Ultrafast switching in an atomic wire system at surfaces
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Introduction Watching the motion of atoms in real time has been a dream for scientists for a long time. The structural dynamics of atoms (i.e., the change of spatial coordinates of the valence electrons and the cores) is one of the key elements for the understanding of catalytic reactions, light to energy conversion such as water splitting, and phase transitions in materials science. Motion on the atomic level is relevant in processes such as film growth, and the inherent fundamental processes of adsorption, desorption, or diffusion of atoms as well as friction and lubrication— the examples are numerous. The time scale for atomic motion is determined by Newton’s Second Law through the mass of atoms and the strength of bonds. This time scale can be roughly estimated from onequarter of a period of the highest frequency phonon modes, which is representative for the motion of atoms of mass, m, and interatomic potential, V(r), in the solid phase, where r is the interatomic distance. With typical phonon frequencies in the THz regime, this translates to the motion of atoms being on a femtosecond time scale. We are especially interested in the ultrafast and nonequilibrium dynamics of atoms at surfaces. For this, we have explored a prototypical one-dimensional (1D) atomic wire system on a surface, namely one atomic layer of indium atoms on a silicon (111) surface. This system exhibits an inherent Peierls instability1,2 (i.e., a structural instability accompanied
by a periodicity doubling along the wire), which was used in this work to demonstrate how ultrafast motion of atoms from one structural state to another can be monitored in the time domain. To watch such processes on the relevant atomistic level requires techniques capable of following the dynamics on a femtosecond time scale and on a sub-Angström length scale. This should finally answer the question—how fast do atoms move?
Time-resolved electron diffraction The method of choice to satisfy these requirements is timeresolved diffraction with wavelengths of less than 2 Å. For studies of structural dynamics in bulk materials or thick films, ultrafast x-ray diffraction is a well-established method employing both tabletop setups as well as free-electron laser facilities.3–11 Owing to their higher scattering cross section, high-energy electron diffraction has been employed in transmission geometry through freestanding thin electron-transparent crystalline films.12–19 Utilizing relativistic energies of a few MeV allows the study of thicker films and avoids space-charge broadening.20,21 For studies at surfaces, electrons are useful because their elastic scattering cross section is 4–6 orders of magnitude larger than for x-rays, thus ensuring enhanced surface sensitivity. The practicability of such an experiment was demonstrated by the early pioneering work of Mourou22 and Elsayed-Ali23,24
Michael Horn-von Hoegen, Department of Physics, Duisburg-Essen University, Germany; [email protected] doi:10.1557/mrs.2018.150
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• VOLUME 43 • JULY 2018 Stellenbosch • www.mrs.org/bull
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