Polymer superconductors

So far no organic polymers have been confirmed to show superconductivity, in spite of several unconfirmed polymer superconductors mentioned below. It should be remembered that the sample should be well oriented, otherwise the disorder inherent to organic polymers will destroy the superconductivity. 

Table 37.1 Summary of Polymer/Superconductor Composite Systems...

This chapter is divided into a number of sections that describe important details related to the conductive polymer/superconductor structures. First, information is provided concerning the preparation and characterization of various polymer/superconductor structures. Chemical and electrochemical deposition methods for localizing the polymers onto a number of cuprate phases are discussed. Section III is devoted to relevant background information related to the induction of superconductivity into metals and semiconductor systems via the proximity effect. More specifically, the four basic methods that have been used to study the occurrence of proximity effects in classical solid-state conductors are described (i.e., contact resistance, modulation of superconductivity in normal/superconductor bilayer structures, passage of supercurrent through superconductor/ normal/superconductor systems, and theoretical analyses). Sections IV and V are devoted to experimental studies of conductive polymer/superconductor interface resistances and modulation of superconductivity in the hybrid systems. Finally, there is a discussion of the initial experimental results that explores the possible induction of superconductivity into organic materials. 

II. SYNTHESIS AND CHARACTERIZATION OF CONDUCTIVE POLYMER/SUPERCONDUCTOR ASSEMBLIES... 

Schottky diodes, metal-insulator-semiconductor diodes, MIS field-effect transistors, and light-emitting diodes have all been prepared by using these polymeric materials [13-17]. With the discovery of high temperature superconductivity, new opportunities exist for the development of hybrid conductive polymer/superconductor systems in which the unique properties of the conducting polymers complement the properties of high-Tc superconductors for novel applications. 

A second approach for the preparation of stable polymer/superconductor systems involves the use of a solution processible polymer such as a poly(3-alkylthio-phene). The advantage of this route is that the high oxidation conditions required for polymerization can be completed prior to exposure to the superconductor [25]. After formation, these polymers can then be dissolved in a dry solvent such as tetrahydrofuran, which is compatible with the cuprate superconductor, and thin films of poly(3-alkylthiophene) can be prepared via spin or spray coating [II]. 

While the growth of conductive polymer layers onto bulk high-Tc ceramic pellets can be accomplished readily, the use of thin films of YBa2Cu307- s is preferred for the construction of polymer/superconductor bilayer structures. Consequently, thin films of YBa2Cu707 is (—200-5(X)0 A in thickness) were deposited onto singlecrystal MgO(KX)) substrates using the pulsed laser ablation method [291, and these films were used to create polymer/superconductor bilayer structures. The result-... 

The physical structure and electrical properties of YBa2Cu307-5 are highly anisotropic. This anisotropy must be considered in both device fabrication using conventional solid-state materials and the construction of polymer/superconductor structures. To explore the extent to which the anisotropy influences the growth of polypyrrole, a number of YBa2Cu307 s thin films were... 

In the following section, contact resistance experiments are described, based on three- and four-point probe measurements that evaluate the chemical compatibility of organic conductors with a number of / -type cuprate phases. These experiments also explore polymer/superconductor charge transfer phenomena at temperatures above and below 7c and are relevant to organic conductor/superconductor proximity effects. 

The temperature dependence of the poly(3-hexylthio-phene)/Pbo.. Bii.7Sri,6Ca 2.4CU3O10 contact resistance above displays an activated behavior, with the contact resistance increasing as the temperature is lowered from room temperature to 110 K (Fig. 37.14A). However. as Tc(onset) is approached, the contact resistance decreases dramatically. Unlike data acquired with normal metal/high-Tc structures, where the values of the contact resistance become vanishingly small below 7, the contact resistance for the polymer/superconductor structure possesses a small but finite value at low temperatures. The presence of a short segment of polymer that extends onto the insulating matrix and is not in di-... 

V. MODULATION OF SUPERCONDUCTIVITY IN CONDUCTIVE POLYMER/SUPERCONDUCTOR BILAYER STRUCTURES... 

Although deposition conditions for the preparation of YBa2Cu307-5 films are often optimized to produce c-axis-oriented films that exhibit high critical currents, polycrystalline films appear to be better suited for the fabrication of sensitive conductive polymer/superconductor structures. These superconductor thin films are more textured and have lower critical currents than the smooth films, which are prepared with what would normally be considered more optimized deposition conditions. The weak link characteristics of these thin films are also enhanced by depositing the superconductor onto cleaved MgO substrates. These substrates possess natural step edges and can be exploited to further disrupt the connection between selected superconductor grains. 

In summary, effective methods have been identified for the preparation of conductive polymer/superconductor and molecular metal/superconductor composite structures. Here both solution-processing strategies and electrochemical deposition techniques for the preparation of the composite structures have been developed. Moreover, a powerful new high-Tc self-assembly method based on the spontaneous adsorption of amine reagents onto cuprate surfaces has been developed that affords precise control of the synthesis of polymer/superconductor composite systems. With these methods, the hybrid structures can be prepared with little chemical or physical damage to either conductor component material. Convincing evidence for the clean combination of the molecular and superconductor components has been obtained from electrochemical, conductivity, contact resistance, and electron microscopic measurements. 

Conductive Polymer/Superconductor Structures as Chemical Sensors... 

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