Point Contact Spectroscopy

Various experimental techniques for point contact production are described. Examples of point-contact spectra are presented for pure metals, alloys and compounds, as well as for semimetals and semiconductors, heavy fermion systems, Kond-lattices, mixed valence compounds and more. Superconducting point contacts are considered in respect to Andreev reflection and Josephson effects. Special attention is paid to contact conductance fluctuation, and new trends of research are outlined.

The characteristics of electrical contacts have long attracted the attention of researchers since these contacts are used in every electrical and electronic device. Earlier studies generally considered electrical contacts of large dimensions, having regions of current concentration with diameters substantially larger than the characteristic dimensions of the material: the interatomic distance, the mean free path for electrons, the coherence length in the superconducting state, etc. [110]. The development of microelectronics presented to scientists and engineers the task of studying the characteristics of electrical contacts with ultra-small dimensions. Characteristics of point contacts such as mechanical stability under continuous current loads, the magnitudes of electrical fluctuations, inherent sensitivity in radio devices and nonlinear characteristics in connection with electromagnetic radiation can not be understood and altered in the required way without knowledge of the physical processes occurring in contacts. Until recently it was thought that the electrical conductivity of contacts with direct conductance (without tunneling or semiconducting barriers) obeyed Ohm's law. Nonlinearities of the current-voltage characteristics were explained by joule heating of the metal in the region of the contact. However, studies of the current-voltage characteristics of metallic point contacts at low (liquid helium) temperatures [142] showed that heating effects were negligible in many cases and the nonlinear characteristics under these conditions were observed to take the form of the energy dependent probability of inelastic electron scattering, induced by various mechanisms.

The investigation and manipulation of matter on the atomic scale have been revolutionised by scanning tunnelling microscopy and related scanning probe techniques. This book is the first to provide a clear and comprehensive introduction to this subject. Beginning with the theoretical background of scanning tunnelling microscopy, the design and instrumentation of practical STM and associated systems are described in detail, as are the applications of these techniques in fields such as condensed matter physics, chemistry, biology, and nanotechnology. Containing 350 illustrations, and over 1200 references, this unique book represents an ideal introduction to the subject for final-year undergraduates in physics or materials science. It will also be invaluable to graduate students and researchers in any branch of science where scanning probe techniques are used.

The understanding of electronic behaviour in solids when (some of) the valence electrons have both localized and band-like characteristics is one of the central problems of physics and chemistry in the second half of this century. Many advances have indeed been made using highly sophisticated techniques and concepts. Our objectives in bringing together specialists from different areas was cross-fertilization of ideas and redefinition of bottlenecks and problems. The testimony of the participants and the book which follows indicate a fair degree of success. This book is a record of discussions aimed at digestion and reassessment of some of the recent major advances in our understanding of narrow bands. Note that we expressly asked participants to give a short readable account of the major problems in their field and not to emphasize their latest results to be as "technical" as they might be in a normal scientific article. We did not ask for complete reviews of what was going on in the field and this book should not be read as such. Neither should it be approached as the sort of educational text which the NATO ASI proceedings are supposed to be. We have tried to produce a useable account of a workshop in which an attempt was made to define real problems and to distinguish them from illusory problems.