Getting back to Na x CoO2 : Spectral and thermoelectric properties

Sodium cobaltate NaxCoO2 as dopable strongly correlated layered material with a triangular sublattice still poses a challenging problem in condensed matter. The intriguing interplay between lattice, charge, spin, and orbital degrees of freedom leads to a complex phase diagram bounded by a nominal Mott (x = 0) regime and a band-insulating (x = 1) phase. By means of the charge self-consistent density functional theory (DFT) plus dynamical mean-field theory (DMFT) scheme, built on a pseudopotential framework combined with a continuous-time quantum Monte-Carlo solver, we here study the one-particle spectral function A(k,ω) as well as the thermopower S(T). The computations may account for the suppression of the pockets in A(k,ω) at lower doping in line with photoemission experiments. Enhancement of the thermopower is verified within the present elaborate multi-orbital method to treat correlated materials. In addition, the two-particle dynamic spin susceptibility is investigated based on a simplified tight-binding approach, yet by including vertex contributions in the DMFT linear response. Besides the identification of paramagnon branches at higher doping, a prominent high-energy antiferromagnetic mode close to x = 0.67 is therewith identified in , which can be linked to extended hopping terms on the CoO2 sublattice.