Volume 37, Issue 12, December 2011
Index of content:
37(2011); http://dx.doi.org/10.1063/1.3674213View Description Hide Description
The current-voltage characteristics (CVCs) of vortexsuperconducting films are investigated in a wide temperature range. It is shown for the first time that the experimental CVCs and their parameters are described well enough by asymptotic formulas within the theory of the vortexresistive state of Aslamazov-Lempitsky (AL). It is established that for wide films, the critical current of which corresponds to the AL theory, the differential resistance of a linear segment of a CVC does not depend on temperature that agrees with the model of the vortexresistive state of AL. In the range of much higher temperatures, where the critical current is I c (T)∝(1−T/T c )3/2, the decrease of the differential resistance with increasing the temperature is observed that corresponds to predictions of the AL theory. However, in the close vicinity of T c the increase of the differential resistance of a linear vortex segment of CVCs was found at T → T c . This effect does not have an explanation in modern theories of a vortex state.
37(2011); http://dx.doi.org/10.1063/1.3674269View Description Hide Description
A possibility of polarization of the gas of hydrogen-like atoms at zero temperature in the Bose–Einstein condensate is studied at equilibrium and nonequilibrium conditions. The Gross–Pitaevskii equation is generalized for a gas of atoms considering their electronic structure. It is shown that the wave function of the system of interacting bosons in a single-particle condensate at zero temperature looks like the Glauber coherent state. With the help of the generalized Gross–Pitaevskii equation the polarization properties of atomic gas in the Bose–Einstein condensation state are investigated. It is shown that the electron stationary states in atoms of a gas in the coherent state differ from the electron stationary states in free atoms. Because of this the superfluidsystem can be spontaneously polarized, even if the free atoms in the stationary states have no intrinsic electric dipole moment. The polarization properties of a system of double-level hydrogen-like atoms in the Bose–Einstein condensate are studied more comprehensively. The propagation of both sound and high-frequency small disturbances in the coherent system of polarized atoms is accompanied by oscillations of polarization. The applicability of the developed idea to the analysis of electrical phenomena in superfluidhelium is discussed.
Critical properties of an antiferromagnetic Ising model on a square lattice with interactions of the next-to-nearest neighbors37(2011); http://dx.doi.org/10.1063/1.3674264View Description Hide Description
The critical properties of an antiferromagneticIsing model on a square lattice with interactions of the next-to-nearest neighbors are investigated by a replica Monte-Carlo method. Using the finite-size scaling theory the static critical exponents of specific heat, ordering parameter, susceptibility, correlation radius as well as the Fisher exponent are calculated. An analysis of data is performed both with and without taking into account a correction to the finite-size scaling. It was found that in the model under consideration the second order phase transition is observed. It is shown that this model belongs to the new class of universality of critical behavior.
Monte Carlo simulation of anisotropic Shastry–Sutherland lattice in the framework of classical Heisenberg model multiferroic37(2011); http://dx.doi.org/10.1063/1.3674184View Description Hide Description
Monte Carlo simulation of two-dimensional Shastry–Sutherland lattice has been carried out using heat-bath method. The dependencies of magnetization M on external field H have been obtained in the framework of classical Heisenberg model. In certain interval of exchange parameters ratio the plateau of magnetization corresponding to M = 1/3 has been observed. The influence of exchange anisotropy of “easy-axis” type on this plateau width is studied. It has been shown that even weak anisotropy (∼1−2%) leads to essential enlargement of the plateau. The dependence of critical temperature on exchange parameters ratio has been established.
37(2011); http://dx.doi.org/10.1063/1.3674182View Description Hide Description
Low-temperature studies of the behavior of the sound velocity and attenuation of acoustic modes have been performed on a single crystal NdFe3(BO3)4Transitions of the magnetic subsystem to the antiferromagnetically ordered state at T N ≈ 30.6 K have been revealed in the temperature behavior of the elasticcharacteristics. The features in the temperature behavior of elasticcharacteristics of the neodymium ferroborate and its behavior in the external magnetic field, applied in the basic plane of the crystal, permit us to suppose that the transition to an incommensurate spiral phase is realized in the system. This phase transition behaves as the first order one. H–T phase diagrams for the cases H ∥ a and H ∥ b have been constructed. The phenomenological theory, which explains observed features, has been developed.
