Periodic Shockley surface states

T= 18.5 K. Obviously, the oscillation period does not depend on the adatom type, but depends on the substrate, more precisely on the Fermi wavelength of the Shockley surface state. (From Ref [51].)... 

The existence of surface states is a consequence of the atomic structure of solids. In an infinite and uniform periodic potential, Bloch functions exist, which explains the band structures of different solids (Kittel, 1986). On solid surfaces, surface states exist at energy levels in the gap of the energy band (Tamm, 1932 Shockley, 1939 Heine, 1963). 

Shockley, W. (1939). On the surface states associated with a periodic potential. Phys. Rev. 56, 317-323. 

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