Diploma - Max Planck Institute for Solid State Research

4.3 Perspective: quasi-linear dispersion in a 4f compound 53
Figure 4.21: dispersion of the linear surface state S1 around Γ measured
for several cuts parallel to Γ−X towards the X-point. The different
cuts are sketched in (c). In (a) are the results for Si terminated surface
and in (b) for Eu terminated surface depicted. (SLS-SIS)
Therefore using atomic excitation spectra is only a zeroth-order approach. In principle, a
model for atomic 4f emission with additional, non-spherical potential could address both
shortcomings. Furthermore, the additional spectral weight at Γ cannot be explained (4).
Nevertheless, the splitting of the surface 4f emission evident in fig. 4.9 can be explained
in the framework of hybridization, because the surface state S2 seems to displace the
spectral weight to higher and lower binding energies with a simultaneously emerging
hybridization gap (5).
As presented, the PE spectra show mainly surface states in hybridization with the
4f multiplet of europium. Since the momentum perpendicular to the surface is not
conserved, shaded bands (part of the projected band structure) emerge additionally. To
disentangle the bulk band structure successfully, a less surface sensitive method has to
be chosen. On the other hand, PE is a good choice to describe surface and edge states.
This is especially important in a new class of solids, the surface states of which have
macroscopically-different properties – e.g. the bulk is an insulator whereat the surface
is a metal. A short outline will be given in the next chapter.
4.3 Perspective: quasi-linear dispersion in a 4f compound
Dimensionality and thereby confined electronic states are of recent interest in current
research [75–78]. Especially after certain topological insulators have been predicted [76]
and experimentally explored [77], the surface of which is stable metallic, a new field of
research has emerged for spintronic and magnetoelectric devices. The topological classification
scheme [76, 79] allows to determine whether the surface states are expected to
be stable under small pertubation (e.g. disorder). It is expected, that due to supressed
back-scattering in the case of strong topological insulators [75], dissipationless transport