Transport and Spectroscopy Studies of Ultrathin Doped Nickelate Films

The notion of a rational, first-principles based design of a novel superconducting material has intrigued physicists for decades. Recently it was suggested that by enforcing a two- dimensional confinement and tensile strain on LaNiO$_{3}$ films, their electronic structure can be made sufficiently similar to that of the Mott-Hubbard system in the cuprates, possibly inducing an antiferromagnetic insulator-superconductor transition [PRL 100, 016404]. We adopt this approach through the synthesis of ultrathin La$_{x}$Nd$_{1-x}$NiO$_{3}$ and hole-doped La$_{x}$Ba$_{1- x}$NiO$_{3}$, Nd$_{x}$Ba$_{1-x}$NiO$_{3}$ films using molecular beam epitaxy. High structural quality is demonstrated by RHEED oscillations and synchrotron x-ray diffraction. Transport measurement show a transition from metallic behavior to localization for films less than 8 uc thick. Tuning of the La/Nd ratio allows the film to be driven into the antiferromagnetic insulating regime. Surprisingly, Ba incorporation increases the localization in the films, which is in contrast to the metallicity-promoting effect of hole doping in bulk nickelates. X-ray absorption measurements allow us to follow the evolution of the Ni and O orbitals and relate it to the observed transport properties.