Organic Superconductors

Materials that contain the element carbon are known as organic compounds and the branch of science concerned with them is termed organic chemistry. This terminology reflects the fact that most organic compounds were initially believed to be solely formed from living organisms and life processes. Since the 1800s, however, scientists have been able to develop many organic compounds via synthetic means, and today almost any compound found in nature can be made in the laboratory. Due to the unusual chemical properties of carbon, which can form long chains or rings and is able to react with both metals and nonmetals, an abundance of widely various carbon compounds exist. Indeed, there are more carbon compounds than compounds of all the other elements collectively.

Organic Superconductor Single Crystal

Superconductivity was discovered more than fifty years before anyone ever seriously theorized about the possibility of organic superconductivity. The birth of this field occurred in 1964 with the work of Bill Little at Stanford University who was exploring potential means of increasing the temperature at which certain materials transition into a superconducting state (termed the critical temperature; T_C) characterized by the lack of resistance to the flow of electric currents. Many more years would pass before the first organic superconducting material was synthesized. Unlike Little had hoped, the organic superconductors that have been discovered do not exhibit higher critical temperatures than most other superconductors, though they do display a number of intriguing properties.

Due to hybridization, which does not leave any unfilled spots in the conduction and valence bands of organic molecules, organic materials normally act as electrical insulators and do not exhibit the properties of metals. Yet as early as 1911 scientists suggested that certain organic substances might be able to display metallic characteristics though they lack metal atoms. It was not until the 1970s that a way to actually produce such materials, known as organic metals, was developed. As was then realized, conduction and valence bands in materials made of organic molecules can be only partially filled if planar organic molecules are combined with anions that are nonorganic. In the materials, the organic molecules serve as electron donors and the nonorganic molecules serve as electron acceptors. The result is a charge transfer salt with metal-like characteristics.

The first true organic metal was tetrathiafulvalene-7,7,8,8-tetracyano-p-quinodimethane (TTF-TCNQ). It was formed from the electron donor tetrathiafulvalene and the electron acceptor tetracyanoquinodimethane in 1973. TTF-TCNQ is an organic conductor, but it is not a superconductor. Charge transfer salts with superconducting capabilities were not achieved for several years after the synthesis of TTF-TCNQ, which was an important step toward the momentous accomplishment. Danish researcher Klaus Bechgaard of the University of Copenhagen and three French colleagues were the first scientists to make the theory of organic superconductivity a reality when in 1980 they synthesized (TMTSF)₂PF₆. In this substance, which was the world’s first organic superconductor, tetramethyltetraselenafulvalene (TMTSF) served as the electron donor and PF₆^- as the electron accepting anion. The critical temperature of the material was an extremely low 1.2 K, but nevertheless, the announcement of its synthesis sparked an explosion of interest in the field of organic superconductivity.

Organic Superconductor Single Crystal

(TMTSF)₂PF₆ and other organic superconductors produced by Bechgaard and his team are commonly referred to as Bechgaard salts. Similar to the initial organic superconductor, subsequent Bechgaard salts that have been synthesized exhibit the general formula (TMTSF)₂X or (TMTTF)₂X (in which case the selenium atoms of the base molecule are replaced with carbon atoms resulting in the organic molecule tetramethyltetrathiofulvalenium. In addition to PF₆^-, TaF₆^-, ClO₄^-, AsF₆^-, and SbPF₆^- are some of the other anions (represented as X in the general formulas above) that were employed to produce early Bechgaard salts. Many of these charge transfer salts required considerable pressure to be applied to them in order for them to enter a superconducting state. The tetrahedral anion perchlorate (ClO₄^-) was utilized as the electron acceptor in the first Bechgaard salt to display superconductivity at an ambient level of pressure. Scientists have suggested that the tighter packing of the solid material associated with the small perchlorate anion affects the salt in a manner similar to applied pressure, thus negating the need for such pressure.

