discussed together with examples of spectroscopic information obtained in various systems. Local temperature gradients in the contact region offer the possibility to study thermo-electric phenomena in small constrictions, such as thermal voltages in non-homogeneous contacts and quenching of the phonon-drag term in the thermo-power in homogeneous contacts. Besides these aspects of the point-contact technique, recent experiments will be shown with applications of point contacts other than just spectroscopy: magneto-resistance of a point contact, high-frequency rectification with a point contact as the non-linear element, electron focusing using a double point-contact set-up, electrical noise in constrictions and generation of phonons by means of point contacts.