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In dieser Arbeit wurde der Quanten-Hall-Effekt mit Hilfe von Phononen untersucht. Ziel der Arbeit war es, die thermodynamischen Eigenschaften von zweidimensionalen Elektronensystemen und Größe der Energielücken im fraktionalen Quanten-Hall-Effekt präzise zu vermessen. So gelang es, die Composite-Fermion-Masse und Landé g-Faktoren zu bestimmen. Darüberhinaus konnte die spezifische Wärme eines zweidimensionalen Elektronensystemens für diverse Füllfaktoren gemessen werden, einer thermodynamischen Größe, für die bisher keine experimentelle Technik zur Verfügung stand.

This is a theoretical investigation of the magnetic, structural and dynamical properties of the ferromagnetic Heusler compound Ni-Mn-Ga. Its physical properties have been studied by means of the Density Functional Theory. The challenge of this work is to explain in terms of the electronic structure and phonon spectra the entire variety of different crystal structures of Ni-Mn-Ga and their stability. It was shown that this crystal exibits anomalies both in the electronic structure and the phonons. Important for Ni-Mn-Ga features include modulation shuffling of the structure, phonon softening, phonon acoustic-optical coupling, electron-phonon coupling and the charge transfer. A combination of these properties allowed to draw a solid physical view on the nature of the Heusler alloys and the alloy Ni-Mn-Ga.

In this thesis clean GaN{0001} as well as AlGaN(0001) surfaces were characterized by high-resolution electron energy-loss spectroscopy (HREELS) and electron spectroscopic means. Moreover, the interaction of hydrogen with GaN{0001} surfaces of both polarities was investigated. At clean GaN surfaces the vibrational and electronic properties were determined, surface cleaning procedures and thermal decomposition were examined. By means of HREELS the energy of the Fuchs-Kliewer (FK)-phonon was determined as 86.8 meV. At Ga-polar surfaces a gap state at 2.7 eV was found. Atomic hydrogen was found to bind to both N and Ga. Deuteration of the surface delivered clear evidence for this behaviour and, thus, resolved a discrepancy published in the literature. Photoemission measurements confirmed this finding. The properties of AlGaN surfaces were investigated as a function of their composition. Over the whole composition range no evidence for a negative electron affinity was found. The FK phonons of AlGaN show a one-mode behaviour.