Volume 36, Issue 4, April 2010
 BOSE EINSTEIN CONDENSATION


Phase transition in the magnetic field in a Bose gas
View Description Hide DescriptionThe thermodynamic properties of a Bose gas of particles with nonzero spin and magnetic moment in states with and without a BoseEinstein condensate are studied as a function of the external magnetic field. It is shown that there exists a temperature interval where a transition into a state with a BoseEinstein condensate occurs in a Bose gas with constant temperature and particle number density when the magnetic field is increasing and reaches a critical magnitude. The temperature and magnetic field derivatives of the specific heat at constant volume and the magnetic susceptibility undergo a jump at such a transition.
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 LOWTEMPERATURE MAGNETISM


EPR in the molecular magnet
View Description Hide DescriptionResonance studies of single crystals of the molecular magnet have been performed in a wide range of frequencies and magnetic fields at liquidhelium temperature. Two nonequivalent magnetic centers of coppernanoclusters with turn angle of the local axes have been found. The ground state of each magnetic center (magnetic molecules, containing a ring of six ferromagnetic interacting ions ) can be represented as a system of energy levels with effective particle spin , split by an axial magnetic field .

Lowtemperature behavior of the magnetoelastic characteristics of praseodymium ferroborate
View Description Hide DescriptionThe behavior of the elastic moduli and sound absorption in a single crystal at low temperatures is studied. A transition of the magnetic subsystem into an antiferromagnetically ordered state is manifested in the temperature behavior of the sound velocities and absorption. The characteristic behavior of the elastic properties of in an external magnetic field is observed. A phenomenological theory that gives a qualitative description of the observed features is constructed. It is proposed that a weak magnetic moment exists in the crystal.

Dissipation function of magnetic media
View Description Hide DescriptionA general method of constructing a dissipation function is developed for disordered magnetic media and for magnetically ordered systems. As an example it is shown for a ferromagnet that not only the invariance with respect to uniform rotations of the body but also the law of conservation of magnetization must be taken into account in order to construct a dissipation function. It is found that in ferromagnets the dissipation term in the equations of motion for the magnetization is a sum of Bloch and Landau–Lifshitz–Gilbert relaxation terms. The region of applicability of the relaxation term in the Landau–Lifshitz form is determined. The damping of spin waves in a ferromagnet with tetragonal symmetry is calculated. A procedure is formulated for transitioning from a ferromagnet with lower symmetry to a ferromagnet with a continuous degeneracy parameter. In this case the relaxation process can be systematically described by means of the dissipation function described in this article. It is shown how the relaxation term of a general form for ferromagnets becomes the Bloch relaxation term for paramagnets. It is shown that the magnetization vector relaxes in two stages. First the magnetic moment relaxes in magnitude quite rapidly as a result of exchange enhancement and then the magnetization relaxes slowly to its equilibrium direction. The second stage qualitatively corresponds to the relaxation picture described by the Landau–Lifshitz model.

Charge transfer along localized states in InSe and InSe⟨Sn⟩ single crystals
View Description Hide DescriptionIt is established that at temperatures hoppingconductivity with variable hopping length exists across natural layers of undoped and doped (0.2 and Sn) InSe single crystals in a constant electric field. The density of states near the Fermi level, the energy spread of the states , the localization radius , and the average hopping length in the temperature interval are evaluated.
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 NANOSTRUCTURES AT LOW TEMPERATURES


New mechanism of the formation of vacancy voids
View Description Hide DescriptionA new mechanism is proposed for the formation of vacancy voids in irradiatedmetal as being due to the successive development of two types of instabilities in a system of interacting point defects produced by neutron irradiation. The first stage of the void formation process involves the nucleation of local vacancyclusters possessing a high, relative to the surroundings, concentration of defects; the second state involves lattice instability inside such clusters.
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 QUANTUM EFFECTS IN SEMICONDUCTORS AND DIELECTRICS


Lowfrequency vibrational spectrum of a crystal
View Description Hide DescriptionThe longwavelength IR transmission spectra of a crystal in the spectral range at low temperatures are measured in polarized light. Two vibrational modes with energies and are found. The structure of the lowfrequency vibrational spectrum in the Brillouin zone is calculated using a onedimensional model.

The temperature dependence of the inelastic scattering time in InGaN grown by MOVPE
View Description Hide DescriptionLow temperature electrical measurements of the resistivity, the Hall effect and the magnetoconductivity were performed on an InGaN sample having an electron concentration far above the critical value for the metal–insulator transition. The weak localization effect and a twoband model were used to analyze the magnetoconductivity data. The temperature dependence of the inelastic scattering time was extracted from the magnetoconductivity data at low temperatures. It was found that the inelastic scattering time is proportional to , suggesting that electron–electron interactions are dominant.

Isothermal slip of a quantum Bose gas with speculardiffuse reflection from the boundary
View Description Hide DescriptionA solution is obtained for Kramers’ problem of isothermal slip of a quantum Bose gas along a flat solid surface. The reflection of molecules from the wall is assumed to be speculardiffuse. A model kinetic equation with a collision integral in the model form and collision frequency proportional to the molecular velocity is used. The dependence of the slip velocity on the value of a parameter equal to the ratio of the chemical potential to the product of Boltzmann’s constant by the absolute temperature is analyzed.

