Doping Evolution of the Superconducting Gap Structure in Heavily Hole-Doped Ba_1?xK_xFe₂As₂: a Heat Transport Study

We perform systematic thermal conductivity measurements on heavily hole-doped Ba_1?xK_xFe₂As₂ single crystals with 0.747≤x ≤0.974. At x=0.747, the κ₀/T is negligible, indicating a nodeless superconducting gap. A small residual linear term κ₀/T (≈0.035 mW?K^?2cm^?1) appears at x=0.826, and it increases slowly up to x=0.974, followed by a substantial 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, possibly associated with the change of Fermi surface topology. The small values of κ₀/T from x=0.826 to 0.974 are consistent with the ?-shaped nodal s-wave gap recently revealed by angle-resolved photoemission spectroscopy experiments at x=0.9. Furthermore, the substantial increase of κ₀/T from x=0.974 to 1.0 is inconsistent with a symmetry-imposed d-wave gap in KFe₂As₂, and a possible nodal gap structure in KFe₂As₂ is discussed.