NdNiO3 Films Research Articles

Synthesis of thin film infinite-layer nickelates by atomic hydrogen reduction: Clarifying the role of the capping layer

We present an integrated procedure for the synthesis of infinite-layer nickelates using molecular-beam epitaxy with gas-phase reduction by atomic hydrogen. We first discuss challenges in the growth and characterization of perovskite NdNiO3/SrTiO3, arising from post growth crack formation in stoichiometric films. We then detail a procedure for fully reducing NdNiO3 films to the infinite-layer phase, NdNiO2, using atomic hydrogen; the resulting films display excellent structural quality, smooth surfaces, and lower residual resistivities than films reduced by other methods. We utilize the in situ nature of this technique to investigate the role that SrTiO3 capping layers play in the reduction process, illustrating their importance in preventing the formation of secondary phases at the exposed nickelate surface. A comparative bulk- and surface-sensitive study indicates that the formation of a polycrystalline crust on the film surface serves to limit the reduction process.

Modulated transport and magnetic behavior in antiferromagnetic NdNiO3/SrIrO3 bilayers

Antiferromagnetic spintronics is intrigued due to its unique properties that could break through the restrictions in ferromagnets. A 3d/5d transition metal oxide heterostructure is a good platform in antiferromagnetic spintronics research since the strong spin–orbit coupling in 5d oxides may bring about delicate interaction with the correlation energy and motivate unconventional phenomena. Here, the transport and magnetic characters of bilayers composed of antiferromagnetic 3d perovskite NdNiO3 and 5d perovskite SrIrO3 were investigated. The decreased metal–insulator transition and Néel temperature associated with suppressed negative magnetoresistance, emerged spin-glass like phenomenon, and the humped nonlinear Hall effect were observed in NdNiO3/SrIrO3 bilayers, which were absent in NdNiO3 and SrIrO3 pure films. It suggests the important role of interfacial interaction between NdNiO3 and SrIrO3 in modulating heterostructure transport and magnetic behavior and also manifests that complex magnetic structures might be realized in NdNiO3/SrIrO3 bilayers.

High-temperature resistivity in rare-earth nickelates films

We expand the temperature interval of the resistivity measurement to clarify the charge transport mechanisms in the rare-earth nickelates ReNiO3. Thin films of LaNiO3 demonstrate an expected metallic behavior at high temperatures: a perfect linear rise of the resistivity ρ∝T is confirmed above room temperature. However, NdNiO3 films characterized by a sharp metal-to-insulator transition display a noticeable deviation of their high-temperature resistivity from the metallic linear relation. This deviation is suggested to originate from an additional thermally activated hopping transport. High-temperature hopping conductivity is also found in SmNiO3 films. Carrier localization due to disorder and the formation of small polarons is discussed as being responsible for hopping.

Optical triggering of a metal-insulator transition in neodymium nickelate films

We have studied photostimulated changes in the electrical resistivity of thin epitaxial films of neodymium nickelate NdNiO3 around its sharp metal-to-insulator transition. It is found that intense light irradiation strongly affects the film resistivity below the transition temperature, where the resistivity is repeatedly switched from high insulating values into low metal-phase levels. By varying the irradiation and thermal conditions, we establish that this strong photoresistive effect is likely caused by light-absorption induced heating rather than electronic excitations. Further analysis of the temperature-dependent resistivity predicts a significant drop in the film thermal conductivity at the metal-to-insulator transition. The simultaneous abrupt change in two physical parameters: electrical resistivity and thermal conductivity, is responsible for a considerable magnitude of the observed photoresistive effect. This effect can be utilized in novel electronics applications, such as thin-film photodetectors or photoswitches.

