Electronic Structure of La1.85Sr0.15CuO4: Characterization of a Fermi Level Band Crossing

We present the results of a new Hubbard model for optimally doped LaSrCuO. This model uses parameters derived from BLYP calculations on the cluster CuO6. It explicitly includes the Cu dx2-y2 and dz2 orbitals, the O psigma orbitals, and the apical O pz orbitals. When correlation is properly included in the Hubbard model, we find that there is a crossing of two bands in the vicinity of the Fermi level for the optimally doped superconductor. This crossing rigorously occurs along the (0,0)-(pi,pi) direction of the 2D Brillouin zone. The crossing arises due to the overlap of a broad "B1g" band dominated by Cu dx2-y2 character and a narrower "A1g" band dominated by Cu dz2 character. We conclude that optimal doping of LaSrCuO and related materials is achieved when the Fermi levels coincides with this crossing. At this point, formation of Cooper pairs between the two bands (i.e. inter-band or IBP) leads to superconductivity. Furthermore, using geometric considerations, we extend our conclusions to YBaCuO and offer a simple explanation for the seemingly complex behavior of Tc as a function of doping in this material. This behavior can be understood on the basis of multiple band crossings.