This week, I returned from the his­toric 50^th Sani­bel Sym­po­sium. Over 350 chemists and physi­cists gath­ered together to cel­e­brate half-centennial suc­cess of quan­tum and com­pu­ta­tional chem­istry. One lec­ture that caught my atten­tion was a ple­nary talk “Con­duct­ing Poly­mers: a saga of more than 50 years” by pro­fes­sor Jean-Marie Andre. Pro­fes­sor Andre empha­sized a role of the­ory in describ­ing the phe­nom­ena of poly­mer con­duc­tiv­ity. The role, unfor­tu­nately, was never prop­erly acknowl­edged… In fact, con­duct­ing poly­mers were prac­ti­cally pre­dicted in 1962 by John Pople and S.H. Walm­s­ley [1] a long before their exper­i­men­tal discovery.

In this clas­si­cal paper Pople and Walm­s­ley intro­duced con­cept of soli­tons in poly­acety­lene. The neu­tral soli­ton is a rad­i­cal mis­fit which exists in the mid­dle of a long poly­ene chain con­tain­ing an odd num­ber of con­ju­gated car­bons and which con­sists of sev­eral suc­ces­sive bonds of sim­i­lar lengths near which the unpaired elec­tron is local­ized. Authors sug­gested that such a defect could be mobile and, if charged, could be respon­si­ble of an high elec­tri­cal conductivity.

This idea is even­tu­ally lead to devel­op­ment Su-Schrieffer-Heeger model [2] and exper­i­men­tal dis­cov­ery of con­duct­ing poly­mers in the late-70s. Finally, in 2000 Hee­gler, Mac­Di­armid and Heeger were awarded by the Nobel Prize in Chem­istry for the dis­cov­ery and devel­op­ment of con­duc­tive poly­mers. Unfor­tu­nately, the work by Pople and Walm­s­ley is not even men­tioned.

Today, almost 50 years later things are dras­ti­cally dif­fer­ent. The atti­tude of exper­i­men­tal­ists is largely changed, we are work­ing together and both ben­e­fit from it. Take, for exam­ple, this week issue of Nature, the­o­ret­i­cal cal­cu­la­tions made small but valu­able con­tri­bu­tion to describe physics behind the first organic super­con­duc­tors [3]!

Efforts to iden­tify and develop new super­con­duct­ing mate­ri­als con­tinue apace, moti­vated by both fun­da­men­tal sci­ence and the prospects for appli­ca­tion. For exam­ple, sev­eral new super­con­duct­ing mate­r­ial sys­tems have been devel­oped in the recent past, includ­ing calcium-intercalated graphite com­pounds, boron-doped dia­mond and—most prominently—iron arsenides such as LaO_1-xF_xFeAs. In the case of organic super­con­duc­tors, how­ever, no new mate­r­ial sys­tem with a high super­con­duct­ing tran­si­tion tem­per­a­ture (Tc) has been dis­cov­ered in the past decade. Here we report that inter­ca­lat­ing an alkali metal into picene, a wide-bandgap semi­con­duct­ing solid hydro­car­bon, pro­duces metal­lic behav­iour and super­con­duc­tiv­ity. Solid potassium-intercalated picene (K_xpicene) shows Tc val­ues of 7 K and 18 K, depend­ing on the metal con­tent. The drop of mag­ne­ti­za­tion in K_xpicene solids at the tran­si­tion tem­per­a­ture is sharp (<2 K), similar to the behaviour of Ca-intercalated graphite. The Tc of 18 K is comparable to that of K-intercalated C₆₀. This dis­cov­ery of super­con­duc­tiv­ity in Kxpicene shows that organic hydro­car­bons are promis­ing can­di­dates for improved Tc values.

Super­con­duc­tiv­ity has been dis­cov­ered in the mate­ri­als that form when alkali met­als react with a solid hydro­car­bon. Now the dis­cov­ery of super­con­duc­tiv­ity at tem­per­a­tures up to 18 K is reported in crys­tals of a sim­ple hydro­car­bon mol­e­cule doped with potas­sium or rubid­ium. The basis for the new com­pound is picene (C₂₂H₁₄), a mol­e­cule con­sist­ing of five ben­zene rings shar­ing edges with one another, which crys­tal­lizes into an ordered mol­e­c­u­lar solid. Inter­ca­la­tion of the alkali met­als into the crys­tal lat­tice induces metal­lic behav­iour and super­con­duc­tiv­ity in what is nor­mally a semi­con­duct­ing mate­r­ial. The Tc of 18 K in potassium-doped picene is high for an organic super­con­duc­tor — only alkali-metal-doped C₆₀ achieves higher. And as picene is one of a large fam­ily of mol­e­cules based on fused ben­zene rings, other super­con­duct­ing hydro­car­bons may be await­ing discovery.

The elec­tronic struc­ture of a pris­tine picene mol­e­cule is shown. The LUMO and LUMO+1 are not degen­er­ate, but the lev­els are very close to each other, sug­gest­ing a high den­sity of states, in the Fermi level as in A_xC₆₀. If three elec­trons are trans­ferred from three K atoms to picene in K₃picene, the LUMO+1 level is half occu­pied. The phonon mode respon­si­ble for electron-phonon cou­pling is prob­a­bly the mol­e­c­u­lar vibra­tion of picene, judg­ing from the phonon mode in the electron-phonon cou­pling of C₆₀ super­con­duc­tors [3].

Ref­er­ences:

1. Pople, J., & Walm­s­ley, S. (1962). Bond alter­na­tion defects in long poly­ene mol­e­cules Mol­e­c­u­lar Physics, 5 (1), 15–20 DOI: 10.1080÷00268976200100021
2. Su, W., Schri­ef­fer, J., & Heeger, A. (1980). Soli­ton exci­ta­tions in poly­acety­lene Phys­i­cal Review B, 22 (4), 2099–2111 DOI: 10.1103/PhysRevB.22.2099
3. Mit­suhashi, R., Suzuki, Y., Yama­nari, Y., Mita­mura, H., Kambe, T., Ikeda, N., Okamoto, H., Fuji­wara, A., Yamaji, M., Kawasaki, N., Maniwa, Y., & Kubo­zono, Y. (2010). Super­con­duc­tiv­ity in alkali-metal-doped picene Nature, 464 (7285), 76–79 DOI: 10.1038/nature08859

6th March, 2010 3 Comments