Watching the birth of a charge density wave order: diffraction study on nanometer-and picosecond-scales

TL;DR

This study uses femtosecond X-ray diffraction to observe the ultrafast transition between two charge density wave states in 1T-TaS2, revealing nanometer-scale domain growth and universal coarsening dynamics.

Contribution

It provides the first real-time observation of the nucleation and growth of incommensurate charge density wave domains on nanometer and picosecond scales.

Findings

NC-CDW order disappears within 400 fs

I-CDW phase develops over 100 ps

I-CDW domains grow following Lifshitz-Allen-Cahn law

Abstract

Femtosecond time-resolved X-ray diffraction is used to study a photo-induced phase transition between two charge density wave (CDW) states in 1T-TaS$_2$, namely the nearly commensurate (NC) and the incommensurate (I) CDW states. Structural modulations associated with the NC-CDW order are found to disappear within 400 fs. The photo-induced I-CDW phase then develops through a nucleation/growth process which ends 100 ps after laser excitation. We demonstrate that the newly formed I-CDW phase is fragmented into several nanometric domains that are growing through a coarsening process. The coarsening dynamics is found to follow the universal Lifshitz-Allen-Cahn growth law, which describes the ordering kinetics in systems exhibiting a non-conservative order parameter.