Direct measurement of diffusion by hot tunneling microscopy: Activation energy, anisotropy, and long jumps

We measure the activation energy for surface diffusion of isolated Pb adatoms on Ge (111) by observing individual atomic interchanges using a scanning tunneling microscope. The diffusion occurs mainly along one direction of the c (28) reconstructed surface with Pb adatoms moving between substitutional Ge adatom sites. Only half of the adatom interchanges are to near-neighbor sites, the rest are ``long jumps. '' The scanning process does not affect the diffusion. For temperatures from 24 to 79 ^ we find the diffusion obeys an Arrhenius law with an activation energy of 0. 540. 03 eV.