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

Abstract

We study the non-equilibrium structural dynamics of the incommensurate and nearly commensurate charge-density wave (CDW) phases in 1T-TaS2. Employing ultrafast low-energy electron diffraction with 1 ps temporal resolution, we investigate the ultrafast quench and recovery of the CDW-coupled periodic lattice distortion (PLD). Sequential structural relaxation processes are observed by tracking the intensities of main lattice as well as satellite diffraction peaks and the diffuse scattering background. Comparing distinct groups of diffraction peaks, we disentangle the ultrafast quench of the PLD amplitude from phonon-related reductions of the diffraction intensity. Fluence-dependent relaxation cycles reveal a long-lived partial suppression of the order parameter for up to 60 ps, far outlasting the initial amplitude recovery and electron-phonon scattering times. This delayed return to a quasi-thermal level is controlled by lattice thermalization and coincides with the population of zone-center acoustic modes, as evidenced by a structured diffuse background. The long-lived non-equilibrium order parameter suppression suggests hot populations of CDW-coupled lattice modes. Finally, a broadening of the superlattice peaks is observed at high fluences, pointing to a non-linear generation of phase fluctuations.

Nonequilibrium charge-density-wave order beyond the thermal limit

“…c 3TM of electronic, hot phonon and lattice temperatures T e , T hot−ph and T l in the regime of strong perturbation ( F = 0.35 mJ cm −2 ). In the 3TM, the optical excitation of the electronic system is followed by an energy transfer to certain strongly coupled optical phonons, widely observed in materials with selective electron-phonon coupling 27 , 51 , 52 , 61 – 63 . Subsequently, this hot phonon subset equilibrates with the remaining lattice phonon bath on a ps timescale ( τ ph−ph ).…”

Coherent Phonon Disruption and Lock-In During a Photoinduced Charge-Density-Wave Phase Transition

“…Finally, though dedicated ultrafast diffraction experiments have been conducted on the NCto-IC transition (35)(36)(37)(38)(39)77), inclusion of real-space measurements provides access to new insights (78). Here, this includes the specific nanoscale spatiotemporal characteristics of how the 1-µm ROI switches between phases during photoexcitation, phonon excitation and diffuse heating, and subsequent cooling back through the critical point.…”

“…Under certain conditions, femtosecond excitation leads to complete suppression of the NC phase, nucleation of the incommensurate (IC) phase, and electron-lattice coupling in less than 5 ps (35,36). Subsequent IC-phase dynamics consist of a fluence-dependent regime spanning tens to hundreds of picoseconds characterized by domain growth and coarsening (37)(38)(39). An additional energy channel in ultrathin, freestanding crystals is the displacive excitation of coherent acoustic phonons (CAPs) also occurring over tens to hundreds of picoseconds (40,41).…”

Coherent Phonon Disruption and Lock-In During a Photoinduced Charge-Density-Wave Phase Transition

“…Finally, though dedicated ultrafast diffraction experiments have been conducted on the NCto-IC transition, [35][36][37][38][39]79 inclusion of real-space measurements provides access to new insights. 80 Here, this includes the specific nanoscale spatiotemporal characteristics of how the 1-µm ROI switches between phases during photoexcitation, phonon excitation and diffuse heating, and subsequent cooling back through the critical point.…”

“…35,36 Subsequent IC-phase dynamics consist of a fluence-dependent regime spanning tens to hundreds of picoseconds characterized by domain growth and coarsening. [37][38][39] An additional energy channel in ultrathin, freestanding crystals is the displacive excitation of coherent acoustic phonons (CAPs) also occurring over tens to hundreds of picoseconds. 40,41 Spatiotemporal commensurability with IC-phase domains and correlation lengths, ξ(t), thus suggests propagating phonon modes will also display anomalous, strongly-correlated behaviors near critical points.…”