Spontaneous orbital-selective Mott transitions and the Jahn-Teller metal of A$_3$C$_{60}$

TL;DR

This paper explains the Jahn-Teller metal phase in alkali-doped fullerides as a spontaneous orbital-selective Mott transition caused by negative Hund coupling, revealing a novel symmetry-broken state with unique orbital properties.

Contribution

It introduces the concept of a spontaneous orbital-selective Mott transition in multiorbital systems with negative Hund coupling, explaining the Jahn-Teller metal phase.

Findings

Identification of a spontaneous orbital-selective Mott state in A$_3$C$_{60}$.

Connection of the Jahn-Teller metal phase to this novel state.

Proposal that Rb$_x$Cs$_{3-x}$C$_{60}$ exhibits this transition.

Abstract

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.