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We used 4D ultrafast electron microscopy (UEM) to directly image femtosecond photoinduced structural dynamics in single-crystal LaFeAsO at initial temperatures of 300 and 100 K, above and below the known structural and magnetic phase-transition temperatures, respectively. With nanometer-picosecond resolution, we resolved an initial (precursor) sigmoidlike response arising from photothermal expansion and lattice reorientation that precedes the onset of propagating coherent acoustic phonons (CAPs). In the specific regions probed, the precursor response at 100 K is shorter than at 300 K (t0.5;100K=11.3ps vs t0.5;300K=17.8ps), and the CAP oscillation frequency is lowered with cooling (fCAP;100K=12GHz vs fCAP;300K=21GHz), correlated to known lattice softening due to the structural phase change. The transient CAP behaviors at 300 K are dispersive, displaying an exponentially decaying phase velocity over the first nanosecond. Further, the CAP symmetry at 300 K matches a first-order antisymmetric shear mode (A1), while at 100 K it is best matched by a mostly nondispersive zero-order symmetric mode (S0). These findings illustrate the sensitivity of UEM imaging to spatially heterogeneous dynamics in the Fe-pnictide materials and more broadly in other quantum materials.
Bibliographical noteFunding Information:
This material is based upon work supported primarily by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award No. DE-SC0018204. This work was supported partially by the National Science Foundation through the University of Minnesota MRSEC under Award No. DMR-2011401 and partially by the U.S. Department of Energy through the UMN Center for Quantum Materials under Grant No. DE-SC-0016371. Partial funding provided by the Arnold and Mabel Beckman Foundation in the form of a Beckman Young Investigator Award. The LaFeAsO crystals were synthesized and provided by Jiaqiang Yan and David Mandrus. We acknowledge a helpful discussion with Rafael Fernandes about differences in the structural dynamics and correlation to ordering and phase behavior.
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