Chemistry Reference
In-Depth Information
Fig. 9.8
Semi-log plots of
T
in
the temperature range below 100 K. Temperature-activated behavior is found, and the activation
energies are given at each curve
σ
⊥
for Pb concentrations between 1.32 and 1.50 ML versus 1
/
δ
the temperature range below 100 K. At all excess concentrations
up to a total
concentration of 1.50 ML we see temperature-activated behavior, i.e., the system
remains in an insulating state in the limit
T
0, as expected for a charge density
wave behavior. The effective band gaps, as determined from plots of Fig.
9.8
, grad-
ually shrink up to a total Pb concentration of 1.50 ML. At this concentration the
measured activation energy is so small that we expect a transition to metallic behav-
ior slightly above this concentration. Here the step decoration is completed, and
adsorption of the second physical monolayer starts, which is most likely metallic.
At the concentration of 1.31 ML (not shown in Fig.
9.8
) the band gap was esti-
mated to be larger than 20 meV, as mentioned above, so that no conductance exceed-
ing the conductance of the substrate can be measured below 80 K and a step-wise
increase was found due to the phase transition at 78 K (see Fig.
9.4
)fromtheinsu-
lating to a conducting state. This phase transition quickly disappears as a function
of increasing Pb coverage both in conductance and in LEED. Even at an excess
concentration as low as 0.01 ML (total Pb concentration 1.32 ML), the transition
in conductance is strongly broadened resulting in an apparent activation of 45 meV
close to the transition, which is not seen anymore at higher concentrations.
Indeed no structural phase transition was found at all for the superlattice struc-
ture of Fig.
9.7
. The peak intensity of the satellite peaks shows only an attenua-
tion according to the Debye-Waller effect up to 220 K. In view of a “classical”
Peierls transition, which is the result of a balance between electric energy gain at
the expense of lattice distortion energy, this result seems to be surprising. However,
for the occupation of step edges no lattice distortion is necessary in the periodic
arrangement of these chains so that only the electronic energy gain remains. This
explains the missing phase transition (apart from final disorder at high temperatures)
and the comparatively high stability.
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