Pseudo-goldstone magnons in the frustrated S=3/2 heisenberg helimagnet ZnCr₂Se₄ with a Pyrochlore Magnetic Sublattice

Low-energy spin excitations in any long-range ordered magnetic system in the absence of magneto-crystalline 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 (00q_h) 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_h 00) and (0 q_h 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.