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1
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Tunneling Spectroscopy Studies of SmB_6 and YbB_{12}

100%
Baťková M., Baťko I., Konovalova E., Shitsevalova N.
Acta Physica Polonica A
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2008
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vol. 113
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issue 1
255-258
EN
Reported electron tunneling studies of SmB_6 and YbB_{12} in the temperature region with a strongly temperature activated transport reveal a qualitative change of tunneling regime upon cooling. While the differential conductance curves above 50 K resemble the tunneling between two different metals, the spectra observed at 4.2 K are typical of the tunneling between a metal and a (narrow-gap) valence fluctuating semiconductor.
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Magnetic Field Enhancement of the Hall Effect in Dilute Magnetic System La_{1 - x}Ce_{x}B_6 (x ≤ 0.1)

76%
Bogach A., Glushkov V., Demishev S., Sluchanko N., Shitsevalova N., Fillipov V., Flachbart K.
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issue 5
931-932
EN
Magnetic field dependences of the Hall coefficient R_{H} and resistivity have been studied in detail for dilute magnetic compounds La_{1-x}Ce_{x}B_6 (x ≤ 0.1) at temperatures 1.8-300 K. It was established that the regime of weak localization of charge carriers, which was observed in these heavy fermion systems below 30 K destroys gradually in magnetic field up to 8 T. Moreover, in addition to the strong negative magnetoresistance (Δρ/ρ ≈ 80%) a drastic enhancement of the negative Hall coefficient in magnetic field (Δ R_{H}/R_{H} ≈ 50%) has been deduced at liquid helium temperatures. The results of comprehensive analysis contradict the predictions of Kondo-impurity approach for this archetypal strongly correlated electron system in the dilute impurity limit. An alternative interpretation of La_{1-x}Ce_{x}B_6 properties is developed on the basis of spin-polaron approach, Pauli paramagnetism and the density of states renormalization effects at low temperatures.
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Point-Contact Spectroscopy of Crystalline Electric Field of Heterocontacts PrB_6 and NdB_6 with Pt

76%
Vasil'ová M., Pristáš G., Reiffers M., Flachbart K., Rusz J., Shitsevalova N.
Acta Physica Polonica A
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2008
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vol. 113
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issue 1
267-270
EN
Point-contact spectra of single crystals PrB_6 and NdB_6 are presented. We observed maxima connected with crystalline electric field excitations in agreement with previous results for applied voltage V>8 mV. Moreover, we observed maxima at 6 mV (PrB_6) and 3 mV (NdB_6), which are probably connected with phonon modes. Moreover, we observed phonon peak at 10.6 mV in PrB_6. Further measurements in magnetic fields and crystalline electric field calculations are necessary.
4
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Investigation of Mixed Valence State of Sm_{1-x}B_{6} and Sm_{1-x}La_{x}B_{6} by XANES

76%
Gabáni S., Flachbart K., Bednarčík J., Welter E., Filipov V., Shitsevalova N.
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vol. 126
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issue 1
338-339
EN
We investigated the valence states of samarium ions in Sm deficient Sm_{1-x}B_{6} (x=0.03, 0.05, 0.08, 0.1 and 0.2) sintered samples, in single crystalline solid solutions Sm_{1-x}La_{x}B_{6} (x=0.16, 0.28, 0.4, 0.55 and 0.7) as well as in a SmB_{6} single crystal by X-ray absorption near edge structure (XANES) between 4.2 and 300 K. It was shown that the mixed valence state v_{Sm} of Sm-ions (v_{Sm} ≈ 2.51 for SmB_{6} at 4.2 K) in deficient samples is changed but maintained up to a concentration of 20% of vacancies and up to a concentration of 70% in samples doped with trivalent La-ions, and that in both cases it increases with temperature. On the other hand, the value of v_{Sm} increases with increasing vacancy level, whereas it decreases with increasing La^{3+} substitution. The obtained dependences of Sm-valence in SmB_{6} on vacancy as well as on La^{3+} ion concentration are in good agreement with susceptibility measurements and exact numerical calculation of the spinless Falicov-Kimball model in two dimensions.
5
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Phase Diagram of TmB_4 Probed by AC Calorimetry

