Prof Radu Coldea

Abstract:

A continuum of magnetic states has been observed by neutron scattering from the spin-1 chain compound CsNiCl3 in its disordered gapped one-dimensional phase. Results using both triple-axis and time-of-flight spectrometers show that around the antiferromagnetic point Qc = pi, the continuum lies higher in energy than the Haldane gapped excitations. At 6 K the integrated intensity of the continuum is about 12(2)% of the total spectral weight. This result is considerably larger than the 1%-3% weight predicted by the nonlinear sigma model for the three-particle continuum.

Abstract:

The magnetic excitation spectrum in the two-dimensional (2D) S=1/2 Heisenberg antiferromagnet copper deuteroformate tetradeuterate (CFTD) has been measured for temperatures up to T\sim J/2, where J=6.31+-0.02 meV is the 2D exchange coupling. For T\ll J, a dispersion of the zone boundary energy is observed, which is attributed to a wavevector dependent quantum renormalization. At higher temperatures, spin-wave-like excitations persist, but are found to broaden and soften. By combining our data with numerical calculations, and with existing theoretical work, a consistent description of the behaviour of the model system is found over the whole temperature interval investigated.