Can a spin chain relate combinatorics to number theory?

TL;DR

This paper introduces a simplified, integrable version of the Motzkin spin chain, revealing connections between quantum integrability, combinatorics, and number theory through the Bethe Ansatz and M"obius function.

Contribution

It constructs an integrable free Motzkin chain and uncovers novel links between quantum spin models and number theory, especially via the M"obius function.

Findings

The free Motzkin chain is integrable and solvable using a generalized Bethe Ansatz.

The energy spectrum involves a new parameter related to roots of unity and the M"obius function.

Patterns of number theory are observed in the quantum integrability context.

Abstract

The Motzkin spin chain is a spin-$1$ frustration-free model introduced by Shor & Movassagh. The ground state is constructed by mapping random walks on the upper half of the square lattice to spin configurations. It has unusually large entanglement entropy [quantum fluctuations]. The ground state of the Motzkin chain can be analytically described by the Motzkin paths. There is no analytical description of the excited states. The model is not solvable. We simplify the model by removing one of the local equivalence moves of the Motzkin paths. The system becomes integrable [similar to the XXX spin chain]. We call it free Motzkin chain. From the point of view of quantum integrability, the model is special since its $R$-matrix does not have crossing unitarity. We solve the periodic free Motzkin chain by generalizing the functional Bethe Ansatz method. We construct a $T-Q$ relation with an additional parameter to formulate the energy spectrum. This new parameter is related to the roots of unity and can be described by the M\"obius function in number theory. We observe further patterns of number theory.