76%
Kačmarčík J., Pribulová Z., Gabáni S., Samuely P., Siemensmeyer K., Shitsevalova N., Flachbart K.
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issue 5
903-904
EN
TmB_4 is a frustrated system based on the Shastry-Sutherland lattice that exhibits complex magnetic properties. In this contribution the magnetic field B vs. temperature T phase diagram of TmB_4 has been studied by ultrasensitive AC calorimetry in the temperature range between 2.9 and 12 K and in magnetic fields up to 8 T. Apart from already known phases our measurements have recognized several new phase transitions suggesting that the phase diagram of TmB_4 is even more complex and deserves further studies.
6
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Magnetic Phase Diagram of TmB_{4} under High Pressure

76%
Gabáni S., Pristáš G., Takáčová I., Flachbart K., Filipov V., Shitsevalova N., Siemensmeyer K.
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vol. 126
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issue 1
356-357
EN
TmB_{4} is a Shastry-Sutherland frustrated system which exhibits very complex magnetic properties. In this contribution the phase diagram of magnetic field vs. temperature of TmB_{4} under hydrostatic pressure up to 26.5 kbar is investigated using sensitive ac-resistance measurements. Temperature and magnetic field dependences of resistance at various pressures were carried out in a piston cylinder pressure cell between 1.7 and 14 K and in magnetic fields up to 6 T. The obtained results exhibit shifts of ordering temperatures T_{N} as well as shifts of boundaries between different magnetic phases. The observed pressure dependences of T_{N} can be described by the relation d lnT_{N}/dp=+(0.16÷0.18) %/kbar. The effect of pressure on various interactions between magnetic ions in this compound is discussed.
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Influence of Pressure on the Electron-Phonon Interaction in Superconductors

76%
Orendáč M., Gabáni S., Pristáš G., Gažo E., Shitsevalova N., Flachbart K.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
1039-1041
EN
The electron-phonon interaction is a very important and ubiquitous process in solids, affecting almost all their physical properties. In metals, where the relaxation processes depend on both electrons and phonons, all thermodynamic and transport properties are dictated by the electron-phonon interaction. A very strong manifestation of the electron-phonon interaction is related with the superconducting state. Here we report the effect of high pressure on the transport electron-phonon interaction, λ_{tr}, in superconducting systems such as YB_6 (T_c ≈ 7.5 K), Pb (T_c ≈7.2 K), Nb bulk or thin film (T_c ≈9.2 K), and in LaB_6 in which superconductivity was not yet observed. The expected pressure effect should correspond to the theoretically predicted negative pressure effect on T_c (except for Nb thin film and LaB_6). To determine the influence of pressure on λ_{tr}, we utilized the Bloch-Grüneisen fit (denoted also as "thermal spectroscopy") of the precise temperature dependence of resistivity measurements in the normal state up to a pressure of 2.8 GPa. Based on this fit the observed negative pressure effect on λ_{tr} values, dλ_{tr}/dp, are as follows: dλ_{tr}/dp ≈-0.045 GPa^{-1} for YB_6, dλ_{tr}/dp ≈-0.13 GPa^{-1} for Pb, dλ_{tr}/dp ≈-0.019 GPa^{-1} or -0.028 GPa^{-1} for Nb bulk or thin film, respectively, and dλ_{tr}/dp ≈-0.003 GPa^{-1} for LaB_6.
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Magnetocaloric Effect in Geometrically Frustrated Magnetic Compound HoB_{12}

76%
Snyman J., Flachbart K., Gabani S., Reiffers M., Shitsevalova N., Siemensmeyer K., Strydom A.
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issue 5
873-874
EN
The magnetocaloric effect of frustrated antiferromagnetic HoB_{12} is calculated. The isothermal entropy change Δ S characterising the magnetocaloric effect shows a small positive change upon magnetisation below the transition temperature T_{N}, indicating a small inverse magnetocaloric effect. At T_{N}, Δ S shows clear scaling behaviour with the applied magnetic field. The adiabatic temperature change Δ T shows paramagnetic behaviour above T_{N}, despite strong magnetic correlations that persist in this temperature region. The adiabatic temperature change calculated for HoB_{12} is appreciable.
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Reentrant Metal-Insulator Transition in Ca_{1-x}Eu_{x}B_{6}

