Ultrafast Probing and Coherent Vibrational Control of a Surface Structural Phase Transition

The present thesis explores the coherent control of surface structural phase transitions by all-optical manipulation of key vibrational modes. To this end, ultrafast low-energy electron diffraction (ULEED) in combination with femtosecond pulse sequences and optical pump-probe spectroscopy (OPP) is harnessed to probe and control the Peierls-like transition between the insulating (8x2) and the metastable, metallic (4x1) phase of atomic indium wires on the (111) surface of silicon. Single-pulse optical excitation is used to drive the (8x2)->(4x1) transition well below the critical temperature of T_c = 125 K via the (de-)population of electronic states coupled to shear and rotational phonon modes connecting both phases. Whereas transient reflectivity measurements point to an acceleration of initial atomic motion at high excitation densities, ULEED underlines the impact of nanoscale heterogeneity on the transition and the subsequent recovery of the ground state for the case of a partially excited surface. In a second set of ULEED experiments, a double-pulse optical excitation scheme is employed to exert coherent control over the transition close to its threshold. Here, pronounced oscillations in the delay-dependent switching efficiency evidence the decisive role of long-lived vibrational coherence in shear and rotation modes for governing the structural transformation. The corresponding lifetimes suggest that these modes act as a phonon bottleneck for energy relaxation between electronic and lattice subsystems. Based on the analysis of mode-specific frequency changes, initial phases and amplitudes, two possible coherent control mechanisms are discussed, involving the ballistic motion of the order parameter across the barrier and absorption modulation by Raman-active phonons, respectively. Multi-pulse experiments demonstrate the selective excitation of shear and rotation phonons and the applicability of 2D spectroscopy schemes for the investigation of possible mode couplings. Furthermore, the joint results of ULEED, OPP and density functional theory (DFT) suggest a description of the transition in terms of a two-dimensional potential energy surface (PES) with an off-diagonal transition state. The outcome of this work shows that coherent atomic motion can be harnessed to affect the efficiencies and thresholds of structural phase transitions. Mode-selective coherent control of surfaces could open new routes to switching chemical and physical functionalities, enabled by metastable and nonequilibrium states.2021-09-0

Structural Dynamics of incommensurate Charge-Density Waves tracked by Ultrafast Low-Energy Electron Diffraction

We study the non-equilibrium structural dynamics of the incommensurate and nearly-commensurate charge-density wave phases in 1T-TaS$_2$. Employing ultrafast low-energy electron diffraction (ULEED) with 1 ps temporal resolution, we investigate the ultrafast quench and recovery of the CDW-coupled periodic lattice distortion. Sequential structural relaxation processes are observed by tracking the intensities of main lattice as well as satellite diffraction peaks as well as 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 picoseconds, 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 nonlinear generation of phase fluctuations.Comment: Main text and Appendice

Surface structure and stacking of the commensurate (√13×√13)R13.9°(√13 × √13)R13.9° charge density wave phase of 1T−TaS2(0001)1T−TaS_{2}(0001)

By quantitative low-energy electron diffraction (LEED) we investigate the extensively studied commensurate charge density wave (CDW) phase of trigonal tantalum disulphide (1T−TaS2), which develops at low temperatures with a (13×13)R13.9∘ periodicity. A full-dynamical analysis of the energy dependence of diffraction spot intensities reveals the entire crystallographic surface structure, i.e., the detailed atomic positions within the outermost two trilayers consisting of 78 atoms as well as the CDW stacking. The analysis is based on an unusually large data set consisting of spectra for 128 inequivalent beams taken in the energy range 20–250 eV and an excellent fit quality expressed by a best-fit Pendry R factor of R=0.110. The LEED intensity analysis reveals that the well-accepted model of star-of-David-shaped clusters of Ta atoms for the bulk structure also holds for the outermost two TaS2 trilayers. Specifically, in both layers the clusters of Ta atoms contract laterally by up to 0.25 Å and also slightly rotate within the superstructure cell, causing respective distortions as well as heavy bucklings (up to 0.23 Å) in the adjacent sulfur layers. Most importantly, our analysis finds that the CDWs of the first and second trilayers are vertically aligned, while there is a lateral shift of two units of the basic hexagonal lattice (6.71 Å) between the second and third trilayers. The results may contribute to a better understanding of the intricate electronic structure of the reference compound 1T−TaS2 and guide the way to the analysis of complex structures in similar quantum materials

Structural phase transitions and phase ordering at surfaces probed by ultrafast LEED

We demonstrate the capability of ultrafast low-energy electron diffraction to resolve phase-ordering kinetics and structural phase transitions on their intrinsic time scales with ultimate surface sensitivity

Structural phase transitions and phase ordering at surfaces probed by ultrafast LEED

We demonstrate the capability of ultrafast low-energy electron diffraction to resolve phase-ordering kinetics and structural phase transitions on their intrinsic time scales with ultimate surface sensitivity