Table of contents

Intensity-fluctuation spectroscopy (IPS) is usually considered to be complementary to conventional spectroscopy and capable of removing technical restrictions on the resolving power. However, the information provided by field spectroscopy is identical to that obtained by IFS only for fields with Gaussian statistics. For non-Gaussian fields, IFS yields essentially new information, and the present review is devoted to this aspect of IFS. It surveys experiments concerned with the investigation of the noise spectrum of resonance fluorescence and of coherent forward scattering by an atomic vapor, which provide data on the width and the structure of levels involved in atomic transitions under the conditions of dominant Doppler broadening. Fundamental and technical limitations of the method are examined. Analogous studies of fluctuation spectra of radiation scattered by macroparticles in liquids can be used to determine the time dependence of the particle form factor independently of the characteristics of translational diffusion.

A number of studies is reviewed that has been conducted in recent years on three current problems of the theory of disordered semiconductors: a) The nature of the electron states in systems of different dimensionalities (including the problem of the so-called "power-law" ("weak") localization), b) Some multielectron effects in disordered semiconductors (including the problem of the Coulomb gap), c) Absorption of light with quantum energies smaller than the optical width of the forbidden band (including an analysis of the role of macroscopic inhomogeneity of thin films).

A review is given of the present state of theoretical and experimental research on second-order phase transitions in anisotropic and nonunifonnly magnetized ferromagnetic and ferrimagnetic materials and on phenomena observed near the Curie point in weak magnetic fields (H < H_A, H_d, where H_A and H_d are the anisotropy field and demagnetizing field, respectively). The nature of these transitions is examined, and it is shown that for anisotropic and nonunifonnly magnetized ferromagnetic materials the Curie point is not an isolated point on the H-T plane. The experimental and theoretical data indicate the existence of a line of second-order phase transitions in a magnetic field applied in certain definite directions with respect to the anisotropy axis. This line of transitions is described by the law T_c(H) = T_c(0) (1 – AH^ω}; theoretical estimates in the molecular-field approximation yield values ω = 2 for ferromagnetic materials of the easy-axis and easy-plane type and ω = 2/3 for cubic ferromagnetic materials. The experimental results on the equilibrium properties (the magnetization, susceptibility, specific heat, magnetostriction, Faraday effect, etc.) and dynamic properties (the speed and attenuation rate of ultrasonic waves, the dynamic susceptibility) not only confirm the existence of a line of phase transitions but also indicate that spin fluctuations play a decisive role in the formation of the transition between ferromagnetic and paramagnetic phases in a weak magnetic field.

The nature of spontaneous radiation is discussed in its technical and historical aspects. The point of view encountered in the literature according to which spontaneous radiation is induced radiation produced by the zero-point (vacuum) oscillations of the electromagnetic field is criticized. Spontaneous radiation is not a purely quantum effect since it occurs also in the classical region (and in this case it is described by classical electrodynamics).