From Hubbard bands to spin-polaron excitations in the doped Mott material Na$_x$CoO$_2$

TL;DR

This study explores the excitation spectrum of doped sodium cobaltate, revealing a transition from Hubbard bands to spin-polaron excitations as doping increases, highlighting the importance of beyond-DMFT effects and magnetic interactions.

Contribution

It provides a realistic many-body analysis of sodium cobaltate's excitation spectrum, demonstrating the emergence of spin-polaron excitations near ferromagnetic ordering at higher doping levels.

Findings

Hubbard bands are captured near half-filling within DMFT.

Beyond-DMFT effects dominate at higher doping levels.

Spin-polaron excitations align with experimental optical data.

Abstract

We investigate the excitation spectrum of strongly correlated sodium cobaltate within a realistic many-body description beyond dynamical mean-field theory (DMFT). At lower doping around $x$=0.3, rather close to Mott-critical half-filling, the single-particle spectral function of Na$_x$CoO$_2$ displays an upper Hubbard band which is captured within DMFT. Momentum-dependent self-energy effects beyond DMFT become dominant at higher doping. Around a doping level of $x\sim 0.67$, the incoherent excitations give way to finite-energy spin-polaron excitations in close agreement with optics experiments. These excitations are a direct consequence of the formation of bound states between quasiparticles and paramagnons in the proximity to in-plane ferromagnetic ordering.