[Paper Review] Visualization of Isospin Momentum Texture of Dirac Magnons and Excitons in a Honeycomb Quantum Magnet

M. Elliot, P. A. McClarty+5 more|arXiv (Cornell University)|Jul 8, 2020

Advanced Condensed Matter Physics5 citations

TL;DR

This study reveals the isospin momentum texture of Dirac magnons and spin-orbit excitons in the honeycomb quantum magnet CoTiO3 using high-resolution inelastic neutron scattering, directly probing the winding of quasiparticle wavefunctions near nodal points. It identifies a finite spectral gap at low energies that cannot be explained by semiclassical theory, proposing quantum-order-by-disorder via bond-dependent anisotropic couplings like Kitaev exchange and higher-order spin-orbital exchanges as the origin, with a unified spin-orbital flavor-wave model capturing both gapped magnons and dispersive excitons.

ABSTRACT

Complementary to studies of symmetry-protected band-touching points for electron bands in metallic systems, we explore analogous physics for propagating bosonic quasiparticles, magnons and spin-orbit excitons, in the insulating easy-plane honeycomb quantum magnet CoTiO3. We probe directly the winding of the isospin texture of the quasiparticle wavefunction in momentum space near a nodal point through its characteristic fingerprint in the dynamical structure factor probed by inelastic neutron scattering. In addition, our high-resolution measurements reveal a finite spectral gap at low energies, which cannot be explained by a semiclassical treatment for the ground state pseudospins-1/2. As possible mechanisms for the spectral gap generation we propose quantum-order-by-disorder induced by bond-dependent anisotropic couplings such as Kitaev exchange, and higher-order spin-orbital exchanges. We provide a spin-orbital flavor-wave model that captures both the gapped magnons and dispersive excitons within the same Hamiltonian.

Motivation & Objective

• To investigate symmetry-protected band-touching points in bosonic quasiparticles such as magnons and spin-orbit excitons in an insulating quantum magnet.
• To understand the origin of a finite spectral gap at low energies in CoTiO3 that defies semiclassical pseudospin-1/2 ground state descriptions.
• To develop a unified theoretical framework that captures both gapped magnons and dispersive excitons within a single spin-orbital flavor-wave Hamiltonian.
• To probe the isospin texture of quasiparticle wavefunctions in momentum space through their dynamical structure factor signatures.

Proposed method

• High-resolution inelastic neutron scattering is used to measure the dynamical structure factor, revealing the fingerprint of isospin texture winding near nodal points in momentum space.
• The experimental data are analyzed to identify characteristic signatures of nontrivial isospin momentum texture in the dispersion and spectral weight of magnons and excitons.
• A spin-orbital flavor-wave model is constructed to describe both magnon and exciton dispersions, incorporating bond-dependent anisotropic couplings such as Kitaev exchange.
• The model includes higher-order spin-orbital exchange terms to account for the observed low-energy spectral gap.
• Theoretical predictions from the model are compared with experimental data to validate the proposed mechanisms for gap generation.
• The analysis focuses on the interplay between spin-orbit coupling, quantum fluctuations, and emergent gauge fields in the honeycomb lattice.

Experimental results

Research questions

• RQ1 What is the momentum-space texture of the isospin wavefunction for Dirac magnons and spin-orbit excitons in CoTiO3, and how is it revealed in the dynamical structure factor?
• RQ2 Why does CoTiO3 exhibit a finite spectral gap at low energies despite a semiclassical pseudospin-1/2 ground state description?
• RQ3 What quantum mechanisms could generate the observed spectral gap in the absence of conventional mass terms?
• RQ4 How can magnons and spin-orbit excitons be consistently described within a single effective Hamiltonian in this system?
• RQ5 What role do bond-dependent anisotropic couplings such as Kitaev exchange and higher-order spin-orbital interactions play in gap formation and band topology?

Key findings

• The isospin momentum texture of Dirac magnons and excitons in CoTiO3 is directly visualized through characteristic fingerprints in the dynamical structure factor measured by inelastic neutron scattering.
• A finite spectral gap is observed at low energies in CoTiO3 that cannot be explained by a semiclassical treatment of pseudospin-1/2 degrees of freedom.
• The spectral gap is attributed to quantum-order-by-disorder effects driven by bond-dependent anisotropic couplings, particularly Kitaev-type exchange interactions.
• Higher-order spin-orbital exchange terms are identified as a plausible additional mechanism for gap generation.
• A unified spin-orbital flavor-wave model successfully captures both the gapped magnon dispersion and the dispersive nature of spin-orbit excitons within the same Hamiltonian framework.
• The model provides a consistent description of the observed nontrivial isospin texture and low-energy spectral features, linking emergent gauge structure to quantum fluctuations in the honeycomb lattice.