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

Y. V. Tymoshenko; Y. A. Onykiienko; T. Mueller; R. Thomale; S. Rachel; A. S. Cameron; P. Y. Portnichenko; D. V. Efremov; V. Tsurkan; D. L. Abernathy; J. Ollivier; A. Schneidewind; A. Piovano; V. Felea; A. Loidl; D. S. Inosov

doi:10.1103/physrevx.7.041049

arXiv · 1705.04642

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

Y. V. Tymoshenko·Y. A. Onykiienko·T. Mueller·R. Thomale·S. Rachel·A. S. Cameron·P. Y. Portnichenko·D. V. Efremov·V. Tsurkan·D. L. Abernathy·J. Ollivier·A. Schneidewind·A. Piovano·V. Felea·A. Loidl·D. S. Inosov

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Abstract

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.

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Y. V. Tymoshenko, Y. A. Onykiienko, T. Mueller, R. Thomale, S. Rachel, A. S. Cameron, P. Y. Portnichenko, D. V. Efremov, V. Tsurkan, D. L. Abernathy, J. Ollivier, A. Schneidewind, A. Piovano, V. Felea, A. Loidl, D. S. Inosov. 2017-05-12. Pseudo-Goldstone magnons in the frustrated S=3/2 Heisenberg helimagnet ZnCr2Se4 with a pyrochlore magnetic sublattice. https://doi.org/10.1103/physrevx.7.041049

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