Table of contents

Experimental and theoretical work on parity violation in nuclear fission is reviewed.

The principal mechanisms of classical diffusion (due to binary collisions) of a partially-ionized plasma in a homogeneous magnetic field are examined. As a result of anisotropy of the transport coefficients of charged particles the problem proves to be far more complicated than the well-known problem of ambipolar diffusion in the absence of a magnetic field. The evolution of diffusion in this case is determined primarily by eddy currents flowing in the plasma. The circuit for these currents can be completed both through the background plasma giving rise to depletion regions, and by the conducting walls of the container of the plasma (the short-circuit effect). Ambipolar diffusion which occurs when the electron and ion fluxes are equal at every point can be realized only in exceptional cases. Experiments are described in which the short-circuit effect and ambipolar diffusion were observed. Using different boundary conditions it was possible to vary the diffusion rate of the plasma by two or three orders of magnitude, and this presents the possibility of controlling the local parameters of the plasma. This review has taken into account references up to July, 1979.

The contemporary state of neutron diffraction of magnetic structures is analyzed from the standpoint of the theory of symmetry of crystals. It is shown that the varied and numerous structures determined in neutron-diffraction studies can be classified and described by the theory of representations of space groups of crystals. This approach is based on expanding the spin density of the crystal in terms of basis functions of the irreducible representations of its space group. Thus the magnetic structure can be specified by the mixing coefficients of the basis functions. Analysis of a large number of different kinds of magnetic structures shows that they arise in the overwhelming majority of cases, in accord with Landau's hypothesis, from a phase transition that follows a single irreducible representation. This means that the number of parameters that fully fix the magnetic structure of an arbitrarily complex crystal is small and equal to the dimensionality of the responsible irreducible representation. This offers great advantages in employing the symmetry approach in deciphering neutron-diffraction patterns of a crystal under study. This is because it reduces the problem of determining a large number of magnetic-moment vectors of the crystal to finding a small number of mixing coefficients. This review presents the fundamentals of such a symmetry analysis of magnetic structures and methods of determining them from neutron-diffraction data. The described method, which is closely allied to Landau's general theory of phase transitions, is illustrated by the most recent neutron-diffraction studies of magnetic structures. They included the so-called multi-k-structures, which are characterized simultaneously by several wave vectors, and structures described simultaneously by several irreducible representations of the space group of the crystal. The article gives a physical explanation of the existence of such structures. The experimental studies of crystal-lattice distortions accompanying the onset of magnetic ordering are reviewed. It is shown how symmetry arguments allow one to determine these distortions as well as the unknown magnetic structure. This review presents in condensed but accessible form the symmetry approach to describing the magnetic structures of crystals and analyzes on this basis the feasibility and degree of reliability of deciphering them by employing the scattering of unpolarized and polarized neutrons.

A review is given of low-temperature studies of the properties of the two-dimensional electron gas in inversion layers on high-index Si surfaces. The long crystallographic periods associated with such surfaces produce the superlattice effect in the spectrum of the quasi-two-dimensional electron gas. The kinetic coefficients are found to exhibit singularities when the Fermi level and minigaps cross. This was established by measuring the static and the high-frequency conductivities, the Shubnikov-de Haas oscillations, the photoresistivity, the emission of thermal electrons, and the cyclotron resonance in n-type inversion layers near (001) for electron concentrations in the range 10¹² – 10¹⁴ cm⁻². The minigap width varies from 1 to 20 meV, depending on the electron concentration. The position of the minigaps in K-space (but not their size) is in quantitative agreement with theory.

Heterotrophic organisms live on accumulated plant and animal biomass. By harvesting the output of a land area that exceeds their coverage of the Earth's surface they attain consumption rates (per unit coverage by the organism) that exceed by several orders of magnitude the rate of production of vegetation (per unit Earth surface area). This occurs as a result of the fact that organisms move over the area they utilize due to the accumulation of production as biomass. Plants use photons, which have zero rest mass. Photons cannot be stored so that plants cannot increase their utilization of photons by moving about. For this reason, plants are stationary and the area they utilize coincides with the Earth surface area that they cover. Under natural conditions, approximately 90% of the vegetation is consumed by immobile microscopic organisms, for which the rate of consumption equals the rate of production of vegetation. As organisms become larger, their rates of consumption increase and the fraction of the production of vegetation that they consume decreases. (All vertebrates in the wild consume about 1% of the vegetation produced.) This results in a rapid increase in the area utilized as the size of the animal increases and increases the energy expended on grazing, which limits the increase in size. Man falls into the class of large organisms and under natural conditions must expend a large amount of energy on locomotion, which leads to all of his energy problems. When man learned how to combust the products of the biosphere and the fossil fuels outside the body and how to use this energy for locomotion, he became a more competitive mammal and he was able to increase his share of the total consumption in the biosphere to 25% of the vegetation produced on land by displacing the natural consumers. Such a high anthropogenic share of the total consumption in the biosphere can only be achieved through the use of nonrenewable energy resources.

A review is given of theoretical concepts and experimental data concerning the spin-Peierls transition in a one-dimensional spin system with antiferromagnetic exchange interaction (an analog of Peierls instability of a one-dimensional metal). Analysis of experimental data confirms the existence of the spin-Peierls transition in TTF-CuBDT, TTF-AuBDT, and MEM(TCNQ)₂ crystals. The magnitude of the spin-phonon interaction in crystals undergoing the spin-Peierls transition at low temperatures is discussed together with the role of fluctuations in transitions of this type. The influence of magnetic fields on spin-Peierls transitions is examined.