Direct visualization of polaron formation in the thermoelectric SnSe

SnSe is a layered material that currently holds the record for bulk thermoelectric efficiency. The primary determinant of this high efficiency is thought to be the anomalously low thermal conductivity resulting from strong anharmonic coupling within the phonon system. Here we show that the nature of the carrier system in SnSe is also determined by strong coupling to phonons by directly visualizing polaron formation in the material. We employ ultrafast electron diffraction and diffuse scattering to track the response of phonons in both momentum and time to the photodoping of free carriers across the bandgap, observing the bimodal and anisotropic lattice distortions that drive carrier localization. Relatively large (18.7 angstrom), quasi-one-dimensional (1D) polarons are formed on the 300-fs timescale with smaller (4.2 angstrom) 3D polarons taking an order of magnitude longer (4 ps) to form. This difference appears to be a consequence of the profoundly anisotropic electron-phonon coupling in SnSe, with strong Frohlich coupling only to zone-center polar optical phonons. These results demonstrate a high density of polarons in SnSe at optimal doping levels. Strong electron-phonon coupling is critical to the thermoelectric performance of this benchmark material and, potentially, high performance thermoelectrics more generally.

Automated summary

SnSe is a layered material with high thermoelectric efficiency due to its anomalously low thermal conductivity caused by strong anharmonic coupling. The carrier system in SnSe is also determined by strong coupling to phonons, as visualized by polaron formation. The response of phonons to photodoping in SnSe shows bimodal and anisotropic lattice distortions that drive carrier localization. Quasi-one-dimensional polarons form quickly (300 fs), while three-dimensional polarons take longer (4 ps) due to the anisotropic electron-phonon coupling in SnSe. These results reveal a high density of polarons in SnSe at optimal doping levels, highlighting the importance of electron-phonon coupling for thermoelectric performance.