Index of content:
Volume 42, Issue 1, January 2016

Fractional quantum Hall quasiparticles are famous for having fractional electric charge. Recent experiments report that the quasiparticle's effective electric charge determined through tunnelingcurrent noise measurements can depend on the system parameters such as temperature or bias voltage. Several works proposed to understand this as a signature for edge theory properties changing with energy scale. I consider two of such experiments and show that in one of them the apparent dependence of the electric charge on a system parameter is likely to be an artefact of experimental data analysis. Conversely, in the second experiment the dependence cannot be explained in such a way.

Polymerization in highly compressed nitrogen (Review Article)
View Description Hide DescriptionThe results of experimental and theoretical studies of the thermodynamic properties of new polymeric phases of nitrogen at ultrahigh pressures, as well as computer modeling of the structure and properties of polymeric nitrogen based on the potential models of interaction of atoms and molecules are reviewed. The location of the phase transition lines for the transitions between the molecular and polymeric crystals, molecular and polymeric fluids as well as the position of the melting line for the polymeric nitrogen in the phase diagram are discussed.

Currentvoltage characteristics of Corbino disks under quantum Hall effect conditions
View Description Hide DescriptionThis is a discussion of the details of the currentvoltage characteristics of a 2D Corbino disk with a uniform density of mobile carriers in the quantum Hall effect plateau. It is shown that diffusion may be one of the reasons for the variety of observed currentvoltage characteristics. The anomalous role of diffusion in the formation of the currentvoltage characteristics of Corbino disks is determined by the dimensionality of the problem, as well as by the “quality” of the density of states of the magnetized 2D conducting system. The diffusion hypothesis is applied to some specific results and provides an explanation of the difference between the currentvoltage characteristics shown in Figs. 1 and 2.

Dissipation of the kinetic energy of a tuning fork immersed in superfluid helium at different oscillation frequencies
View Description Hide DescriptionThe drag coefficient characterizing the dissipation of the energy of oscillating tuning forks immersed in liquid helium is studied experimentally. The experiments are done at temperatures from 0.1 to 3.5 K, a range that covers both hydrodynamicflow and the ballistic transport of thermal excitations in superfluid helium below 0.6 K. It is found that a frequency dependence of the drag coefficient exists in the hydrodynamic limit, where the main dissipation mechanism is viscousfriction of the liquid against the surface of the oscillating object at temperatures above 0.7 K. In this case, the drag coefficient is proportional to the square root of the oscillation frequency and its temperature dependence in He II is determined by the corresponding relationships between the density of the normal component and the viscosity of the liquid. At lower temperatures, there is no frequency dependence of the drag coefficient and the magnitude of the dissipative losses is determined only by the temperature dependence of the density of the normal component. At the same time, over the entire range of temperatures studied here, the magnitude of the dissipative losses depends on the geometrical dimensions of the oscillating object.

Features of the electronic structure of the ternary superconductors RRh4B4 (R = Y, Lu)
View Description Hide DescriptionThe electronic structure and a number of thermodynamic characteristics of the ternary RRh4B4 (R = Y, Lu) systems of superconductors in the normal phase are calculated from first principles. The electronic states and interactions responsible for the superconducting and magnetic properties of these systems are analyzed. It is found that the Fermi level in the compounds YRh4B4 and LuRh4B4 is immediately adjacent to a peak in the density of electronic states and about 1 eV above a pseudogap in the electronic spectrum. The existence of a number of groups of quasidegenerate electronic states with low effective masses near the Fermi level is revealed. These states may be the cause of a significant diamagnetic contribution of the conduction electrons to the magnetic susceptibility and may be responsible for the strong temperature dependence of the susceptibility in the rhodiumborides.

Analysis of nonlinear conductivity of point contacts on the base of FeSe in the normal and superconducting state
View Description Hide DescriptionNonlinear conductivity of point contacts (PCs) on the base of FeSe single crystals has been investigated. Measured dV/dI dependencies demonstrate the prevailing contribution to the PC conductivity caused by the degraded surface. Superconducting (SC) feature in dV/dI like a sharp zerobias minimum develops for relatively low ohmic PCs, where the deep areas of FeSe are involved. Analysis of dV/dI has shown that the origin of the zerobias minimum is connected with the Maxwell part of the PCresistance, what masks energy dependent spectral peculiarities. Even so, we have found the specific features in dV/dI—the sharp side maxima, which may have connection to the SC gap, since their position follows the BCS temperature dependence. Exploring the dV/dI spectra of the rare occurrence with Andreevlike structure, the two gaps with Δ = 2.5 and 3.5 meV were identified.

