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The thermal conductivity k of the iron-arsenide superconductorBa(Fe_{1-x}Co_x)_2As_2 was measured down to 50 mK for a heat current parallel(k_c) and perpendicular (k_a) to the tetragonal c axis, for seven Coconcentrations from underdoped to overdoped regions of the phase diagram (0.038< x < 0.127). A residual linear term k_c0/T is observed in the T = 0 limit whenthe 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 compatiblewith an s_{+/-} state, for example. The fact that the in-plane residual linearterm k_a0/T is negligible at all x implies that the nodes are located inregions of the Fermi surface that contribute strongly to c-axis conduction andvery 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 aswell as the nodal quasiparticles conduct along the c axis. This shows that thegap must be very small (but non-zero) in regions of the Fermi surface whichcontribute significantly to in-plane conduction. These findings can beunderstood in terms of a strong k dependence of the gap Delta(k) which producesnodes on a Fermi surface sheet with pronounced c-axis dispersion and deepminima on the remaining, quasi-two-dimensional sheets.

Nodes in the gap structure of the iron-arsenide superconductor Ba(Fe_{1-x}Co_x)_2As_2 from c-axis heat transport measurements