Metal-Insulator Transition of strained SmNiO3 Thin Films: Structural, Electrical and Optical Properties

Valence-Ordered Thin-Film Nickelate with Tri-component Nickel Coordination Prepared by Topochemical Reduction

The metal-hydride-based "topochemical reduction" process has produced several thermodynamically unstable phases across various transition metal oxide series with unusual crystal structures and nontrivial ground states. Here, by such an oxygen (de-)intercalation method we synthesis a samarium nickelate with ordered nickel valences associated with tri-component coordination configurations. This structure, with a formula of Sm9Ni9O22 as revealed by four-dimensional scanning transmission electron microscopy (4D-STEM), emerges from the intricate planes of {303}pc ordered apical oxygen vacancies. X-ray spectroscopy measurements and ab initio calculations show the coexistence of square planar, pyramidal, and octahedral Ni sites with mono-, bi-, and tri-valences. It leads to an intense orbital polarization, charge-ordering, and a ground state with a strong electron localization marked by the disappearance of ligand-hole configuration at low temperature. This nickelate compound provides another example of previously inaccessible materials enabled by topotactic transformations and presents an interesting platform where mixed Ni valence can give rise to exotic phenomena.

Small-polaron transport in perovskite nickelates

Knowledge of the explicit mechanisms of charge transport is preeminent for a fundamental understanding of the metal-to-insulator transition in ABO₃-type perovskite rare-earth nickelates and for potential applications of these technologically promising materials. Here we suggest that owing to intrinsic Jahn-Teller-driven carrier localization, small-polaron transport is innate in nickelates. We demonstrate experimental evidence for such transport by investigating AC conductivity over a broad range of temperatures and frequencies in epitaxial SmNiO₃ films. We reveal the hopping mechanism of conductivity, Holstein-type activation energy for hopping, nonclassical relaxation behavior, and nonclassical consistency between activation and relaxation. By analyzing these observations, we validate small-polaron transport. We anticipate that our findings can lead to precise tailoring of the DC and AC conductivity in nickelates as requested for fruitful employment of these materials. We also believe that further investigations of self-trapped small polarons are essential for a comprehensive understanding of nickelates.

A Review of Phase-Change Materials and Their Potential for Reconfigurable Intelligent Surfaces

Phase-change materials (PCMs) and metal-insulator transition (MIT) materials have the unique feature of changing their material phase through external excitations such as conductive heating, optical stimulation, or the application of electric or magnetic fields, which, in turn, results in changes to their electrical and optical properties. This feature can find applications in many fields, particularly in reconfigurable electrical and optical structures. Among these applications, the reconfigurable intelligent surface (RIS) has emerged as a promising platform for both wireless RF applications as well as optical ones. This paper reviews the current, state-of-the-art PCMs within the context of RIS, their material properties, their performance metrics, some applications found in the literature, and how they can impact the future of RIS.

Band Gaps and Optical Properties of RENiO3 upon Strain: Combining First-Principles Calculations and Machine Learning

Rare earth nickel-based perovskite oxides (RENiO₃) have been widely studied over recent decades because of their unique properties. In the synthesis of RENiO₃ thin films, a lattice mismatch frequently exists between the substrates and the thin films, which may affect the optical properties of RENiO₃. In this paper, the first-principles calculations were employed to study the electronic and optical properties of RENiO₃ under strain. The results showed that with the increase in tensile strength, the band gap generally shows a widening trend. For optical properties, the absorption coefficients increase with the enhancement of photon energies in the far-infrared range. The compressive strain increases the light absorption, while the tensile strain suppresses it. For the reflectivity spectrum in the far-infrared range, a minimum reflectivity displays around the photon energy of 0.3 eV. The tensile strain enhances the reflectivity in the range of 0.05-0.3 eV, whereas it decreases it when the photon energies are larger than 0.3 eV. Furthermore, machine learning algorithms were applied and found that the planar epitaxial strain, electronegativity, volume of supercells, and rare earth element ion radius play key roles in the band gaps. Photon energy, electronegativity, band gap, the ionic radius of the rare earth element, and the tolerance factor are key parameters significantly influencing the optical properties.

