Unusual Electron Correlations in Na_xCoO₂Due to the Spin-State Quasidegeneracy of Cobalt Ions

Abstract

Recent studies exposed many remarkable properties of layered cobaltates NaxCoO2. Surprisingly, many-body effects have been found to increase at sodium-rich compositions of NaxCoO2 where one expects a simple, nearly free motion of the dilute S = 1/2 holes doped into a band insulator NaCoO2. Here we discuss the origin of enigmatic correlations that turn a doped NaCoO2 into a strongly correlated electronic system. A minimal model including orbital degeneracy is proposed and its predictions are discussed. The model is based on a key property of cobalt oxides -the spin-state quasidegeneracy of CoO6 octahedral complex -which has been known, e.g., in the context of an unusual physics of LaCoO3 compound. Another important ingredient of the model is the 90• Co-O-Co bonding in NaxCoO2 which allows nearest-neighbor t2g − eg hopping. This hopping introduces a dynamical mixture of electronic configurations t 6 2g , S = 0 and t 5 2g e 1 g , S = 1 of neighboring cobalt ions. We show that scattering of charge carriers on spin-state fluctuations suppresses their coherent motion and leads to the spin-polaron physics at x ∼ 1. At larger doping when coherent fermionic bands are formed, the model predicts singlet superconductivity of extended s-wave symmetry. The presence of low-lying spin states of Co 3+ is essential for the pairing mechanism. Implications of the model for magnetic orderings are also discussed. §1. IntroductionThe physics of transition metal oxides offers a wealth of interesting phenomena related to their strongly correlated nature. This is because the bandwidth is relatively small compared to the intraionic Coulomb repulsion between the 3d electrons. The understanding of many unique properties of oxides, such as high-T c superconductivity and colossal magnetoresistivity is therefore based on the well-known Mott physics. 1)Among the various families of transition metal oxides, attention has recently focused on layered cobaltates, in particular on the sodium cobaltate Na x CoO 2 . The interest was initiated by the large thermoelectric power observed in this compound, 2)-4) i.e. the capability of an efficient conversion of heat energy to electricity. The research on these systems was further boosted by the unexpected discovery of superconductivity (SC) in water-intercalated Na x CoO 2 . 5) Soon after, many remarkable properties were found 6) such as spin-sensitive thermopower, 3) unusual charge and spin orderings, 7)-11) very narrow quasiparticle bands 12)-16) and especially a very unusual phase diagram. 7) While the strongly correlated nature of Na x CoO 2 is no longer at doubt, the mechanisms by which the correlated electrons design such an exotic phase diagram are not fully understood even on a qualitative level.Layered cobaltates consist of CoO 2 planes with a triangular lattice of Co ions

Pseudospin exchange interactions ind7cobalt compounds: Possible realization of the Kitaev model

“…For a c-bond pair, hopping between c and 3z 2 − r 2type orbitals, shown in Fig. 8(a), is only finite [9,36]; the other t 2g -e g matrix elements vanish due to quantum interference between pd virtual hoppings through the upper and lower oxygen ions. Since e g states are half-filled in d 7 configuration, the exchange interaction should be antiferromagnetic, according to Goodenough-Kanamori rules [37].…”

Specific features of spin, charge, and orbital ordering in cobaltites

“…All this leads to the energy of the HS state lying higher than that of the LS state by 1.75 eV [216], and hence to stabilization of the low-spin state as the ground state at any Na concentration. However, in subsequent paper [217], a hypothesis was advanced that even in the case of realization of the LS state as the ground level, the dynamic IS and HS states can affect the magnetic properties of quasiparticles.…”

Evolution of Co charge disproportionation with Na order inNaxCoO2

“…This suggest that in the cobaltates the holes are distributed over the three orbital states within the t 2g -shells due to the thermal/quantum fluctuations. 31 Such effect could explain the recovery of the isotropy of the local wave functions. 30,32 A plausible possibility for the reduction could come from the band structure of the holes in the triangular CoO 2 layers, which would intermix the three planar t 2g orbitals (xy, yz, xz).…”