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Sample records for dodecaborides dinamika kristallicheskoj

  1. Magnetic structure of rare-earth dodecaborides

    SciTech Connect

    Siemensmeyer, K. . E-mail: siemensmeyer@hmi.de; Flachbart, K.; Gabani, S.; Matas, S.; Paderno, Y.; Shitsevalova, N.

    2006-09-15

    We have investigated the magnetic structure of HoB{sub 12}, ErB{sub 12} and TmB{sub 12} by neutron diffraction on isotopically enriched single-crystalline samples. Results in zero field as well as in magnetic field up to 5T reveal modulated incommensurate magnetic structures in these compounds. The basic reflections can be indexed with q=(1/2+/-{delta}, 1/2+/-{delta}, 1/2+/-{delta}), where {delta}=0.035 both for HoB{sub 12} and TmB{sub 12} and with q=(3/2+/-{delta}, 1/2+/-{delta}, 1/2+/-{delta}), where {delta}=0.035, for ErB{sub 12}. In an applied magnetic field, new phases are observed. The complex magnetic structure of these materials seems to result from the interplay between the RKKY and dipole-dipole interaction. The role of frustration due to the fcc symmetry of dodecaborides and the crystalline electric field effect is also considered.

  2. Specific features of magnetoresistance during the antiferromagnet—paramagnet transition in Tm1 - x Yb x B12

    NASA Astrophysics Data System (ADS)

    Sluchanko, N. E.; Azarevich, A. N.; Bogach, A. V.; Glushkov, V. V.; Demishev, S. V.; Levchenko, A. V.; Filippov, V. B.; Shitsevalova, N. Yu.

    2013-05-01

    The transverse magnetoresistance Δρ/ρ( H, T) of Tm1 - x Yb x B12 single crystals is studied in the ytterbium concentration range corresponding to the antiferromagnet-paramagnet transition in a magnetic field up to 80 kOe at low temperatures. A magnetic H- T phase diagram is constructed for the antiferromagnetic state of substitutional Tm1 - x Yb x B12 solid solutions with x ≤ 0.1. The contributions to the magnetoresistance in the antiferromagnetic and paramagnetic phases of the dodecaborides under study are separated. Along with negative quadratic magnetoresistance -Δρ/ρ ∝ H 2, the magnetically ordered phase of these compounds is found to have component Δρ/ρ ∝ H that linearly changes in a magnetic field. The negative contribution to the magnetoresistance of Tm1 - x Yb x B12 is analyzed in terms of the Yosida model for a local magnetic susceptibility.

  3. Anomalous charge transport in RB12 (R = Ho, Er, Tm, Lu)

    NASA Astrophysics Data System (ADS)

    Sluchanko, N.; Bogomolov, L.; Glushkov, V.; Demishev, S.; Ignatov, M.; Khayrullin, Eu.; Samarin, N.; Sluchanko, D.; Levchenko, A.; Shitsevalova, N.; Flachbart, K.

    High precision measurements of Hall RH(T) and Seebeck S(T) coefficients have been carried out for the first time on single crystals of rare earth dodecaborides RB12 (R D Ho, Er, Tm, Lu) at temperatures 1.8-300 K. Low temperature anomalies detected on the temperature dependencies of RH(T) and S(T) are associated with antiferromagnetic phase transitions in HoB12, ErB12 and TmB12 compounds. The observed discrepancy between the change of charge carriers' mobility and de-Gennes factor (g - 1)2 J(J + 1) (J - angular momentum of the 4f shell) in the set of HoB12-TmB12 allows us to conclude about the appreciable influence of spin fluctuations on the charge transport in these compounds with B12 atomic clusters.

  4. Hall and transverse even effects in the vicinity of a quantum critical point in Tm{sub 1-x}Yb{sub x}B{sub 12}

    SciTech Connect

    Sluchanko, N. E. Azarevich, A. N.; Bogach, A. V.; Glushkov, V. V.; Demishev, S. V.; Anisimov, M. A.; Levchenko, A. V.; Filipov, V. B.; Shitsevalova, N. Yu.

