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Terahertz conductivity of charge carriers in semiconductors
Hendrych, Erik ; Kužel, Petr (advisor) ; Nádvorník, Lukáš (referee)
In this thesis, we investigate the conductivity spectra of samples of weakly doped gal- lium arsenide in the terahertz region as a function of temperature. These are silicon-doped gallium arsenide of type N and zinc-doped gallium arsenide of type P. The temperature dependence in the terahertz region for these materials has not yet been described in the literature. We observe conductivity maxima corresponding to the admixture energies in the forbidden band. We use the Drude-Lorentz model to process the data. We achieve a good fit of the measured data. The carrier concentration at room temperature corre- sponds to the nominal value. The relaxation time and high-frequency permittivity match the tabulated values. The oscillator frequency for the N type approximately matches the admixture energy. For the P type, this value is outside the measured range and only the rise to this maximum is seen. 1
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Ultrafast photoconductivity and charge carrier transport in semiconductor nanostructures: a study by terahertz spectroscopy
Pushkarev, Vladimir ; Kužel, Petr (advisor) ; Nádvorník, Lukáš (referee) ; Winnerl, Stephan (referee)
Title: Ultrafast photoconductivity and charge carrier transport in semiconductor nanostructures: a study by terahertz spectroscopy Author: Vladimir Pushkarev Institute: Institute of Physics of the Czech Academy of Sciences Supervisor of the doctoral thesis: doc. RNDr. Petr Kužel, Ph.D. Abstract: The thesis is devoted to a study of charge transport in nanostructured systems by means of time-resolved terahertz spectroscopy. We address various aspects of electron confinement in such systems, and the role of depolarization fields on the effective photoconductive response of samples. Accurate evaluation of localized carrier properties has been achieved using quantum mechanical calculations of the mobility. Theoretical interpretation of the terahertz conductivity spectra is based on the solution of the wave equation for the terahertz wave in inhomogeneous photoexcited media described in terms of VBD effective medium model developed in our laboratory. This approach is employed for investigation of the terahertz photoconductivity in two types of nanomaterials: superlattices of Si nanocrystals prepared by thermal decomposition of SiOx layers and lithographically prepared GaAs nanobars. Experimental spectra were measured at 300 and 20 K and for a wide range of photocarrier densities. A careful analysis of electron...
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Socio-economic transformation of kicker societies and formation of football clubs in Prague cities and suburbs before The Great War
Kužel, Petr ; Čechurová, Jana (advisor) ; Kvaček, Robert (referee)
The most popular game all over the world has entered the territory of Bohemia already in the last decades of the 19th century, when especially in the cities and suburbs of Prague many Czech or German societies, engaged in new game from England called "football", was founded. Sudden and long lasting interruption of positive development of young sport by mobilization in the summer of 1914 and deep political and social changes after conflict isolated prewar events and made unique relict environment that creates the main sources for ideas of work. However chapters leaving sport performances aside and try to describe the period culminating after year 1900, when profesional player was born of student-enthusiast and when club loyalities based on nationality or social inclusion of spectator have been created. To achive a comprehensive view is also important to describe ideological orientation and economy of clubs, topography of Prague grounds or relationships between biggest clubs SK Slavia, AC Sparta, SK Viktoria Žižkov and DFC Prag, which was litmus paper of attitudes with German etnicity. Keywords Football, Prague, Czechs, Germans, Slavia, Sparta, The Great War
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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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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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Ultrafast laser spectroscopy of antiferromagnets
Saidl, Vít ; Němec, Petr (advisor) ; Kužel, Petr (referee) ; Hamrle, Jaroslav (referee)
This work is dedicated to the study of two antiferromagnetic materials that are suitable for use in spintronic devices. In series of FeRh samples we studied the transition temperature between the antiferromagnetic and ferromagnetic phases. We developed a method based on material optical response for a quick determination of this temperature, which enabled us to study with a spatial resolution of 1 μm a magnetic inhomogeneity of prepared samples.We also developed a method for a determination of the Néel temperature and the magnetization easy axis position in thin films prepared from compensated antiferromagnetic metal. We successfully applied this method on an uniaxial sample of CuMnAs and we discussed its applicability for a research of samples with a biaxial magnetic anisotropy.
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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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Cluster studies in molecular beams
Profant, Václav ; Dian, Juraj (advisor) ; Kužel, Petr (referee)
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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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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