Quantum magnetism and quantum phase transitions

Abstract:

The spin excitations in La2CuO4, a two-dimensional spin- 1/2 Heisenberg antiferromagnet and parent compound of high-Tc superconductors, have been measured throughout the Brillouin zone using high-resolution inelastic neutron scattering. The observed dispersion relations show evidence for substantial second- or third-nearest-neighbour exchange couplings in addition to the strong first nearest-neighbour antiferromagnetic exchange.

Abstract:

Measurements of the magnetic-field dependent millimetre-wave response of Cs2CuCl4, a quasi-one-dimensional Heisenberg antiferromagnet, are presented. The evolution of the electron spin resonance between high temperatures (the paramagnetic regime) and temperatures below T-N, at which there is an onset of long-range magnetic order (the antiferromagnetic regime), is studied for a range of crystal orientations. The magneto-optical data suggest that there is an onset of local magnetic order at temperatures considerably higher than T-N as determined from neutron scattering measurements, and that the evolution towards a long-range ordered state is gradual. A phenomenological model that describes the general features is suggested. (C) 1998 Elsevier Science B.V. All rights reserved.