Epitaxial growth and electron doping of Ba$_{\mathrm{2}}$IrO$_{\mathrm{4}}$ films by molecular beam epitaxy*

Sr$_{\mathrm{2}}$IrO$_{\mathrm{4}}$ has been shown to share many common key signatures of superconductivity with cuprates, such as Fermi arcs and $d$-wave band gap. However, it is difficult to effectively dope enough charge carriers into Sr$_{\mathrm{2}}$IrO$_{\mathrm{4}}$ and drive them into the potential superconducting state. Ba$_{\mathrm{2}}$IrO$_{\mathrm{4}}$ has a number of distinct advantages over Sr$_{\mathrm{2}}$IrO$_{\mathrm{4}}$, such as the straight Ir-O-Ir bonds, providing another great system to be explored. Here, using reactive molecular beam epitaxy, we successfully grew epitaxial thin films of Ba$_{\mathrm{2}}$IrO$_{\mathrm{4}}$ and doped them through chemical substitutions and surface doping with alkaline metal.