PLANE DIMPLING AND SADDLE-POINT BIFURCATION IN THE BAND STRUCTURES OFOPTIMALLY DOPED HIGH-TEMPERATURE SUPERCONDUCTORS - A TIGHT-BINDING MODEL

We argue that extended saddle points observed at the Fermi level for o ptimally doped superconductors are essentially the bifurcated saddle p oints predicted by density-functional [local density approximation (LD A)] calculations. Such saddle points are caused by the dimple of the C uO2 planes and are enhanced by plane-plane hopping. Dimpling may provi de a mechanism for pinning the Fermi level to the saddle points. Simpl e tight-binding Hamiltonians and analytical expressions for the consta nt-energy contours are derived from the LDA bands of YBa2Cu3O7. In add ition to the usual O2 x, O3 y, and Cu x2-y2 orbitals, we find that O2 z and O3 z are crucial and Cu s, xz, and yz important. The O z orbital s allow the pdsigma antibond to tilt with the dimple.