Multi Time Scale Mesoscale Domain Dynamics in Ferroelectric Superstructures Observed Using Pump-Probe Coherent X-Ray Diffraction

SrTiO3/PtTiO3 (STO/PTO) superlattices are a unique material system that is with a large phase space of ferroelectric configurations, including polar vortices [1], supercrystals [2], and skyrmions [3]. Recent work has revealed that in addition to these equilibrium structures, there exist multiple nonequilibrium ferroelectric superstructure phases that exhibit unique dynamics. A mixture of two of these nonequilibrium (and metastable) phases - the zig-zag flux closure (ZFC) phase and the zig-zag supercrystal (ZSC) phase - are generated using optical excitation above a fluence threshold. The pathway by which these phases are generated is of interest as a case study of the dynamics of phase transformation and phase competition more broadly. The superstructures present in (STO/PTO) create satellite diffraction peaks that can be monitored and measured to reveal the phase dynamics of the material. Optically pumping the phase mixture of ZFC and ZSC phases induces dynamics at multiple length and time scales. Generated free charges interact with the polar order parameter in PTO which further alters the atomic and mesoscopic configuration. Despite the tremendous progress, it remains elusive how and how fast optical excitations reconfigure the polar order on the mesoscale, e.g., the scale of domains. Such knowledge is critical for understanding the size, the rate limit in memory, communication, and transduction. In this work we observe changes induced by an optical pump in the speckle patterns of these satellite peaks generated by coherent x-ray diffraction to understand the mesoscale domain interactions between the ZFC and ZSC phases. We performed first-of-its-kind stroboscopic pump-probe x-ray photon correlation spectroscopy (XPCS) measurements to measure repeatable ultrafast (~10s ps) mesoscopic domain dynamics. The satellite diffraction peaks show evidence of distinct atomic and mesoscale evolution induced by the optical pulse. In addition, these measurements reveal irreversible long-time scale (~1000s s) aging of the sample. Enabled by the pump-probe XPCS, our work demonstrates mesoscale domain growth as well as the non-Markovian domain wall annihilation, which arises fundamentally from the dedicated energy balance between domains and domain walls in a dynamical system. The insights and methodology we developed will shed light not only on the driven dynamics of ferroelectric materials but a wide range of heterogeneous quantum and functional materials.
References
1. Yadav, A.K., et al., Observation of polar vortices in oxide superlattices. Nature, 2016. 530(7589): p. 198-201.
2. Stoica, V.A., et al., Optical creation of a supercrystal with three-dimensional nanoscale periodicity. Nature Materials, 2019. 18(4): p. 377-383.
3. Das, S., et al., Observation of room-temperature polar skyrmions. Nature, 2019. 568(7752): p. 368-372.
This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Science and Engineering Division.