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.
Abstract: 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.
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Maple, M. B., B. W. Lee, J. J. Neumeier, G. Nieva, L. M. Paulius, and C. L. Seaman. "Hybridization, hole localization and pair breaking in the high Tc superconducting system Y1-xPrxBa2Cu3O7-δ." Journal of Alloys and Compounds 181, no. 1-2 (1992): 135–152.
Abstract: The Y1-xPrxBa2Cu3O7-δ (δ ≈ 0.05) system has a rich temperature—praseodymium concentration (T-x) phase diagram consisting of a metal—insulator transition at xcr≈0.6, high Tc superconductivity in the metallic phase (0x≤xcr), and antiferromagnetic ordering of the Cu2+ and praseodymium ions at Néel temperatures TN(Cu) >TN(Cu)>TN(Pr) in the insulating phase (xcr≤x1). In the metallic phase, Tc decreases monotonically with x and vanishes near xcr, the dependence of Tc on pressure P and the upper critical field on temperature are both anomalous, and the low temperature specific heat can be resolved into a large γΓ contribution, reminiscent of heavy fermion behavior, and a spin Kondo anomaly; in the insulating phase, the values of TN(Pr) are anomalously large. These phenomena are briefly reviewed within the framework of a picture that is based on appreciable hybridization of the praseodymium localized 4f states and the CuO2 valence band states. A phenomenological model that incorporates the annihilation of mobile holes in the CuO2 planes and superconducting electron pair breaking by the praseodymium ions can describe Tc (x,P) for x ≤ 0.2.
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Maple, M. B., N. Y. Ayoub, J. Beille, T. Bjornholm, Y. Dalichaouch, E. A. Early, S. Ghamaty, B. W. Lee, J. T. Markert, J. J. Neumeier et al. "Experiments on Electron-and Hole-Doped High Tc Copper Oxide Superconductors." Transport Properties of Superconductors (1990): 536.
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