Structural dynamics of incommensurate charge-density waves tracked by ultrafast low-energy electron diffraction

Area of Science:

• Condensed Matter Physics
• Materials Science
• Ultrafast Dynamics

Background:

• Charge-density waves (CDWs) are fundamental electronic states in low-dimensional materials.
• Understanding their non-equilibrium behavior is crucial for novel electronic applications.
• 1T-TaS2 exhibits complex CDW phases with unique structural dynamics.

Purpose of the Study:

• To probe the non-equilibrium structural dynamics of CDW phases in 1T-TaS2.
• To investigate the ultrafast quench and recovery of the CDW-coupled periodic lattice distortion (PLD).
• To elucidate the relaxation mechanisms and timescales governing CDW order.

Main Methods:

• Ultrafast low-energy electron diffraction (ULEED) with 1 ps temporal resolution.
• Tracking intensities of main lattice, satellite diffraction peaks, and diffuse scattering.
• Analyzing fluence-dependent relaxation cycles.

Main Results:

• Observed sequential structural relaxation processes after an ultrafast quench.
• Disentangled PLD amplitude quench from phonon-induced intensity reduction.
• Revealed a long-lived (up to 60 ps) partial suppression of the CDW order parameter.
• Identified lattice thermalization and acoustic mode population controlling delayed recovery.
• Observed superlattice peak broadening at high fluences, indicating phase fluctuations.

Conclusions:

• The long-lived non-equilibrium order parameter suppression suggests hot populations of CDW-coupled lattice modes.
• Lattice thermalization plays a key role in the delayed recovery of CDW order.
• Non-linear phase fluctuations are generated at high excitation fluences.