Heisenberg-Kitaev model in a magnetic field: $1/S$ expansion

TL;DR

This paper investigates the quantum effects in the Heisenberg-Kitaev model under magnetic fields using a $1/S$ expansion, revealing significant modifications to the classical phase diagram and providing a method to analyze quantum corrections in spin models.

Contribution

It introduces a comprehensive $1/S$ expansion approach to study quantum corrections in the Heisenberg-Kitaev model with broken spin-rotational symmetry under magnetic fields.

Findings

Quantum corrections significantly alter the classical phase diagram.

The $1/S$ expansion effectively captures leading quantum effects.

The study provides a general framework for analyzing quantum corrections in similar spin models.

Abstract

The exact solution of Kitaev's spin-$1/2$ honeycomb spin-liquid model has sparked an intense search for Mott insulators hosting bond-dependent Kitaev interactions, of which $\mathrm{Na}_{2}\mathrm{IrO}_{3}$ and $\alpha-\mathrm{RuCl}_{3}$ are prime examples. Subsequently, it has been proposed that also spin-$1$ and spin-$3/2$ analogs of Kitaev interactions may occur in materials with strong spin-orbit coupling. As a minimal model to describe these Kitaev materials, we study the Heisenberg-Kitaev Hamiltonian in a consistent $1/S$ expansion, with $S$ being the spin size. We present a comprehensive study of this model in the presence of an external magnetic field applied along two different directions, [001] and [111], for which an intricate classical phase diagram has been reported. In both settings, we employ spin-wave theory in a number of ordered phases to compute phase boundaries at the next-to-leading order in $1/S$ and show that quantum corrections substantially modify the classical phase diagram. More broadly, our work presents a consistent route to investigate the leading quantum corrections in spin models that break spin-rotational symmetry.