Knight shift in superconducting oxides BaPb1−xBixO3 (x < 0.35)

The Knight shift ²⁰⁷ K _s for the ²⁰⁷Pb nuclei in the metal phase of the oxides BaPb_1−x Bi_xO₃ (x < 0.35) has been analyzed as a function of the concentration. The shift, which is proportional to the density of states near the Fermi energy: ²⁰⁷ K _s ∼ N(E _F), reaches a maximum for an oxide with the maximum superconducting transition temperature T _c(x ≈ 0.25) = 12 K. A significant increase in the width of the shift distribution with the Bi concentration testifies to the formation of a nonuniform state of the electronic system in the conduction band of superconducting oxides, which is accompanied by an increase in short-wavelength contributions to the spin susceptibility. To detect the ²⁰⁷Pb NMR spectra in superconducting oxides with x > 0.2, the ¹⁷O-²⁰⁷Pb spin-echo double-resonance method is used, which provides successful detection of the ²⁰⁷Pb NMR signal with an anomalously high rate of spin-spin relaxation T ₂ ⁻¹ > 500 ms⁻¹. Thus, fundamental restrictions arising in investigations of rapidly relaxing ²⁰⁷Pb nuclei, which are “unobservable” in superconducting oxides BaPb_1−x Bi_xO₃ when they are studied by traditional single-resonance methods of pulse NMR spectroscopy, have been overcome.