Three-dimensional electronic structures and the metal-insulator transition in Ruddlesden-Popper iridates

In this study, we systematically investigate three-dimensional (3D) momentum (k)-resolved electronic structures of Ruddlesden-Popper-type iridium oxides Srn+1IrnO3n+1 using soft-x-ray (SX) angle-resolved photoemission spectroscopy (ARPES). Our results provide direct evidence of an insulator-to-metal transition that occurs upon increasing the dimensionality of the IrO2-plane structure. This transition occurs when the spin-orbit-coupled jeff=1/2 band changes its behavior in the dispersion relation and moves across the Fermi energy. In addition, an emerging band along the Γ(0,0,0)-R(π,π,π) direction is found to play a crucial role in the metallic characteristics of SrIrO3. By scanning the photon energy over 350 eV, we reveal the 3D Fermi surface in SrIrO3 and kz-dependent oscillations of photoelectron intensity in Sr3Ir2O7. In contrast to previously reported results obtained using low-energy photons, folded bands derived from lattice distortions and/or magnetic ordering make significantly weak (but finite) contributions to the k-resolved photoemission spectrum. At the first glance, this leads to the ambiguous result that the observed k-space topology is consistent with the unfolded Brillouin zone (BZ) picture derived from a nonrealistic simple square or cubic Ir lattice. Through careful analysis, we determine that a superposition of the folded and unfolded band structures has been observed in the ARPES spectra obtained using photons in both ultraviolet and SX regions. To corroborate the physics deduced using low-energy ARPES studies, we propose to utilize SX-ARPES as a powerful complementary technique, as this method surveys more than one whole BZ and provides a panoramic view of electronic structures.