Abstract
Magnetization plateaus in quantum magnets—where bosonic quasiparticles crystallize into emergent spin superlattices—are spectacular yet simple examples of collective quantum phenomena escaping classical description. While magnetization plateaus have been observed in a number of spin-1/2 antiferromagnets, the description of their magnetic excitations remains an open theoretical and experimental challenge. Here, we investigate the dynamical properties of the triangular-lattice spin-1/2 antiferromagnet Ba3CoSb2O9 in its one-third magnetization plateau phase using a combination of nonlinear spin-wave theory and neutron scattering measurements. The agreement between our theoretical treatment and the experimental data demonstrates that magnons behave semiclassically in the plateau in spite of the purely quantum origin of the underlying magnetic structure. This allows for a quantitative determination of Ba3CoSb2O9 exchange parameters. We discuss the implication of our results to the deviations from semiclassical behavior observed in zero-field spin dynamics of the same material and conclude they must have an intrinsic origin.
Highlights
Notwithstanding the progress in the search of quantum plateaus, much less is known about their excitation spectra
We demonstrate that the modified nonlinear spin wave (NLSW) approach reproduces the mplaatgenaeut3ic0–3e9x.citTahtieonexscpeelcletrnutmagorfeeBmae3nCtoSbbe2tOw9eewniththineotrhye
We present a comprehensive study of magnon excitations in the 1/3 magnetization plateau phase of a quasi-two-dimensional TLHAFM with easy-plane exchange anisotropy
Summary
Notwithstanding the progress in the search of quantum plateaus, much less is known about their excitation spectra. Given that a sizable reduction of |〈Sr〉| is unlikely within the plateau because of the gapped nature of the spectrum, a spin wave description could be adequate. This may seem in conflict with the order-by-disorder mechanism[1,2,3] stabilizing the plateau[40,41,42], this energy correction phenomenonPis Ezp 1⁄4 ð1=2Þ q produced by the zero-point ωq þ OðS0Þ (ωq is the spin wave dispersion), which does not necessarily produce a large moment size reduction. The resulting model parameters confirm that the anomalous zero-field dynamics reported in two independent experiments[37,39] must be intrinsic and non-classical
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