Spin-Orbital Locking, Emergent Pseudo-Spin, and Magnetic order in Honeycomb Lattice Iridates

TL;DR

This paper investigates the effective spin Hamiltonian and magnetic order in honeycomb iridates, emphasizing the role of trigonal crystal fields and spin-orbit coupling, leading to emergent pseudo-spins and a zig-zag magnetic ground state.

Contribution

It introduces a new effective pseudo-spin-1/2 model derived from a Hubbard model considering trigonal distortions and spin-orbit coupling, differing from previous jeff=1/2 approaches.

Findings

Emergent pseudo-spins from spin-orbital locking.

Anisotropic, frustrated spin Hamiltonian with further neighbor interactions.

Mean field theory predicts a zig-zag magnetic order consistent with experiments.

Abstract

The nature of the effective spin Hamiltonian and magnetic order in the honeycomb iridates is explored by considering a trigonal crystal field effect and spin-orbit coupling. Starting from a Hubbard model, an effective spin Hamiltonian is derived in terms of an emergent pseudo-spin-1/2 moment in the limit of large trigonal distortions and spin-orbit coupling. The present pseudo-spins arise from a spin-orbital locking and are different from the jeff = 1/2 moments that are obtained when the spin-orbit coupling dominates and trigonal distortions are neglected. The resulting spin Hamiltonian is anisotropic and frustrated by further neighbour interactions. Mean field theory suggests a ground state with 4-sublattice zig-zag magnetic order in a parameter regime that can be relevant to the honeycomb iridate compound Na2IrO3, where similar magnetic ground state has recently been observed. Various properties of the phase, the spin-wave spectrum and experimental consequences are discussed. The present approach contrasts with the recent proposals to understand iridate compounds starting from the strong spin-orbit coupling limit and neglecting non-cubic lattice distortions.