Thermally Enhanced Pseudo-Goldstone Spin Gap in the Isotropic Helimagnet ZnCr$_2$Se$_4$
In magnetically ordered systems, the spin gap in the magnon spectrum is normally expected to decrease on warming and close at the transition out of the long-range order. The presence of a pseudo-Goldstone gap is commonly associated with nonlinear terms in spin-wave theory. However, its precise origin and detailed properties can vary and remain under debate. Here we show by means of inelastic neutron scattering that the pseudo-Goldstone gap in the Heisenberg helimagnet ZnCr$_2$Se$_4$ exhibits an atypical, nonmonotonic temperature dependence: the gap increases over much of the ordered phase before closing near $T_\mathrm{N}$. These results align with recent spin-dynamic simulations based on an effective quartic potential in an isotropic system~[Phys. Rev. B \textbf{113}, L220411 (2026)]. As the pseudo-Goldstone spin gap in ZnCr$_2$Se$_4$ is dominated by quantum effects at subkelvin temperatures, its characteristic growth on warming is caused by thermal fluctuations.