Quantum magnetism and quantum phase transitions

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.

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.