Interplay betwen spin, orbital, and lattice in metal and transition-metal-oxide surfaces

Abstract

Significant progress in materials science of the past decade is associated with the emerging paradigm of phase complexity and cross-coupling phenomena in functional complex materials, in which various degrees of freedom are intricately coupled and mutually interacting. Such mutual interactions couple the spin-orbital-lattice, and often induce various emerging phenomena such as colossal magnetoresistance, multiferroicity, strain induced ferroelectricity, etc. Fundamental understanding of such phenomena requires detailed microscopic information on each physical degree of freedom for different physical qualities of spin, orbital, and lattice. In this thesis, we discuss how to approach to the physical quantities on spin, orbital and lattice degree of freedom and report theoretical analyses on emerging phenomena in surface of both transition metal oxides and metals. Firstly, thickness and strain dependent magnetism in SrRuO$_3$ ultra thin film shows a dimensional crossover character of surface electronic states. We observed that the magnetism of SrRuO$_3$ films is sensitive to the thickness and the applied strain. Stoner theory of itinerant magnetism is successfully applied to explain this behavior. Secondly, surface play a role as a boundary between normal insulator (vacuum) and non-trivial $Z_2$ topological insulator in possible topological insulator phase of $5d$ transition metal oxides Na$_2$IrO$_3$. Non-trivial topological states which is obtained by increasing the inter-layer distance make the topological surface states which is robust against the perturbation preserving time-reversal symmetry. Topological surface states of transition metal compound is special among other topological insulators since it contains the moderate electron correlation effects to make novel topological states such as topological Mott insulators and Weyl semimetal phases. Finally, chiral orbital angular momentum on the metal surfaces is originated from the inversion symmetry breaking nature of surfaces. We propose that the existence of chiral orbital angular momentum is a general consequence of inversion symmetry breaking potential, regardless of strength of spin-orbit coupling. And this local orbital angular momentum on the surfaces of high-$Z$ materials play a crucial role in the formation of Rashba-type surface band splitting. To detect such orbital angular momentum, circular dichroism experiment using the angle-resolved photoemission could be used.