Doping Dependence of Heat Transport in the Iron-Arsenide Superconductor Ba(Fe1-xCox)2As2: From Isotropic to a Strongly k-Dependent Gap Structure

The temperature and magnetic field dependence of the in-plane thermal conductivity kappa of the iron-arsenide superconductor Ba(Fe1-xCox)(2)As-2 was measured down to T similar or equal to 50 mK and up to H = 15 T as a function of Co concentration x in the range 0.048 <= x <= 0.114. At H = 0, a negligible residual linear term in kappa/T as T -> 0 at all x shows that the superconducting gap has no nodes in the ab plane anywhere in the phase diagram. However, while the slow H dependence of kappa(H) at T -> 0 in the underdoped regime is consistent with a superconducting gap that is large everywhere on the Fermi surface, the rapid increase in kappa(H) observed in the overdoped regime shows that the gap acquires a deep minimum somewhere on the Fermi surface. Outside the antiferromagnetic-orthorhombic phase, the superconducting gap structure has a strongly k-dependent amplitude.