**6. Single component superconductors**

124 Superconductors – Materials, Properties and Applications

pressure, which is indicative of the non-BCS behavior.

**Scheme 8.**

superconductors.

Considering the fact that Ca1.5picene also shows a superconducting phase below 7 K [185], three-fold charge transfer from dopants to one picene molecule would be responsible for emergence of the superconductivity. At present, although the crystal structures of the doped compounds are unclear, the refined lattice parameters are indicative of the deformation of the herringbone structure of pristine picene and the intercalation of dopants within the twodimensional picene layers. After the discovery of the picene-based superconductors, several superconductors have been found for alkali metal (*T*c = 7 K) [186] and alkaline-earth metal (*T*c ~ 5.5 K) [187] doped phenanthrene, potassium-doped 1,2:8,9-dibenzopentacene (*T*c = 33 K; partially decomposed) [188], and potassium-doped coronene (*T*c < 15 K) [185]. Among them, phenanthrene-based superconductors shows an enhancement of *T*c with increasing

Contrary to the electron-doped system above described, it has been found that a cation radical salt (perylene)2Au(mnt)2, in which each perylene has an average charge of +0.5 and form segregated columns, show a superconductivity with *T*c = 0.3 K when the hydrostatic pressure above 0.5 GPa was applied to suppress CDW phase [189]. So far 8 aromatic

In 2001, first superconducting carbon nanotubes were discovered for ropes of singlewalled carbon nanotubes (SWNTs) with diameters of the order of 1.4 nm (*T*c = 0.4 K) [190], and immediately after that SWNT with diameters of 0.4 nm embedded in a zeolite matrix (*T*c = 15 K) [191,192]. The drop in magnetic susceptibility is more gradual than expected for three-dimensional superconductors, and superconducting gap estimated from the *I*-*V* plot shows the temperature dependency characteristic of one-dimensional fluctuations. It is apparent that the isolation of carbon nanotubes from each other is responsible for the realization of the almost ideal one-dimensional system. Multi-walled carbon nanotubes (MWNTs) also show the superconductivity; namely, MWNT with diameters of 10–17 nm that were grown in nanopores of alumina templates was found to show superconductivity with *T*c = 12 K [193]. We note that this superconducting system is classified into single component superconductors, contrary to the C60 and graphite (*vide infra*) based

with sp2 character. First-stage alkali metal doped graphite intercalation compounds (GICs)

π


hydrocarbon superconductors were prepared with the highest *T*c of 33 K at AP.

**5. Carbon nanotubes, graphite, and diamond superconductors** 

Graphite has a layered structure composed of infinite benzene-fused

Since the discovery of the first metallic CT solid, TTF•TCNQ, in 1973 [8], much attention for organic (super)conductors has been devoted to plural component CT solids. Besides numerous studies on multi-component CT solids, several single-component organic conductors have been developed. Even though pentacene is known to be the first organic metal (semimetal) showing a decrease of resistivity down to ca. 200 K at 21.3 GPa [206], no superconductivity was reported so far on the solids composed of aromatic hydrocarbon solely. Electric conductivity increases by the enhancement of intermolecular interactions by appropriate use of hetero-atomic contacts. There are two single-component superconductors under extremely high pressure, *p*-iodanil (σRT = 1 × 10–12 S cm–1 at AP, σRT = 2 × 10 S cm–1 at 25 GPa, and superconductor at *T*c ~ 2 K at 52 GPa) [207,208] and hexaiodobenzene (*T*c = 0.6–0.7 K at around 33 GPa and ca. 2.3 K at 58 GPa) [209]. Both have peripheral chalcogen atoms, iodine, which may cause the increased electronic dimensionality of the solid under pressure owing to intermolecular iodine∙∙∙iodine contacts.

Development and Present Status of Organic Superconductors 127

**Author details** 

**7. References** 

pp. 175-202.

(1982) 76: 5497-5501.

Liq. Cryst. 452: 99-108.

118: 8604-8622.

Verlag, Berlin.

Englewood Cliffs, NJ.

Gunzi Saito and Yukihiro Yoshida

[1] Little W A (1964) Phys. Rev. A134: 1416-1424.

*Faculty of Agriculture, Meijo University, Shiogamaguchi 1-501 Tempaku-ku, Nagoya, Japan* 

[3] Kawamura H, Shirotani I, Tachikawa K (1984) Solid State Commun. 49: 879-881. [4] Lee K, Cho S, Park S H, Heeger A J, Lee C-W, Lee S-H (2006) Nature 441: 65-68.

[5] As a review see for example, Prassides K (2000) In: Andreoni W, editor. The Physics of Fullerene-Based and Fullerene-Related Materials. Boston: Kluwer Academic Publishers.

[7] As a review see for example, Hertler W R, Mahler W, Melby L R, Miller J S, Putscher R

[8] Ferraris J, Cowan D O, Walatka V, Perlstein J H (1973) J. Am. Chem. Soc. 95: 948-949. [9] Jerome D, Mazaud A, Ribault M, Bechgaard K (1980) J. Phys. Lett. 41: L95-L98.

[10] Lee I J, Naughton M J, Tanner G M, Chaikin P M (1997) Phys. Rev. Lett. 78: 3555-3558. [11] Wudl F, Aharon-Shalom E, Nalewajek D, Waszeczak J V, Walsh Jr. W M, Rupp Jr. L W, Chaikin P M, Lacoe R, Burns M, Poehler T O, Williams J M, Beno M A, J. Chem. Phys.

[12] Sakata M, Yoshida Y, Maesato M, Saito G, Matsumoto K, Hagiwara R (2006) Mol. Cryst.

[15] Horiuchi S, Yamochi H, Saito G, Sakaguchi K, Kusunoki M (1996) J. Am. Chem. Soc.

[18] Williams J M, Ferraro J R, Thorn R J, Carlson K D, Geiser U, Wang H H, Kini A M, Whangbo M -H (1992) Organic Superconductors (Including Fullerenes): Prentice Hall,

[19] Wosnitza J (1996) Fermi Surfaces of Low-Dimensional Organic Metals and

[22] Balicas L, Behnia K, Kang W, Canadell E, Auban-Senzier P, Jerome D, Ribault M, Fabre

[21] Chem. Rev. (2004) 104, No. 11, Special Issue on Molecular Conductors.

[16] Saito G, Yoshida Y (2007) Bull. Chem. Soc. Jpn. 80: 1-137 and references cited therein. [17] Ishiguro T, Yamaji K, Saito G (1998) Organic Superconductors, 2nd ed.: Springer-

[2] Greene R L, Street G B, Suter L J (1975) Phys. Rev. Lett. 34: 577-579.

[6] Akamatu H, Inokuchi H, Matsunaga Y (1954) Nature 173: 168-169.

E, Webster O W (1989) Mol. Cryst. Liq. Cryst. 171: 205-216.

[13] Jerome D, Schulz H J (1982) Adv. Phys. 31: 299-490.

[14] Jerome D (2004) Chem. Rev. 104: 5565-5591.

Superconductors: Springer-Verlag, Berlin. [20] Singleton J (2000) Rep. Prog. Phys. 63: 1111-1207.

J M (1994) J. Phys. I (France) 4: 1539-1549.

**Scheme 9.**
