2011/06/30 by Yogesh Singh, S. Manni, J. Reuther +5
Physics and Astronomy · #cond-mat.str-el #cond-mat.mtrl-sci
paper · pdf · doi:10.1103/physrevlett.108.127203
published as Phys. Rev. Lett. 108, 127203 (2012) · Version accepted for publication in PRL. Additional DFT and thermodynamic calculations have been included. 6 pages of supplementary material included
arxiv created 2012/02/02 · arxiv updated 2012/03/26
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 θ= -125 K for Na2IrO3 to θ= -33 K for Li2IrO3, while the antiferromagnetic ordering temperature remains roughly the same TN = 15 K for both materials. Using finite-temperature functional renormalization group calculations we show that this evolution of θ, TN, the frustration parameter f = θ/TN, 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 (α≈ 0.25), while Li2IrO3 appears to be close to a spin-liquid regime (0.6 < α< 0.7).