Daniele Preziosi

The difficulty of determining cation concentrations and oxygen stoichiometry in infinite-layer nickelate thin films has so far prevented clear experimental identification of the nickel electron configuration in the superconducting phase. We used soft x-ray absorption spectroscopy to study the successive changes in PrNiO x thin films at various intermediate stages of topotactic reduction with x = 2 – 3 . By comparing the Ni L -edge spectra to single- and double-cluster ligand-field calculations, we find that none of our samples exhibits a pure d 9 configuration. Our quantitative analysis using the charge sum rule shows that even when films are maximally reduced, the spatially averaged number of nickel 3 d holes is 1.38. Superconducting samples have even higher values, calling into question the previously assumed limit of hole doping. Concomitant changes in the oxygen K -edge absorption spectra upon reduction indicate the presence of oxygen 2 p holes, even in the most reduced films. Overall, our results suggest a complex interplay of hole-doping mechanisms resulting from self-doping effects, structural defects, and oxygen nonstoichiometry.

Two-dimensional electron gases (2DEGs) at oxide interfaces are promising for electronics because of desirable ingredients such as spin–orbit coupling and strong correlations that can be leveraged to bridge into spintronics or photonics. In this context, the ability to manipulate oxide 2DEGs via external knobs is particularly important. Here we show that a volatile high-mobility 2DEG can be photogenerated at the interface between SrTiO₃ (001) and infinite-layer NdNiO₂, where such an electronic state is otherwise absent. This allows us to optically switch the 2DEG between ON and OFF, leading to a giant, instantaneous conductivity variation. The key ingredients for this effect are the structural and electronic reconstructions at the NdNiO₂//SrTiO₃ interface, together with a built-in interfacial electric field that promotes the occupation of the Ti 3d_xy band by the photogenerated carriers. By contributing to understanding photoconductance at complex-oxide interfaces, our results pave the way to engineer the photoresponse of strongly correlated electrons.

In transition metal oxides, electron–electron interaction and lattice degree of freedom are basic ingredients of emergent phenomena, such as metal-to-insulator transition (MIT) and superconductivity. Perovskite rare-earth nickelates are largely studied for their temperature-driven MIT, which is accompanied by a breathing-mode distortion and associated with a bond-disproportionation of the expanded (3d8L0) and compressed (3d8L2) NiO6 octahedra. Steric effects control the onset temperature of the MIT, the latter being concomitant or not with a complex antiferromagnetic spin arrangement depending upon the choice of the rare-earth ion (TMIT ≥ TNéel). Interface engineering of oxygen octahedra tilting, as imposed by the symmetry and orientation of the substrate, has resulted in an efficient pathway to modify both TMIT and TNéel, hence suggesting a key role of the electron–phonon coupling for both transport and magnetic properties in nickelate thin films. Here, via a combination of resonant elastic x-ray scattering and transport experiments, we show control over both TMIT and TNéel in heteroepitaxial PbZr0.2Ti0.8O3(d)/NdNiO3(7 nm)//SrTiO3 heterostructures, which are characterized by different strains and polarization states of the PbZr0.2Ti0.8O3 layer grown at different thicknesses d. We found the expected NdNiO3 bulk behavior (TMIT = TNéel), for a fully relaxed PbZr0.2Ti0.8O3 layer showing a monodomain polarization state. On the other side, an almost 30 K difference (TMIT > TNéel), is found for a fully strained PbZr0.2Ti0.8O3 characterized by a multidomain texture of the polarization state. We discuss our results in terms of an altered breathing distortion pattern of the underlying nickelate layer as supported by x-ray absorption spectroscopy measurements. We infer that locally different polar distortions controlled by a combination of polarization direction and strength of the strain state play the main role in the observed TMIT and TNéel variations.

Several reports about infinite‐layer nickelate thin films suggest that the superconducting critical temperature versus chemical doping phase diagram has a dome‐like shape, similar to cuprates. Here, a highly reproducible superconducting state in undoped PrNiO₂ thin films grown on SrTiO₃ are demonstrated. Scanning transmission electron microscopy measurements show coherent infinite‐layer phase with no visible stacking‐fault defects, an overall high structural quality where possible unintentional chemical doping or interstitial oxygen, if present, sum well below the measurable threshold of the technique. X‐ray absorption measurements show very sharp features at the Ni L_3,2‐edges with a large linear dichroism, indicating the preferential hole occupation of Ni¹⁺‐3 orbitals in a square planar geometry. Resonant inelastic X‐ray scattering measurements reveal sharp magnon excitations of 200 meV energy at the magnetic Brillouin zone boundary, highly resonant at the Ni^{1 +} absorption peak. The results indicate that, when properly stabilized, infinite‐layer nickelate thin films are superconducting without chemical doping.

In transition metal oxides electron-electron interaction and lattice degree of freedom are basic ingredients of emergent phenomena, such as metal-to-insulator transition (MIT) and superconductivity. Perovskite rare-earth nickelates are largely studied for their temperature-driven MIT which is accompanied by a breathing mode distortion, and associated to a bond-disproportionation of the expanded (3d8L0) and compressed (3d8L2) NiO6 octahedra. Steric effects control the onset temperature of the MIT, the latter being concomitant or not with a complex antiferromagnetic spin arrangement depending upon the choice of the rare earth ion (TMIT>TNeel). Interface engineering of oxygen octahedra tilting, as imposed by the symmetry and orientation of the substrate, has resulted in an efficient pathway to modify both TMIT and TNeel, hence, suggesting a key role of the electron-phonon coupling for both transport and magnetic properties in nickelate thin films. Here, via a combination of resonant elastic X-ray scattering and transport experiments, we show a control over both TMIT and TNeel in heteroepitaxial PZT(d)/NNO(7 nm)//STO heterostructures, which are characterized by different strains and polarization states of the PZT layer grown at different thicknesses d. We found the expected NNO bulk behaviour, for a fully relaxed PZT layer showing a monodomain polarization state. On the other side, an almost 30 K difference, is found for a fully strained PZT characterized by a multidomain texture of the polarization state. We discuss our results in terms of an altered breathing distortion pattern of the underlying nickelate layer as supported by X-ray absorption spectroscopy measurements. We infer that locally different polar distortions controlled by a combination of polarization direction and strength of the strain state play the main role in the observed TMIT and TNeel variations.