The True Nature of High-Temperature Superconductivity

The question of whether high‐temperature superconductivity is s‐wave or d‐wave has been resolved in favor of a nodeless gap function consistent with s‐wave pairing [Phys. Rev. B 69, 174505 (2004)]. In the case of YBa2Cu3O7, where the CuO2 and the CuO layers both contain Cu d‐bands, this result implies that the superconducting hole condensate resides in the BaO layers. Thus the theory for YBa2Cu3O7 also describes systems without cuprate‐planes, such as doped Ba2YRuO6 (Tc ∼ 93 K) and materials such as GdSr2Cu2RuO8 or Gd2−zCezSr2Cu2RuO10 with onset Tc’s near 45 K, whose cuprate‐planes are either weak‐ferromagnetic or antiferromagnetic, but whose BaO or SrO layers superconduct. Clearly high‐temperature superconductivity (1) is in BaO or SrO layers, or in interstitial regions, rather than in cuprate‐planes, (2) is s‐wave, not d‐wave, and (3) usually involves unit cells with both normal and superconducting planes.