Apparent anomaly in electron scattering in Ag nanostructures*

Electron scattering from diffusing atoms can be visualized via the effects of the corresponding force that biases atomic motion, the ``electromigration force''. Using a combination of scanning tunneling microscopy and scanning electron microscopy, we investigate electron scattering via the biased motion of monatomic islands and C$_{60}$-decorated steps on Ag(111) surfaces in the presence of large current density (j$_{bulk}$ = 6.7x10$^{9}$ A/m$^{2})$. For monatomic adatom islands, the biased motion is opposite to the current direction and thus parallel to the direction of momentum transfer (the ``wind force'' direction), while vacancy islands move oppositely. The measured drift velocity v as a function of the island radius R, vR = 1.9nm$^{2}$/s, yields an anomalously large$^{1}$ effective force per boundary atom $\sim $0.06 meV/nm . An effective scattering force of similar magnitude is also observed via current-induced curvature of C$_{60}$ decorated line-boundaries. Possible mechanisms for this effect, including current crowding, charge transfer and local heating, will be discussed. 1. A. Bondarchuk, et al. Phys. Rev. Lett. \textbf{99, 206801 (2007)}