Abstract
We spectroscopically investigate a series of pyrochlore iridates (: rare-earth and Y ions) where the metal-insulator transitions are induced by systematic bandwidth control via chemical substitutions of ions. We establish the phase diagram of , as endorsed by the variation of the optical conductivity spectra, in which the competing phases including paramagnetic insulator (PI), paramagnetic metal (PM), and antiferromagnetic insulator (AFI) show up as a function of bandwidth and temperature. For small -ionic radius ( Y-Sm), i.e., strongly correlated region, pronounced peaks on the edge of the optical gap are discerned below the magnetic transition temperature , which is attributable to exciton and magnon sideband absorptions. It turns out that the estimated nearest-neighbor exchange interaction increases as -ionic radius increases, whereas monotonically decreases, indicating that the all-in all-out magnetic order arises from the interplay among several exchange interactions inherent to extended orbitals on the frustrated lattice. For larger -ionic radius ( Sm-Pr), i.e., relatively weakly correlated region, the optical conductivity spectra markedly change below 0.3 eV in the course of PM-AFI transition, implying that the magnetic order induces the insulating state. In particular, we have found distinct electrodynamics in the composition of which is located on the boundary of the quantum PM-AFI transition, pointing to the possible emergence of unconventional topological electronic phases related possibly to the correlated Weyl electrons.
- Received 29 March 2016
- Revised 25 May 2016
DOI:https://doi.org/10.1103/PhysRevB.93.245120
©2016 American Physical Society