[Paper Review] Floquet Weyl Magnons

S. A. Owerre|arXiv (Cornell University)|Jan 10, 2018

Topological Materials and Phenomena74 references3 citations

TL;DR

This paper proposes a mechanism to realize magnonic Weyl points in three-dimensional quantum magnets via circularly-polarized light irradiation, using Floquet-Bloch theory to induce effective time-reversal symmetry breaking. The key result is the emergence of tunable, topologically protected magnonic Weyl points in both Dirac nodal-line and gapped trivial insulator systems, confirmed through band structure, Berry curvature, and anomalous thermal Hall effect calculations.

ABSTRACT

In three-dimensional (3D) quantum magnets, magnonic Weyl points (WPs) featuring linear band crossing of two non-degenerate magnon branches can emerge in certain lattice geometry when time-reversal symmetry is broken macroscopically. Unfortunately, there are very limited 3D quantum magnets that host magnonic WPs, and they are yet to be observed experimentally because the intrinsic perturbative interactions that break time-reversal symmetry macroscopically can be very negligible. Here, we present an alternative means via photo-irradiation, in which magnonic WPs can emerge in 3D quantum magnets without relying on intrinsic perturbative interactions to break time-reversal symmetry. By utilizing the magnonic Floquet-Bloch theory, we put forward the general theory of magnonic Floquet WPs in 3D quantum magnets. We show that periodically driven 3D magnonic Dirac nodal-line (DNL) and 3D magnonic gapped trivial insulators can generate 3D magnonic Floquet WPs, which can be tuned by the incident circularly-polarized light. We demonstrate the existence of magnonic Floquet WPs by combining the study of the magnon dispersions, Berry curvatures, and the anomalous thermal Hall effect. The general theoretical formalism can be applied to different magnetic insulators, and thus extending the concept of magnonic WPs to a broader class of 3D magnetically ordered systems.

Motivation & Objective

• To address the experimental challenge of observing magnonic Weyl points in 3D quantum magnets due to weak intrinsic time-reversal symmetry-breaking interactions.
• To explore alternative mechanisms for realizing topologically protected magnonic Weyl points beyond intrinsic perturbative interactions.
• To extend the concept of magnonic Weyl points to a broader class of magnetic insulators using periodic photo-irradiation.
• To establish a general theoretical framework for magnonic Floquet Weyl points using Floquet-Bloch theory and topological invariants.
• To demonstrate the existence of these states through magnon dispersions, Berry curvature, and the anomalous thermal Hall effect.

Proposed method

• Utilizes magnonic Floquet-Bloch theory to describe periodically driven 3D quantum magnets under circularly-polarized light irradiation.
• Derives the effective Floquet Hamiltonian using time-dependent perturbation theory, incorporating light-induced vector potentials and Aharonov-Casher phases.
• Models the system as a massless neutral particle with magnetic dipole moment coupled to time-periodic electromagnetic fields via the Dirac-Pauli Lagrangian.
• Computes the effective Hamiltonian in momentum space, showing light-tunable Weyl point formation through Bessel function modulations of hopping integrals.
• Analyzes topological invariants such as Berry curvature and quantifies the anomalous thermal Hall effect to confirm Weyl point signatures.
• Applies the formalism to both Dirac nodal-line and gapped trivial insulator systems to demonstrate universal emergence of magnonic Weyl points.

Experimental results

Research questions

• RQ1 Can magnonic Weyl points be induced in 3D quantum magnets without relying on intrinsic time-reversal symmetry-breaking interactions?
• RQ2 How does circularly-polarized light drive the formation of topologically protected magnonic Weyl points in magnetic insulators?
• RQ3 What is the role of the Aharonov-Casher effect in generating effective gauge fields for magnons in periodically driven systems?
• RQ4 Can the anomalous thermal Hall effect serve as a robust experimental signature of magnonic Weyl points in photo-irradiated systems?
• RQ5 To what extent can the Floquet engineering approach be generalized across different classes of 3D magnetic insulators?

Key findings

• Magnonic Weyl points emerge in 3D quantum magnets under circularly-polarized light irradiation, even in the absence of intrinsic time-reversal symmetry breaking.
• The Weyl points are tunable via the intensity and polarization of the incident light, enabling dynamic control of topological magnon states.
• The effective Hamiltonian derived from the Floquet-Bloch theory shows light-induced Bessel function-modulated hopping terms that generate linear band crossings.
• The system exhibits a non-zero Berry curvature centered at the Weyl points, confirming their topological nature as monopoles in momentum space.
• A finite anomalous thermal Hall signal is predicted, providing a measurable experimental signature of the magnonic Weyl points.
• The mechanism applies universally to both Dirac nodal-line and gapped trivial insulator systems, significantly broadening the scope of potential host materials.