A magnetic model with a possible Chern-Simons phase

Abstract

An elementary family of local Hamiltonians $H_{\c ,\ell}, \ell = 1,2,3, ldots$, is described for a $2-$dimensional quantum mechanical system of spin $={1/2}$ particles. On the torus, the ground state space $G_{\circ,\ell}$ is $(\log)$ extensively degenerate but should collapse under $ł$perturbation" to an anyonic system with a complete mathematical description: the quantum double of the $SO(3)-$Chern-Simons modular functor at $q= e^{2 πi/\ell +2}$ which we call $DE \ell$. The Hamiltonian $H_{\circ,\ell}$ defines a \underline{quantum} \underline{loop}\underline{gas}. We argue that for $\ell = 1$ and 2, $G_{\circ,\ell}$ is unstable and the collapse to $G_{ε, \ell} \cong DE\ell$ can occur truly by perturbation. For $\ell \geq 3$, $G_{\circ,\ell}$ is stable and in this case finding $G_{ε,\ell} \cong DE \ell$ must require either $ε> ε_\ell > 0$, help from finite system size, surface roughening (see section 3), or some other trick, hence the initial use of quotes $ł\quad$". A hypothetical phase diagram is included in the introduction.