Variation of optical conductivity spectra in the course of bandwidth-controlled metal-insulator transitions in pyrochlore iridates

Abstract

We spectroscopically investigate a series of pyrochlore iridates ${R}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ ($R$: rare-earth and Y ions) where the metal-insulator transitions are induced by systematic bandwidth control via chemical substitutions of $R$ ions. We establish the phase diagram of ${R}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$, 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 $R$-ionic radius ($R=$ Y-Sm), i.e., strongly correlated region, pronounced peaks on the edge of the optical gap are discerned below the magnetic transition temperature ${T}_{\mathrm{N}}$, which is attributable to exciton and magnon sideband absorptions. It turns out that the estimated nearest-neighbor exchange interaction increases as $R$-ionic radius increases, whereas ${T}_{\mathrm{N}}$ monotonically decreases, indicating that the all-in all-out magnetic order arises from the interplay among several exchange interactions inherent to extended $5d$ orbitals on the frustrated lattice. For larger $R$-ionic radius ($R=$ 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 $R=\phantom{\rule{4pt}{0ex}}{\mathrm{Nd}}_{0.5}{\mathrm{Pr}}_{0.5}$ 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.