**1. Introduction**

104 Superconductors – Materials, Properties and Applications

611, ISSN 1051-8223

Yinming Dai et al. An 8T Superconducting Split Magnet System with Large Crossing Warm Bore, *IEEE Transactions on Applied Superconductivity*, Vol. 20, No.3, (June 2010), pp. 608-

> Since the first observation of superconductivity by K. Ones with a critical temperature of superconductivity (*T*c) of 4.2 K on mercury (1911), many researchers have persuaded such exciting system on organic materials with vain. Even metallic behavior was hardly seen on the organic materials. Little's theoretical proposal (1964) for high *T*c superconductivity (*T*c > 1000 K) was based on a polymer system having both a conduction path and highly polarizable pendants, which mediate the formation of Cooper pairs in the conduction path by electronexciton coupling [1]. There are at least two inorganic polymer superconductors without doping (graphite and diamond are superconductors by doping: see Section 5), poly(sulfur nitride) (SN)*x* (1975, *T*<sup>c</sup> ≤ 3 K) [2] and black phosphorus (1984, *T*c ~ 6 K at 16 GPa and 10.7 K at 29 GPa) [3], with crystalline forms. However, so far no organic polymers have been confirmed to show superconductivity which is easily destroyed by a variety of disorder. Only crystalline polymers were reported to exhibit metallic behavior: a doped polyaniline by chemical oxidation of monomers [4] and MC60 (Scheme 1) having linearly polymerized C60•– with onedimensional (M = Rb, Cs) or three-dimensional (M = K) metallic behavior [5].

### **Scheme 1.**

The Little's model accelerated the exploration of the conducting organic materials of low molecular weight, that had been started by the finding of highly conductive perylene•halides charge-transfer (CT) solids (100–10–3 S cm–1) in early 1950s [6] and TCNQ

© 2012 Saito and Yoshida, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Saito and Yoshida, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

CT solids from the 1960s [7]. The first metallic CT solid TTF•TCNQ appeared in 1973 [8] based on the two main requirements for the conductivity, namely, (1) a uniform segregated stacking of the same kind of component molecules, and (2) the fractional CT state (uniform partial CT) of the molecules. Since TTF•TCNQ has a low-dimensional segregated stacking, it showed a metal-insulator (MI) transition (Peierls transition) below about 60 K. For TTF•TCNQ, the Peierls transition occurs by the nesting of the one-dimensional Fermi surface causing lattice distortion associated with the strong electron-phonon interaction and forms charge density wave (CDW). There are also several one-dimensional organic metals which show MI transitions by the formation of spin density wave (SDW) when the periodicity of the SDW coincides with the nesting vector of Fermi surface and no lattice distortion occurs in this case. An increase in the electronic dimensionality is inevitable to prevent the nesting of Fermi surfaces and develop superconductors. Several attempts have been made through "pressure", "heavy atom substitution", or "peripheral addition of alkylchalcogen groups" (Fig. 1). The latter two correspond to the enhancement of the self-assembling ability of the molecules.

Development and Present Status of Organic Superconductors 107

In this review, we mainly introduce the development and present status of organic superconductors of CT type based on electron donor molecules such as ET, electron acceptor molecules such as C60 (highest *T*c = 38 K at 0.7 GPa), aromatic hydrocarbons (highest *T*c = 33 K at ambient pressure (AP)), M(dmit)2 (highest *T*c = 8.4 K at 0.45 GPa //*b*) and graphite (highest *T*c = 15.1 K at 7.5 GPa), carbon nanotube (*T*c ~ 15 K in zeolite), and B doped diamond (*T*c = 11 K at AP). Besides those, single component organic compounds show superconductivity (*T*<sup>c</sup> ≤ 2.3 K at 58 GPa). The most reported *T*c values of CT solids of C60, aromatic hydrocarbons, those recently prepared, and those under pressure are the on-set values that are approximately 0.5–1 K higher than the mid-point *T*c values. All donor based superconductors are stable in open air, however, only M(dmit)2 superconductors are stable

Most of the superconducting phases of TMTSF, ET, and C60 materials and also of oxide superconductors reside spin-ordered phases such as SDW and antiferromagnetic (AF) phases. We briefly describe the recent development of superconductors having

CT solids are prepared mainly by the following three redox reactions: (1) electrocrystallization (galvanostatic and potentiostatic), (2) direct reaction of donors (D) and acceptors (A) in the gaseous, liquid, or solid phase, and (3) metathesis usually in solution (D•X + M•A → D•A + MX, M: cation, X: anion). In the latter two cases, single crystals are produced by the diffusion, concentration, slow cooling, or slow cosublimation methods.

