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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.

Revealing the bonding and time-evolving atomic dynamics in functional materials with complex lattice structures can update the fundamental knowledge on rich physics therein, and also help to manipulate the material properties as desired. As the most prototypical chalcogenide phase change material, Ge2Sb2Te5 has been widely used in optical data storage and non-volatile electric memory due to the fast switching speed and the low energy consumption. However, the basic understanding of the structural dynamics on the atomic scale is still not clear. Using femtosecond electron diffraction, structure factor calculation and TDDFT-MD simulation, we reveal the photoinduced ultrafast transition of the local correlated structure in the averaged rock-salt phase of Ge2Sb2Te5. The randomly oriented Peierls distortion among unit cells in the averaged rock-salt phase of Ge2Sb2Te5 is termed as local correlated structures. The ultrafast suppression of the local Peierls distortions in individual unit cell gives rise to a local structure change from the rhombohedral to the cubic geometry within ~ 0.3 ps. In addition, the impact of the carrier relaxation and the large amount of vacancies to the ultrafast structural response is quantified and discussed. Our work provides new microscopic insights into contributions of the local correlated structure to the transient structural and optical responses in phase change materials. Moreover, we stress the significance of femtosecond electron diffraction in revealing the local correlated structure in the subunit cell and the link between the local correlated structure and physical properties in functional materials with complex microstructures.

Manipulating crystal structure and the corresponding electronic properties in quantum materials provides opportunities for the exploration of exotic physics and practical applications. Here, by ultrafast electron diffraction, structure factor calculation and TDDFT-MD simulations, we report the photoinduced concurrent intralayer and interlayer structural transitions in the Td and 1T' phase of XTe2 (X=Mo, W). Concomitant with the interlayer structural transition by shear displacement, the ultrafast suppression of the intralayer Peierls distortion within 0.3 ps is demonstrated and attributed to Mo-Mo (W-W) bond stretching. We discuss the modification of multiple quantum electronic states associated with the intralayer and interlayer structural transitions, such as the topological band inversion and the higher-order topological state. The twin structure and the stacking fault in XTe2 are identified by the ultrafast structural response. Our work elucidates the pathway of the photoinduced intralayer and interlayer structural transitions in atomic and femtosecond spatiotemporal scale. Moreover, the concurrent intralayer and interlayer structural transitions reveals the traversal of all double-well potential energy surfaces (DWPES) by laser excitation in material system, which may be an intrinsic mechanism in the field of photoexcitation-driven symmetry engineering, beyond the single DWPES transition model and the order-disorder transition model.

The ultrafast dynamics of magnetic order in a ferromagnet are governed by the interplay between electronic, magnetic and lattice degrees of freedom. In order to obtain a microscopic understanding of ultrafast demagnetization, information on the response of all three subsystems is required. A consistent description of demagnetization and microscopic energy flow, however, is still missing. Here, we combine a femtosecond electron diffraction study of the ultrafast lattice response of nickel to laser excitation with ab initio calculations of the electron-phonon interaction and energy-conserving atomistic spin dynamics simulations. Our model is in agreement with the observed lattice dynamics and previously reported electron and magnetization dynamics. Our approach reveals that the spin system is the dominating heat sink in the initial few hundreds of femtoseconds and implies a transient non-thermal state of the spins. Our results provide a clear picture of the microscopic energy flow between electronic, magnetic and lattice degrees of freedom on ultrafast timescales and constitute a foundation for theoretical descriptions of demagnetization that are consistent with the dynamics of all three subsystems.

We combine femtosecond electron diffuse scattering experiments and first-principles calculations of the coupled electron-phonon dynamics to provide a detailed momentum-resolved picture of the ultrafast lattice thermalization in a thin film of black phosphorus. The measurements reveal the emergence of highly anisotropic non-thermal phonon populations which persist for several picoseconds following excitation of the electrons with a light pulse. Combining ultrafast dynamics simulations based on the time-dependent Boltzmann formalism and calculations of the structure factor, we reproduce the experimental data and identify the vibrational modes primarily responsible for the carrier relaxation via electron-phonon coupling and the subsequent lattice thermalization via phonon-phonon scattering. In particular, we attribute the non-equilibrium lattice dynamics of black phosphorus to highly-anisotropic phonon-assisted scattering processes, which are primarily mediated by high-energy optical phonons. Our approach paves the way towards unravelling and controlling microscopic energy-flow pathways in two-dimensional materials and van der Waals heterostructures, and may also be extended to other non-equilibrium phenomena involving coupled electron-phonon dynamics such as superconductivity, phase transitions or polaron physics.

Black phosphorus has recently attracted significant attention for its highly anisotropic properties. A variety of ultrafast optical spectroscopies has been applied to probe the carrier response to photoexcitation, but the complementary lattice response has remained unaddressed. Here we employ femtosecond electron diffraction to explore how the structural anisotropy impacts the lattice dynamics after photoexcitation. We observe two timescales in the lattice response, which we attribute to electron-phonon and phonon-phonon thermalization. Pronounced differences between armchair and zigzag directions are observed, indicating a nonthermal state of the lattice lasting up to ~60 ps. This nonthermal state is characterized by a modified anisotropy of the atomic vibrations compared to equilibrium. Our findings provide insights in both electron-phonon as well as phonon-phonon coupling and bear direct relevance for any application of black phosphorus in nonequilibrium conditions.