Competition between superconductivity and magnetic/nematic order as a source of anisotropic superconducting gap in underdoped Ba₁₋ₓKₓFe₂As₂

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The in-plane London penetration depth, Δλ(T), was measured using a tunnel diode resonator technique in single crystals of Ba₁₋ₓKₓFe₂As₂ with doping levels x ranging from heavily underdoped, x=0.16 (Tc=7 K) to nearly optimally doped, x= 0.34 (Tc=39 K). Exponential saturation of Δλ(T) in the T→0 limit is found in optimally doped samples, with the superfluid density ρₛ(T)≡(λ(0)/λ(T))² quantitatively described by a self-consistent γ-model with two nodeless isotropic superconducting gaps. As the doping level is decreased towards the extreme end of the superconducting dome at x=0.16, the low-temperature behavior of Δλ(T) becomes non-exponential and best described by the power-law Δλ(T)∝ T², characteristic of strongly anisotropic gaps. The change between the two regimes happens within the range of coexisting magnetic/nematic order and superconductivity, x<0.25, and is accompanied by a rapid rise in the absolute value of Δλ(T) with underdoping. This effect, characteristic of the competition between superconductivity and other ordered states, is very similar to but of significantly smaller magnitude than what is observed in the electron-doped Ba(Fe₁₋ₓCoₓ)₂As₂ compounds. Our study suggests that the competition between superconductivity and magnetic/nematic order in hole-doped compounds is weaker than in electron-doped compounds, and that the anisotropy of the superconducting state in the underdoped iron pnictides is a consequence of the anisotropic changes in the pairing interaction and in the gap function promoted by both magnetic and nematic long-range order.