College Park, Maryland      June 6 - 10 , 2004

W2-D6 (3:00 PM): Bound spinons in an antiferromagnetic S=1/2 chain with a staggered field (Invited)

M. Kenzelmann (Department of Physics & Astronomy, Johns Hopkins University; NIST Center for Neutron Research), Y. Chen (Los Alamos National Laboratory), C. Broholm (Department of Physics & Astronomy, Johns Hopkins University; NIST Center for Neutron Research), D. Reich (Department of Physics & Astronomy, Johns Hopkins University), C.D. Batista (Los Alamos National Laboratory), Y. Qiu, S. Park (Department of Materials Science and Engineering, University of Maryland)

Strongly-correlated charge and spin degrees of freedom can lead to cooperative many-body states which are believed to have wide implications for the macroscopic properties of complex solids. The emergent character of such states can give rise to non-linear excitations which are very different from those in more conventional systems like the Fermi liquids or ordered magnetic structures.We have recently studied the antiferromagnetic S=1/2 chain in high uniform and staggered fields using various cold neutron scattering techniques. We have found that the spinon excitations bind under the influence of the staggered field and appear as well-defined excitations in the neutron spectra. This is in stark contrast to broad neutron scattering spectra observed from S=1/2 chains in zero field. In the long wave-length limit, the experiments are in excellent quantitative agreement with predictions of soliton and breather excitations of the quantum sine-Gordon model. On finite length scales, we find that a mapping of the Hamiltonian to a gas of interacting fermions with spin explains the dispersion of the observed excitations surprisingly well.

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