Multi-scale spectral statistics of intermediate quantum chaos in ultracold erbium collisions
Characterizing intermediate quantum chaos in systems with mixed phase spaces requires probing spectral correlations across multiple scales. Here, we present a multi-scale statistical analysis of the resonance spectra of ultracold 166Er and 168Er atoms using higher-order spacing ratios, spacing increments, and the accumulated power spectral density of decimated level sequences. Our analysis reveals that while the spectra exhibit clear level repulsion, they are consistently better described by semi-Poisson than by standard Wigner-Dyson ensembles, with this intermediate character persisting across higher-order correlations. The accumulated power spectrum reveals an intermediate 1/f^alpha regime, with alpha approximately 1.5 at full spectral resolution and a tendency toward the integrable random-walk limit alpha = 2 at intermediate decimation scales, indicating a scale-dependent loss of spectral rigidity. Furthermore, the spacing-increment distributions independently reveal statistics intermediate between Laplace and Gaussian limits. Together, these complementary diagnostics show that the semi-Poisson character reflects persistent non-Wigner-Dyson correlations beyond the nearest-neighbor scale, providing a consistent picture of intermediate quantum chaos and a possible connection to hierarchical trapping in the underlying collision dynamics.