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Observing structural dynamics at surfaces with Ultrafast Low-Energy Electron Diffraction

Periodic Reporting for period 4 - ULEED (Observing structural dynamics at surfaces with Ultrafast Low-Energy Electron Diffraction)

Reporting period: 2019-09-01 to 2020-02-29

This project addresses the establishment of Ultrafast Low-Energy Electron Diffraction (ULEED) as a novel and versatile tool to investigate ultrafast structural dynamics at surfaces and in ultrathin films. Two-dimensional systems such as surfaces and molecular monolayers exhibit a multitude of intriguing phases and complex transitions. Studying the ultrafast dynamics of these materials elucidates correlations and microscopic couplings, but to date, access to structural degrees of freedom with ultrahigh time resolution remains challenging. Low-Energy Electron Diffraction (LEED) is a powerful technique in surface science to determine the atomic-scale structure and symmetry of surfaces. However, time-resolved LEED has proven exceedingly difficult to realize, owing to the problems in realizing suitable low-energy electron pulses of sufficiently short pulse duration and high beam quality.

This project targets both of these present limitations by using laser-triggered nanoscopic electron sources to generate high-brightness beams of low-energy electrons. Specifically, nanotip cathodes driven by nonlinear photoemission are integrated in compact micro- and nanofabricated electrostatic lens assemblies. This allows for a drastic reduction of electron beam propagation distances while maintaining a high level of beam control and focusing ability. Using this electron source, we employ a laser-pump/electron-diffraction-probe scheme at low electron energies with a temporal resolution in the picosecond and femtosecond range. A number of strategies are followed to improve the temporal resolution of the setup, including wavelength-tuning of the laser excitation, active spectral compression of the electron pulses using locally enhanced THz fields, as well as radio-frequency compression. ULEED is being applied in the investigation of the structural dynamics within a range surface systems, including molecular monolayers, intrinsic surface reconstructions and adsorbate-induced charge-density waves.
In the first half of the project, major steps towards the project goals were successfully implemented, and a large fraction of the technological advancements establishing ULEED were taken. The project has led to a laboratory infrastructure that provides for unique experimental capabilities. Specifically, as its key achievements, and in-line with the project plan, the project group has:
• implemented a custom-built ultrahigh vacuum (UHV) system for ultrafast low-energy electron diffraction using ultrashort photoelectron pulses, designed two ultrafast low-energy electron gun concepts and successfully implemented both of them in experiment,
• demonstrated the first realization of ULEED in backscattering diffraction from surfaces, and implemented spot-profile analysis in ULEED to determine structural correlation lengths with ultrafast temporal resolution [S. Vogelgesang et al., Nature Physics 14, 184 (2018), published 2017],
• achieved by far the highest temporal resolution of any ultrafast electron diffraction experiment employing sub-keV electrons (1.3 ps), [G. Storeck et al., Structural Dynamics 4, 044024 (2017)],
• employed these capabilities to observe, for the first time, the phase-ordering kinetics of a charge density wave system driven across a structural phase transitions [S. Vogelgesang et al., Nature Physics 14, 184 (2018)],
• devised and experimentally demonstrated a scheme for the THz-induced phase space manipulation of low-energy electron pulses [L. Wimmer et al., Phys. Rev. B 95, 165416 (2017)].

In the second half of the project, we harnessed the high temporal and momentum resolution of ULEED to studied ultrafast structural dynamics in several solid-state surface systems. At the same time, we developed techniques for electron pulse compression and tailored optical excitation of structural phase transitions. In this respect, the project group has:
• uncovered the nonequilibrium dynamics of incommensurate CDWs, disentangling in the time domain the amplitude quench from the excitation of fluctuation modes [Storeck et al., Structural Dynamics 7, 034304 (2020)],
• demonstrated coherent control over a surface structural phase transition by combining ULEED with a multi-pulse optical excitation scheme [J. G. Horstmann et al., Nature 582, 232 (2020)],
• examined the nonequilibrium heat-transfer of a physisorbed molecular layer on graphene [B. Wit et al., Adv. Mater. Interfaces 7, 2000473 (2020)],
• harnessed the µm-spot size of our electron guns to resolve local reconstructions of the lanthanum hexaboride (001) surface [P. Buchsteiner et al., Phys. Rev. B 100, 205407 (2019)],

Further work was conducted to on specific sample systems to be studied by ULEED. For example, molecular dynamics simulations were employed to elucidate mechanisms involved in an order to disorder transition of polymer monolayers on graphene [M. Gulde et al., Nano Lett. 16, 6994 (2016). Moreover, in collaboration with the University of Erlangen-Nürnberg (Dr. Lutz Hammer), we used LEED as a function of the incident electron energy (“IV-LEED”) to determine the surface structure and stacking of the commensurate charge density wave state of tantalum disulphite (TaS2) [G. von Witte et al., Phys. Rev. B 100, 155407 (2019)]. Beyond these results, we are presently investigating the photo-induced structural dynamics of several promising surface systems, including charge-ordered states, molecular monolayers and surface reconstructions.
At the core of the project lies the development of a new methodology, which facilitates experimental capabilities in the analysis of ultrafast structural changes at surfaces beyond the state of the art. To date, the setup funded within the project is the only operating ultrafast low-energy electron diffraction apparatus [see, e.g. S. Vogelgesang et al., Nature Physics 14, 184 (2018)]. The methodology permits observations of ultrafast changes in the symmetry, orientation and long-range order of lowdimensional systems with unprecedented surface sensitivity and signal-to-noise ratio. The key methodical advance in this project is the use of nanoscale photoelectron emitters housed in very compact photoelectron sources. This facilitates the generation of highly coherent low-energy electron pulses. Furthermore, the compact outer diameter of the electron gun environment allows us to minimize the propagation distance between the electron source and the sample, thus drastically limiting the temporal spreading of the electron pulses from the source to the sample. An essential technological advance was realized by the micro- and nanofabrication of laser-triggered electron sources. Within the project, we have developed a manufacturing process that yields a unique set of complex microscopic electron optics via a combination of optical lithography and focused ion beam milling [G. Storeck et al., Structural Dynamics 4, 044024 (2017)]. Based on these technological developments, the project has been able to study structural dynamics in several materials systems. Combined with tailored optical excitation, this has not only enabled the observation ultrafast structural transformations, but also their control. After completion of the project, the developed methodology will be further extended and systematically refined. Based on the results achieved, we expect broader proliferation in the scientific community and beyond.
Artist's impression of the phase transition of indium atoms on a silicon crystal observed by ULEED