Electronic structure and strain-induced Lifshitz transition in epitaxial Ba$_{2}$RuO$_{4}$ thin films as studied by ARPES

We employ oxide molecular beam epitaxy and \emph{in situ} ARPES to synthesize epitaxial thin films of Ba$_{2}$RuO$_{4}$, which is isostructural and isoelectronic to the unconventional superconductor Sr$_{2}$RuO$_{4}$, and characterize its Fermi surface topology and multiorbital quasiparticle dynamics. Although Ba$_{2}$RuO$_{4}$ cannot be synthesized as bulk single crystals, we epitaxially stabilize thin films on TbScO$_{3}$ or SrTiO$_{3}$ substrates. We report a full parametrization of the band structure and compare our results to first-principles calculations as well as our data on Sr$_{2}$RuO$_{4}$. Unlike in Sr$_{2}$RuO$_{4}$ we do not observe a surface reconstruction in line with our expectations for the larger Ba cations. We use ARPES to demonstrate that the combination of a larger cation radius, together with epitaxial strain, can be employed to drive a Lifshitz transition in the d$_{xy}$-like $\gamma$ band from electron-like in Sr$_{2}$RuO$_{4}$ to hole-like in Ba$_{2}$RuO$_{4}$. The ability to control the Fermi surface topology by epitaxial strain is a promising tool for investigating the role of the near-E$_{F}$ van Hove singularity in superconductivity and magnetism in ruthenates, as well as a general tool for controlling and studying correlated electronic materials.