Additional quantum many-body scars of the spin-$1$ $XY$ model with Fock-space cages and commutant algebras

TL;DR

This paper uncovers new quantum many-body scar states in the spin-1 XY model, revealing interference and algebraic mechanisms that lead to weak ergodicity breaking and non-thermal eigenstates.

Contribution

The work introduces multiple new families of exact scar eigenstates in the spin-1 XY model, including Fock space cage states and algebraically characterized states, expanding understanding of non-ergodic behavior.

Findings

Identification of interference-protected Fock space cage states

Discovery of long-lived fidelity oscillations under magnetic field

Uncovering algebraic structure behind nonthermal eigenstates

Abstract

Quantum many-body scars (QMBS) represent a mechanism for weak ergodicity breaking, characterized by the coexistence of atypical non-thermal eigenstates within an otherwise thermalizing many-body spectrum. In this work, we revisit the spin-$1$ $XY$ model on a periodic chain and construct several new families of exact scar eigenstates embedded within its extensively degenerate manifolds that owe their origins to an interplay of $U(1)$ magnetization conservation and chiral symmetries. We go beyond previously studied towers of states and first identify a novel set of interference-protected eigenstates resembling Fock space cage states, where destructive interference confines the wave function to sparse subgraphs of the Fock space. These states exhibit subextensive entanglement entropy, and when subjected to a transverse magnetic field, form equally spaced ladders whose coherent superpositions display long-lived fidelity oscillations. We further reveal a simpler organizing principle behind these nonthermal states by using the commutant algebra framework, in particular by showing that they are simultaneous eigenstates of non-commuting local operators. Moreover, in doing so, we uncover two more novel families of exact scars: a tower of volume-entangled states, and a set of mirror-dimer states with some free local degrees of freedom. Our results illustrate the power and interplay of interference-based and algebraic mechanisms of non-ergodicity, offering systematic routes to identifying and classifying QMBS in generic many-body quantum systems.