Analysis of Low-Energy Hamiltonians with Extended DMFT

Abstract

To understand the mechanism of the superconductivity and to obtain a theoretical phase diagram including the determination of \(T_\mathrm{c}\), we analyze the derived low-energy Hamiltonians (Chap. 3) for the fcc \(\mathrm{A}_3\mathrm{C}_{60}\) systems with the extended dynamical mean-field theory. We show that the theoretical phase diagram reproduces the experimental phase diagram not only qualitatively but also quantitatively. In particular, the calculated \(T_\mathrm{c}\)’s agree with the experimental data within an accuracy of 10 K. The agreement suggests that the present scheme successfully captures the essential physics. Our theoretical observations in the insulating state are completely consistent with the experimentally observed low-spin states and the dynamical Jahn-Teller effects. We demonstrate that the s-wave superconductivity surprisingly benefits from strong correlations under the unusual multi-orbital intramolecular interactions having the weakly negative exchange interaction (Chap. 3). This confirms an unconventional nature of the superconductivity in fullerides, where a surprising cooperation between electron correlations and phonons drives high-\(T_\mathrm{c}\) s-wave superconductivity.