Shockley states

Problem 2.2. The electronic surface states are shown in Fig. 2.26 by dashed lines. The parabolic shape of the upper surface band implies that it is well described by the nearly-free electron approximation and hence this state is classified as a Shockley state. The lower surface state has the character of a surface resonance in the region where it intersects the electronic bulk band. An electronic transition between the surface states is possible if the upper state is unoccupied, i.e., it is located above the Fermi level. The minimum energy of such a transition is about 1.8 eV. 

Shockley states Electronic surface states which are obtained in a model implementing an inverted forbidden energy gap. 

As discussed in Section 5.2, the very origin of this behavior is the electron-electron interaction. Zo denotes the position of the image plane. The precise position of Zo is not known a priori, although it is clear that it is located quite close to the surface. Attempts have been made to derive its position from the spectroscopy of surface states, which - as we will see - exist for such a long-range potential in addition to the Shockley states discussed before and the Tamm states discussed in Section 5.3.6 the image-potential states (see also Chapter 3.2.4). 

In Figure 3.2.2.33a, panel a shows angle-resolved ARPES spectra measured at five difierent angles probing the dispersion of the Au(lll) Shockley state. The energy resolution has been much relaxed because of the low efficiency of the spin... 

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