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TT: Fachverband Tiefe Temperaturen

TT 19: Focus Session: New Routes to Localization and Quantum Non-Ergodicity II (joint session TT/DY)

TT 19.3: Vortrag

Montag, 9. März 2026, 15:30–15:45, CHE/0091

Fock space fragmentation in quenches of disordered interacting fermions — •Ishita Modak¹, Rajesh Narayanan², Ferdinand Evers³, and Soumya Bera¹ — ¹Department of Physics, Indian Institute of Technology Bombay, Mumbai, India. — ²Department of Physics, Indian Institute of Technology Madras, Chennai, India — ³Institute of Theoretical Physics and Halle-Berlin-Regensburg Cluster of Excellence CCE, University of Regensburg, Regensburg, Germany

Hilbert space fragmentation primarily originates from specific kinematic constraints or emergent conservation laws in many-body systems with translation invariance. It leads to non-ergodic dynamics and breakdown of the eigenstate thermalization hypothesis. We demonstrate that also in disordered systems (e.g. random-field XXZ model), fragmentation appears as a natural concept offering fresh perspectives on many-body delocalization (MBdL). We split the Fock-space into 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 shell-mass. Upon crossing over 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 a manifestation of Fock-space fragmentation introduced here.

Keywords: Hilbert space Fragmentation; Role of Fock space geometry in dynamics; Many Body Delocalization; Ergodicity breaking; Sample to sample fluctuation in disordered systems