Table of contents

Results of investigating the temperature dependences of the surface impedance Z(T) = R(T) + iX(T) and complex conductivity σ(T) = σ'(T) – iσ''(T) in the ab-planes and along the c-axis of high-T_c superconductors (HTSCs) are reviewed and critically analyzed. An electrodynamic method is considered for extracting all components of (T) and (T) tensors from quantities measured in microwave experiments. The main attention is focused on the evolution of (T) and (T) dependences in a YBa₂Cu₃O_7–x crystal with the oxygen content variation. Possible mechanisms of conductivity are discussed in the framework of models for normal, superconducting, and pseudogap states of HTSCs.

The origins and physical consequences of the traditionally used relation between the position measurement error and the momentum perturbation, Δ_m²xΔ_p²p ≥ ²/4, are discussed. It is demonstrated that the corresponding increase in the momentum variance for the aposteriori state occurs only in some special cases. The product of Δ_m²A and Δ_p²B is shown to essentially differ from the one given by the uncertainty relation if the commutator is an operator. The error quantum limits for the joint homodyne measurement of quadrature amplitudes for an optical mode are found. It is shown that similar results can be obtained if the quadratures of a harmonic oscillator are estimated by means of continuous position measurement.

It is shown that the 'new' runaway criterion for electrons in dense gases suggested by Tarasenko and Yakovlenko (Usp. Fiz. Nauk174 953 (2004) [Phys. Usp. 47 887 (2004)]) is actually not a criterion, and the 'upper' curve of the U(Pd) dependence does not exist. Only the Z-shaped segment of U(Pd) in the region of small Pd, known for helium since the early 1930s, agrees with the reality. The decrease in the ionization coefficient with E/P increasing and the existence of Pd_min for helium have been known since the same time. Doubt is cast upon the 'record' runaway-electron currents at P = 1 atm. The acceleration mechanism suggested in the above article has been known for a long time, and the interpretation of the 'record' runaway-electron currents on this basis is the result of fitting the data to the formula that implies the lack of electron multiplication but is 'understood' by Tarasenko and Yakovlenko as a runaway criterion. Nothing new has been added to the mechanism of volumetric discharge formation, but mistakes have been made.

Matvei Bronstein's 1935 work on quantum gravity, the first in-depth study of the problem, is analyzed in the context of the history of physics and the scientist's career. Bronstein's analysis of field measurability revealed "an essential difference between quantum electrodynamics and the quantum theory of the gravitational field" and showed that general relativity and quantum theory are fundamentally difficult to unify. Featured in the story are Planck, Einstein, Heisenberg, Pauli, Rosenfeld, Landau, and Bohr. The methodological uniqueness of the quantum gravity problem is discussed.