Charge density waves and soft phonon evolution in the superconductor BaNi$_2$(As$_{1-x}$P$_x$)$_2$

TL;DR

This study investigates the complex charge density wave phenomena and phonon behavior in BaNi$_2$(As$_{1-x}$P$_x$)$_2$, revealing that CDW instabilities are not driven by soft phonons and are weakly affected by phosphorus doping.

Contribution

It provides a detailed experimental and theoretical analysis of CDW and phonon evolution in BaNi$_2$(As$_{1-x}$P$_x$)$_2$, clarifying their relationship with structural transitions.

Findings

Most diffuse scattering explained by lattice dynamics calculations.

Phosphorus substitution weakly affects CDW correlation length.

No evidence of soft phonon-driven CDW or structural transitions.

Abstract

The superconductor BaNi$_2$As$_2$ exhibits a soft-phonon-driven, incommensurate charge density wave (I-CDW) which is accompanied by a small orthorhombic structural phase transition. Upon further cooling, BaNi$_2$As$_2$ undergoes a first-order structural transition to a triclinic phase in which a commensurate CDW (C-CDW) appears. The relationship and interplay between the I-CDW, C-CDW and structural phase transitions has remained elusive. To investigate this issue, we present a complementary study of thermal diffuse X-Ray scattering and inelastic X-Ray scattering for phosphorus substituted BaNi$_2$(As$_{1-x}$P$_x$)$_2$ $(x\lessapprox0.12)$ and down to 2.2 K. We show that most of the diffuse scattering signal can be well described by first-principles lattice dynamics calculations. Furthermore, we find that although phosphorus substitution rapidly suppresses the structural transition temperatures, the temperature dependence of the correlation length of the I-CDW fluctuations and the formation of Bragg-like superstructure peaks associated with long-range ordering of this order depends only weakly on the substitution level. Finally, we present the absence of signatures of the I-CDW to C-CDW or triclinic transition in the lattice dynamics, indicating that these instabilities are not (soft) phonon driven.