Snapshots of cooperative atomic motions in the optical suppression of charge density waves

Abstract

Macroscopic quantum phenomena such as high-temperature superconductivity, colossal magnetoresistance, ferrimagnetism and ferromagnetism arise from a delicate balance of different interactions among electrons, phonons and spins on the nanoscale. The study of the interplay among these various degrees of freedom in strongly coupled electron-lattice systems is thus crucial to their understanding and for optimizing their properties. Charge-density-wave (CDW) materials, with their inherent modulation of the electron density and associated periodic lattice distortion, represent ideal model systems for the study of such highly cooperative phenomena. With femtosecond time-resolved techniques, it is possible to observe these interactions directly by abruptly perturbing the electronic distribution while keeping track of energy relaxation pathways and coupling strengths among the different subsystems. Numerous time-resolved experiments have been performed on CDWs, probing the dynamics of the electronic subsystem. However, the dynamics of the periodic lattice distortion have been only indirectly inferred. Here we provide direct atomic-level information on the structural dynamics by using femtosecond electron diffraction to study the quasi two-dimensional CDW system 1T-TaS(2). Effectively, we have directly observed the atomic motions that result from the optically induced change in the electronic spatial distribution. The periodic lattice distortion, which has an amplitude of ∼0.1 Å, is suppressed by about 20% on a timescale (∼250 femtoseconds) comparable to half the period of the corresponding collective mode. These highly cooperative, electronically driven atomic motions are accompanied by a rapid electron-phonon energy transfer (∼350 femtoseconds) and are followed by fast recovery of the CDW (∼4 picoseconds). The degree of cooperativity in the observed structural dynamics is remarkable and illustrates the importance of obtaining atomic-level perspectives of the processes directing the physics of strongly correlated systems.

Time-domain classification of charge-density-wave insulators

“…The observed hierarchy of melting times ultimately seems to reflect three different non-thermal destruction mechanisms following photoexcitation (Fig. 1d): the Mott insulator collapses due to an ultrafast non-equilibrium rearrangement of the electronic states taking place on the elementary timescale of electron hopping 44 , the excitonic insulator breaks down because the Coulomb attraction causing electrons and holes to form excitons is screened by the added free carriers as soon as screening has built up on the timescale of the inverse plasma frequency 19 , and the Peierls insulator melts via a displacive excitation of coherent atomic motions resulting in a suppressed periodic lattice distortion within about half an oscillation of the amplitude mode 40 .…”

“…Yet, the ARPES snapshots in Fig. 3a-d display qualitative electronic structure changes happening within 65-210 fs after excitation, which is much faster than the 0.35-4 ps it takes to transfer the excess energy in the electron system to the phonon bath 16,17,19,[39][40][41] . These changes therefore do not reflect transitions to quasi-equilibrium states at elevated effective temperatures, although the band maps look fairly similar to temperature-dependent equilibrium ARPES data 9,[12][13][14] .…”

“…However, what is important for the following quantitative analysis is that the experimentally determined values of τ amp (or some fraction thereof) define an intrinsic timescale for the fastest possible structural response in the two systems 16,17,35,36,40 . Figure 5 shows characteristic ARPES intensity transients, extracted from the data in Fig.…”

Aperiodic Spin State Ordering of Bistable Molecules and Its Photoinduced Erasing

“…The control of the functionality of a material requires understanding the complex organization of its atomic or electronic constituents, but also understanding the bi-stable or multi-stable processes addressable by external stimulation, including light for the photo-control 1,2,3,4 . On the one hand, macroscopic ordering is manifested by the appearance of regular patterns, which in some cases never repeat themselves periodically in 3D space but only in a higher dimension space: this so-called aperiodicity 5,6,7 plays a central role in the structure and physical properties of materials as diverse as quasicrystals 8 , charge-density 1 and spin-density waves 9,10 , or new superconductors 11 for instance.…”

Ultrafast time resolved reflection high energy electron diffraction with tilted pump pulse fronts

“…Time resolved transmission electron diffraction (TR-TED) has been used, for example, to study bulk phenomena such as ultrafast laser-induced structural phase transitions [1], cooperative rearrangement of crystalline structures [2], and dynamics of charge density waves [3]. Time resolution of less than 100 fs [4] has been achieved.…”