Table of contents

The macroscopic electrodynamics of time-dependent coherent processes in active media is reviewed. Dicke super-radiance in the two-level model and cyclotron super-radiance in a model consisting of an electron beam in a magnetic field are investigated. Similar phenomena in other media are discussed. Particular attention is devoted to polarization waves, i.e., a type of normal wave that coexists with electromagnetic waves and in many ways determines the character of coherent effects. Super-radiance is considered as a dissipative instability of negative-energy polarization waves, which occurs in an active sample as a result of losses by emission of positive-energy electromagnetic waves into the ambient space. The method of phenomenological quantization is developed for unstable modes in active samples, and directly describes the quantum-mechanical properties of collective excitations. The procedure is used to analyze macroscopic quantum-mechanical fluctuations of super-radiance.

Recent experimental and theoretical investigations of frustrated triangular-lattice antiferromagnets are reviewed. An analysis is made of phases with different structures: partly disordered and double incommensurate states, helical configurations, triangular superstructures, etc. These phases appear because of an instability of the conical high-symmetry point in the Brillouin zone as a result of the dipole interaction, or they may be due to other factors. An analysis is made of magnetic transitions belonging to new universal classes as well as Berezinskiĭ–Kosterlitz–Thouless transitions with an exponential decay of correlations in the low-temperature phase. The chiral symmetry and the associated problem of transition accompanied by simultaneous ordering of discrete and continuous components of the order parameter space are considered for triangular antiferromagnets with different spins. The spectrum of collective modes of some incommensurate structures is analyzed.

The current state of studies of solid surfaces by EPR is reviewed. The methodological problems of theory and experiment that are specific or important for studying surfaces are examined. Concrete results are presented for most studied substances. Comparison of the levels of study of paramagnetic centers in the bulk and at the surface has shown that studies lag substantially in the latter case in comparison with the former. The reasons for this situation are analyzed and pathways for further studies are discussed.

The purpose of this review is to show how the development of ideas regarding classical fluctuations in a system of charged particles initially led to an understanding of the nature of collective collisions of particles, transition scattering, and transition bremsstrahlung and also to an understanding of why the cross sections for emission and scattering in a system of many particles are fundamentally different from those for individual charged particles. It is then shown how the development of these concepts and their generalization to the relativistic and quantum-mechanical cases necessarily lead to the appearance of collective radiative-resonant interactions.