37(2011); http://dx.doi.org/10.1063/1.3674267View Description Hide Description
The band structure of a carbon-nitrogen nanotube of “zigzag” type (8, 0) with nitrogen atom concentrations of 6.25%, 12.5%, and 25% and “armchair” (5, 5) carbon-nitrogen nanotube with nitrogen atom concentrations of 10% and 20% is calculated within the density functional theory approach. The calculations are carried out taking into account optimizations of both the longitudinal and transverse nanotube structural parameters. The structure is distorted significantly both lengthwise and across with increasing nitrogen concentrations. In particular, their cross-sectional profile ceases to be a circle. Dependence of the energies of interband electronic transitions on the concentration of nitrogen can be observed in the form of a peak shift in the optical absorption spectrum.
Free and bound states of excitons in a percolation cluster of ZnSe quantum dots in a dielectric matrix37(2011); http://dx.doi.org/10.1063/1.3674196View Description Hide Description
Two-phase systems (borosilicate glass with ZnSequantum dots) were studied. The results obtained provided an insight both into the mechanism of formation of a phase percolation transition of excitons in the array of quantum dots at the critical concentration and radii and the contribution of dielectric mismatch between ZnSe and matrix to the transition. Using the above two-phase system as an example, it was found for the first time that such a complex fractal object as the percolation cluster of quantum dots had not only free (mobile) states, but bound (immobile) ones also. These bound (immobile) states resemble the type-II excitons in superlattices or the impurity states in bulk semiconductors. It is shown that the polarization of the two-phase system interface is the main source of these immobile states whose energy is dependent on the size of quantum dots and their average separation.
37(2011); http://dx.doi.org/10.1063/1.3674185View Description Hide Description
Magnetopolaronic effects are considered in electron transport through a single-level vibrating quantum dot subjected to a transverse (to the current flow) magnetic field. It is shown that the effects are most pronounced in the regime of sequential electron tunneling, where a polaronic blockade of the current at low temperatures and an anomalous temperature dependence of the magnetoconductance are predicted. In contrast, for resonant tunneling of polarons the peak conductance is not affected by the magnetic field.
37(2011); http://dx.doi.org/10.1063/1.3674189View Description Hide Description
The orientational order parameter, rotational ground-state energy, and lattice distortion parameter (the deviation of the c/a ratio from the ideal hcp value 1.633) in hcp lattice of phase I of p-H2 and o-D2 are calculated using a semi-empirical approach. It is shown that the lattice distortion in these J-even species is small compared with that found in n-H2, and n-D2. The difference presumably is caused by the J-odd species.
37(2011); http://dx.doi.org/10.1063/1.3674268View Description Hide Description
The bulk titanium with a grain size from nanometer to submicron was obtained using cryomechanical treatment. An investigation of nanostructure, determining unusual mechanical properties of the new material, was performed by methods of transmission electron microscopy and X-ray diffraction. The results of a comparative study of the structures formed by deformation at temperatures close to that of liquid nitrogen and room temperature in the case of different activity of sliding and twinning, showed that large degree of grain diminishment at the cryodeformation is caused by mechanical twinning. In this case, the twinning generating a set of diverse orientations due to fragmentation of grains leads to increasing the number of reflexes in X-ray diffraction patterns, providing a random character of the structure/texture. The data on a mean size of a region of coherent scattering (RCS) in crystallites and values of average microdiformations Δɛ were obtained. It is assumed that much smaller values of microdeformations Δɛ after cryorolling are caused by activated relaxation processes due to warming up to room temperature. It is shown that an X-ray amorphous phase presented in nanostructured titanium is not truly amorphous, and corresponds to RCS with the size d 15 nm.
37(2011); http://dx.doi.org/10.1063/1.3674200View Description Hide Description
The magnetic susceptibility and magnetization measured along the b axis of a KTb(WO4)2 single crystal was investigated experimentally in the temperature range 70 mK–6 K and in magnetic fields up to 8 T. The results allow us to make conclusion that singlet magnet KTb(WO4)2 undergoes an antiferromagneticphase transitionT c = 0.65 K.
Low-temperature magnetic and thermal properties of the frustrated two-dimensional S = 1 compound Ni5(TeO3)4Cl237(2011); http://dx.doi.org/10.1063/1.3674203View Description Hide Description
The temperature and magnetic field dependent magnetization of the two-dimensional quantum spin system Ni5(TeO3)4Cl2 has been investigated using single crystals in temperature range 1.8–100 K and in magnetic fields up to 14 T. The magnetization below the magnetic phase transition demonstrates an unusual temperature behavior with considerable anisotropy. Combined magnetization and specific heat data allowed to determine the critical temperature of the magnetic phase transition,T c = 28.4 K. Magnetic fields shift this temperature toward lower temperatures.