64%
Glushkov V., Anisimov M., Baybakov R., Demishev S., Filippov V., Flachbart K., Kuznetsov A., Shitsevalova N., Sluchanko N.
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vol. 126
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issue 1
294-295
EN
Resistivity, Hall and Seebeck effects have been studied on single crystals of Ca_{1-x}Eu_{x}B_{6} (0 ≤ x ≤ 1) at temperatures 2-300 K and in magnetic fields up to 8 T. An insulating ground state is found to be limited by narrow range of Eu doping 0.6 ≤ x ≤ 0.8. This region is characterized by an enhanced colossal magnetoresistance (CMR), which reaches values of ρ(0)/ρ(7T) > 10^{6} for x = 0.63 at T < 10 K. Decreasing of Eu content in Ca_{1-x}Eu_{x}B_{6} below x* ≈ 0.6 restores the metallic ground state with moderate resistivity (ρ ~ 1 ÷ 5 mΩ·cm) and CMR amplitude (ρ(0)/ρ(7T) < 7). The second metal-insulator transition (MIT) in Ca_{1-x}Eu_{x}B_{6} is observed beyond the whole conductivity region found earlier in the narrow range of Eu doping (0.7 ≤ x ≤ 0.8). The correlation between the enhanced CMR amplitude, the onset of positive diffusive thermopower and the elevation of anomalous Hall effect, determined for Eu content 0.6 ≤ x ≤ 0.85, favors the idea that a smooth change of band structure is the main factor governing the reentrant MIT in Ca_{1-x}Eu_{x}B_{6}.
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Anomalous Transport Properties of Carbon-Doped EuB_6

64%
Batkova M., Batko I., Filipov V., Flachbart K., Sechovský V., Shitsevalova N., Šantavá E., Šebek J.
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issue 5
893-894
EN
In the presented work we report electrical, magnetic and thermal properties of EuB_{6-x}C_{x} single crystals with an estimated value of x ≈ 0.07. Our studies reveal an antiferromagnetic phase transition at T_{N} ≈ 6.7 K. Electrical resistivity at zero magnetic field shows a pronounced resistivity maximum at T_{M} ≈ 7 K, just above the antiferromagnetic phase transition temperature. With increasing applied magnetic field the maximum moves to lower temperature and becomes totally suppressed at the field of 9 T. Observed magnetoresistance is negative in the whole studied temperature range 2-20 K, yielding a ratio of ρ(0 T, 7 K)/ρ(9 T, 7 K) ≈ 2.5. The origin of such magnetoresistance is associated with formation of mixed magnetic structure in the system due to fluctuation of carbon concentration.
11
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^{11}B-NMR Study of SmB_6 under Pressure

64%
Pristáš G., Mito T., Kohara T., Gabáni S., Reiffers M., Flachbart K., Takeshita N., Shitsevalova N.
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issue 5
895-896
EN
We present first experimental results of ^{11}B-NMR of SmB_6 under applied pressure. From measurement of nuclear spin-lattice relaxation time (T_1) we find that with applied pressure the value of activation gap E_{g} is decreasing. This decrease is larger than in case of other experimental techniques. We suppose that the enhancement of 1/T_1 in temperature range 20-100 K with applied pressure reflects not only a suppression of hybridization gap, but also changes in spin correlations.
12
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Anomalies of Heat Capacity and Phase Transitions in Tm_{1-x}Yb_{x}B_{12}

64%
Sluchanko N., Azarevich A., Bogach A., Glushkov V., Demishev S., Gavrilkin S., Shitsevalova N., Fillipov V., Gabani S., Flachbart K.
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issue 5
929-930
EN
In the system Tm_{1 - x}Yb_{x}B_{12} the specific heat has been studied in a wide range of Yb-concentration in the vicinity of the quantum critical point x_{C} ≈ 0.3. The results were obtained on high quality single crystalline samples of Tm_{0.7}Yb_{0.3}B_{12} compound placed near quantum critical point, both for antiferromagnetic metals (x < x_{C}) as well as for paramagnetic insulators (x > x_{C}) within a wide temperature range of 1.9-300 K in magnetic field up to 9 T. The temperature dependence of the magnetic contribution to specific heat for Tm_{0.74}Yb_{0.26}B_{12} shows a logarithmic divergence of the form C_{mag}/T∿-lnT at T<4 K, which may be attributed to the quantum critical regime, and it is suppressed by strong external magnetic field. The Schottky anomaly of the magnetic contribution to specific heat in Tm_{1 - x}Yb_{x}B_{12} has been established and analyzed in detail.
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Magnetic Field Influence on the Thermal Conductivity of PrB_6

