BROWSE VOLUME

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July 2009

Volume 35, Issue 7,  pp. 517-595

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SUPERCONDUCTIVITY, INCLUDING HIGH-TEMPERATURE SUPERCONDUCTIVITY

Superconducting and magnetic properties of a new superconductor: EuAsFeO0.85F0.15

V. M. Dmitriev, I. E. Kostyleva, E. P. Khlybov, A. J. Zaleski, A. V. Terekhov, L. F. Rybaltchenko, E. V. Khristenko, L. A. Ishchenko, O. E. Omel'yanovskiy, and A. V. Sadakov

Low Temp. Phys. 35, 517 (2009) (4 pages)

Online Publication Date: 11 September 2009

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Polycrystalline samples of the new superconducting compound EuAsFeO0.85F0.15, with critical temperature Tc=11  K, are prepared by solid state synthesis. Its electric and magnetic properties are investigated in magnetic fields from 0.1  to  140000  Oe. The critical magnetic fields Hc1 and Hc2 are measured, and thence the magnetic penetration depth lambda and the coherence length xi are estimated. The temperature dependence Hc2(T) exhibits clear hyperbolic-type behavior starting with the lowest fields. The data derived were used to estimate probable high Tc and Hc2 in compounds doped with rare earths having small atomic radii.
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74.10.+v, 81.05.-t, 81.20.-n, 75.60.Ej, 74.70.-b, 74.25.Ha, 74.25.Fy
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LOW-TEMPERATURE MAGNETISM

Monte Carlo investigation of the critical properties of a three-dimensional frustrated Heisenberg model on a triangular lattice

A. K. Murtazaev, M. K. Ramazanov, and M. K. Badiev

Low Temp. Phys. 35, 521 (2009) (5 pages)

Online Publication Date: 11 September 2009

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The Monte Carlo replica method is used to investigate the critical properties of a three-dimensional frustrated antiferromagnetic Heisenberg model on a triangular lattice. The static magnetic and chiral critical exponents are calculated within the theory of finite-dimensional scaling: specific heat alpha=0.05(2); magnetization beta=0.30(1), betak=0.52(2); susceptibility gamma=1.36(2), gammak=0.93(3); and, correlation radius nu=0.64(1), nuk=0.64(2). The critical Fisher indices eta=−0.06(3) and etak=0.63(4) for this model are calculated for the first time. It is shown that the three-dimensional 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.
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75.40.Cx, 75.30.Cr, 75.30.Et, 75.10.Jm, 75.30.Kz, 75.60.Ej

Field dependences of the magnetization and exchange bias in ferro/antiferromagnetic systems. II. Continuum model of a ferromagnetic layer

A. G. Grechnev, A. S. Kovalev, and M. L. Pankratova

Low Temp. Phys. 35, 526 (2009) (5 pages)

Online Publication Date: 11 September 2009

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A 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 easy-plane anisotropy is described in the continuum approximation. It is shown that collinear and canted structures of the ferromagnetic layer can co-exist 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 M(H) is possible. The results of the present work are compared with our previous results obtained using simpler model of a two-layer FM subsystem.
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75.60.Ej, 75.50.Ee, 75.70.Cn, 75.30.Et, 75.30.Gw

Effect of pressure on the magnetic properties of CrB2

G. E. Grechnev, A. S. Panfilov, A. V. Fedorchenko, V. B. Filippov, A. B. Lyashchenko, and A. N. Vasiliev

Low Temp. Phys. 35, 531 (2009) (5 pages)

Online Publication Date: 11 September 2009

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The magnetic susceptibility chi of the itinerant antiferromagnet CrB2 with TN~=87  K is studied as a function of the hydrostatic pressure up to 2  kbar at fixed temperatures 78 and 300  K. The pressure effect on chi 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, dTN/dP=0.1±0.1  K/kbar, is roughly two orders of magnitude smaller than the corresponding value for the pure chromium. The main contributions to chi 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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62.50.-p, 75.25.+z, 75.30.Kz, 75.10.Lp, 75.50.Ee, 75.30.Cr
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ELECTRONIC PROPERTIES OF CONDUCTING SYSTEMS

Grüneisen's law and particularities of electronic excitations in quasicrystals

A. F. Prekul, N. I. Shchegolikhina, and S. M. Podgornykh

Low Temp. Phys. 35, 536 (2009) (3 pages)

Online Publication Date: 11 September 2009

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Grüneisen'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 Al63Cu25Fe12. The specific heat CP(T) and linear thermal expansion coefficient alphaL(T) are measured in the temperature interval 1.8–400  K. It is found that Grüneisen's law holds quite satisfactorily in the temperature range 200–400  K. 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.
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61.44.Br, 73.30.+y, 65.40.De, 65.40.Ba

