Potential Lifshitz transition at optimal substitution in nematic pnictide BaSrNiAs.

BaNiAs is a structural analog of the pnictide superconductor BaFeAs, 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 (). To understand the mechanisms responsible for enhancement of , we present high-resolution angle-resolved photoemission spectroscopy (ARPES) measurements of the BaSrNiAs 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 is maximum. These nested pockets host Ni 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 .