KSEF RDE: Search for Topological Insulators in Pyrochlore Iridate Thin Films

The recent discovery of topological insulators among bismuth compounds has triggered intense research efforts to identify similar phenomena in materials with strong spin-orbit coupling. The theory describing topological insulators assumes time-reversal symmetry; however experiments on pyrochlore iridates have revealed magnetic order. Therefore these materials have been considered unlikely candidates as topological insulators, and hence they have received little recent attention. However, recent theoretical calculations involving local-spin-density approximation, Coulomb interaction, and spin-orbit coupling predict that topologically protected surface states can occur, even in gapless systems, in so-called 'Weyl' semimetal phases. The characteristic property of a 'Weyl' semimetal is a linear dispersion of the electronic density of states at the Fermi level, which can be viewed as a three-dimensional analog of graphene. The topological 'Weyl' semimetal phase can occur within a three-dimensional magnetic solid; thus pyrochlore iridates re-emerge as ideal candidates for investigation of the interplay between strong electronic correlation, strong spin-orbit coupling, and the topology of solids. We propose to synthesize and investigate pyrochlore iridate thin-films and heterostructures, particularly along the [111]-direction, along which pyrochlores consist of alternating layers of Kagome and triangular lattices of Ir ions. The Kagome lattice of Ir ions leads to half-filled 5d-electron state with strong correlation and spin-orbit coupling, and is of particular interest due to their geometrical frustration, spin-spin interaction, and spin-lattice interactions. In particular, we will synthesize sandwich heterostructures of pyrochlore iridate layers, in which a Z2 topological insulator and a Chern insulator are theoretically predicted. We, as a materials physics team at University of Kentucky (UK), have lead researches of iridate crystals and thin films. The outcome of this project will provide with indispensable insights to the physics of this system, and strengthen our leadership in this newly emerging field as pioneers of novel electronic materials physics.