Critical field of magnesium diboride in substituted and irradiated samples

We studied the upper critical field in poly-crystalline MgB/sub 2/ samples in which disorder was introduced by different ways: Al substitution and neutron irradiation. We observed that by Al doping critical temperature T/sub c/ decreases, the resistivity increases but the upper critical field, H/sub c2/, decreases. This behavior can be explained considering that Al substitutions affect only /spl pi/ bands leaving /spl sigma/ bands in clean regime; the decrease of H/sub c2/ can be correlated with suppression of the larger gap in doped samples. On the other hand neutron irradiation produces an increase of H/sub c2/. Neutron irradiation was carried on up to fluences of 1.4/spl middot/10/sup 20/ n/cm/sup 2/. To obtain a disorder structure homogeneously distributed, the experiment was carried out on bulk samples prepared with the /sup 11/B isotope. Up to fluences of 10/sup 18/ n/cm/sup 2/ the critical temperature is slightly diminished (36 K) and the superconducting properties are improved; the upper critical field is significantly increased: from 13.5 T to 20.3 T at 12 K. For larger neutron fluences the critical temperature is suppressed down to 9.2 K and the superconducting properties come out strongly degraded.