Evidence of Sn Adatoms Quantum Tunneling at the α-Sn/Si(111) Surface

We present a low-temperature scanning tunneling microscopy study of the α-Sn/Si(111) surface that demonstrates the fluctuating behavior of the Sn adatoms. The dynamical fluctuation model, successfully applied in describing the α-Sn/Ge(111) surface, is proposed for the related α-Sn/Si(111) surface too, although with a much lower transition temperature. In addition, a new phenomenon appears responsible for the unexpected evidence that the average oscillation frequency remains constant at temperatures lower than 15 K, in contradiction to the Arrhenius law. We explain this phenomenon as quantum tunneling of Sn adatoms.