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Critical state response in hard type II superconductors (comparison of transverse and longitudinal geometries)
Youssef, Ahmed Elmetwally ; Janů, Zdeněk (advisor) ; Koláček, Jan (referee) ; Gömöry, Fedor (referee)
In this work we have studied the Bean critical state model (BCSM) in hard type II superconductors. The study has been aimed towards the determination of the critical current density Jc and its temperature dependence using analyses of a temperature dependence of a magnetic susceptibility χac measured by a contactless method using SQUID magnetometer. We have compared the predictions of the BCSM for the four geometries: slab in a parallel field, long cylinder in an axial field, and disk and strip in a transverse field. The main interest of the present study is the χac of the superconducting thin films (disk and strip). We have measured the magnetic response of Nb thin films and second-generation high temperature superconductor YBa2Cu3Ox wire in a perpendicular AC field as a function of temperature as well as bulk NEG-123 sample in parallel magnetic field. We have presented a method of linking model and experimental susceptibility. The good agreement of experimental susceptibility with model of 2D disk allows noncontact estimation of the critical depinning current density and its temperature dependence. Harmonic analysis of the temperature dependence of the nonlinear χac gives excellent agreement with the susceptibility calculated on basis of 2D BCSM to disk.
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Low-field excitations in magnetite
Švindrych, Zdeněk ; Janů, Zdeněk (advisor) ; Novák, Pavel (referee) ; Svoboda, Pavel (referee)
We have performed detailed measurements of magnetic and dielectric properties of high quality magnetite (Fe3O4) single crystals in weak magnetic and electric fields. These measurements can reveal details of phase transitions and other features that are not yet fully understood. We focused not only at the Verwey transition - a well known phase transition taking place at about 120 K in stoichiometric samples - but we also explored and described new relaxation effects in magnetite at low temperatures. The low-temperature properties were also found to be exceptionally sensitive to sample quality, stoichiometry and homogeneity. The results presented in this thesis were acquired on sensitive non-commercial SQUID magnetometer complemented by four-probe dielectric spectroscopy and dc conductivity measurements.
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Non-metallic resistive low temperature thermometers
Duda, Daniel ; Rotter, Miloš (advisor) ; Janů, Zdeněk (referee)
temperature calibration has been carried out on series of resistors made of RuO2 of SMD type in the bath of normal and superfluid helium in temperatures at 1.4 - 4.2 K with reference thermometer of saturated vapor pressure. Measured temperature dependence of electrical resistance of resistors was analyzed using the exponential function and Chebychev's polynomials. It was established an optimum calibration for the use of resistors as thermometers.
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Low-field excitations in magnetite
Švindrych, Zdeněk ; Janů, Zdeněk (advisor) ; Novák, Pavel (referee) ; Svoboda, Pavel (referee)
We have performed detailed measurements of magnetic and dielectric properties of high quality magnetite (Fe3O4) single crystals in weak magnetic and electric fields. These measurements can reveal details of phase transitions and other features that are not yet fully understood. We focused not only at the Verwey transition - a well known phase transition taking place at about 120 K in stoichiometric samples - but we also explored and described new relaxation effects in magnetite at low temperatures. The low-temperature properties were also found to be exceptionally sensitive to sample quality, stoichiometry and homogeneity. The results presented in this thesis were acquired on sensitive non-commercial SQUID magnetometer complemented by four-probe dielectric spectroscopy and dc conductivity measurements.
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Critical state response in hard type II superconductors (comparison of transverse and longitudinal geometries)
Youssef, Ahmed Elmetwally ; Janů, Zdeněk (advisor) ; Koláček, Jan (referee) ; Gömöry, Fedor (referee)
In this work we have studied the Bean critical state model (BCSM) in hard type II superconductors. The study has been aimed towards the determination of the critical current density Jc and its temperature dependence using analyses of a temperature dependence of a magnetic susceptibility χac measured by a contactless method using SQUID magnetometer. We have compared the predictions of the BCSM for the four geometries: slab in a parallel field, long cylinder in an axial field, and disk and strip in a transverse field. The main interest of the present study is the χac of the superconducting thin films (disk and strip). We have measured the magnetic response of Nb thin films and second-generation high temperature superconductor YBa2Cu3Ox wire in a perpendicular AC field as a function of temperature as well as bulk NEG-123 sample in parallel magnetic field. We have presented a method of linking model and experimental susceptibility. The good agreement of experimental susceptibility with model of 2D disk allows noncontact estimation of the critical depinning current density and its temperature dependence. Harmonic analysis of the temperature dependence of the nonlinear χac gives excellent agreement with the susceptibility calculated on basis of 2D BCSM to disk.
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|
|
Non-metallic resistive low temperature thermometers
Duda, Daniel ; Rotter, Miloš (advisor) ; Janů, Zdeněk (referee)
temperature calibration has been carried out on series of resistors made of RuO2 of SMD type in the bath of normal and superfluid helium in temperatures at 1.4 - 4.2 K with reference thermometer of saturated vapor pressure. Measured temperature dependence of electrical resistance of resistors was analyzed using the exponential function and Chebychev's polynomials. It was established an optimum calibration for the use of resistors as thermometers.
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