Critical field of magnesium diboride in substituted and irradiated samples

We studied the upper critical field in poly-crystalline MgB2 samples in which disorder was introduced by different ways: Al substitution and neutron irradiation. We observed that by Al doping critical temperature Tc decreases, the resistivity increases but the upper critical field, Hc2, decreases. This behavior can be explained considering that Al substitutions affect only π bands leaving σ bands in clean regime; the decrease of Hc2 can be correlated with suppression of the larger gap in doped samples. On the other hand neutron irradiation produces an increase of Hc2. Neutron irradiation was carried on up to fluences of 1.4·1020 n/cm2. To obtain a disorder structure homogeneously distributed, the experiment was carried out on bulk samples prepared with the 11B isotope. Up to fluences of 1018 n/cm2 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.