Dynamical properties of the honeycomb-lattice Iridates ${\rm Na_2IrO_3}$

TL;DR

This paper explores the dynamical magnetic properties of Na2IrO3 using numerical and analytical methods, revealing discrepancies with experiments due to its proximity to a quantum spin-liquid phase.

Contribution

It compares multiple effective models for Na2IrO3 and identifies the model that best matches experimental data, highlighting the importance of phase boundary proximity.

Findings

Effective ab initio model matches neutron scattering data

Discrepancy between spin-wave theory and dynamical structure factors

Na2IrO3 is near a Kitaev spin-liquid phase boundary

Abstract

We investigate the dynamical properties of ${\rm Na_2IrO_3}$. For five effective models proposed for ${\rm Na_2IrO_3}$, we numerically calculate dynamical structure factors (DSFs) with an exact diagonalization method. An effective model obtained from $ab$ $initio$ calculations explains inelastic neutron scattering experiments adequately. We further calculate excitation modes based on linearized spin-wave theory. The spin-wave excitation of the effective models obtained by $ab$ $initio$ calculations disagrees with the low-lying excitation of DSFs. We attribute this discrepancy to the location of ${\rm Na_2IrO_3}$ in a parameter space close to the phase boundary with the Kitaev spin-liquid phase.