We test whether a next-nearest-neighbor coupling supplies one finite-size control scale for both the spectral crossover from integrability and the onset of eigenstate thermalization in an open spin-1/2 XXZ chain at Δ=1. Dense… Read moreRead less
We test whether a next-nearest-neighbor coupling supplies one finite-size control scale for both the spectral crossover from integrability and the onset of eigenstate thermalization in an open spin-1/2 XXZ chain at Δ=1. Dense exact diagonalization was executed for L=8,10,12 in total-Sz=0 sectors resolved by reflection parity, with λ∈{0,0.05,0.1,0.2,0.3,0.5,0.7,1.0}. The computed diagnostics do not support a clean Poisson-to-Gaussian-orthogonal-ensemble rise in the mean spacing ratio: the reflection-even high-λ values remain near 0.35--0.40 rather than approaching 0.5307. Off-diagonal matrix elements of a parity-preserving central-spin observable show partial 1/D consistency, but the adjacent-size ETH exponents drift from near one at small λ to about 0.78 and 0.72 at λ=1. Half-chain entanglement increases toward a Page-like comparator, reaching about 0.86--0.92 of the full-space Page value at λ=1. Independently extracted crossover scales do not coincide: spectral midpoint crossings are absent for L=10,12, whereas ETH midpoint estimates occur near λ≃0.31--0.36. At the executed sizes, the evidence is a finite-size failure atlas for the proposed single-scale collapse rather than confirmation of a universal crossover law.