K. Pawlik: A new phase of matter in the Anderson–quantum sun model

The quantum sun model, originally introduced in the context of quantum avalanches, is a toy model for the ergodicity-breaking transition, and consists of an ergodic dot connected to a localized spin chain. It exhibits a wide variety of intriguing quantum phenomena, such as a many-body mobility edge and fading ergodicity, while remaining significantly simpler to analyze numerically than other paradigmatic models of ergodicity breaking, such as the disordered Heisenberg XXZ model. By replacing the localized chain with the Anderson model, we discover an unexpected mismatch between eigenstate properties and spectral statistics in this interacting system. This leads to the emergence of a new, robust phase between localized and ergodic phases, characterized by volume-law entanglement entropy and intermediate level statistics.