Vacancy-mediated and exchange diffusion in a Pb/Cu(111) surface alloy: Concurrent diffusion on two length scales

Pb diffuses in a Pb/Cu(111) surface alloy predominantly by exchange with surface vacancies and, much less frequently, by exchange with thermal Cu adatoms. Because the infrequent adatom exchanges transport Pb atoms much farther, both processes affect observations of Pb transport in the Pb/Cu(111) surface alloy.