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

摘要

Low-energy spin excitations in any long-range ordered magnetic system in theabsence of magnetocrystalline anisotropy are gapless Goldstone modes emanatingfrom the ordering wave vectors. In helimagnets, these modes hybridize into theso-called helimagnon excitations. Here we employ neutron spectroscopy supportedby theoretical calculations to investigate the magnetic excitation spectrum ofthe isotropic Heisenberg helimagnet ZnCr2Se4 with a cubic spinel structure, inwhich spin-3/2 magnetic Cr3+ ions are arranged in a geometrically frustratedpyrochlore sublattice. Apart from the conventional Goldstone mode emanatingfrom the (0 0 q) ordering vector, low-energy magnetic excitations in thesingle-domain proper-screw spiral phase show soft helimagnon modes with a smallenergy 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 thempseudo-Goldstone magnons, as they appear gapless within linear spin-wave theoryand only acquire a finite gap due to higher-order quantum-fluctuationcorrections. Our results are likely universal for a broad class of symmetrichelimagnets, opening up a new way of studying weak magnon-magnon interactionswith accessible spectroscopic methods.