Magnetization processes of zigzag states on the honeycomb lattice: Identifying spin models for α−RuCl3 and Na2IrO3

Abstract

We study the field-induced magnetization processes of extended Heisenberg-Kitaev models on the honeycomb lattice, taking into account off-diagonal and longer-range exchange interactions, using a combination of Monte-Carlo simulations, classical energy minimization, and spin-wave theory. We consider a number of different parameter sets, previously proposed to describe the magnetic behavior of α-RuCl3 and Na2IrO3 with their antiferromagnetic zigzag ground states. By classifying these parameter sets, we reveal the existence of three distinct mechanisms to stabilize zigzag states, which differ in the sign of the nearest-neighbor Kitaev interaction, the role of longer-range interactions, and the magnitude of the off-diagonal Γ1 interaction. While experimentally hardly distinguishable at zero field, we find that the three different scenarios lead to significantly different magnetization processes in applied magnetic fields. In particular, we show that a sizable off-diagonal interaction Γ1 > 0 naturally explains the strongly anisotropic field responses observed in α-RuCl3 without the need for a strong anisotropy in the effective g tensor. Moreover, for a generic field direction, it leads to a high-field state with a finite transversal magnetization, which should be observable in α-RuCl3. Scenario 1: If the most dominant interactions are between nearest neighbors only, a zigzag-ordered state occurs if the Kitaev interaction is antiferromagnetic, K 1 > 0. 30 Scenario 2: Zigzag order can be stabilized also for ferromagnetic Kitaev coupling, K 1 < 0, if a sizable arXiv:1706.05380v2 [cond-mat.str-el] 1 Sep 2017

Magnetic field-dependent low-energy magnon dynamics in α–RuCl3

“…Here, B is the Zeeman term including the g factor and the Bohr magneton. This relation is consistent with that found in [40]. We note that the LSWT analysis for such strong spin-orbit coupling was recently compared with exact diagonalization [55,58], which shows agreement with the dispersion at low energies and additional magnon breakdown effects at higher frequencies.…”

“…Correspondingly, next-leading-order corrections to the linear spin-wave Hamiltonian would not fully capture the highly nonlinear effects that arise in the real quantum system. While additional higher-order exchange terms have been shown to produce a good description of the spin dynamics (especially further-neighbor Heisenberg interactions) [19,38,40,55,58], we remark that such corrections are only expected to modify the dispersion away from the zone center. Below we focus only on the two lowest-energy modes, where our spin-wave analysis is expected to be robust.…”

Magnon damping in the zigzag phase of the Kitaev-Heisenberg- Γ model on a honeycomb lattice

“…Strong Kitaev-like bond-dependent couplings between effective pseudospins-1/2 have been identified in iridium oxides, such as α-Li 2 IrO 3 and Na 2 IrO 3 , α-RuCl 3 , and other materials [40][41][42][43][44][45][46][47]. In these systems, magnetic ions form the two-dimensional honeycomb lattices stacked along the [111]-direction.…”

Pressure-induced dimerization and valence bond crystal formation in the Kitaev-Heisenberg magnet α−RuCl3

“…A structural phase transition was reported at T S 60 K under cooling and T S 166 K upon warming, but the low-temperature crystal structure is still under debate and could be either rhombohedral (R3) [10,11] or monoclinic (C2/m) [12,13]. The onset of long-range magnetic order at T N 7 K [9] in α-RuCl 3 implies that other magnetic interactions have to be considered in addition to the Kitaev interaction K: a NN Heisenberg J , an off-diagonal coupling , as well as next-NN interactions J 2 and J 3 [7,8,14,15]. While electronic-structure calculations indicate that K is ferromagnetic in α-RuCl 3 and indeed defines the largest exchange energy scale [7,8,14,15], the debate on the minimal effective spin model and precise magnitude of the different couplings is not fully settled yet.…”

“…The onset of long-range magnetic order at T N 7 K [9] in α-RuCl 3 implies that other magnetic interactions have to be considered in addition to the Kitaev interaction K: a NN Heisenberg J , an off-diagonal coupling , as well as next-NN interactions J 2 and J 3 [7,8,14,15]. While electronic-structure calculations indicate that K is ferromagnetic in α-RuCl 3 and indeed defines the largest exchange energy scale [7,8,14,15], the debate on the minimal effective spin model and precise magnitude of the different couplings is not fully settled yet. By applying a magnetic field in the basal plane, the magnetic zigzag ground state can be suppressed [6,16,17] and the phase above this transition was identified as a quantum spin liquid, by NMR [18], thermal conductivity [19][20][21], terahertz spectroscopy [22], and neutron scattering experiments [23].…”