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

Abstract

The nature of superconductivity in ${\mathrm{BiS}}_{2}$-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 ${\mathrm{NdO}}_{1\ensuremath{-}x}{\mathrm{F}}_{x}{\mathrm{BiS}}_{2}$ superconductor $(x=0.37 \mathrm{and} 0.3)$. The CDW wave vector in ${\mathrm{NdO}}_{0.63}{\mathrm{F}}_{0.37}{\mathrm{BiS}}_{2}$ correspond ${\mathbf{Q}}_{\mathrm{CDW}}=(0.17, 0.17, 0.5)$ 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 ${\mathrm{NdO}}_{0.7}{\mathrm{F}}_{0.3}{\mathrm{BiS}}_{2}$, where the superconducting transition temperature is highest, the CDW satellites are slightly broader and weaker compared to ${\mathrm{NdO}}_{0.63}{\mathrm{F}}_{0.37}{\mathrm{BiS}}_{2}$, 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 ${\mathrm{BiS}}_{2}$-based superconductor.