@article{ab5505a200f941b9b23fd2cb69eaa3c6,
title = "Direct visualization of polaron formation in the thermoelectric SnSe",
abstract = "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 {\AA}), quasi-one-dimensional (1D) polarons are formed on the 300-fs timescale with smaller (4.2 {\AA}) 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 Fr{\"o}hlich 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.",
keywords = "Electron-phonon coupling, Polarons, Thermoelectric",
author = "{Ren{\'e} de Cotret}, {Laurent P.} and Otto, {Martin R.} and P{\"o}hls, {Jan Hendrik} and Zhongzhen Luo and Kanatzidis, {Mercouri G.} and Siwick, {Bradley J.}",
note = "Funding Information: ACKNOWLEDGMENTS. This work was supported by the National Science and Engineering Research Council, the Canadian Foundation for Innovation, and Fonds de Recherche du Qu{\'e}bec – Nature et Technologies. L.P.R.d.C. thanks H. Seiler, D. Zahn, B. J. Dringoli, and D. G. Cooke for enlightening discussions. We thank McGill University{\textquoteright}s Facility for Electron Microscopy Research for preparing samples via ultramicrotome. The Northwestern University personnel and research work are supported by the Department of Energy, Office of Science, Basic Energy Sciences under Grant DE-SC0014520 (design and synthesis of thermoelectric materials). Funding Information: This work was supported by the National Science and Engineering Research Council, the Canadian Foundation for Innovation, and Fonds de Recherche du Qu{\'e}bec - Nature et Technologies. L.P.R.d.C. thanks H. Seiler, D. Zahn, B. J. Dringoli, and D. G. Cooke for enlightening discussions. We thank McGill University's Facility for Electron Microscopy Research for preparing samples via ultramicrotome. The Northwestern University personnel and research work are supported by the Department of Energy, Office of Science, Basic Energy Sciences under Grant DE-SC0014520 (design and synthesis of thermoelectric materials). Publisher Copyright: {\textcopyright} This article is distributed under Creative Commons Attribution-NonCommercialNoDerivatives License 4.0 (CC BY-NC-ND).",
year = "2022",
month = jan,
day = "18",
doi = "10.1073/pnas.2113967119",
language = "English (US)",
volume = "119",
journal = "Proceedings of the National Academy of Sciences of the United States of America",
issn = "0027-8424",
publisher = "National Academy of Sciences",
number = "3",
}