Point-contact spectroscopy in neutron-irradiated Mg(11)B(2)

We report on recent results of point-contact spectroscopy measurements in Mg11B2 polycrystals irradiated at different neutron fluences up to =1.41020 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, Tc A. The resulting Tc A and Tc A curves show a clear merging of the gaps when Tc A9 K that perfectly confirms the findings of specific-heat measurements in the same samples. Anomalous contacts with Tc ATc being Tc the bulk critical temperature were often obtained, particularly in samples irradiated at very high fluences. Their fit gave a different dependence of on Tc 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.