Cobaltate superconductor to papers

In early 2003, experimentalists at NIMS announced the discovery of a new superconductor, NaxCoO2・yH20, with a Tc of about 5 Kelvins. (K. Takada et al, Nature 422 53 (2003).) The material consists of cobalt oxide planes stacked along the c-axis and intercalated by sodium and water. These layers resemble the copper oxide planes in cuprate high Tc superconductors. However the cobalt ion sites (where the carriers reside) form a triangular lattice in contrast to the square lattice formed by the cuprate’s copper sites. A number of theorists took the carrier-doped 2d Heisenberg model (t-J model) on a triangular lattice as a starting point for investigating the physics of this superconductor. Intense activity followed, since doping into a frustrated 2D antiferromagnet is widely believed to be the requisite ingredient for realizing the long-sought Resonating-Valence-Bond superconductor, proposed by P. W. Anderson in 1987. We took a somewhat more general approach of combining group theoretical analysis of the hexagonal system with energetics based on fermiology (Fig.1), a method which yields the correct dx2-y2 symmetry for the cuprates. We found within a single band picture, that among the likely pairing states is a novel chiral spin-triplet state (Fig.2), which takes advantage of the nesting among different segments of the Fermi surface. This is similar to the pairing mechanism proposed for the tetragonal material SrRuO4, though the hexagonal case would benefit further from the nesting effect. More recent developments incorporate orbital effects. There again, the physical picture of pairing induced by scattering between Fermi segments, resulting in a triplet state continues to be an important possibility pursued both experimentally and theoretically.

Fig. 1  Pairing symmetries based on single-band picture

VS Fig. 2   Schematic comparison of the pairing in cuprates (singlet-pairs) and cobaltates (triplet pairs) which derive from our approach.