Diploma - Max Planck Institute for Solid State Research

36 4 EuRh 2 Si 2 – semi-localized electrons
ground state is reasonable (enabling 4f 8 →4f 7 transitions) – following an arguement of
Hund’s rules that half-filled shells are energetically favourable – nor the hybridization
with the valence band at the Fermi level (admitting 4f 7 →4f 6 →VB −1 4f 7 transitions)
seems to be substantial in EuRh 2 Si 2 (cf. ch. 4.2).
Atomic-like surface emission
The presence of Eu atoms at the surface is identified by a shift of the 4f emission by
approx. 1 eV towards higher binding energies (see fig. 4.9), which reflects the altered
bonding properties with respect to bulk Eu and can be determined by a Born-Haber
process because the 4f level is rather localized and therefore the PE spectrum behaves
similar to that of a corelevel. The shift is comparable to other divalent Eu compounds
(cf. EuPd x in [54], EuPd 2 Si 2 in [38]), since it is mainly determined by the coordination
number at the surface. As PE is rather surface sensitive, in principle solely the
emission of the first (labelled surface) and second (labelled subsurface) Eu layer have
a reasonable contribution to the spectrum (cf. fig. 2.3). Hence, they will be used as
an approximation for the surface and bulk signal of Eu, respectively. In addition, the
spectrum of the surface state seems to deviate from the seven final states of the bulk
signal. There are two possible explanations for that behaviour: on the one hand, the
potential cannot be regarded as spherically symmetric on the surface (lower coordination
number) anymore, and therefore the final states can differ severely. On the other
hand, the energy broadening increases linearly with binding energy because lifetimes of
final states decrease due to stronger relaxation. It will be shown later, that the splitting
of the Eu surface emission is probably related with hybridization (see ch. 4.2.3).
Examining the effect of surface termination on 4f bulk emission, only a part of the
angle-resolved spectrum (integrated spectrum between |8 ◦ − 9 ◦ | in order to reduce the
impact of hybridization, marked in fig. 4.8) has been chosen for evaluation of the multiplet
maxima. After the integration, the Fermi level of both spectra were aligned (cf. inset
in fig. 4.9). Although the 4f multiplet is formally attributed to the bulk, the positions
of the multiplet lines in fig. 4.9 seem to depend on the surface termination. Thereby,
the position of the 4f 6 subsurface multiplet for a Si terminated surface is shifted by
about 33 meV towards higher binding energies as compared to the respective emission
from a Eu terminated surface. On the one hand, it may be related with the charge
transfer from the Eu surface layer to the outermost subsurface layers, that is missing
for a Si terminated surface. On the other hand, comparing the binding energies of the
multiplet lines relative to the highest level in binding energy (cf. in fig. 4.9, schemata on
the right-hand side) suggests that the 4f state of the Eu subsurface atom hybridizes at
the selected emission angle partly with the VB states. Therefore a detailed discussion
on bulk / surface hybrid states will be given in ch. 4.2.