Chemistry Reference
In-Depth Information
9.3.4 Temperature-Driven Phase Transitions
Nevertheless, Fermi nesting is removed above 78 K by a structural phase transition,
as we will show now. In fact it turns out that this phase transition is an order-order
transition with a relatively small change in average step separation. We again used
LEED for the quantitative determination of changes in the step density and measured
1D profiles in the direction normal to the steps, e.g., along a vertical line in the center
of Fig.
9.1
b.
Contrary to earlier assumptions of an order-disorder phase transition at 78 K,
the LEED profiles in the top part of Fig.
9.6
demonstrate that in fact a temperature-
driven order-order transition occurs at 1.31 ML Pb concentration. The original spot
(20)
11
10.5
10
(662)
3.8
3.7
3.6
3.5
k
|| [112]
(Å
-1
)
Fig. 9.6
Top
: Line scans through LEED spots perpendicular to step directions (see Fig.
9.1
)below
(
T
80 K) seen in conductance showing peak shifts
and splittings.
Bottom
: Section of an Ewald construction close to the (662) 3D Bragg point.
Squares
are measured below,
circles
above the phase transition. The
lines
interpolate the data points and
correspond to (111) [
vertical line
], (223), and (17 17 25) orientations, respectively (see also text)
=
75 K) and above the phase transition (
T
=
