nodes found of the paring gap in pnictides

The thermal conductivity of the iron-arsenide superconductor Ba Fe1−xCox 2As2 was measured down to 50 mK for a heat current parallel c and perpendicular 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 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 Tc, 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 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., Hc2 /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 k which produces nodes on a Fermi-surface sheet with pronounced
c-axis dispersion and deep minima on the remaining, quasi-two-dimensional sheets.

PHYSICAL REVIEW B 82, 064501 2010