Qualitative understanding of the highest Tc cuprates

We use two old ideas for which layered structures are particularly relevant. The first is that the pairing can occur in different parts of the unit cell and interact symbiotically, behaving in some ways as a natural realization of the sandwich models discussed in the early days of this conference. Pairing and large current flow in the CuO2 layers are enhanced by additional pairing in other layers. The second idea is that additional pairing interactions can be due to negative U-atoms (Tl+3-Tl+1), molecules (Hg2+2−2Hg+2), or due to interactions in the quasi one-dimensional CuO chain layers of the YBCO-123 and 248 cuprates. Further, coulomb interactions in the CuO2 planes between pairs moving in a and b directions lead to d-wave superconductivity.