Role of the incommensurate CDW in the nematic response of Sr-doped BaNi₂As₂

Ba_1-xSr_xNi₂As₂ (BSNA) is a charge-ordered pnictide superconductor [1,2] that exhibits a six-fold enhancement of superconductivity near a nematic quantum phase transition [3]. This system was also shown to exhibit a diverging nematic susceptibility in elastoresistivity experiments under applied uniaxial strain [3]. This resistive response becomes strain-hysteretic at an incommensurate charge-density-wave (I-CDW) transition, and the hysteresis temperature dependence matches the temperature dependence of I-CDW intensity as seen by x-ray scattering. However the relationship between the CDW and the fluctuating nematic state has not yet been established. To understand the role of the CDW in the nematic response, we performed an x-ray scattering study of the CDW in BSNA x=0.27 under applied uniaxial strain, with an emphasis on the incommensurate CDW present when the nematic response is strongest. We find a large nematic response in the CDW as well, detected as a breaking of the C4 symmetry induced by the applied strain field. The response follows a Curie-Weiss law closely resembling that observed in transport measurements carried out under the same conditions. Our results indicate that the CDW is strongly coupled to the nematic order parameter and may even be the driver of nematic phenomena in this material.

[1] Lee, S. et al. (2019). Unconventional charge density wave order in the pnictide superconductor Ba(Ni1-xCox)2As2. Physical Review Letters, 122(14), 147601. https://doi.org/10.1103/PHYSREVLETT.122.147601

[2] Lee, S. et al. (2021). Multiple Charge Density Waves and Superconductivity Nucleation at Antiphase Domain Walls in the Nematic Pnictide Ba1-xSrxNi2As2. Physical Review Letters, 127(2), 027602. https://doi.org/10.1103/PHYSREVLETT.127.027602

[3] Eckberg, C. et al. (2019). Sixfold enhancement of superconductivity in a tunable electronic nematic system. Nature Physics 2019 16:3, 16(3), 346–350. https://doi.org/10.1038/s41567-019-0736-9