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

G Storeck,K Rossnagel,T Diekmann,J G Horstmann,S V Yalunin,S Vogelgesang,C Ropers,G Von Witte

doi:10.1063/4.0000018

Abstract

We study the non-equilibrium structural dynamics of the incommensurate and nearly commensurate charge-density wave (CDW) phases in 1T-. 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.

Highlights

The spontaneous breaking of a continuous symmetry is a fundamental concept of physics with broad relevance in such diverse areas as particle physics,[1] cosmology,[2,3] and condensed matter physics.[4,5]An essential consequence of this symmetry breaking is the emergence of new amplitude and phase excitations of the fields considered, exemplified in the Higgs mechanism[6] and massless Nambu–Goldstone bosons,[7,8] respectively
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)

Summary

Introduction

The spontaneous breaking of a continuous symmetry is a fundamental concept of physics with broad relevance in such diverse areas as particle physics,[1] cosmology,[2,3] and condensed matter physics.[4,5]An essential consequence of this symmetry breaking is the emergence of new amplitude and phase excitations of the fields considered, exemplified in the Higgs mechanism[6] and massless Nambu–Goldstone bosons,[7,8] respectively. Electron–lattice interaction is an important source of symmetry breaking in solids, most prominently in superconductivity and the formation of charge-density wave (CDW) phases.[9,10,11,12] CDWs constitute a periodic modulation of the charge density by electron–hole pairing,[12] coupled to a periodic lattice distortion (PLD)[13,14,15] and an electronic gap.[16,17,18,19] The emergence, correlations, and fluctuations of symmetry-broken CDW states can be revealed in the time domain by ultrafast measurement techniques In this way, quenches of the electronic gap coupled to coherent amplitude oscillations,[20,21,22,23,24,25,26] lightinduced PLD dynamics,[27,28,29,30] and phase transitions have been investigated.[20,31,32] In particular, ultrafast structural probes trace changes of structural symmetry[33,34] and long-range ordering following a phase transformation.[35,36]

Results

Discussion

Conclusion