Proximate quantum spin liquids and Majorana continua in magnetically ordered Kitaev magnets

TL;DR

This paper investigates the high-energy spin excitation spectra in Kitaev magnets, revealing broad continua and spectral features consistent with experimental observations, using a Majorana parton approach within an extended Kitaev model.

Contribution

It introduces a Majorana parton-based theory to analyze high-energy features in proximate Kitaev quantum spin liquids, capturing experimental continua and magnetic field effects.

Findings

Broad multi-spinon continua at high energies across the Brillouin zone.

Qualitative agreement of zigzag ground state with experiments on $ ext{RuCl}_3$.

Anisotropic stability of magnetic order under external magnetic fields.

Abstract

We study the spin excitation spectra in magnetically ordered phases proximate to the Kitaev quantum spin liquid (KQSL). Although the low-energy universal features should be governed by the magnetic orders, the $\textit{non-universal}$ high-energy features of the KQSL and adjacent phases can be remarkably similar. Therefore, we study the extended Kitaev model within a Stoner-like theory using Majorana partons, and compute the inelastic neutron scattering (INS) intensities in the random phase approximation. First, we benchmark against the antiferromagnetic (AFM) Heisenberg model and recover the AFM order with linear Goldstone modes. We then explore the phase diagram which agrees qualitatively with previous numerical results. In particular, the Majorana parton theory accurately captures Order-by-Disorder effects in the Kitaev-Heisenberg limit. We also find large INS intensities near the associated high-symmetry Brillouin zone (BZ) points of the magnetic orders. At intermediate and high energies, broad multi-spinon continua emerge across the BZ, providing a distinct mechanism for magnon decay and spectral broadening beyond the conventional multi-magnon decay scenario. Finally, we study the model Hamiltonian of candidate Kitaev material $\alpha$-RuCl$_3$. The zigzag ground state agrees qualitatively with experiments, its stability under external magnetic field also exhibits strong anisotropy in the field directions, and broad scattering continua are recovered similar to those observed experimentally.