Superconductivity-induced Phonon Renormalization on   NaFe$_{1-x}$Co$_{x}$As
  We report a study of the lattice dynamics in superconducting NaFeAs (Tc = 8 K) and doped NaFe0.97Co0.03As (Tc = 20 K) using Raman light scattering. Five of the six phonon modes expected from group theory are observed. In contrast with results obtained on iso-structural and iso-electronic LiFeAs, anomalous broadening of Eg(As) and A1g(Na) modes upon cooling is observed in both samples. In addition, in the Co-doped sample, a superconductivity-induced renormalization of the frequency and linewidth of the B1g(Fe) vibration is observed. This renormalization can not be understood within a single band and simple multi-band approaches. A theoretical model that includes the effects of SDW correlations along with sign-changing s-wave pairing state and interband scattering has been developed to explain the observed behavior of the B1g(Fe) mode. 
Y.J. Um, Yunkyu Bang, B.H. Min, Y.S. Kwon, M. Le Tacon
cond-mat.supr-con cond-mat.str-el