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Materials with field-tunable polarization are of broad interest to condensed matter sciences and solid-state device technologies. Here, using hydrogen (H) donor doping, we modify the room temperature metallic phase of a perovskite nickelate NdNiO3 into an insulating phase with both metastable dipolar polarization and space-charge polarization. We then demonstrate transient negative differential capacitance in thin film capacitors. The space-charge polarization caused by long-range movement and trapping of protons dominates when the electric field exceeds the threshold value. First-principles calculations suggest the polarization originates from the polar structure created by H doping. We find that polarization decays within ~1 second which is an interesting temporal regime for neuromorphic computing hardware design, and we implement the transient characteristics in a neural network to demonstrate unsupervised learning. These discoveries open new avenues for designing ferroelectric materials and electrets using light-ion doping.

Solid-state ion shuttles are of broad interest in electrochemical devices, nonvolatile memory, neuromorphic computing, and biomimicry utilizing synthetic membranes. Traditional design approaches are primarily based on substitutional doping of dissimilar valent cations in a solid lattice, which has inherent limits on dopant concentration and thereby ionic conductivity. Here, we demonstrate perovskite nickelates as Li-ion shuttles with simultaneous suppression of electronic transport via Mott transition. Electrochemically lithiated SmNiO₃ (Li-SNO) contains a large amount of mobile Li⁺ located in interstitial sites of the perovskite approaching one dopant ion per unit cell. A significant lattice expansion associated with interstitial doping allows for fast Li⁺ conduction with reduced activation energy. We further present a generalization of this approach with results on other rare-earth perovskite nickelates as well as dopants such as Na⁺. The results highlight the potential of quantum materials and emergent physics in design of ion conductors.

Materials with field-tunable polarization are of broad interest to condensed matter sciences and solid-state device technologies. Here, using hydrogen (H) donor doping, we modify the room temperature metallic phase of a perovskite nickelate NdNiO 3 into an insulating phase with both metastable dipolar polarization and space-charge polarization. We then demonstrate transient negative differential capacitance in thin film capacitors. The space-charge polarization caused by long-range movement and trapping of protons dominates when the electric field exceeds the threshold value. First-principles calculations suggest the polarization originates from the polar structure created by H doping. We find that polarization decays within ~1 second which is an interesting temporal regime for neuromorphic computing hardware design, and we implement the transient characteristics in a neural network to demonstrate unsupervised learning. These discoveries open new avenues for designing ferroelectric materials and electrets using light-ion doping. Hydrogen-doping driven metal to ferroelectric phase transition in a complex oxide NdNiO 3 is demonstrated. Transient negative differential capacitance and implementation of polarization decay into neural network for learning are then presented.

Active switching, which enables multifunctionality within a single optical component, is essential for reconfigurable infrared photonic systems such as radiation engineering, sensing, and communication. Metamaterials offer a solution but involve complex design and fabrication. A simpler approach with a planar layered structure becomes promising for offering economical manufacturing, easier integration, and scalability. However, it requires an active medium with giant tunability and effective modulation mechanisms. Here, an electrically controlled reversible infrared switching is demonstrated via a single layer of perovskite nickelate on an opaque substrate. Driven by the evolution of the refractive index during an electrically triggered proton‐mediated metal‐to‐insulator transition, the device transforms from a high reflective (R ≈0.74) to a low reflective state (R ≈0.09) at λ = 7–10 µm. A temperature‐independent perfect absorption (A > 0.99 at λ = 11.6–12.1 µm) emerges in the partially hydrogenated state with the mixture of the metal and insulator phases, which results in a modulation of emissivity ≈0.623 at λ = 7–14 µm. The switching behavior is tunable over a wide temperature and wavelength range, offering a versatile path for adaptive infrared applications. Electrically controlled optical switch and perfect absorption are achieved in the long wavelength infrared range by modulating the proton‐mediated metal‐insulator phase mixture of a single layer of SmNiO3 on an opaque sapphire substrate. Such an infrared switch is reversible and tunable over a broad temperature and wavelength range, which could shed light on the design of reconfigurable photonics.

Transition-metal oxide perovskites usually exhibit mixed ionic and electronic conductivity and have been widely investigated as electrode materials for use in solid-oxide fuel cells. Recently, samarium nickelate SmNiO 3 was found experimentally to show promising potential for use in proton-conducting fuel cells. To understand the ionic conductivity of SmNiO 3 , the oxygen and proton diffusion therein are investigated via density functional theory calculations in this work. Based on the vacancy hopping mechanism, oxygen diffusion in SmNiO 3 shows a migration barrier of 0.84 eV. The proton diffusion is studied in terms of different diffusion mechanisms, including reorientation, intraoctahedral, and interoctahedral hopping. The migration barrier for intraoctahedral migration is calculated to be lower than that for interoctahedral hopping. To realize long-range diffusion, the proton is predicted to exhibit reorientation and intraoctahedral hopping. These findings provide a theoretical guide for the development of mixed ionic and electronic perovskite conductors.

