Origin, symmetry and interplay of charge density waves in superconducting BaNi2(As1-xPx)2

Superconductivity and charge density waves (CDWs) are often observed in close proximity in the phase diagrams of many quantum materials [1], and competition is observed for example in the superconducting cuprates [2]. An example of a material exhibiting both superconductivity and CDWs is BaNi2As2 (Tc≈0.6 K), an isostructural but non-magnetic analog of the Fe-based superconductor BaFe2As2 [3]. In BaNi2As2 an incommensurate (I-CDW) and a commensurate CDW (C-CDW) are observed, which are associated with an orthorhombic and a triclinic structural phase transition, respectively. It has been shown that the I-CDW is unconventional in nature [4]. Upon substitution with strontium or phosphorus, the triclinic phase transition and the corresponding (C-CDW) can be suppressed, and at the point of suppression, the superconducting transition temperature is drastically increased up to 3.5 K [5-6]. The response of the I-CDW formation mechanism to substitution, the origin of the C-CDW, the orbital order of both CDWs, and the interplay of the I-CDW with superconductivity are still unknown.
To address the above questions, we present a detailed diffuse X-ray scattering, inelastic X-ray scattering and resonant elastic X-ray scattering study of pure and P-substituted BaNi2As2 single crystals. ... mehrWe find only a minor alteration of the soft phonon origin of the I-CDW upon phosphorus substitution even at phosphorus contents where the C-CDW is completely suppressed and superconductivity is enhanced. Only the transition temperature is lowered. In contrast to the I-CDW, we could not identify a similar soft phonon origin for the C-CDW. However, both CDWs clearly show an orbital ordering, most likely related to the nickel dxz,yz orbitals. Finally, measurements of the I-CDW in the superconducting phase (2.2 K) showed no competition between the I-CDW and superconductivity.