Order-by-Disorder from Bond-Dependent Exchange and Intensity Signature of Nodal Quasiparticles in a Honeycomb Cobaltate

TL;DR

This study combines neutron scattering experiments and theoretical analysis to reveal bond-dependent anisotropic interactions and topological features of quasiparticles in a honeycomb cobaltate, highlighting quantum order-by-disorder effects.

Contribution

It provides experimental evidence of bond-dependent exchange couplings and topological quasiparticle signatures in CoTiO3, advancing understanding of 3d transition metal magnetism beyond 4d and 5d systems.

Findings

Detection of a finite energy gap indicating bond-dependent anisotropic couplings.

Observation of universal winding in scattering intensity related to quasiparticle topology.

Demonstration of quantum order-by-disorder involving virtual crystal field fluctuations.

Abstract

Recent theoretical proposals have argued that cobaltates with edge-sharing octahedral coordination can have significant bond-dependent exchange couplings thus offering a platform in 3$d$ ions for such physics beyond the much-explored realizations in 4$d$ and 5$d$ materials. Here we present high-resolution inelastic neutron scattering data within the magnetically ordered phase of the stacked honeycomb magnet CoTiO$_3$ revealing the presence of a finite energy gap and demonstrate that this implies the presence of bond-dependent anisotropic couplings. We also show through an extensive theoretical analysis that the gap further implies the existence of a quantum order-by-disorder mechanism that, in this material, crucially involves virtual crystal field fluctuations. Our data also provide an experimental observation of a universal winding of the scattering intensity in angular scans around linear band-touching points for both magnons and dispersive spin-orbit excitons, which is directly related to the non-trivial topology of the quasiparticle wavefunction in momentum space near nodal points.