Ultrafast Electron Diffraction at Surfaces:From Nanoscale Heat Transport to Driven Phase Transitions

Abstract: Many fundamental processes of structural changes at surfaces occur on a pico- or femtosecond time scale. In order to study such ultra-fast processes, we have combined modern surface science techniques with fs-laser pulses in a pump-probe scheme. Reflection high energy electron diffraction (RHEED) with grazing incident electrons ensures surface sensitivity for the probing electron pulses. Utilizing the Debye-Waller effect, we studied the nanoscale heat transport from an ultrathin film through a hetero-interface or the damping of vibrational excitations in monolayer adsorbate systems on the lower ps-time scale. By means of spot profile analysis the different cooling rates of epitaxial Ge nanostructures of different size and strain state were determined. The excitation and relaxation dynamics of a driven phase transition far away from thermal equilibrium is demonstrated using the In-induced (8x2) reconstruction on Si(111). This Peierls-distorted surface charge density wave system exhibits a discontinuous phase transition at 130 K from a (8x2) insulating ground state to (4x1) metallic excited state. Upon excitation by a fs-laser pulse, this structural phase transition is non-thermally driven in only 700 fs into the excited state. A small barrier of 40 meV hinders the immediate recovery of the groundstate and the system is found in a metastable supercooled state for up to few nanoseconds.