Thermally Enhanced Pseudo-Goldstone Spin Gap in the Isotropic Helimagnet ZnCr$_2$Se$_4$

N. D. Andriushin; A. Sanders; J. Reuther; D. C. Peets; P. Steffens; A. Hiess; J. Lass; L. Prodan; V. Tsurkan; V. Felea; D. S. Inosov

arXiv · 2610.16439

Thermally Enhanced Pseudo-Goldstone Spin Gap in the Isotropic Helimagnet ZnCr$_2$Se$_4$

N. D. Andriushin·A. Sanders·J. Reuther·D. C. Peets·P. Steffens·A. Hiess·J. Lass·L. Prodan·V. Tsurkan·V. Felea·D. S. Inosov

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Abstract

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.

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N. D. Andriushin, A. Sanders, J. Reuther, D. C. Peets, P. Steffens, A. Hiess, J. Lass, L. Prodan, V. Tsurkan, V. Felea, D. S. Inosov. 2026-10-12. Thermally Enhanced Pseudo-Goldstone Spin Gap in the Isotropic Helimagnet ZnCr$_2$Se$_4$. https://arxiv.org/abs/2610.16439

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