3:45 PM - QN07.12.03
Atomic Dynamics in VO2 Across the Metal-Insulator Transition—Ultrafast Transition and Equilibrium Thermodynamics
Olivier Delaire1,Simon Wall2,Shan Yang1,Luciana Vidas2,Matthieu Chollet3,Michael Glownia3,Michael Kozina3,Tetsuo Katayama4,Thomas Henighan3,Mason Jiang3,Timothy Miller2,David Reis5,Lynn Boatner6,Mariano Trigo3
Duke University1,ICFO–The Institute of Photonic Sciences2,SLAC National Accelerator Laboratory3,Japan Synchrotron Radiation Research Institute4,Stanford University5,Oak Ridge National Laboratory6
Vanadium dioxide (VO2) can be switched from an insulating to a metallic state with either ultrafast laser pulses  or heating above TMIT=340K [2,3]. In both the photoexcited and thermal transitions, the insulator-to-metal transition (IMT) is accompanied by a structural change from a monoclinic (M1) to a rutile (R) structure, and numerous prior efforts have focused on elucidating the evolution of both the electronic and lattice degrees of freedom. The photoexcited transition occurs in a time scale of hundred femtoseconds, and while ultrafast x-ray diffraction has provided tremendous insight into the atomic dynamics during such ultrafast transformations, diffraction peaks alone probe only an average over multiple unit cells and are less sensitive to deviations from the average atomic path connecting the initial and final state. To overcome this limitation, we use femtosecond total x-ray scattering (diffuse and Bragg) from the LCLS x-ray free-electron laser to study the dynamics of the structural transition of bulk VO2 at all length-scales . We observe that the structural transition proceeds by uncorrelated disordering of the vanadium ions from their initial dimerized distribution , rather than the previously proposed synchronized motion along an optical phonon mode. After photoexcitation, the system explores a large volume of the available phase-space in a timescale comparable with a single phonon oscillation [1,2]. Our ab-initio molecular dynamics simulation quantitatively match our ultrafast x-ray scattering measurements, and show an unusual highly anharmonic, flat potential energy surface for the quasi-rutile structure in the photoexcited state, developing on femtosecond timescales and disrupting the vanadium dimers of the M1 phase by populating a continuum of modes. The rapid evolution after photoexcitation is enabled by the large phase space of phonon modes with low-frequency V-vibrations, which was also noted in  to yield a large phonon entropy gain stabilizing the rutile phase, and the strong damping of phonons in the rutile phase. Our current observations thus reveal an interesting parallel between the ultrafast and the thermal transitions. These results overturn our current understanding of an archetypal ultrafast phase transition and provide new microscopic insights into rapid evolution toward equilibrium in photoexcited matter.
 S. Wall*†, S. Yang,* L. Vidas, M. Chollet, M. Glownia, M. Kozina, T. Katayama, T. Henighan, M. Jiang, T. A. Miller, D. A. Reis, L. Boatner, O. Delaire† and Mariano Trigo†, “Ultrafast disordering of vanadium dimers in photoexcited VO2”, Science (2018). DOI:10.1126/science.aau3873
 J. D. Budai*, J. Hong*, M. E. Manley, E. D. Specht, C. W. Li, J. Z. Tischler, D. L. Abernathy, A. H. Said, B. M. Leu, L. A. Boatner, R. J. McQueeney, and O. Delaire, “Metallization of vanadium dioxide driven by large phonon entropy”, Nature 515, 535–539 (2014).
 S. Lee, K. Hippalgaonkar, F. Yang, J. Hong, C. Ko, J. Suh, K. Liu, K. Wang, J. J. Urban, X. Zhang, C. Dames, S. A. Hartnoll, O. Delaire†, J. Wu†, “Anomalously low electronic thermal conductivity in metallic vanadium dioxide”, Science, 355 (6323): 371 (2017)