Potential Lifshitz transition at optimal substitution in nematic pnictide Ba(1−x)Sr(x)Ni(2)As(2)

BaNi(2)As(2) is a structural analog of the pnictide superconductor BaFe(2)As(2), 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(c)). To understand the mechanisms responsible for enhancement of T(c), we present high-resolution angle-resolved photoemission spectroscopy (ARPES) measurements of the Ba(1−x)Sr(x)Ni(2)As(2) 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(c) is maximum. These nested pockets host Ni d(xz)/d(yz) 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(c).