Snapshots of cooperative atomic motions in the optical suppression of charge density waves

Macroscopic quantum phenomena such as high-temperature superconductivity, colossal magnetoresistance, ferrimagnetism and ferromagnetism arise from a delicate balance of different interactions among electrons, phonons and spins on the nanoscale¹. The study of the interplay among these various degrees of freedom in strongly coupled electron–lattice systems is thus crucial to their understanding and for optimizing their properties. Charge-density-wave (CDW) materials², with their inherent modulation of the electron density and associated periodic lattice distortion, represent ideal model systems for the study of such highly cooperative phenomena. With femtosecond time-resolved techniques, it is possible to observe these interactions directly by abruptly perturbing the electronic distribution while keeping track of energy relaxation pathways and coupling strengths among the different subsystems^{3, 4, 5, 6, 7}. Numerous time-resolved experiments have been performed on CDWs^{8, 9, 10, 11, 12, 13}, probing the dynamics of the electronic subsystem. However, the dynamics of the periodic lattice distortion have been only indirectly inferred¹⁴. Here we provide direct atomic-level information on the structural dynamics by using femtosecond electron diffraction¹⁵ to study the quasi two-dimensional CDW system 1T-TaS₂. Effectively, we have directly observed the atomic motions that result from the optically induced change in the electronic spatial distribution. The periodic lattice distortion, which has an amplitude of ~0.1 Å, is suppressed by about 20% on a timescale (~250 femtoseconds) comparable to half the period of the corresponding collective mode. These highly cooperative, electronically driven atomic motions are accompanied by a rapid electron–phonon energy transfer (~350 femtoseconds) and are followed by fast recovery of the CDW (~4 picoseconds). The degree of cooperativity in the observed structural dynamics is remarkable and illustrates the importance of obtaining atomic-level perspectives of the processes directing the physics of strongly correlated systems.