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1
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Magnetic Ordering in Boron-Rich Borides TbB_{66} and GdB_{66}

100%
Flachbart K., Gabani S., Mori T., Siemensmeyer K.
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issue 5
875-876
EN
Magnetic ordering in TbB_{66} and GdB_{66} was investigated at very low temperatures. Measurements of ac susceptibility have shown rather clear features of magnetic ordering below 1 K, at 0.34 K for TbB_{66} and at 0.20 K for GdB_{66}. However, no clear evidence of long range magnetic order was found by neutron scattering experiments. Reasons leading to these observations are discussed.
2
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Microstructural Analysis and Transport Properties of RuO_2-Based Thick Film Resistors

88%
Gabáni S., Flachbart K., Pavlík V., Pietriková A., Gabániová M.
Acta Physica Polonica A
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2008
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vol. 113
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issue 1
625-628
EN
RuO_2-based low temperature sensors appear as very good secondary thermometers, mainly in the temperature range below 4.2 K. This is due to their high temperature sensitivity and small magnetoresistance. Both properties are strongly influenced by the manufacturing process (mainly by firing temperature and firing time). In our contribution we show that the microstructure of sensors and the temperature dependence of their resistance R(T) down to 50 mK, in case when all sensors are prepared from the same paste, can be strongly influenced by change of the firing temperature from 800°C to 900°C. The paper also presents results on the X-ray microanalysis and the analysis of electrical conductivity of these sensors.
3
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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.
4
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Low Temperature Transport and Magnetic Properties of SmB_{6}

76%
Gabani S., Flachbart K., Pavlík V., Konovalova E., Paderno Y., Herrmannsdörfer T.
Acta Physica Polonica A
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2000
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vol. 97
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issue 3
419-422
EN
We present results of transport and magnetic properties of three single-crystalline samples of the intermediate valence small-gap semiconductor SmB_{6} at low temperatures. The received resistivity dependences of the samples below 0.5 K exhibit an activated behavior with an energy gap of a few mK. The temperature dependences of the magnetic susceptibility show an increase below 15 K which can be accounted for by impurities, by bare Sm^{3+} ions or by a small amount of in-gap magnetic 4f^{5}5d^{1} states.
5
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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.
6
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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.
7
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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.
8
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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.
9
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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.
10
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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.
11
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Phonon Drag and Magnetic Anomalies of Thermopower in RB_{12} (R = Ho, Er, Tm, Lu)

64%
Glushkov V., Demishev S., Ignatov M., Khayrullin E., Sluchanko N., Shitsevalov 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
275-278
EN
High precision measurements of the Seebeck coefficient S(T) were carried out on the single crystals of RB_{12} (R = Ho, Er, Tm, Lu) at temperatures 2-300 K. It was shown that the effects of phonon drag result from vibrations of rare earth ions (ħω_E≈10-33 meV) in the rigid framework structure of the B_{12} clusters and determine the main contribution to thermopower at intermediate temperatures (30-300 K). The correlated behavior of transport parameters favors the appreciable enhancement of spin fluctuations in the sequence of magnetic compounds (HoB_{12}-TmB_{12}) when approaching to the valence instability state in YbB_{12}. The giant increase in S(T) detected in the vicinity of the Néel temperature T_N for HoB_{12}, ErB_{12}, and TmB_{12} seems to result from the density of states renormalization caused by antiferromagnetic ordering.
12
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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}.
13
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Influence of Pressure on the Electrical Transport Properties of Carbon-Doped EuB₆

64%
Pristáš G., Gabáni S., Baťko I., Baťková M., Orendáč M., Gažo E., Filipov V., Konovalova E., Diko P., Flachbart K.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
982-984
EN
We have studied the influence of hydrostatic pressure on the electrical resistivity of carbon-doped semimetal EuB₆ which orders ferromagnetically at T_{C}=3.9 K and is intrinsically inhomogeneous due to fluctuations of carbon content. We observed a shift of the low-temperature resistivity maximum from 4.6 K (at 1 bar) to 5.2 K (at 30.3 kbar) with increasing pressure. However, the maximum of the derivative dρ/dT(T), which reveals the temperature of ferromagnetic ordering, does not change its position with increasing pressure. This behaviour is different from stoichiometric EuB₆, where pressure increases the ferromagnetic ordering temperature. The origin of this discrepancy may lie in the increase of volume fraction of the non-ferromagnetic phase with increase of pressure. Additional magnetoresistance measurements at various pressures between 1.5 K and 30 K have shown that with increase of magnetic field the resistivity is monotonically decreasing, and above 1 T a transition to a monotonic resistivity behaviour (dρ/dT(T)>0) is observed. Our results support the picture that carbon-rich regions play a role of "spacers", which prevent the percolation of the ferromagnetic phase.
14
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Influence of Pressure on Superconductivity in YB_{6}

64%
Gabáni S., Pristáš G., Takáčová I., Flachbart K., Gažo E., Mori T., Braithwaite D., Samuely P.
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vol. 126
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issue 1
340-341
EN
Magnetoresistivity measurements on a superconducting system of YB_{6} (T_c ≈7.5 K) down to 60 mK at hydrostatic pressures up to 47 kbar are presented. The superconducting transition temperature, as well as the third critical field H_{c3} reveal a linear decrease with increasing pressure with slopes of d ln H_{c3}/dp=-1.1 %/kbar, and d ln T_c/dp=-0.59 %/kbar. From the latter a critical pressure, p_c ≈ 170 kbar, at which T_c vanishes, is determined.
15
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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.
16
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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.
17
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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.
18
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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.
19
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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
20
Content available remote

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.
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