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Nonequilibrium superconductivity
Hopjan, Miroslav
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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Superconductivity and Magnetism of Uranium Compounds
Vališka, Michal
This thesis studies the UCo1−xRuxGe system on the border of magnetic instabil- ity. Proper study of magnetic and transport properties of poly crystals with dif- ferent ruthenium content revealed complex change of these parameters through the series. UCoGe is a weak ferromagnet with TC ∼ 3 K and low magnetic moment 0.035 µB/f.u.. We observed rapid increase of these two quantities up to the con- centration of ruthenium of xmax ≈ 0.1. Background of this increase was studied on the single crystal grown by oating zone method. Polarized neutron diraction experiment performed on this single crystal with composition of UCo0.88Ru0.12Ge ex- plained strengthening of magnetism with a change of mutual orientation of magnetic moments on the uranium and cobalt site. Previously reported antiparallel alignment for the UCoGe is changed to the parallel orientation for the doped UCo0.88Ru0.12Ge when cobalt moment is reoriented. Strong anisotropy of the UCo0.88Ru0.12Ge projects to the temperature dependence of resistivity and to the thermal expansion proper- ties. This primal increase of TC and magnetic moment is followed by decrease when approaching the concentration xcr ≈ 0.31 where magnetic order disappears. Closer study of the critical exponents of temperature dependencies of resistivity, specic heat and the...
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Elektronové vlastnosti sloučenin RPd5Al2
Zubáč, Jan ; Javorský, Pavel (advisor)
We have studied magnetic properties of the intermetallic NdPd5Al2 com- pound by means of specific heat and magnetization measurements and neutron scattering. The compound crystallizes in the tetragonal I4/mmm space group with lattice parameters a =4.147 ˚A and c =14.865 ˚A, orders antiferromagnet- ically below TN =1.3 K and presents large magnetocrystalline anisotropy due to the crystal-field effects. The obtained magnetic phase diagram is charac- terized by two distinct magnetically ordered phases similarly to structurally re- lated tetragonal RTX5 and R2TX8 compounds. The zero-field antiferromagnetic phase is characterized by the propagation vector k = (1 2 00) and antiferromag- netic coupling of Nd moments along the teragonal c-axis with the amplitude of magnetic moments of 2.22 µB/Nd as was revealed by neutron diffraction. The transition from the paramagnetic to magnetically ordered in zero field is the first-order phase transition. The CF excitations in NdPd5Al2 were detected by means of INS at 3.0 meV, 7.4 meV, 8.6 meV and 17.1 meV. We further compare our findings about CF in NdPd5Al2 obtained from INS, susceptibility analysis and first-principles calculations and confront them with the experimental mag- netization and magnetic specific heat data. Our results will be also discussed with respect to related...
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Pressure effects on physical properties of a selected Ce-based compound
Staško, Daniel ; Klicpera, Milan (advisor) ; Míšek, Martin (referee)
Presented thesis focuses on the study of transport properties of two intermetallic compounds, CeRhSi3 and CeCuAl3, under high external pressure. High-pressure dependence of previously reported pressure induced superconductivity in CeRhSi3 is studied closely up to 4.6 GPa, focusing on the, so-far-unstudied, pressure region above 3 GPa. After reaching maximal value at 2.9 GPa, the critical temperature of superconducting transition is gradually suppressed by further pressure application. Extensive search for possible pressure induced superconductivity in CeCuAl3 is conducted, employing solid-pressure-medium Bridgman anvil cells with theoretical pressure limit of 12 GPa. No pressure induced superconductivity is observed up to 4.5 GPa and down to 8 mK. Additionally, a calibration study of three liquid pressure exchange media from Daphne Oil 7000 series is carried out in order to advance high pressure experimental techniques. Comprehensive results about hydrostaticity, solidification, pressure drop and compressibility of individual pressure media are presented.
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Superconductivity in the fourth state of matter
Mádrová, Tereza ; Klíma, Miloš (referee) ; Fiala, Pavel (advisor)
This master thesis is focused on description and research of electromagnetic waves in plasma state. Some simple cases of no-loss mode distribution of electromagnetic waves in plasma are known, for example around the coaxial cable. In other words, the properties of superconducting state in materials are shown. This view of plasma properties can significantly help to clarify observed phenomena in experimental plasma research or to outline new trends in plasma usage in this mode. The introduction and the theoretical parts describe the known theory related to the propagation of electromagnetic waves in a non-homogeneous or anisotropic environment, in the next parts of the thesis the mathematical model of the solved problem of wave propagation in plasma is mentioned and the design and realization of the experiment is followed. The model includes processing of Maxwell's reduced equations in Haeviside notation for the state of matter referred to as plasma, the design of a model for numerical modeling and analysis, and the design of a simple experiment to verify numerically analyzed results.
