Effective Field Theories of Frustrated Antiferromagnets: Helical Edge Modes and Spontaneous Chiral Symmetry Breaking in the Heisenberg Triangular Antiferromagnet

Date

Aug 1, 2025

Time

2:00 PM - 3:00 PM

Speaker

Dr. Bastian Pradenas

Affiliation

Johns Hopkins University, Baltimore

Language

en

Main Topic

Materialien

Host

Grit Rötzer

Description

. We investigate the emergence of helical edge modes and new quantum numbers in a Heisenberg antiferromagnet on a triangular lattice, uncovering novel symmetry-driven phenomena in noncollinear spin systems. Using a spin-frame effective field theory for three-sublattice order, we demonstrate that the system admits a topological term in its energy functional which modifies the boundary conditions for spin waves, giving rise to helical edge modes - counterpropagating spin-wave excitations with opposite circular polarizations—reminiscent of edge states in two-dimensional topological insulators. Notably, this mechanism requires neither Dzyaloshinskii–Moriya interactions nor nontrivial magnon band topology; rather, the edge modes are stabilized by a disparity between inequivalent superexchange paths. In addition, we identify a previously overlooked symmetry structure: the spin order parameter transforms under an enlarged group, SO(3)L x SO(2)R, rather than the conventional spin-rotation group SO(3). Although this extended symmetry is spontaneously broken by the ground state, a residual subgroup persists, giving rise to conserved Noether charges. Upon quantization, these charges endow magnons with an emergent quantum number—isospin—in addition to energy and momentum, drawing unexpected parallels to chiral symmetry breaking in particle physics. References: [1] T. Dombre and N. Read, Phys. Rev. B 39, 6797 (1989). [2] Bastián Pradenas and Oleg Tchernyshyov, Phys. Rev. Lett. 132, 096703 (2024). [3] Bastián Pradenas, Grigor Adamyan, and Oleg Tchernyshyov, Phys. Rev. B 110, 224426 (2024). [4] Bastián Pradenas, Grigor Adamyan, and Oleg Tchernyshyov, Emergent Isospin from Chiral Symmetry Breaking in Noncollinear Antiferromagnets, arXiv:2504.12411. [5] Journal Club for Condensed Matter Physics, May 2025 selection. .

Links

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Last modified: Aug 1, 2025, 7:35:35 AM