Matthias Schmitt (PhD thesis): High Energy Spin- and Momentum-Resolved Photoelectron Spectroscopy of Complex Oxides

Spin- and k-resolved hard X-ray photoelectron spectroscopy (HAXPES) is a powerful tool to probe bulk electronic properties of complex metal oxides. Due to the low efficiency of common spin detectors of ≈ 10⁻⁴ , such experiments have been rarely performed within the hard X-ray regime since the notoriously low photoionization cross sections further lower the performance tremendously. This thesis is about a new type of spin detector, which employs an imaging spin-filter with multichannel electron recording. This increases the efficiency by a factor of 10⁴ and makes spin- and k-resolved photoemission at high excitation energies possible. Two different technical approaches were pursued in this thesis: One using a hemispherical deflection analyzer (HDA) and a separate external spin detector chamber, the other one resorting to a momentum- or k-space microscope with time-of-flight (TOF) energy recording and an integrated spin-filter crystal. The latter exhibits significantly higher count rates and—since it was designed for this purpose from scratch—the integrated spin-filter option found out to be more viable than the subsequent upgrade of an existing setup with an HDA. This instrumental development is followed by the investigation of the complex metal oxides (CMOs) KTaO₃ by angle-resolved HAXPES (HARPES) and Fe₃O₄ by spin-resolved HAXPES (spin-HAXPES), respectively.
KTaO₃ (KTO) is a band insulator with a valence-electron configuration of Ta 5d⁰ . By angle- and spin-integrated HAXPES it is shown that at the buried interface of LaAlO₃/KTO— by the generation of oxygen vacancies and hence effective electron doping—a conducting electron system forms in KTO. Further investigations using the momentum-resolution of the k-space TOF microscope show that these states are confined to the surface in KTO and intensity is only obtained from the center or the Γ-point of each Brillouin zone (BZ). These BZs are furthermore square-like arranged reflecting the three-dimensional cubic crystal structure of KTO. However, from a comparison to calculations it is found that the band structure deviates from that of electron-doped bulk KTaO₃ due to the confinement to the interface.
There is broad consensus that Fe₃O₄ is a promising material for spintronics applications due to its high degree of spin polarization at the Fermi level. However, previous attempts to measure the spin polarization by spin-resolved photoemission spectroscopy have been hampered by the use of low photon energies resulting in high surface sensitivity. The surfaces of magnetite, though, tend to reconstruct due to their polar nature, and thus their magnetic and electronic properties may strongly deviate from each other and from the bulk, dependent on their orientation and specific preparation. In this work, the intrinsic bulk spin polarization of magnetite at the Fermi level (E_F) by spin-resolved photoelectron spectroscopy, is determined by spin-HAXPES on (111)-oriented thin films, epitaxially grown on ZnO(0001) to be P(E_F) = −80⁺¹⁰₋₂₀ %