Renormalized classical theory of quantum magnets

Abstract

We derive a renormalized classical spin (RCS) theory for 𝑆>1/2 quantum magnets by constraining a generalized classical theory that includes all multipolar fluctuations to a reduced CP1 phase space of dipolar SU(2) coherent states. When the spin Hamiltonian ˆℋ⁡(𝑆) is linear in the spin operators ˆ𝑺𝑗 for each lattice site 𝑗, the RCS Hamiltonian ˜ℋcl coincides with the usual classical model ℋcl=lim𝑆→∞⁡ˆℋ⁡(𝑆). In the presence of nonlinear terms, however, the RCS theory is more accurate than ℋcl. For the many materials modeled by spin Hamiltonians with (nonlinear) single-ion anisotropy terms, the use of the RCS theory is essential to accurately model phase diagrams and to extract the correct Hamiltonian parameters from neutron-scattering data.