Table of contents

Experimental and theoretical work on the magnetoacoustics of rare-earth orthoferrites near reorientation phase transitions (RPTs) is reviewed. The temperature and field dependence of magnetoresonant soft-mode frequencies and of the sound velocity and attenuation near various RPTs, as obtained by RF and ultrasonic spectroscopy, are given. A spin-wave approximation theoretical analysis of orthoferrite magnetoacoustics is presented, accounting as fully as possible for interactions between all the subsystems involved, including the ordered ferrous, elastic, paramagnetic rare-earth, and dipole (electromagnetic) subsystems. The origin of energy gaps in the spin-wave spectrum is discussed in detail, as are the ultrasonic dispersion, propagation velocity, and attenuation changes at RPT points. The energy gaps measured and the sound velocity behaviour observed are shown to result from the interaction of the orthoferrite subsystems. In most cases experimental data agree well with theoretical predictions.

Properties of the nonclassical light (NCL) have been considered with an emphasis on experimentally-observed features, which are underlain by the well-known Mandel's formula connecting the statistics of photocounts with that of light falling on the detector. A systematic operational approach is presented to study the NCL using two parallel sets of numbers measured: probabilities of photocounts {p_m} and normalised factorial moments of counts {g_k}. Two particular examples are examined in detail: a 'heated' squeezed vacuum and a 'heated' one-photon state. An alternative method is proposed to discover the week nonclassicality using 'generalised' moments {a_k(s)}. The effect of the linear absorption (amplification) and of the beam-splitting of the NCL, and the relation between the NCL and the absolute calibration of photodetectors are considered. The conditions are elucidated whereat the beam-splitter realises a mathematical operation of superposition of two one-mode fields useful in studying the NCL.

We consider the stability problem for a plane gas layer whose nonequilibrium state is maintained by pumping energy into the molecular vibrational degrees of freedom and by heat removal through the walls. Two approaches to the study of gas stability are discussed, which consider the evolution of small hydrodynamic perturbations and thermal explosion. We have studied in detail the Rayleigh–Benard problem of convection instability and the Semenov–Frank-Kamenetskii problem of thermal explosion, and generalised them to the case of a nonequilibrium gas. Some unsolved problems of the physical hydrodynamics of a nonequilibrium gas have been outlined as well.

The collapse of the wave function has recently reemerged as a subject of extensive discussion in the quantum mechanical literature. In the present paper, wave function collapses occuring during the irreversible evolution of complex quantum systems, including those involved in measurement procedures, are described.

The conceptual foundations of quantum physics are discussed based on the hypothesis that physics has progressed towards the basic laws of nature by isolating objects from their environment. Observable objects are coupled to their environment by spontaneous quantum jumps and, therefore, their motion is affected by statistical laws. A completely deterministic evolution is possible only in the idealised case of isolated objects. The evolution of these unobservable idealised objects is described by the dynamical laws of quantum mechanics.

Measurements on the anomalous temperature behaviour of the spontaneous magnetisation in various ferrimagnets (N-, M-, and P-type Neel curves) are analysed and summarised. The concept of a 'weak' magnetic sublattice is applied to explain the anomalies as well as manifestations of the 'low-temperature point' T_B and the antiferromagnetic-type paraprocess. The paraprocess phenomenon causes sign anomalies in the magnetocaloric effect, magnetostriction, and the 'first' component of the isotropic magnetoresistance. It is suggested that in magnetite (Fe₃O₄) the role of a 'weak' sublattice is played by the spin-ordered subsystem of hopping electrons ('magnetoelectron' sublattice), and the appearing point T_B is nothing else but the low-temperature transformation point T_t(100-120 K).