Effect of Magnetic Impurities in a Two-Band Superconductor: A Point-Contact Study of Mn-Substituted MgB_2 Single Crystals

摘要

We present the first results of directional point-contact measurements in Mg_(1-x)Mn_xB_2 single crystals, with x up to 0.015 and bulk T_c down to 13.3 K. The order parameters Δ_σ and Δ_π were obtained by fitting the conductance curves with the two-band Blonder-Tinkham-Klapwijk model. Both Δ_π and Δ_σ 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 σ band, as also confirmed by first-principles band-structure calculations.