Spin-Polarized Surface Electronic States

These limitations, most urgently felt in solid state theory, have stimulated the search for alternative approaches to the many-body problem of an interacting electron system as found in solids, surfaces, interfaces, and molecular systems. Today, local density functional (LDF) theory (3-4) and its generalization to spin polarized systems (5-6) are known to provide accurate descriptions of the electronic and magnetic structures as well as other ground state properties such as bond distances and force constants in bulk solids and surfaces. 

The electronic states of surfaces can be spin-polarized. They have been studied by means of spin-polarized ARPES for the past twenty years [66]. Recently spin polarized STM [67] has been developed as a powerful tool... 

Fig. 3 Spin polarization of the secondary electrons emitted from an Fe(001) surface following irradiation with 150 keV nitrogen ions in various charge states and 2 keV electrons. Reprinted with permission from [47]. Copyright 2001 by the American Physical Society...

A final comment is required about the problem of the description of isolated TM atoms on oxide surfaces in general. TM atoms have complex spin states which are not properly described within the DFT approach. Spin-polarized calculations provide a way to take into account the spin properties of the system but some details of the interaction may be described incorrectly. In this respect, the use of wave function-based methods is particularly important for benchmarks and comparisons. This problem is less severe when one considers small clusters where several electronic states exist separated by small energies. But one should be well aware of the fact that the treatment of isolated atoms, dimers, and very small aggregates with DFT methods requires spin-polarized approaches and special care in evaluating the results [199,200]. 

The spin splitting of the energy bands of semiconductors without inversion symmetry has other interesting and presumably also technological consequences. Photoemitted electrons from GaAs are spin polarized when the light used for the irradiation is circularly polarized.[49,50] (such effects may also be observed for metals, see for example [51]). The so-called three step model for photomission considers the three processes, optical excition from a valence-band state to a state in the conduction band, transport of the excited electron to the surface, and — finally — emission through the... 

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