Electron-phonon interaction function in the layered dichalcogenide 2Ha-TaSe2

G. V. Kamarchuk, A. V. Khotkevich, A. V. Savitskii, P. Molinié, A. Leblanc, and E. Falques

Low Temp. Phys. 35, 539 (2009) (5 pages)

Online Publication Date: 11 September 2009

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Point-contact spectroscopy is used to investigate experimentally the electron-phonon interaction spectrum in the layered quasi-two-dimensional dichalcogenide 2Ha-TaSe2. The measurements are performed on 2Ha-TaSe2/Cu heterocontacts. The point-contact spectra reflecting the effects of the electron-phonon interaction are recorded in the spectral regime of current flow and delocalization of the electronic states near point contacts for small momentum free-path lengths of electrons. The electron-phonon interaction function in this compound is determined for the first time. Calculations of important integral parameters of the electron-phonon system of the experimental material confirm the good reproducibility of the results.
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63.20.kd, 71.38.-k

Hall-effect anomalies near the quantum critical point in CeCu6−xAux

N. E. Sluchanko, D. N. Sluchanko, N. A. Samarin, V. V. Glushkov, S. V. Demishev, A. V. Kuznetsov, G. S. Burkhanov, and O. D. Chistyakov

Low Temp. Phys. 35, 544 (2009) (12 pages)

Online Publication Date: 11 September 2009

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The results of Hall-effect and resistance measurements on the substitutional solid solutions CeCu6−xAux with concentrations 0<=x<=0.3, corresponding to a wide neighborhood of the quantum critical point (QCP) at x=0.1, are presented. The measurements are made by rotation of the sample in a constant magnetic field of up to 70  kOe in a temperature interval of 1.8–300  K. For the classic heavy-fermion compound CeCu6 the temperature dependence of the Hall coefficient RH(T) exhibits a complex activational form with activation energies Ea1/kB[approximate]110  K and Ea2/kB[approximate]1.5  K in the temperature intervals 50–300  K and 3–10  K, respectively. It is shown that the anomalous behavior of the Hall effect can be explained in a spin-polaron approach, in which the values Ea1,2 can be associated to the binding energy of many-body and one can obtain estimates of the effective mass (meff1,2[approximate]130–150m0) and localization radius (a<sub>p1,2</sub><sup>*</sup>[approximate]1.7 and 14  Å) of the charge carriers in CeCu6. For the compound CeCu5.9Au0.1, corresponding to the QCP, one observes correlated power-law behavior of the temperature dependence of the Hall coefficient RH(T)~T−0.4 and magnetic susceptibility chi(T)~T−0.4, as is characteristic of the regime of quantum critical behavior. For compounds in the immediate vicinity of the QCP an anomalous, even contribution RH2 to the angular dependence of the Hall voltage appears at temperatures below T*~24  K and becomes stronger with increasing magnetic field. Different scenarios for passage through the QCP and their applicability for describing the Hall-effect anomalies in the substitutional solid solutions CeCu6−xAux are discussed.
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72.15.Gd, 64.60.F-, 71.18.+y, 71.27.+a, 71.38.-k, 75.30.Cr

Evolution and collapse of quasistationary states of an electron in planar symmetric three-barrier resonance-tunneling structures

N. V. Tkach and Yu. A. Seti

Low Temp. Phys. 35, 556 (2009) (9 pages)

Online Publication Date: 11 September 2009

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A 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 three-barrier structure. Analytical and numerical calculations of the spectral parameters (resonance energies and widths) are performed by the transmission coefficient and probability distribution function method using the transfer and S matrices. It is shown that the collapse of the resonance energies and widths of all quasistationary states in a symmetric three-barrier 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 square-barrier model the delta model overestimates the resonance energies by 10% and the resonance widths by almost a factor of 2.
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73.21.-b

Hall effect and magnetic ordering in RB12

A. E. Baranovskiy, G. E. Grechnev, N. Yu. Shitsevalova, D. N. Sluchanko, V. V. Glushkov, S. V. Demishev, and N. E. Sluchanko

Low Temp. Phys. 35, 565 (2009) (3 pages)

