Point-contact spectroscopy in neutron-irradiated Mg11B2

We report on recent results of point-contact spectroscopy measurements in (MgB2)-B-11 polycrystals irradiated at different neutron fluences up to Phi=1.4x10(20) cm(-2). The point contacts were made by putting a small drop of Ag paint-acting as the counterelectrode-on the cleaved surface of the samples. The gap amplitudes were extracted from the experimental conductance curves, showing Andreev-reflection features, through a two-band Blonder-Tinkham-Klapwijk fit and reported as a function of the Andreev critical temperature of the junctions, T-c(A). The resulting Delta(sigma)(T-c(A)) and Delta(pi)(T-c(A)) curves show a clear merging of the gaps when T-c(A)similar or equal to 9 K that perfectly confirms the findings of specific-heat measurements in the same samples. Anomalous contacts with T-c(A)> T-c (being T-c the bulk critical temperature) were often obtained, particularly in samples irradiated at very high fluences. Their fit gave a different dependence of Delta(pi) on T-c(A). The possible origin of these contacts is discussed in terms of local current-induced annealing and/or nanoscale inhomogeneities observed by STM in the most irradiated samples.