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Fock-space fragmentation in quenches of disordered interacting fermions

Ishita Modak1, Rajesh Narayanan2, Ferdinand Evers3, and Soumya Bera1,*

  • *Contact author: soumya.bera@iitb.ac.in
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Phys. Rev. Research 8, 043030 – Published 9 October, 2026

DOI: https://doi.org/10.1103/hvqr-6n3b

Abstract

Fock-space fragmentation, as it is currently investigated, primarily originates from specific kinematic constraints or emergent conservation laws in many-body systems with translation invariance. It leads to nonergodic dynamics and possible breakdown of the eigenstate thermalization hypothesis. Here, we demonstrate that also in disordered systems, such as the XXZ model with random on-site fields, fragmentation appears as a natural concept offering fresh perspectives, for example, on many-body localization. Specifically, for a given quenched initial state, we identify a potential-energy shell in Fock space, defined by its energy and energy uncertainty, which represents the energetically accessible part of the many-body configuration space. For example, the drastic sample-to-sample fluctuations observed in the weak disorder regime, W<Wfrag, represent fluctuations of the mass of the shell. Upon increasing disorder, the potential-energy shell becomes increasingly fragmented. Its finite-size evolution exhibits a turnaround flow; extrapolating this flow to the disorder at which the turnaround vanishes provides a heuristic estimate of the strong-fragmentation scale Wfrag. We conjecture that the fragmentation identified here provides a geometric mechanism contributing to the finite-time slowing down of relaxation observed in studies of disordered interacting systems.

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Phys. Rev. Research 8, 043030– Published 9 October, 2026
  • Received 31 October 2025
  • Accepted 15 September 2026
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