Point-contact Andreev-reflection spectroscopy in MgB2: The role of substitutions

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 TcA), but remain clearly distinct with no evidence of gap merging even when TcA 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 TcA (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).