Imaging coherent phonons and precursor dynamics in LaFeAsO with 4D ultrafast electron microscopy

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 (t_0.5;1000.16em0exK=11.30.16em0exps vs t_0.5;3000.16em0exK=17.80.16em0exps), and the CAP oscillation frequency is lowered with cooling (f_CAP;1000.16em0exK=120.16em0exGHz vs f_CAP;3000.16em0exK=210.16em0exGHz), 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.