Nodes in the gap structure of the iron arsenide superconductorBa(Fe1−xCox)

Abstract

Nodes in the gap structure of the iron-arsenide superconductor Ba(Fe 1−x Co x ) 2 As 2 from c-axis heat transport measurements The thermal conductivity κ of the iron-arsenide superconductor Ba(Fe1−xCox)2As2 was measured down to 50 mK for a heat current parallel (κc) and perpendicular (κa) 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 κc0/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 κa0/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 non-zero) 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.

Millimeter-wave surface impedance of optimally-doped Ba(Fe1−xCox)2Barannik1, Cherpak2, Tanatar3 et al. 2013Phys. Rev. BSelf Cite284320View full textAdd to dashboardCite

“…Such behavior was ultimately attributed to the effect of strong pair -breaking scattering [13,21,22], which actually supported s ± pairing model [13], although a possibility remains that nodes in predominantly c-axis direction may influence the in -plane penetration depth as well, since the latter is calculated by a full average over the Fermi surface [23]. Fully gapped superconductivity in BaCo122 at the optimal has been confirmed by measurements of thermal conductivity [24,25].…”

Angular-dependent upper critical field of overdoped Ba(Fe1−xNix)2Murphy1, Tanatar2, Graf3 et al. 2013Phys. Rev. BSelf Cite235150View full textAdd to dashboardCite

“…We speculate, that the dependence may reflect c-axis modulation of the superconducting gap, as suggested by anisotropic penetration depth and thermal conductivity measurements. 53,54,59 VI. ACKNOWLEDGEMENTS …”

Nodal superconductivity in isovalently substituted SrFe2(As1−xPxMurphy1, Strehlow2, Cho3 et al. 2013Phys. Rev. BSelf Cite229160View full textAdd to dashboardCite

“…The superconducting gap in charge -doped BaK122 and BaCo122 is full and isotropic at the optimal doping [7,8]. It becomes anisotropic upon departure from the optimal doping toward either end of the "superconducting dome" and even develops nodes at the extreme doping levels [9][10][11][12][13][14][15]. In sharp contrast, the superconducting gap of isovalently substituted BaP122 reveals line nodes irrespective of the doping level [16][17][18].…”