Ferromagnetic and triplet-pairing instabilities controlled by the trigonal distortion of the CoO_6 octahedra in Na_xCoO_2*yH_2O

TL;DR

This study investigates how trigonal distortion of CoO6 octahedra influences magnetic and superconducting properties in Na_xCoO_2*yH_2O, revealing that increased distortion enhances ferromagnetic fluctuations and pairing instabilities.

Contribution

It demonstrates that trigonal distortion modifies the electronic structure, promoting hole-pocket formation and increasing magnetic and pairing instabilities in the material.

Findings

Trigonal distortion pushes Co e'g bands upward.

Hole-pocket Fermi surfaces are enlarged with increased distortion.

Ferromagnetic fluctuations and pairing instabilities grow as distortion increases.

Abstract

In the recently discovered Co-oxide superconductor Na_xCoO_2*yH_2O, the edge-shared CoO6 octahedra are trigonally contracted along the c-axis in the CoO2-plane. We study how this CoO6 distortion affects the magnetic properties and superconductivity in this compound by analyzing the multiorbital Hubbard model using the fluctuation-exchange approximation. It is shown that through generating the trigonal crystal field, the distortion pushes the Co e'g bands up and consequently gives rise to the hole-pocket Fermi surfaces, which have been predicted in the band calculations. As the distortion increases, the hole pockets are enlarged and the ferromagnetic fluctuation as well as the pairing instability increases, which is in good agreement with recent NQR results.