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

1989; Elsevier BV; Volume: 202; Linguagem: Inglês

10.1016/0166-1280(89)87029-0

1872-7999

Barbara Szpunar, Vedene H. Smith, R. W. Smith,

Superconductivity in MgB2 and Alloys

Spin polarized LMTO-ASA calculations have been performed for antiferromagnetic ordering in the semiconductor YBa2Cu3O6 (123) and for forced ferromagnetic ordering in the superconductors YBa2Cu3O7 (123) and Bi2Sr2CaCu2O8 (2212). The influence on the electronic structure of oxygen content and magnetic ordering in YBa2Cu3O6 and YBa2Cu3O7 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 Bi2Sr2CaCu2O8 than for YBa2Cu3O7. 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 YBa2Cu3O6 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.