Is there a common theme behind the correlated-electron superconductivity in organic charge-transfer solids, cobaltates, spinels and fullerides?

TL;DR

This paper explores deep connections between various correlated-electron superconductors, proposing a unified theoretical framework based on the extended Hubbard model and orbital effects, highlighting common features like quarter-filling and strong correlations.

Contribution

It introduces a unified model explaining electronic behaviors across organic, cobaltate, spinel, and fulleride superconductors, emphasizing the role of orbital ordering and electron interactions.

Findings

Similar carrier dependence in organics and cobaltates explained by extended Hubbard model.

Superconductivity occurs near quarter-filling in cobaltates and organics.

Orbital ordering and Jahn-Teller effects relate fullerides and spinels to other superconductors.

Abstract

We posit that there exist deep and fundamental relationships between the above seemingly very different materials. The carrier concentration-dependences of the electronic behavior in the conducting organic charge-transfer solids and layered cobaltates are very similar. These dependences can be explained within a single theoretical model, - the extended Hubbard Hamiltonian with significant nearest neighbor Coulomb repulsion. Interestingly, superconductivity in the cobaltates seems to be restricted to bandfilling exactly or close to one-quarter, as in the organics. We show that dynamic Jahn-Teller effects and the resultant orbital ordering can lead to 1/4-filled band descriptions for both superconducting spinels and fullerides, which show evidence for both strong electron-electron and electron-phonon interactions. The orbital orderings in antiferromagnetic lattice-expanded bcc M$_3$C$_{60}$ and the superconductor are different in our model. Strong correlations, quarter-filled band and lattice frustration are the common characteristics shared by these unusual superconductors.