Chemistry Reference
In-Depth Information
hand, these measurements have to be carried out ex situ. This requires a protective
cap layer to be deposited, which potentially can affect the superconducting proper-
ties. Also, in order to pick up a measurable magnetic signal, the macroscopic coher-
ence must be established over all sample. DC transport measurements, however,
detect the onset of the superconductivity via establishement of a single filamentary
supercurrent.
Magnetic data in Fig.
4.24
exhibit a clear 1
d
dependence of
T
C
, which can be
explained in terms of the suppressed order parameter close to the film boundaries
according to Simonin's theory [
64
]. Possible bilayer
T
C
oscillations cannot be seen
in this data set because only the stable thicknesses dictated by QSE and separated
by 2ML have been prepared.
T
C
values extrapolate to 7.2 K in the bulk limit which
is the true bulk value. Same 1
/
d
thickness dependence of
T
C
is observed also for
Pb-Bi alloys films. Pb
89
Bi
11
and Pb
80
Bi
20
films exhibit extrapolated bulk
T
C
values
of 7.69 and 8.05 K, respectively [
31
,
65
]. Same figure also shows the transport mea-
surements with monolayer resolution indicating that most of these films were grown
in the classical layer-by-layer growth regime. In this data set, bilayer oscillations
of
T
C
are evident and it can be explained by oscillating density of states and/or
electron-phonon coupling due to QSE [
12
].
We note that the magnetic and transport data show significantly different slopes
in these
T
C
versus 1
/
d
plots. Only the magnetic data extrapolate to the correct bulk
value. Metalic Au protective layer employed for transport data may cause this dif-
ference because of a possible proximity effect. Note that amorphous Ge layer was
used to protect the samples prepared for magnetic measurements. In situ supercon-
ducting gap measurements [
13
] shown in Fig.
4.25
exhibit another contrast quite
different in character: this time
T
C
does not exhibit a significant thickness depen-
dence and bilayer oscillations depicted in the figure are much smaller than those
observed in transport measurements. This radically different behaviors may be due
to the fact that magnetic and transport measurements rely on the establishement of
the macroscopic coherence, i.e., they are sensitive to both amplitude and phase of
the order parameter. On the other hand,
local
scanning tuneling spectroscopy (STS)
measurements detect only the amplitude of the order parameter.
A recent study reported superconducting gap opening in 2ML thick Pb films
[
66
,
67
]. This is interesting because in a 2ML Pb film, only one 2D subband is
allowed to exist due to the relation 3
/
λ
F
/
2
≈
2ML. Also, this work
r
eports two
different
T
C
values for 2ML films residing on phase separated
√
3
√
3 and 1
1
substrate reconstructions. This last point shows the effect of the interface on the thin
film superconductivity.
Film morphology obtained by quantum growth also allows the incorporation of
nanostructures into the picture providing a control knob of some superconducting
parameters [
15
]. STM images of 9ML thick Pb films are shown in Fig.
4.15
.In
these images, slight overdose of Pb causes 2ML tall mesas (a) and slight underdose
of Pb creates 2ML deep voids (b) as dictated by QSE (see Sect.
4.4.1
). Pb is a
type I supeconductor in bulk form; however, it becomes type II character below a
crtitical thickness of
×
×
250 nm [
68
]. As a result, magnetization loops show strong
hysteresis as shown in Fig.
4.15
c, d. Wide-open loops, belonging to the films with
∼
