Giant vibrations of impurity atoms on a crystal surface

Abstract
We have measured extremely large anisotropic thermal vibrations of gallium adatoms on a Si(111) surface using x-ray standing waves. At 830 K the rms amplitude normal to the surface is 0.47±0.02 Å while the parallel amplitude is 0.30±0.02 Å. The temperature dependence of these amplitudes deviates significantly from that expected from Debye theory and suggests a significant thermal softening of the surface bonding at elevated temperatures.