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.
Highlights
We have used thermal- and cold-neutron time-of-flight (TOF) and triple-axis spectroscopy (TAS) techniques to map out dispersions of magnetic excitations in ZnCr2Se4 in a broad range of energies
A remarkable hallmark of the observed pseudoGoldstone modes is that linear spin-wave theory (LSWT) predicts them to be gapless in the absence of magnetocrystalline anisotropy [43] at wave vectors where no magnetic Bragg reflections are found below TN
This is in contrast to magnetic soft modes observed, for example, in α-CaCr2O4 [44], which originate from a proximity to another phase with a different magnetic ordering vector and have a much larger gap that is well captured by LSWT
Summary
Considering only the antiferromagnetic nearest-neighbor (NN) interactions results in a classical spin liquid [1], exhibiting no longrange magnetic order down to zero temperature. This is explained by strong geometric frustration that leads to a highly degenerate classical ground state. Depending on the chemical composition, chromium spinels exhibit different mechanisms of frustration, such as geometric frustration that occurs if dominant NN interactions are antiferromagnetic, or bond frustration, which originates from competition between ferromagnetic NN and antiferromagnetic further-neighbor exchange. The importance of the two third-nearest-neighbor exchange paths on the pyrochlore lattice has been emphasized for the spin-1=2 molybdate Heisenberg antiferromagnet Lu2Mo2O5N2 [11], where J03 and J030 have opposite signs and dominate over J2. It was recently conjectured that this may lead to an unusual “gearwheel” type of a quantum spin liquid [12]
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