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The effects of vacuum and oxygen annealing on Sr2-xNbxIrO4 samples have been systematically investigated. The annealing under vacuum leads to an enhanced insulating state of the Sr2-xNbxIrO4 compounds, which could be due to the evaporation of oxygen atoms which breaks the superexchange interaction between Ir and O ions. The annealing under oxygen atmosphere results in substantial straightening of the in-plane Ir–O–Ir bond and rapid depression of the canted antiferromagnetic ordering state. Importantly, the insulator-to-metal transition has been achieved by oxygen annealing of the Sr2-xNbxIrO4 samples, which could be due to the enhanced hybridization of Ir 5d orbitals with the neighboring O 2p orbitals. The present results suggest that the annealing treatment could be an effective way for exploring of novel physical phenomena in Sr2IrO4 and related compounds. [ABSTRACT FROM AUTHOR]

The effects of vacuum and oxygen annealing on Sr2-xNbxIrO4 samples have been systematically investigated. The annealing under vacuum leads to an enhanced insulating state of the Sr2-xNbxIrO4 compounds, which could be due to the evaporation of oxygen atoms which breaks the superexchange interaction between Ir and O ions. The annealing under oxygen atmosphere results in substantial straightening of the in-plane Ir–O–Ir bond and rapid depression of the canted antiferromagnetic ordering state. Importantly, the insulator-to-metal transition has been achieved by oxygen annealing of the Sr2-xNbxIrO4 samples, which could be due to the enhanced hybridization of Ir 5d orbitals with the neighboring O 2p orbitals. The present results suggest that the annealing treatment could be an effective way for exploring of novel physical phenomena in Sr2IrO4 and related compounds. [ABSTRACT FROM AUTHOR]

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Rare-earth nickelates (RENiO3) show widely tunable metal-to-insulator transition (MIT) properties with ignorable variations in lattice constants and small latent heat across the critical temperature (TMIT). Particularly, it is worth noting that compared with the more commonly investigated vanadium oxides, the MIT of RENiO3 is less abrupt but usually across a wider range of temperatures. This sheds light on their alternative applications as negative temperature coefficient resistance (NTCR) thermistors with high sensitivity compared with the current NTCR thermistors, other than their expected use as critical temperature resistance thermistors. In this work, we demonstrate the NTCR thermistor functionality for using the adjustable MIT of NdxSm1−xNiO3 within 200–100 K, which displays larger magnitudes of NTCR (e.g., more than 7%/K) that is unattainable in traditional NTCR thermistor materials. The temperature dependence of resistance (R–T) shows sharp variation during the MIT of NdxSm1−xNiO3 with no hysteresis via decreasing the Nd content (e.g., x ≤ 0.8), and such a R–T tendency can be linearized by introducing an optimum parallel resistor. The sensitive range of temperature can be further extended to 210–360 K by combining a series of NdxSm1−xNiO3 with eight rare-earth co-occupation ratios as an array, with a high magnitude of NTCR (e.g., 7%–14%/K) covering the entire range of temperatures. [ABSTRACT FROM AUTHOR]

Phase transitions occurring in nonequilibrium conditions can evolve through high-energy intermediate states inaccessible via equilibrium adiabatic conditions. Because of the subtle nature of such hidden phases, their direct observation is extremely challenging and requires simultaneous visualization of matter at subpicoseconds and subpicometer scales. Here, we show that a magnetite crystal in the vicinity of its metal-to-insulator transition evolves through different hidden states when controlled via energy-tuned ultrashort laser pulses. By directly monitoring magnetite's crystal structure with ultrafast electron diffraction, we found that upon near-infrared (800 nm) excitation, the trimeron charge/orbital ordering pattern is destroyed in favor of a phase-separated state made of cubic-metallic and monoclinic-insulating regions. On the contrary, visible light (400 nm) activates a photodoping charge transfer process that further promotes the long-range order of the trimerons by stabilizing the charge density wave fluctuations, leading to the reinforcement of the monoclinic insulating phase. Our results demonstrate that magnetite's structure can evolve through completely different metastable hidden phases that can be reached long after the initial excitation has relaxed, breaking ground for a protocol to control emergent properties of matter. [ABSTRACT FROM AUTHOR]

