Sweet spot in the RuCl$_3$ magnetic system: nearly ideal $j_{\mathrm{eff}}\!=\!1/2$ moments and maximized $K/J$ ratio under pressure

TL;DR

This study reveals that under pressure, a crystalline phase of RuCl$_3$ exhibits nearly ideal $j_{eff}=1/2$ moments and a maximized $K/J$ ratio of about 100, making it a promising candidate for realizing Kitaev spin-liquid behavior.

Contribution

The paper demonstrates that a specific high-symmetry phase of RuCl$_3$ under pressure achieves an exceptionally high $K/J$ ratio and minimal deviation from ideal $j_{eff}=1/2$ moments, advancing the understanding of Kitaev materials.

Findings

Very small residual Heisenberg interaction $J$ under pressure.

$K/J$ ratio as high as approximately 100.

Minimal trigonal splitting indicating near-ideal $j_{eff}=1/2$ moments.

Abstract

Maximizing the ratio between Kitaev and residual Heisenberg interactions is a major goal in nowadays research on Kitaev-Heisenberg quantum magnets. Here we investigate Kitaev-Heisenberg exchange in a recently discovered crystalline phase of RuCl$_3$ under presure -- it displays unusually high symmetry, with only one type of Ru-Ru links, and uniform Ru-Cl-Ru bond angles of $\approx$93$^{\circ}$. By quantum chemical calculations in this particular honeycomb-lattice setting we find a very small $J$, which yields a $K/J$ ratio as large as $\sim$100. Interestingly, we also find that this is associated with vanishingly small $d$-shell trigonal splittings, i.\,e., minimal departure from ideal $j_{\mathrm{eff}}\!=\!1/2$ moments. This reconfirms RuCl$_3$ as a most promising platform for materializing the much sought-after Kitaev spin-liquid phase and stimulates further experiments under strain and pressure.