Electrocrystallization (main procedures in detail and corresponding references are described in Section 11 of Ref. 17) is performed with a variety of glass cells, as shown in Fig. 2. Strictly speaking, the potentiostatic method is the proper way, in which a three-compartments cell is employed and one of the compartments contains the reference electrode, such as saturated calomel or Ag/AgCl electrode. However, this method is troublesome when a large number of crystal-growth runs are performed for a long period of time due to the following: 1) the contamination through the use of a reference electrode cell, and 2) the limited space for the experiment. The galvanostatic method is much more convenient than the potentiostatic one from these points of view. An H-cell (20 ml or 50 ml capacity) and an Erlenmeyer-type cell (100 ml) with a fine- porosity glass-frit equipped with two platinum wire electrodes (1–5

There are many factors and tricks to grow single crystals of good quality. The important factors besides both the purity and the concentration of the component materials are the kinds of solvent and electrolyte, the surface of the electrode, the current (0.5–5 μA), and temperature. THF (tetrahydrofuran), CH2Cl2, TCE (1,1,2-trichloroethane), chlorobenzene, CH3CN, and benzonitrile are commonly utilized solvents. The addition of 1–10 v/v% ethanol occasionally accelerates the crystal growth. As for the electrolyte, solubility in organic solvent is an important factor and usual electrolytes are tetrabutylammonium (TBA) or tetraphenylphosphonium salt of anion X. Sometimes, the electrolyte is a combination of

superconducting phase next to spin-disorder state (quantum spin liquid state).

among the acceptor based superconductors.

mm in diameter) have been used (Fig. 2).

**2. Preparation of organic superconductors** 

Appropriate examples taking TTF derivatives are shown in Fig. 1. Based on TMTSF molecules several superconductors under pressure have been prepared with warped one-dimensional Fermi surface since 1980 (a in Fig. 1) [9–14]. In general, the ratio of transfer energies (*t*// / *t*⊥) is larger than 3 for one-dimensional Fermi surface and a closed two-dimensional Fermi surface is formed when *t*// ≤ 3*t*⊥, where *t*// and *t*<sup>⊥</sup> are the transfer energies along the directions of the largest and second largest intermolecular interactions. The BO (BEDO-TTF) molecules afforded stable two-dimensional metals having two-dimensional Fermi surface (b in Fig. 1) owing to the strong self-assembling ability by intermolecular S∙∙∙S and hydrogen-bonds [15], and only two superconductors are known since 1990 (*T*<sup>c</sup> ≤ 1.5 K). The substitution of an ethylenedioxy group with an ethylenedithio group (BO → ET (BEDT-TTF)) destabilized the metallic state of BO compounds and provided unstable two-dimensional conductors (c in Fig. 1). Consequently, variety of superconductors and other functional solids have been developed based on twodimensional metals of ET since 1982 (*T*<sup>c</sup> ≤ 13.4 K) [16–20] and its analogues (*T*<sup>c</sup> ≤ 10 K) [21].

**Figure 1.** Strategy for chemical modification of the TTF molecule to increase (arrows) or decrease (dotted arrow) the electronic dimensionality by the aid of enhancement or suppression of the selfassembling ability of the molecules, respectively [16]. Typical Fermi surfaces of TMTSF (a: (TMTSF)2NbF6), BO (b: (BO)2.4I3), and ET (c: β-(ET)2I3) CT solids are depicted.

In this review, we mainly introduce the development and present status of organic superconductors of CT type based on electron donor molecules such as ET, electron acceptor molecules such as C60 (highest *T*c = 38 K at 0.7 GPa), aromatic hydrocarbons (highest *T*c = 33 K at ambient pressure (AP)), M(dmit)2 (highest *T*c = 8.4 K at 0.45 GPa //*b*) and graphite (highest *T*c = 15.1 K at 7.5 GPa), carbon nanotube (*T*c ~ 15 K in zeolite), and B doped diamond (*T*c = 11 K at AP). Besides those, single component organic compounds show superconductivity (*T*<sup>c</sup> ≤ 2.3 K at 58 GPa). The most reported *T*c values of CT solids of C60, aromatic hydrocarbons, those recently prepared, and those under pressure are the on-set values that are approximately 0.5–1 K higher than the mid-point *T*c values. All donor based superconductors are stable in open air, however, only M(dmit)2 superconductors are stable among the acceptor based superconductors.

Most of the superconducting phases of TMTSF, ET, and C60 materials and also of oxide superconductors reside spin-ordered phases such as SDW and antiferromagnetic (AF) phases. We briefly describe the recent development of superconductors having superconducting phase next to spin-disorder state (quantum spin liquid state).
