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A. Wisniewski, R. Puzniak, J. Judek, C. Krutzler, M. Eisterer, H. W. Weber, J. Jun, S. Kazakov, J. Karpinski:
"Comparison of the influence of carbon substitution and neutron-induced defects on the upper critical field and flux pinning in MgB₂ single crystals";
Superconductor Science and Technology, 20 (2007), 256 - 260.

The superconducting properties of pure and carbon-substituted MgB2 single
crystals were studied prior to and after fast neutron irradiation. The C
substitution significantly enhances the upper critical field Hc2. In comparison
with an unsubstituted crystal, Hc2 parallel to the c-axis in Mg(B0.948C0.052)2,
e.g. at 15 K, is enhanced by a factor of about 2 and the Hc2 anisotropy is
reduced. After fast neutron irradiation very pronounced changes in flux
pinning are observed, both for the pure and for the substituted crystal, and the
impact of the induced defects on the critical current density is significantly
stronger than that of substitution. On the other hand, the changes in Hc2 of
the carbon-substituted crystal are much more pronounced than those caused
by irradiation. The observed changes in Hc2 can be explained by taking the
characteristic length scale of the defects introduced by C substitution and by
neutron irradiation into account.