    2012-09-15

    The angular, temperature, and magnetic field dependences of the resistance recorded in the Hall effect geometry are studied for the rare-earth dodecaboride Tm{sub 1-x}Yb{sub x}B{sub 12} solid solutions where the metal-insulator and antiferromagnetic-paramagnetic phase transitions are observed in the vicinity of the quantum critical point x{sub c} Almost-Equal-To 0.3. The measurements performed on high-quality single crystals in the temperature range 1.9-300 K for the first time have revealed the appearance of the second harmonic contribution, a transverse even effect in these fcc compounds near the quantum critical point. This contribution a is found to increase drastically both under the Tm-to-ytterbium substitution in the range x > x{sub c} and with an increase in the external magnetic field. Moreover, as the Yb concentration x increases, a negative peak of a significant amplitude appears on the temperature dependences of the Hall coefficient R{sub H}(T) for the Tm{sup 1-x}Yb{sub x}B{sub 12} compounds, in contrast to the invariable behavior R{sub H}(T) Almost-Equal-To const found for TmB{sub 12}. The complicated activation-type behavior of the Hall coefficient is observed at intermediate temperatures for x {>=} 0.5 with activation energies E{sub g}/k{sub B} Almost-Equal-To 200 K and E{sub a}/k{sub B} 55-75 K, and the sign inversion of R{sub H}(T) is detected at liquid-helium temperatures in the coherent regime. Renormalization effects in the electron density of states induced by variation of the Yb concentration are analyzed. The anomalies of the charge transport in Tm{sub 1-x}Yb{sub x}B{sub 12} solid solutions in various regimes (charge gap formation, intra-gap many-body resonance, and coherent regime) are discussed in detail and the results are interpreted in terms of the electron phase separation effects in combination with the formation of nanosize clusters of rare earth ions in the cage-glass state of the studied dodecaborides. The data obtained allow

  5. Effects of disorder and isotopic substitution in the specific heat and Raman scattering in LuB{sub 12}

    SciTech Connect

    Sluchanko, N. E. Azarevich, A. N.; Bogach, A. V.; Vlasov, I. I.; Glushkov, V. V.; Demishev, S. V.; Maksimov, A. A.; Tartakovskii, I. I.; Filatov, E. V.; Flachbart, K.; Gabani, S.; Filippov, V. B.; Shitsevalova, N. Yu.; Moshchalkov, V. V.

    2011-09-15

    Precision measurements of the specific heat and spectral intensity I({omega}) of Raman scattering for Lu{sup N}B{sub 12} single crystal samples with various boron isotopes (N = 10, 11, nat) have been performed at low and intermediate temperatures. A boson peak in the low-frequency part of the I({omega}) spectrum has been observed for the first time for lutetium dodecaboride at liquid nitrogen temperatures. It has been shown that low-temperature anomalies in the specific heat, along with the features of Raman spectra, can be interpreted in terms of the transition to a cageglass state at T* = 50-70 K, which appears when Lu{sup 3+} ions are displaced from the centrosymmetric position in cavities of a rigid covalent boron sublattice towards the randomly located boron vacancies. The concentrations of various two-level systems that correspond to two types of vibrational clusters with correlation lengths of 12-15 and 18-22 A, respectively, have been estimated. The vibrational density of states of LuB{sub 12} has been calculated from Raman spectra in the model of soft atomic potentials. An approach has been proposed to explain the dielectrization of the properties of the YbB{sub 12} compound at T < T*, as well as the features of the formation of magnetic structures in RB{sub 12} antiferromagnets (R = Tb, Dy, Ho, Er, Tm) and the suppression of superconductivity in LuB{sub 12}.

  6. Physical properties of cage-like compound UB12

    NASA Astrophysics Data System (ADS)

    Troć, R.; Wawryk, R.; Pikul, A.; Shitsevalova, N.

    2015-07-01

    Boron and uranium form three metallic borides having the chemical formulae UB2, UB4 and UB12. In this study, we present the temperature variations of magnetic susceptibility, specific heat, electrical resistivity (performed in magnetic fields of 0 and up to 9 T), thermoelectric power and thermal conductivity measured on the bulk sample of UB12. This dodecaboride behaves as a typical metal, being a Pauli paramagnet and exhibiting a large variety of physical properties due to its specific close-packed structure containing B12 groups. We describe also an uncommon phenomenon observed in UB12, that is, a fairly large scattering of the experimental resistivity data under application of a magnetic field at low temperatures and its systematic vanishing during heating of the sample. This effect is probably caused by inharmonious movement (rattling) of the uranium atoms inside the oversized coordination cage, B24, reflected by applying the magnetic field. The specific heat, resistivity, thermoelectric power and heat transport data have been analysed in the framework of the low-frequency Einstein modes, which are mainly responsible for the phonon spectra behaviour in the system studied here.

  7. Fabrication and Characterization of Squeezed Cast Aluminum Matrix Composites Containing Boride Reinforcements

    NASA Astrophysics Data System (ADS)

    Olaya-Luengas, L.; Estremera-Pérez, E.; Muñoz, L.; Suárez, O. M.

