## Pseudo-Goldstone magnons in the frustrated $$S=3/2$$ Heisenberg helimagnet $$ZnCr_2Se_4$$ with a pyrochlore magnetic sublattice

Please always quote using this URN: urn:nbn:de:bvb:20-opus-172770
• 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 $${ZnCr_2Se_4}$$ with a cubic spinel structure, in which spin$$-3/2$$ magnetic $${Cr^{3+}}$$ ions are arranged in aLow-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 $${ZnCr_2Se_4}$$ with a cubic spinel structure, in which spin$$-3/2$$ magnetic $${Cr^{3+}}$$ ions are arranged in a geometrically frustrated pyrochlore sublattice. Apart from the conventional Goldstone mode emanating from the $$(0~ 0~ {q_h})$$ 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 $$({q_h}~ 0~ 0)$$ and $${(0~ {q_h}~ 0)}$$ where no magnetic Bragg peaks are present. We term them pseudo-Goldstone magnons, as they appear gapless within linear spinwave 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.