Magnetism and electronic structure in the (123) and (2212) copper oxides

Abstract

Spin polarized LMTO-ASA calculations have been performed for antiferromagnetic ordering in the semiconductor YBa 2Cu 3O 6 (123) and for forced ferromagnetic ordering in the superconductors YBa 2Cu 3O 7 (123) and Bi 2Sr 2CaCu 2O 8 (2212). The influence on the electronic structure of oxygen content and magnetic ordering in YBa 2Cu 3O 6 and YBa 2Cu 3O 7 is examined. The primary difference in the electronic structure of these two compounds is that for the superconducting compound π states on the O2 atoms in the barium planes are created. They may be responsible for the absence ofantiferromagnetic ordering in the crucial Cu2-O (3,4) planes. The contribution of the out-of-plane states to the density of states at the Fermi energy is smaller for Bi 2Sr 2CaCu 2O 8 than for YBa 2Cu 3O 7. To obtain the same magnetic moment on the planar copper atoms the field required is twice as large in the former compound as in the latter. Our symmetry-breaking calculations show that for antiferromagnetic ordering of the copper magnetic moments in the Cu2-O (3,4) plane, a Peierls-type gap opens up around the Fermi energy and YBa 2Cu 3O 6 becomes semiconducting. The Cul sites in both of the (123) compounds are not likely to form substantial local magnetic moments and thus probably are not directly involved in the mechanism for superconductivity. The possibility of an anharmonic effect owing to magnetic interactions is discussed.