Direct observation of an incommensurate charge density wave in the BiS2-based superconductor NdO1-xFxBiS2

The nature of superconductivity in BiS2-based superconductors has been controversial while ab initio calculations proposed this system in close proximity to a charge-density-wave (CDW) phase. Using high-energy high-flux x-ray diffraction, we reveal an intrinsic and long-range CDW phase coexisting with superconductivity in NdO1-xFxBiS2 superconductor (x = 0.37 and 0.3). The CDW wave vector in NdO0.63F0.37BiS2 correspond Q(CDW) = (0.17, 0.17, 0.5) and is associated with transverse atomic displacements. Interestingly, this wave vector does not match theoretical expectations based on either phonon softening or Fermi surface nesting. In NdO0.7F0.3BiS2, where the superconducting transition temperature is highest, the CDW satellites are slightly broader and weaker compared to NdO0.63F0.37BiS2, possibly suggesting the competition with the superconductivity. Last, we measure a thermal diffuse scattering across the superconducting transition temperature and find no meaningful changes. Our result suggests the importance of understanding CDW which might hold a key to the superconductivity in the BiS2-based superconductor.