Pressure tuning of charge density waves in superconducting BaNi2(As1-xPx)2

Hydrostatic and uniaxial pressure have proven to be good tools for tuning competing orders, including superconductivity and charge density waves (CDW), in a wide variety of materials [1-2]. These techniques allow to gain new insights into the origin of the different orders and their interplay. The superconductor BaNi2As2 (Tc≈0.6 K) is isostructural with the Fe-based superconductor BaFe2As2 at ambient conditions but does not exhibit long-range magnetic order [3]. In analogy to the spin density wave in BaFe2As2, a degenerate incommensurate CDW is observed, and has been shown to be unconventional in nature [4-5]. Consequently, BaNi2As2 is a potential candidate system for charge driven nematicity, and recent Raman study further unveiled an unusual coupling of nematic fluctuations to the lattice, forming a nematic liquid [6]. Nematicity enhanced Cooper pairing has also been proposed in the Sr-substituted system [7], yet overall the interplay between lattice, charge density waves and superconductivity remains unclear.
To better understand their relation, we present high-resolution X-ray diffraction studies of pure and P-substituted BaNi2As2 single crystals under hydrostatic and uniaxial pressure. ... mehrPressure was applied using a diamond anvil cell and a piezoelectric strain cell, respectively. We found a strong dependence of the charge density waves and the underlying lattice under applied pressure. Our DFT calculations find a phonon instability for all observed CDWs under hydrostatic pressure with similar wave vectors to the experimental one. The calculations suggest an unconventional nature of the CDWs.
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