64%
Priputen P., Reiffers M., Gabáni S., Flachbart K., Šantavá E., Šebek J., Alekseev P., Nefeodova E., Sadikov I., Shitsevalova N.
Acta Physica Polonica A
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2008
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vol. 113
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issue 1
383-386
EN
Results of thermal conductivity measurements on single crystalline PrB_6 sample in the temperature range of 2-30 K and in magnetic field up to 14 T are presented. The obtained results are discussed in order to estimate the electron, phonon, and magnetic contributions to thermal conductivity. Taking into account the results of electrical resistivity of this compound the temperature dependence of the reduced Lorentz function is determined.
14
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Angular Dependences of ESR Parameters in Antiferroquadrupolar Phase of CeB₆

64%
Semeno A., Gilmanov M., Sluchanko N., Krasnorussky V., Shitsevalova N., Filipov V., Flachbart K., Demishev S.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
1060-1062
EN
Angular dependences of ESR line parameters (g-factor and linewidth Δ H) were experimentally explored in the antiferroquadrupolar phase of heavy fermion system CeB₆ at T=1.8 K. The data were obtained in two experimental geometries with different mutual directions of the wavevector k and the external magnetic field H at frequency of f=60 GHz. A g-factor anisotropy was found: while g-factors for [110] and [111] directions are close to each other (g ≈1.6), it is considerably higher (g ≈1.75) for [100]. The obtained angular dependence g(Θ) was compared with the theoretically predicted g-factor behavior for the Γp₈ state of Ce³⁺ ion in antiferroquadrupolar phase of CeB₆. It turns out that the experimental g-factor is considerably smaller at all angles than the theoretically calculated limits (2
15
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Magnetic Structure and Phase Diagram of TmB_4

64%
Gabáni S., Maťaš S., Priputen P., Flachbart K., Siemensmeyer K., Wulf E., Evdokimova A., Shitsevalova N.
Acta Physica Polonica A
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2008
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vol. 113
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issue 1
227-230
EN
Magnetic structure of single crystalline TmB_4 has been studied by magnetization, magnetoresistivity, and specific heat measurements. A complex phase diagram with different antiferromagnetic phases was observed below T_{N1}=11.7 K. Besides the plateau at half-saturated magnetization (1/2 M_S), also plateaus at 1/9, 1/8 and 1/7 of M_S were observed as a function of applied magnetic field Bparallel c. From additional neutron scattering experiments on TmB_4, we suppose that these plateaus arise from a stripe structure which appears to be coherent domain boundaries between antiferromagnetic-ordered blocks of 7 or 9 lattice constants. The received results suggest that the frustration among the Tm^{3+} magnetic ions, which maps to a geometrically frustrated Shastry-Sutherland lattice, leads to a strong competition between antiferromagnetic and ferromagnetic order. Thus, stripe structures in intermediate field appear to be the best way to minimize the magnetostatic energy against other magnetic interactions among the Tm ions combined with very strong Ising anisotropy.
16
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Bulk and Local Magnetic Susceptibility of ErB_{12}

53%
Bogach A., Bogomolov L., Glushkov V., Demishev S., Sluchanko D., Sluchanko N., Shitsevalova N., Levchenko A., Filipov V., Flachbart K., Siemensmeyer K.
Acta Physica Polonica A
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2008
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vol. 113
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issue 1
271-274
EN
High precision measurements of magnetoresistance Δρ/ρ = f(T,H) and magnetization M(T,H) were carried out on single crystals of rare-earth dodecaboride ErB_{12} at temperatures in the interval 1.8-30 K in magnetic fields up to 70 kOe. The high accuracy of the experiments allowed us to perform numerical differentiation and analyze quantitatively the behavior of the derivative d(Δρ/ρ)/dH = f(T,H) and of the magnetic susceptibility χ(T,H) = dM/dH in paramagnetic and magnetically ordered (antiferromagnetic, T_N ≈ 6.7 K and T_M ≈ 5.85 K) phases of ErB_{12}. It was shown that negative magnetoresistance anomalies observed in present study in paramagnetic state of ErB_{12} may be consistently interpreted in the framework of a simple relation between resistivity and magnetization -Δρ/ρ ~ M^2.
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Defect Mode in LaB_{6}

