Plane dimpling and saddle-point bifurcation in the band structures of optimally doped high-temperature superconductors: a tight-binding model.

Andersen O.K., Jepsen O., Liechtenstein A.I., Mazin I.I.

We argue that extended saddle points observed at the Fermi level for optimally doped superconductors are essentially the bifurcated saddle points predicted by density-functional (local density approximation (LDA)) calculations. Such saddle points are caused by the dimple of the CuO2 planes and are enhanced by plane-plane hopping. Dimpling may provide a mechanism for pinning the Fermi level to the saddle points. Simple tight-binding Hamiltonians and analytical expressions for the constant-energy contours are derived from the LDA bands of YBa2Cu3O7. In addition 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 orbitals allow the pd sigma antibond to tilt with the dimple.

Physical Review B, 49 4145-57, 1994.

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