Orbital Ordering in the Charge Density Wave Phases of BaNi$_2$(As$_{1-x}$P$_x$)$_2$

Abstract: We use resonant X-ray scattering at the nickel L${2,3}$ edges to investigate the interplay between orbital degrees of freedom and charge density waves (CDW) in the superconductor BaNi$_2$(As${1-x}$P$x$)$_2$. Both the incommensurate and commensurate CDWs in this system exhibit strong resonant enhancement with distinct energy and polarization dependencies, indicative of orbital ordering. Azimuthal-angle-dependent measurements reveal a lowering of the local Ni site symmetry, consistent with monoclinic or lower point group symmetry. The scattering signatures of both CDWs are dominated by contributions from Ni d${xz,yz}$ orbitals, with similar orbital character despite their distinct wave vectors. These findings point to a shared orbital-driven formation mechanism and provide new insight into the symmetry breaking and orbital/nematic fluctuations in the high-temperature regime of the superconductor BaNi$2$(As${1-x}$P$_x$)$_2$.