Optical Spectroscopic Studies on the Electronic Structures Coupled to Lattice/Spin Degrees of Freedom in 5d Transition Metal Oxides

Abstract

Novel phenomena in 5d transition metal oxides (TMOs) are strongly linked to how spin/lattice degrees of freedoms affect their electronic structures. In these compounds, exotic phases have been expected including a superconductivity, a topological insulator, a quantum spin-liquid, and a Weyl semimetal. To search for the phases, we should understand the coupling between spin/lattice and electronic structures. Weyl semimetal, for example, is expected to originate from the interplay between spin and band structures. In this dissertation, I will report optical spectroscopy studies on electronic structures affected by spin/lattice in 5d layered perovskite, pyrochlores, and honeycomb lattice systems. Sr2IrO4, the most well-known 5d layered perovskite, have been believed to show superconductivity due to many similarities with La2CuO4, a mother compound of high TC superconductivity. To search for new superconductivities, we should find the possible bosonic glues which strongly renormalize their electronic structures. Using temperature-dependent optical spectra, I will show that strong electron-phonon interaction indeed exists in Sr2IrO4. The coupling constants of electron-phonon interaction are comparable to those in cuprates and maganites. Further, I will demonstrate that electron-phonon coupling in this compounds occurs in orbital-dependent ways. Namely, electrons/holes in different orbital symmetries are coupled to the different phonon symmetries. The effects of spin ordering on band structures of 5d pyrochlores is essential for novel phenomena such as Weyl semimetal and a new type of quantum criticality. In many of these compounds, magnetic ordering significantly affects their electronic structure and leads to metal-insulator transitions. Such phase transitions are not understood in terms of Mott, Slater, and density wave scenario, because the transitions are continuous without structural transitions, q=0 magnetic pattern, and 3D band structures. I will show that, in Cd2Os2O7, one of the 5d pyrochlores, metal-insulator transition is composed of two sequential phase transitions, from paramagnetic metal to antiferromagnetic metal, and then form antiferromagnetic metal to antiferromagnetic insulator. I will demonstrate that the transitions are originate from upward/downward shifts of bands similar to the case of Lifshitz transition. Interplay between spin-orbit entangled electronic structures and honeycomb lattice will be presented. Spin-orbit entangled Jeff = 1/2 states in honeycomb lattices are expected to induce non-trivial topological band structures and quantum spin-liquid ground states. The one of the promising candidates is Na2IrO3. The basic assumption for novel phenomena is that orbital characters near the Fermi level are described by a single Jeff = 1/2 orbital. To elucidate the orbital characters, optical and x-ray absorption spectroscopy were employed. I revealed that anisotropic hopping interaction raised by honeycomb lattice can modify the expected Jeff = 1/2 states.