Table of contents

Neutrino oscillations occur if neutrinos have masses and if they mix among each other. This phenomenon would be in close analogy to the experimentally observed mixing in the quark sector. Neutrino oscillations are therefore very interesting in the context of elementary particle physics. Further neutrino oscillation in matter is probably the reason for the observed deficit of the solar neutrino flux. As an interference effect, the search for neutrino oscillations is the only way to investigate even very small differences of neutrino masses. In the present article, after an introduction of the theoretical background concerning massive neutrinos, the authors describe important experiments which have been performed in this field. Experiments looking for neutrino oscillations have been carried out using accelerators, nuclear power plants, cosmic rays and the Sun as neutrino sources. Also the observed neutrino burst of supernova SN 1987A has implications for the oscillation parameter. The authors show the interesting link of neutrino oscillations to related phenomena, like neutrino decay and the possible existence of a 17 keV neutrino, coupled weakly to the electron.

A scanning tunnelling microscope can image surfaces down to the atomic scale. Its very high resolution is caused by its local probing mechanism: a tunnel current which flows through the outermost atom of a scanning tip to the sample. Its invention, about ten years ago, was based on an ingenious blend of quantum physics, mechanical design and electronic control. Since then, it has proved to be a very versatile instrument. Its applications range from the characterization of surface roughness to atomic reconstructions, and from doped semiconductors to cell membranes. It can operate in ultra high vacuum, in air, in reactive gases, in corrosive solutions or at cryogenic temperatures. This paper reviews the evolution of scanning tunnelling microscopy in the first decade after its invention. Attention is focused on the basis of STM: tunnelling theory, mechanical design and modes of operation. Representative examples of applications in various fields of research are discussed.

An overview is given of theory and experiments on liquid crystal phases which appear in solutions of elongated colloidal particles or stiff polymers. The Onsager (1949) virial theory for the isotropic-nematic transition of thin rodlike particles is treated comprehensively along with extensions to polydisperse solutions and soft interactions. Computer simulations of liquid crystal phases in hard particle fluids are summarized and used to assess the quality of statistical mechanical theories for stiff particles at higher volume fraction-like the inclusion of higher virial coefficients, y-expansion, scaled particle theory and density functional theory. Both computer simulations and density functional theory indicate formation of more highly ordered smectic phases. The range of experimental applicability is strongly widened by the extension of the virial theory to wormlike chains by Khokhlov and Semenov (1981,1982). Finally, experimental results for a number of carefully studied, charged and uncharged colloids and polymers are summarized and compared to theoretical results. In many cases the agreement is semi-quantitative.

The authors review conductance effects in small samples at low temperatures where quantum confinement and quantum interference are significant perturbations on the classical Drude conductance. In disordered materials, the elastic scattering of the carriers from impurities leads to random conductance fluctuations or resistance fluctuations. The fluctuations arise because of interference among the scattered waves, and they are random and sample specific because the impurity potential is. The fluctuations appear in response to changes in many extrinsic parameters such as the carrier density, the applied measuring current, external electric fields and external fields. The interface fluctuations have consequences for much larger samples, particularly in flicker noise, even though quantum coherence is obtained only over regions much smaller than the sample size, in completely phase coherent conductors a number of purely quantum effects are observed, including non-local response and Aharonov-Bohm effects. Other 'applications' of the quantum fluctuations include studies of reciprocity (which is related to time-reversal symmetry) and of the effects of the measurement probes on a quantum system. Interestingly, these are two areas where disagreements remain with theoretical calculations. The size and correlation scale of the fluctuations, however, are mainly in agreement with theoretical calculations of the same quantities, although one or two other small disagreements of detail remain. In very clean semiconductor heterostructures, the mean free path length between scattering events is large enough to allow for studies of ballistic transport that reveal a variety of conductance anomalies that result from device shape (as opposed to fortuitous placement of impurities as in the metals). These ballistic effects are reviewed briefly and connection is made to the effects of disorder in ballistic systems, and experiments on disordered metal samples are reviewed in detail.