Ultrafast optical melting of trimer superstructure in layered 1T'-TaTe2

TL;DR

This study uses ultrafast electron diffraction to observe the rapid melting and recovery of trimer superstructure in layered TaTe2, revealing charge transfer dynamics that destabilize atomic clusters on a sub-picosecond timescale.

Contribution

It provides the first ultrafast structural investigation of TaTe2's trimer order, demonstrating charge transfer-induced melting distinct from other charge density wave materials.

Findings

Rapid 1.4 ps melting of trimer order observed

Charge transfer destabilizes trimer clusters

Recovery involves thermalisation into a hot superstructure

Abstract

Quasi-two-dimensional transition-metal dichalcogenides are a key platform for exploring emergent nanoscale phenomena arising from complex interactions. Access to the underlying degrees-of-freedom on their natural time scales motivates the use of advanced ultrafast probes sensitive to self-organised atomic-scale patterns. Here, we report the first ultrafast investigation of TaTe2, which exhibits unique charge and lattice trimer order characterised by a transition upon cooling from stripe-like chains into a $(3 \times 3)$ superstructure of trimer clusters. Utilising MeV-scale ultrafast electron diffraction, we capture the photo-induced TaTe2 structural dynamics -- exposing a rapid $\approx\!1.4$ ps melting of its low-temperature ordered state followed by recovery via thermalisation into a hot cluster superstructure. Density-functional calculations indicate that the initial quench is triggered by intra-trimer Ta charge transfer which destabilises the clusters, unlike melting of charge density waves in other TaX2 compounds. Our work paves the way for further exploration and ultimately rapid optical and electronic manipulation of trimer superstructures.