On structural invariants of energy spectrum of S=1 Heisenberg antiferromagnets with single-ion anisotropy
Abstract
We study the relationship of the energy spectrum of finite S=1 Heisenberg antiferromagnets with their structure in the presence of single-ion anisotropy. We show that in the limit of strong easy-plane anisotropy magnets with the structure of adjacency cospectral graphs have equal ground state energies with magnetization M=0. We derive additional necessary condition for equality of lowest energy levels with M=±1. For strong easy-axis anisotropy we found that bipartite S=1 magnets with structures, for which S=1/2 Ising models have equal spectra for arbitrary longitudinal magnetic field, have close energy spectra of S=1 antiferromagnets for arbitrary parameter of single-ion anisotropy. For moderate easy-axis anisotropy bipartite S=1 antiferromagnets with equal energies of spin waves in linear approximation are also approximately isoenergetic. Overall, this explains the remarkable similarity of energy spectra in M=0 subspace for S=1 antiferromagnetic Heisenberg model on bipartite cospectral regular graphs.
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