  The apparently counterintuitive carrier concentration-dependent electronic
properties of layered cobaltates have attracted wide interest. Here we point
out that very similar carrier-concentration dependence has previously been
noted in strongly correlated quasi-one dimensional (quasi-1D) organic
charge-transfer solids. The normal states of both families can be understood,
over the entire range of carrier concentration of interest, within the extended
Hubbard Hamiltonian with significant intersite Coulomb interaction. As with the
charge-transfer solids, superconductivity in the cobaltates appears to be
limited to bandfilling of one-quarter. We point out further that there exist
other families of correlated superconductors, such as spinels, where too strong
correlations, geometric lattice frustration and bandfilling of one-quarter seem
to be the essential features of the unconventional superconductors.
