Point-Contact Andreev-Reflection Spectroscopy in Neutron-Irradiated Mg$^{11}$B$_2$

We report recent results of point-contact spectroscopy (PCS) in Mg$^{11}$B$_2$ polycrystalline samples irradiated with neutrons at different fluences up to $\Phi$ = 1.4 $\cdot$ 10$^ {20}$ cm$^{-2}$. A strong depression of the bulk critical temperature $T_c$ down to about 8.7 K was observed after irradiation. The gaps $\Delta_{\pi}$ and $\Delta_{\sigma}$ were obtained from the experimental Andreev-reflection conductance curves 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 $\Delta_{\pi}$($T_c^A$) and $\Delta_{\sigma}$($T_c^A$) curves clearly show a merging of the gaps when $T_c^A <$ 9 K, which perfectly confirms the findings of recent specific-heat measurements in the same samples. ``Anomalous'' contacts with $T_c^A > T_c$ and a different dependence of the gaps on $T_c^A$ with respect to ``standard'' ones were obtained in samples irradiated at the highest fluences. The possible origin of these anomalies is discussed in terms of local current-induced annealing and/or nanoscale inhomogeneities - indeed observed by STM in the most irradiated samples.