Том 106, № 7 (2017)

In this work, we address a challenging problem of a competition of charge and spin orders for high-T_c cuprates within a simplified 2D spin-pseudospin model which takes into account both conventional Heisenberg Cu²⁺−Cu²⁺ antiferromagnetic spin exchange coupling (J) and the on-site (U) and intersite (V) charge correlations in the CuO₂ planes with the on-site Hilbert space reduced to only three effective charge states (nominally Cu^1+;2+;3+). We performed classical Monte Carlo calculations for large square lattices implying the mobile doped charges and focusing on a case of a small intersite repulsion V ≪ J. The on-site attraction (U < 0) does suppress the antiferromagnetic ordering and gives rise to a checkerboard charge order with the doped charge distributed randomly over a system in the whole temperature range. However, under the on-site repulsion (U > 0) the homogeneous ground state antiferromagnetic solutions of the doped system found in a mean-field approximation are shown to be unstable with respect to a phase separation with the charge and spin subsystems behaving like immiscible quantum liquids. Puzzlingly, with lowering the temperature one can observe two sequential phase transitions: first, an antiferromagnetic ordering in the spin subsystem diluted by randomly distributed charges, then, a charge condensation in the charge droplets. The effects are illustrated by the Monte Carlo calculations of the specific heat and longitudinal magnetic susceptibility.

In the space-charge-limited current regime at T = 4.2 K, the magnetoresistance of PbSnTe:In/(111)BaF₂ films has been studied at various mutual orientation of the magnetic field B (up to 4 T), electric field E (up to ~10³ V/cm), and normal to the surface n. At B ‖ n, the reduction of the current reaches a factor of ~10⁵, whereas at B ‖ E, the current increases by a factor of ~10³. The angular dependences of the magnetoresistance have been studied at the “rotation” of B in three different planes. The angular dependences of the magnetoresistance for the plane corresponding to the orientation B ⊥ E exhibit local maxima near the orientations B ⊥ n, at which charge carriers are deflected by the magnetic field to one of the boundaries of the film. At the deviation to the free surface, the half-width of maxima is several degrees. At the deviation to the interface with the substrate, the half-width of maxima is about an order of magnitude larger and their amplitude is one or two orders of magnitude smaller. Possible mechanisms of giant positive and negative magnetoresistance, as well as the effect of the boundaries of the film on the angular dependences of the magnetoresistance, have been discussed.

The temperature dependence of the electrical resistance has been studied for heterostructures formed by antiferromagnetic LaMnO₃ single crystals of different orientations with epitaxial films of ferroelectric Ba_0.8Sr_0.2TiO₃ deposited onto them. The measured electrical resistance is compared to that exhibited by LaMnO₃ single crystals without the films. It is found that, in the samples with the film, for which the axis of polarization in the ferroelectric is directed along the perpendicular to the surface of the single crystal, the electrical resistance decreases significantly with temperature, exhibiting metallic behavior below 160 K. The numerical simulations of the structural and electronic characteristics of the BaTiO₃/LaMnO₃ ferroelectric−antiferromagnet heterostructure has been performed. The transition to the state with two-dimensional electron gas at the interface is demonstrated.

The temporal dynamics of a spinor exciton-polariton condensate in a high-quality anisotropic GaAs microcavity under pulsed resonant excitation with light possessing a nonzero orbital angular momentum is investigated. The phenomenon of spatial separation of the spin components of the polariton condensate upon pumping with a coherent superposition of two beams with opposite circular polarizations and orbital angular momenta is observed. The key factors for the observation of this effect are the lateral anisotropy of the microcavity that causes a splitting between the linear components of the polariton ground state and the occurrence of opposite orbital angular momenta for the two spin components of the condensate. The experimental results are in qualitative agreement with the theoretical model of the phenomenon developed in JETP Lett. 104, 827 (2016).

A new channel of radiative losses in fusion plasma with impurities of multielectron ions associated with their excitation by fast alpha particles has been studied. It has been shown that this loss channel for tungsten impurity is comparable with standard radiative losses of plasma electrons.

To implement quantum random number generators, it is fundamentally important to have a mathematically provable and experimentally testable process of measurements of a system from which an initial random sequence is generated. This makes sure that randomness indeed has a quantum nature. A quantum random number generator has been implemented with the use of the detection of quasi-single-photon radiation by a silicon photomultiplier (SiPM) matrix, which makes it possible to reliably reach the Poisson statistics of photocounts. The choice and use of the optimal clustering of photocounts for the initial sequence of photodetection events and a method of extraction of a random sequence of 0’s and 1’s, which is polynomial in the length of the sequence, have made it possible to reach a yield rate of 64 Mbit/s of the output certainly random sequence.

The energy for creating a temporal train of nearly harmonic superoscillations is studied. It is shown that the power of sources necessary for creating the train of such superoscillations increases exponentially with the number of periods in the train and with the frequency of superoscillations.