1201.2966 (K. Cho et al.)

Nodal superconducting gap in optimally charge-doped
Na$_{1-δ}$(Fe$_{1-x}$Co$_x$)As    [PDF]

K. Cho, M. A. Tanatar, N. Spyrison, H. Kim, G. Tan, P. Dai, C. L. Zhang, R. Prozorov

The London penetration depth was measured in single crystals of self-doped Na$_{1-\delta}$FeAs (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 0 to 0.1 (from underdoped to overdoped compositions). In all samples, the low - temperature variation of the penetration depth exhibits a power-law dependence, $\Delta \lambda (T) = AT^n$, with the exponent $n$ as low as $n = 1.1$, which is close to the value expected for a superconducting gap with line nodes. Moreover, even at the optimal doping, the results are incompatible with fully-gapped superconductivity. The full - temperature range superfluid density, $\rho_s(T) = (\lambda(0)/\lambda(T))^2$, is distinctly different from other charge - doped pnictides and is similar to isovalently - substituted BaFe$_2$(As$_{1-x}$P$_x$)$_2$, believed to be a nodal pnictide at the optimal doping. This result breaks the monopoly of isovalently - substituted materials for nodal superconductivity in optimally doped Fe-based superconductors, and is against a recent suggestion that the pnictrogen height controls nodal vs. nodeless structure of the superconducting gap.

View original: http://arxiv.org/abs/1201.2966