Doping - dependent anisotropic superconducting gap in Na${}_{1-\hat{I}´}$(Fe${}_{1-x}$Co${}_x$)As

Abstract

The London penetration depth was measured in single crystals of self-doped Na${}_{1-\hat{I}´}$FeAs (from under - to optimal - doping, $T_c$ from 14 to 27 K) and electron-doped Na(Fe${}_{1-x}$Co${}_x$)As with $x$ ranging from undoped, $x=0$ to overdoped $x=0.1$. In all samples, the low - temperature variation of the penetration depth exhibits a power-law dependence, $\hat{I}”\hat{I}»(T)=A{T}^n$, with the exponent that varies in a dome - like fashion from $n\sim 1.1$ in the underdoped, reaching a maximum of $n\sim 1.9$ in the optimally doped and decreasing again to $n\sim 1.3$ on the overdoped side. While the anisotropy of the gap structure follows universal dome-like evolution, the exponent at the optimal doping, $n\sim 1.9$, is lower than in other charge - doped Fe-based superconductors (FeSCs). The full - temperature range superfluid density, $\ddot{I}{}_s(T)=(\hat{I}»(0)/\hat{I}»(T){)}^2$, at the optimal doping is also distinctly different from other charge - doped FeSCs but is similar to isovalently - substituted BaFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$, believed to be a nodal pnictide at the optimal doping. These results suggest that the superconducting gap in Na(Fe${}_{1-x}$Co${}_x$)As is highly anisotropic even at the optimal doping.