Confinement, Banding, and Meson Spectroscopy in the Staggered-Field XXZ Chain

Confinement of fractionalized excitations can qualitatively reshape many-body spectra. We report numerical and analytic results for the spin-1/2 XXZ chain in a staggered magnetic field deep in the antiferromagnetic regime, where spinons bind into domain-wall ``mesons''. Exact diagonalization in fixed symmetry sectors reveals a crossover from Gaussian-orthogonal (chaotic) level statistics at modest anisotropy to nonergodic behavior at large negative anisotropy. In parallel, correlation and entanglement diagnostics show a striking banding of eigenstates by domain-wall number W: a Page-like entanglement dome at small |\Delta| gives way to suppressed, band-resolved entanglement consistent with emergent quasi-conservation of W.

We then perform meson spectroscopy at total momentum P=0 and benchmark the low-lying spectrum against the near-threshold analytic ladder of Rutkevich [Phys. Rev. B 106, 134405 (2022)]. Three checks support quantitative agreement for the first several levels: (i) continuum-relative ``bindings'' versus Airy magnitudes yield consistent slope/intercept within finite-size uncertainty; (ii) an offset-removed Airy scaling collapses theory and ED points onto a single line; and (iii) two-meson thresholds, drawn in the same reference, account for the observed termination of the one-meson ladder and its field dependence. Varying the staggered field modulates the Airy scale while preserving the overall ladder structure until subleading and finite-size effects appear. Together these results provide a coherent, non-scar-centric picture of confinement-induced nonergodicity and a practical framework for meson spectroscopy in quantum spin chains.