We have measured the transverse field muon spin precession in the flux-lattice (FL) state of the twogap superconductor MgB 2 and of the electron doped compounds Mg 1ÿx Al x B 2 in magnetic fields up to 2.8 T. We show the effect of the two gaps on the internal field distribution in the FL, from which we determine two coherence length parameters and the doping dependence of the London penetration depth. This is an independent determination of the complex vortex structure already suggested by the STM observation of large vortices in a MgB 2 single crystal. Our data agree quantitatively with STM and we thus validate a new phenomenological model for the internal fields.

We measured the 27 Al and 11 B NMR spin lattice relaxation rates and the isotropic Knight shifts in powder samples of Mg 1−x Al x B 2 , as a function of the Al concentration, x. The temperature independence of the Knight shifts and the linear temperature dependence of the relaxation are verified throughout the compositions explored. The variation with x of the measured quantities is discussed in terms of the projected densities of states at the Fermi energy, finding good qualitative as well as quantitative agreement with recent band structure calculations.

We present a systematic characterization of Mg 1−x Li x B 2 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 π (0) and σ (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 π (0) that may be related to an inhomogeneous distribution of Li content.

No abstract

No abstract