Thermodynamics of quantum 2D Heisenberg magnets with intermediate spin

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 map the original quantum problem to a classical one ruled by an effective classical spin Hamiltonian. Such approach has revealed especially valuable to investigate systems with S>1/2 which cannot be easily addressed by other methods. This has made possible to quantitatively interpret experimental data for intermediate-spin compounds and to study how different observables reach the classical limit by increasing S. Here, we focus on the spin-flop phase of a quantum 2D antiferromagnet frustrated by an applied magnetic field that acts as an effective easy-plane anisotropy and determines Berezinskii-Kosterlitz-Thouless (BKT) behavior. By acting on the field one can tune the BKT transition temperature, giving a unique opportunity to observe the otherwise elusive BKT critical behavior in real magnetic systems. The calculated data are shown to well concur with the experimental findings for the S=5/2 compound manganese-formate-dihydrate.