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

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Abstract

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 subsystems3,4,5,6,7. Numerous time-resolved experiments have been performed on CDWs8,9,10,11,12,13, 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.

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Figure 1: FED data in the NCCP in 1T-TaS2.
Figure 2: Time evolution of the diffraction intensities following photoexcitation with a fluence of 2.4 mJ cm−2.
Figure 3: Early time dynamics and emerging time evolution of the CDW state in 1T-TaS 2 on photoexcitation.

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Acknowledgements

We would like to acknowledge discussions with V. V. Kabanov, T. Dekorsy, D. Mihailovic, U. Bovensiepen and M. Wolf, and thank A. Nagy for help in preparing the video in Supplementary Material. This research was supported by the Sofja Kovalevskaja Award of the Alexander von Humboldt Foundation, the Center for Applied Photonics and Zukunftskolleg at the University of Konstanz, the Natural Science and Engineering Research Council of Canada and the Canada Foundation for Innovation. M.E. acknowledges financial support through the Stiftung der Deutschen Wirtschaft. H.B. acknowledges financial support from the Swiss NSF and the NCCR MaNEP.

Author information

R.J.D.M. and J.D. directed this work. H.B. grew 1T-TaS2 single crystals. M.E. and M.K. prepared thin films and performed transmission electron microscopy characterization. G.S., G.M., M.E. and H.S. performed the FED experiments at the University of Toronto. M.B., M.E. and H.S. performed the optical pump–probe experiments at the University of Konstanz. M.E., H.S. and J.D. performed the data analysis. J.D., G.S. and R.J.D.M. wrote the paper. All authors contributed to discussions.

Correspondence to Jure Demsar or R. J. Dwayne Miller.

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Supplementary information

Supplementary Information

This file contains Supplementary Information comprising Experimental Details and Detailed Data Analysis, Supplementary Figures 1-6 with legends, Legends for Supplementary Movies 1-3 and additional references. (PDF 2126 kb)

Supplementary Movie 1

This movie shows the time evolution of the differential diffraction from -1 to 6 ps with 100 fs time steps, recorded at the excitation density F = 2.4 mJ/cm2. (AVI 2122 kb)

Supplementary Movie 2

This movie shows the animation of the emerging time-evolution of the real-space structure, including both lattice dynamics and the dynamics of the conduction electron density. (AVI 10360 kb)

Supplementary Movie 3

This movie shows the animation of the corresponding changes in the momentum space. (AVI 10175 kb)

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Eichberger, M., Schäfer, H., Krumova, M. et al. Snapshots of cooperative atomic motions in the optical suppression of charge density waves. Nature 468, 799–802 (2010) doi:10.1038/nature09539

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