Surface states

Surface states are electronic states found at the surface of materials and are part of condensed matter physics. They are formed due to the sharp transition from solid material that ends with a surface and are found only at the atom layers closest to the surface. The termination of a material with a surface lead to a change of the electronic band structure from the bulk material to the vacuum. In the weakened potential at the surface new electronic states can be formed, so called surface states.

There are two types of surface states defined: Shockley and Tamm states. Although they might look similar in a recorded spectra, they have different origins. The Shockley state come from a state which is created at the surface in a gap of the bulk band structure created by the termination of the crystal at the surface. This is modeled by adding a weak potential to the free electron gas of a solid, and thus emerge from a formalism that is generally applied to the description of $sp$-bands in metals. The Tamm states are also found at the surface, as Shockley states, but they are split-off states of $d$- and $f$- valence band states in the weaker potential at the surface.

An experimental technique to measure surface states is angle resolved Ultraviolet Photoelectron Spectroscopy (UPS).

See also

[ Fermi surfaces of the two-dimensional surface states on vicinal Cu(111),F. Baumberger, T. Greber and J. Osterwalder, Phys. Rev. B, 64(2001)195411. http://www.physik.unizh.ch/groups/grouposterwalder/new/publications.php ]