Measurement of the spin-excitation continuum in one-dimensionalKCuF3using neutron scattering

In this, and the following paper, we present the results of a detailed experimental investigation into the spin dynamics of the quasi-one-dimensional S=1/2 Heisenberg antiferromagnet ${\mathrm{KCuF}}_3$. In this paper we report results of inelastic neutron-scattering measurements made using the MARI time-of-flight spectrometer at the Rutherford Appleton Laboratory. Measurement of the magnetic excitation spectrum was made in both the low-temperature phase, where the chains are ordered together, and in the high-temperature phase, where the correlations between chains become unimportant. We compare our measurements with the Müller ansatz, which is identified with a two-spinon cross section, and find excellent agreement with this. The effect of temperature on the excitation continuum was also investigated. A series of measurements were made at temperatures ranging from 20 to 200 K. These results are compared to the predictions of field theory for the one-dimensional Heisenberg chain. Again good agreement is found with the theoretical predictions. In the following paper measurements of the low-energy scattering in the ordered phase, made on a triple-axis spectrometer, are presented.