  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.
