Element-Specific Ultrafast Lattice Dynamics in Monolayer WSe₂

Area of Science:

• Condensed Matter Physics
• Materials Science
• Ultrafast Spectroscopy

Background:

• Understanding atomic vibrations (phonons) is crucial for material properties.
• Femtosecond excitation creates nonthermal phonon populations.
• Element-specific vibrational dynamics are challenging to resolve.

Purpose of the Study:

• To develop a method for element-specific analysis of atomic vibrations.
• To investigate the nonthermal evolution of phonon populations in monolayer WSe2.
• To identify energy transfer pathways between different phonon modes.

Main Methods:

• Utilized ultrafast electron diffraction (UED) on monolayer tungsten diselenide (WSe2).
• Developed a novel approach to quantitatively extract atomic-site-specific information from UED data.
• Combined experimental results with calculated phonon dispersion for interpretation.

Main Results:

• Achieved an element-specific view of incoherent atomic vibrations after femtosecond excitation.
• Identified distinct stages in the nonthermal evolution of phonon populations by analyzing W and Se vibrations separately.
• Observed a long-lasting overpopulation of specific optical phonons.

Conclusions:

• The element-specific vibrational information provides insights into energy transfer processes between phonon groups.
• Demonstrated that resolving element-specific vibrational dynamics in the ultrafast domain is highly valuable.
• This technique opens new avenues for studying transient states in materials.