Work

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

Year: 2010

Type: article

Abstract: The thermal conductivity $\ensuremath{\kappa}$ of the iron-arsenide superconductor $\text{Ba}{({\text{Fe}}_{1\ensuremath{-}x}{\text{Co}}_{x})}_{2}{\text{As}}_{2}$ was measured down to 50 mK for a heat current parallel $({\ensuremath{\kappa}}_{c})$ and perpendicular $({\ensuremath{\kappa}}_{a})$ to the tetragonal $c$ axis for seven Co concentrations from underdoped to overdoped regions of the phase diagram $(0.038\ensuremath{\le}x\ensuremath{\le}0.127)$. A residual linear term ${\ensuremath{\kappa}}_{c0}/T$ is observed in the $T\ensuremath{\rightarrow}0$ 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 ${s}_{\ifmmode\pm\else\textpm\fi{}}$ state, for example. The fact that the in-plane residual linear term ${\ensuremath{\kappa}}_{a0}/T$ 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 $\mathbf{k}$ dependence of the gap $\ensuremath{\Delta}(\mathbf{k})$ which produces nodes on a Fermi-surface sheet with pronounced $c$-axis dispersion and deep minima on the remaining, quasi-two-dimensional sheets.

Source: Physical Review B

DOI: 10.1103/physrevb.82.064501