Doping-induced vertical line nodes in the superconducting gap of the iron arsenide Ba1−xKxFe2As2 from directional thermal conductivity 

J.-Ph. Reid,¹ M. A. Tanatar,² X. G. Luo,¹ H. Shakeripour,¹ S. Ren ́e de Cotret,¹ N. Doiron-Leyraud,¹ J. Chang,¹ B. Shen,³ H.-H. Wen,^{3, 4} H. Kim,^{2, 5} R. Prozorov,^{2, 5} and Louis Taillefer^{1, 4, ∗}

1.Departement de physique & RQMP, Universit ́e de Sherbrooke, Sherbrooke, Quebec, Canada J1K 2R1
2.Ames Laboratory, Ames, Iowa 50011, USA
3.National Laboratory for Superconductivity, Institute of Physics and Beijing National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, People’s Republic of China
4.Canadian Institute for Advanced Research, Toronto, Ontario, Canada M5G 1Z8
5.Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA
(Dated: August 17, 2011) 

Abstract

The thermal conductivity κ of the iron-arsenide superconductor Ba1−xKxFe2As2 was measured down to 50 mK in a magnetic field up to 15 T, for a heat current parallel and perpendicular to the tetragonal c axis. In the range from optimal doping (x ≃ 0.4) down to x = 0.16, there is no residual linear term in κ(T ) as T → 0, showing that there are no nodes in the superconducting gap anywhere on the Fermi surface. Upon crossing below x = 0.16, a large residual linear term suddenly appears, signaling the onset of nodes in the superconducting gap, most likely vertical line nodes running along the c axis. We discuss two scenarios: (1) accidental nodes in an s-wave gap, resulting from a strong modulation of the gap around the Fermi surface, in which minima deepen rapidly with underdoping; (2) a phase transition from a nodeless s-wave state to a d-wave state, in which nodes are imposed by symmetry.