The development of tabletop femtosecond electron-diffraction sources has provided an alternative way of observing atomic motions in crystalline materials. This technique has now been applied to the charge density wave material 1T-TaS_2, in which modulation of the electron density is accompanied by a periodic lattice distortion. Previous time-resolved studies have revealed the dynamics of the electronic charge density wave, but until now the dynamics ofrnthe lattice system has been only indirectly inferred. In this new experiment, atomic motions were observed in response to a 140 femtosecond optical pulse. Periodic lattice distortion was seen to collapse on an exceptionally fast timescale (about 250 fs), indicative of an electronically driven process involving a high degree of cooperativity.
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