Abstract Oxide semimetals exhibiting both nontrivial topological characteristics stand as exemplary parent compounds and multiple degrees of freedom, offering a promise for the realization of novel electronic states. In this work, we report the structural and transport phase transition in an oxide semimetal, SrNbO3, achieved through effective anion doping. Notably, the resistivity increased by more than three orders of magnitude at room temperature upon nitrogen‐doping. The extent of electronic modulation in SrNbO3 is strongly correlated with misfit strain, underscoring its phase instability to both chemical doping and crystallographic symmetry variations. Using first‐principles calculations, we discern that elevating the level of nitrogen doping induces an upward shift in the conductive bands of SrNbO3−δNδ. Consequently, a transition from a metallic state to an insulating state becomes apparent as the nitrogen concentration reaches a threshold of 1/3. This investigation shows effective anion engineering in oxide semimetals, offering pathways for manipulating their physical properties.

Although rare-earth nickelates (Re NiO 3) exhibit widely adjustable metal-to-insulator transition (MIT) properties, it is yet difficult to control their material resistivities to cater for potential applications in correlated electronics. Herein, we demonstrate a strategy to regulate the material resistivities of Re NiO 3 while maintaining their relative abrupt MIT properties by compositing with the NiO nanoparticles and afterward high oxygen pressure sintering. Introducing the more insulating NiO as a secondary phase within Re NiO 3 (e.g., NdNiO 3 and SmNiO 3) elevates their material resistivities across two orders of magnitudes while maintaining a similar critical temperature (T MIT) and its resultant resistive change at a moderate compositing ratio (e.g., <80 %). The nearest-neighbors-hopping (NNH) model and linear model were used to fit the low-temperature electric transportation properties for both the insulating and metallic phases of Re NiO 3 , and it indicates that the pristine carrier transportation modes can be well preserved despite a high compositing ratio of NiO up to 80 %. Similar effects cannot be achieved by using other more insulating compositing oxides, such as BaBiO 3 , LaAlO 3 , and NiFe 2 O 4 , as the abruption in resistive change across T MIT was not preserved at a similar high compositing ratio (e.g., >20 %), while their carrier transportation modes varied at much smaller composition ratio. It is worth noticing that the compositing with NiO also improves the mechanical strength of Re NiO 3 , and this benefits their practical applications in electronic devices. [ABSTRACT FROM AUTHOR]

The insulator‐to‐metal transition (IMT) of vanadium dioxide (VO2) at ≈68 °C enables a variety of optical applications, including switching and modulation, and tuning of optical resonators. This work designs and demonstrates a novel thermally activated optical switch consisting of a SiN/VO2/SiN multilayer sandwich structure with an AMONIL‐based grating with a reduced transition temperature of 47 °C. The optical switching in the multilayer is due to the IMT of the VO2‐embedded layer. Here, the asymmetrical TE1 mode exhibiting a quasi‐zero electric field in the center of the multilayer waveguide is excited in the resonant waveguide grating (RWG) structure under normal incidence via the AMONIL‐based grating printed on top, leading to high resonant transmittance (75%) at room temperature. Increasing the temperature to more than 47 °C causes VO2 to undergo an insulator‐to‐metal transition accompanied by optical modifications in the IR region, completely canceling the resonance effect, while reducing the transmittance to 30%. Further, the modeling results aimed at optimizing the design of the experimental structure. These results demonstrate good performance of the proposed design and pave the way to fabricate VO2‐based optical switches for photonics applications including lasers, sensors, and detectors, in which external stimuli such as heat affect the transmittance or reflectance spectrum. [ABSTRACT FROM AUTHOR]

