Photoemission of switchable mirrors and quantum wells

This thesis focuses on the electronic properties of materials, which were explored with Angle Resolved Photoemission (ARPES) and Density Functional Theory (DFT). The natural fingerprint of electronic phenomena in crystalline solids, e.g. in this thesis the hydrogen-induced metal-insulator transition and the formation of standing electron waves in quantum wells, is the k-resolved band structure or in short the E(k) relation in the solid. The experimental technique to explore the occupied band structure in a k-resolved fashion is angle-resolved photoemission (ARPES). We emphasize in this work the importance of extensive angle-scanning (equivalent to extensive k-space sampling, see below) to analyze the complex band structure of solids. The full-hemispherical approach of photoemission has proven it's success mainly in conjunction with high-temperature superconductivity, where it has been shown that an experimental restriction to high symmetry directions might lead to a restricted understanding of materials properties. The comparison of DFT results and experimental measurements requires the adaptation of DFT to full-hemispherical photoemission, as it has been pursued in this work. The extensive introduction covers the experimental technique and the theoretical foundations of DFT by using the example of the surface system YSi2. Later on, in subsequent chapters the electronic properties of switchable mirrors and quantun wells are discussed. The photoemission experiments on the switchable mirror material YHx and the comparison to DFT, which yielded generally less convincing agreement for Hydrogen related states, allowed us to understand in greater detail the actual cause and nature of the switching process. The transition can be traced to the unique electronic properties of hydrogen in solids, which is very hard to describe theoretically due to its present electron correlation. Last but not last the aforementioned methods were applied to quantum well state spectroscopy. Here the unique combination of high quality quantum well growth on a substrate with high atomic mass allowed for the observation of spin seperation without magnetism due to the Rashba effect at the confining boundary of the quantum well. The work was motivated partly by the challenging combination of experiment and theory but nevertheless by a strong application oriented background as well. Both switchable mirrors and spintronic devices might be companions of everyday life in the future and call for a detailed understanding of their underlying work principles.

Thèse de doctorat : Université de Neuchâtel, 2004 ; 1826