Volume 35, Issue 7, July 2009
 SUPERCONDUCTIVITY, INCLUDING HIGHTEMPERATURE SUPERCONDUCTIVITY


Superconducting and magnetic properties of a new superconductor:
View Description Hide DescriptionPolycrystalline samples of the new superconducting compound, with critical temperature , are prepared by solid state synthesis. Its electric and magnetic properties are investigated in magnetic fields from . The critical magnetic fields and are measured, and thence the magnetic penetration depth and the coherence length are estimated. The temperature dependence exhibits clear hyperbolictype behavior starting with the lowest fields. The data derived were used to estimate probable high and in compoundsdoped with rare earths having small atomic radii.
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 LOWTEMPERATURE MAGNETISM


Monte Carlo investigation of the critical properties of a threedimensional frustrated Heisenberg model on a triangular lattice
View Description Hide DescriptionThe Monte Carlo replica method is used to investigate the critical properties of a threedimensional frustrated antiferromagnetic Heisenberg model on a triangular lattice. The static magnetic and chiral critical exponents are calculated within the theory of finitedimensional scaling: specific heat ; magnetization , ; susceptibility , ; and, correlation radius , . The critical Fisher indices and for this model are calculated for the first time. It is shown that the threedimensional frustrated Heisenberg model on a triangular lattice forms a new universality class of critical behavior. It is found that the universality class of the antiferromagnetic Heisenberg model on a triangular lattice depends on the type of interlayer exchange interaction.

Field dependences of the magnetization and exchange bias in ferro/antiferromagnetic systems. II. Continuum model of a ferromagnetic layer
View Description Hide DescriptionA model giving a qualitative explanation of the results of an experimental investigation of the field dependences of the magnetization in ferro and antiferromagnetic media in contact with one another is proposed. In this model a thin ferromagnetic(FM) film with strong easyplane anisotropy is described in the continuum approximation. It is shown that collinear and canted structures of the ferromagnetic layer can coexist in the system studied. The ranges of the parameters (exchange interactions in the FM layer and through the FM/AFM interface, thickness of the FM layer, and magnitude of the external magnetic field) where a nonuniform state of the FM exists are found and explicit analytic expressions are obtained for the distribution of the magnetization in this state. The dependences of the total magnetization of the system on the magnitude of the external field are constructed in the physically important cases of “thin” and “thick” FM layers, and it is demonstrated that the experimentally observed unsymmetric dependence is possible. The results of the present work are compared with our previous results obtained using simpler model of a twolayer FM subsystem.

Effect of pressure on the magnetic properties of
View Description Hide DescriptionThe magnetic susceptibility of the itinerant antiferromagnet with is studied as a function of the hydrostatic pressure up to at fixed temperatures 78 and . The pressure effect on is found to be negative in sign and weakly dependent on the magnetic state of the compound. In addition, the measured pressure dependence of the Néel temperature, , is roughly two orders of magnitude smaller than the corresponding value for the pure chromium. The main contributions to and their volume dependence are calculated ab initio within the local spin density approximation, and they turn out to be in close agreement with the experimental data.
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 ELECTRONIC PROPERTIES OF CONDUCTING SYSTEMS


Grüneisen’s law and particularities of electronic excitations in quasicrystals
View Description Hide DescriptionGrüneisen’s law according to which the temperature dependence of the lattice specific heat at constant volume is similar to that of the thermal expansion coefficient is checked experimentally for the icosahedral phase of . The specific heat and linear thermal expansion coefficient are measured in the temperature interval . It is found that Grüneisen’s law holds quite satisfactorily in the temperature range . Significant deviations from this law are found at lower temperatures. These deviations are due to the presence of “excess” specific heat of the Schottky anomaly type. It is shown that this contribution is of electronic origin.

Electronphonon interaction function in the layered dichalcogenide
View Description Hide DescriptionPointcontactspectroscopy is used to investigate experimentally the electronphonon interaction spectrum in the layered quasitwodimensional dichalcogenide . The measurements are performed on heterocontacts. The pointcontact spectra reflecting the effects of the electronphonon interaction are recorded in the spectral regime of current flow and delocalization of the electronic states near point contacts for small momentum freepath lengths of electrons. The electronphonon interaction function in this compound is determined for the first time. Calculations of important integral parameters of the electronphonon system of the experimental material confirm the good reproducibility of the results.

Halleffect anomalies near the quantum critical point in
View Description Hide DescriptionThe results of Halleffect and resistance measurements on the substitutional solid solutions with concentrations , corresponding to a wide neighborhood of the quantum critical point (QCP) at , are presented. The measurements are made by rotation of the sample in a constant magnetic field of up to in a temperature interval of . For the classic heavyfermion compound the temperature dependence of the Hall coefficient exhibits a complex activational form with activation energies and in the temperature intervals and , respectively. It is shown that the anomalous behavior of the Hall effect can be explained in a spinpolaron approach, in which the values can be associated to the binding energy of manybody and one can obtain estimates of the effective mass and localization radius ( and ) of the charge carriers in . For the compound , corresponding to the QCP, one observes correlated powerlaw behavior of the temperature dependence of the Hall coefficient and magnetic susceptibility , as is characteristic of the regime of quantum critical behavior. For compounds in the immediate vicinity of the QCP an anomalous, even contribution to the angular dependence of the Hall voltage appears at temperatures below and becomes stronger with increasing magnetic field. Different scenarios for passage through the QCP and their applicability for describing the Halleffect anomalies in the substitutional solid solutions are discussed.

