Electron-phonon interaction dressed by electronic correlations near charge ordering as the origin for superconductivity in cobaltates

TL;DR

This paper explores how electron-phonon interactions combined with electronic correlations near charge ordering can lead to unconventional superconductivity in hydrated cobaltates, revealing specific pairing symmetries and phase behaviors.

Contribution

It introduces a theoretical framework combining the t-J-V model and phonons to explain superconductivity and phase competition in cobaltates, highlighting the role of correlations and electron-phonon coupling.

Findings

Superconductivity with nnn-f wave symmetry near x ~ 0.35 doping.

Phase separation and bond-ordered phases at low doping.

Tc exhibits a dome shape consistent with experimental observations.

Abstract

We consider possible routes to superconductivity in hydrated cobaltates Na_xCoO_2.yH_2O on the basis of the t-J-V model plus phonons on the triangular lattice. We studied the stability conditions for the homogeneous Fermi liquid (HFL) phase against different broken symmetry phases. Besides the sqrt(3)xsqrt(3)-CDW phase, triggered by the nearest-neighbour Coulomb interaction V, we have found that the HFL is unstable, at very low doping, against a bond-ordered phase due to J. We also discuss the occurrence of phase separation at low doping and V. The interplay between the electron-phonon interaction and correlations near the sqrt(3)xsqrt(3)-CDW leads to superconductivity in the unconventional next-nearest neighbour f-wave (NNN-f) channel with a dome shape for Tc around x ~ 0.35, and with values of a few Kelvin as seen in experiments. Near the bond-ordered phase at low doping we found tendencies to superconductivity with d-wave symmetry for finite J and x<0.15. Contact with experiments is given along the paper.