Doping evolution of the superconducting gap structure in heavily hole-doped Ba₁₋ₓKₓFe₂As₂: heat transport study

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We performed systematic thermal conductivity measurements on heavily hole-doped Ba₁₋ₓKₓFe₂As₂ single crystals with 0.747 ≤ x ≤ 0.974. At x = 0.747, the κ₀/T is negligible, indicating nodeless superconducting gap. A small residual linear term κ₀/T (≈ 0.035 mW/K² cm) appears at x = 0.826, and it increases slowly up to x = 0.974, followed by a drastic increase of more than 20 times to the pure KFe₂As₂ (x = 1.0). This doping dependence of κ₀/T clearly shows that the nodal gap appears near x = 0.8, likely associated with the change of Fermi surface topology. The small values of κ₀/T from x = 0.826 to 0.974 support a ""-shaped nodal s-wave gap recently revealed by angle-resolved photoemission spectroscopy experiments at x = 0.9. Furthermore, the drastic increase of κ₀/T from x = 0.974 to 1.0 is inconsistent with a symmetry-imposed d-wave gap in KFe₂As₂, and the possible nodal gap structure in KFe₂As₂ is discussed.