Intercalant-Driven Superconductivity inYbC6andCaC6

Abstract

Recently discovered superconductivity in and , at temperatures substantially higher than previously known for intercalated graphites, raises several new questions. (1) Is the mechanism considerably different from that of previously known intercalated graphites? (2) If superconductivity is conventional, what are the relevant phonons? (3) Given the extreme similarity between and , why are their critical temperatures so different? We address these questions on the basis of first-principles calculations and conclude that coupling with intercalant phonons is likely to be the main force for superconductivity in and , but not in alkaline-intercalated compounds, and explain the difference in by the “isotope effect” due to the difference in Yb and Ca atomic masses.