The history is described of how one of the most commonly used electric circuit components, an ordinary electric contact, has become a powerful tool for the physicists to study various mechanisms of electron scattering in metals. The physical principles of spectroscopy of quasi-particle excitations in metals by means of point contacts (PCs) whose dimensions range from only a few to tens of nanometers are presented in a popular form.PACS 73.40.Jn

A review of heavy-fermion compounds investigated in their normal and superconducting states by point-contact spectroscopy is presented. The Joule heating appears to have a considerable influence on the conductivity in metallic point contacts in both the normal and superconducting states. Nonetheless, some spectroscopic information can be obtained from the point-contact measurements, and Josephson effects are observed for certain heavyfermion superconductors. We review in detail very recent development in this field, including experiments using mechanically controllable break junctions covering the regime from metallic conductivity to truly vacuum tunnelling.

Analysis of the point-contact spectroscopy (PCS) data on the new dramatic high-Tc superconductor MgB2 reveals quite different behavior of two disconnected σ and π electronic bands, deriving from their anisotropy, different dimensionality, and electron-phonon interaction. PCS allows direct registration of both the superconducting gaps and electron-phonon-interaction spectral function of the two-dimensional σ and three-dimensional π band, establishing correlation between the gap value and intensity of the high-Tc driving force -the E2g boron vibration mode. PCS data on some nonsuperconducting transition-metal diborides are surveyed for comparison.