Vol 108, No 4 (2018)

An unusual frequency dependence of a coercive field observed in polycrystalline ferroelectric films of Pb(Zr_{1 − x}Ti_x)O₃ solid solutions in a wide frequency range has been explained within the model of surface sources emitting thin ferroelectric domains with reversed polarization. This model makes it possible to interpret experimental data as the manifestation of the coercivity paradox in polycrystalline films and predicts the existence of the limiting frequency for switching of domains in agreement with the experiment. The proposed mechanism of polarization switching explains the observed temperature dependence of the activation field E_a ∝ T^−1/2. Furthermore, it predicts an increase in the activation energy for the nucleation of domains with an increase in the size of the source, indicating that the coercive field increases with the degradation of small

It has been shown that potassium dihydrogen phosphate crystals change their microhardness reversibly after their exposure to a magnetic field of B = 0.8 or 1.2 T for t_m = 7–90 min. It has been found that the magnetic effect can be conveniently characterized by the quantity B²t_m, because the variation of the parameters conserving B²t_m=const does not change the result. At B²t_m < 10 T² min, the effect is almost absent. Above this threshold, the amplitude of changes in the microhardness increases and approaches a constant value of ~10% at B²t_m ≈ 19 T² min. The responses of samples of the same crystal from the faces of the prismatic and pyramidal growth sectors to exposure are different. In the former case, they soften; in the latter case, the hardening stage follows the softening stage. However, in both cases, the microhardness returns to the initial value. At B²t_m values from 19 to 37 T² min, the amplitudes and durations of the effect do not change, but in the narrow range of 37–43 T² min, the lifetime of the modified state increases sharply with transition to a new level: “sharp” peaks with a half-width of ~2 days are transformed to trapezoids with the width of the horizontal side of ~1–2 weeks. A physical scheme of the observed effects has been proposed.

The possibility of implementing a Berezinskii–Kosterlitz–Thouless topological phase transition in a Josephson medium of two-level high-temperature superconductors induced by a magnetic field has been studied. The effect of temperature and external magnetic field on the transport properties of the YBa₂Cu₃O_7–δ granular high-temperature superconductor considered as a model object is studied experimentally. For the first time, an anomalous behavior of the magnetoresistance isotherms is detected in a narrow range of temperature and external magnetic field indicating a topological phase transition in the Josephson medium of granular high-temperature superconductors.

The evolution of the microstructure of nonstoichiometric niobium carbides NbC_y (y = 0.77, 0.84, 0.96) and vanadium carbide V₈C₇ subjected to high-energy milling has been studied by the time-of-flight neutron diffraction method. It has been found that milled nanocrystalline powders have microinhomogeneous structure: two fractions with different sizes of particles have been identified in them. The content of the nanofraction is more than 90 wt %; the size of particles of this fraction varies from 90 to 250 Å, depending on the composition of the initial carbide and the duration of milling. The size of particles of the second fraction is more than 2000 Å. Anisotropic deformation distortions have been revealed. The mean size of coherent scattering regions and microstrains in nanocrystallites have been estimated.

The structure of the joint phase diagram of high-temperature superconducting cuprates has been studied within the theory of fermion condensation. Prerequisites of the topological rearrangement of the Landau state with the formation of a flat band adjacent to the nominal Fermi surface have been established. The related non-Fermi-liquid behavior of cuprates in the normal phase has been studied with focus on the non-Fermi-liquid behavior of the resistivity ρ(T), including the observed crossover from the linear temperature behavior ρ(T, x) = A₁(x)T at doping levels x below the critical value x_c^h corresponding to the boundary of the superconducting region to the quadratic temperature behavior at x > x_c^h, which is incompatible with predictions of the conventional quantum-critical-point scenario. It has been demonstrated that the slope of the coefficient A₁(x) is universal and is the same on both boundaries of the joint phase diagram of cuprates in agreement with available experimental data. It has also been shown that the fermion condensate is responsible for pairing in the D-wave state in cuprates. The effective Coulomb repulsion in the Cooper channel, which prevents the existence of superconductivity in normal metals in the S channel, leads to high-temperature superconductivity in the D channel.