Electronic structure of PrNiO3 studied by photoemission and x-ray-absorption spectroscopy: Band gap and orbital ordering

Effect of precipitant on pro-oxidative and antibacterial properties of CeO2 nanoparticles - an experimental study

In this study, the synthesis of pro-oxidative cerium-oxide nanozymes (CeO2 NZs) is reliably performed via the co-precipitation method using ceric ammonium nitrate as a precursor and ammonium carbonate as a precipitating agent. Different samples of CeO2 NZs were prepared by varying the amount of the precipitant. The synthesized NZs were characterized by ultraviolet-visible (UV-vis) spectroscopy, particle size analysis, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HRTEM) and then checked for their pro-oxidative (peroxidase and oxidase) activity. Furthermore, we studied the NZ kinetics and antibacterial properties of synthesized samples.

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.

Structural transformation, Griffiths phase and metal-insulator transition in polycrystalline Nd2-x Sr x NiMnO6 (x = 0, 0.2, 0.4, 0.5 and 1) compound

Polycrystalline double perovskite Nd_2-x Sr _x NiMnO₆ (x  =  0, 0.2, 0.4, 0.5 and 1) samples were synthesized using the solid state reaction method. There occurs a structural transformation from monoclinic (P2₁/n, for x  =  0 to x  =  0.5) to cubic (Fm [Formula: see text] m, for x  =  1) with increasing Sr doping. Raman spectroscopy reveals the increase in static disorder with doping. The Curie temperature (T _C) shows a small increase from x  =  0 to 0.5 (T _C ~ 200 K), but for x  =  1, T _C increases drastically upto ~264 K. The deviation of 1/χ(T) from Curie-Weiss behaviour for doped samples with exponent less than one, indicates a development of the Griffiths phase with doping. The systematic reduction in magnetic moment at 5 K suggests an increase in anti-site disorders with doping. Mn 3s x-ray photoemission spectra show an increase in exchange splitting, indicating a decrease in the valency of Mn. The x-ray absorption spectra at Ni and Mn 2p edges show that the formal valence remains 2+  (Ni) and 4+  (Mn) for all the samples, with changes in spectral weights. Ni 2p x-ray photoemission spectra show characteristic feature similar to Ni³⁺ systems, only for x  =  1 sample. Our GGA-based calculations for the ordered supercell, predict half metallic character for doping (x  >  0) samples due to delocalization of Ni e_g orbitals. The calculations with anti-site disorders yield drastic reduction in Ni moments, with the disordered anti-ferromagnetic phase having lowest energy at maximum doping. Temperature dependent resistivity measurements exhibit a clear metallic region for x  =  0.2 sample, while for higher dopings (x  >  0.2), the metallicity gets suppressed due to increase in anti-site disorders in these samples.

Disentangled Cooperative Orderings in Artificial Rare-Earth Nickelates

Coupled transitions between distinct ordered phases are important aspects behind the rich phase complexity of correlated oxides that hinder our understanding of the underlying phenomena. For this reason, fundamental control over complex transitions has become a leading motivation of the designer approach to materials. We have devised a series of new superlattices by combining a Mott insulator and a correlated metal to form ultrashort period superlattices, which allow one to disentangle the simultaneous orderings in RENiO_{3}. Tailoring an incommensurate heterostructure period relative to the bulk charge ordering pattern suppresses the charge order transition while preserving metal-insulator and antiferromagnetic transitions. Such selective decoupling of the entangled phases resolves the long-standing puzzle about the driving force behind the metal-insulator transition and points to the site-selective Mott transition as the operative mechanism. This designer approach emphasizes the potential of heterointerfaces for selective control of simultaneous transitions in complex materials with entwined broken symmetries.

Rare-earth nickelates RNiO3: thin films and heterostructures

This review stands in the larger framework of functional materials by focussing on heterostructures of rare-earth nickelates, described by the chemical formula RNiO₃ where R is a trivalent rare-earth R  =  La, Pr, Nd, Sm, …, Lu. Nickelates are characterized by a rich phase diagram of structural and physical properties and serve as a benchmark for the physics of phase transitions in correlated oxides where electron-lattice coupling plays a key role. Much of the recent interest in nickelates concerns heterostructures, that is single layers of thin film, multilayers or superlattices, with the general objective of modulating their physical properties through strain control, confinement or interface effects. We will discuss the extensive studies on nickelate heterostructures as well as outline different approaches to tuning and controlling their physical properties and, finally, review application concepts for future devices.