Infrared Nano-Imaging of Electronic Phase across the Metal–Insulator Transition of NdNiO3 Films

NdNiO3 is a typical correlated material with temperature-driven metal–insulator transition. Resolving the local electronic phase is crucial in understanding the driving mechanism behind the phase transition. Here we present a nano-infrared study of the metal–insulator transition in NdNiO3 films by a cryogenic scanning near-field optical microscope. The NdNiO3 films undergo a continuous transition without phase coexistence. The nano-infrared signal shows significant temperature dependence and a hysteresis loop. Stripe-like modulation of the optical conductivity is formed in the films and can be attributed to the epitaxial strain. These results provide valuable evidence to understand the coupled electronic and structural transformations in NdNiO3 films at the nano-scale.

Atomic-scale observation of spontaneous hole doping and concomitant lattice instabilities in strained nickelate films

Thin oxide films are of vast opportunities for modern electronics and can facilitate emergent phenomena by factors absent in the bulk counterparts, such as the ubiquitous epitaxial strain and interfacial charge doping. Here, we demonstrate the twisting of intended bulk-metallic phases in 10-unit-cell LaNiO3, PrNiO3, and NdNiO3 films on (001)-oriented SrTiO3 into distinct charge-lattice entangled states by epitaxial strains. Using atomically-resolved electron microscopy and spectroscopy, the interfacial electron doping into SrTiO3 in the conventional context of band alignments are discounted. Instead, spontaneously doped holes that are localized and at the order of 1013 cm−2 are atomically unraveled across all three heterointerfaces and associated with strain mitigations by the accompanied atomic intermixing with various ionic radii. The epitaxial strains also lead to condensations of monoclinic-C2/c lattice instabilities, which are hidden to the bulk phase diagram. The group-theoretical analysis of characteristic transition pathways unveils the strain resurrection of the hidden C2/c symmetry. While this strain-induced monoclinic phase in LaNiO3 remains metallic at room temperature, those in PrNiO3 and NdNiO3 turn out to be insulating. Such strain-induced monoclinic lattice instabilities and parasitic localized holes go beyond the classical elastic deformations of films upon epitaxial strains and hint on plausible hidden orders in versatile oxide heterostructures with unexpected properties, of which the exploration is only at the infancy and full of potentials.

Observation of Josephson-like Tunneling Junction Characteristics and Positive Magnetoresistance in Oxygen Deficient Nickelate Films of Nd0.8Sr0.2NiO3-δ.

Nickelate films have recently attracted broad attention due to the observation of superconductivity in the infinite layer phase of NdSrNiO (obtained by reducing Sr doped NdNiO films) and their similarity to the cuprates high temperature superconductors. Here, we report on the observation of a new type of transport in oxygen poor NdSrNiO films. At high temperatures, variable range hopping is observed while at low temperatures a novel tunneling behavior is found where a Josephson-like tunneling junction characteristic with serial resistance is revealed. We attribute this phenomenon to coupling between superconductive (S) surfaces of the grains in our Oxygen poor films via the insulating (I) grain boundaries, which yields SIS junctions in series with the normal (N) resistance of the grains themselves. The similarity of the observed conductance spectra to the tunneling junction characteristic with Josephson-like current is striking, and seems to support the existence of superconductivity in our samples.

Impact of Cation Stoichiometry on the Crystalline Structure and Superconductivity in Nickelates

The recent discovery of superconductivity in infinite-layer nickelate films has aroused great interest since it provides a new platform to explore the mechanism of high-temperature superconductivity. However, superconductivity only appears in the thin film form and synthesizing superconducting nickelate films is extremely challenging, limiting the in-depth studies on this compound. Here, we explore the critical parameters in the growth of high-quality nickelate films using molecular beam epitaxy. We found that stoichiometry is crucial in optimizing the crystalline structure and realizing superconductivity in nickelate films. In precursor NdNiO3 films, optimal stoichiometry of cations yields the most compact lattice while off-stoichiometry of cations causes obvious lattice expansion, influencing the subsequent topotactic reduction and the emergence of superconductivity in infinite-layer nickelates. Surprisingly, in-situ reflection high energy electron diffraction indicates that some impurity phases always appear once Sr ions are doped into NdNiO3 although the X-ray diffraction data are of high quality. While these impurity phases do not seem to suppress the superconductivity, their impacts on the electronic and magnetic structure deserve further studies. Our work demonstrates and highlights the significance of cation stoichiometry in the superconducting nickelate family.

