Abstract
The recent observation of superconductivity in infinite-layer ${\mathrm{Nd}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{NiO}}_{2}$ thin films has attracted a lot of attention, since this compound is electronically and structurally analogous to the superconducting cuprates. Due to the challenges in the phase stabilization upon chemical doping with Sr, we synthesized artificial superlattices of ${\mathrm{LaNiO}}_{3}$ embedded in insulating ${\mathrm{LaGaO}}_{3}$, and we used layer-selective topotactic reactions to reduce the nickelate layers to ${\mathrm{LaNiO}}_{2}$. Hole doping is achieved via interfacial oxygen atoms and tuned via the layer thickness. We used electrical transport measurements, transmission electron microscopy, and x-ray spectroscopy together with ab initio calculations to track changes in the local nickel electronic configuration upon reduction, and we found that these changes are reversible. Our experimental and theoretical data indicate that the doped holes are trapped at the interfacial quadratic pyramidal Ni sites. Calculations for electron-doped cases predict a different behavior, with evenly distributed electrons among the layers, thus opening up interesting perspectives for interfacial doping of transition-metal oxides.
Highlights
Ever since the discovery of unconventional superconductivity in high-Tc cuprates, the search has been ongoing for other 3d transition-metal oxides that exhibit this intriguing quantum state of matter
We used electrical transport measurements, transmission electron microscopy, and x-ray spectroscopy together with ab initio calculations to track changes in the local nickel electronic configuration upon reduction, and we found that these changes are reversible
Based on our combined results from electrical transport, x-ray spectroscopy, and ab initio calculations, we draw the following conclusions: (i) The oxygen reduction of the superlattice is accompanied by a change in the electronic structure, which is characterized by a transition from a negative charge-transfer to a Mott-Hubbard type system
Summary
Ever since the discovery of unconventional superconductivity in high-Tc cuprates, the search has been ongoing for other 3d transition-metal oxides that exhibit this intriguing quantum state of matter. The collapse of the c-axis parameter makes the reduction process trackable by x-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM), even for very thin films and multilayers Theoretical interest in this new, supposedly unconventional superconductor naturally arises from the possible similarity to, or insightful differences from, high-Tc cuprates [14,15,16,17,18]. A recent report shows that optimally self-doped, epitaxial RP thin films exhibit superconductivity [27]. Another advantage of this approach is that the growth of the precursor superlattices is stable and the optimal conditions have been well established [28]
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