Large-scale behavior of energy spectra of the quantum random antiferromagnetic Ising chain with mixed transverse and longitudinal fields

In recent years, it became clear that the metallic regime of systems that exhibit a many-body localization (MBL) behavior shows properties that are quite different than the vanilla metallic region of the single-particle Anderson regime. Here we show that the large-scale energy spectrum of a canonical microscopical model featuring MBL displays a nonuniversal behavior at intermediate scales, which is distinct from the deviation from universality seen in the single-particle Anderson regime. The crucial step in revealing this behavior is a global unfolding of the spectrum performed using the singular value decomposition (SVD) which takes into account the sample to sample fluctuations of the spectra. The spectrum properties may be observed directly in the singular value amplitudes via the scree plot, or by using the SVD to unfold the spectra and then perform a number of states variance calculation. Both methods reveal an intermediate scale of energies that follow super-Poissonian statistics.