Fig. 3.

Polaron formation in SnSe visualized with UEDS. (A) Wavevector-dependent scattering intensity for a Gaussian displacement field model of the polaron lattice distortion as a function of size (polaron FWHM), as described in the text. (B) Measured change in diffuse intensity at 1 ps (black triangles) and 5 ps (orange circles) fit to the Gaussian displacement model above (solid curves). Best-fit FWHM polaron dimensions are $f=18.7\pm 0.3$ Å (1 ps) and $f=4.2\pm 0.1$ Å (5 ps). The larger polaron is consistent with uniaxial displacements along ${\mathbf{c}}^{\star }$, and the smaller polaron is consistent with uniform displacements in the b– c plane. The boundary between regions 2 and 3 from Fig. 2B are indicated. The shaded region represents the SE in the mean of intensity across the integration region. (C) Differential scattering intensity across the BZ due to the large polaron lattice distortion. (D) Differential scattering intensity across the BZ due to the small polaron lattice distortion. (E) The measured differential diffuse intensity across the BZ at $\tau =5\hspace{0.17em}\text{ps}$ is in excellent agreement with the predicted scattering vector dependence of the model.