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1. Parametric dependence of hot electron relaxation timescales on electron-electron and electron-phonon interaction strengths

   https://doi.org/10.1038/s42005-020-00442-x  

   Wilson, Richard B.; Coh, Sinisa (December 2020, Communications Physics)

   null (Ed.)

   Abstract Understanding how photoexcited electron dynamics depend on electron-electron (e-e) and electron-phonon (e-p) interaction strengths is important for many fields, e.g. ultrafast magnetism, photocatalysis, plasmonics, and others. Here, we report simple expressions that capture the interplay of e-e and e-p interactions on electron distribution relaxation times. We observe a dependence of the dynamics on e-e and e-p interaction strengths that is universal to most metals and is also counterintuitive. While only e-p interactions reduce the total energy stored by excited electrons, the time for energy to leave the electronic subsystem also depends on e-e interaction strengths because e-e interactions increase the number of electrons emitting phonons. The effect of e-e interactions on energy-relaxation is largest in metals with strong e-p interactions. Finally, the time high energy electron states remain occupied depends only on the strength of e-e interactions, even if e-p scattering rates are much greater than e-e scattering rates. 

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2. Real-time GW -BSE investigations on spin-valley exciton dynamics in monolayer transition metal dichalcogenide

   https://doi.org/10.1126/sciadv.abf3759  

   Jiang, Xiang; Zheng, Qijing; Lan, Zhenggang; Saidi, Wissam A.; Ren, Xinguo; Zhao, Jin (March 2021, Science Advances)

   We develop an ab initio nonadiabatic molecular dynamics (NAMD) method based on GW plus real-time Bethe-Salpeter equation ( GW + rtBSE-NAMD) for the spin-resolved exciton dynamics. From investigations on MoS 2 , we provide a comprehensive picture of spin-valley exciton dynamics where the electron-phonon (e-ph) scattering, spin-orbit interaction (SOI), and electron-hole (e-h) interactions come into play collectively. In particular, we provide a direct evidence that e-h exchange interaction plays a dominant role in the fast valley depolarization within a few picoseconds, which is in excellent agreement with experiments. Moreover, there are bright-to-dark exciton transitions induced by e-ph scattering and SOI. Our study proves that e-h many-body effects are essential to understand the spin-valley exciton dynamics in transition metal dichalcogenides and the newly developed GW + rtBSE-NAMD method provides a powerful tool for exciton dynamics in extended systems with time, space, momentum, energy, and spin resolution. 

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3. Mapping static core-holes and ring-currents with X-ray scattering

   https://doi.org/10.1039/D0FD00124D  

   Moreno Carrascosa, Andrés; Yang, Mengqi; Yong, Haiwang; Ma, Lingyu; Kirrander, Adam; Weber, Peter M.; Lopata, Kenneth (May 2021, Faraday Discussions)

   null (Ed.)

   Measuring the attosecond movement of electrons in molecules is challenging due to the high temporal and spatial resolutions required. X-ray scattering-based methods are promising, but many questions remain concerning the sensitivity of the scattering signals to changes in density, as well as the means of reconstructing the dynamics from these signals. In this paper, we present simulations of stationary core-holes and electron dynamics following inner-shell ionization of the oxazole molecule. Using a combination of time-dependent density functional theory simulations along with X-ray scattering theory, we demonstrate that the sudden core-hole ionization produces a significant change in the X-ray scattering response and how the electron currents across the molecule should manifest as measurable modulations to the time dependent X-ray scattering signal. This suggests that X-ray scattering is a viable probe for measuring electronic processes at time scales faster than nuclear motion. 

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4. Observation of Multi-Phonon Emission in Monolayer WS2 on Various Substrates

   https://doi.org/10.3390/nano14010037  

   Adler, Eli R; Le, Thy_Doan Mai; Boulares, Ibrahim; Boyd, Robert; He, Yangchen; Rhodes, Daniel; Van_Keuren, Edward; Barbara, Paola; Najmaei, Sina (January 2024, Nanomaterials)

   Transition metal dichalcogenides (TMDs) have unique absorption and emission properties that stem from their large excitonic binding energies, reduced-dielectric screening, and strong spin–orbit coupling. However, the role of substrates, phonons, and material defects in the excitonic scattering processes remains elusive. In tungsten-based TMDs, it is known that the excitons formed from electrons in the lower-energy conduction bands are dark in nature, whereas low-energy emissions in the photoluminescence spectrum have been linked to the brightening of these transitions, either via defect scattering or via phonon scattering with first-order phonon replicas. Through temperature and incident-power-dependent studies of WS2 grown by CVD or exfoliated from high-purity bulk crystal on different substrates, we demonstrate that the strong exciton–phonon coupling yields brightening of dark transitions up to sixth-order phonon replicas. We discuss the critical role of defects in the brightening pathways of dark excitons and their phonon replicas, and we elucidate that these emissions are intrinsic to the material and independent of substrate, encapsulation, growth method, and transfer approach. 

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5. Ab initio calculations of spin-nonconserving exciton–phonon scattering in monolayer transition metal dichalcogenides

   https://doi.org/10.1088/1361-648X/ac6649  

   Zhang, Xiao-Wei; Cao, Ting (April 2022, Journal of Physics: Condensed Matter)

   Abstract We investigate the spin-nonconserving relaxation channel of excitons by their couplings with phonons in two-dimensional transition metal dichalcogenides using ab initio approaches. Combining GW-Bethe–Salpeter equation method and density functional perturbation theory, we calculate the electron–phonon and exciton–phonon coupling matrix elements for the spin-flip scattering in monolayer WSe 2 , and further analyze the microscopic mechanisms influencing these scattering strengths. We find that phonons could produce effective in-plane magnetic fields which flip spin of excitons, giving rise to relaxation channels complimentary to the spin-conserving relaxation. Finally, we calculate temperature-dependent spin-flip exciton–phonon relaxation times. Our method and analysis can be generalized to study other two-dimensional materials and would stimulate experimental measurements of spin-flip exciton relaxation dynamics. 

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