Experimental evidence for bipolaron condensation as a mechanism for the metal-insulator transition in rare-earth nickelates

Many-body effects produce deviations from the predictions of conventional band theory in quantum materials, leading to strongly correlated phases with insulating or bad metallic behavior. One example is the rare-earth nickelates RNiO3, which undergo metal-to-insulator transitions (MITs) whose origin is debated. Here, we combine total neutron scattering and broadband dielectric spectroscopy experiments to study and compare carrier dynamics and local crystal structure in LaNiO3 and NdNiO3. We find that the local crystal structure of both materials is distorted in the metallic phase, with slow, thermally activated carrier dynamics at high temperature. We further observe a sharp change in conductivity across the MIT in NdNiO3, accompanied by slight differences in the carrier hopping time. These results suggest that changes in carrier concentration drive the MIT through a polaronic mechanism, where the (bi)polaron liquid freezes into the insulating phase across the MIT temperature.

Gap-state engineering of visible-light-active ferroelectrics for photovoltaic applications

Photoferroelectrics offer unique opportunities to explore light energy conversion based on their polarization-driven carrier separation and above-bandgap voltages. The problem associated with the wide bandgap of ferroelectric oxides, i.e., the vanishingly small photoresponse under visible light, has been overcome partly by bandgap tuning, but the narrowing of the bandgap is, in principle, accompanied by a substantial loss of ferroelectric polarization. In this article, we report an approach, ‘gap-state’ engineering, to produce photoferroelectrics, in which defect states within the bandgap act as a scaffold for photogeneration. Our first-principles calculations and single-domain thin-film experiments of BiFeO₃ demonstrate that gap states half-filled with electrons can enhance not only photocurrents but also photovoltages over a broad photon-energy range that is different from intermediate bands in present semiconductor-based solar cells. Our approach opens a promising route to the material design of visible-light-active ferroelectrics without sacrificing spontaneous polarization.

Overcoming the optical transparency of wide bandgap of ferroelectric oxides by narrowing its bandgap tends to result in a loss of polarization. By utilizing defect states within the bandgap, Matsuo et al. report visible-light-active ferroelectrics without sacrificing polarization.

Ground-state oxygen holes and the metal-insulator transition in the negative charge-transfer rare-earth nickelates

The metal–insulator transition and the intriguing physical properties of rare-earth perovskite nickelates have attracted considerable attention in recent years. Nonetheless, a complete understanding of these materials remains elusive. Here we combine X-ray absorption and resonant inelastic X-ray scattering (RIXS) spectroscopies to resolve important aspects of the complex electronic structure of rare-earth nickelates, taking NdNiO₃ thin film as representative example. The unusual coexistence of bound and continuum excitations observed in the RIXS spectra provides strong evidence for abundant oxygen holes in the ground state of these materials. Using cluster calculations and Anderson impurity model interpretation, we show that distinct spectral signatures arise from a Ni 3d⁸ configuration along with holes in the oxygen 2p valence band, confirming suggestions that these materials do not obey a conventional positive charge-transfer picture, but instead exhibit a negative charge-transfer energy in line with recent models interpreting the metal–insulator transition in terms of bond disproportionation.

Rare-earth perovskite nickelates show intriguing metal–insulator transitions, whose mechanism remains elusive. Here, Bisogni et al. evidenced a 3d⁸ Ni configuration together with abundance of oxygen 2p holes in the ground state of a NdNiO₃ thin film, suggesting a negative charge-transfer scenario.

