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 by 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 by 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.