Potential Lifshitz transition at optimal substitution in nematic pnictide Ba_1-SrNi₂As₂

BaNi₂As₂ is a structural analog of the pnictide superconductor BaFe₂As₂, which, like the iron-based superconductors, hosts a variety of ordered phases including charge density waves (CDWs), electronic nematicity, and superconductivity. Upon isovalent Sr substitution on the Ba site, the charge and nematic orders are suppressed, followed by a sixfold enhancement of the superconducting transition temperature (T). To understand the mechanisms responsible for enhancement of T, we present high-resolution angle-resolved photoemission spectroscopy (ARPES) measurements of the Ba_1-SrNi₂As₂ series, which agree well with our density functional theory (DFT) calculations throughout the substitution range. Analysis of our ARPES-validated DFT results indicates a Lifshitz transition and reasonably nested electron and hole Fermi pockets near optimal substitution where T is maximum. These nested pockets host Ni d orbital compositions, which we associate with the enhancement of nematic fluctuations, revealing unexpected connections to the iron-pnictide superconductors. This gives credence to a scenario in which nematic fluctuations drive an enhanced T.