Do Longitudinal Magnons Exist in Weakly-Coupled Quantum Spin Chains: A Spin-Acrobatics Experiment

An unusual series of low-field spin reorientation transitions in the quasi one-dimensional antiferromagnet BaCu2Si2O7 is exploited to study the polarization dependence of low-energy magnetic excitations.

The measured longitudinal spectrum is best described as single broad continuum, with no sharp ``longitudinal mode'', in apparent contradiction with established theoretical results. The observed behavior is also quite different than that previously seen in the related KCuF3 system, and highligts the quantum-critical nature of spin dynamics in isolated chains.

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