Rock Salt Layers and Their Effect on Electronic Properties

0 DISTORTIONS IN THE ROCK SALT LAYERS AND THEIR EFFECT ON ELECTRONIC PROPERTIES 

Tight-binding band electronic structure calculations have been 

Oxidation of the Cu02 layer removes electrons from the x2-y2 bands which have antibonding character in the in-plane Cu-O bonds. Therefore, as the number of holes (nH) increases, the in-plane Cu-O bond length (rCu 0) is shortened. In addition to this electronic factor, the in-plane rCu 0 is also controlled by a nonelectronic factor (i.e., steric strain) associated with the cations located at the 9-coordinate sites adjacent to the Cu02 layers (e.g., La, Sr, Ba) (40). With the increasing size of the 9-coordinate site cations, the in-plane Cu-O bond is lengthened to reduce the extent of the resulting steric strain. 

Plots of Tc vs in-plane rCu 0 for the series of p-type cuprate superconductors are grouped into three classes distinguished by the size of the 9-coordinate site cations (that is, La-, Sr- and Ba-classes) because of the combined electronic and nonelectronic effects. Every class of the Tc vs in-plane rCu 0 plot shows a maximum, so that every class of the p-type cuprate superconductors possesses an optimum hole density for which the Tc is maximum (40). 

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