Ground states of the infinite q-deformed Heisenberg ferromagnet

TL;DR

This paper analyzes frustration-free ground states of infinite quantum spin systems, establishing a framework for their structure and showing that for certain models, all zero-energy states are translationally invariant or periodic, with specific results for the q-deformed Heisenberg ferromagnet.

Contribution

It introduces a general structure for analyzing zero-energy states in lattice models and characterizes the limit space of these states for the q-deformed Heisenberg ferromagnet.

Findings

Limit space is an abelian C*-algebra for certain models.

All zero-energy states are translationally invariant or periodic.

For the q-deformed chain, only two translationally invariant states exist, with others converging to these states.

Abstract

We set up a general structure for the analysis of ``frustration-free ground states'', or ``zero-energy states'', i.e., states minimizing each term in a lattice interaction individually. The nesting of the finite volume ground state spaces is described by a generalized inductive limit of observable algebras. The limit space of this inductive system has a state space which is canonically isomorphic (as a compact convex set) to the set of zero-energy states. We show that for Heisenberg ferromagnets, and for generalized valence bond solid states, the limit space is an abelian C*-algebra, and all zero-energy states are translationally invariant or periodic. For the $q$-deformed spin-$1/2$ Heisenberg ferromagnet in one dimension (i.e., the XXZ-chain with S$_q$U(2)-invariant boundary conditions) the limit space is an extension of the non-commutative algebra of compact operators by two points, corresponding to the ``all spins up'' and the ``all spins down'' states, respectively. These are the only translationally invariant zero-energy states. The remaining ones are parametrized by the density matrices on a Hilbert space, and converge weakly to the ``all up'' (resp.\ ``all down'') state for shifts to $-\infty$ (resp.\ $+\infty$).