Scaling relations in anisotropic superconductors with strong pair-breaking

Abstract

Following the work of Abrikosov and Gor'kov on the pair-breaking effects, one can derive the temperature dependencies of the electronic specific heat $C_s/T=\gamma^\prime+\mu T^2$ (with the jump at the superconducting transition $\Delta C \propto T_c^3$) for materials with zero Fermi surface average of the order parameter $<\Delta>=0$ (e.g. d-wave) or for those with $<\Delta> \ll \Delta_{max}$ (e.g., $\pm s$ of iron-pnictides) in the presence of strong pair-breaking. Moreover, the London penetration depth satisfies $\lambda^{-2}=\lambda_0^{-2}(1-T^2/T_c^2)$ (or $\lambda -\lambda_0=\beta T^2 $ at low temperatures) and the slope of the upper critical field near $T_c$ is $H_{c2}^\prime \propto T_c$. Remarkably simple relations between these at first sight unrelated quantities take place: $\mu \lambda_0^2 T_c^3/|H_{c2}^\prime|=3\phi_0/8\pi^2$ and $\Delta C \beta^2 T_c^4/|H_{c2,c}^\prime| = \phi_0/16\pi^2 $ are universal constants. The prediction is checked on two samples of Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ and on CeCoIn$_5$ for which the data needed are available.