Crossover from Quantum Chaos to a Reversed Quantum Disentangled Liquid in a Disorder-Free Spin Ladder

TL;DR

This paper explores a disorder-free quantum spin ladder system, revealing a transition from chaos to a reversed quantum disentangled liquid where different species thermalize differently, driven by tunable interactions.

Contribution

It identifies the microscopic origin of quasi-MBL and the emergence of a reversed-QDL phase in a disorder-free setting, expanding understanding of nonergodic quantum phases.

Findings

Reentrant dynamical regimes as rung coupling varies: integrable, chaotic, and nonthermal phases.

Discovery of a reversed-QDL where light species thermalize and heavy species localize.

Emergence of local integrals of motion in the strong-coupling limit.

Abstract

The mechanisms by which isolated interacting quantum systems evade thermalization extend beyond disorder-induced many-body localization, encompassing a growing class of interaction-driven phenomena. We investigate a spin-1/2 ladder with asymmetric XY leg couplings and tunable Ising interactions on the rungs, and identify the microscopic origin of quasi many-body localization (quasi-MBL) in this setting. Through a suite of diagnostics -- including entanglement dynamics, fidelity susceptibility, adiabatic gauge potential norms, level-spacing statistics and entropy of eigenstates -- we uncover a reentrant progression of dynamical regimes as the rung coupling Jz is varied: integrable behavior at Jz=0, quantum chaos at intermediate Jz, and a robust nonthermal regime at strong coupling. In the latter regime, we demonstrate the emergence of a reversed quantum disentangled liquid (reversed-QDL), where the light species thermalizes while the heavy species remains localized. The strong-coupling limit further yields emergent local integrals of motion anchored in a fixed-point structure, providing a microscopic origin of the observed quasi-MBL dynamics. These results establish reversed-QDL as a distinct, disorder-free route to nonergodicity and broaden the classification of dynamical phases in quantum matter.