Enhancement of superconductivity by an external magnetic field in magnetic alloys

Abstract

Abstract. An infinite-volume limit solution of the thermodynamics of a BCS superconductor containing spin 1/2 and 7/2 magnetic impurities, obtained recently in [D. Borycki, J. Maćkowiak, Supercond. Sci. Technol. 24, 035007 (2011)] is exploited to derive the expressions for critical magnetic field Hc(T ). The credibility of the resulting thermodynamically limited theoretical equations, which depend on the magnetic coupling constant g and impurity concentration c, is verified on the experimental data for the following superconducting alloys: LaCe, ThGd and SmRh4B4. Good quantitative agreement with experimental data is found for sufficiently small values of c. The discrepancies between theoretical and experimental values of Hc(T ) for larger values of c in case of LaCe and ThGd are reduced by introducing the concept of the effective temperatureT , which accounts for the Coulomb interactions between the electron gas and impurity ions. At low temperatures, the critical magnetic field is found to increase with decreasing temperature T . This enhancement of the critical magnetic field provides evidence of the Jaccarino-Peter effect, which was experimentally observed in the Kondo systems like LaCe, (La1−xCex)Al2 and also in the pseudoternary compounds, including Sn1−xEuxMo6S8, Pb1−xEuxMo6S8 and La1.2−xEuxMo6S8. The effect of an external magnetic field H on a BCS superconductor perturbed by magnetic impurities was also studied. On these grounds, by analyzing the dependence of superconducting transition temperature Tc on H of (La1−xCex)Al2, we have shown, that for certain parameter values, external magnetic field compensates the destructive effect of magnetic impurities.

Magnetic field dependence of the precursor diamagnetism in La superconductors with magnetic Pr impurities

“…Therefore, it could be expected that a time independent (static) observable like the fluctuation magnetic susceptibility ∆χ will be unaffected by magnetic impurities, apart from the effects associated to possible changes in the superconducting parameters (for a recent theoretical work on this issue see, e.g., Ref. [11]). This is in fact the case of high-T c cuprates: recent measurements [12] have shown that the fluctuation diamagnetism of optimally-doped YBa 2 Cu 3 O 7−δ is not appreciably affected by the presence of magnetic impurities (both in the CuO 2 planes or intercalated between them).…”