Evolution and collapse of quasistationary states of an electron in planar symmetric threebarrier resonancetunneling structures
View Description Hide DescriptionA theory of the evolution and collapse of pairs of resonances because of a change in the strength (thickness) of the inner barrier is developed in a model of effective masses and symmetric, square, potential barriers for an electron in a planar threebarrier structure. Analytical and numerical calculations of the spectral parameters (resonance energies and widths) are performed by the transmission coefficient and probability distributionfunction method using the transfer and matrices. It is shown that the collapse of the resonance energies and widths of all quasistationary states in a symmetric threebarrier structure occurs for practically the same thicknesses of the inner barriers, somewhat greater than the total thickness of the outer barriers. It is established that with respect to the squarebarrier model the model overestimates the resonance energies by 10% and the resonance widths by almost a factor of 2.

Hall effect and magnetic ordering in
View Description Hide DescriptionThe concentration of carriers in is evaluated theoretically by applying ab initio FPLMTO calculations. Theoretical results are found to be in agreement with highprecision measurements of the Hall coefficient carried out on single crystals of the rareearth dodecaborides at temperatures . The nature of the antiferromagnetic ordering in is investigated within an RKKYlike model supplemented by comprehensive electronic structure calculations for paramagnetic, ferromagnetic, and antiferromagnetic phases.
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 NANOSTRUCTURES AT LOW TEMPERATURES


Nanoparticle size effect on the magnetic and transport properties of manganites
View Description Hide DescriptionMagnetic and transport thermal measurements of nanosize manganite are reported. The nanoparticles are synthesized with use of the coprecipitation method at different (800, 900, and ) temperatures. Their crystal structure is determined to be perovskitelike with a rhombohedral distortion (the space group ). The phase composition and specific surface nanopowders are determined. The average size of synthesized nanoparticles (from ) is estimated by both the method of lowtemperature adsorption of argon and xray diffraction measurements. All the nanosize samples show ferromagneticlike ordering with close phase transition temperatures. Their magnetization decreases with decreasing particle size. Comparison of experimental and calculated temperature dependences of the spontaneous magnetic moment shows that the spontaneous magnetization both in magnetic field and without field is well described in the framework of the double exchange model. The decrease of the magnetization with decreasing particle size is due to the increasing surface contribution to the magnetization. The magnetic entropy is shown to increase with increasing applied magnetic field and to be smaller for the small particles. The resistivity is found to become higher with decreasing particles size at any temperatures.
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 PHYSICAL PROPERTIES OF CRYOCRYSTALS


Isochoric thermal conductivity of solid alkanes: propane
View Description Hide DescriptionThe isochoric thermal conductivity of solid propane is investigated on three samples of different densities in the temperature interval from to the onset of melting. In all cases the isochoric thermal conductivity exhibits dependences that are weaker than . The results obtained are compared with those for other representatives of alkanes. The experimental data are discussed within a model in which the heat is transported by lowfrequency phonons and by “diffusive” modes above the phonon mobility edge.
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 LATTICE DYNAMICS


Structure and timeresolved phosphorescence spectra of crystalline and glassy 2bromobenzophenone
View Description Hide DescriptionMicrosecondlevel timeresolved photoluminescence spectra of 2bromobenzophenone are measured on crystalline (at 1.6, 52, and ) and glassy (at 1.6 and ) samples. For each of these physical situations the characteristic decay times are determined for the 0–0 band of monomer emission from metastable (blue 0–0 band) and global (red 0–0 band) excited states of the 2benzophenone molecule in the crystal and also for the bimolecular excimer emission. The time dependence of the intensity of the red 0–0 band at in the crystal is found to be nonmonotonic, a fact that provides yet another beautiful demonstration of the presence of two excited states and is indicative of thermally activated nonradiative transitions from the metastable excited state to the global. The corresponding kinetics is well described by a system of equations for joint radiative decay. To gain a better understanding of the nature of the anomalies of the phosphorescence, the crystal and molecular structures of 2benzophenone at 100 and are determined by the singlecrystal xray diffraction method. Within the temperature interval the crystal structure of 2benzophenone remains monoclinic, with symmetry space group . The coefficient of linear thermal expansion is anisotropic and of a magnitude typical for organic molecular crystals. The conformation parameters of the molecule vary relatively strongly with temperature; the behavior of the C–Br and bonds is most interesting: they become shorter with increasing temperature, possibly indicating a weakening of the weak hydrogen bonds of the peripheral oxidizer atoms with the acceptors of the neighboring molecules.

Quasitwodimensional features in the phonon spectrum of graphite
View Description Hide DescriptionThe phononspectrum of graphite is analyzed in detail at the microscopic level and the partial contributions from the displacement of atoms in and perpendicular to the plane of the layers to the phonondensity of states are calculated. The main distinctive features of the phononspectrum of graphite are determined; they are due to the quasitwodimensional character of phonon propagation as is characteristic for graphite, specifically, the feature arising in the spectral density as a result of the displacement of atoms along the axis, analogous to the Dirac singularity in the electron spectrum of graphene. This makes it possible to predict the general changes occurring in the phonon and electron spectra as a result of the intercalation of different metals in graphite as well as to explain the change of the superconducting transition temperature in intercalated graphite.
