Solvable models for unitary and nonunitary topological phases

Abstract

We introduce a broad class of simple models for quantum Hall states based on the expansion of their parent Hamiltonians near the one-dimensional limit of "thin cylinders", i.e. when one dimension $L_y$ of the Hall surface becomes comparable to the magnetic length $\ell_B$. Formally, the models can be viewed as topological generalizations of the 1D Hubbard model with center-of-mass-preserving hopping of multiparticle clusters. In some cases, we show that the models can be exactly solved using elementary techniques, and yield simple wave functions for the ground states as well as the entire neutral excitation spectrum. We study a large class of Abelian and non-Abelian states in this limit, including the Read-Rezayi $\mathbb{Z}_k$ series, as well as states deriving from non-unitary conformal field theories -- the "Gaffnian", "Haffnian", Haldane-Rezayi, and the "permanent" state. We find that the thin-cylinder limit of unitary states is "classical": their effective Hamiltonians reduce to only Hartree-type terms, the ground states are trivial insulators, and excitation gaps result from simple electrostatic repulsion. In contrast, for states deriving from non-unitary conformal field theories, the thin-cylinder limit is found to be intrinsically \emph{quantum} -- it contains hopping terms that play an important role in the structure of the ground states and in the energetics of the low-lying neutral excitations.