Non-thermal phonon dynamics and a quenched exciton condensate probed by surface-sensitive electron diffraction

Area of Science:

• Condensed Matter Physics
• Materials Science
• Ultrafast Spectroscopy

Background:

• Electron-phonon interactions are crucial for energy flow and phase emergence in materials.
• Existing techniques lack monolayer sensitivity for 2D materials, especially for out-of-plane phonon polarizations.
• Investigating non-equilibrium dynamics in anisotropic 2D materials requires advanced methods.

Purpose of the Study:

• To resolve non-equilibrium phonon dynamics in 1T-TiSe2 with monolayer sensitivity.
• To quantify the excitonic contribution to the structural order parameter.
• To introduce and validate ultrafast low-energy electron diffuse scattering for 2D materials.

Main Methods:

• Ultrafast low-energy electron diffuse scattering (ULEDS).
• Tracking non-equilibrium structural dynamics with electron pulses.
• Analyzing momentum- and fluence-dependent phonon populations.

Main Results:

• ULEDS resolves phonon dynamics and excitonic contributions in 1T-TiSe2.
• Phonon-phonon scattering mediates a build-up of zone boundary modes, followed by slower zone-centre acoustic modes.
• Excitonic and Peierls contributions to lattice distortion are found in a 30:70 ratio.

Conclusions:

• The surface-sensitive ULEDS technique complements existing ultrafast structural tools.
• Phonon scattering significantly delays equilibration in layered materials.
• This method can elucidate phonon impacts in other 2D material phenomena, like interlayer excitons.