Brewton-Parker College: A Private Christian/Baptist College located in Southeast Georgia Mathematics and Natural Sciences: Biology, Chemistry, Mathematics, PhysicsMathematics and Natural Sciences: Biology, Chemistry, Mathematics, PhysicsMathematics and Natural Sciences: Biology, Chemistry, Mathematics, PhysicsMathematics and Natural Sciences: Biology, Chemistry, Mathematics, Physics

Home / Academics / Division of Math & Science / Faculty / Christopher Jones / Dissertation / 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

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 Å and the polymer thickness of ~2 mm.

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 and the polymer thickness of ~2 mm.

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 Å 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 and the polymer thickness of ~2 mm.

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.

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 . Instead, this modulation has only been found in YBa2Cu3O7-x thin films with thicknesses less than 1000 or in areas of the thin films where the thickness is less than 1000 . 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.

Schematic diagram showing weak links that exist on a superconducting film deposited on a cleaved MgO substrate.

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.

Mathematics and Natural Sciences: Biology, Chemistry, Mathematics, Physics
Brewton-Parker College | Located on U.S. 280 at 201 David-Eliza Fountain Circle, P. O. Box 197, Mount Vernon, GA 30445
with a site in Newnan
912-583-2241, 1-800-342-1087
Get directions to the main campus.
Contact Technology Services if you have any comments, questions or issues.

The mission of Brewton-Parker College, a Georgia Baptist college, is to develop the whole student through the application of Biblically-centered truth to a liberal arts curriculum in a community of shared Christian values.
 
Brewton-Parker College is accredited by the Commission on Colleges of the Southern Association of Colleges and Schools to award associate and baccalaureate degrees. Contact the Commission on Colleges at 1866 Southern Lane, Decatur, Georgia 30033-4097 or call 404-679-4500 for questions about the accreditation of Brewton-Parker College.
 
Updated on: April 15, 2010 8:26 PM