Vol 105, No 9 (2017)

The conical effect in Smith–Purcell radiation arising from electrons moving at non-zero angle to the direction of grating periodicity has been observed for the first time. It was found that the maximum of radiation intensity for ψ ≠ 0 shifts in both polar and azimuth angles. The experimental and theoretical results were compared, and the good agreement was shown. The experiment has been performed for 6 MeV electrons and at millimeter wavelengths.

The heat capacity in a La_0.8 Ag_0.15 MnO₃ manganite has been measured near the Curie temperature T_C in applied magnetic fields up to 26 kOe to study the scaling critical behavior and to obtain the universality class. The conventional scaling fails in application to the manganites with a hysteresis and the strong sensitivity of T_C to a magnetic field. However, the application of the improved scaling procedure designed by us allows yielding the good scaling the magnetic heat =0.23 capacity in La_0.85Ag_0.15MnO₃, which may belong to a new universality class for systems with the strong spin-orbital coupling of t_2g -electrons, namely, double -Heisenberg with the critical exponent of the heat capacity α = −0.23 and the critical exponent of the correlation radius v=0.7433. This new universality class is consistent with the crystal, magnetic and orbital symmetries for the La_0.85Ag_0.15MnO₃. Scaling failure in the vicinity of T_C in the range of t/H^1/2ν ≈ [−0.033;0.024] is understood by finite-size and other disordering effects when T →T_C. It is remarkable that finite-size effect is consistent with grain size, L ≈ 50 μm, in the La_0.85Ag_0.15MnO₃. The correlation radius, Lt^ν ≈ 30.28 Å, estimated from the finite-size effect is of the same order of magnitude with the sizes of the ferromagnetic fluctuations and drops in manganites.

In the framework of the 14-band k • p model, the intensity of the impact ionization processes in direct gap semiconductors is studied and explicit expressions for the impact ionization rate are obtained. It is shown that the rate of the process near the threshold energy is determined by the sum of the isotropic and strongly anisotropic contributions. The former contribution is proportional to the cube of the distance from the threshold, whereas the latter is a quadratic one arising only because of the coupling with remote bands. The comparison of these contributions under averaging over the nondegenerate isotropic distribution of nonequilibrium electrons characterized by some effective temperature T* demonstrates that the cubic contribution rather than the commonly used quadratic one is dominant in the direct gap semiconductors with E_g<1−1.5 eV up to T* = 300 K. This should be taken into account in the calculations of the operating characteristics of the devices based on the avalanche multiplication of charge carriers.

We consider dark matter consisting of long-living particles with masses 10⁷ GeV ≲ M ≲10¹⁶ GeV decaying through hadronic channel as a source of high-energy neutrino. Using recent data on high-energy neutrino from IceCube and Pierre Auger experiments, we derive the upper-limits on neutrino flux from dark matter decay and constraints on dark matter parameter space. For the dark matter masses of order 10⁸ GeV the constraints derived are slightly stronger than those obtained for the same dark matter model using the highenergy gamma-ray limits.

The process of merging of three X-ray photons into one photon in the field of a light atomic ion is theoretically studied. A pronounced resonance structure and a strong angular anisotropy of the differential cross section for merging are predicted in the region of the incident photon energy ħω ≅ I_1s /2 (I_1s is the threshold energy of ionization of the -shell of the ion). The magnitude of the observed merging cross section is estimated.

A model describing a plasmonic nanopatch antenna based on triangular silver nanoprisms and multilayer cadmium chalcogenide quantum dots is introduced. Electromagnetic-field distributions in nanopatch antennas with different orientations of the quantum-dot dipoles are calculated for the first time with the finite element method for numerical electrodynamics simulations. The energy flux through the surface of an emitting quantum dot is calculated for the configurations with the dot in free space, on an aluminum substrate, and in a nanopatch antenna. It is shown that the radiative part of the Purcell factor is as large as 1.7 × 10₂ The calculated photoluminescence lifetimes of a CdSe/CdS/ZnS colloidal quantum dot in a nanopatch antenna based on a silver nanoprism agree well with the experimental results.

The interaction between particles in the mean-field approximation of the many-body theory is often taken into account with the use of the semiclassical description of the particle motion. However, quantization of a part of the degrees of freedom becomes essential in certain cases. In this work, two such cases where nonlinear wave equations appear have been considered: electrons in a quantum well and excitons in a trap. In the case of indirect excitons in an annular trap, the one-dimensional Gross–Pitaevskii equation permits an analytical solution and it turns out that there can be no bound state in a one-dimensional symmetric potential well. This makes the problem qualitatively different from a similar one-body problem. In the case of electrons in a quantum well, the nonlinear integro-differential equation does not have an exact solution and the allowed energy levels have been found by the direct variational method.

A quantum cryptography system based on a 4-basis protocol with geometrically uniform states is tested in a series of experiments. Quantum states of light transmitted through real fiber optic communication channels to a distance of 32 km in the presence of uncontrolled external actions are prepared, transformed, and measured. It is shown that the chosen algorithms of processing quantum information are adequate and can be used as foundations of practical devices in protected communication lines.₁

The representation of the tunneling conductance G(T) of the “dirty” (with low concentrations of the same nonmagnetic impurities in the I layer) N–I–N junction (where N is a normal metal and I is an insulator) in the form of a sum of conductances of random quantum jumpers penetrating the disordered I layer is obtained in the low-temperature region. It is shown that the axis of the parameter δ = |ε₀ − ε_F| giving the deviation of the energy of ε₀ the quasi-local electron state on the impurity in the I layer from the Fermi energy of εF the dirty N–I–N junction contains a series of bifurcation points, at the transition through each of which (in the direction of the increase in δ) the number of maxima on the temperature dependence G(T) increases by unity; i.e., a new maximum is “born” on the curve G(T). Numerical estimates are given for the characteristic parameters of dirty N–I–N junctions indicating the possibility of the experimental observation of at least the first of these maxima.