Doping-evolution of the superconducting gap in single crystals of (Ca1−xLax)10(Pt3As8)(Fe2As2)5 superconductor from London penetration depth measurements

Abstract

The doping-evolution of the superconducting gap structure in iron-based superconductor (CaLax)10(Pt3As8)(Fe2As2)5 (x = 0.04, 0.06, 0.09, 0.11, and 0.18) was probed by high—resolution measurements of the London penetration depth, . The samples spanned compositions from underdoped to slightly overdoped with superconducting critical temperatures, Tc, from 12.7 K (x = 0.04) through (optimal) 23.3 K (x = 0.11) to 21.9 K (x = 0.18). The low-temperature variation (up to 0.3 Tc) of was analysed using a power-law function, . For compositions close to the optimal doping, (x = 0.09, 0.11, and 0.18), characterized by , shows a tendency to saturation, indicative of a full gap on the Fermi surface. Fitting over the lowest temperature range () gives n = 2.6. This value is well outside the range expected for the line-nodal superconductor. The exponent n decreased to in the two most underdoped compositions x = 0.04 (Tc = 12.7 K) and 0.06 (Tc = 18.2 K), implying the development of a notable gap anisotropy revealed by the enhanced influence of pair-breaking scattering. This decrease is accompanied by a significant increase of the total variation of the penetration depth in a fixed temperature interval (e.g., ). Both the decrease of the exponent and the increase of the absolute value of in the underdoped regime are similar to the observations in other charge-doped iron-based superconductors, such as doped BaFe2As2 and NaFeAs, suggesting a universal behavior in iron-based superconductors.