Possibility of Horizontal Line Node in KFe$_2$As$_2$ Probed by Muon Spin   Rotation
  The anisotropy and temperature dependence of the magnetic field penetration depth in the iron-based superconductor KFe$_2$As$_2$ have been measured down to 20 mK by means of muon spin rotation. We have observed that the flux-line lattice forms triangular symmetry when we applied the field parallel to c-axis, suggesting the interaction between vortices should be isotropic. The temperature dependence of the penetration depth observd with an applied field parallel and perpendicular to c-axis are different. These results can be accounted for by a superconducting gap function with a horizontal line node in the basal plane. 
Kazuki Ohishi, Yasuyuki Ishii, Hedeto Fukazawa, Taku Saito, Isao   Watanabe, Yoh Kohori, Takao Suzuki, Kunihiro Kihou, Chul-...
cond-mat.supr-con