"Anisotropic structural dynamics of monolayer crystals revealed by femtosecond surface X-ray scattering"

I-Cheng Tung: Aravind Krishnamoorthy, Sridhar Sadasivam, Hua Zhou, Qi Zhang, Kyle L. Seyler, Genevieve Clark, Ehren M. Mannebach, Clara Nyby, Friederike Ernst, Diling Zhu, James M. Glownia, Michael E. Kozina, Sanghoon Song, Silke Nelson, Hiroyuki Kumazoe, Fuyuki Shimojo, Rajiv K. Kalia, Priya Vashishta, Pierre Darancet, Tony F. Heinz, Aiichiro Nakano, Xiaodong Xu , Aaron M. Lindenberg and Haidan Wen; Nature Photonics, 03/11/19.

Additional Authors: Aravind Krishnamoorthy, Sridhar Sadasivam, Hua Zhou, Qi Zhang, Kyle L. Seyler, Genevieve Clark, Ehren M. Mannebach, Clara Nyby, Friederike Ernst, Diling Zhu, James M. Glownia, Michael E. Kozina, Sanghoon Song, Silke Nelson, Hiroyuki Kumazoe, Fuyuki Shimojo, Rajiv K. Kalia, Priya Vashishta, Pierre Darancet, Tony F. Heinz, Aiichiro Nakano, Xiaodong Xu , Aaron M. Lindenberg and Haidan Wen

Abstract:

Ultrafast X-ray scattering is one of the primary tools to track intrinsic crystallographic evolution with atomic accuracy in real time. However, its application to study nonequilibrium structural properties at the two-dimensional limit remains a long-standing challenge due to a significant reduction of diffraction volume and complexity of data analysis. Here, we report femtosecond surface X-ray diffraction in combination with crystallographic model-refinement calculations to quantify the ultrafast structural dynamics of monolayer WSe2 crystals supported on a substrate. We found the absorbed optical photon energy is preferably coupled to the in-plane lattice vibrations within one picosecond whereas the out-of-plane lattice vibration amplitude remains unchanged during the first ten picoseconds. The model-assisted fitting suggests an asymmetric intralayer spacing change upon excitation. The observed nonequilibrium anisotropic structural dynamics agrees with first-principles modelling in both real and momentum space, marking the distinct structural dynamics of monolayer crystals from their bulk counterparts.