Emergent orbitals in the cluster Mott insulator on a breathing kagome lattice

Abstract

Motivated by the recent developments on cluster Mott insulating materials such as the cluster magnet LiZn2Mo3O8, we consider the strong plaquette charge ordered regime of the extended Hubbard model on a breathing Kagome lattice and reveal the properties of the cluster Mottness. The plaquette charge order arises from the inter-site charge interaction and the collective motion of three localized electrons on the hexagon plaquettes. This model leads naturally to a reduction of the local moments by 2/3 as observed in LiZn2Mo3O8. Furthermore, at low temperatures each hexagon plaquette contains an extra orbital-like degree of freedom in addition to the remaining spin 1/2. We explore the consequence of this emergent orbital degree of freedom. We point out the interaction between the local moments is naturally described by a Kugel-Khomskii spin-orbital model. We develop a parton approach and suggest a spin liquid ground state with spinon Fermi surfaces for this model. We further predict an emergent orbital order when the system is under a strong magnetic field. Various experimental consequences for LiZn2Mo3O8 are discussed, including an argument that the charge ordering much be short ranged if the charge per Mo is slightly off stoichiometry.

Scaling and data collapse from local moments in frustrated disordered quantum spin systems

“…2/3 of the spin disappears into singlets, leaving behind 1/3 of the spins which behave almost as free spins [11][12][13]. The mechanism for this behavior is not well understood, since various theoretical proposals [14][15][16] all require some form of short range tripling of the unit cell size, which has been searched for and not found [13]. However on symmetry grounds we again expect the prevalent nonmagnetic Li/Zn site mixing disorder [11] to generate bond randomness, whose competition with singlets requires [7] low energy spin excitations to appear.…”

Higher-Order Topological Mott Insulators

“…Model and topological invariant.-Let us begin by describing our model and introducing the Z 3 spin-Berry phase. We consider the system of spinful interacting electrons on the kagome lattice [53][54][55][56][57][58][59][60][61]. The Hamiltonian is given by…”

Quantum spin liquid and cluster Mott insulator phases in the Mo3O8 magnets

“…(2) can also be realised when g 1 and g 2 have the same sign (g 1 < 0 and g 2 < 0) and the antiferromagnetic coupling between next-nearest neighbours is large, J nn > 2 3 ðÀg 1 À g 2 ÀgÞ, as was suggested in refs. 22,34 . Although the PCO can be destroyed by strong spin fluctuations 32 , the calculated thermodynamic properties for a single hexagon shown in Fig.…”