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We report on a study of the superconducting order parameter in thin films (with different Se contents: ) by means of point-contact Andreev-reflection spectroscopy (PCARS). The PCARS spectra show reproducible evidence of multiple structures, namely two clear conductance maxima associated to a superconducting gap of amplitude and additional shoulders at higher energy that, as we show, are the signature of the strong interaction of charge carriers with a bosonic mode whose characteristic energy coincides with the spin-resonance energy. The details of some PCARS spectra at low energy suggest the presence of a smaller and not easily discernible gap of amplitude . The existence of this gap and its amplitude are confirmed by PCARS measurements in single crystals. The values of the two gaps and , once plotted as a function of the local critical temperature , turn out to be in perfect agreement with the results obtained by various experimental techniques reported in literature. [ABSTRACT FROM AUTHOR] more...

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 p bands are filled with holes and/or affected by disorder, as predicted by theory. We also measured the magnetic penetration depth l by an inductance technique. Data fits based on the standard Bardeen-Cooper-Schrieffer two-band model yield the zero-temperature limit of both l and superconducting gaps: we find that l(0) increases weakly and quickly saturates with increasing x, whereas Dp(0) and Ds(0) decrease with x. This analysis suggests that lithium substitution induces disorder mainly in the p 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 Dp(0) that may be related to an inhomogeneous distribution of Li content. [ABSTRACT FROM AUTHOR] more...