Carrier Distribution in Oxide Superconductors

Abstract

Based on the ionic crystal model, we have calculated the Madelung energies of several Cu–O based compounds: Tl₂Ba₂(Ca, Y)_n−1Cu_nO_2n+4, Bi₂Sr₂(Ca, Y)_n−1Cu_nO_2n+4 and TlBa₂(Ca, Y)_n−1Cu_nO_2n+3. We have determined the carrier distribution which makes the sum of the Madelung energy and the ionic energies of a compound the smallest. Our main concern is what the carriers are (holes or electrons) and which sites they occupy. We have found several different types of carrier distribution. When a large fraction of the Ca is replaced by Y in the substances with n≥3, electrons will be doped in the CuO₂ planes where the Cu ions are in square-planar coordination.