Stoichiomery control and electronic and transport properties of pyrochlore Bi2Ir2O7 thin films

Synthesizing stoichiometric and epitaxial thin films of pyrochlore iridates is an essential step toward the experimental realization of unusual topological and magnetic states that are theoretically predicted in this unique spin-orbit coupled material system. Here, we report on the stoichiometry control and electronic and transport properties of pyrochlore iridate Bi$_2$Ir$_2$O$_7$ thin films grown by pulsed laser deposition. The as-grown films form a bilayerlike structure, in which the top surface is highly Ir deficient while the bottom layer is mainly composed of iridium metal. By postannealing the as-deposited films in IrO$_2$ + O$_2$ atmosphere, we improved the stoichiometry and homogeneity through the film thickness with the lattice constant close to the bulk value. Density functional theory calculation in the bulk limit shows a fourfold degenerate Dirac node slightly below the Fermi energy at the X point, along with trivial bands around the point. The projected partial density of states suggests that the states in the vicinity of the Fermi energy (−3 to 0 eV) mainly consist of highly hybridized Ir 5$d$ and O 2$p$ with minor contributions from Bi 6$s$ and 6$p$, while those far below the Fermi energy (−9 to –3 eV) are contributed primarily by the O bands. Transport measurements revealed a weakly metallic behavior at higher temperatures transitioning to a weakly insulating behavior below 150 K, and a low-temperature magnetoresistance qualitatively ascribed to multicarrier and band-structural effects. The transport features are influenced by a density of states sharply peaked at the Fermi energy, and by the coexistence of trivial bands with the Dirac node, as revealed by the density functional theory calculations