Real-Space Observation of Unidirectional Charge Density Wave and Complex Structural Modulation in the Pnictide Superconductor Ba 1- x Sr x Ni 2 As 2 .

Here we use low-temperature and variable-temperature scanning tunneling microscopy to study the pnictide superconductor, Ba 1- x Sr x Ni 2 As 2 . In the low-temperature phase (triclinic phase) of BaNi 2 As 2 , we observe the unidirectional charge density wave (CDW) with Q = 1/3 on both the Ba and NiAs surfaces. On the NiAs surface of the triclinic BaNi 2 As 2 , there are structural-modulation-induced chain-like superstructures with distinct periodicities. In the high-temperature phase (tetragonal phase) of BaNi 2 As 2 , the NiAs surface appears as the periodic 1 × 2 superstructure. Interestingly, in the triclinic phase of Ba 0.5 Sr 0.5 Ni 2 As 2 , the unidirectional CDW is suppressed on both the Ba/Sr and NiAs surfaces, and the Sr substitution stabilizes the periodic 1 × 2 superstructure on the NiAs surface, which enhance the superconductivity in Ba 0.5 Sr 0.5 Ni 2 As 2 . Our results provide important microscopic insights for the interplay among the unidirectional CDW, structural modulation, and superconductivity in this class of pnictide superconductors.