Superconductivity of Li 2 MC 60 and Na 2 MC 60 Fullerides

The superconductivity of Li 2 MC 60 and Na 2 MC 60 (where M is Cs, Rb and K) compounds with small lattice parameters is studied in detail. The results reveal that Na 2 CsC 60 and Na 2 RbC 60 are superconducting at 12 K and 3.5 K with large superconducting fractions (70% for Na 2 CsC 60 and more than 20% for Na 2 RbC 60 ), while Li 2 MC 60 compounds are not superconducting down to 2 K. The large variation in T c observed over a wide range of lattice parameter is explained in the framework of the McMillan equation, and the phonon frequency involved in electron pairing is discussed.