CAREER: Artificial 2D Superlattices of Epitaxial Pyrochlore Iridates

The objective of this research project is to investigate iridates in order to uncover their novel electronic phenomena, which are hidden in the bulk, but emerge in the thin-film artificial superstructures. Recent pioneering studies on iridates (e.g. Sr2IrO4, Na2IrO3) have revealed Jeff = 1/2 Mott ground states resulting from the interplay between strong electron-correlation and the relativistic spin-orbit interaction (1-3). The strong electron-correlation is key physics of intriguing collective-quantum phenomena such as high-Tc superconductivity, and the strong spin-orbit coupling is believed to provide topologically-protected novel electronic states (4-9). Therefore, the unique circumstance of coexisting strong electron-correlation and spin-orbit coupling in iridate is leading to an exciting new field of condensed matter physics and has attracted enormous interest in the system (10). There have been experimental studies in finding the topological phenomena in iridates, mostly in the bulk materials. Despite the efforts, the presence of the nontrivial topological properties is not discovered yet. Here we note that the topological invariant of a system is governed by two essential ingredients, the dimensionality and the symmetry of the system. We propose to study artificial superstructure thin-films of pyrochlore iridates (R2Ir2O7, R: Rare-earth elements, i.e. complex iridium oxides with the Kagome lattices). Our approach of studying thin-film-superstructures offers useful control parameters such as dimensionality, lattice-strain, and interfacial coupling. Tuning these parameters is expected to compel strongly interacting 5d electrons in pyrochlore iridates to exhibit unprecedented exotic collective states such as quantum spin-Hall insulators (4, 11) and Weyl semimetals (12), which are largely supported by existing theoretical studies.