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Chapter 4-Section 4.4
4.4 Modification of
Superconducting Properties of the Bilayer Assembly
Conducting polymer / high-temperature
superconductor assemblies were prepared by depositing YBa2Cu3O7-x onto
a MgO (100) substrate with the pulsed laser ablation methods described
in Chapter 2. A superconductor microbridge was patterned with dimensions
of ~3 mm long by ~100 mm wide. Then,
approximately 2 mm
of polypyrrole was deposited electrochemically onto the microbridge.
A schematic diagram of these steps is presented in Illustration 4.2.
The conducting properties of the polypyrrole
were then modified by oxidation and reduction. Since polypyrrole deposited
on thin films of YBa2Cu3O7-x displays
reversible voltammetry similar to that of Pt,24, 25 electronic “communication” between
polypyrrole and YBa2Cu3O7-x has
been established. The effect of oxidizing the polypyrrole layer of
the assembly is that the transition temperature, Tc, of
the superconductor dropped from Tc(zero) = 79 K to Tc(zero)
= 64 K, a decrease of 15 K. There is also a notable decrease in the
critical current upon oxidation. Upon reduction of the polymer, the
transition temperature almost returns to its original value. Additional
cycling of the oxidation state of the polymer can yield similar results The
reversibility of this method was found to be dependent on maintaining
a clean interface and strong adhesion of the polymer as the assembly
is cycled between 30K and 300K.
Electrochemical techniques
have been exploited to modulate the superconducting transition temperature
of a polypyrrole coated YBa2Cu3O7-x microbridge
assembly. Likewise, the McDevitt group has observed for the first
time that the superconducting properties can be modified in a controllable
manner by a conducting polymer layer. It has been found that the deposition
of neutral polypyrrole, an insulator, has little effect on the electrical
and superconducting properties of the underlying superconductor. Upon
oxidation of polypyrrole to its conducting state, there is a depression
in the transition temperature of the superconducting thin-film assembly
by at least 15 K, as seen in Figure 4.1 and Figure 4.2. This was reported
as the first “molecular switch” for controlling superconductivity
in high-temperature superconductors.25
Figure 4.1: Resistivity
vs. temperature plots of a) bare YBa2Cu3O7-x microbridge,
b) YBa2Cu3O7-x coated with conductive
polypyrrole, and c) YBa2Cu3O7-x coated
with insulating polypyrrole 0-300 K. The microbridge is
100 mm
wide x 3 mm long with the superconductor thickness varying between
500 - 3000 Ć and the polymer thickness of ~2 mm.
Figure 4.2: Resistivity
vs. temperature plots of a) bare YBa2Cu3O7-x microbridge,
b) YBa2Cu3O7-x coated with conductive
polypyrrole, and c) YBa2Cu3O7-x coated
with insulating polypyrrole from 60-100 K. The microbridge
is 100 mm
wide x 3 mm long with the superconductor thickness varying between
500 - 3000 Ć and the polymer thickness of ~2 mm.
Illustration
4.2: Schematic of the steps to fabricate a conducting polymer
coated microbridge where a) is the substrate, b) is the a YBa2Cu3O7-x thin
film, c) is a patterned microbridge, and d) is the conducting polymer
coated microbridge assembly.
The ability
to modulate Tc depends upon several factors: polymer and
superconductor film thickness, oxidation level, number of weak links, crystallographic
orientation. The thickness of both the superconductor and conducting
polymer seems to be critical in the modulation of superconductivity. In
conventional superconductor / metal bilayer structures, it has been
found26, 27 that to lower the transition temperature throughout
the entire layer, the superconductor must be sufficiently thin. In addition,
the metal layer must be thick enough to cause such a shift. In the
case of the polypyrrole / YBa2Cu3O7-x assembly,
oxidized polypyrrole plays the part of the metal while YBa2Cu3O7-x is
the superconducting component. Modulation of superconductivity has
not been found in YBa2Cu3O7-x thin
films with thicknesses greater than 3000 Ć. Instead, this modulation
has only been found in YBa2Cu3O7-x thin
films with thicknesses less than 1000 Ć or in areas of the thin films
where the thickness is less than 1000 Ć. Since the metallic component
of the bilayer structure must be thick, polypyrrole was always grown
to thicknesses greater than 1-2 mm.
The oxidation level
of the conducting polymer is one factor which seems to be directly
correlated to the depression
of superconductivity. When the conductive polymer is undoped, it
is in its insulating state. Here, it seems to have little or no
effect on the properties of the underlying superconductor. Upon
doping the polypyrrole layer to a conductive state, however, there
is a depression in the transition temperature of the superconductor
/ conducting polymer assembly.
The number
of weak links may also play a role in the modulation of Tc. Weak
links are defined as areas in a superconductor structure where two
superconducting regions are weakly coupled to one another.28 Examples of weak links are grain boundaries in polycrystalline
samples, twin boundaries, or other defect sites. Weak links are
known to be created when superconductors are deposited on an uneven
surface. Cleaved MgO substrates can be utilized to produce weak-link
structures as seen in Illustration 4.3. Weak links in the laser
ablated thin films would be the most sensitive regions for the modulations
in superconductivity.
Illustration 4.3: Schematic
diagram showing weak links that exist on a superconducting film deposited
on a cleaved MgO substrate.
Crystallographic orientation
may also be a factor in modifying the superconducting properties
of this type of assembly. As discussed in Chapter 2, access to
the copper oxide planes and chains is one critical factor in obtaining
good electron transfer between solution redox species and YBa2Cu3O7-x thin
films. In this case of Figures 4.1 and Figure 4.2, there were
pits in the c-axis film that provided direct access to a-axis orientations. Additionally,
thin a-axis films have produced even larger shifts in Tc of
up to 40 K. However, the good electron transfer seen in the electrochemical
response of polypyrrole coated c-axis thin films shown in Illustration
3.3 may indicate that polypyrrole is electrically “hard-wired”
into the superconductor. The same type of electrochemical response
has been seen for ferrocene-tagged amine reagents on YBa2Cu3O7-x thin
films.29
The novel effect of
modulating the transition temperature of this hybrid assembly raises
fundamental questions concerning its mechanism. Currently, there
is not a definitive explanation for how superconductivity is modified
by the doping of the conducting polymer. There are a number of
influences that must be considered in this complex system. Chemical
degradation could easily account for the initial depression of
superconductivity since YBa2Cu3O7-x readily
reacts with H2O, CO, CO2, and acids.30-32 The restoration of superconductivity to higher temperatures,
though, is not consistent with such behavior which would involve
repairing the damage caused by chemical degradation at room temperature. Additionally,
uncoated thin films electrochemically cycled under similar conditions
do not display this behavior. This would seem to rule out changes
in the oxygen content by electrochemical methods.33
One proposed mechanism for the modulation
of the superconducting properties of this bilayer assembly is the
proximity effect.26,
27 In the proximity effect, a thin layer of superconductor
can have its superconducting properties suppressed by a thick metallic
layer that is in intimate contact with the superconductor. Conversely,
a thick superconductor could induce superconductivity into a thin
metal layer. Some evidence for the induction of superconductivity
has been presented in the form of contact resistance measurements
where there is an unexplained drop in the resistance of contacts
made to superconductors by an organic conductor. Direct evidence
such as obtaining a supercurrent between two superconductor pads
separated by a bridging organic conductor has yet to be established.22 Alternative
explanations based on electric field effects are also plausible.
