Attempt at Extending BCS-Like Theory to Explain Fullerene Superconductors(Condensed Matter Physics)

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In order to interpret the recent experimental discovery of superconductivity (T_c=38K) in Cs_3C_<60> with the A15 structure under pressure, the mechanism of phonon-mediated electron-electron coupling is reconsidered for fullerene superconductors. A primary concern is whether or not Cs_3C_<60> can be understood in a unified picture including previously studied alkali-metal-doped fullerene superconductors, i.e., fcc A_3C_<60> (A=K, Rb, etc.). By extending the Appel-Kohn integral equation in the site representation formalism, the effect of three types (acoustic I_a, optical I_o and intramolecular high-frequency I_H) of phonons on T_c are analyzed for both fcc and A15 symmetries. The essential point is that the optical phonon mode by which alkali-metal ions at tetrahedral sites couple with electrons holds the key to adjusting whether superconductivity is promoted or prevented. Following the present approach, the following previously unexplained unusual properties are explained naturally for C_<60>-based superconductors: (i) large Coulomb repulsion, (ii) the small isotope effect on T_c and (iii) two types of isotope shift affecting T_c upon different methods of partial substitution. Hence, the stage is set for understanding A15-type fullerene superconductors by applying the same equation as that for fcc-type fullerene superconductors regardless of their structure.
2011-05-25