Epitaxial growth, optical and electrical conductivity of the metallic pyrochlore Bi2Ru2O7 on Y-stabilized ZrO2 substrate

Epitaxial heterostructures composed of complex correlated metal oxides, grown along specific crystallographic orientations, offer a route to investigating emergent phenomena such as topological states and spin liquids through geometrical lattice engineering. A2Ru2O7 pyrochlore ruthenates, in particular, exhibit a metal–insulator transition with varying A cation, whose mechanism is not fully understood. We report on the epitaxial growth, and structural and electrical properties of metallic pyrochlore bismuth ruthenate heterostructures, grown along both the [001] and [111] directions. Ordered pyrochlore thin films were obtained with a highly oriented texture along the [001] and [111] crystallographic directions. Density functional theory calculations of the electronic band structure and density of states indicated that Bi2Ru2O7 is semimetallic and that hybridization of the Ru 4d and Bi 6p orbitals via the anion network at the Fermi energy was responsible for the metallicity. Electrical conductivity measurements confirmed that the compound is weakly metallic, in agreement with the reported conductivity for the stoichiometric bulk compound. The carrier concentration and mobility of the electrons compared favorably with previous reports on bulk material and indicate strong electron–electron interactions. The measured and computed optical conductivities were found to share coincident spectral features and confirm the electronic correlation. Comparison of the electrical and optical properties of the two distinct orientations indicates differences that cannot be attributed to differences in crystalline quality or dislocations and may indicate anisotropy in the electronic structure of Bi2Ru2O7. This study will enable access to the kagome lattice arising naturally in the 111 planes of the pyrochlore B cation sublattice, which may be used to uncover emergent topological properties.