Search

We investigated the system Mg 1− x Al x B 2 , in which the heterovalent substitution of Al for Mg changes both the band filling (different valence) and the bandwidth (lattice contraction due to different ionic radii). The result is the disappearance of superconductivity at about x =0.5. Contrary to earlier findings, our carefully annealed powder samples show no evidence of phase separation. We determined the μSR lineshape in a flux line lattice. We obtain a nearly linear dependence of the second moment of the flux lattice field distribution upon doping, indicating a strong reduction of the super carrier density with increasing x . Muon diffusion is detected by zero field measurements. It persists down to the lowest temperatures also at x =0.4. The doping dependent activation energies are reported.

We present a systematic characterization of Mg1−xLixB2 polycrystalline samples with nominal Li content x up to 0.30. We explore the effectiveness of the substitution and investigate its influence on superconducting and normal state properties. The structural analyses by neutron and x-ray diffraction indicate that, despite the lattice parameters remaining unchanged within the experimental accuracy, around 30% of the nominal Li content actually enters the structure. Also the transition temperature is only weakly affected by Li substitution, but its relationship with the residual resistivity and magnetoresistivity data is compatible with a picture where π bands are filled with holes and/or affected by disorder, as predicted by theory. We also measured the magnetic penetration depth λ by an inductance technique. Data fits based on the standard Bardeen–Cooper–Schrieffer two-band model yield the zero-temperature limit of both λ and superconducting gaps: we find that λ(0) increases weakly and quickly saturates with increasing x, whereas Δπ(0) and Δσ(0) decrease with x. This analysis suggests that lithium substitution induces disorder mainly in the π band. Point contact spectroscopy results on samples obtained from the same batch are in full agreement with magnetic penetration depth data on the undoped sample, but give superconducting gaps almost insensitive to Li substitution, showing at most a small increase in Δπ(0) that may be related to an inhomogeneous distribution of Li content.

We measured the 2 7 Al and 1 1 B NMR spin lattice relaxation rates and the isotropic Knight shifts in powder samples of Mg 1 - x Al x B 2 . The comparison with band structure calculations allows the experimental determination of the partial DOS and its dependence upon the Al concentration x. Agreement is obtained for its absolute values as well as for its behavior upon doping. The satisfactory understanding of the electronic wave function around the probe nuclei validates our determination of a surprisingly large Korringa ratio, from the comparison between 2 7 Al relaxations and shifts, which can only partially be attributed to Al disorder.