Abstract
The strongly correlated insulator is considered as a paradigmatic realization of both spin-orbital physics and a band-Mott insulating phase, characterized by orbitally selective coexistence of a band and a Mott gap. We present a high resolution oxygen -edge resonant inelastic x-ray scattering study of the antiferromagnetic Mott insulating state of . A set of low-energy (about 80 and 400 meV) and high-energy (about 1.3 and 2.2 eV) excitations are reported, which show strong incident light polarization dependence. Our results strongly support a spin-orbit coupled band-Mott scenario and explore in detail the nature of its exotic excitations. Guided by theoretical modeling, we interpret the low-energy excitations as a result of composite spin-orbital excitations. Their nature unveils the intricate interplay of crystal-field splitting and spin-orbit coupling in the band-Mott scenario. The high-energy excitations correspond to intra-atomic singlet-triplet transitions at an energy scale set by Hund’s coupling. Our findings give a unifying picture of the spin and orbital excitations in the band-Mott insulator .
- Received 4 July 2017
- Revised 28 January 2018
DOI:https://doi.org/10.1103/PhysRevX.8.011048
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Mott insulators are materials that should conduct electricity, according to conventional theories, but become insulating because of strong interactions between electrons. A particularly interesting situation arises in some materials where the spin and orbital momentum of electrons within atoms also interact (a behavior known as spin-orbit coupling). The compound is one such case, where multiple atomic-level interactions give rise to a wealth of exotic phenomena. We used x-ray scattering experiments to investigate in its Mott-insulating state and explore these interactions in detail.
Specifically, we conducted resonant inelastic x-ray scattering (RIXS) experiments on the oxygen resonance. These experiments provided access to the low-energy sectors of orbital excitations that have a spin-orbit-coupled nature. We identified four different sectors of excitations. These results suggest that one orbital turns into an almost insulating band and hence becomes a “silent” channel. The remaining (now nearly half-filled) orbitals responsible for the Mott transition constitute the spin-orbit-coupled excitations that are unique fingerprints of the band-Mott-insulating ground state.
Comparing theoretical modeling of with our data suggests that the reported excitation sectors have an internal structure that we only partially resolved. Future improvements to the RIXS instrumentation should allow access to the excitation spectrum from both the ruthenium and oxygen site directly, with improved energy resolution. This will uncover another layer of complexity of this intricate Mott-insulating ground state.