From hidden metal-insulator transition to Planckian-like dissipation by tuning the oxygen content in a nickelate

Heavily oxygen-deficient NdNiO3 (NNO) films, which are insulating due to electron localization, contain pristine regions that undergo a hidden metal-insulator transition. Increasing oxygen content increases the connectivity of the metallic regions and the metal-insulator transition is first revealed, upon reaching the percolation threshold, by the presence of hysteresis. Only upon further oxygenation is the global metallic state (with a change in the resistivity slope) eventually achieved. It is shown that sufficient oxygenation leads to linear temperature dependence of resistivity in the metallic state, with a scattering rate directly proportional to temperature. Despite the known difficulties to establish the proportionality constant, the experiments are consistent with a relationship 1/τ = αkBT/ℏ, with α not far from unity. These results could provide experimental support for recent theoretical predictions of disorder in a two-fluid model as a possible origin of Planckian dissipation.

Electrochromic Properties of Perovskite NdNiO3 Thin Films for Smart Windows

Semiconductors with electrically tunable band gaps are of great interest in controlling transparency to electromagnetic radiation. Thin films of perovskite nickelate NdNiO3 (NNO), a class of correl...

Interfacial eg orbital reconstruction: Modulation of Metal–Insulator transitions of ultrathin NdNiO3 films by two-dimensional electronic gas

Tunable metal–insulator transitions (MIT) of ultrathin NdNiO3 (NNO) film grown on the LaAlO3/SrTiO3 two-dimensional electronic gas structures have been investigated by interfacial eg orbital reconstruction. Spectroscopy studies suggest that the polar catastrophe of the two-dimensional electronic gas results in Ni2+ to Ni3+ charge transfer of the NNO owing to the band-alignment condition. The orbital-hybridization of Ni 3d and O 2p can directly increase in-plane eg orbital occupancy, and the physical mechanism and modulation of the MIT can be clearly elucidated. This study not only provides new insight into the relationship between the electronic properties and orbital occupation, but it will also promote future application of epitaxial NNO thin films.

Nucleation and Growth Bottleneck in the Conductivity Recovery Dynamics of Nickelate Ultrathin Films.

We investigate THz conductivity dynamics in NdNiO3 and EuNiO3 ultrathin films (15 unit cells, u.c., ∼5.7 nm thick) following a photoinduced thermal quench into the metallic state and reveal a clear contrast between first- and second-order dynamics. While in EuNiO3 the conductivity recovers exponentially, in NdNiO3 the recovery is nonexponential and slower than a simple thermal model. Crucially, it is consistent with first-order dynamics and well-described by a 2d Avrami model, with supercooling leading to metastable phase coexistence on the nano- to mesoscopic scale. This novel observation is a fundamentally dynamic manifestation of the first-order character of the insulator-to-metal transition, which the nanoscale thickness of our films and their fast cooling rate enable us to detect. The large transients seen in our films are promising for fast electronic (and magnetic) switching applications.

Tunable resistivity exponents in the metallic phase of epitaxial nickelates

We report a detailed analysis of the electrical resistivity exponent of thin films of NdNiO3 as a function of epitaxial strain. Thin films under low strain conditions show a linear dependence of the resistivity versus temperature, consistent with a classical Fermi gas ruled by electron-phonon interactions. In addition, the apparent temperature exponent, n, can be tuned with the epitaxial strain between n = 1 and n = 3. We discuss the critical role played by quenched random disorder in the value of n. Our work shows that the assignment of Fermi/Non-Fermi liquid behaviour based on experimentally obtained resistivity exponents requires an in-depth analysis of the degree of disorder in the material.