Synthesis of rhombohedral LaCuO3 thin films using the oxidation effect of NaClO solution

LaCuO₃ is one of the most fundamental cuprate perovskite blocks and has a similar structure to the cuprate high-temperature superconductor. Therefore, some consider that a study on LaCuO₃ is one of the clues to clarify the mechanism of cuprate high-temperature superconductivity. However, since the synthesis of bulk LaCuO₃ is difficult due to the need for the high-pressure and high-temperature (HPHT) method, not enough research on LaCuO₃ has been done so far. In this paper, I report the successful synthesis of rhombohedral LaCuO₃ thin films using the oxidation effect of NaClO solution without using the HPHT method. The nanobeam electron diffraction pattern of the cross-section of LaCuO_3-δ thin film oxidized in NaClO solution at 80 °C for 72 hours demonstrates the formation of rhombohedral LaCuO₃. The amount of oxygen defects in the rhombohedral LaCuO₃ thin films has not been measured directly because it is difficult to do so. However, considering the relationship between the amount of oxygen defects and the symmetry of bulk LaCuO_3-δ, these rhombohedral LaCuO₃ thin films are considered to be very close to the stoichiometric composition. The temperature dependence of the resistance of the thin films indicates that rhombohedral LaCuO₃ is an insulator. Considering the previous studies on the first-principles calculation of stoichiometric LaCuO₃ and this study, the rhombohedral LaCuO₃ close to the stoichiometric composition is presumed to be a Mott insulator.

First principles study on hydrogen doping induced metal-to-insulator transition in rare earth nickelates RNiO3 (R = Pr, Nd, Sm, Eu, Gd, Tb, Dy, Yb)

Rare earth nickelates (RNiO3), consisting of a series of correlated transition metal oxides, have received increasing attention due to their sharp metal-to-insulator transition (MIT). Previous reports focused on understanding the origin and modulation of thermally driven MIT by strain effects, cation doping, or external electric field. Recently, it was reported that isothermal chemical doping of hydrogen can induce MIT and increase resistivity by ∼8 orders of magnitude, which opens up the possibility of utilizing these oxides to develop advanced electronic and sensing devices. In this study, we applied first principles methods to study geometric and electronic structures of MIT driven by hydrogen doping in a series of rare earth nickelates RNiO3 (R = Pr, Nd, Sm, Eu, Gd, Tb, Dy, Yb). Hybrid functional HSE06 calculations predict that all oxides under study exhibit sharp MIT, opening up an ∼3 eV band gap after hydrogen doping, with band gap values slightly increasing from Pr to Yb. We find that the R site elements play a key role in determining hydrogen adsorption energies and hydrogen migration barriers, which controls how difficult it would be for the hydrogen atoms to migrate inside the oxides. Detailed information on geometries, electronic structures, migration barriers and adsorption energies of hydrogen provides guidance for further optimizing these materials for future experiments and applications.

Ultrafast X-Ray Absorption Spectroscopy of Strongly Correlated Systems: Core Hole Effect

In recent years, ultrafast pump-probe spectroscopy has provided insightful information about the nonequilibrium dynamics of excitations in materials. In a typical experiment of time-resolved x-ray absorption spectroscopy, the systems are excited by a femtosecond laser pulse (pump pulse) followed by an x-ray probe pulse after a time delay to measure the absorption spectra of the photoexcited systems. We present a theory for nonequilibrium x-ray absorption spectroscopy in one-dimensional strongly correlated systems. The core hole created by the x ray is modeled as an additional effective potential of the core hole site, which changes the spectrum qualitatively. In equilibrium, the spectrum reveals the charge gap at half-filling and the metal-insulator transition in the presence of the core hole effect. Furthermore, a pump-probe scheme is introduced to drive the system out of equilibrium before the x-ray probe. The effects of the pump pulse with varying frequencies, shapes, and fluences are discussed for the dynamics of strongly correlated systems in and out of resonance. The spectrum indicates that the driven insulating state has a metallic droplet around the core hole. The rich structures of the nonequilibrium x-ray absorption spectrum give more insight into the dynamics of electronic structures.

Strongly correlated perovskite lithium ion shuttles

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.