Tracking Nuclear and Electronic Orders in a Strongly Coupled Charge-Density-Wave Phase

By combining two complementary probes—time-resolved ultrafast electron diffraction (UED) and femtosecond broadband transient reflectivity (bb-TR)—we resolve the dynamics of the coupled order parameters in photoexcited NbTe2. UED directly captures cooperative atomic displacements, revealing a transient 2% suppression of the periodic lattice distortion following the collapse of the electronic order.

In contrast, bb-TR identifies an 800-nm probe response that is sensitive to the electronic order parameter, showing a prompt quench within the experimental time resolution, followed by fluence-dependent recovery and coherent amplitude-mode oscillations. Comparison with density-functional-theory calculations and the wavelength-dependent optical response identifies the 81 cm–1 Ag2 phonon as the primary amplitude mode coupled to the CDW order.