Electronic Structure of the Electron Nematic BaNi₂As₂

The interplay between competing ordered states and superconductivity has long been identified as key to understanding high-temperature superconductivity. In the case of the iron pnictide superconductors, superconductivity coexists with SDW order and electronic nematicity. BaNi₂As₂, a structural analogue of the high temperature pnictide superconductor BaFe₂As₂ has recently been reported to host CDW order and electronic nematicity in conjunction with superconductivity. Interestingly, as a function of Sr doping, CDW and nematicity in the system is suppressed. Upon suppression of this order to absolute zero, a dramatic, six-fold enhancement of the superconducting transition temperature is observed. This system could thus serve as a model system to study the enhancement of superconductivity in the vicinity of quantum critical fluctuations. Here we present detailed high-resolution ARPES measurements of the Sr_xBa_1-xNi₂As₂ series as a function of temperature, comparing the measured band dispersion and Fermi surfaces to density functional theory calculations of the electronic structure carried out in different phases. Utilizing both the experimental and theoretical data, we discuss various mechanisms for the CDW and electronic nematicity.