ELECTRONIC-STRUCTURE CALCULATION OF THE HOLE-CARRIER-DENSITY DISTRIBUTION IN BI2SR2CUO6, BI2SR2CACU2O8, AND BI2SR2CA2CU3O10 SUPERCONDUCTORS

Results of electronic structure calculations for the hole density in the two-dimensional CuO2 planes are presented in this paper for the three bismuth cuprate superconductors Bi2Sr2CuO6, Bi2Sr2CaCu2O8, and Bi2Sr2Ca2Cu3O10. The hole density in these compounds arises due to electron transfer from the CuO2 Planes to the Bi-O layers. Our calculations show that Bi2Sr2CuO6 is so highly overdoped that it is really a metal and at the limit of superconductivity. Also Bi2Sr2CaCu2O8 has a hole density which is higher than is required for a maximum value of T(c). On the other hand, the hole density in the three-layer compound Bi2Sr2Ca2Cu3O10 is nearly optimum, which indicates that apart from the number of layers, an optimum value of the hole density is important for maximizing T(c). The hole densities in the two types of CuO2 layers in this compound are nearly equal although the central layer has a slightly lower value. As the number of CuO2 Planes increases in this family of superconductors the total amount of hole carrier density per chemical formula increases due to an enhanced electron transfer to the Bi-O layers. However, this increase is small so that the hole density per CuO2 drops rapidly as the number of CuO2 planes increases.