Effect of impurities on the superheating field of type-II superconductors

Abstract

We consider the effect of nonmagnetic and magnetic impurities on the superheating field Hs in a type-II superconductor. We solved the Eilenberger equations, which take into account the nonlinear pairbreaking of Meissner screening currents, and calculated Hs(T) for arbitrary temperatures and impurity concentrations in a single-band s-wave superconductor with a large Ginzburg-Landau parameter. At low temperatures, nonmagnetic impurities suppress a weak maximum in Hs(T), which has been predicted for the clean limit, resulting, instead, in a maximum of Hs as a function of impurity concentration in a moderately clean limit. It is shown that nonmagnetic impurities weakly affect Hs even in the dirty limit, while magnetic impurities suppress both Hs and the critical temperature Tc. The density of quasiparticles states N(ε) is strongly affected by an interplay of impurity scattering and current pairbreaking. We show that a clean superconductor at H=Hs is in a gapless state, but a quasiparticle gap εg in N(ε) at H=Hs appears as the concentration of nonmagnetic impurities increases. As the nonmagnetic scattering rate α increases above αc=0.36, the quasiparticle gap εg(α) at H=Hs increases, approaching εg≈0.32Δ0 in the dirty limit α≫1, where Δ0 is the superconducting gap parameter at zero field. The effects of impurities on Hs can be essential for the nonlinear surface resistance and superconductivity breakdown by strong RF fields.

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