Low-lying quasiparticle states and hidden collective charge instabilities in parent cobaltate superconductors (NaxCoO2)

TL;DR

This study uses ARPES to analyze low-energy quasiparticle states in NaxCoO2, revealing small Fermi velocities and potential collective charge instabilities that differentiate cobaltates from other unconventional superconductors.

Contribution

It provides detailed ARPES measurements of NaxCoO2, highlighting the small Fermi velocity and identifying possible charge instabilities linked to its unique electronic structure.

Findings

Small Fermi velocity (< 0.5 eV·A) across dopings

Weak anisotropy of Fermi velocity

Emergence of 120-type fluctuations at low doping

Abstract

We report a state-of-the-art photoemission (ARPES) study of high quality single crystals of the NaxCoO2 series focusing on the fine details of the low-energy states. The Fermi velocity is found to be small (< 0.5 eV.A) and only weakly anisotropic over the Fermi surface at all dopings setting the size of the pair wavefunction to be on the order of 10-20 nanometers. In the low doping regime the exchange inter-layer splitting vanishes and two dimensional collective instabilities such as 120-type fluctuations become kinematically allowed. Our results suggest that the unusually small Fermi velocity and the unique symmetry of kinematic instabilities distinguish cobaltates from other unconventional oxide superconductors such as the cuprates or the ruthenates.