Orbital Engineering in Nickelate Heterostructures Driven by Anisotropic Oxygen Hybridization rather than Orbital Energy Levels

Resonant inelastic x-ray scattering is used to investigate the electronic origin of orbital polarization in nickelate heterostructures taking LaTiO_{3}-LaNiO_{3}-3×(LaAlO_{3}), a system with exceptionally large polarization, as a model system. We find that heterostructuring generates only minor changes in the Ni 3d orbital energy levels, contradicting the often-invoked picture in which changes in orbital energy levels generate orbital polarization. Instead, O K-edge x-ray absorption spectroscopy demonstrates that orbital polarization is caused by an anisotropic reconstruction of the oxygen ligand hole states. This provides an explanation for the limited success of theoretical predictions based on tuning orbital energy levels and implies that future theories should focus on anisotropic hybridization as the most effective means to drive large changes in electronic structure and realize novel emergent phenomena.

Volume-wise destruction of the antiferromagnetic Mott insulating state through quantum tuning

RENiO3 (RE=rare-earth element) and V2O3 are archetypal Mott insulator systems. When tuned by chemical substitution (RENiO3) or pressure (V2O3), they exhibit a quantum phase transition (QPT) between an antiferromagnetic Mott insulating state and a paramagnetic metallic state. Because novel physics often appears near a Mott QPT, the details of this transition, such as whether it is first or second order, are important. Here, we demonstrate through muon spin relaxation/rotation (μSR) experiments that the QPT in RENiO3 and V2O3 is first order: the magnetically ordered volume fraction decreases to zero at the QPT, resulting in a broad region of intrinsic phase separation, while the ordered magnetic moment retains its full value until it is suddenly destroyed at the QPT. These findings bring to light a surprising universality of the pressure-driven Mott transition, revealing the importance of phase separation and calling for further investigation into the nature of quantum fluctuations underlying the transition.

Facet-Independent Electric-Field-Induced Volume Metallization of Tungsten Trioxide Films

Reversible metallization of band and Mott insulators by ionic-liquid gating is accompanied by significant structural changes. A change in conductivity of seven orders of magnitude at room temperature is found in epitaxial films of WO3 with an associated monoclinic-to-cubic structural reorganization. The migration of oxygen ions along open volume channels is the underlying mechanism.

Pure electronic metal-insulator transition at the interface of complex oxides

In complex materials observed electronic phases and transitions between them often involve coupling between many degrees of freedom whose entanglement convolutes understanding of the instigating mechanism. Metal-insulator transitions are one such problem where coupling to the structural, orbital, charge, and magnetic order parameters frequently obscures the underlying physics. Here, we demonstrate a way to unravel this conundrum by heterostructuring a prototypical multi-ordered complex oxide NdNiO₃ in ultra thin geometry, which preserves the metal-to-insulator transition and bulk-like magnetic order parameter, but entirely suppresses the symmetry lowering and long-range charge order parameter. These findings illustrate the utility of heterointerfaces as a powerful method for removing competing order parameters to gain greater insight into the nature of the transition, here revealing that the magnetic order generates the transition independently, leading to an exceptionally rare purely electronic metal-insulator transition with no symmetry change.

Hybridization-controlled charge transfer and induced magnetism at correlated oxide interfaces

At interfaces between conventional materials, band bending and alignment are classically controlled by differences in electrochemical potential. Applying this concept to oxides in which interfaces can be polar and cations may adopt a mixed valence has led to the discovery of novel two-dimensional states between simple band insulators such as LaAlO3 and SrTiO3. However, many oxides have a more complex electronic structure, with charge, orbital and/or spin orders arising from strong Coulomb interactions between transition metal and oxygen ions. Such electronic correlations offer a rich playground to engineer functional interfaces but their compatibility with the classical band alignment picture remains an open question. Here we show that beyond differences in electron affinities and polar effects, a key parameter determining charge transfer at correlated oxide interfaces is the energy required to alter the covalence of the metal-oxygen bond. Using the perovskite nickelate (RNiO3) family as a template, we probe charge reconstruction at interfaces with gadolinium titanate GdTiO3. X-ray absorption spectroscopy shows that the charge transfer is thwarted by hybridization effects tuned by the rare-earth (R) size. Charge transfer results in an induced ferromagnetic-like state in the nickelate, exemplifying the potential of correlated interfaces to design novel phases. Further, our work clarifies strategies to engineer two-dimensional systems through the control of both doping and covalence.

