Raman scattering in superconducting NdO1-xFxBiS2 crystals

The recently discovered layered BiS2-based superconductors have attracted a great deal of interest due to their structural similarity to cuprate and iron-pnictide superconductors. We have performed Raman scattering measurements on two superconducting crystals NdO0.5F0.5BiS2 (Tc = 4.5 K) and NdO0.7F0.3BiS2 (Tc = 4.8 K). The observed Raman phonon modes are assigned with the aid of first-principles calculations. The asymmetrical phonon mode around 118 cm-1 reveals a small electron-phonon (e-ph) coupling constant 0.16, which is insufficient to generate superconductivity at ~ 4.5 K. In the Raman spectra there exists a clear temperature-dependent hump around 100 cm-1, which can be well understood in term of inter-band vertical transitions around Fermi surface. The transitions get boosted when the particular rectangular-like Fermi surface meets band splitting caused by spin-orbit coupling. It enables a unique and quantitative insight into the band splitting.