# Pseudo-Goldstone magnons in the frustrated S=3/2 Heisenberg helimagnet ZnCr2Se4 with a pyrochlore magnetic sublattice

## TL;DR

This study investigates low-energy magnetic excitations in the frustrated S=3/2 Heisenberg helimagnet ZnCr2Se4, revealing pseudo-Goldstone magnons with small energy gaps, supported by neutron spectroscopy and theoretical calculations.

## Contribution

It uncovers the existence of pseudo-Goldstone magnons with finite gaps in a cubic spinel helimagnet, highlighting their universal presence and implications for studying magnon interactions.

## Key findings

- Identification of soft helimagnon modes with ~0.17 meV gap
- Observation of pseudo-Goldstone magnons emerging from orthogonal wave vectors
- Support for the universality of these excitations in symmetric helimagnets

## Abstract

Low-energy spin excitations in any long-range ordered magnetic system in the absence of magnetocrystalline anisotropy are gapless Goldstone modes emanating from the ordering wave vectors. In helimagnets, these modes hybridize into the so-called helimagnon excitations. Here we employ neutron spectroscopy supported by theoretical calculations to investigate the magnetic excitation spectrum of the isotropic Heisenberg helimagnet ZnCr2Se4 with a cubic spinel structure, in which spin-3/2 magnetic Cr3+ ions are arranged in a geometrically frustrated pyrochlore sublattice. Apart from the conventional Goldstone mode emanating from the (0 0 q) ordering vector, low-energy magnetic excitations in the single-domain proper-screw spiral phase show soft helimagnon modes with a small energy gap of ~0.17 meV, emerging from two orthogonal wave vectors (q 0 0) and (0 q 0) where no magnetic Bragg peaks are present. We term them pseudo-Goldstone magnons, as they appear gapless within linear spin-wave theory and only acquire a finite gap due to higher-order quantum-fluctuation corrections. Our results are likely universal for a broad class of symmetric helimagnets, opening up a new way of studying weak magnon-magnon interactions with accessible spectroscopic methods.