Conductive polymer-superconductor structures

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. 

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. 

Conductive Polymer/Superconductor Structures as Chemical Sensors... 

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

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. 

The optical properties of conducting polymers are important to the development of an understanding of the basic electronic structure of the material. These and other problems were described in various books and review papers [90-93]. Raman spectroscopy is also an ideal tool for predicting many important electronic properties of molecular materials, organic conductors, and superconductors as well as for understanding their different physical properties, since it is a nondestructive tool, which can be used in situ and with spatial resolution as good as 1 xm. 

We start our discussion with simple concepts from the band theory for solids, discuss what can break the symmetry of one-dimensional systems, introduce electrical conductivity and superconductivity, present the Mulliken charge transfer theory for solution complexes and its extension to solids, then discuss briefly the simple tt electron theory for long polyenes. Other articles in this volume review the detailed interplay between structure and electronic properties of conductors and superconductors [206], and electrical transport in conducting polymers [207],... 

Quantum-chemical treatment of these systems is indispensable for discussing the conduction mechanism based on the electronic structure of the polymers concerned with their physicochemical properties and, furthermore, for providing us with guidelines to design novel, conductive polymers or even superconductors. 

As in polycrystalline pressed at-(BEDT-TTF)2l3, in pressed samples of Pp-(BEDT-l lF)2l3 a pressure-induced structural phase transition plays an important role. As a consequence of the structural phase transition in pp-(BEDT-TTF)2l3, the superconducting transition temperature is increas. This behavior re-emphasizes that organic superconductors might also be of interest for industrial applications, since polyciystalline materials are easier to use than single crystals. In addition, the discovery of bulk superconductivity in large pressed samples of crystallites of organic metals, of the typical diameter of 1 p.m and below, indicates that the observation of superconductivity in conducting polymers should be possible as well. 

Except for the sp hybridized diamond structure, all of these forms of carbon conduct electricity (and the fullerenes, with help from the alkali metals, can even become superconductors, with zero electrical resistance). The ease with which carbon forms conjugated tt electron systems leads to several geometries that can carry electricity, including conducting polymers. Carbon continues to draw our attention more than any other element, as research continues to search for ways to efficiently capitalize on the electronic and structural properties of these novel materials. 

The assembly of conductive polymer/supercon-ductor structures may lead to the development of new prototype devices and sensors in which the properties of the superconductor are controlled by the influence of the conductive polymer element. 

Finally, the most exciting new opportunity afforded by research in this area is that under appropriate conditions superconductivity may be induced in conductive polymer structures. Since high-Tc superconductor systems can be used as the source of the superconductivity, it may be possible to drive conductive polymer systems into the superconducting state at temperatures well above 100 K. The search for superconductivity in organic polymeric systems has been an important goal in the field of conductive polymers and has attracted the attention of scientists for more than three decades [12]. 

Fig. 37.2 (A) Schematic illustration showing a conductive polymer/high-temperature superconductor sandwich device. To create such a structure, a YBa2Cu307-s thin film is deposited onto a MgO(lOO) substrate via laser ablation, a microbridge is patterned on the central portion of the film, and a conductive polymer layer is deposited electrochemi-cally onto the microbridge area. (B) Cyclic voltammetry (5 mV/s) recorded at room temperature in 0.1 M Et4NBp4/ acetonitrile for a YBa2Cu307-s thin-film electrode assembly coated with polypyrrole. Well-behaved voltammetry is observed, indicating that electronic charge flows readily between the superconductor and the polymer layer. (Adapted from Ref. 11.)...

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-... 

Having identified methods to deposit conductive polymer and molecular metal systems onto cuprate superconductor structures without damage to either material, it becomes important now to consider the electronic interactions that occur when the two conductors are in contact with one another. Of particular importance is the interaction that occurs between the polymer-derived charge carriers and the superconducting Cooper pairs. Important background information related to this area can be obtained from the well-documented behavior of the more classical metal/superconductor and semicon-ductor/superconductor systems. Thus, prior to considering experimental data and theoretical treatments for organic conductor proximity effects, we review previous studies of proximity effects in the more classical systems. 

The fact that good electrical contact is made between the conductive polymer and the superconductor is noted from decreases in the four-point sample resistance with an onset temperature near 110 K and zero resistance close to 85 K for Pbo.3Bii.7Sri.6Ca2.4Cu30io samples measured with poly(3-hexylthiophene) contacts, as shown in Fig. 37.14A. Almost identical four-point sample resistance results are acquired with the use of silver contacts on the same specimen. Importantly, these results demonstrate that conductive polymer components can be used to prepare superconductor circuits that operate at temperatures both above and below 7c. Moreover, the values of the contact resistance acquired for the poly(3-hexylthiophene)/YBa3Cu307 5 structure are comparable to values acquired for systems in which the superconductor component is replaced with a noble metal material such as platinum (see Table 37.3). Al-... 

The preparation of superconductor/organic conduc-tor/superconductor structures suitable for supercurrent measurements is important for the continued growth of this new area of research. Toward this objective, methods have been developed for the controlled growth of conductive polymer and vapor-phase deposition of (BEDT-TTF Ia systems. Molecular engineering of high-7c structures and devices made possible with the newly discovered high-7c self-assembly procedure will aid in the further development of these studies. 

S. G. Haupt, D. R. Riley, and J. T. McDevitt, Conductive polymers/high-temperature superconductors composite structures, Adv. Mater. 5 755 (1993). 

For the polymer/supeiconductor structures there are a number of important interactions that could be responsible for the observed behavior. For example, changes in the oxygen content have been shown previously to be promoted by electrochemical means and such changes can result in modest alterations of Tc. Consequently, we have conducted a number of electrochemical control studies in which devices were polarized without conductive polymer layers. No modulations of superconductivity were noted from these studies. Moreover, chemical degradation of the superconductor components have been shown previously to decrease both and The return of superconductivity to higher temperatures which occurs upon r uction of the polymer layer, however, is not consistent with this explanation eidier. 

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