Nodeless multiband superconductivity in stoichiometric single   crystalline CaKFe$_4$As$_4$
  Measurements of the London penetration depth and tunneling conductance in single crystals of the recently discovered stoicheometric, iron - based superconductor, CaKFe$_4$As$_4$ (CaK1144) show nodeless, two effective gap superconductivity with a larger gap of about 6-9 meV and a smaller gap of about 1-4 meV. Having a critical temperature, $T_{c,onset}\approx$35.8 K, this material behaves similar to slightly overdoped Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ (e.g. $x=$0.54, $T_c \approx$ 34 K)---a known multigap $s_{\pm}$ superconductor. We conclude that the superconducting behavior of stoichiometric CaK1144 demonstrates that two-gap $s_{\pm}$ superconductivity is an essential property of high temperature superconductivity in iron - based superconductors, independent of the degree of substitutional disorder. 
Kyuil Cho, A. Fente, S. Teknowijoyo, M. A. Tanatar, T. Kong, W. Meier,   U. Kaluarachchi, I. Guillam\'on, H. Suderow, S. L. ...
cond-mat.supr-con