Photoinduced anisotropic lattice dynamic response and domain formation in thermoelectric SnSe

Wei Wang,Yu Liu,Junjie Li,Yimei Zhu,Ian K Robinson,Cedomir Petrovic,Mark P M Dean,Mikhail Fedurin,Weiguo Yin,Niraj Aryal,Jing Tao,Lijun Wu,Marcus Babzien,Xilian Jin,Rotem Kupfer,Mark Palmer

doi:10.1038/s41535-021-00400-y

Abstract

Identifying and understanding the mechanisms behind strong phonon–phonon scattering in condensed matter systems is critical to maximizing the efficiency of thermoelectric devices. To date, the leading method to address this has been to meticulously survey the full phonon dispersion of the material in order to isolate modes with anomalously large linewidth and temperature-dependence. Here we combine quantitative MeV ultrafast electron diffraction (UED) analysis with Monte Carlo based dynamic diffraction simulation and first-principles calculations to directly unveil the soft, anharmonic lattice distortions of model thermoelectric material SnSe. A small single-crystal sample is photoexcited with ultrafast optical pulses and the soft, anharmonic lattice distortions are isolated using MeV-UED as those associated with long relaxation time and large displacements. We reveal that these modes have interlayer shear strain character, induced mainly by c-axis atomic displacements, resulting in domain formation in the transient state. These findings provide an innovative approach to identify mechanisms for ultralow and anisotropic thermal conductivity and a promising route to optimizing thermoelectric devices.

Highlights

Thermoelectric materials provide a platform to study phonon–phonon interactions, which is a central topic in condensed matter physics[1,2,3,4]
Combining quantitative ultrafast diffraction analysis and firstprinciples calculations, we find that a soft phonon mode, transverse optical (TO) Ag phonon mode at the Brillouin zone center in the Pnma phase, is highly excited[23]
The intensity decreases with the increase of the displacement Δz of Sn for {002} but increases for {004}

Summary

INTRODUCTION

Thermoelectric materials provide a platform to study phonon–phonon interactions, which is a central topic in condensed matter physics[1,2,3,4]. The SnSe crystal structure features Sn-Se zigzag-chains along the b axis and armchair-chains along the c axis[19] These b–c planes are stacked along the a axis to form an orthorhombic unit cell with strong anisotropy in electronic conductivity, phonon dispersion relation, and optical properties[9,16,20,21]. Phase, which is associated with the condensation of a soft optical phonon mode with an Ag symmetry[22,23]. The lattice distortion corresponding to the Ag phonon modes introduces different strain effects to the system Their competition gives rise to a shear strain between the layers, which significantly reduces the correlation length along the c axis via lattice distortion, yielding in-plane domain nucleation. The contributions from these phonon modes are sensitive to the sample temperature as the strain generation and the domain formation in SnSe are suppressed at 300 K

RESULTS AND DISCUSSION

METHODS

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