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The properties of Haldane excitations and multi-particle states in the antiferromagnetic spin-1 chain compound CsNiCl

M. Kenzelmann·R. A. Cowley·W. J. L. Buyers·Z. Tun·R. Coldea·M. Enderle
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Abstract

We report inelastic time-of-flight and triple-axis neutron scattering measurements of the excitation spectrum of the coupled antiferromagnetic spin-1 Heisenberg chain system CsNiCl3. Measurements over a wide range of wave-vector transfers along the chain confirm that above T_N CsNiCl3 is in a quantum-disordered phase with an energy gap in the excitation spectrum. The spin correlations fall off exponentially with increasing distance with a correlation length xi=4.0(2) sites at T=6.2K. This is shorter than the correlation length for an antiferromagnetic spin-1 Heisenberg chain at this temperature, suggesting that the correlations perpendicular to the chain direction and associated with the interchain coupling lower the single-chain correlation length. A multi-particle continuum is observed in the quantum-disordered phase in the region in reciprocal space where antiferromagnetic fluctuations are strongest, extending in energy up to twice the maximum of the dispersion of the well-defined triplet excitations. We show that the continuum satisfies the Hohenberg-Brinkman sum rule. The dependence of the multi-particle continuum on the chain wave-vector resembles that of the two-spinon continuum in antiferromagnetic spin-1/2 Heisenberg chains. This suggests the presence of spin-1/2 degrees of freedom in CsNiCl3 for T < 12K, possibly caused by multiply-frustrated interchain interactions.

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M. Kenzelmann, R. A. Cowley, W. J. L. Buyers, Z. Tun, R. Coldea, M. Enderle. 2001-12-10. The properties of Haldane excitations and multi-particle states in the antiferromagnetic spin-1 chain compound CsNiCl. https://doi.org/10.1103/physrevb.66.024407

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