Absorption spectra of thin films of the solid solutions
View Description Hide DescriptionThe excitonic absorption spectra of thin films of the solid solutions have been studied for the first time. It is determined that the spectra of the solid solutions are stable and consist of two bands which genetically originate from the initial bands of the compounds and . It is shown on the basis of the structure of the crystal lattices of the compounds that the concentration dependence of the spectral position of the excitonic bands and is related with the transfer of excitons between and tetrahedra along the axis of the solid solutions. The experimental result is in good agreement with the calculation of the energies and on the basis of a theory similar to the theory of Davydov splitting of excitonic bands in molecular crystals.
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 PHYSICAL PROPERTIES OF CRYOCRYSTALS


Lattice distortion in hcp rare gas solids
View Description Hide DescriptionThe lattice distortion parameter has been calculated as a function of molar volume for the hcp phases of He, Ar, Kr, and Xe. Results from both semiempirical potentials and density functional theory are presented. Our study shows that is negative for helium in the entire pressure range. For Ar, Kr, and Xe changes sign from negative to positive as the pressure increases, growing rapidly in magnitude at higher pressures.
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 LATTICE DYNAMICS


Twoparameter dynamical solitons in thin elastic plates
View Description Hide DescriptionThe dynamics of twoparameter elasticsolitons with almost shear polarization, which are localized in a thin elastic plate consisting of an anharmonic material, is investigated. Onedimensional nonlinear integrodifferential equations for the shear displacements are derived and a variant of an asymptotic procedure making it possible to find approximate analytical solutions for such solitons is proposed.

Vibrational characteristics of niobium diselenide and graphite nanofilms
View Description Hide DescriptionThe phonon spectra of niobium diselenide nanofilms, consisting of several structural elements of this compound, as well as graphite nanofilms starting with bigraphene are analyzed at the microscopic level. The partial contributions of the displacements of atoms along the strong and weak coupling directions (i.e. along and perpendicular to the layers) to the phonondensity of states are calculated. The characteristic distinguishing features of the vibrational spectral of these structures are analyzed. The fact that our calculations are practically identical to the data obtained from neutron diffraction, acoustic, and optical experiments confirms that the description of the phonon spectra of the compounds is highly accurate. The temperature dependences of the meansquare shifts for bulk samples and nanofilms along different crystallographic directions, making it possible to evaluate the stability of the graphite and niobium diselenide nanofilms at low temperatures, are calculated for each compound studied.
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 LOWTEMPERATURE PHYSICS OF PLASTICITY AND STRENGTH


Effect of impurities on the quantum plasticity of tin single crystals
View Description Hide DescriptionThe effect of small additions of indium, cadmium, and zinc impurities on the temperature dependences of the plasticitycharacteristics of tin single crystals in the interval is studied. Plastic deformation of the samples occurred by means of dislocation slip in the system (100)⟨010⟩. It has been shown in previous work that Peierls barriers are the main obstacles to the motion of dislocations of this system in pure tin. There exists a threshold temperature above which dislocations overcome barriers by means of thermal fluctuations (thermally activated plasticity) and below which by means of quantummechanical tunneling (quantum plasticity). The present investigation shows that for weak doping of tin by various types of impurities the influence of Peierls relief on dislocation mobility remains dominant at low temperatures but shifts appreciably in the direction of high temperatures , i.e. the temperature interval of quantum plasticity becomes wider. The experimentally observed shift of the threshold temperature depends in a complicated manner on the size of the impurities atoms, which create local barriers for dislocation slip. Current theory cannot give an unequivocal interpretation of such a dependence.
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 BRIEF COMMUNICATIONS


Coercive force of nanocrystalline manganites
View Description Hide DescriptionSamples of and with particle sizes ranging from are obtained using cold isostatic pressing. The coercive force of the experimental samples is determined from the field dependences of the resistance and dynamic magnetic susceptibility. It is determined by two methods that the contribution of the surface layer to the magnetic characteristics of manganites is composition dependent. It is shown experimentally for the first time that the coercive force in manganites reaches its highest values with particle size of the order of for both compositions and vanishes completely for lanthanum manganite with particles as a result of reaching a superparamagnetic state.

Interaction of two substitutional impurity atoms in an hcp crystal
View Description Hide DescriptionMolecular dynamicscomputer simulation with a Lennard–Jones potential is used to investigate the interaction of two identical substitutional impurity atoms in an hcp crystal lattice. Different atomic radii of the impurities atoms, interaction energy of the atoms and the lattice atoms, and initial distances between the impurity atoms at zero temperature and pressure. It is found that in a number of cases for small distances between the impurity atoms not exceeding five interatomic distances these atoms attract one another contrary to the wellknown laws of the continuum theory of elasticity. Good agreement between the computational results and the theory of elasticity obtains for short distances between impurity atoms.
