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A time-dependent order parameter for ultrafast photoinduced phase transitions


Strongly correlated electron systems often exhibit very strong interactions between structural and electronic degrees of freedom that lead to complex and interesting phase diagrams1,2. For technological applications of these materials it is important to learn how to drive transitions from one phase to another. A key question here is the ultimate speed of such phase transitions, and to understand how a phase transition evolves in the time domain3,4,5,6,7,8,9,10,11,12,13. Here we apply time-resolved X-ray diffraction to directly measure the changes in long-range order during ultrafast melting of the charge and orbitally ordered phase in a perovskite manganite. We find that although the actual change in crystal symmetry associated with this transition occurs over different timescales characteristic of the many electronic and vibrational coordinates of the system, the dynamics of the phase transformation can be well described using a single time-dependent ‘order parameter’ that depends exclusively on the electronic excitation.

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Figure 1: Ultrafast melting of charge and orbital order.
Figure 2: Measured (top) and simulated (bottom) evolution of the normalized diffracted X-ray intensity for three superlattice reflections.
Figure 3: Fluence dependence of charge order dynamics.
Figure 4: Optical phonon modes related to the charge and orbital order phase.

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This work was supported by the NCCR Molecular Ultrafast Science and Technology (NCCR MUST), a research instrument of the Swiss National Science Foundation (SNSF). A.C. and T.K. acknowledge financial support by SNSF, Grants No. 200021_124496 and 200021_144115, respectively. Portions of this research were carried out at the Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory. LCLS is an Office of Science User Facility operated for the US Department of Energy Office of Science by Stanford University. Portions of this research received grants from the Japan Society for the Promotion of Science (JSPS) through the ‘Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST Program)’, initiated by the Council for Science and Technology Policy (CSTP). This work was partially supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan (X-ray Free Electron Laser Priority Strategy Program).

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U.S., S.L.J. and P.B. conceived the experiment; A.C., S.O.M., S-W.H., C.D., H.W., H.T.L., M.C., G.I., T.H., J.A.J., A.F., T.K., M.R., S.L.J., U.S. and P.B. carried out the experiment; H.T.L., M.C., D.Z., J.M.G., M.S. and A.R. set up and operated the beamline including the optical pump laser; A.C., S.O.M., J.A.J., C.D., T.H. and H.T.L. analysed the data online during the experiments; A.C. analysed the data; H.W., M.N., M.K. and Y.T. conceived, designed and characterized the sample; S.O.M., L.R., A.C. and U.S. performed the static diffraction experiments; P.B. developed the model and performed the simulations with input from S.L.J.; P.B., S.L.J. and U.S. wrote the manuscript with discussions and improvements from all authors.

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Correspondence to P. Beaud.

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Beaud, P., Caviezel, A., Mariager, S. et al. A time-dependent order parameter for ultrafast photoinduced phase transitions. Nature Mater 13, 923–927 (2014).

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