Fock space fragmentation in quenches of disordered interacting fermions

• URL: http://arxiv.org/abs/2510.19510v1
• Date: Wed, 22 Oct 2025 12:05:02 GMT
• Title: Fock space fragmentation in quenches of disordered interacting fermions
• Authors: Ishita Modak, Rajesh Narayanan, Ferdinand Evers, Soumya Bera,
• Abstract summary: We show that in disordered systems, such as the XXZ model with random on-site fields, fragmentation appears as a natural concept.<n>We conjecture that the slowing down of relaxation dynamics reported in traditional MBdL studies is (essentially) a manifestation of Fock-space fragmentation introduced here.
• Score: 27.151780750119556
• License: http://creativecommons.org/licenses/by/4.0/
• Abstract: Hilbert 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 non-ergodic 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 delocalization (MBdL). Specifically, we split the Fock-space into subspaces, potential-energy shells, which contain the accessible phase space for the relaxation of a quenched initial state. In this construction, dynamical observables reflect properties of the shell geometry, e.g., the drastic sample-to-sample fluctuations observed in the weak disorder regime, $W<W_c$, represent fluctuations of the mass of the shell. Upon crossing over from weak to strong disorder, $W>W_c$, the potential-energy shell decays into fragments; we argue that, unlike percolation, fragmentation is a strong-coupling scenario with turn-around flow: $W_c(L)$ diverges with increasing system size. We conjecture that the slowing down of the relaxation dynamics reported in traditional MBdL studies is (essentially) a manifestation of Fock-space fragmentation introduced here.

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