$α$-RuCl3: a Spin-Orbit Assisted Mott Insulator on a Honeycomb Lattice (1403.0883v2)

Abstract: We examine the role of spin-orbit coupling in the electronic structure of $\alpha$-RuCl3, in which Ru ions in 4d5 configuration form a honeycomb lattice. The measured optical spectra exhibit a clear optical gap and excitations within the t2g orbitals. The spectra can be described very well with first-principles electronic structure calculations obtained by taking into account both spin orbit coupling and electron correlations. Furthermore, our X-ray absorption spectroscopy measurements at the Ru L-edges exhibit distinct spectral features associated with the presence of substantial spin- orbit coupling, as well as an anomalously large branching ratio. We propose that $\alpha$-RuCl3 is a spin-orbit assisted Mott insulator, and the bond-dependent Kitaev interaction may be relevant for this compound.

• The paper demonstrates that spin-orbit coupling and electronic correlations yield a 220 meV optical gap, aligning first-principles theory with optical spectroscopy.
• The paper employs LDA+U+SOC calculations and X-ray absorption spectroscopy to highlight the significant impact of SOC on the 4d electrons in α-RuCl₃.
• The paper identifies α-RuCl₃ as an ideal platform for exploring Kitaev interactions and potential quantum spin liquid states, paving the way for novel spintronic applications.

Overview of the Study on $\alpha$-RuCl$_3$ as a Spin-Orbit Assisted Mott Insulator

Plumb et al. explore the electronic and magnetic properties of $\alpha$-RuCl$_3$, a compound described as a spin-orbit assisted Mott insulator, residing on a honeycomb lattice. This paper examines the critical role of spin-orbit coupling (SOC) in shaping the electronic structure of $\alpha$-RuCl$_3$, where the Ru ions in a 4d$^5$ electronic configuration form part of a layered honeycomb architecture. This investigation brings to light the interplay between electronic correlations and spin-orbit coupling that leads to the emergence of exotic topological phases, offering a fertile platform for further exploration of Kitaev physics and potential quantum spin liquid states.

Key Findings

• Electronic Structure and Optical Spectroscopy: The authors report an optical gap of approximately 220 meV in $\alpha$-RuCl$_3$, characterized by transitions within the $\rm t_{2g}$ bands. Through first-principles electronic calculations, it was shown that both strong electronic correlations and SOC are pivotal in presenting an accurate depiction of the band structure, successfully aligning with higher energy optical transitions observed experimentally.
• X-ray Absorption Spectroscopy: Distinct features observed at the Ru L edges support the substantial influence of SOC in the 4d electrons, exhibiting an anomalously large branching ratio indicative of strong SOC effects.
• Theoretical and Computational Insights: The paper combines LDA+U+SOC calculations to map the band structure, revealing the metallic nature absent SOC. In contrast, the SOC presence, alongside electronic correlations, captures the insulating state realistically, critical in approximating the observed optical gap.

Implications and Future Directions

• Kitaev Interaction and Quantum Spin Liquids: The findings reinforce the notion of $\alpha$-RuCl$_3$ as a promising platform for examining Kitaev-type interactions, given its ideal two-dimensional honeycomb geometry. The potential for bond-dependent exchange interactions presents a compelling case for future in-depth explorations into its quantum magnetic properties and realization of a quantum spin liquid phase.
• Exploration Beyond Iridates: Unlike 5d transition metal iridates significantly affected by distortions and further neighbor interactions, $\alpha$-RuCl$_3$ offers a near-ideal playground devoid of such complexities. The research suggests a shift towards studying 4d systems where SOC, albeit weaker, maintains substantial influence owing to reduced bandwidths compared to iridates.
• Advancements in Material Applications: Understanding the intrinsic SOC and correlation effects in $\alpha$-RuCl$_3$ might pave pathways for applications in developing novel materials with tunable magnetic and electronic properties, which are essential in realizing functional devices in spintronics and quantum computing domains.

This paper by Plumb et al. contributes rigorously to the existing body of research on the role of SOC in transition metal compounds, establishing $\alpha$-RuCl$_3$ as a critical material for further explorations in unconventional magnetic state realizations and beyond. The groundwork laid in this paper could potentially guide future theoretical and experimental endeavors in elucidating the complex interplay of electronic interactions in similar compounds.