Direct evidence of a zigzag spin-chain structure in the honeycomb lattice: A neutron and x-ray diffraction investigation of single-crystal Na2IrO3

Abstract

We have combined single crystal neutron and x-ray diffractions to investigate the magnetic and crystal structures of the honeycomb lattice Na2IrO3. The system orders magnetically below 18.1(2) K with Ir 4+ ions forming zigzag spin chains within the layered honeycomb network with an ordered moment of 0.22(1) µB/Ir site. Such a configuration sharply contrasts with the Néel or stripe states proposed in the Kitaev-Heisenberg model. The structure refinement reveals that the Ir atoms form a nearly ideal two-dimensional honeycomb lattice while the IrO6 octahedra experience a trigonal distortion that is critical to the ground state. The results of this study provide much needed experimental insights into the magnetic and crystal structure that are crucial to the understanding of the exotic magnetic order and possible topological characteristics in the 5d-electron-based honeycomb lattice.

Tuning into the Kitaev spin liquid phase: A spin model on the honeycomb lattice with two types of Heisenberg exchange couplings

“…The zigzag AFM order was found in experiments for the iridates [6][7][8][9]. This zig-zag order phase in the phase diagram appears in the third quarter of Fig.…”

“…A phase transition from a stripy antiferromagnetic(AFM) order to the Kitaev SL phase was predicted. Experiments for the iridates, however, found a zig-zag AFM order [6][7][8][9]. These progresses drive a number of further researches [10][11][12][13][14][15][16][17][18].…”

Zigzag Magnetic Order in the Iridium OxideNa2IrO3

“…It is also established now [3][4][5] that Ir-moments in Na 2 IrO 3 undergo antiferromagnetic (AF) order at T N ≃ 15 K. The fact that T N is much smaller than paramagnetic Curie temperature (−125 K) [6] and spin-wave energies [4] implies that the underlying interactions are strongly frustrated. This is natural in so-called KitaevHeisenberg (KH) model [16] where long range order is suppressed by the proximity to the Kitaev spin-liquid (SL) state.…”

“…Magnetic susceptibility, spin wave spectra, and zigzag order parameter are calculated and compared to the experimental data, obtaining thereby the spin coupling constants in Na2IrO3 and Li2IrO3. In the quest for the materials with novel electronic phases, iridium oxide Na 2 IrO 3 came into focus recently [1][2][3][4][5][6][7] due to theoretical predictions [8,9] that this system may host Kitaev model physics and quantum spin Hall effect. Na 2 IrO 3 is an insulator with sizable and temperature independent optical gap ≃ 0.35 eV [7], and shows CurieWeiss type susceptibility [1,6] with moments corresponding to effective spin one-half of Ir 4+ ion with t 5 2g configuration [10].…”

Finite-temperature phase diagram of the classical Kitaev-Heisenberg model

“…Recently, Jackeli and Khaliullin 1 suggested that the Kitaev model could actually be realized within the honeycomb iridates with general formula A 2 IrO 3 , since the anisotropic part of the interactions among Ir-ions has the same form as the Kitaev coupling. This suggestion has triggered a lot of experimental [7][8][9][10][11] and theoretical [12][13][14][15][16][17][18] activity in the study of Na 2 IrO 3 and Li 2 IrO 3 compounds.…”