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.

Point-Contact Andreev-Reflection Spectroscopy in Neutron-Irradiated Mg^(11)B_2 / Gonnelli, Renato; Calzolari, Andrea; Daghero, Dario; Tortello, Mauro; Ummarino, Giovanni; V. A., Stepanov; C., Tarantini; P., Manfrinetti. - ELETTRONICO. - 52:(2007). ((Intervento presentato al convegno APS March Meeting 2007 tenutosi a Denver, Colorado nel 5-9 marzo 2007.

### Point-Contact Andreev-Reflection Spectroscopy in Neutron-Irradiated Mg^(11)B_2

#### Abstract

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.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11583/2503750
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