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We summarize the results of point-contact Andreev-reflection (PCAR) spectroscopy in MgB2 doped by chemical substitutions, either magnetic (Mn) or non-magnetic (Al, C), obtained by us and by other groups in the last four years. Despite the variety of samples used (crystals and polycrystals of various origin) and some minor differences in the experimental techniques, these measurements have directly provided a complete and consistent picture of the effects of chemical substitutions on the gaps of MgB2 shedding light on other relevant parameters (scattering rates, DOSs) affected by doping. In Al-doped crystals and polycrystals, the gap amplitudes Δσ and Δπ – obtained through a two-band Blonder-Tinkham-Klapwijk (BTK) fit of the Andreev-reflection conductance curves – decrease on increasing the Al content x (i.e. on decreasing the critical temperature of the contacts T c A ), but remain clearly distinct with no evidence of gap merging even when T c A is of the order of 10 K. The case of C-substituted MgB2 is less clear: the merging of the two gaps at x ≃ 0.13 was observed in single crystals grown at ETH, while the persistence of two gaps up to x = 0.10 – with apparently no tendency to merging at higher x – was found in other crystals and polycrystals. Finally, in Mn-doped single crystals the effects of pair-breaking magnetic scattering strongly suppress the critical temperature and the gaps, which however remain clearly distinct from each other down to Tc ≈ 10 K. In all cases, the analysis of the Δσ and Δπ data as a function of T c A (or of the doping content x) within the two-band Eliashberg theory gives interesting information about the effects of substitutions on the different scattering channels (interband and intraband, magnetic or non-magnetic).

We studied the effects of co-doping with Li and Al on the energy gaps of MgB2 by performing point-contact Andreev-reflection spectroscopy (PCAR) in polycrystalline Mg1−x(AlαLi1−α)xB2 samples with x≤0.4. Even though the lattice parameters and the critical temperature of the compound simply scale with the effective Al content αx, irrespective of the Li concentration, the energy gaps do not. In particular, for a given effective Al content, the comparison with Mg1−y(Al)yB2 with y = αx shows that the σ bandgap is practically the same while the π bandgap is higher. A clear gap merging is observed in the most doped sample (x = 0.4) when Tc

We present the first results of directional point-contact spectroscopy in high quality CaC6 samples both along the ab plane and in the c-axis direction. The superconducting order parameter \Delta(0), obtained by fitting the Andreev-reflection (AR) conductance curves at temperatures down to 400 mK with the single-band 3D Blonder-Tinkham-Klapwijk model, presents two different distributions in the two directions of the main current injection, peaked at 1.35 and 1.71 meV, respectively. By ab-initio calculations of the AR conductance spectra, we show that the experimental results are in good agreement with the recent predictions of gap anisotropy in CaC6., Comment: 4 pages, 4 color eps figures. New results at 400 mK added; figures and discussion modified accordingly. Phys. Rev. Lett., accepted

We present new results of point-contact Andreev-reflection (PCAR) spectroscopy in single-phase Mg_{1-x}Al_{x}B_{2} single crystals with x up to 0.32. Fitting the conductance curves of our point contacts with the two-band Blonder-Tinkham-Klapwijk model allowed us to extract the gap amplitudes Delta_{sigma} and Delta_{pi}. The gap values agree rather well with other PCAR results in Al-doped crystals and polycrystals up to x=0.2 reported in literature, and extend them to higher Al contents. In the low-doping regime, however, we observed an increase in the small gap Delta_{pi} on increasing x (or decreasing the local critical temperature of the junctions, T_{c}^{A}) which is not as clearly found in other samples. On further decreasing T_{c}^{A} below 30 K, both the gaps decrease and, up to the highest doping level x=0.32 and down to T_{c}^{A}= 12 K, no gap merging is observed. A detailed analysis of the data within the two-band Eliashberg theory shows that this gap trend can be explained as being mainly due to the band filling and to an increase in the interband scattering which is necessary to account for the increase in Delta_{pi} at low Al contents (x < 0.1). We suggest to interpret the following decrease of Delta_{pi} for T_{c}^{A} < 30 K as being governed by the onset of inhomogeneity and disorder in the Al distribution that partly mask the intrinsic effects of doping and is not taken into account in standard theoretical approaches., 22 pages, 9 eps figures, Elsevier style. Theoretical details added in appendix. Characterization of crystals included

We summarize and compare the results obtained by point-contact spectroscopy (PCS) in high-quality single crystals and polycrystals of MgB2 with partial Al or C substitutions or irradiated with neutrons. We determined the values of the gaps Delta(sigma) and Delta(pi) by fitting the conductance curves of point contacts in the Andreev-reflection regime with the generalized BTK model. We always studied the temperature and magnetic-field dependencies of the conductance curves, so as to determine the critical temperature of the contact, T-c(A) and highlight whether one or two gaps were present. The dependence of the gaps on TcA shows a dominant effect of the band filling not only in Mg1-xAlxB2 and Mg(B1-xCx)(2) but also, unexpectedly, in irradiated MgB2. The increase in interband scattering is instead smaller than expected in Al-doped and irradiated MgB2 but is very large in C-doped single crystals, probably for extrinsic effects. (c) 2007 Elsevier B.V. All rights reserved.

We present the results of point-contact spectroscopy (PCS) measurements in single-phase Mg1−x Al x B2 crystals and polycrystals with x up to 0.32, recently grown at ETH (Zurich) and University of Genova, respectively. The differential conductance curves of our “soft” point contacts were measured as a function of temperature from 4.2 K up to the Andreev critical temperature T , at which the normal-state conductance is recovered. The gap amplitudes, Δ σ and Δ π , were obtained through a two-band BTK fit of the normalized conductance curves. The results extend previous data to the range of heavy Al doping and show that contrary to expectations, no gap merging is observed up to x=0.32 (i.e., down to T =9 K). The analysis of the gap trends within the two-band Eliashberg theory shows that band filling is the main effect of Al doping, even though a small increase in interband scattering might be necessary to account for the experimental data in single crystals.

Spectroscopic abundances of sand r-process enriched very metal-poor stars are interpreted as the result of mass transfer in a binary system from an AGB companion assuming an initial composition of the parental cloud pre-enriched in r elements. The spectroscopic determination of [Na/Fe], [Mg/Fe] and [ls/Fe] permits an estimate of the initial AGB stellar mass, while a value [Zr/Nb] ≈ 0 is a nuclear indicator of an extrinsic AGB in a binary system.