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Superconductivity and electronic properties of y-U alloys and their hydrides
Tkach, Ilya ; Havela, Ladislav (advisor) ; Mihálik, Marián (referee) ; Schneeweiss, Oldřich (referee)
Title: Superconductivity and electronic properties of γ-U alloys and their hydrides. Author: Ilya Tkach Department / Institute: Department of Condensed Matter Physics, Faculty of Mathematics and Physics, Charles University Supervisor of the doctoral thesis: Doc. RNDr. Ladislav Havela, CSc., Department of Condensed Matter Physics, Faculty of Mathematics and Physics, Charles University, Prague, The Czech Republic Abstract: Low-temperature electronic properties for U-Mo and U-Zr splats stabilized in γ-U were investigated. Magnetic measurements revealed Pauli paramagnetic behavior with temperature independent susceptibility for U-Mo alloys. U-Mo and U- Zr splats become superconducting at low temperatures with Tc up to 2.1 K and critical field 5-6 T. The superconductivity of -U can be considered as a bulk effect and can be described by the BCS theory, while -U superconductivity is not a real bulk effect. U-Mo and U-Zr alloys absorb hydrogen at high pressures (p ≥ 4.5 bar) and form hydrides with stoichiometry analogous to UH3. The hydrides with Mo have an amorphous structure based on β-UH3 phase, while hydrides with Zr have a crystalline structure of the -UH3 type. (UH3)1-xMox hydrides are ferromagnetic with enhanced TC up to 202 K and magnetic moments 1.1 μB/U in comparison with pure β-UH3 (175 K; 0.87 μB/U)....
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Kooperativní jevy v cerových sloučeninách na hranici magnetismu
Moudřík, Jan ; Prokleška, Jan (advisor) ; Javorský, Pavel (referee)
This work reports on physical properties of a novel CeCo0.715Si2.285 compound. The compound crystallizes in the I-4m2 space group structure with extremely elongated unit cell (a = 4.13˚A , c = 32.84˚A) containing BaAl4 structural patterns. In zero magnetic field it orders antiferromagnetically at TN = 10.0K. Under application of magnetic field along the c-axis it manifests numerous magnetic transitions in small fields (B < 0.5T), resembling the so-called 'devil's staircase' behaviour (e.g. CeSb, CeCoGe3). The magnetization is almost constant from 1T up to 45T for H||c but considerably reduced (0.3µB/Ce) with respect to the free Ce3+ ion. For fields applied along the a-axis typical behaviour for a hard axis in a material with uniaxial anisotropy is observed. The performed single crystal neutron diffraction experiment did not allow complete determination of magnetic structure. 1
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Supravodivost a magnetismus uranových sloučenin
Vališka, Michal ; Pospíšil, Jiří (advisor) ; Custers, Jeroen (referee)
Cílem této práce je studium systému UCo1−xRuxGe nacházejícího se na rozhraní magnetické nestability. Podrobná studie magnetických a transportních vlastností po- lykrystal· s r·zným obsahem ruthenia odhalila komplexní změnu těchto parametr· v rámci celé série. UCoGe je slabý ferromagnet s TC ∼ 3 K a malým magnetickým momentem 0.035 µB/f.u. Pozorovali jsme strmý nár·st těchto hodnot až po hodnotu koncentrace ruthenia xmax ≈ 0.1(TC,max = 9 K a µ0 = 0.11 µB). Podstata tohoto ná- r·stu byla studována na mono krystalu o složení UCo0.88Ru0.12Ge získaném metodou plovoucí zóny. Difrakce polarizovaných neutron· provedená na tomto krystalu se slo- žením UCo0.88Ru0.12Ge vysvětlila posílení magnetismu změnou vzájemné orientace magnetických moment· na iontu uranu a kobaltu. Antiparalelní uspořádání, které bylo dříve publikováno pro UCoGe je změněno na paralelní uspořádání v dopova- ném UCo0.88Ru0.12Ge kdy dojde k reorientaci momentu na kobaltu. Silná anizotropie UCo0.88Ru0.12Ge se promítá do teplotní závislosti odporu a teplotní roztažnosti, které se dramaticky liší pro všechny tři osy. Tento prvotní nár·st TC a magnetického mo- mentu je následován poklesem směřujícím ke koncentraci xcr ≈ 0.31 kde magnetické uspořádání mizí. Podrobná studie kritických exponent· teplotních závislostí elek- trického odporu,...
