More bridging ligands activate direct exchange: the case of anisotropic Kitaev effective magnetic interactions

Abstract

A magnet is a collection of magnetic moments. How those interact is determined by what lies in between. In transition-metal and rare-earth magnetic compounds, the configuration of the ligands around each magnetic center and the connectivity of the ligand cages are therefore pivotal -- for example, the mutual interaction of magnetic species connected through one single ligand is qualitatively different from the case of two bridging anions. Two bridging ligands are encountered in Kitaev magnets. The latter represent one of the revelations of the 21st century in magnetism research: they feature highly anisotropic intersite couplings with seemingly counterintuitive directional dependence for adjacent pairs of magnetic sites and unique quantum spin-liquid ground states that can be described analytically. Current scenarios for the occurrence of pair-dependent magnetic interactions as proposed by Kitaev rely on $indirect$ exchange mechanisms based on intersite electron hopping. Analyzing the wavefunctions of Kitaev magnetic bonds at both single- and multi-configuration levels, we find however that $direct$, Coulomb exchange may be at least as important, in 5$d$ and 4$d$ $t_{2g}^5$, 3$d$ $t_{2g}^5e_g^2$, and even rare-earth 4$f^1$ Kitaev-Heisenberg magnets. Our study provides concept clarification in Kitaev magnetism research and the essential reference points for reliable computational investigation of how novel magnetic ground states can be engineered in Kitaev, Kitaev-Heisenberg, and Heisenberg edge-sharing systems.

Figures (5)

• Figure 1: Exchange contributions to the intersite magnetic couplings in 5$d^5$ Na$_2$IrO$_3$: Coulomb exchange (SC results, in red), Ir($t_{2g}$)--Ir($t_{2g}$) kinetic exchange (as the difference between CASSCF and SC data, in blue), plus contributions related to Ir-O$_2$-Ir superexchange, Ir($t_{2g}$)$\rightarrow$Ir($e_g$) excitations, and so called dynamical correlation effects olsen_bible (as the difference between MRCI and CASSCF, in green).
• Figure 2: Contributions to the intersite magnetic couplings in 4$d^5$ RuCl$_3$: Coulomb exchange (red bars), Ru($t_{2g}$)--Ru($t_{2g}$) kinetic exchange (blue), plus contributions related to Ru-Cl$_2$-Ru superexchange, Ru($t_{2g}$)$\rightarrow$Ru($e_g$) excitations, and dynamical correlation (green).
• Figure 3: Exchange mechanisms contributing to the intersite magnetic couplings in 3$d^7$ Li$_3$Co$_2$SbO$_6$: $d$-$d$ Coulomb exchange (red bars), renormalization due to on-site intra-3$d$ excitations (light blue), $d$-$d$ kinetic exchange (dark blue), plus contributions related to Co-O$_2$-Co superexchange and dynamical correlation (green).
• Figure 4: 4$f$-4$f$ Coulomb exchange (red) and 4$f$-4$f$ kinetic exchange (blue) in RbCeO$_2$.
• Figure :