Femtosecond electron diffraction reveals local disorder and local anharmonicity in thermoelectric SnSe

TL;DR

This study uses femtosecond electron diffraction combined with simulations to uncover static local disorder and anharmonicity in SnSe, providing new insights into its ultralow thermal conductivity relevant for thermoelectric applications.

Contribution

It introduces a novel ultrafast structural dynamics approach to directly observe local disorder and anharmonicity in thermoelectric materials like SnSe.

Findings

Identified static off-symmetry displacement of Sn (~0.4 Å)

Observed ultrafast atomic displacement within 100 fs after photoexcitation

Revealed local disorder and anharmonicity as key to low thermal conductivity

Abstract

The microscopic arrangement of atoms and molecules is the determining factor in how materials behave and perform. Beyond the long-range periodicity, the local disorder with local structures deviating from the average lattice structure plays a vital role in determining the physical properties of the phonon, electron and spin subsystems in crystalline functional materials. Experimentally characterizing the 3D atomic configuration of such local disorder and correlating it with the advanced functions remain a big challenge. Time-domain evolution of the local disorder, either static or dynamical, is lost due to the characterization at equilibrium state with conventional probing techniques. With the combination of femtosecond electron diffraction, structure factor calculation and TDDFT-MD simulation, we exclusively identify the static local disorder and the local anharmonicity of it in thermoelectric SnSe. The ultrafast structural dynamics in time domain reveal a dominant static off-symmetry displacement of Sn (~0.4 angstrom) and the anharmonicity of this local disorder induces an ultrafast atomic displacement within 100 fs after photoexcitation. The microscopic picture of the local anharmonicity indicates a direct and first signature of the THz Einstein oscillators in real space. Therefore, a glass-like thermal transport channel with the local disorder, the Einstein oscillators and the local anharmonicity, updates the fundamental insight into the long-debated ultralow thermal conductivity in SnSe. The local disorder over one to a few unit cells is pervasive and indispensable in thermoelectric materials, multiferroic materials and correlated electronic materials. Our method of revealing the 3D local disorder and the local correlated interactions by ultrafast structural dynamics will inspire broad interest in construction of the structure-property relationship in material science.