Traversing Double-Well Potential Energy Surfaces: Photoinduced Concurrent Intralayer and Interlayer Structural Transitions in XTe₂ (X = Mo, W)

Abstract

The microscopic arrangement of atoms and molecules is the determining factor in how materials behave and perform; i.e., the structure determines the property, a traditional paradigm in materials science. Photoexcitation-driven manipulation of the crystal structure and associated electronic properties in quantum materials provides opportunities for the exploration of exotic physics and practical applications; however, a generalized mechanism for such symmetry engineering is absent. 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′ phases of XTe2 (X = Mo, W). 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 structures and the stacking faults in XTe2 are also identified by ultrafast structural responses. The comprehensive study of the ultrafast structural response in XTe2 suggests the traversal of all double-well potential energy surfaces (DWPES) by laser excitation, which is expected to be an intrinsic mechanism in the field of photoexcitation-driven global/local symmetry engineering and also a critical ingredient inducing the exotic properties in the non-equilibrium state in a large number of material systems.

Femtosecond Electron Diffraction Reveals Local Disorder and Local Anharmonicity in Thermoelectric SnSe

“…Photoexcitation‐enhanced thermal vibration (the Debye‐Waller effect) is expected to dominate this behavior. [ 15,21 ] The calculated intensity changes of the three reflections follow the s 2 dependence (−ln( I / I 0 ) = a · s 2 , s is the scattering vector and a is constant) with the ratio of ≈1:4:9, that is, significantly larger intensity changes of the higher order reflections than that of the lower order reflections, as shown in the inset of Figure 2a and Section S3.1 in the Supporting Information. Therefore, the Debye‐Waller effect could not reproduce the experimental phenomenon.…”

“…[ 3h ] Structural evolution in the time domain is a potentially suitable method for identifying the local structure contained in the ground state. [ 3f,15 ] Based on this perspective, we propose the use of femtosecond electron diffraction to quantify the local structure and the local disorder. The local disorder considered here is intrinsic to material systems, contrary to photoinduced disorder and defects.…”

“…[ 18 ] The Bragg reflections have a greater intensity than diffuse scattering by several orders of magnitude; therefore, they has a higher signal‐to‐noise ratio and higher priority in quantifying the local structure. [ 3f,15,19 ] The local structure has a limited correlation length over one to a few unit cells, and the Bragg reflection intensity is determined by such a supercell. [ 16a,20 ] The parameters associated with the local structure, such as the thermal vibration, static local disorder, and photoexcited displacement, can be identified by the ultrafast structural dynamics with the structure factor calculation (refer to the Experimental Section).…”

Photoinduced Structural Dynamics of 2H-MoTe2Under Extremely High-Density Excitation Conditions

“…We demonstrated ultrafast time-resolved electron diffraction measurements on an ultrathin 2H-MoTe 2 sample. Ultrafast time-resolved electron diffraction measurements can explore the photoinduced structural dynamics of TMDs. − The electron pulse traveled perpendicular to the ab -plane of the 2H-MoTe 2 sample. The obtained electron diffraction pattern from the sample is shown in Figure a, where six symmetric diffraction spots from the {100}, {110}, and {200} planes are clearly observed.…”

Probing the electronic topological transitions of WTe2 under pressure using ultrafast spectroscopy

“…3(e), the phase ϕ at ambient pressure is ∼0, indicating a cosine-type CP oscillation, which highlights that the coherent shear phonon stems from the displacive excitation. [52,53] The displacive CP excitation is associated with the shift of the potential energy surface induced by the nonradiative energy delivery through the e-ph coupling. [54] Thereby, we suggest that the sudden changes of CP oscillation around 3.5 GPa reflect an abrupt reduction in the e-ph coupling.…”