Doping dependence of heat transport in the iron-arsenide superconductor Ba(Fe₁₋ₓCoₓ)₂As₂: from isotropic to strongly k-dependent gap structure

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The temperature and magnetic field dependence of the in-plane thermal conductivity κ of the iron-arsenide superconductor Ba(Fe₁₋ₓCoₓ)₂As₂ was measured down to T ≃ 50 mK and up to H = 15 T as a function of Co concentration x in the range 0.048 ≤ x ≤ 0.114. In zero magnetic field, a negligible residual linear term in κ/T as T → 0 at all x shows that there are no zero-energy quasiparticles and hence the superconducting gap has no nodes in the ab-plane anywhere in the phase diagram. However, the field dependence of κ reveals a systematic evolution of the superconducting gap with doping x, from large everywhere on the Fermi surface in the underdoped regime, as evidenced by a flat κ(H) at T → 0, to strongly k-dependent in the overdoped regime, where a small magnetic field can induce a large residual linear term, indicative of a deep minimum in the gap magnitude somewhere on the Fermi surface. This shows that the superconducting gap structure has a strongly k-dependent amplitude around the Fermi surface only outside the antiferromagnetic/orthorhombic phase.