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Numerical optimization of solid-state NMR experiments
Šmelko, Andrej ; Tošner, Zdeněk (advisor) ; Chlan, Vojtěch (referee)
Numerical optimizations of solid-state nuclear magnetic resonance experiments are one of the possible solutions to its low sensitivity. Using the Average Hamiltonian Theory, the negative effects of the radiofrequency field inhomogeneity on magnetization transfer were investigated. To overcome these effects, the Average Hamiltonian Theory was employed in the optimization of commonly used techniques such as Ramp-CP and Adiabatic-CP, optimization of pulses with arbitrarily modulated amplitude and new periodic pulses with arbitrarily modulated amplitudes and phases. Both types of shaped pulses achieved significantly higher efficiencies than Ramp- CP and Adiabatic-CP. The highest efficiency enhancement was calculated at high spinning frequencies and low dipolar coupling constants where the effect of the inhomogeneity is the strongest.
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Spectroscopic studies of new generation of optical and magneto-optical materials
Butková, Eva ; Veis, Martin (advisor) ; Chlan, Vojtěch (referee) ; Zemen, Jan (referee)
Title: Spectroscopic studies of new generation of optical and magneto-optical materials Author: Eva Butková Department / Institute: Institute of Physics of Charles University. Department of Materials Science and Technology of Nagaoka University of Technology. Supervisors of the doctoral thesis: RNDr. Martin Veis, Ph.D., Division of Magneto-optics, Institute of Physics of Charles University Dr. Takayuki Ishibashi, Ph.D., Department of Materials Science and Technology, Nagaoka University of Technology Abstract: In recent years, novel optical and magneto-optical devices have been proposed. This ranges from integrated photonic devices such as 3D holographic displays to magnetic recording, non- reciprocal photonic devices such as optical isolators and circulators or high-energy X-ray/gamma ray detectors. These devices, however, require suitable materials with tunable optical and magneto-optical properties. Presented thesis aims to systematically study such materials, namely GdxFe(100-x), magnetic garnets (Y3-xBixFe5O12, Nd2BiFe(5-x)GaxO12, Nd0.5Bi2.5Fe(5-x)GaxO12) and Ce(0.95-x)HfxCo0.05O(2-δ). Systematic study is carried out by the combination of experimental methods of spectroscopic ellipsometry, magneto-optical Kerr effect spectroscopy and Faraday effect spectroscopy. Experimental data are confronted to...
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Description of the non-linear oscillators for study of superfluid helium flows
Talíř, Marek ; Midlik, Šimon (advisor) ; Chlan, Vojtěch (referee)
The main goal of this Thesis was to develop and optimize fitting procedures for the detailed modeling of resonant curves characterizing physical behaviour of micron sized electro-mechanical resonators (MEMS). These devices are frequently used for the detec- tion of hydrodynamical properties of quantum fluids such as He II. We provide the model originating from the equation of motion of linear harmonic oscillator introducing extra parameters describing nonlinear drag forces and nonlinear restoring force. By testing of our fitting procedures on the real measured experimental data, we verify their applica- bility. For this purpose we chose two types of experiments namely; resonant behaviour of silicon vibrating wires measured at sub-kelvin temperatures in vacuum omitting any nonlinear drag and; resonant behaviour of NbTi vibrating wire submerged in superfluid helium at 1K experiencing both types of nonlinearities. Presented results prove universal- ity and robustness of developed procedures and are able to capture physical effects such as turbulent transition in a flow past the oscillating body or the Duffing like nonlinearity. 1
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Nuclear magnetic resonance of Pb(x)Ba(1-x)(FeNb)0.5O3 perovskites
Adamec, Martin ; Chlan, Vojtěch (advisor) ; Kohout, Jaroslav (referee)
Temperature dependences of nuclear magnetic resonance spectra of isotopes 207 Pb, 137 Ba, and 93 Nb in polycrystalline samples of PbxBa1-x(FeNb)0.5O3, with x = 0 (BFN), 0.5 (PBFN), 1 (PFN) and in Pb(FeNb)0.5O3 single crystal (PFN SC) were acquired. Measured nuclear magnetic resonance spectra are analyzed in this work. The temperature dependence of the spectra show strong broadening with decreasing temperature. Part of the temperature dependences of nuclear magnetic resonance spectra did not comply with expected characteristics. Possible hypotheses for interpretation of such behavior are discussed in the text. Probable arrangement of Fe and Nb cations in PFN and BFN phases is deduced and, based on these differences, the contrast in Néel temperatures of these phases is explained.
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Study of LaCuAl3 structure by means of nuclear magnetic and quadrupole resonance
Sgallová, Ráchel ; Chlan, Vojtěch (advisor) ; Englich, Jiří (referee)
In the presented bachelor thesis the structure of LaCuAl3 was studied on a series of metallic powder samples LaCuxAl4−x, x = 0,75; 0,9; 1 and 1,1 especially with respect to the local symetry of La atoms. 63 Cu, 65 Cu, 27 Al and 139 La nuclear magnetic resonance spectra were measured at room tempera- ture in external magnetic eld of 9,4 T. Some of the measured spectra were compared with simulated ones in order to access the number of nonequiva- lent atoms in the structure. Furthermore, the effect of Al/Cu stoichiometry on nuclear magnetic resonance spectra was observed and diccussed. 1
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Locally probed superflows of 4He.
