Multiband model of high-Tc superconductors

We propose an extension to other high-ir, compounds of a model introduced earlier for YBa2Cu3O7. In the "self-doped" compounds we assume that the doping part [namely the BiO, HgO, and TlO planes in Bi2Sr2CaCu2O8+delta (BSCCO), HgBa2Can-1CunO2n+2 (HBCCO), and Tl2Ba2Can-1CunO4+2n (TBCCO), respectively] is metallic, which leads to a multiband model. This assumption is supported by band structure calculations. Taking a repulsive pairing interaction between these doping bands and the CuO2 bands leads to opposite signs for the order parameter on these bands and to nodes whenever the Fermi surfaces of these bands cross. We show that in BSCCO the low-temperature dependence of the penetration depth is reasonably ac counted for. In this case the nodes are not located near the 45 degrees direction, which makes the experimental determination of the node locations an important test for our model. The situation in HBCCO and TBCCO is rather analogous to BSCCO. We consider the indications given by NMR and Rnd that they rather favor a metallic character for the doping bands. Finally we discuss the cases of Nd2-xCexCuO4 and La2-xSrxCuO4 which are not "self-doped" and where our model does not give nodes.