Spin fractionalization and zero modes in the spin-$\frac{1}{2}$ XXZ chain with boundary fields

TL;DR

This paper demonstrates that the antiferromagnetic spin-1/2 XXZ chain with boundary fields hosts fractional spin-1/4 edge states, which are sharp quantum observables and relate to zero energy modes in the low-energy subspace.

Contribution

It reveals the existence of fractional spin-1/4 edge states in the XXZ chain with boundary fields and connects them to strong zero energy modes, using Bethe ansatz and DMRG techniques.

Findings

Fractional spins are sharp quantum observables at edges.

Edge fractional spins relate to zero energy modes.

Results hold in both ground and first excited states.

Abstract

In this work we argue that the antiferromagnetic spin $\frac{1}{2}$ XXZ chain in the gapped phase with boundary magnetic fields hosts fractional spin $\frac{1}{4}$ at its edges. Using a combination of Bethe ansatz and the density matrix renormalization group we show that these fractional spins are sharp quantum observables in both the ground and the first excited state as the associated fractional spin operators have zero variance. In the limit of zero edge fields, we argue that these fractional spin operators once projected onto the low energy subspace spanned by the ground state and the first excited state, identify with the strong zero energy mode discovered by P. Fendley \cite{Fendley}.