We have carried out inelastic neutron scattering on CuC12 2N(C5D5), at T =1.2 K and at magnetic fields up to 70 kOe. The spin dynamics of this typical s =one-dimensional Heisen-2 berg antiferromagnet have previously been investigated at zero magnetic field by Endoh et al. , using neutron scattering. They observed a spectrum of magnetic excitations in close agreement with the spectrum of lowest excited states as calculated exactly by des Cloizeaux and Pearson (dCP). The marked asymmetry in the line shape of the neutron response previously observed is carefully reexamined and is shown to be a true effect, in agreement with several theoretical predictions. At high magnetic field, a broadening of the neutron response is observed, especially pronounced at the antiferromagnetic zone boundary, where the peak smears out at 70 kOe. For wave vectors near an antiferromagnetic Bragg point a decrease in the peak energy is observed for increasing field, lending qualitative support to the calculations of Ishimura and Shiba of the field dependence of the dCP states.

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We present in this paper the results of a detailed neutron scattering investigation at various temperatures of the dynamical properties of the linear-mercury-chain compound Hg,~AsF, . The main emphasis has been put on the nature of the response function of the Hg chains at higher temperatures (T & 150 K). The results are analyzed and discussed in terms of new theoretical results obtained by Emery and Axe, and it is found that the behavior. of the. Hg chains in this temperature region is one-dimensional liquidlike, in accordance with theoretical predictions. Below T, = 120 K where three-dimensional long-range order is established among the Hg ions, the transverse phononlike excitations in the Hg chains with displacement along the chain develop a small (-0.1 meV) energy gap at finite wave vector. This feature makes it straightforward to demonstrate the absence of elastic scattering in the sheets of scattering from the Hg chains. We have measured at room temperature acoustic AsF6 sublattice phonons in the 6((00) and the A(00() directions. The observed dispersion slope of the. transverse branch along 5 polarized along A is about 25% smaller than that of the transverse branch along A polarized along 6, in apparent disagreement with elasticity theory which predicts both modes to be governed by C44. We show that this behavior can be qualitatively explained by the anisotropic coupling between Hg chains and the host lattice, leading to a much smaller elastic regime in the A direction than in the 5 direction.

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