Superconductivity-induced phonon renormalization on NaFe1-xCoxAs

We report a study of the lattice dynamics in superconducting NaFeAs (T-c = 8 K) and doped NaFe0.97Co0.03As (T-c = 20 K) using Raman light scattering. Five of the six phonon modes expected from group theory are observed. In contrast with results obtained on isostructural and isoelectronic LiFeAs, anomalous broadening of E-g(As) and A(1g)(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 B-1g (Fe) vibration is observed. This renormalization cannot be understood within a single band and simple multiband approaches. A theoretical model that includes the effects of spin-density wave correlations along with sign-changing s-wave pairing state and interband scattering has been developed to explain the observed behavior of the B-1g (Fe) mode.