  The alkali-doped fullerides A$_3$C$_{60}$ are half-filled three-orbital
Hubbard systems which exhibit an unconventional superconducting phase next to a
Mott insulator. While the pairing is understood to arise from an effectively
negative Hund coupling, the highly unusual Jahn-Teller metal near the Mott
transition, featuring both localized and itinerant electrons, has not been
understood. This property is consistently explained by a previously
unrecognized phenomenon: the spontaneous transition of multiorbital systems
with negative Hund coupling into an orbital-selective Mott state. This
symmetry-broken state, which has no ordinary orbital moment, is characterized
by an orbital-dependent two-body operator (the double occupancy) or an
orbital-dependent kinetic energy, and may be regarded as a diagonal-order
version of odd-frequency superconductivity. We propose that the recently
discovered Jahn-Teller metal phase of Rb$_x$Cs$_{3-x}$C$_{60}$ is an
experimental realization of this novel state of matter.
