Structural dynamics of two-dimensional charge-density waves in CeTe3investigated by ultrafast electron crystallography

Abstract

The electron-phonon mechanism that gives rise to various charge-ordered systems is often controversial because of the cooperative nature of the transformation and that the structural aspect of the transformation is generally poorly understood. Using femtosecond electron crystallography, we reveal a two-step (≈400 fs and 3.3 ps) suppression of the structural order parameter of a 2D charge density wave (CDW) that clearly decouples from its electronic counterpart following fs optical quenching. Through atomic fluctuational analysis on Bragg reflections and satellite features, we identify important momentum-dependent electron-phonon couplings appearing on both timescales that can be related to interactions between the unidirectional CDW collective modes, the lattice phonons, and the perturbed electronic subsystem. We show that the characteristic timescales of these couplings and relative fluctuational amplitudes as characterized by fs crystallography jointly determine the cooperativity between the electronic and structural subsystems and from this it is possible to elucidate the underlying mechanism of the charge-ordered system.

Anisotropic electron-phonon coupling investigated by ultrafast electron crystallography: Three-temperature model

“…The diffraction pattern not only records the nearly ideal 2D square host lattice, but also unveils the well defined CDW long-range order, as shown from the sharp satellite peaks highlighted in the inset at regions near (3,0) along the c-axis. The weak periodic distortion induced by the CDW (∼ 0.1Å) gives a relatively low satellite intensity, 2-3 orders of magnitude lower than the intensity of the neighboring Bragg reflections 18 . In contrast, in the study of Mott insulator VO 2 the fs electron beam is directed along the crystal monoclinic b (b M ) axis 30 where strong dimer peaks emerge along the a M -axis during metal-insulator transition.…”

“…Singleparticle dynamics, the electronic temperature T e can also be investigated via trARPES [8][9][10] , and combined with optical reflectance/transmission investigation, and terahertz (THz) spectroscopy 11,12 yielding rich characterization data on the electronic evolution. Equally important are diffraction techniques [13][14][15] to validate the critical coupling mode through tracking long-range ordering (amplitude and period) and short-range fluctuations of the lattice, necessary to describe the structure and dynamics in the lattice counterpart [16][17][18] . Recent trARPES studies of pristine 1T-TaS 2 , intercalated 1T-TaS 2 , and 1T-TiSe 2 , have shown subtle differences in the optical quenching timescales, which are used to explain their underlying different charge ordering mechanisms 19 .…”

“…Recent trARPES studies of pristine 1T-TaS 2 , intercalated 1T-TaS 2 , and 1T-TiSe 2 , have shown subtle differences in the optical quenching timescales, which are used to explain their underlying different charge ordering mechanisms 19 . Yet, their respective relaxation timescales in relation to the energy transfer into the lattice and the CDW amplitude fluctuations have not been clarified, due to the probe's limited sensitivity to the lattice responses, although the nonhomogeneous phononic relaxation pathways are central to identify the electron-lattice couplings in these systems [16][17][18]20 . Rather unexpectedly, following the fs quenching a nearly universal sub-ps (partial) recovery has been found from all-optical studies of various CDWs 21 .…”

Ultrafast Formation of a Charge Density Wave State in 1T−TaS2 : Observation at Nanometer Scales Using Time-Resolved X-Ray Diffraction

“…Diffraction techniques are especially well adapted to study CDW compounds [5,10,12,14,15,22,[24][25][26][27][28][29]. Indeed, a structural modulation with wavevector q gives rise to satellite peaks located at positions ± q with respect to each regular lattice peak (Fig.…”

Unoccupied electronic structure and momentum-dependent scattering dynamics in Pb/Si(557) nanowire arrays

“…In addition to step-induced scattering, these surface systems provide an interesting field of study for one-dimensional physics including spin-charge separation and charge-density-wave (CDW) formation leading to metalinsulator transitions. While pump-induced melting of CDWs in layered crystals has been investigated in the time domain using femtosecond optical [25], time-and angle-resolved direct photoemission [26,27], and transmission electron diffraction [28,29] pump-probe methods, the investigation of photoinduced melting of CDWs on 1D surface reconstructions has so far been limited to surface-sensitive electron diffraction [30]. Rügheimer et al have published a first time-resolved 2PPE study of the one-dimensional Si(557)-Au atomic chain system [31].…”