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Studium magnetických a termodynamických vlastností ternárních sloučenin se silně korelovanými 4f elektrony.
Kratochvílová, Marie
We have studied two different systems of compounds, YPd2Al3 and CenTmIn3n+2m. Polycrystalline YPd2Al3, a new member of the REPd2Al3 system, was prepared by arc melting. The X-ray diffraction confirmed that YPd2Al3 crystallizes in the hexagonal PrNi2Al3-type structure as the other REPd2Al3 compounds. Magnetization, AC susceptibility, specific heat and resistivity measurements revealed superconductivity below Ts ≈ 2.2 K. The second part of the work was focused on studies of Ce2PdIn8 and Pd-doped CenRhIn3n+2. Single crystals of Ce2Rh1-xPdxIn8 with x = 0, 0.10, 0.15, 0.30, 0.45, 0.85, 1 and CeRh1-xPdxIn5 with x = 0, 0.1, 0.25 were prepared by solution growth method. The quality of crystals was confirmed by microprobe analysis and X-ray diffraction. The effect of Pd doping on magnetism of CenRh1-xPdxIn3n+2 was studied by specific heat, magnetization and resistivity measurements. The Pd doping gradually suppresses the Néel temperature in both systems, however the effect is stronger in Ce2Rh1-xPdxIn8. Temperature dependence of resistivity of CeRh0.75Pd0.25In5 was studied in pressure up to 2.2 GPa. Similar to CeRhIn5, the antiferromagnetism is gradually suppressed by the applied pressure, while superconductivity is induced and coexists with antiferromagnetism.
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Anizotropic magnetic properties of the tetragonal R2TIn8 compounds
Pajskr, Karel ; Javorský, Pavel (advisor) ; Svoboda, Pavel (referee)
This thesis deals with termodynamic and magnetic properities of the intermetalic compound Er2RhIn8. This compound is a member of the wide intermetalics family of RX3, RTX5 and R2TX8, crystallizing in related structures and having non-trivial magnetic properities and phase diagrams, including the Ce-based heavy-fermion superconductivity. The thesis is a part of the broader research of these compounds. Single Er2RhIn8 crystals were prepared from flux with their stoichiometry and structure verified by energy-dispersive X-ray spectrometry and Laue method, respectively. Heat capacity and magnetization measurements were performed on the produced crystals and the measured data were analyzed. As a result, the compound was classified as antiferromagnetic under the order temperature of 3,8(1) K with an easy magnetization axis of [110] and relatively low magnetocrystalline anisotropy. Heat capacity measurements indicate a relatively small splitting of the energy levels of the rare-earth ion ground state due to the crystal field.
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Properties of superconductors in the terahertz frequency region
Šindler, Michal ; Skrbek, Ladislav (advisor) ; Lipavský, Pavel (referee) ; Samuely, Peter (referee)
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Nerovnovážná supravodivost
Hopjan, Miroslav ; Lipavský, Pavel (advisor) ; Morawetz, Klaus (referee)
In the present thesis we study superconductivity using approaches based on the two-particle T-matrix. With the multiple scattering corrections the Galitskii- Feynman ladder T-matrix approximation becomes applicable to the supercon- ducting state. This theory describes the superconducting and normal states within the same approximation. In this thesis, the original equilibrium theory is generalized to nonequilibrium systems using the generalized Kadanoff-Baym formalizm. The obtained theory of nonequilibrium superconductors is suitable for bulk systems where the momentum is a good quantum number. We have reformulated the theory for nanosystems, where the momentum is no longer a good quantum number. The modification was aimed at nanospheres, where one can benefit from the expansion in eigenstates of the angular momentum. High degeneracy of energy levels leads to high critical temperatures of sheres with a magical number of electrons, which makes them good candidates for observa- tion of phenomena beyond the weak coupling limit. As a suitable experimental technique we discuss the tunneling spectroscopy. 1
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Vliv struktury na magnetické vlastnosti vzácně zeminných intermetalických sloučenin.
