Effect of impurity scattering on superconductivity in K$_2$Cr$_3$As$_3$
  Impurity scattering in a superconductor may serve as an important probe for the nature of superconducting pairing state. Here we report the impurity effect on superconducting transition temperature $T_\mathrm{c}$ in the newly discovered Cr-based superconductor K$_2$Cr$_3$As$_3$. The resistivity measurements show that the crystals prepared using high-purity Cr metal ($\geq$99.99\%) have an electron mean free path much larger than the superconducting coherence length. For the crystals prepared using impure Cr that contains various \emph{nonmagnetic} impurities, however, the $T_\mathrm{c}$ decreases significantly, in accordance with the generalized Abrikosov-Gor'kov pair-breaking theory. This finding supports a non-$s$-wave superconductivity in K$_2$Cr$_3$As$_3$. 
Y. Liu, J. K. Bao, H. K. Zuo, A. Ablimit, Z. T. Tang, C. M. Feng, Z.   W. Zhu and G. H. Cao
cond-mat.supr-con