Sub-angstrom many-body localization driven by phononic flat bands in real quantum materials

Abstract

Defects, fluctuations, degenerate states and correlated interactions facilitate the emergence of exotic properties in condensed matter systems while also inducing atomic-scale local correlated structures that deviate from the average long-range order. Establishing the structure-property relationship from the perspective of these atomic-scale local correlated structures remains ambiguous and controversial due to the lack of direct methods for identifying such local correlated structures. In this work, based on the photoexcited ultrafast structural response, we propose a Bragg scattering phase breaking regime to identify sub-angstrom local correlated structures in quantum materials. With this regime, we unambiguously identify the many-body-interaction driven local correlated structures in the low temperature ground state of AgCrSe2, characterized by static off-center displacements of Ag atoms ranging from 0 to 0.5 angstrom. The competition between Ag-Ag Coulomb correlations and potential wells induced by CrSe2 layers, leading to phononic flat bands and driving the system into a many body localization (MBL) regime. As temperature rising, these static local correlated structures transform to a dynamic state where the thermal fluctuations overwhelm the multiple localized states. These distinctive local correlated structures constitute the first experimental observation of MBL with vortex-like topological characteristic in a real material system. Emergent vibrational modes arising from MBL have been confirmed and show excellent agreement with inelastic neutron scattering experiments. Our work not only offers a universal approach to characterize sub-angstrom local correlated structures across a wide range of quantum materials but also deepens our understanding of the fundamental mechanism behind exotic properties from the perspective of atomic-scale local correlated structures.