Quantum magnetism and quantum phase transitions

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.

Abstract:

Elastic and inelastic neutron scattering were used to study the ordered phase of the quasi-one-dimensional spin-1/2 antiferromagnet ${\rm BaCu_{2}Si_{2}O_{7}}$. The previously proposed model for the low-temperature magnetic structure was confirmed. Spin wave dispersion along several reciprocal-space directions was measured and inter-chain, as well as in-chain exchange constants were determined. A small gap in the spin wave spectrum was observed and attributed to magnetic anisotropy effects. The results are discussed in comparison with existing theories for weakly-coupled quantum spin chain antiferromagnets.