Enhanced Electrocatalytic Oxygen Evolution Activity by Tuning Both the Oxygen Vacancy and Orbital Occupancy of B‐Site Metal Cation in NdNiO3

AbstractPerovskite oxides have been explored as promising electrocatalysts for the oxygen evolution reaction (OER), while a lack of understanding of key factors impacting the catalytic activity restricts their further design and development. Here, for the first time, the contributions of oxygen vacancy (VO) and orbital occupancy of B‐site cations to the catalytic activity of NdNiO3 films are systematically investigated. It is found that OER activity follows a typical volcano‐shaped dependence on the oxygen pressure. In the range of 0.2–10 Pa, proper concentration of VO can provide a moderate bonding strength with intermediate hydroxyl OH* and the increased ratio of Ni3+/Ni2+ provides a more favorable occupancy of eg orbital for the catalytic activity; while in the range of 10–60 Pa, insufficient concentration of VO leads to an enhanced strength of hybridization between Ni 3d and O 2p band and thus deteriorated catalytic activity. The superior OER catalytic performance can be only achieved with both appropriate concentration of VO and the ratio of B‐site metal cations with different valences.

Strain Effect on Oxygen Evolution Reaction Activity of Epitaxial NdNiO3 Thin Films.

Epitaxial strain can cause both lattice distortion and oxygen nonstoichiometry, effects that are strongly coupled at heterojunctions of complex nickelate oxides. Here we decouple these structural and chemical effects on the oxygen evolution reaction (OER) by using a set of coherently strained epitaxial NdNiO3 films. We show that within the regime where oxygen vacancies (VO) are negligible, compressive strain is favorable for the OER whereas tensile strain is unfavorable; the former induces orbital splitting, resulting in a higher occupancy in the d3 z2- r2 orbital and weaker Ni-O chemisorption. However, when the tensile strain is sufficientlylarge to promoteVO formation,an increase in the OER is also observed. The partial reduction of Ni3+ to Ni2+ due to VOmakes the eg occupancy slightly larger than unity, which is thought to account for the increased OER activity. Our work highlights that epitaxial-strain-induced lattice distortion and VO generation can be individually or collectively exploited to tune OER activity, which isimportant for the predictive synthesis of high-performance electrocatalysts.

Coexisting first- and second-order electronic phase transitions in a correlated oxide

The explanation and control of phase transitions remain cornerstones of contemporary physics. Landau provided an invaluable insight into the thermodynamics of complex systems by formulating their phase transitions in terms of an order parameter. Within this formulation, continuous evolution of the order parameter away from zero classifies the phase transition as second-order, whereas a discontinuous change signals a first-order transition. Here we show that the temperature-tuned insulator–metal transition in the prototypical correlated electron system NdNiO3 defies this established binary classification. By harnessing a nanoscale optical probe of the local electronic conductivity, we reveal two physically distinct yet concurrent phase transitions in epitaxial NdNiO3 films. Whereas the sample bulk exhibits a first-order transition between metal and insulator phases, we resolve anomalous nanoscale domain walls in the insulating state that undergo a distinctly continuous insulator–metal transition, with hallmarks of second-order behaviour. We ascribe these domain walls to boundaries between antiferromagnetically ordered domains within the charge ordered bulk. The close correspondence of these observations to predictions from a Landau theory of coupled charge and magnetic orders highlights the importance of coupled order parameters in driving the complex phase transition in NdNiO3. A phase transition between metallic and insulating states is observed to simultaneously happen in two ways at once. The bulk of the sample shows an instantaneous jump in the conductivity, while 1D domain walls show a slow switching into a metallic state.

Light Tuning of the Resistance of NdNiO3 Films With CoFe2O4 Capping

Metal–insulator transition (MIT) is generally thought to be difficult to alter once the material has been prepared. In this study, visible light is used to tune the resistance of NdNiO3 films, which are covered by granular CoFe2O4 capping, in an insulating state. With light illumination, the resistance of the NdNiO3 films is enhanced significantly at a low temperature of ≈9 K, which shows non‐volatile behavior. The magnitude of light‐induced resistance increase is dependent on the light wavelength and illumination time, irrespective of whether the time is consecutive or intermittent. Light control of the resistance can be ascribed to photon‐induced release of the monoclinic lattice of NdNiO3, which is clamped by granular CoFe2O4 capping before the light illumination. Our finding provides insight into the optical manipulation of physical properties on an interface and applications of this method in optoelectronics.

