Prof Radu Coldea

Abstract:

The magnetic excitations of the square-lattice spin-1/2 antiferromagnet and high- T(c) parent compound La2CuO4 are determined using high-resolution inelastic neutron scattering. Sharp spin waves with absolute intensities in agreement with theory including quantum corrections are found throughout the Brillouin zone. The observed dispersion relation shows evidence for substantial interactions beyond the nearest-neighbor Heisenberg term which can be understood in terms of a cyclic or ring exchange due to the strong hybridization path around the Cu4O4 square plaquettes.

Abstract:

The ground-state ordering and dynamics of the two-dimensional S = 1/2 frustrated Heisenberg antiferromagnet Cs(2)CuCl(4) are explored using neutron scattering in high magnetic fields. We find that the dynamic correlations show a highly dispersive continuum of excited states, characteristic of the resonating valence bond state, arising from pairs of S = 1/2 spinons. Quantum renormalization factors for the excitation energies (1.65) and incommensuration (0.56) are large.

Abstract:

Neutron scattering measurements have been made of the excitations of the S = 1 antiferromagnetic chain compound CSNiCl3. The new results show the existence of a continuum component to the excitation spectrum at energies higher than the Haldane gap and that the Haldane gap excitation increases in energy and remains resonant as the temperature is increased. © 2001 Elsevier Science B.V. All rights reserved.