Spontaneous Orbital-Selective Mott Transitions and the Jahn-Teller Metal of A3C60

Abstract

The alkali-doped fullerides A3C60 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 orbitaldependent 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 RbxCs3−xC60 is an experimental realization of this novel state of matter.The appearance of long-range order by spontaneous symmetry breaking (SSB) is a fundamental and widely studied concept in physics. In condensed matter systems, the ordered state is typically characterized by an order parameter measuring the charge, magnetic moment, orbital angular momentum, or the pair amplitude in a superconductor. There may however exist more complex types of ordering phenomena. Here we demonstrate that in a certain class of multi-orbital systems one observes an orbital symmetry breaking into a state without conventional order parameter, but with an orbital-dependent double occupancy (a composite order parameter) and an orbital-dependent kinetic energy. The resulting ordered phase is a spontaneous orbital-selective Mott (SOSM) state, in which itinerant and localized electrons coexist. As this state combines properties of the metal (connected to the weak-interaction limit) and Mott insulator (connected to the strong-interaction limit) it cannot be detected by perturbative methods from either limits, and its study requires the use of sophisticated techniques. The ordering phenomenon can be discussed in terms of a symmetry-breaking field or order parameter with odd time (frequency) dependence, and is hence related to the concept of odd-frequency superconductivity [1-4].We will argue that this unconventional SOSM state is realized in alkali-doped fullerides [5][6][7][8][9][10][11][12][13], which are promising candidates for diagonal odd-frequency orders. Several compounds in this class of materials can be regarded as strongly correlated systems since a Mott transition occurs as a function of pressure. In the case of an fcc lattice [12], the Mott insulator stays paramagnetic in a wide range of temperature due to geometrical frustration, while it is antiferromagnetically ordered in the case of a bcc lattice, which is bipartite [11]. With increasing pressure, the system turns into a paramagnetic metal and is unstable to superconductivity below a maximum T c ≃ 38 K [11]. Recently, a so-called Jahn-Teller metal (JTM) state has bee...

Visualizing molecular orientational ordering and electronic structure in CsnC60 fulleride films

“…1(e), bottom panel). In Cs n C 60 alkalides of even stoichiometry (n = 2 and 4), the strong JT coupling splits the triply degenerate LUMO states into subbands and results into a charge-disproportionated insulator [24][25][26][27][28], whereas in odd stoichiometries of n = 1 and 3 the strong electronic correlations further split one subband and lead to the Mott-insulating character. This has been well schematically illustrated by the inserted band structure in Figs.…”

Charge Disproportionation, Mixed Valence, and Janus Effect in Multiorbital Systems: A Tale of Two Insulators

“…For small interactions all multiplets are populated. Then, a sharp crossover leads to a highly correlated JT metal [33,37] (in analogy with Hund's metal), where the only populated multiplets are those already present in the insulating states, namely the low-spin states with 2, 3, or 4 electrons per site. The emergent JT metal can be regarded as a mixed-valence state where, due to the degeneracy between insulating states, the systems gains kinetic energy via hopping processes connecting the local configu-rations of the two insulators.…”

Nonequilibrium steady states and transient dynamics of conventional superconductors under phonon driving

“…Since alkali doped fullerides are multiorbital systems and an effective negative Hund's coupling from the Jahn-Teller screening plays an important role [56][57][58][59], we need to go beyond the Holstein model to fully understand the mechanism.…”