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Tunable materials and structures for terahertz spectral range
Skoromets, Volodymyr ; Kužel, Petr (advisor) ; Hlídek, Pavel (referee) ; Pashkin, Alexei (referee)
This thesis is devoted to an experimental study of dielectric properties of incipient fer- roelectrics. The terahertz time-domain spectroscopy was used to investigate the complex permittivity spectra in both single crystals and various strained thin film structures versus temperature and applied electric bias. Namely, it allowed characterizing the ferroelectric soft-mode dynamics and its coupling to a central mode. An electric-field tunability of bulk single crystals of SrTiO3 was determined up to room temperature for the first time. The phenomenon is governed by soft-mode stiffening under applied field. As an application we proposed and characterized a tunable one-dimensional photonic structure with a thin SrTiO3 plate inserted as a defect layer. The importance of the soft-mode dynamics was stressed also in the study of bulk high density KTaO3 ceramics. A systematic study was performed of a KTaO3 thin film and especially of a set of strained multilayers consisting of SrTiO3/DyScO3 bilayers grown on DyScO3 substrate. Strain-induced ferroelectric transition was observed in these films governed by the soft mode coupled to a lower-frequency relaxation. A general model was developed describing the whole family of the studied samples. Effect of the composition stoichiometry of SrTiO3 films grown on DyScO3 was...
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Dynamics of delocalized states in molecular systems studied by time-resolved THz spectroscopy
Mics, Zoltán ; Kužel, Petr (advisor) ; Jepsen, Peter Uhd (referee) ; Kůsová, Kateřina (referee)
This thesis is devoted to a study of the conductivity of confined charge carriers in various nanocrystalline materials. The experimental method -- THz spectroscopy -- involves measurements of the conductivity of carriers generated by doping or by optical excitation. In the theoretical interpretation of measured data we address important aspects of carrier transport, connected to the microscopic inhomogeneity of samples and the nanoscale confinement of carriers. We focus on the study of nanocrystalline ZnO and CdS using simulations and experiments in a wide range of carrier densities and for several temperatures. In ZnO, the interplay of highly mobile electron-hole gas, dense electron-hole plasma and population of excitons was observed. In CdS the investigation reveals the importance of clusters of nanocrystals in the electron confinement. The dynamics of the electron mobility at different carrier densities show the role of the kinetic energy of electrons and its relaxation in the carrier transport in nanostructured systems. At temperatures 20 K and at low carrier densities a crossover between localized transport and Drude-like free electron behaviour was observed for the first time.
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Spin dynamics in GaAs-based semiconductor structures
Schmoranzerová, Eva ; Němec, Petr (advisor) ; Postava, Kamil (referee) ; Kužel, Petr (referee)
This work is dedicated to the study of spin dynamics in systems based on the semiconductor gallium arsenide (GaAs) that are suitable for use in spintronic devices. We explored two types of model structures using experimental methods of ultrafast laser spectroscopy and transport measurements. In the ferromagnetic semiconductor (Ga,Mn)As, we investigated laser-induced magnetization precession. We found out that transfer of both energy and angular momentum from the circularly polarized laser light can trigger magnetization precession, the latter one being identified as a new phenomenon, the "optical spin transfer torque". Furthermore, we demonstrate the possibility to control the energy-transfer-induced magnetization dynamics both optically and electrically using piezo-stressing. When dealing with purely non-magnetic structures for spintronics, we studied the Spin-Injection Hall Effect (SIHE) in GaAs/AlGaAs heterostructures with a special type of spin- orbit (SO) coupling that are lithographically patterned to create nanodevices. We managed to observe precession of the electron spin in the SO field directly in the space domain by extending the original detection method. This finding, together with the direct detection of a pure spin current, helped to propose a working spin Hall effect transistor.
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Builders and defenders of artillery fortresses of Czechoslovak fortificatinon 1935 - 1938
Kužel, Petr ; Čechurová, Jana (advisor) ; Kvaček, Robert (referee)
Still burning question of defense of the Czechoslovakia in September 1938 is constantly living in minds of many researchers even after seventy years since the humiliating Munich Agreement has been signatured. Most massive elements of the Czechoslovak defense system were called artillery fortresses including an extensive underground corridors, connecting infantry blocks designed for advanced technical mechanisms of the interwar period. Chapters of the thesis try to open new perspectives on the reason for the construction of fortification in our latitudes, great organization of groups providing construction and excellent qualities of special trained soldiers showed not only during preparations to defend the fortresses, but also after dissolution of theirs border guard units. Keywords Czechoslovakia 1938, Munich agreement, fortification, artillery fortress
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Cluster studies in molecular beams
Profant, Václav ; Kužel, Petr (referee) ; Dian, Juraj (advisor)
In the present work we study properties of clusters of small heteroatomic molecules with biological relevance by several experimental methods based on molecular beams. In the rst experiment structure and dynamics of size-selected charged pyrrole clusters have been studied by means of molecular beam scattering experiment. Small neutral P yn clusters were produced in Py/He expansions and larger mixed P ynArm clusters in Py/Ar expansions, and the scattering experiment with a secondary beam of He atoms was used to select the neutral clusters of dierent sizes. The complete size-selected fragmentation patterns for the neutral dimer to tetramer after an electron impact ionization at 70 eV from the measurements of the angular and velocity distributions at dierent fragment masses. In second experiment photolysis of size selected pyrrole, imidazole and pyrazole clusters has been investigated. Comparison with the photolysis of an isolated molecules and between studied systems has been made. Clusters were photolyzed at 243 and 193 nm and the kinetic energy distributions of the H-photofragments have been measured and analyzed. Finally the mass spectra of the fragments after multiphoton ionization have been measured. The signicant inuence of the cluster environment to the photolytic behavior was observed and discussed.
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Laser photolysis of molecules in free nanoparticles
Tkáč, Ondřej ; Dian, Juraj (advisor) ; Kužel, Petr (referee)
Several experiments with various clusters were performed on a molecular beam apparatus. (1) The beam velocities were measured for rare gases and water clusters under various expansion conditions to learn about the supersonic expansions and cluster generation processes. (2) The photodissociation of (HBr)n clusters was measured for calibration purposes. (3) The mass spectrometry of clusters of small biomolecules imidazole, pyrazole and pyrrole was investigated. The stabilization of these hydrogen bonded species in the excited states by hydrogen transfer process was revealed, relevant to the stability of biomolecules in general. (4) Finally, the photodissociation of an HI molecule on water clusters (H2O)n was studied and compared to the photodissociation on (Ar)n clusters. It was shown that HI molecules acidically dissociate on (H2O)n and generate zwitterionic species H3O+ I-(H2O)n-1, which are then excited into biradical states with the neutral hydronium molecule H3O.
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Application of metamaterial structures in terahertz spectral range
Šibík, Juraj ; Kužel, Petr (advisor) ; Čtyroký, Jiří (referee)
The thesis is devoted to a research on metamaterials for terahertz spectral range based on deeply etched silicon. The aim of the work is a theoretical conception and experimental realization of wave plates for selected terahertz frequencies. These wave plates are made of silicon substrates with an etched two-dimensional periodic microstructure where the dimensions of an elementary cell are below the considered wavelength. Theoretical proposal is optimized using the transfer matrix formalism. We designed and experimentally characterized quarter-wave plates for frequencies 0.5 THz and 1 THz and a half-wave plate for frequency 1 THz.
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