Role of equilibrium fluctuations in light-induced order

TL;DR

This study reveals that light-induced transient order in materials arises from equilibrium fluctuations, with ultrafast electron diffraction showing that these fluctuations are crucial for the emergence of hidden phases after laser excitation.

Contribution

It demonstrates that transient orders induced by light are driven by equilibrium fluctuations, providing a nonperturbative model linking fluctuation strength to light-induced order.

Findings

Light-induced charge density wave consists of order parameter fluctuations.

The strength of photoinduced order correlates with equilibrium fluctuation amplitude.

Materials with strong equilibrium fluctuations are promising for hidden order realization.

Abstract

Engineering novel states of matter with light is at the forefront of materials research. An intensely studied direction is to realize broken-symmetry phases that are "hidden" under equilibrium conditions but can be unleashed by an ultrashort laser pulse. Despite a plethora of experimental discoveries, the nature of these orders and how they transiently appear remain unclear. To this end, we investigate a nonequilibrium charge density wave (CDW) in rare-earth tritellurides, which is suppressed in equilibrium but emerges after photoexcitation. Using a pump-pump-probe protocol implemented in ultrafast electron diffraction, we demonstrate that the light-induced CDW consists solely of order parameter fluctuations, which bear striking similarities to critical fluctuations in equilibrium despite differences in the length scale. By calculating the dynamics of CDW fluctuations in a nonperturbative model, we further show that the strength of the light-induced order is governed by the amplitude of equilibrium fluctuations. These findings highlight photoinduced fluctuations as an important ingredient for the emergence of transient orders out of equilibrium. Our results further suggest that materials with strong fluctuations in equilibrium are promising platforms to host "hidden" orders after laser excitation.