题 目：Unconventional superconductivity: Hydrocarbon superconductors
Organic materials are believed to be potential superconductor with high transition temperature. Organic superconductors mainly belong to one of two families: the quasi-one dimensional (TMTSF)2X and two dimensional (BEDT-TTF)2X[1,2], in which TMTSF is tetramethyltetraselenafulvalene (C10H12Se4) and BEDT-TTF or “ET” is bis(ethylenedithio)tetrathiafulvalene (C10H8S8). Recently, superconductivity up to 18 K in alkali-metal doped picene with π-electron networks was reported[3]. We have discovered superconductivity at 5 K in alkali-metal-doped phenanthrene[4], 5.5 K in (Ba, Sr)-doped phenanthrene[5] and 6.1 K in (La, Sm) -doped phenanthrene. A 1-GPa pressure leads to a 20% increase of Tc for all the samples, suggesting that this class of materials shows unconventional superconductivity. The Raman spectra of all the superconducting samples show about 7 cm-1 electron downshifts generated by charge transfer, which is similar to A3C60 (A = K and Rb). The shielding fraction of Ba1.5phenanthrene can reach to 100%, it makes the investigation of the intrinsic physical properties and superconductivity mechanism in these organic,π-molecular materials possible. We successfully measure specific heat of Ba1.5phenanthrene and discuss the superconducting pairing symmetry in this newly found organic superconductor. The result of specific heat suggests that Ba1.5phenanthrene has s-wave superconducting gap.
[1] Ishiguro, T., Yamaji, K. & Saito, G. in Organic Superconductors, 2nd edn (Springer-Verlag, 1997).
[2] Singleton, J. Rep. Prog. Phys. 63, 1111–1207 (2000).
[3 ]Ryoji Mitsuhashi et al. Nature, 464, 76 (2010).
[4] X. F. Wang et al. Nature Communications 2, 507 (2011) (2011).
[5] X. F. Wang et al. Phys. Rev. B 84, 214523 (2011)
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