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

TL;DR

This study investigates the doping-dependent anisotropic superconducting gap in Na$_{1- ext{delta}}$(Fe$_{1-x}$Co$_x$)As, revealing a dome-like evolution of the gap anisotropy and power-law behavior of the penetration depth, indicating highly anisotropic gaps even at optimal doping.

Contribution

It provides detailed measurements of the London penetration depth across doping levels, showing unique gap anisotropy behavior compared to other Fe-based superconductors.

Findings

Power-law dependence of penetration depth with doping-dependent exponent.

Dome-like evolution of gap anisotropy with doping.

Superfluid density at optimal doping resembles nodal pnictides.

Abstract

The London penetration depth was measured in single crystals of self-doped Na$_{1-\delta}$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, $\Delta \lambda (T) = AT^n$, 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, $\rho_s(T) = (\lambda(0)/\lambda(T))^2$, 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.