Observation of Charge Density Waves in Superconducting Ba(Ni_1-xCo_x)₂As₂

Nickel-pnictide compounds exhibit superconductivity at relatively low T_c, compared to isostructural iron-pnictide high-T_c superconductors. As an example, BaNi₂As₂, a non-magnetic analogue of BaFe₂As₂, has T_c of 0.7 K and Co-substitution raises it up to 2.5 K, whereas T_c of BaFe₂As₂ can be as high as 38 K upon doping. This strong suppression of the superconductivity in BaNi₂As₂ suggests the possible existence of competing phases. Here, we present the first observation of charge density wave (CDW) order in BaNi₂As₂. Using x-ray reciprocal space mapping, we observed an incommensurate CDW below 148 K in tetragonal phase and a commensurate CDW below 128 K in triclinic phase. On Co-substitution, the onset of the CDW is suppressed to 70 K at x=0.07 and 0.08, and completely suppressed at x=0.12. We also observe that CDW order is strongly tied to the tetragonal to triclinic phase transition, implying a role of the structural distortion in stabilizing the electronic ordering. These results prove the existence of novel ordered phases in Ni-based superconductors and support a competing order scenario for the coexistence of charge order and superconductivity in nickel-pnictide materials.