The fate of local order in topologically frustrated spin chains

TL;DR

This paper investigates how topological frustration affects local order in antiferromagnetic XY spin chains, revealing conditions under which local order persists or decays, with implications for understanding quantum many-body systems.

Contribution

It classifies all possible behaviors of local order in topologically frustrated spin chains and identifies conditions for their existence or decay, using analytical and numerical methods.

Findings

Finite local order exists only in degenerate ground states with momentum difference of π.

In non-degenerate cases, local order decays algebraically or faster with chain length.

The realization of local order depends on the sequence of chain lengths approaching the thermodynamic limit.

Abstract

It has been recently shown that the presence of topological frustration, induced by periodic boundary conditions in an antiferromagnetic $XY$ chain made of an odd number of spins, prevents the realization of a perfectly staggered local order. Starting from this result and exploiting a recently introduced approach which enables the direct calculation of the expectation value of any operator with support over a finite range of lattice sites, in this work we investigate the possible fates of local orders. We show that, regardless of the variety of possible situations, they can be all arranged in two different cases. A system admits a finite local order only if the ground state is degenerate, with at least two elements whose momenta differ, in the thermodynamic limit, by $\pi$, and this order breaks translational symmetry. In all other cases, any local order decays to zero, algebraically (or faster) in the chain length. Moreover, we show that, in some cases, which of the two possibilities is realized, may depend on the sequence of chain lengths with which the thermodynamic limit is reached. These results are established both analytically and by exact diagonalization and illustrated through examples.