Universal crossover from ground state to excited-state quantum criticality

TL;DR

This paper investigates a quantum chain where excited states show critical behavior similar to ground states, revealing a universal crossover between different quantum critical regimes through analytical and renormalization techniques.

Contribution

It demonstrates a universal crossover in a nonergodic quantum chain from ground state to excited-state criticality using strong-disorder renormalization group analysis.

Findings

Universal crossover between ground and excited state criticality.

Derivation of a flow equation for system dynamics.

Identification of universal scaling functions.

Abstract

We study the nonequilibrium properties of a nonergodic random quantum chain in which highly excited eigenstates exhibit critical properties usually associated with quantum critical ground states. The ground state and excited states of this system belong to different universality classes, characterized by infinite-randomness quantum critical behavior. Using strong-disorder renormalization group techniques, we show that the crossover between the zero and finite energy density regimes is universal. We analytically derive a flow equation describing the unitary dynamics of this isolated system at finite energy density from which we obtain universal scaling functions along the crossover.