Chemistry Reference
In-Depth Information
Fig. 5.4
Time correlation functions,
G
data) extracted from
repeated measurements. The data for the step-down current (
open circles
) are the average over 10
separate measurements; for the step-up current (
open squares
) the average is over 9. Fits to (
5.3
)
are shown as the
solid lines
for each data set. Also shown is the measured correlation function
(average of 19 separate measurements) for an unstressed sample (
open triangles
) fit to a single
power law [
25
]
(
t
)
, for the step fluctuations (
x
(
t
)
the bulk current densities and the bulk resistivity of Ag, which is approximately
1
10
−
6
10
6
cm at 370 K, respectively [
50
,
51
],
confirms the surprising character of the result. Taking into account all possible
measurement biases, the
lower limit
on the effective valence required to explain
the observation is
Z
∗
=−
(
.
8
×
cm at 325 K and 2
.
2
×
10
2
, which is
at least
a factor of 5 higher than
2
±
1
)
×
expected [
25
,
34
].
5.2.3.2 Island Motion
As shown in Fig.
5.2
, it is possible to tailor the surface to contain a substantial
density of monolayer island stacks. Slightly above room temperature, the top layer
island in each stack shrinks because the small radius of curvature increases the local
chemical potential (Gibbs-Thompson effect) relative to lower islands in the stack
[
52
]. In addition the islands, particularly the top layer island, move in a fashion
similar to Brownian motion [
53
,
54
] as a result of atomic motion at their edges. In
the presence of an electromigration force, the atomic diffusion underlying the island
motion will be biased, with concomitant effects on the motion of the island [
55
,
56
].
Island diffusion can occur due to motion of free atoms on the terraces (
D
T
)or
on the island edges (
D
L
). If the island moves because the electromigration force
induces a drift velocity (
v
EM
=
/
kT
) of atoms on the terrace onto (or away
from) the front (or back) island edge, then the island motion will be in the direction
opposite the electromigration force. Alternatively, if the island moves because the
force preferentially moves atoms along its edges in one direction, then the island
D
T
F
