Relevance of the Heisenberg-Kitaev model for the honeycomb lattice iridates A_2IrO_3

TL;DR

This study combines experiments and theory to show that honeycomb lattice iridates A2IrO3 are magnetically ordered Mott insulators whose magnetic properties can be described by an extended Heisenberg-Kitaev model, revealing different regimes for Na2IrO3 and Li2IrO3.

Contribution

It demonstrates that the magnetic behavior of A2IrO3 iridates can be explained by an extended Heisenberg-Kitaev model, integrating experimental data with theoretical calculations.

Findings

Na2IrO3 has a Curie-Weiss temperature of -125 K, Li2IrO3 has -33 K.

Both materials exhibit similar antiferromagnetic ordering temperatures (~15 K).

Na2IrO3 is deep in the magnetically ordered regime, while Li2IrO3 is near a spin-liquid regime.

Abstract

Combining thermodynamic measurements with theoretical density functional and thermodynamic calculations we demonstrate that the honeycomb lattice iridates A2IrO3 (A = Na, Li) are magnetically ordered Mott insulators where the magnetism of the effective spin-orbital S = 1/2 moments can be captured by a Heisenberg-Kitaev (HK) model with Heisenberg interactions beyond nearest-neighbor exchange. Experimentally, we observe an increase of the Curie-Weiss temperature from \theta = -125 K for Na2IrO3 to \theta = -33 K for Li2IrO3, while the antiferromagnetic ordering temperature remains roughly the same T_N = 15 K for both materials. Using finite-temperature functional renormalization group calculations we show that this evolution of \theta, T_N, the frustration parameter f = \theta/T_N, and the zig-zag magnetic ordering structure suggested for both materials by density functional theory can be captured within this extended HK model. Combining our experimental and theoretical results, we estimate that Na2IrO3 is deep in the magnetically ordered regime of the HK model (\alpha \approx 0.25), while Li2IrO3 appears to be close to a spin-liquid regime (0.6 < \alpha < 0.7).