Unconventional Pairing from Local Orbital Fluctuations in Strongly Correlated A3C60

The pairing mechanism in A$_3$C$_{60}$ is investigated by studying the properties of
a three-orbital Hubbard model with antiferromagnetic Hund coupling in the normal and superconducting phase.
Local orbital fluctuations are shown to be substantially enhanced in the superconducting state, with a fluctuation energy scale that matches the low-energy peak in the spectral weight of the order parameter.
Our results demonstrate that local orbital fluctuations provide the pairing glue in strongly correlated fulleride superconductors and support the spin/orbital freezing theory of unconventional superconductivity.
They are also consistent with the experimentally observed universal relation between the gap energy and local susceptibility in a broad range of unconventional superconductors.