  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.
