Robust d -wave pairing symmetry in multiorbital cobalt high-temperature superconductors

The pairing symmetry of the cobalt high-temperature (high-Tc) superconductors formed by vertex-shared cation-anion tetrahedral complexes is studied by the methods of mean-field, random phase approximation (RPA), and functional renormalization-group (FRG) analyses. The results of all of these methods show that the dx2-y2 pairing symmetry is robustly favored near half filling. The RPA and FRG methods, which are valid in weak-interaction regions, predict that the superconducting state is also strongly orbital selective, namely, the dx2-y2 orbital that has the largest density near half filling among the three t2g orbitals dominates superconducting pairing. These results suggest that these materials, if synthesized, can provide an indisputable test of the high-Tc pairing mechanism and the validity of different theoretical methods.