High Harmonic Tracking of Ultrafast Electron Dynamics across the Mott to Charge Density Wave Phase Transition

TL;DR

This paper demonstrates that strong-field ultrafast optics can effectively track and reveal the nature of the phase transition from Mott insulator to charge density wave in strongly correlated materials by analyzing high harmonic spectra.

Contribution

It introduces a novel approach using high harmonic generation spectroscopy to identify the order of the phase transition and observe complex quasiparticles during the transition.

Findings

High harmonic spectra reveal a first-order phase transition.

Complex excitations like excitons and biexcitons are tracked.

Ultrafast electron dynamics are characterized during the transition.

Abstract

Different insulator phases compete with each other in strongly correlated materials with simultaneous local and non-local interactions. It is known that the homogeneous Mott insulator converts into a charge density wave (CDW) phase when the non-local interactions are increased, but there is ongoing debate on whether and in which parameter regimes this transition is of first order, or of second order with an intermediate bond-order wave phase. Here we show that strong-field optics applied to an extended Fermi-Hubbard system can serve as a powerful tool to reveal the nature of the quantum phase transition. Specifically, we show that in the strongly interacting regime characteristic excitations such as excitons, biexcitons, excitonic strings, and charge droplets can be tracked by the non-linear optical response to an ultrafast and intense laser pulse. Subcycle analysis of high harmonic spectra unravels the ultrafast dynamics of these increasingly complex objects, which partially escape the scrutiny of linear optics. Their appearance in the high harmonic spectrum provides striking evidence of a first-order transition into the CDW phase, and makes a strong case for using strong-field optics as a powerful tool to reveal the nature of quasiparticles in strongly correlated matter, and to track the electron dynamics during a first-order quantum phase transition.