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Elektronická struktura materiálů na bázi grafenu
Nádvorník, Lukáš ; Orlita, Milan (advisor) ; Němec, Hynek (referee)
In last two years, the proposal to create artificial graphene in standard semiconducting 2D systems via surface patterning has emerged. This way, an alternative system would be created, allowing us to study phenomena related to Dirac-type particles in a fully carbon free system. The main idea of the concept assumes the creation of an additional potential in a quantum well by nanopatterning of the specimen surface or by using local electrodes. The additionally introduced modulation can transform the conventional (i.e. parabolic) energy dispersion into separated minibands with possible appearance of Dirac cones. In the theoretical part, we introduce four basic criteria that estimate appropriate technological parameters and the required experimental conditions. Experimentally, we study the cyclotron resonance of prepared heterostructures AlGaAs/GaAs with induced hexagonal potential via the etching lateral holes. The observed multi-mode resonance response is discussed with respect to the expected appearance of Dirac cones.
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Ultrafast laser spectroscopy of semiconductors
Reichlová, Helena ; Němec, Petr (advisor) ; Orlita, Milan (referee)
This work is devoted to the study of the ferromagnetic semiconductor GaMnAs by magneto-optical methods, namely we used the magnetic linear dichroism method (MLD). First, we found a suitable method for the measurement of the magneto-optical coefficient, which describes MLD. Then we used MLD method to study the hysteresis loops in the sample with almost cubic magnetic anisotropy and in the sample with both cubic and uniaxial anisotropy - we measured the polarisation, temperature and intensity dependence of the hysteresis loops and we derived a formula for the polarisation dependence of the amplitude of the hysteresis loops. Using timeresolved MLD method we investigated the ultrafast demagnetization process in the sample on various time-scales after the laser-pulse excitation. We observed that the demagnetisation dynamics depends on the pump fluence and the temperature.
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Dynamics of spin polarization in ferromagnetic semiconductors
Šubrt, Jiří ; Němec, Petr (advisor) ; Orlita, Milan (referee)
This work was devoted to the measurement of magnetooptical effects in ferromagnetic semiconductors. We analyzed in detail the measurement technique based on a photoelastic modulator and we constructed an experimental setup that enables to measure the spectra of magnetic circular dichroism (MCD), magnetic circular birefringence (MCB), magnetic linear dichroism (MLD), and magnetic linear birefringence (MLB). This setup was used for the measurement of magnetooptical spectra of several samples of ferromagnetic semiconductors (Ga,Mn)As and (Ga,Mn)(As,P).
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Infrared magneto-spectroscopy of the Rashba-type semiconductors
Šikula, Marek ; Dubroka, Adam (referee) ; Orlita, Milan (advisor)
Optická odezva BiTeX (X = I, Cl, Br) polovodičových materiálů s obřím spinovým štěpením Rashbova typu je studována za nízkých teplot do vysokých magnetických polí, kde je směr magnetické indukce kolmý na povrch vzorku (Faradayova konfigurace). Na rozdíl od reflexního uspořádání nám transmisní uspořádání umožňuje přímé pozorování přechodů mezi Landauovými hladinami v blízkosti křižiště vodivostních pásů - Diracův bod. Optická odezva BiTeX sloučenin je srovnána s teoretickým modelem spočteným v rámci Kubo-Greenwoodova formalismu z Rashbova hamiltonianu.
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