Chemistry Reference
In-Depth Information
Fig. 9.5
Left
: Angular-resolved photoemission data in the direction normal to the steps after
adsorption of 1.3 ML of Pb on Si(557).
Right
: Schematic of the Umklapp process leading to
insulating behavior in this direction (for details, see text)
fore, 1D conductance cannot be due to 1D bandstructures close to
E
F
,butmust
be caused by special properties of the 2D band structure. This is indeed found in
angular-resolved photoemission (ARPES) measurements [
23
]. Pronounced disper-
sion is seen in both directions, in particular in the direction normal to the steps. An
example of the photoemission intensity close to the Fermi energy as a function
k
⊥
at
k
=
.
24 Å is shown in the left part of Fig.
9.5
. A detailed analysis reveals that
only bands associated with (Pb-modified) surface states have intensity close to the
Fermi level (
E
F
), whereas valence band states of Si reach their maximum close to
E
F
−
0
3 eV. A characteristic repetition of bands can be also seen, which is due to
the periodicity introduced by the steps. The connecting vector is marked by yellow
arrows. Its length 2
0
.
π/
d
with the characteristic terrace length
d
corresponds exactly
4
3
a
0
, in agreement with our findings above (see also schematic in the right
part of Fig.
9.5
).
Further inspection of the ARPES data reveals that for exactly this step separation
the condition 2
k
F
=
to
d
=
d
is fulfilled in the direction normal to the steps (red arrow
in Fig.
9.5
). Thus the topmost band is completely filled in this direction leading
to strong Umklapp scattering, to Fermi nesting and to gap opening of a gap of
≈
2
π/
20meV. Along the step direction we found two split bands [
23
]. This splitting
may be caused by the Rashba effect. Only one of them can carry current. The other
falls again on the edge of the small Brillouin zone due to the 10-fold periodicity
parallel to the mini-terraces, which is formed at this Pb concentration. Thus only
electronic states with very long wavelengths cross the Fermi level and lead to the
1D conductance observed at temperatures below 78 K.
We note here that this 1D band filling is directly related to the Pb-induced forma-
tion of (223) facets and must be a result of electronic stabilization of the associated
terrace length, therefore. It thus corresponds to a modified Peierls mechanism, where
step formation energies take over the role of lattice deformations in the direction
perpendicular to the steps. Indeed the transition to an insulating state in this direction
can be considered as formation of a 1D charge density wave. Because of the very
different nature of step interactions and Pb-modified step formation energies, no
period doubling, etc., should be expected.