Goleňa, Maximilián ; Midlik, Šimon (advisor) ; Chlan, Vojtěch (referee)
The main goal of this Thesis was the manufacture and characterisation of a new type of local probe of quantum turbulence in He II. Our MEMS devices are semicircular loops 3 mm in diameter made of 40 µm superconducting NbTi wire placed in magnetic field, whose motion is driven by applied alternating current. To detect quantum turbulence, we measure the hydrodynamic properties of the probe, in particular changes in its reso- nant frequency and amplitude depending on the applied external flow. In this work we specifically investigate the generation of quantum turbulence in thermal counterflow of He II for temperatures between 1.45 K and 2.1 K, representing a well-known and under- stood system suitable for the characterisation of our devices. Additionally, the response of the detectors is calibrated against the intensity of turbulence as given by the vortex line density L, measured simultaneously using second sound attenuation. This technique is more sensitive than the microresonators, but precludes any attempts at local detection of L necessary to study inhomogeneous systems of quantized vortices. Analysis of the results based on the Donnelly number describing instabilities in the normal component has confirmed the sensitivity of the probe to quantized vortices in the superfluid. 1
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Hyperfine Interactions in Ferrites with Spinel Structure
Chlan, Vojtěch
Title: Hyperfine Interactions in Ferrites with Spinel Structure Author: Vojtěch Chlan Faculty of Mathematics and Physics, Charles University in Prague Supervisor: Prof. RNDr. Helena Štěpánková, CSc. Abstract: Ferrite systems with spinel structure, manganese ferrite, lithium ferrite and magnetite, are studied experimentally by nuclear magnetic resonance (NMR) spectroscopy and from the first principles by electron structure calculations based on density functional theory (DFT). Manganese ferrites with various degrees of manganese-iron inversion and with different manganese contents are investigated, with respect to magnetic structure and cationic arrangement. An approach for NMR spectra interpretation is elaborated on ordered lithium ferrite, consisting in matching the calculated hyperfine field anisotropy with experimental results. This method is then applied to low temperature structure of magnetite, showing a promising path towards solving its complicated structure, orbital and charge ordering. The switching of magnetic easy axis induced by external magnetic field is observed in magnetite and the accompanying structural changes are studied by means of magnetic and NMR measurements. Magnetite with cationic vacancies is investigated by analyzing satellite structure induced by the vacancies; it is shown that...
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Nuclear magnetic resonance of Pb(x)Ba(1-x)(FeNb)0.5O3 perovskites
Adamec, Martin ; Chlan, Vojtěch (advisor) ; Kohout, Jaroslav (referee)
Temperature dependences of nuclear magnetic resonance spectra of isotopes 207 Pb, 137 Ba, and 93 Nb in polycrystalline samples of PbxBa1-x(FeNb)0.5O3, with x = 0 (BFN), 0.5 (PBFN), 1 (PFN) and in Pb(FeNb)0.5O3 single crystal (PFN SC) were acquired. Measured nuclear magnetic resonance spectra are analyzed in this work. The temperature dependence of the spectra show strong broadening with decreasing temperature. Part of the temperature dependences of nuclear magnetic resonance spectra did not comply with expected characteristics. Possible hypotheses for interpretation of such behavior are discussed in the text. Probable arrangement of Fe and Nb cations in PFN and BFN phases is deduced and, based on these differences, the contrast in Néel temperatures of these phases is explained.
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Study of LaCuAl3 structure by means of nuclear magnetic and quadrupole resonance
Sgallová, Ráchel ; Chlan, Vojtěch (advisor) ; Englich, Jiří (referee)
In the presented bachelor thesis the structure of LaCuAl3 was studied on a series of metallic powder samples LaCuxAl4−x, x = 0,75; 0,9; 1 and 1,1 especially with respect to the local symetry of La atoms. 63 Cu, 65 Cu, 27 Al and 139 La nuclear magnetic resonance spectra were measured at room tempera- ture in external magnetic eld of 9,4 T. Some of the measured spectra were compared with simulated ones in order to access the number of nonequiva- lent atoms in the structure. Furthermore, the effect of Al/Cu stoichiometry on nuclear magnetic resonance spectra was observed and diccussed. 1
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Relaxivity of magnetic iron oxide nanoparticles containing diamagnetic cations
Kubíčková, Lenka ; Kohout, Jaroslav (advisor) ; Chlan, Vojtěch (referee)
Magnetic nanoparticles have received extensive attention in the biomedical research, e.g. as prospective contrast agents for T2-weighted magnetic resonance imaging. The ability of a contrast agent to enhance the relaxation rate of 1 H in its vicinity is quantified by relaxivity. The main aim of this thesis is to evaluate the transversal re- laxivity of ε-Fe2−x Alx O3 nanoparticles coated with amorphous silica or citrate - its dependence on external magnetic field, temperature and thickness of silica coating - by means of nuclear magnetic resonance. The aluminium content x = 0.23(1) was determined from XRF, the material was further characterised by XRPD, Möss- bauer spectroscopy, DLS, TEM and magnetic measurements. The size of magnetic cores was ∼ 21 nm, the thickness of silica coating ∼ 6,10,17 and 21 nm. Magne- tization of the ε-Fe2−x Alx O3 nanoparticles increased by ∼ 30 % when compared to ε-Fe2O3. The saturating dependence of relaxivity on external magnetic field and on the linear decrease with increase of thickness of silica coating contravene the theo- retical model of motional averaging regime (MAR); nevertheless, the temperature dependence acquired in 0.47 T and 11.75 T may be explained by MAR. In compari- son to ε-Fe2O3 nanoparticles, the relaxivity of examined samples was higher for par-...
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