We investigate a family of spin-
S chain Hamiltonians recently introduced by one of us [Greiter, Mapping of Parent Hamiltonians, Springer Tracts in Modern Physics, Vol. 244 (Springer, Berlin, 2011)]. For
S=1/2, it corresponds to the Haldane-Shastry model. For general spin
S, we find indication that the low-energy theory of these spin chains is described by the SU(2)
k Wess-Zumino-Witten model with coupling
k=2S. In particular, we investigate the
S=1 model whose ground state is given by a Pfaffian for even number of sites
N. We reconcile aspects of the spectrum of the Hamiltonian for arbitrary
N with trial states obtained by Schwinger projection of two Haldane-Shastry chains.
The authors have identified critical points in the Wess-Zumino-Witten (WZW) SU(2)
5 universality class in the phase diagram of a spin-5/2 chain with frustrated next-nearest-neighbor and 3-site interactions, providing a concrete example of this universality class in a realistic, nonintegrable model. These points lie on a transition line to a dimerized phase and occur when the transition turns from first order into WZW SU(2)
3 in agreement with the generic possibilities allowed by field theory arguments.
We study the long-wavelength limit of a spin
S Heisenberg antiferromagnetic chain. The fermionic Lagrangian obtained corresponds to a perturbed level
2S SU(2) Wess-Zumino-Witten (WZW) model. This effective theory is then mapped into a compact
U(1) boson interacting with
Z2S parafermions. The analysis of this effective theory allows us to show that when
S is an integer there is a mass gap to all excitations, whereas this gap vanishes in the half-odd-integer spin case and the
SU(2)2S WZW model flows towards the
SU(2)1 stable fixed point. This gives a field theory treatment of the so-called Haldane's conjecture for arbitrary values of the spin
S.