Probable hole-doped superconductivity without apical oxygens in (Ca, Na)₂CuO₂CI₂

Abstract

THE presence of apical oxygens (above or below the CuO₂ planes) has been regarded as indispensable for the occurrence of hole-doped superconductivity in the high-transition-temperature (high-T_c) copper oxide superconductors^1,2. We have opposed this view^3,4, on the basis of studies in compounds such as (Ca, Sr)_{1 - x}CuO_{2 - y} and Sr_n+1CU_nO_2n+1+δ; however, our conclusions have been ques-tioned in subsequent work^5,6. Here we provide strong evidence for hole-doped superconductivity in the absence of apical oxygens, by showing that Ca₂CuO₂Cl₂ can be rendered superconducting by doping with sodium. The compound contains CuO₂ planes as in La₂CuO₄, but all of the oxygen atoms at the apices of the CuO₆ octahedra found in La₂CuO₄ are replaced by chlorine, and the lanthanum atoms by calcium₇. The apparent difference in mass and electronic character between chlorine and oxygen might strongly affect the electronic structure of the CuO₂ planes if the contributions of the apical atoms were significant. But we find hole-doped superconductivity in (Ca, Na)₂CuO₂Cl₂ at rather high transition temperatures (26 K at maximum), which are only slightly less than those of doped La₂CuO₄ (ref. 8). This implies that the apical atoms do not play a significant role in the electron pairing mechanism leading to high-T_c superconductivity.