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Microstructures fabrication for transport properties measurements of heavy fermion compounds
Volný, Jiří ; Uhlířová, Klára (advisor) ; Prchal, Jiří (referee)
In this thesis, we focus on preparation of micro-devices for resistivity measure- ment of CeCoIn5 using focused ion beam technique. Single crystal samples of ∼ 50 µm length were carved out by focused ion beam and transported on silicon sub- strates with prepared gold structures for electrical resistivity measurement. The samples were conductivily attached to gold contact pads by focused ion beam in- duced platinum deposition. Several steps and techniques were optimized: electron litography, golden pads electrical resitivity measurement and platinum structures electrical resitivity measurement and microstructure transport. Electrical resisti- vity measurement of CeCoIn5 single crystal showed superconductive transition at Tc = 2.27 K and H-T phase diagram was created from measurement in magnetic fields. The result obtained from bulk sample are in agreement with literature. On the other hand measurement of micro-devices showed no superconducting transition, suggesting unsufficient quality of samples. 1
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Elektronové vlastnosti sloučenin RPd5Al2
Zubáč, Jan ; Javorský, Pavel (advisor)
We have studied magnetic properties of the intermetallic NdPd5Al2 com- pound by means of specific heat and magnetization measurements and neutron scattering. The compound crystallizes in the tetragonal I4/mmm space group with lattice parameters a =4.147 ˚A and c =14.865 ˚A, orders antiferromagnet- ically below TN =1.3 K and presents large magnetocrystalline anisotropy due to the crystal-field effects. The obtained magnetic phase diagram is charac- terized by two distinct magnetically ordered phases similarly to structurally re- lated tetragonal RTX5 and R2TX8 compounds. The zero-field antiferromagnetic phase is characterized by the propagation vector k = (1 2 00) and antiferromag- netic coupling of Nd moments along the teragonal c-axis with the amplitude of magnetic moments of 2.22 µB/Nd as was revealed by neutron diffraction. The transition from the paramagnetic to magnetically ordered in zero field is the first-order phase transition. The CF excitations in NdPd5Al2 were detected by means of INS at 3.0 meV, 7.4 meV, 8.6 meV and 17.1 meV. We further compare our findings about CF in NdPd5Al2 obtained from INS, susceptibility analysis and first-principles calculations and confront them with the experimental mag- netization and magnetic specific heat data. Our results will be also discussed with respect to related...
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Practical applications of superconductivity
Mádrová, Tereza ; Langerová, Petra (referee) ; Baumgartnerová, Alena (advisor)
Předložená bakalářská práce je zaměřena na zpracování souhrnného přehledu využití supravodivosti v technických řešeních a aplikacích, čímž se rozumí nikoliv experimentální využití, nýbrž využití v reálném technickém řešení. Tato práce se věnuje zejména popisu, v první části základním atributům supravodivosti, bez nichž není možné princip supravodivosti pochopit a jsou tudíž nezbytnou součástí práce, ve druhé části rozdělení do kategorií supravodičů podle určitých kritérií, a v poslední části popisuje příklady aplikací supravodivosti tak, jak se s nimi můžeme setkat v každodenním životě.
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Elektronové vlastnosti sloučenin RPd5Al2
Zubáč, Jan ; Javorský, Pavel (advisor) ; Čermák, Petr (referee)
We have studied magnetic properties of the intermetallic NdPd5Al2 com- pound by means of specific heat and magnetization measurements and neutron scattering. The compound crystallizes in the tetragonal I4/mmm space group with lattice parameters a =4.147 ˚A and c =14.865 ˚A, orders antiferromagnet- ically below TN =1.3 K and presents large magnetocrystalline anisotropy due to the crystal-field effects. The obtained magnetic phase diagram is charac- terized by two distinct magnetically ordered phases similarly to structurally re- lated tetragonal RTX5 and R2TX8 compounds. The zero-field antiferromagnetic phase is characterized by the propagation vector k = (1 2 00) and antiferromag- netic coupling of Nd moments along the teragonal c-axis with the amplitude of magnetic moments of 2.22 µB/Nd as was revealed by neutron diffraction. The transition from the paramagnetic to magnetically ordered in zero field is the first-order phase transition. The CF excitations in NdPd5Al2 were detected by means of INS at 3.0 meV, 7.4 meV, 8.6 meV and 17.1 meV. We further compare our findings about CF in NdPd5Al2 obtained from INS, susceptibility analysis and first-principles calculations and confront them with the experimental mag- netization and magnetic specific heat data. Our results will be also discussed with respect to related...
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Nonequilibrium superconductivity
Hopjan, Miroslav
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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