| By Hamiltonian path-integration a purely-quantum, self-consistent, spin wave approximation can be developed for spin models on a lattice, that finally allows to remap the original quantum problem to a classical one ruled by an effective classical spin Hamiltonian. Such approach has revealed especially valuable to investigate the thermodynamic behaviour of systems characterized by an intermediate value of the spin, that can not be easily addressed by other methods, both numerical and analytical. This has made possible to quantitatively interpret experimental data available for S=1 and higher spin compounds and to study the behaviour of different observables when, by incresing S, the quantum behaviour approaches the classical one. The outlined method has been usefully employed to fill the gap between the predictions of field-theoretical approaches and semiclassical and experimental results on 2D isotropic antiferromagnets. Moreover, we have focused our attention on the spin-flop phase of a quantum 2D antiferromagnet frustrated by an applied external, uniform magnetic field. The resulting field-tunable Berezinskii-Kosterlitz-Thouless (BKT) behaviour has been shown to well concur with the experimental findings on the S=5/2 compound Mn(HCOO)_2.2H_2O: the possibility to tune the effective easy-plane anisotropy driving the transition by acting on the external magnetic field is proved to represent an unique opportunity to observe the otherwise elusive BKT critical behaviour in real magnetic systems. |
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