Doping dependence of heat transport in the iron-arsenide superconductor Ba(Fe\(_{1-x}\)Co\(_x\))\(_2\)As\(_2\): from isotropic to strongly \(k\)-dependent gap structure

Abstract

The temperature and magnetic field dependence of the in-plane thermal conductivity \(\kappa\) of the iron-arsenide superconductor Ba(Fe\(_{1-x}\)Co\(_x\))\(_2\)As\(_2\) was measured down to \(T \simeq 50\) mK and up to \(H = 15\) T as a function of Co concentration \(x\) in the range 0.048 \( \leq x \leq \) 0.114. In zero magnetic field, a negligible residual linear term in \(\kappa/T\) as \(T \to 0\) 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 \(\kappa\) 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 \(\kappa (H)\) at \(T \to 0\), 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.