Ab initio periodic approach to electronic structure and magnetic exchange in A2CUO2X2 (A=Ca,Sr and X=F,Cl) High-Tc superconductor parent compounds

Abstract

The electronic structure of A2CuO2X2 (A5Ca, Sr and X5F ,Cl) compounds is investigated by means of the periodic unrestricted Hartree-Fock ~UHF! method using the linear combination of atomic orbitals approach ~LCAO!. The relative stability of different alternative structures is discussed. All the systems are described as insulators with strong ionic character and well localized spin moments on the Cu atoms. The O 2p nature of the highest occupied bands is clear, supporting the charge transfer nature of this kind of narrow band systems; optical gaps are however, overestimated. The magnetic ordering of these materials is two dimensional in nature with almost independent antiferromagnetic CuO2 planes with Jc /J1;1023. The in–plane nearest (J1) and next-nearest (J2) magnetic coupling constants are antiferromagnetic and much larger than the interplane interactions (Jc), with J2 /J1;0.02. The relative values of the magnetic constants are in qualitative agreement with available experimental results. None of the analyzed properties provide differences between the various compounds so relevant to limit the possible development of a superconducting phase. On the contrary, the close similarity between them suggest that kinetic limitations in the doping process can be responsible of the fact that no superconducting transition has been observed in Sr2CuO2Cl2 and in the T8 phase of Sr2CuO2F2 and Ca2CuO2F2.