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arXiv:cond-mat/0210568 · Physical Review Letters

Dominance of the Excitation Continuum in the Longitudinal Spectrum of Weakly Coupled Heisenberg S=1/2 Chains

A. Zheludev, Kazuhisa Kakurai, Takatsugu Masuda, K. Uchinokura, Kenji Nakajima

Published Oct 22, 2002• 29 citations
arXivHTMLPDF

Abstract

A low-field spin-flop transition in the quasi-one-dimensional antiferromagnet $\mathrm{B}\mathrm{a}\mathrm{C}{\mathrm{u}}_{\mathrm{2}}\mathrm{S}{\mathrm{i}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{7}}$ is exploited to study the polarization dependence of low-energy magnetic excitations. The measured longitudinal spectrum is best described as a single broad continuum, with no sharp ``longitudinal mode,'' in apparent contradiction with the commonly used chain-mean-field and random phase approximation (MF/RPA) theories. The observed behavior is also quite different than that previously seen in the related $\mathrm{K}\mathrm{C}\mathrm{u}{\mathrm{F}}_{\mathrm{3}}$ material, presumably due to a large difference in the relative strength of interchain interactions. The results highlight the limitations of the chain-MF/RPA approach.

DOI
10.1103/physrevlett.89.197205

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Inelastic neutron scattering is used to study transverse-polarized magnetic excitations in the quasi-one-dimensional S=1/2S\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1/2S=1/2 antiferromagnet BaCu2Si2O7{\mathrm{BaCu}}_{2}{\mathrm{Si}}_{2}{\mathrm{O}}_{7}BaCu2​Si2​O7​, where the saturation value for the N\'eel order parameter is ${m}_{0}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.12{\ensuremath{\mu}}_{\mathrm{B}}$ per spin. At low energies the spectrum is totally dominated by resolution-limited spin-wave-like excitations. An excitation continuum sets in above a well-defined threshold frequency. Experimental results are discussed in the context of current theories for weakly interacting quantum half-integer-spin chains.
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