Point-Contact Spectroscopy

In a small metallic constriction ("point contact") the transport of electrons is ballistic. The applied voltage V tunes a well-defined non-equilibrium energy eV of electrons. The nonlinear current-voltage characteristics can be used to perform energy-resolved spectroscopy of the inelastic scattering of electrons with elementary excitations in metal (e.g. phonons, magnons, crystal-field levels, paramagnetic impurities). The basic elements of the point--contact method and its applications will be discussed. In recent point-contact experiments the observed phenomena (weak localization, resistance fluctuations) need a description that goes beyond the classical Boltzmann approach of electronic transport in a point contact. In analogy to observed effects in the diffusive transport in mesoscopic systems, these phenkena are explained by considering quantum-interference effects in the ballistic transport near the contact region related to the wave character of the electrons. For a macroscopic sample in an applied electric field E, the acceleration of the electrons during a mean free path t is only a very small perturbation with respect to the thermal excitations at a temperature Τ (i.e eEl < kΒΤ). This thermal equilibrium of the electronic system in an applied electric field is one of the reasons for the linear relation between the current density and the electric field as given by Ohm,s law. For a small metallic constriction (size of the constriction small compared to the electronic mean free path), all electrons passing the contact gain a well-defined energy eV given by the applied voltage V. In such a ballistic point contact the electronic system is out of equilibrium with an excess energy eV that can be bigger than kBT. Sharvin [1] proposed the realization of this ballistic transport mechanism in metallic point contacts. In his pioneering experiments on shorted tunnel junctions, Yanson The method of point-contact spectroscopy will be discussed and examples of point-contact investigations will be given. The given description of the electronic transport in a point contact is based on a classical solution of the Boltzmann trans-. port equation. In anology to the phenomena of weak-localization and conductanc