We synthesized crystalline films of neodymium nickel oxide (NdNiO ), a perovskite quantum material, switched the films from a metal phase (intrinsic) into an insulator phase (electron-doped) by field-driven lithium-ion intercalation, and characterized their structural and optical properties. Time-of-flight secondary-ion mass spectrometry (ToF-SIMS) showed that the intercalation process resulted in a gradient of the dopant concentration along the thickness direction of the films, turning the films into insulator–metal bilayers. We used variable-angle spectroscopic ellipsometry to measure the complex refractive indices of the metallic and insulating phases of NdNiO . The insulator phase has a refractive index of ∼ 2 and low absorption in the visible and near-infrared, and analysis of the complex refractive indices indicated that the band gap of the insulating phase is roughly 3–4 eV. Electrical control of the optical band gap, with corresponding large changes to the optical refractive indices, creates new opportunities for tunable optics.

We present two ideas to simplify the measurement and analysis of terahertz time-domain spectroscopic ellipsometry data of ultrathin films. The measurement is simplified by using a specially designed sample holder with mirrors, which can be mounted on a cryostat. It allows us to perform spectroscopic ellipsometry by simply inserting the holder into a conventional terahertz spectroscopy system for measurements in transmission geometry. The analysis of the obtained data is simplified by considering a single interface with a certain sheet conductivity σ s (since the film thickness is significantly smaller than the wavelength of the terahertz light). We demonstrate the application of these ideas by evaluating the sheet conductivities of two perovskite rare-earth nickelate thin films in the temperature range 78–478 K. The use of this particular analytical method and the sample holder design will help to establish terahertz time-domain spectroscopic ellipsometry as a characterization technique for ultrathin films.

Point defects, such as oxygen vacancies, control the physical properties of complex oxides, relevant in active areas of research from superconductivity to resistive memory to catalysis. In most oxide semiconductors, electrons that are associated with oxygen vacancies occupy the conduction band, leading to an increase in the electrical conductivity. Here we demonstrate, in contrast, that in the correlatedelectron perovskite rare-earth nickelates, RNiO₃ (R is a rare-earth element such as Sm or Nd), electrons associated with oxygen vacancies strongly localize, leading to a dramatic decrease in the electrical conductivity by several orders of magnitude. This unusual behavior is found to stem from the combination of crystal field splitting and filling-controlled Mott–Hubbard electron–electron correlations in the Ni 3d orbitals. Furthermore, we show the distribution of oxygen vacancies in NdNiO₃ can be controlled via an electric field, leading to analog resistance switching behavior. This study demonstrates the potential of nickelates as testbeds to better understand emergent physics in oxide heterostructures as well as candidate systems in the emerging fields of artificial intelligence.

Point defects, such as oxygen vacancies, control the physical properties of complex oxides, relevant in active areas of research from superconductivity to resistive memory to catalysis. In most oxide semiconductors, electrons that are associated with oxygen vacancies occupy the conduction band, leading to an increase in the electrical conductivity. Here we demonstrate, in contrast, that in the correlated-electron perovskite rare-earth nickelates, RNiO3 (R is a rare-earth element such as Sm or Nd), electrons associated with oxygen vacancies strongly localize, leading to a dramatic decrease in the electrical conductivity by several orders of magnitude. This unusual behavior is found to stem from the combination of crystal field splitting and filling-controlled Mott–Hubbard electron–electron correlations in the Ni 3d orbitals. Furthermore, we show the distribution of oxygen vacancies in NdNiO3 can be controlled via an electric field, leading to analog resistance switching behavior. Here, this study demonstrates the potential of nickelates as testbeds to better understand emergent physics in oxide heterostructures as well as candidate systems in the emerging fields of artificial intelligence.

The discovery of superconducting states in the nickelate thin film with infinite-layer structure has paved a new way for studying unconventional superconductivity. So far, research in this field is still very limited due to difficulties in sample preparation. Here we report the successful preparation of the superconducting state of Nd 0.8 Sr 0.2 NiO 2 thin film ( T c = 8.0–11.1 K) and study the stability of such films in the ambient environment, water, and under electrochemical conditions. Our work demonstrates that the superconducting state of Nd 0.8 Sr 0.2 NiO 2 is remarkably stable, which can last for at least 47-day continuous exposure to air at 20°C and 35% relative humidity. We also show that the superconductivity disappears after being immersed in de-ionized water at room temperature for 5 h. Surprisingly, it can also survive under ionic liquid gating conditions with an applied voltage of about 4 V, which is even more stable than conventional perovskite complex oxides.