Femtosecond trimer quench in the unconventional charge-density-wave material 1T′−TaTe2

Ultrafast optical switching of materials properties promises future technological applications, enabled by fundamental insights about microscopic couplings and nonequilibrium phenomena. Transition-metal dichalcogenides (TMDCs) combine photosensitivity with strong correlations, furthering rich phase diagrams and enhanced tunability. The compound <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mn>1</a:mn><a:msup><a:mi>T</a:mi><a:mo>′</a:mo></a:msup><a:mtext>−</a:mtext><a:msub><a:mi mathvariant="normal">TaTe</a:mi><a:mn>2</a:mn></a:msub></a:math> exhibits an electronically and structurally unique set of charge density waves (CDWs), featuring an unusual increase in conductivity and amplitude modes of low prominence. 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 <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mn>1</c:mn><c:msup><c:mi>T</c:mi><c:mo>′</c:mo></c:msup><c:mtext>−</c:mtext><c:msub><c:mi mathvariant="normal">TaTe</c:mi><c:mn>2</c:mn></c:msub></c:math> by means of ultrafast nanobeam electron diffraction at an unprecedented repetition rate of <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mrow><e:mn>2</e:mn><e:mspace width="0.16em"/><e:mi>MHz</e:mi></e:mrow></e:math>. 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 <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"><g:mrow><g:mn>500</g:mn><g:mspace width="0.16em"/><g:mi>fs</g:mi></g:mrow></g:math>, 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 <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"><i:mn>1</i:mn><i:msup><i:mi>T</i:mi><i:mo>′</i:mo></i:msup><i:mtext>−</i:mtext><i:msub><i:mi mathvariant="normal">TaTe</i:mi><i:mn>2</i:mn></i:msub></i:math> to chemical doping, we thus expect the material to serve as a versatile platform for tunable structural control by optical stimuli.