  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.
