Interplay among different degrees of freedom including electrons, phonons and spins is of paramount importance in understanding and optimizing the properties of quantum materials. Optical excitation is a powerful tool to distinguish different interactions and to manipulate the state of matter. Furthermore, dominant interactions can be identified and meaningful insights into ground-state properties, phase transitions, and hidden phases can be obtained. Photoexcitation is particularly useful for complex quantum systems, where a variety of degrees of freedom and quantum interactions coexist and are strongly coupled. A particular example is charge density wave (CDW) materials. The layered transition-metal dichalcogenides such as 1T-TaS₂ have been widely investigated to understand CDW physics in real materials.

A group from Institute of Physics, Chinese academy of sciences led by Prof. Sheng Meng recently discovered a new type of atomistic mechanism and photoinduced ultrafast dynamics of CDW in 1T-TaS₂, using nonadiabatic molecular dynamics (MD) quantum simulations based on time-dependent density functional theory (TDDFT). The first-principles excited state simulations provide for the first time the intrinsic electron-nuclei coupled dynamics of 1T-TaS₂. Amplitude mode and nonthermal melting of CDW are successfully reproduced at low laser intensities. At higher laser intensity, a laser-induced new collective mode has been discovered with distinctive electronic properties (Fig. 1).

This study entitled "Photoexcitation Induced Quantum Dynamics of Charge Density Wave and Emergence of a Collective Mode in 1T-TaS₂" was recently published in Nano Letters. The scientists have also studied the CDW physics and the interactions between multiple degrees of freedom therein from the perspective of ultrafast dynamics, revealing the nature of photoexcitation induced phases in bulk 1T-TaS₂. They discovered a novel collective mode induced by photodoping, which is significantly different from thermally induced phonon mode in 1T-TaS₂. The results provide compelling evidence that the ultrafast dynamics in CDW state of bulk 1T-TaS₂ is a nonthermal process, where hot electron model is not sufficient to describe this novel phenomenon because of the lack of electron-electron scatterings (Fig. 2). The work provides new insights into laser induced insulator-to-metal transition in the CDW state of 1T-TaS₂, and the methods adopted here might be useful for understanding a wide range of laser-modulated quantum materials.

In addition, the group led by Prof. Sheng Meng and their collaborators further investigated the hot-carrier dynamics at the interfaces of semiconductors and nanoclusters, which is of significant importance for photovoltaic and photocatalytic applications. Plasmon-driven charge separation processes are considered to be dependent on the type of donor–acceptor interactions, that is, the conventional hot-electron-transfer mechanism for van der Waals interactions and the plasmon-induced interfacial charge-transfer transition mechanism for chemical bonds. They demonstrated that the two mechanisms can coexist in a nanoparticle–semiconductor hybrid nanomaterial, both leading to faster transfer than carrier relaxation. The origin of the two mechanisms is attributed to the spatial polarization of the excited hot carriers, where the longitudinal state couples to semiconductors more strongly than the transverse state. These findings provided a new insight into the photoinduced carrier dynamics for many applications including water splitting, photocatalysis, and photovoltaics.