    2010-12-01

    Aluminum alloys containing 2 and 4 wt.% copper reinforced with aluminum dodecaborides were successfully fabricated by squeeze casting employing pressures up to 62 MPa. The distribution of reinforcements throughout the composites was quantified, whereas the effect of pressure on the composites density and hardness was determined. In addition, the reinforcement chemical stability was examined using high-temperature x-ray diffraction which permitted to confirm the formation of AlB2 phase a result of AlB12 decomposition. The pressure effect on the squeezed composites was investigated by optical microscopy, Vickers microhardness testing, superficial Rockwell hardness testing, and differential thermal analysis. At the highest applied pressures, the castings density increased when the shrinkage porosity was practically eliminated. The composite superficial hardness improved by increasing the applied squeeze. The indirect squeeze cast technique caused Al2Cu phase segregation to upper regions of the specimens, while conversely, the AlB12 reinforcements segregated preferentially at lower regions of the casting.

  8. Specific features of magnetoresistance during the antiferromagnet-paramagnet transition in Tm{sub 1-x}Yb{sub x}B{sub 12}

    SciTech Connect

    Sluchanko, N. E. Azarevich, A. N.; Bogach, A. V.; Glushkov, V. V.; Demishev, S. V.; Levchenko, A. V.; Filippov, V. B.; Shitsevalova, N. Yu.

    2013-05-15

    The transverse magnetoresistance {Delta}{rho}/{rho}(H, T) of Tm{sub 1-x}Yb{sub x}B{sub 12} single crystals is studied in the ytterbium concentration range corresponding to the antiferromagnet-paramagnet transition in a magnetic field up to 80 kOe at low temperatures. A magnetic H-T phase diagram is constructed for the antiferromagnetic state of substitutional Tm{sub 1-x}Yb{sub x}B{sub 12} solid solutions with x {<=} 0.1. The contributions to the magnetoresistance in the antiferromagnetic and paramagnetic phases of the dodecaborides under study are separated. Along with negative quadratic magnetoresistance -{Delta}{rho}/{rho} {proportional_to} H{sub 2}, the magnetically ordered phase of these compounds is found to have component {Delta}{rho}/{rho} {proportional_to} H that linearly changes in a magnetic field. The negative contribution to the magnetoresistance of Tm{sub 1-x}Yb{sub x}B{sub 12} is analyzed in terms of the Yosida model for a local magnetic susceptibility.

  9. Experimental and Numerical Study of Swirling Flows and Flame Dynamics

    NASA Astrophysics Data System (ADS)

    Abricka, M.; Barmina, I.; Valdmanis, R.; Zake, M.

    2014-08-01

    The effect of swirling air on the flow dynamics was investigated for the cold non-reacting flows and the flame arising at thermo-chemical conversion of biomass pellets downstream of a cylindrical channel. Under experimental and numerical investigation was the swirling flow dynamics with the primary axial air supply below a biomass layer and swirling air supply above it. The results indicate that for cold flows the swirling air jet outflow from tangential nozzles leads to the formation of a complex flow dynamics which is influenced both by upstream and downstream air swirl propagation near the channel walls, with correlating swirl-enhanced formation of the upstream and downstream axial flows close to the flow centreline depending on the swirling air supply rate. These axial flows can be completely balanced at their stagnation within the axial recirculation zone. It is shown that at equal boundary conditions for the swirling flame and the cold flows the swirling flow dynamics is influenced by the upstream air swirl-enhanced mixing of the reactants below the air swirl nozzles. This determines the formation of a downstream reaction zone with correlating development of the flow velocity, temperature and composition profiles in the downstream flame regions with improved combustion stability. The low swirl intensity in these regions prevents the formation of a recirculation zone Ir veikti kompleksi aukstu nereaģējošu un liesmas virpuļplūsmu dinamikas veidošanās eksperimentālie pētījumi, izvērtējot galvenos faktorus, kas ietekmē šo plūsmu dinamikas veidošanos cilindriskā kanālā virs granulēta biomasas slāņa pie aksiālas primārā gaisa padeves zem granulu slāņa un gaisa virpuļplūsmas padeves virs tā. Auksto virpuļplūsmu pētījumi apliecina, ka plūsmas dinamiku būtiski ietekmē divu savstarpēji konkurējošu un pretēji vērstu virpuļplūsmu veidošanās pie tangenciālās gaisa padeves sprauslas izejas. Lejupvērstā virpuļplūsma, kas