Vol 105, No 12 (2017)

We present the first results of precision measurements of tritium -decay spectrum in the electron energy range 16–18.6 keV by the Troitsk nu-mass experiment. The goal is to find distortions that may be caused by the existence of heavy sterile neutrinos. A signature would correspond to a kink in the spectrum with characteristic shape and end point shifted by the value of a heavy neutrino mass. We set new upper limits to the neutrino mixing matrix element U_e4², which improve existing limits by a factor of 2 to 5 in the mass range of 0.1–2 keV.

It has recently been realized that the focusing of high-energy particle beams at a distance of about 1 cm is promising. A new idea is proposed in this work to focus the beam at a short distance by using a bent plane–parallel silicon plate whose side edges are rotated at a small angle with respect to crystallographic planes. At the U-70 accelerator (IHEP, Protvino), a 50-GeV proton beam has been focused to a narrow line with a width of no more than 30 μm at a distance of 17 cm.

Ultralong ZnO microbushes have been synthesized using a simple thermal evaporation and condensation method by Cu catalysts. The lengths of the ZnO microbushes range from several millimeters to more than one centimeter and the diameters of the branches’ teeth are about 300 nm. The growth mechanism of the ultralong microbushes and the catalytic behavior of the copper are discussed. Room-temperature photoluminescence spectra of the ultralong ZnO microbushes showed a UV emission band at about 388 nm and wide green emissions at around 525 nm. It may be very attractive for commercial applications such as electrical devices, microelectromechanical systems and sensors.

The effect of the exchange interaction between excited high-spin terms of transition metal ions in magnetic Mott–Hubbard insulators on the thermodynamics of the system with singlet terms of ions in the ground state near the crossover of singlet and high-spin terms with the increase in the pressure is studied with the effective Hamiltonian. It is shown that the crossover at the temperature below the critical T* < T_N is a first-order phase transition and is accompanied by a volume jump. The crossover at the temperature above T* is accompanied by a smooth change in the crystal volume.

Photocurrents in the Weyl semimetals belonging to the gyrotropic symmetry classes have been theoretically studied. It has been shown that the circular photocurrent transverse to the direction of light incidence appears in weakly gyrotropic crystals with the C_nv (n = 3, 4, 6) symmetry only when spin-dependent terms both linear and quadratic or cubic in the quasimomentum, as well as a spin-independent term resulting in the tilt of the cone dispersion, are taken into account in the electron effective Hamiltonian. A polarization-independent magnetic-field-induced photocurrent, which is allowed only in gyrotropic systems, has been predicted. For crystals with the C_2v symmetry, a microscopic mechanism of the photocurrent in a quantized magnetic field, which is generated in direct optical transitions between the ground and first excited magnetic subbands, has been considered. It has been shown that this photocurrent becomes nonzero in the presence of the anisotropic tilt of dispersion cones.

A “differential” method for local diagnostics of superconductors has been developed. Regular steps with identical heights through certain intervals of an external field have been revealed on the magnetic-field dependences of the trapped magnetic flux density B_tr(H₀) and the effective demagnetizing factor n_eff (H₀) of bulk and film YBCO samples. It has been shown that the sample in high magnetic fields “decays” stepwise into subcrystallites and nanocrystallites whose size is much smaller than the depth of penetration of the magnetic field λ.

The security of quantum cryptography protocols after a quantum key distribution (QKD) session is formulated in terms of proximity between two situations: quantum states corresponding to real and ideal situations after QKD. The measure of proximity is the trace distance. It is more reasonable to formulate security directly in terms of the smallness of probability of successive guessing of keys by an eavesdropper after an arbitrary number of QKD sessions. There is a fundamental question the answer to which is a priori very unobvious: Is the security criterion in terms of the proximity of the real and ideal situations for a single QKD session sufficient to guarantee the security of keys in terms of the smallness of probability of guessing of keys by the eavesdropper after an arbitrary number of QKD sessions? It has been shown that the answer to this question is positive.

A brief overview of the theoretical studies on the electronic phase separation in the systems with the imperfect nesting of Fermi surface sheets is presented. Among these systems, there are chromium and its alloys, superconducting iron-based pnictides, bilayer graphene, and some other materials.