Thin Film Alchemy: Using Epitaxial Engineering to Unleash the Hidden Properties of Oxide

Unparalleled properties—those of hidden ground states—are being unleashed by exploiting large strains in concert with the ability to precisely control dimensionality and stabilize metastable phases in epitaxial oxide heterostructures. For example, materials that are not ferroelectric or ferromagnetic in their unstrained state can be transmuted into materials that are both at the same time. Similarly, we exploit epitaxial engineering to tune the band structure of the complex oxide ruthenates: CaRuO₃, SrRuO₃, and BaRuO₃, with the perovskite structure as well as their twodimensional counterparts Ca₂RuO₄, Sr₂RuO₄, and Ba₂RuO₄. The band structure is revealed by high-resolution angle-resolved photoemission (ARPES) on pristine as-grown surfaces of these complex oxides made possible by a direct ultra-high vacuum connection between molecular-beam epitaxy (MBE) and ARPES systems. Our work demonstrates the possibility of utilizing strain engineering as a disorder-free means to manipulate emergent properties and many-body interactions in correlated materials.