Dynamical quantum phase transitions and broken-symmetry edges in the many-body eigenvalue spectrum

TL;DR

This paper explores the energy spectrum structure of many-body quantum systems undergoing phase transitions, revealing energy edges that separate symmetry-breaking states and influence out-of-equilibrium dynamics, with specific models demonstrating these phenomena.

Contribution

It predicts and demonstrates the existence of energy edges separating symmetry-breaking and invariant eigenstates in many-body models, affecting dynamical phase transitions and out-of-equilibrium behavior.

Findings

Energy edges separate symmetry-breaking and invariant eigenstates.

Dynamical phase transitions are linked to these energy edges.

High-energy eigenstates can exhibit superfluidity in repulsive models.

Abstract

Many body models undergoing a quantum phase transition to a broken-symmetry phase that survives up to a critical temperature must possess, in the ordered phase, symmetric as well as non-symmetric eigenstates. We predict, and explicitly show in the fully-connected Ising model in a transverse field, that these two classes of eigenstates do not overlap in energy, and therefore that an energy edge exists separating low-energy symmetry-breaking eigenstates from high-energy symmetry-invariant ones. This energy is actually responsible, as we show, for the dynamical phase transition displayed by this model under a sudden large increase of the transverse field. A second situation we consider is the opposite, where the symmetry-breaking eigenstates are those in the high-energy sector of the spectrum, whereas the low-energy eigenstates are symmetric. In that case too a special energy must exist marking the boundary and leading to unexpected out-of-equilibrium dynamical behavior. An example is the fermionic repulsive Hubbard model Hamiltonian H. Exploiting the trivial fact that the high energy spectrum of H is also the low energy one of -H, we conclude that the high energy eigenstates of the Hubbard model are superfluid. Simulating in a time-dependent Gutzwiller approximation the time evolution of a high energy BCS-like trial wave function, we show that a small superconducting order parameter will actually grow in spite of the repulsive nature of interaction.