Nodes in the gap structure of the iron arsenide superconductor Ba(Fe1-xCox)2As2 from c-axis heat transport measurements

The thermal conductivity kappa of the iron-arsenide superconductor Ba(Fe1-xCox)(2)As-2 was measured down to 50 mK for a heat current parallel (kappa(c)) and perpendicular (kappa(a)) to the tetragonal c axis for seven Co concentrations from underdoped to overdoped regions of the phase diagram (0.038 <= x <= 0.127). A residual linear term kappa(c0)/T is observed in the T -> 0 limit when the current is along the c axis, revealing the presence of nodes in the gap. Because the nodes appear as x moves away from the concentration of maximal T-c, they must be accidental, not imposed by symmetry, and are therefore compatible with an s +/- state, for example. The fact that the in-plane residual linear term kappa(a0)/T is negligible at all x implies that the nodes are located in regions of the Fermi surface that contribute strongly to c-axis conduction and very little to in-plane conduction. Application of a moderate magnetic field (e.g., H-c2/4) excites quasiparticles that conduct heat along the a axis just as well as the nodal quasiparticles conduct along the c axis. This shows that the gap must be very small (but nonzero) in regions of the Fermi surface which contribute significantly to in-plane conduction. These findings can be understood in terms of a strong k dependence of the gap Delta(k) which produces nodes on a Fermi-surface sheet with pronounced c-axis dispersion and deep minima on the remaining, quasi-two-dimensional sheets.