In this work, NbOx-based selector devices were fabricated by sputtering deposition systems. Metal-to-insulator transition characteristics of the device samples were investigated depending on the oxygen flow rate (3.5, 4.5, and 5.5 sccm) and the deposition time. The device stack was scanned by transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS). The yields, including MIT, nonlinear, and Ohmic, in working devices with different deposition conditions were also evaluated. Moreover, we observed the trend in yield values as a function of selectivity. In addition, the current–voltage (I–V) curves were characterized in terms of DC and pulse endurance. Finally, the switching speed and operating energies were obtained by applying a triangular pulse on the devices, and the recovery time and drift-free characteristics were obtained by the paired pulses. [ABSTRACT FROM AUTHOR]

Fundamental understanding and control of the electronic structure evolution in rare‐earth nickelates is a fascinating and meaningful issue, as well as being helpful to understand the mechanism of recently discovered superconductivity. Here the dimensionality effect on the ground electronic state in high‐quality (NdNiO3) m/(SrTiO3)1 superlattices is systematically studied through transport and soft X‐ray absorption spectroscopy. The metal‐to‐insulator transition temperature decreases with the thickness of the NdNiO3 slab decreasing from bulk to 7 unit cells, then increases gradually as m further reduces to 1 unit cell. Spectral evidence demonstrates that the stabilization of insulating phase can be attributed to the increase of the charge‐transfer energy between O 2p and Ni 3d bands. The prominent multiplet feature on the Ni L3 edge develops with the decrease of NdNiO3 slab thickness, suggesting the strengthening of the charge disproportionate state under the dimensional confinement. This work provides convincing evidence that dimensionality is an effective knob to modulate the charge‐transfer energy and thus the collective ground state in nickelates. [ABSTRACT FROM AUTHOR]

Abstract Fundamental understanding and control of the electronic structure evolution in rare‐earth nickelates is a fascinating and meaningful issue, as well as being helpful to understand the mechanism of recently discovered superconductivity. Here the dimensionality effect on the ground electronic state in high‐quality (NdNiO3) m/(SrTiO3)1 superlattices is systematically studied through transport and soft X‐ray absorption spectroscopy. The metal‐to‐insulator transition temperature decreases with the thickness of the NdNiO3 slab decreasing from bulk to 7 unit cells, then increases gradually as m further reduces to 1 unit cell. Spectral evidence demonstrates that the stabilization of insulating phase can be attributed to the increase of the charge‐transfer energy between O 2p and Ni 3d bands. The prominent multiplet feature on the Ni L3 edge develops with the decrease of NdNiO3 slab thickness, suggesting the strengthening of the charge disproportionate state under the dimensional confinement. This work provides convincing evidence that dimensionality is an effective knob to modulate the charge‐transfer energy and thus the collective ground state in nickelates.

Coupling dynamical charges to gauge fields can result in highly non-local interactions with a linear confining potential. As a consequence, individual particles bind into mesons which, in one dimension, become the new constituents of emergent Luttinger liquids (LLs). Furthermore, at commensurate fillings, different Mott-insulating states can be stabilized by including nearest-neighbour (NN) interactions among charges. However, rich phase diagrams expected in such models have not been fully explored and still lack comprehensive theoretical explanation. Here, by combining numerical and analytical tools, we study a simple one-dimensional $\mathbb{Z}_2$ lattice gauge theory at half-filling, where U(1) matter is coupled to gauge fields and interacts through NN repulsion. We uncover a rich phase diagram where the local NN interaction stabilizes a Mott state of individual charges (or partons) on the one hand, and an LL of confined mesons on the other. Furthermore, at the interface between these two phases, we uncover a highly frustrated regime arising due to the competition between the local NN repulsion and the non-local confining interactions, realizing a pre-formed parton plasma. Our work is motivated by the recent progress in ultracold atom experiments, where such simple model could be readily implemented. For this reason we calculate the static structure factor which we propose as a simple probe to explore the phase diagram in an experimental setup.