Charge disproportionation without charge transfer in the rare-earth-element nickelates as a possible mechanism for the metal-insulator transition

We study a model for the metal-insulator (M-I) transition in the rare-earth-element nickelates RNiO3, based upon a negative charge transfer energy and coupling to a rocksaltlike lattice distortion of the NiO6 octahedra. Using exact diagonalization and the Hartree-Fock approximation we demonstrate that electrons couple strongly to these distortions. For small distortions the system is metallic, with a ground state of predominantly d8L character, where L_ denotes a ligand hole. For sufficiently large distortions (δdNi-O∼0.05-0.10  Å), however, a gap opens at the Fermi energy as the system enters a periodically distorted state alternating along the three crystallographic axes, with (d8L_2)S=0(d8)S=1 character, where S is the total spin. Thus the M-I transition may be viewed as being driven by an internal volume "collapse" where the NiO6 octahedra with two ligand holes shrink around their central Ni, while the remaining octahedra expand accordingly, resulting in the (1/2, 1/2, 1/2) superstructure observed in x-ray diffraction in the insulating phase. This insulating state is an example of charge ordering achieved without any actual movement of the charge.

Effects of Ni vacancies and crystallite size on the O 1s and Ni 2p x-ray absorption spectra of nanocrystalline NiO

We have studied the electronic structure of nanocrystalline NiO thin films, grown by radio-frequency magnetron sputtering under different experimental conditions, using x-ray absorption spectroscopy. The O 1s and Ni 2p spectra showed distinct changes as a function of O2 content in the plasma, which were reproduced with cluster model calculations. These changes are attributed to the incrementing of the surface contribution due to a decrease of the crystallite size as the O2 content in the plasma increases, and to the presence of induced nickel vacancies. Thus, the changes in the electronic structure can be related to the modification of structural and transport properties of these nanocrystalline films.

Theory of the magnetic and metal-insulator transitions in RNiO3 bulk and layered structures

A slave rotor--Hartree-Fock formalism is presented for studying the properties of the p-d model describing perovskite transition metal oxides, and a flexible and efficient numerical formalism is developed for its solution. The methodology is shown to yield, within a unified formulation, the significant aspects of the rare-earth nickelate phase diagram, including the paramagnetic metal state observed for the LaNiO3 and the correct ground-state magnetic order of insulating compounds. It is then used to elucidate ground state changes occurring as morphology is varied from bulk to strained and unstrained thin-film form. For ultrathin films, epitaxial strain and charge transfer to the apical out-of-plane oxygen sites are shown to have significant impact on the phase diagram.

Metal-insulator transition and orbital reconstruction in Mott-type quantum wells made of NdNiO3

The metal-insulator transition and the underlying electronic and orbital structure in e(g)(1) quantum wells based on NdNiO3 were investigated by dc transport and resonant soft x-ray absorption spectroscopy. By comparing quantum wells of the same dimension but with two different confinement structures, we explicitly demonstrate that the quantum well boundary condition of correlated electrons is critical to selecting the many-body ground state. In particular, the long-range orderings and the metal-insulator transition are found to be strongly enhanced under quantum confinement by sandwiching NdNiO(3) with the wide-gap dielectric LaAlO(3), while they are suppressed when one of the interfaces is replaced by a surface (interface with vacuum). Resonant spectroscopy reveals that the reduced charge fluctuations in the sandwich structure are supported by the enhanced propensity to charge ordering due to the suppressed e(g) orbital splitting when interfaced with the confining LaAlO3 layer.

Excitonic insulator state in Ta2NiSe5 probed by photoemission spectroscopy

We report on a photoemission study of Ta2NiSe5 that has a quasi-one-dimensional structure and an insulating ground state. Ni 2p core-level spectra show that the Ni 3d subshell is partially occupied and the Ni 3d states are heavily hybridized with the Se 4p states. In angle-resolved photoemission spectra, the valence-band top is found to be extremely flat, indicating that the ground state can be viewed as an excitonic insulator state between the Ni 3d-Se 4p hole and the Ta 5d electron. We argue that the high atomic polarizability of Se plays an important role to stabilize the excitonic state.