Knight shift in superconducting oxides BaPb1−x BixO3 (x < 0.35)

Abstract

The Knight shift 207 K s for the 207Pb nuclei in the metal phase of the oxides BaPb1−x BixO3 (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 BaPb1−x BixO3 when they are studied by traditional single-resonance methods of pulse NMR spectroscopy, have been overcome.