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1
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Study of Niobium Thin Films under Pressure

100%
Pristáš G., Gabáni S., Orendáč M., Komanický V., Gažo E.
Acta Physica Polonica A
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2014
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vol. 126
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issue 1
346-347
EN
Niobium is widely used in many important superconducting applications. At ambient pressure, bulk Nb has the highest critical temperature, T_c ≈ 9.25 K among the superconducting elements. Thin films of Nb show several differences in behavior in comparison with bulk Nb, e.g. substantial increase in the upper critical field (H_{C2}). Critical temperature of superconducting transition is usually lower for thin films than in bulk sample and depends on thickness of the film, size of grains etc. We prepared 100 nm thick niobium thin films in the high vacuum DC magnetron sputtering system, with T_c=8.95 K at ambient pressure. In this study, we performed measurements of superconducting transition temperature by electrical resistivity measurements of Nb thin film under hydrostatic pressure of up to 30 kbar. We observed an increase of T_c with increasing value of pressure (dT_c/dp=7.3 mK/kbar). On the other side in the case of bulk sample of Nb we observed a decrease of T_c value (dT_c/dp=-2.5 mK/kbar) with increasing applied pressure. Difference in superconducting properties between niobium bulk and thin film under pressure is discussed.
2
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Influence of Pressure on the Electron-Phonon Interaction in Superconductors

86%
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.
3
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Point-Contact Spectroscopy of Crystalline Electric Field of Heterocontacts PrB_6 and NdB_6 with Pt

86%
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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Magnetic Phase Diagram of TmB_{4} under High Pressure

86%
Gabáni S., Pristáš G., Takáčová I., Flachbart K., Filipov V., Shitsevalova N., Siemensmeyer K.
Acta Physica Polonica A
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2014
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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.
5
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Influence of Pressure on the Electrical Transport Properties of Carbon-Doped EuB₆

73%
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.
6
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Influence of Pressure on Superconductivity in YB_{6}

73%
Gabáni S., Pristáš G., Takáčová I., Flachbart K., Gažo E., Mori T., Braithwaite D., Samuely P.
Acta Physica Polonica A
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2014
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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.
7
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^{11}B-NMR Study of SmB_6 under Pressure

73%
Pristáš G., Mito T., Kohara T., Gabáni S., Reiffers M., Flachbart K., Takeshita N., Shitsevalova N.
Acta Physica Polonica A
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2010
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vol. 118
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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.
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