Critical fields of superconductors with magnetic impurities

Abstract

The upper critical field $H_{c2}$, the field $H_{c3}$ for nucleation of the surface superconductivity, and the thermodynamic $H_c$ are evaluated within the weak-coupling theory for the isotropic s-wave case and arbitrary transport and pair-breaking scattering. We find that for the standard geometry of a half-space sample in a magnetic field parallel to the surface, the ratio $\mathcal{R}=H_{c3}/H_{c2}$ is within the window $1.55\lesssim \mathcal{R}\lesssim 2.34$, regardless of temperature, magnetic or non-magnetic scattering. While the non-magnetic impurities tend to flatten the $\mathcal{R}\left(T\right)$ variation, the magnetic scattering merely shifts the maximum of $\mathcal{R}\left(T\right)$ to lower temperatures. Surprisingly, while reducing the transition temperature, magnetic scattering has a milder impact on $\mathcal{R}$ than the non-magnetic scattering. The surface superconductivity is quite robust; in fact, the ratio $\mathcal{R}\approx 1.7$ even in the gapless state. We used Eilenberger's energy functional to evaluate the condensation energy $F_c$ and the thermodynamic critical field $H_c$ for any temperature and scattering parameters. By comparing $H_{c2}$ and $H_c$, we find that unlike the transport scattering, the pair-breaking pushes materials toward type-I behavior. We find a peculiar behavior of $F_c$ as a function of the pair-breaking scattering parameter at the low-$T$ transition from gapped to gapless phases, which has recently been associated with the topological transition in the superconducting density of states.

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