Point-contact study of the role of non-magnetic impurities and disorder in the superconductivity of MgB2

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.