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

TL;DR

This paper uses soft-x-ray ARPES to explore the 3D electronic structures of Ruddlesden-Popper iridates, revealing an insulator-metal transition linked to changes in the $j_{eff}$=1/2 band and the importance of 3D Fermi surface features.

Contribution

It provides the first detailed 3D momentum-resolved electronic structure analysis of Ruddlesden-Popper iridates, highlighting the role of dimensionality and band behavior in the insulator-metal transition.

Findings

Insulator-to-metal transition occurs with increasing IrO$_2$-plane dimensionality.

The $j_{eff}$=1/2 band crosses the Fermi energy during the transition.

3D Fermi surface and $k_z$-dependent oscillations are revealed in SrIrO$_3$.

Abstract

In this study, we systematically investigate 3D momentum($\hbar k$)-resolved electronic structures of Ruddlesden-Popper-type iridium oxides Sr$_{n+1}$Ir$_n$O$_{3n+1}$ 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 IrO$_2$-plane structure. This transition occurs when the spin-orbit-coupled $j_{\rm eff}$=1/2 band changes its behavior in the dispersion relation and moves across the Fermi energy. In addition, an emerging band along the $\Gamma$(0,0,0)-R($\pi$,$\pi$,$\pi$) direction is found to play a crucial role in the metallic characteristics of SrIrO$_3$. By scanning the photon energy over 350 eV, we reveal the 3D Fermi surface in SrIrO$_3$ and $k_z$-dependent oscillations of photoelectron intensity in Sr$_3$Ir$_2$O$_7$. 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 non-realistic 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.