Anomalous Fermi-Surface Dependent Pairing in a Self-Doped High-Tc   Superconductor
  We report the discovery of a self-doped multi-layer high Tc superconductor Ba2Ca3Cu4O8F2(F0234) which contains distinctly different superconducting gap magnitudes along its two Fermi surface(FS) sheets. While formal valence counting would imply this material to be an undoped insulator, it is a self-doped superconductor with a Tc of 60K, possessing simultaneously both electron- and hole-doped FS sheets. Intriguingly, the FS sheet characterized by the much larger gap is the electron-doped one, which has a shape disfavoring two electronic features considered to be important for the pairing mechanism: the van Hove singularity and the antiferromagnetic (Pi/a, Pi/a) scattering. 
Yulin Chen, Akira Iyo, Wanli Yang, Xingjiang Zhou, Donghui Lu, Hiroshi   Eisaki, Thomas P. Devereaux, Zahid Hussain, and Z.-...