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.
