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We present transverse field μ SR measurements of the flux lattice field distribution in dense pellets of MgB 2 and of the alloy Mg 1 - x Al x B 2 . In the present work we focus on the analysis of the data, comparing the second moment of the calculated and experimental field distributions. Starting from the standard model, we modified the calculation to include the presence of two length scales, the diffusion lengths of the σ and π pairs. We further introduce a partially anisotropic single gap model, adapted to the electronic structure of MgB 2 , and a partially anisotropic two-band model. We confirm that the two distinct gaps can better describe the field dependence of the data.

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 measured the 27Al and 11B NMR spin lattice relaxation rates and the isotropic Knight shifts in powder samples of Mg1−xAlxB2, 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 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 have measured the transverse field muon spin precession in the flux-lattice (FL) state of the two-gap superconductor $\mathrm{M}\mathrm{g}{\mathrm{B}}_{2}$ and of the electron doped compounds ${\mathrm{M}\mathrm{g}}_{1\ensuremath{-}x}{\mathrm{A}\mathrm{l}}_{x}{\mathrm{B}}_{2}$ 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 $\mathrm{M}\mathrm{g}{\mathrm{B}}_{2}$ single crystal. Our data agree quantitatively with STM and we thus validate a new phenomenological model for the internal fields.

We have measured the transverse field muon spin precession in the flux-lattice (FL) state of the two-gap superconductor MgB2 and of the electron doped compounds Mg(1-x)AlxB2 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 MgB2 single crystal. Our data agree quantitatively with STM and we thus validate a new phenomenological model for the internal fields.

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.

Il libro presenta, articola e dettaglia storicamente cosa dia il 'Fuoco sacro' dei gesuati, ovvero l'osmotica capacità dei fratres vestiti di bianco di entrare negli interstizi dei contesti in cui si sono di volta in volta trovati a vivere, e di creare, a partire da quelli, nuove realtà e contesti he avranno vita autonoma, innescando così processi storici destinati a vivere ben oltre la congregazione, sia nella sincronia, sia sulla diacronia