Femtosecond trimer quench cycled at megahertz rates in the unconventional charge-density wave material $1\textit{T'}-\text{TaTe}_2$

Abstract

Ultrafast optical switching of materials properties is of great relevance both for future technological applications as well as gaining fundamental physical insights to microscopic couplings and nonequilibrium phenomena. Transition-metal dichalcogenides (TMDCs) combine photo-sensitivity with strong correlations, furthering rich phase diagrams and enhanced tunability. Owing to its chemical composition, $1\textit{T'}-\text{TaTe}_2$ exhibits an electronically and structurally unique set of charge-density waves (CDWs), featuring increased conductivity and a reduced prominence of amplitude modes. Compared to other charge-ordered TMDCs, only very few studies addressed the ultrafast response of this material to optical excitation. In particular, the question whether such unconventional properties translate to unusual quench dynamics remains largely unresolved. Here, we investigate the structural dynamics in $1\textit{T'}-\text{TaTe}_2$ by means of ultrafast nanobeam electron diffraction. The experiments are carried out with a tailored sample design that allows for excitation at 2$\,$MHz repetition rate, higher than any structural phase transformation probed thus far. Harnessing the enhanced resolution and sensitivity of this approach, we reveal a strongly directional cooperative atomic motion during the one-dimensional quench of the low-temperature trimer lattice. These dynamics are completed within less than 500$\,$fs, substantially faster than reported previously. In striking contrast, the periodic lattice distortion of the room-temperature phase is unusually robust against high-density electronic excitation. In conjunction with the known sensitivity of $1\textit{T'}-\text{TaTe}_2$ to chemical doping, we thus expect the material to serve as a versatile platform for tunable structural control by optical stimuli.