Effective Hamiltonian and excitation spectrum of harmonically trapped bosons
View Description Hide DescriptionAn approach is proposed to obtain an effective Hamiltonian of a harmonically trapped Bosesystem. Such a Hamiltonian is quadratic in the creation–annihilation operators and certain approximations allow to simplify higher (three and four operator) products to the required form. After the Hamiltonian diagonalization, the expression for the excitation spectrum is obtained containing in particular temperaturedependent corrections. Numerical calculations are made for a onedimensional system. Some prospects towards the extension of the suggested approach to study binary bosonic mixtures are briefly discussed.

Quantum oscillations of the Bloch point in domain walls of magnetic bubbles
View Description Hide DescriptionIt is shown that quantum oscillations of the Bloch point in the domain walls of cylindrical magnetic domains occur at low temperatures. The conditions for this phenomenon are established.

Competition of magnetization mechanisms in (NdDy)(FeCo)B alloys, doped with samarium
View Description Hide DescriptionIt is found that magnetic hysteresis in sinteredmagnets (Nd0.62Dy0.33Sm0.05)16.2(Fe0.77Co0.23)78.1B5.7 for the temperature range T = 150–350 K, is caused by a mechanism associated with the displacement of domain walls, which ceases at temperatures below 150 K. In addition, the formation of magnetic hysteresis in this temperature range is affected by the mechanism involving the nucleation of the reverse magnetization phase, which is observed at temperatures down to 36 K.

Twokink excitation in a spiral magnetic structure
View Description Hide DescriptionTwokink excitations in the spiral structures of magnets and multiferroics are found and analyzed within the framework for the sineGordon model. It is shown that the movement and interaction of the kinks is accompanied by macroscopic translations of the spiral structure. The ways of observing and exciting kinks in the external magnetic field are discussed.

The effect of the temperature of graphene oxide reduction on lowtemperature sorption of ^{4}He
View Description Hide DescriptionThe sorption of ^{4}He by graphene oxide powders thermally reduced at T = 200, 300, 500, 700, 900 °C has been investigated in the interval 1.5–290 K. The measured dependence of the quantity of sorbed helium upon the reduction temperature shows up as a nonmonotonic curve. The highest quantities of helium were sorbed by the samples reduced at T = 300 and 900 °C. It is assumed that the thermal reduction of graphite oxide by heating it to 300 °C causes evaporation of the water intercalated in the spacings of the carbon layers, this results in exfoliation of the graphene planes, which enhances the sorptive capacity. Heating the samples to 900 °C generates numerous defects in the carbon planes, as a result, the interlayer spacings become accessible for sorption, which enhances the sorptive capacity.

Tunneling current noise in the fractional quantum Hall effect: When the effective charge is not what it appears to be
View Description Hide DescriptionFractional quantum Hall quasiparticles are famous for having fractional electric charge. Recent experiments report that the quasiparticle's effective electric charge determined through tunnelingcurrent noise measurements can depend on the system parameters such as temperature or bias voltage. Several works proposed to understand this as a signature for edge theory properties changing with energy scale. I consider two of such experiments and show that in one of them the apparent dependence of the electric charge on a system parameter is likely to be an artefact of experimental data analysis. Conversely, in the second experiment the dependence cannot be explained in such a way.

Thermal conductivity of solid thiophene in an incommensurate orientational state
View Description Hide DescriptionThe thermal conductivity of solid thiophene at equilibrium vapor pressure between 2 K < T < 170 K, has been measured in a sequence of incommensurate metastable orientationally disordered phases II, II1, II2, and II2g with different degrees of orientational ordering of the molecules. It is found that in phase states II, II1 and II2 with dynamic orientational disorder of the molecules, the thermal conductivity does not depend on the temperature. It is shown that the temperature dependence of the thermal conductivity κ(T) of orientational glass Vg and II2g (incommensurate) does not have any of the anomalies that are typical for amorphous materials and glasses. The temperature dependence κ(T) of the incommensurate state of orientational glass II2g is bellshaped, which is typical for the thermal conductivity of crystals with longrange orientational order. In the II2g state, as temperature drops from Tg to almost 10 K, the thermal conductivity increases according to κ(T) = A/T + B, where the first term describes the input of the propagating phonons, wherein the average length of their mean free path is greater than half of the phonon wavelength. The B term is associated with the input of localized shortwave, or “diffuse” vibrational modes. At low temperatures T ≤ 7 K, κ(T) ∝ T^{3} is observed with increasing temperatures, which corresponds to the boundary scattering of phonons.