53%
Anisimov M., Bogach A., Glushkov V., Demishev S., Samarin N., Gavrilkin S., Mitsen K., Shitsevalova N., Levchenko A., Filippov V., Gabani S., Flachbart K., Sluchanko N.
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vol. 126
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issue 1
350-351
EN
The specific heat of high quality La^{N}B_{6} (N=10, 11, natural) single crystals is investigated in a wide range of temperatures 2 - 300 K. The obtained data allow to estimate correctly (i) the electronic γ· T term of specific heat (γ ≈ 2.4 mJ/(mol·K^{2})), (ii) the contribution from quasilocal vibrating mode of La^{3+} ions (Θ_{E} ≈ 150 - 152 K), (iii) the Debye-type term from rigid boron cages (Θ_{D} ≈ 1160± 40 K). Our data also suggest an additional defect-mode component (iv) which may be attributed to a contribution of 1.5% boron vacancies in LaB_{6}. The obtained results may be interpreted in terms of formation of two level systems, which appear when La^{3+} ions are displaced from their centrosymmetric positions in the cavities of rigid boron cages, apart from randomly distributed boron vacancies in the LaB_{6} matrix.
18
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Hall Effect in GdB_{6}

53%
Anisimov M., Bogach A., Glushkov V., Demishev S., Samarin N., Shitsevalova N., Levchenko A., Filippov V., Kuznetsov A., Flachbart K., Sluchanko N.
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vol. 126
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issue 1
348-349
EN
The Hall effect of GdB_{6} has been studied on high quality single crystals in the temperature range 2-150 K and in magnetic field of 1 T. The obtained data allow to detect anomalies in the antiferromagnetic (AF) phase including (i) a drastic enhancement of negative Hall coefficient below T_{N1} ≈ 15.5 K and (ii) the appearance of an anomalous Hall effect at T_{N2} ≈ 4.7 K. Possible scenarios of the AF ground state formation are discussed.
19
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Charge Transport and Magnetism in Tm_{0.03}Yb_{0.97}B₁₂

53%
Glushkov V., Azarevich A., Anisimov M., Bogach A., Demishev S., Dukhnenko A., Filipov V., Flachbart K., Gabáni S., Gavrilkin S., Kondrin M., Shitsevalova N., Sluchanko N.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
985-987
EN
Transport and magnetic properties of polycrystalline Tm_{0.03}Yb_{0.97}B₁₂ samples were investigated at temperatures 1.8-300 K in magnetic fields up to 9 T. The activated behavior of resistivity, the Hall coefficient and thermopower is described in terms of a narrow gap ε_g ≈ 16.6 meV, which controls the charge transport in Tm_{0.03}Yb_{0.97}B₁₂ at T>40 K. The maximum of magnetic susceptibility found at 50 K is shown to be induced by a spin gap Δ ≈ 4.7 meV being close to the half of the spin fluctuation energy in YbB₁₂. Large diffusive thermopower S=AT, A=-29.1 μV/K² and the Pauli susceptibility χ₀ ≈ 7.2×10¯³ emu/mol found below 20 K seem to be associated with the many-body resonance, which corresponds to states with an enhanced effective mass m* ≈ 250m₀ (m₀ - free electron mass). The effective parameters of magnetic centers and the analysis of anomalies favor the nonequivalent states of substitute Tm ions.
20
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Magnetoresistance Anisotropy and Magnetic H-T Phase Diagram of Tm_{0.996}Yb_{0.004}B_{12}

53%
Sluchanko N., Azarevich A., Bogach A., Glushkov V., Demishev S., Gavrilkin S., Gabani S., Flachbart K., Shitsevalova N., Filipov V., Vanacken J., Moshchalkov V., Stankiewicz J.
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vol. 126
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issue 1
332-333
EN
The antiferromagnetic ground state has been studied by transverse magnetoresistance, heat capacity and magnetization measurements, which were carried out on high quality single crystals of Tm_{0.996}Yb_{0.004}B_{12} dodecaboride in strong magnetic fields at liquid helium temperatures. Both antiferromagnetic-paramagnetic (AF-P) and spin-orientation (AF1-AF2) phase transitions have been observed, and allowed to construct a complicated magnetic H-T phase diagram for this compound. Strong magnetoresistance anisotropy was found both in AF states (ρ(H||[110])/ρ(H||[111])~ 1.2 at H~ 20 kOe) and at the critical field of AF-P transition (H_{N}[100]/H_{N}[111]~ 1.25) in this magnetic metal with a simple fcc crystal structure.
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