Vališka, Michal ; Pospíšil, Jiří (advisor) ; Javorský, Pavel (referee)
Náplní této práce je zkoumání vlivu symetrie na magnetické vlastnosti a supravodi- vost vzácně zeminných intermetalických sloučenin. Byly úspěšně připraveny polykrys- talické vzorky sloučenin YIr2Si2, LaIr2Si2 a SmIr2Si2 a to v podobě vysokoteplotní i nízkoteplotní fáze. Nízkoteplotní fáze byla připravena žíháním, jehož korektnost byla potvrzena pomocí DTA analýzy. Složení obou polymorfních fází bylo potvrzeno pomocí EDX analýzy a pomocí práškové rentgenové difrakce. V případě YIr2Si2 byly mřížové parametry získané difrakcí srovnány s teoretickými výpočty s velmi uspokojivým souhlasem. Z analýzy měrných tepel vyplývá, že vysokoteplotní fáze YIr2Si2 je supravodivá při TSC =2,50 K a LaIr2Si2 při TSC =1,49 K. Nebyly nalezeny žádné stopy po supravodivosti nízkoteplotních fází obou sloučenin až do teploty 0,35 K. Tato měření byla podpořena výsledky teplotní závislosti elektrického odporu. V případě sloučeniny SmIr2Si2 byly pomocí analýzy měrného tepla nalezeny magnet- ické přechody při teplotách 1,7 K, 6,2 K a 18,8 K pro vysokoteplotní fázi a při 1,9 K, 6,1 K a 38,9 K pro fázi nízkoteplotní. Dále byl shledán náznak přechodu při teplotě nižší než 0,5 K a to v obou fázích. M·že se jednat o další magnetický pře- chod nebo jaderný Schottkyho příspěvek. Magnetismus obou Sm fází je dán charak- teristickým...
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Electronic properties and structure of selected rare earth and uranium compounds: influence of impurities
Pospíšil, Jiří ; Sechovský, Vladimír (advisor) ; Schneeweiss, Oldřich (referee) ; Sedmidubský, David (referee)
Title: Electronic properties and structure of selected rare earth and uranium compounds; influence of impurities Author: Jiří Pospíšil Abstract: This thesis is devoted to studies of influence of impurities and/or chemical varieties on magnetic and superconducting properties of selected metallic materials containing rare earth and uranium. The research generally consisted of the defined preparation of studied materials, detailed composition and structure characterisation, experiments focussed on determination of mutually related magnetic, transport and thermal properties followed by data analysis of relations between material composition/quality and material properties. The technology phase was carried out in two ways - 1) preparation of the best pure materials as possible and 2) controlled doping of pure materials. Three main study cases have been chosen: SmPd2Al3 - the representative of the complex magnetism of the Sm3+ ion, the so far unexplored superconductor YPd2Al3 were selected as the RE materials candidates and the controversial ferromagnetic superconductor UCoGe. The key ingredient of the initial technology phase was the refining the commercially available best element metals to the laboratory best achievable purity grade by the unique Solid State Electrotransport (SSE) method. SSE has been also...
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Superconductivity in disordered systems
Šopík, Břetislav ; Lipavský, Pavel (advisor) ; Novotný, Tomáš (referee) ; Munzar, Dominik (referee)
Author: Mgr. Břetislav Šopík Thesis advisor: doc. Pavel Lipavský, CSc. Ph.D. thesis Superconductivity in disordered systems Abstract We study some aspects of the superconductivity in disordered systems - namely the superconductivity in a boron-doped diamond. We also apply theoretical methods originally developed in the field of the disordered systems to the theory of superconductivity. In the case of the boron-doped diamond we focus on the question of the dependence of the critical temperature Tc on boron doping. We discuss the impact of the boron distribution correlations on the Tc as well. First, we evaluate the density of states at the Fermi energy N0 within the dynamical cluster approximation. We discuss the Tc as a function of N0 within the BCS, the McMillan and the Belitz theory. In the case of 100 samples, the simplified Belitz theory gives the best agreement with experimental data. For 111 samples the McMillan theory is sufficient. We also show that the difference of 100 and 111 samples in the N0 dependence of Tc can be explained as given by attractive correlations in the boron distribution. Applying the concept of the coherent potential approximation, we re- move a self-interaction from the Galitskii-Feynman T-matrix approxima- tion. This correction has no effect in the normal state but makes the...
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