Iwasaki, H., Y. Dalichaouch, J. T. Markert, G. Nieva, C. L. Seaman, and M. B. Maple. “On narrowing of the resistive superconducting transition in applied magnetic fields and positive curvature in Hc2(T) in the (Gd1−xPrx)Ba2Cu3O7−δ system.” *Physica C: Superconductivity* 169, no. 1-2 (1990): 146–151.

The superconducting transition temperature Tc and the temperature dependence of the upper critical field Hc2 were measured resistively on polycrystalline samples in the (Gd1−xPrx)Ba2Cu3O7−δ system. The experimental results reveal that Tc decreases monotonically with increasing x and vanishes at x≈0.49. Whereas Hc2 of a sample with x=0.10 has an almost linear temperature dependence and the superconducting transition broadens with increasing magnetic field, a sample with x=0.45 shows strong positive curvature in the Hc2 versus temperature curve and narrowing of the superconducting transition width with increasing magnetic field. Both phenomena observed in the sample with x=0.45 can be explained qualitatively in terms of a negative exchange interaction between the superconducting charge carriers and the Pr 4f electrons.