Honeycomb lattice spin-orbit Mott insulators*

Iridates displaying a Mott insulating state caused by the interplay of electronic correlations and strong spin-orbit coupling have recently attracted considerable attention. We focus on the honeycomb material A$_{2}$IrO$_{3}$ (A=Na, Li), in which the topology of the underlying lattice leads to interesting magnetic properties [1]. The strong spin-orbit coupling in this 5d transition metal system is expected to result in orbital-dependent highly anisotropic magnetic in-plane exchange [2]. The combination of J$_{eff}$ = 1/2 and the underlying honeycomb lattice makes A$_{2}$IrO$_{3}$ a promising candidate for the Kitaev model, which is exactly solvable and has a spin-liquid ground state. Our experimental data on Na$_{2}$IrO$_{3}$ prove a Mott insulating state of effective J=1/2 moments with predominant antiferromagnetic coupling, indicated by a Weiss temperature of $\theta =-$120 K. A bulk antiferromagnetic transition occurs at a much reduced temperature of T$_{N}$ = 15 K and the reduced magnetic entropy suggests strong magnetic frustration and/or low-dimensional magnetic interactions. The nature of the ordered phase has also been studied by resonant x-ray spectroscopy near the Ir-L3 edge, providing evidence for an unconventional, most-likely zig-zag-type spin ordering [3]. The latter may be related to next-nearest neighbour exchange and/or a substantial Kitaev contribution in the Heisenberg-Kitaev model [2]. Upon replacing Na with the smaller Li, one may enhance the relative importance of the Kitaev contribution. For Mott insulating Li$_{2}$IrO$_{3}$ we observe a similar ordering temperature of 15 K, while the negative Weiss temperature is drastically reduced. These observations are compatible with an enhancement of the Kitaev contribution compared to the Na-system, suggesting that Li$_{2}$IrO$_{3}$ could be located close to the Kitaev limit [5]. \\[4pt] [1] Yogesh Singh and P. Gegenwart, Phys. Rev. B. 82, 064412 (2010). \\[0pt] [2] G. Jackeli and G. Khaliullin, Phys. Rev. Lett. 102, 017205, (2009). \\[0pt] [3] X. Liu et al., Phys. Rev. B 83, 220403(R) (2011). \\[0pt] [4] Yogesh Singh, S. Manni, P. Gegenwart, arXiv:1106.0429v1. \\[0pt] [5] J. Reuther, R. Thomale, S. Trebst, Phys. Rev. B 84, 100406 (2011).