Metal-insulator-metal transition in NdNiO3 films capped by CoFe2O4

Metal-insulator transition features as a transformation from a highly charge conductive state to another state where the charge conductivity is greatly suppressed when decreasing the temperature. Here, we demonstrate two consecutive transitions in NdNiO3 films with CoFe2O4 capping, in which the metal-insulator transition occurs at ∼85 K, followed by an unprecedented insulator-metal transition below 40 K. The emerging insulator-metal transition associated with a weak antiferromagnetic behavior is observed in 20 unit cell-thick NdNiO3 with more than 5 unit cell CoFe2O4 capping. Differently, the NdNiO3 films with thinner CoFe2O4 capping only exhibit metal-insulator transition at ∼85 K, accompanied by a strong antiferromagnetic state below 40 K. Charge transfer from Co to Ni, instead of from Fe to Ni, formulates the ferromagnetic interaction between Ni–Ni and Ni–Co atoms, thus suppressing the antiferromagnetic feature and producing a metallic conductive behavior. Furthermore, a phase diagram for the metal-insulator-metal transition in this system is drawn.

Investigation of the metal-insulator transition in NdNiO3 films by site-selective X-ray absorption spectroscopy.

In this work, multifunctional oxide NdNiO3 (NNO) thin films grown on a SrTiO3 (STO) substrate using pulsed-laser deposition are studied. Temperature dependent resistivity measurements revealed that NNO/STO samples exhibit a sharp thickness dependent metal-insulator transition (MIT) over a range of 150-200 K. It is known that the electronic properties of correlated oxides are extremely complex and sensitive to changes in orbital occupancy. To evaluate the changes in the electronic and/or crystallographic structure responsible for the MIT, a site-selective (O, Ni and Nd) X-ray absorption near edge structure (XANES) analysis is performed above and below the transition temperature. Analysis of XANES spectra suggests that: (i) in NNO films nominally trivalent Ni ions exhibit multiple valency (bond disproportionation), (ii) intermetallic hybridization plays an important role, (iii) the presence of strong O 2p-O 2p hole correlation at low temperature results in the opening of the p-p gap and (iv) the valency of Nd ions matches well with that of Nd3+. For NNO films exhibiting a sharp MIT, Ni 3d electron localization and concurrent existence of Ni 3d8 and Ni 3d8L[combining low line]2 states are responsible for the observed transition. At temperatures below the MIT the O 2p-O 2p hole correlation is strong enough to split the O 2p band stabilizing insulating phase. Temperature and thickness dependent differences observed in the site-selective XANES data are discussed in terms of possible mechanisms for the MIT (negative charge-transfer type).

Reproducibility and off-stoichiometry issues in nickelate thin films grown by pulsed laser deposition

Rare-earth nickelates are strongly correlated oxides displaying a metal-to-insulator transition at a temperature tunable by the rare-earth ionic radius. In PrNiO3 and NdNiO3, the transition is very sharp and shows an hysteretic behavior akin to a first-order transition. Both the temperature at which the transition occurs and the associated resistivity change are extremely sensitive to doping and therefore to off-stoichiometry issues that may arise during thin film growth. Here we report that strong deviations in the transport properties of NdNiO3 films can arise in films grown consecutively under nominally identical conditions by pulsed laser deposition; some samples show a well-developed transition with a resistivity change of up to five orders of magnitude while others are metallic down to low temperatures. Through a detailed analysis of in-situ X-ray photoelectron spectroscopy data, we relate this behavior to large levels of cationic off-stoichoimetry that also translate in changes in the Ni valence and bandwidth. Finally, we demonstrate that this lack of reproducibility can be remarkably alleviated by using single-phase NdNiO3 targets.