  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
$T_c$.
