On the suppression of superconductivity in ErBa2Cu3Ox

Powder samples of ErBa₂Cu₃O_x prepared by a desorption-absorption procedure were studied by susceptibility and neutron-diffraction experiments for variousx(6x7). The suppression of superconductivity does not coincide with the orthorhombic-to-tetragonal phase transition, but with a significant charge redistribution between the Cu–O chains and planes. Upon oxygen removal, asT _c (x)O, step-like anomalies are observed in some bonding lengths and atomic distances, which support an all-electronic mechanism for the oxide superconductivity.     

Antiferromagnetic order in PrBa2Cu3O x (x≈6, 7)

Zero and weak transverse fieldμ ⁺ SR measurements on PrBa₂Cu₃O _x samples withx≈6 andx≈7 show ordered magnetism in both oxygen concentration limits. As expected for equivalent doping (nominally PrBa₂Cu₃O _x ∼YBa₂Cu₃O _x −0.5), neither sample is superconducting. Two muon signals are observed in thex≈6 sample, as in YBa₂Cu₃O₆, but only one (the weaker) has the same local magnetic field as in YBa₂Cu₃O₆. In thex≈7 sample, only one site is observed; its local field is reduced with respect to that of the primary site in YBa₂Cu₃O₆ by a factor roughly consistent with the carrier-density dependence of the Cu ion moment in antiferromagnetic YBa₂Cu₃O _x .     

Zero-field μSR study of ErBa2Cu3Ox for various oxygen contentx

Results from zero-field μSR experiments are presented, performed on the high-T _c compounds ErBa₂Cu₃O_x for various oxygen contentx with 6≤x≤7 at temperatures between 40 mK and 300 K. The aim was to study the magnetic ordering behavior of the Cu moments and the Er moments, and its interplay with superconductivity.     

Microwave absorption spectra and the problem of the crystal field in tetragonal compounds HoBa2Cu3O x (x = 6.0, 6.3)

The resonance microwave absorption at wavelengths 871, 406, 305, and 118 μm in tetragonal layered perovskites HoBa₂Cu₃O _x (x = 6.0, 6.3) in pulsed magnetic fields up to 40 T has been detected. This absorption is caused by the electronic transitions between low lying levels of the Ho³⁺ ion in the crystal field. The positions and intensities of the basic resonance absorption lines for the crystal with x = 6.0 in the magnetic field oriented along the tetragonal axes are adequately described in terms of the tetragonal crystal field with known interaction parameters. To explain the weaker absorption lines, it is necessary to take into account the effect of the inhomogeneous orthorhombic and monoclinic components of the crystal field, which are due to disorder in the oxygen subsystem. This effect is more pronounced in the absorption spectra of the crystal with x = 6.3 for which the oxygen disorder is more pronounced.     

Mössbauer absorption in some high Tc superconductors:170Yb in YbBa2Cu3O7−x,166Er in ErBa2Cu3O7−x,and57Fe in YBa2Cu3O7−x

For the high T_c compound YbBa₂Cu₃O_7−x, Mossbauer absorption measurements show that magnetic ordering occurs within the Yb³⁺ sublattice at 0.35K (μ_sat=1.7μ_B). For ErBa₂Cu₃O_7−x the Er³⁺ sublattice orders at 0.7K (μ_sat=4μ_B). Measurements on⁵⁷Fe diluted into YBa₂Cu₃O_7−x shows that no ordered magnetic moments exist within the Cu sublattices down to at least 4.2K. Instituto de Fisica, Porto-Alegre, Brasil (supported by CNPq)