Online Publication Date: 11 September 2009

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The concentration of carriers in LuB12 is evaluated theoretically by applying ab initio FP-LMTO calculations. Theoretical results are found to be in agreement with high-precision measurements of the Hall coefficient RH(T) carried out on single crystals of the rare-earth dodecaborides RB12 (R=Ho,Er,Tm,Lu) at temperatures 1.8–300  K. The nature of the antiferromagnetic ordering in RB12 is investigated within an RKKY-like model supplemented by comprehensive electronic structure calculations for paramagnetic, ferromagnetic, and antiferromagnetic phases.
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72.20.My, 72.80.Sk, 71.15.-m
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NANOSTRUCTURES AT LOW TEMPERATURES

Nanoparticle size effect on the magnetic and transport properties of (La0.7Sr0.3)0.9Mn1.1O3 manganites

V. Dyakonov, A. Ślawska-Waniewska, J. Kazmierczak, K. Piotrowski, O. Iesenchuk, H. Szymczak, E. Zubov, S. Myronova, V. Pashchenko, A. Pashchenko, A. Shemjakov, V. Varyukhin, S. Prilipko, V. Mikhaylov, Z. Kravchenko et al.

Low Temp. Phys. 35, 568 (2009) (9 pages)

Online Publication Date: 11 September 2009

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Magnetic and transport thermal measurements of nanosize (La0.7Sr0.3)0.9Mn1.1O3 manganite are reported. The nanoparticles are synthesized with use of the co-precipitation method at different (800, 900, and 950  °C) temperatures. Their crystal structure is determined to be perovskite-like with a rhombohedral distortion (the space group R[overline 3]c). The phase composition and specific surface nanopowders are determined. The average size of synthesized nanoparticles (from 40  to  100  nm) is estimated by both the method of low-temperature adsorption of argon and x-ray diffraction measurements. All the nanosize samples show ferromagnetic-like 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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72.20.My, 75.30.Kz, 75.30.Cr, 75.50.Dd, 61.50.Ah, 81.30.Mh, 81.16.-c, 65.80.+n, 75.50.Tt, 81.07.Bc
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PHYSICAL PROPERTIES OF CRYOCRYSTALS

Isochoric thermal conductivity of solid n-alkanes: propane C3H8

V. A. Konstantinov, V. P. Revyakin, and V. V. Sagan

Low Temp. Phys. 35, 577 (2009) (3 pages)

Online Publication Date: 11 September 2009

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The isochoric thermal conductivity of solid propane C3H8 is investigated on three samples of different densities in the temperature interval from 50  K to the onset of melting. In all cases the isochoric thermal conductivity exhibits dependences that are weaker than Lambda[proportional]1/T. The results obtained are compared with those for other representatives of n-alkanes. The experimental data are discussed within a model in which the heat is transported by low-frequency phonons and by “diffusive” modes above the phonon mobility edge.
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66.70.Lm, 66.30.H-, 64.70.dj, 63.20.-e
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LATTICE DYNAMICS

Structure and time-resolved phosphorescence spectra of crystalline and glassy 2-bromobenzophenone

O. S. Pyshkin, L. M. Buravtseva, R. V. Romashkin, M. A. Strzhemechny, D. I. Zloba, and V. N. Baumer

Low Temp. Phys. 35, 580 (2009) (9 pages)

Online Publication Date: 11 September 2009

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Microsecond-level time-resolved photoluminescence spectra of 2-bromobenzophenone are measured on crystalline (at 1.6, 52, and 300  K) and glassy (at 1.6 and 95  K) 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 2-benzophenone molecule in the crystal and also for the bimolecular excimer emission. The time dependence of the intensity of the red 0–0 band at 52  K 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 2-benzophenone at 100 and 200  K are determined by the single-crystal x-ray diffraction method. Within the temperature interval 100–300  K the crystal structure of 2-benzophenone remains monoclinic, with symmetry space group P21/a (Z=4). The coefficient of linear thermal expansion is anisotropic and of a magnitude (10−5−10−4  K−1) typical for organic molecular crystals. The conformation parameters of the molecule vary relatively strongly with temperature; the behavior of the C–Br and C=O 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.
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61.66.Hq, 61.43.Fs, 78.55.Kz, 78.55.Qr, 78.47.Cd, 65.40.De

Quasi-two-dimensional features in the phonon spectrum of graphite

I. A. Gospodarev, K. V. Kravchenko, E. S. Syrkin, and S. B. Feodos'ev

Low Temp. Phys. 35, 589 (2009) (7 pages)

Online Publication Date: 11 September 2009

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The phonon spectrum 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 phonon density of states are calculated. The main distinctive features of the phonon spectrum of graphite are determined; they are due to the quasi-two-dimensional 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 c 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.
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63.20.D-, 63.22.Np, 74.10.+v, 74.25.Kc