Stacking order dynamic in the quasi-two-dimensional dichalcogenide 1T-TaS$_2$ probed with MeV ultrafast electron diffraction

TL;DR

This study uses MeV ultrafast electron diffraction to investigate how the stacking order of charge density waves in 1T-TaS$_2$ changes on ultrafast timescales, revealing phase transitions within a few picoseconds.

Contribution

First-time measurement of out-of-plane stacking order dynamics in 1T-TaS$_2$ using ultrafast electron diffraction, showing rapid phase transitions between CDW states.

Findings

The commensurate CDW phase disappears in 0.5 ps.

The incommensurate CDW phase emerges in 2.0 ps.

Results shed light on the metastable 'hidden' state in the material.

Abstract

Transitions between different charge density wave (CDW) states in quasi-two-dimensional materials may be accompanied also by changes in the inter-layer stacking of the CDW. Using MeV ultrafast electron diffraction, the out-of-plane stacking order dynamics in the quasi-two-dimensional dichalcogenide 1T-TaS$_2$ is investigated for the first time. From the intensity of the CDW satellites aligned around the commensurate $l$ = 1/6 characteristic stacking order, it is found out that this phase disappears with a 0.5 ps time constant. Simultaneously, in the same experiment, the emergence of the incommensurate phase, with a slightly slower 2.0 ps time constant, is determined from the intensity of the CDW satellites aligned around the incommensurate $l$ = 1/3 characteristic stacking order. These results might be of relevance in understanding the metallic character of the laser-induced metastable "hidden" state recently discovered in this compound.