Charge density waves and soft phonon evolution in the superconductor BaNi2⁢(As1−𝑥⁢P𝑥)2

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 have 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 (As1−x P x )2(x 0.12) and down to 2.2 K. ... mehrWe 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 temper-
atures, 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.

The superconductor BaNi2⁢As2
exhibits a soft-phonon-driven, incommensurate charge density wave (I-CDW) which is accompanied by a small orthorhombic structural phase transition. Upon further cooling, BaNi2⁢As2
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 have 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 BaNi2⁢(As1−x⁢Px)2⁢(𝑥≲0.12
... mehr
) 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.