Charge density wave transitions, soft phonon, and possible electronic nematicity in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>BaNi</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:msub><mml:mi>As</mml:mi><mml:mrow><mml:mn>1</mml:mn><mml:mo>−</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub><mml:msub><mml:mi mathvariant="normal">P</mml:mi><mml:mi>x</mml:mi></mml:msub><mml:msub><mml:mrow><mml:mo>)</mml:mo></mml:mrow><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math>
C. Meingast, Anmol Shukla, Liran Wang, R. Heid, F. Hardy, Mehdi Frachet, Kristin Willa, Tom Lacmann, M. Le Tacon, Michael Merz, Amir A. Haghighirad, Thomas Wolf
Abstract
A detailed investigation of ${\mathrm{BaNi}}_{\text{2}}{({\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{P}}_{x})}_{\text{2}}$ single crystals using high-resolution thermal-expansion, heat-capacity, Young's-modulus, and resistivity measurements is presented. The phase diagram of ${\mathrm{BaNi}}_{\text{2}}{({\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{P}}_{x})}_{\text{2}}$ is shown to be much richer than suggested by the original data of Kudo et al. [Phys. Rev. Lett. 109, 097002 (2012)]. The transition to the commensurate charge density wave (C-CDW) is always preceded by a fourfold symmetry-breaking transition associated with the long-range ordering of a strongly fluctuating unidirectional incommensurate charge density wave (I-CDW). Significant precursors above the I-CDW and C-CDW transitions are seen in the thermal expansion and resistivity and are particularly evident in the temperature dependence of the $c/a$ ratio of the lattice parameters. Heat-capacity measurements of the crystals with a higher P content and a higher critical temperature of 3.2 K uncover a Debye-like behavior of a soft-phonon mode with a very low ${\mathrm{\ensuremath{\Theta}}}_{\text{Debye}}$ of roughly 50 K. Associated with this soft phonon are unusually large thermal-expansion anomalies, resulting in logarithmically diverging uniaxial phonon Gr\"uneisen parameters. Young's-modulus data of these higher-${T}_{c}$ crystals exhibit a significant softening in both ${B}_{1g}$ and ${B}_{2g}$ channels, which is argued to be incompatible with nematic criticality and is rather associated with a broad phase transition to an hitherto unknown structure. Possible origins of the increase in the superconducting critical temperature with P substitution are discussed.