Coverage dependence and isotope effect in quantum surface diffusion

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Abstract
We consider tunneling of an adatom through a substrate potential barrier in the presence of phonons. The phonon-adatom interaction leads to a concentration-dependent diffusion constant; the effective tunneling potential is significantly changed by the presence of nearby adatoms. We explain the coverage dependence of the low-temperature diffusion coefficient observed for hydrogen on the (110) plane of tungsten and the isotope effect as tritium is substituted for hydrogen.