Bechgaard salts are grown as very thin crystals and are usually described as quasi-one dimensional. The molecules that comprise them are stacked into segregated sheets of electron acceptors and donors. Due to their formation, electrical conductivity of Bechgaard salts is extremely anisotropic. Conductivity is greatest along the axis upon which the donor molecules are stacked. Orbital overlap in each layer is poor and along the axis perpendicular to the stacking axis, electrical conductivity is reduced by several orders of magnitude. It is because superconduction can only occur along a single axis in Bechgard salts that they are considered quasi-one dimensional materials.

After the development of quasi-one dimensional organic superconductors, additional research resulted in the development of quasi-two dimensional systems. Superconductor properties are exhibited in two dimensions in quasi-two dimensional systems, hence their moniker. The first of these systems was produced utilizing a new donor molecule bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF). Eight sulfur atoms are found in BEDT-TTF, half of which are located at the periphery of the donor molecule. Due to this structure, orbital overlap is better between donor molecule stacks than it is within the stacks themselves

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BEDT-TTF-based charge transfer salts have been produced with a substantial array of compositions. The complexes have included a variety of electron acceptors, including anions in linear, tetrahedral, and polymeric form. When certain anions have been employed, however, superconductivity does not occur. Scientists have suggested that this may be due to increased disorder in the salts. The critical temperatures of BEDT-TTF-based charge transfer salts range from about 1.5 K to 12.8K, with higher temperatures being associated longer, polymeric anions.

Quasi-two dimensional organic superconductors are not all formed with a BEDT-TTF donor. Other donors have also been employed. Particularly notable are quasi-two dimensional systems that are produced with a donor that contains the element oxygen, such as (bisethylenedioxy)tetrathiafulvalene (BEDO-TTF). It was once thought that organic superconductors formed with oxygen or other relatively light elements would have higher critical temperatures than organic superconductors containing heavier atoms, such as sulfur or selenium. This hope has thus far proved false, with donor molecules containing oxygen producing superconductors with transition temperatures comparable to other one-dimensional and two-dimensional organic superconductors.

Higher critical temperatures have been obtained with a completely different kind of organic superconducting material, the fullerene. Fullerenes are hollow clusters of carbon atoms that were named for their similarity of appearance to the geodesic domes designed by R. Buckminster Fuller. The most heavily studied fullerene consists of 60 carbon atoms and is often referred to as a buckyball. Sometimes this term is alternatively applied to any spherical fullerene. The first fullerenes exhibiting a different shape were discovered in 1991 by a microscopist examining the deposits on the cathode during the arc-evaporation synthesis of fullerenes. These fullerenes were comprised of graphene cylinders capped at both ends. They are commonly known as carbon nanotubes or buckytubes. Both spherical fullerenes and nanotubes display interesting electrical properties and have been shown to be capable of superconductivity.

In its natural state the buckyball is an insulator, but by doping (a process of intentionally introducing an impurity) C₆₀ with alkali metals, superconductors with critical temperatures up to 40 K, a value much higher than those yet obtained with quasi-one and –two dimensional organic systems, can be produced. Generally the formula for buckyballs doped in this manner is A₃C₆₀, with A representing some alkali metal. The critical temperatures of nanotubes thus far appear to be somewhat lower, hovering nearer 15 K.

The highest critical temperatures observed in organic superconducting materials to date were achieved in fullerenes via a technique employing a field-effect transistor to introduce charge or holes. Hole doped fullerenes exhibit higher critical temperatures than electron doped fullerenes. The greatest T_C for an organic superconductor, 117 K, was achieved with a buckyball doped with holes and intercalated with CHBr₃. This value is remarkably close to the highest critical temperature reported for any superconductor. Further research in this area may eventually lead to organic superconductors that surpass nonorganic superconductors in the race toward higher and higher critical temperatures, providing hope that Little’s high ambitions for the materials could one day be realized.

Moreover, additional work may eventually lead to the supplanting of various inorganic materials by organic superconductors in certain applications. Key advantages of organic superconductors over inorganic electrical conductors include their reduced weight and their potential versatility due to an increased ability to modify their electrical attributes via chemical methods. This modification is facilitated by numerous free parameters available for adjustment in organic salts. Though certain problems will likely have to be overcome if organic superconductors are to become practical for widespread use, their inclusion in many future technologies appears to be a goal well within the grasp of humanity.