Condensed Matter Seminar: Magnetic excitations in honeycomb and pyrochlore iridates with strong spin-orbit coupling and electron correlation
Recent discovery of topological electronic states has shed light on the role of strong spin-orbit coupling in condensed mater. When the coupling is combined with electron correlation effect, their combination leads to a new class of topological states accompanying exotic magnetism. The 5d transition metal iridates have received considerable attention as the candidates that harness Kitaev quantum spin liquid, Wyle semimetal or Axion insulator phases. For the first part of this talk, I discuss resonant inelastic x-ray scattering (RIXS) investigation of a honeycomb Na2IrO3. The observation of diffuse magnetic scattering points to presence of short-range magnetic orders resulting from competition of bond-directional magnetic anisotropies. This validates the novel route to realize Kitaev spin liquid and evidences the proximity to the spin liquid phase. In the second part, I present RIXS study of magnetic excitation in a pyrochlore Eu2Ir2O7. Its metal-insulator transition driven by all-in-all-out (AIAO) magnetic order is regarded as possible realization of topological Weyl semimetal. We observe gradual softening of the magnon excitations in the entire Brillouin zone while warming, whose temperature evolution shows a direct relationship with the metal-insulator transition. This result suggests substantial change of magnetic exchange due to the varying electronic structure, and thereby classifies intermediate electron correlation strength: a requisite for realizing Weyl semimetal state.