Strong Correlations Enhanced by Charge Ordering in Highly Doped Cobaltates

Abstract

We present an explanation for the puzzling spectral and transport properties of layered cobaltates close to the band-insulator limit, which relies on the key effect of charge ordering. Blocking a significant fraction of the lattice sites deeply modifies the electronic structure in a way that is shown to be quantitatively consistent with photoemission experiments. It also makes the system highly sensitive to interactions (especially to intersite ones), hence accounting for the strong correlations effects observed in this regime, such as the high effective mass and quasiparticle scattering rate. These conclusions are supported by a theoretical study of an extended Hubbard model with a realistic band structure on an effective kagomé lattice.The layered cobaltate metals are famous for their remarkable electronic properties [1], ranging from a large thermoelectric response [2] charge-ordering [3,4] and puzzling magnetic behavior, to superconductivity when intercalated with water [5]. Because of the universality of the physical properties throughout the cobaltate family, it is believed that most of these properties have their roots in the electronic structure of the CoO 2 layers. Those are formed of edge-sharing octahedra, with the Co ions forming a triangular lattice. Either alkali(-earth) metals (Na, Li, Ca etc., as in Na x CoO 2 ) or more complex building blocks (e.g. rocksalt BiO or SrO planes in misfit cobaltates [6]) separate the layers and serve as electron donors. Varying the composition of the intercalated compounds controls the doping x ∈ [0, 1], resulting in a nominal Co (4−x)+ (3d 5+x ) valence in the low-spin configuration within the t 2g manifold. The latter is split by the local trigonal symmetry into one a 1g and two e g states. Strong Coulomb interactions have been documented [7] to occur within this orbital subspace.

Competing orders in NaxCoO2from strong correlations on a two-particle level

“…Several theoretical works have dealt with the influence of the sodium arrangements on the electronic properties of Na x CoO 2 , both from the viewpoint of disordered sodium ions [12] as well as from orderings for certain dopings. [14][15][16] However, wether such sodium patterns are due to sole (effective) single-particle potentials or mainly originating from many-body effects within the CoO 2 planes is still a matter of debate [17,18].In this letter, we report the fact that a large part of the electronic (spin and charge) phase diagram of sodium cobaltate may be well described within a Hubbard model using realistic dispersions, and without invoking the details of the sodium arrangement. Thereby most of the observed crossovers and instabilities are truly driven by strong correlation effects and cannot be described within weak-coupling scenarios.…”

“…Several theoretical works have dealt with the influence of the sodium arrangements on the electronic properties of Na x CoO 2 , both from the viewpoint of disordered sodium ions [12] as well as from orderings for certain dopings. [14][15][16] However, wether such sodium patterns are due to sole (effective) single-particle potentials or mainly originating from many-body effects within the CoO 2 planes is still a matter of debate [17,18].…”

“…[14][15][16] However, wether such sodium patterns are due to sole (effective) single-particle potentials or mainly originating from many-body effects within the CoO 2 planes is still a matter of debate [17,18].…”

Multiorbital kinetic effects on charge ordering of frustrated electrons on the triangular lattice

“…3(c) are also similar to those recently predicted theoretically in the kagome lattice at n = 1/3 filling 34,42 , but in that case they are a natural consequence of the larger unit cell of the underlying lattice (see also Refs. [43,44]). …”

Origin of electron disproportionation in metallic sodium cobaltates

“…12, 13 The non-uniform Coulomb potential introduced by the sodium layers could substantially affect the hole states in the cobaltates. 19,22,31 The disproportionation per se, when the holes are almost completely pushed out from the Co1 sites, was not achieved in this model. Besides, the employed a 1g states of the holes 23 do not produce the anisotropy of the d-shells, which was observed experimentally.…”