Potential Lifshitz transition at optimal substitution in nematic pnictide Ba${}_{1-x}$Sr${}_x$Ni${}_2$As${}_2$

Abstract

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 $T_c$.

19 pages, 12 figures, 1 table