The characteristic features of the Ca${}_{10}$(Pt${}_3$As${}_8$)[(Fe${}_{1-x}$Pt${}_x$)${}_2$As${}_2$]${}_5$ (the “10-3-8” phase) superconductor are triclinic symmetry, high anisotropy, and a clear separation of superconducting and antiferromagnetic regions in the $T$ versus doping ($x$) phase diagram, which enables the superconducting gap to be studied without complications due to the coexisting magnetic order. The London penetration depth, measured on the underdoped side of the superconducting “dome” ($x=$ 0.028, 0.041, 0.042, and 0.097), shows behavior remarkably similar to other Fe-based superconductors, exhibiting a robust power law, $\Delta \lambda \left(T\right)=A{T}^n$. The exponent $n$ decreases from 2.36 ($x=$ 0.097, close to the optimal doping) to 1.7 ($x=$ 0.028, a heavily underdoped composition), suggesting that the superconducting gap becomes more anisotropic at the dome edge. A similar trend is found in the lower anisotropy BaFe${}_2$As${}_2$ (“122”) -based superconductors, implying that it is an intrinsic property, unrelated to the coexistence of magnetic order and superconductivity or the anisotropy of the normal state.

• Received 3 November 2011

DOI:https://doi.org/10.1103/PhysRevB.85.020504