Suppression of ferromagnetism and observation of quantum well states in epitaxial thin films of the cubic ruthenate BaRuO$_{3}$.

The pseudocubic perovskite ruthenates ARuO$_{3}$, where A is alkaline earth metal, are correlated materials where Hund's coupling drives correlations and leads to a low coherence scale, large renormalization, and formation of local moments. The ferromagnetic BaRuO$_{3}$ has an ideal cubic structure and a larger bandwidth, compared to its GdFeO$_{3}$-distorted counterparts, CaRuO$_{3}$ and SrRuO$_{3}$. In stark contrast to SrRuO$_{3}$, which is a Fermi liquid below T$_{C}$, BaRuO$_{3}$ exhibits critical fluctuations near T$_{C}$ that are enhanced under hydrostatic pressure, which suppresses the Fermi liquid coherence scale and T$_{C}$ and drives a crossover into non-FL regime. Here we use ARPES to characterize the momentum-resolved electronic structure of strained ultrathin BaRuO$_{3}$ films grown in situ by molecular beam epitaxy. The films on STO (001) are metallic down to 2 u.c. thickness and manifest clearly defined subbands of well-defined quasiparticles which arise due to quantum confinement effects. We observe that the bands are moderately renormalized compared to bare GGA bands and discover that the ferromagnetism can be suppressed in the atomically thin limit. We discuss our results on BaRuO$_{3}$ in the context of our recent ARPES studies of the other perovskite ruthenates, SrRuO$_{3}$ and CaRuO$_{3}$.