Phase Fluctuations and the Absence of Topological Defects in Photo-excited Charge Ordered Nickelate

TL;DR

This study uses advanced spectroscopy techniques to reveal that photo-induced phase fluctuations in charge-ordered nickelates preserve long-range order without defect formation, with phase relaxation being significantly slower than amplitude recovery.

Contribution

It uncovers a novel type of phase fluctuation in photo-excited charge order that maintains order without topological defects, expanding understanding of quantum material dynamics.

Findings

Photo-induced phase fluctuations preserve long-range order.

Phase fluctuations relax slower than amplitude fluctuations.

No topological defects are formed during fluctuations.

Abstract

The dynamics of an order parameter's amplitude and phase determines the collective behaviour of novel states emerged in complex materials. Time- and momentum-resolved pump-probe spectroscopy, by virtue of its ability to measure material properties at atomic and electronic time scales and create excited states not accessible by the conventional means can decouple entangled degrees of freedom by visualizing their corresponding dynamics in the time domain. Here, combining time-resolved femotosecond optical and resonant x-ray diffraction measurements on striped La1.75Sr0.25NiO4, we reveal unforeseen photo-induced phase fluctuations of the charge order parameter. Such fluctuations preserve long-range order without creating topological defects, unlike thermal phase fluctuations near the critical temperature in equilibrium10. Importantly, relaxation of the phase fluctuations are found to be an order of magnitude slower than that of the order parameter's amplitude fluctuations, and thus limit charge order recovery. This discovery of new aspect to phase fluctuation provides more holistic view for the importance of phase in ordering phenomena of quantum matter.