Effect of Magnetic Impurities in a Two-Band Superconductor: A Point-Contact Study of Mn-Substituted MgB2 Single Crystals

We present the first results of directional point-contact measurements in Mg(1-x)Mn(x)B(2) single crystals, with x up to 0.015 and bulk T(c) down to 13.3 K. The order parameters Delta(sigma) and Delta(pi) were obtained by fitting the conductance curves with the two-band Blonder-Tinkham-Klapwijk model. Both Delta(pi) and Delta(sigma) decrease with the critical temperature of the junctions T(c)(A), but remain clearly distinct up to the highest Mn content. Once analyzed within the Eliashberg theory, the results indicate that spin-flip scattering is dominant in the sigma band, as also confirmed by first-principles band-structure calculations.