Correlation tuned cross-over between thermal and nonthermal states following ultrafast transient pumping

TL;DR

This study investigates how electron relaxation dynamics differ across the Mott transition following ultrafast excitation, revealing a transition from thermal to nonthermal states that impacts modeling of correlated materials.

Contribution

It demonstrates the correlation-dependent crossover from thermal to nonthermal relaxation states in a Mott system after ultrafast pumping.

Findings

In metallic regimes, relaxation leads to a Fermi-Dirac distribution with increased effective temperature.

In insulating regimes, relaxation results in nonthermal, non-monotonic electronic distributions.

The behavior is observable on short time scales where electrons are effectively isolated.

Abstract

We examine electron-electron mediated relaxation following excitation of a correlated system by an ultrafast electric field pump pulse. The results reveal a dichotomy in the temporal evolution as one tunes through a Mott metal-to-insulator transition: in the metallic regime relaxation can be characterized by evolution toward a steady-state electronic distribution well described by Fermi-Dirac statistics with an increased effective temperature; however, in the insulating regime this quasithermal paradigm breaks down with relaxation toward a nonthermal state with a more complicated electronic distribution that does not vary monotonically as a function of energy. We characterize the behavior by studying changes in the energy, photoemission response, and electronic distribution as functions of time. Qualitatively these results should be observable on short enough time scales that the electrons behave like an isolated system not in contact with additional degrees of freedom which can act as a thermal bath. Importantly, proper modeling used to analyze experimental findings should account for this behavior, especially when using strong driving fields or studying materials whose physics may manifest the effects of strong correlations.