In this work, NbOx-based selector devices were fabricated by sputtering deposition systems. Metal-to-insulator transition characteristics of the device samples were investigated depending on the oxygen flow rate (3.5, 4.5, and 5.5 sccm) and the deposition time. The device stack was scanned by transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS). The yields, including MIT, nonlinear, and Ohmic, in working devices with different deposition conditions were also evaluated. Moreover, we observed the trend in yield values as a function of selectivity. In addition, the current–voltage (I–V) curves were characterized in terms of DC and pulse endurance. Finally, the switching speed and operating energies were obtained by applying a triangular pulse on the devices, and the recovery time and drift-free characteristics were obtained by the paired pulses.

Complex oxides provide rich physics related to ionic defects. For the proper tuning of functionalities in oxide heterostructures, it is highly desired to develop fast, effective, and low‐temperature routes for the dynamic modification of defect concentration and distribution. Herein, the use of helium implantation to efficiently control the vacancy profiles in epitaxial La0.7Sr0.3MnO3−δ thin films is reported. The viability of this approach is supported by lattice expansion in the out‐of‐plane lattice direction and dramatic change in physical properties, i.e., a transition from ferromagnetic metallic to antiferromagnetic insulating. In particular, a significant increase of resistivity up to four orders of magnitude is evidenced at room temperature, upon implantation of highly energetic He ions. The result offers an attractive means for tuning the emergent physical properties of oxide thin films via strong coupling between strain, defects, and valence. [ABSTRACT FROM AUTHOR]

This review aims to summarize more than 100 years of research on spinel compounds, mainly focusing on the progress in understanding their magnetic, electronic, and polar properties during the last two decades. Over the years, more than 200 different spinels, with the general formula AB 2 X 4 , were identified or synthesized in polycrystalline or single-crystalline form. Many spinel compounds are magnetic insulators or semiconductors; however, a number of spinel-type metals exists including superconductors and some rare examples of d -derived heavy-fermion compounds. In the early days, they gained importance as ferrimagnetic or even ferromagnetic insulators with relatively high saturation magnetization and high ordering temperatures, with magnetite being the first magnetic mineral known to mankind. From a technological point of view, spinel-type ferrites with the combination of high electrical resistance, large magnetization, and high magnetic ordering temperature made them promising candidates for many applications. However, spinels are also known as beautiful gemstones, with the famous "Black Prince's Ruby" in the front centre of the Imperial State Crown. In addition, spinels are important for the earth tectonics, and the detection of magnetite in a Martian meteorite even led to the speculation of life on Mars. However, most importantly in the perspective of this review, spinels played an outstanding role in the development of concepts of magnetism, in testing and verifying the fundamentals of magnetic exchange, in understanding orbital-ordering and charge-ordering phenomena including metal-to-insulator transitions, in developing the concepts of magnetic frustration, in establishing the importance of spin–lattice coupling, and in many other aspects. The still mysterious Verwey transition in magnetite was one of the very first illuminating examples of this complexity, which results from the fact that some ions can exist in different valence states in spinels, even at a given sublattice. In addition, the A -site as well as the B -site cations in the spinel structure form lattices prone to strong frustration effects resulting in exotic ground-state properties. The A -site ions are arranged in a diamond lattice. This bipartite lattice shows highly unusual ground states due to bond-order frustration, with a strength depending on the ratio of inter- to intra-sublattice exchange interactions of the two interpenetrating face-centred cubic lattices. The occurrence of a spiral spin-liquid state in some spinels is an enlightening example. Very recently, even a meron (half-skyrmion) spin structure was identified in MnSc 2 S 4 at moderate external magnetic fields. In case the A -site cation is Jahn–Teller active, additional entanglements of spin and orbital degrees of freedom appear, which can give rise to a spin–orbital liquid or an orbital glass state. In systems with such a strong entanglement, the occurrence of a new class of excitations – spin–orbitons – has been reported. The B -site cations form a pyrochlore lattice, one of the strongest contenders of frustration in three dimensions. A highly degenerate ground state with residual zero-point entropy and short-range spin ordering according to the ice rules is one of the fascinating consequences, which is known already for more than 50 years. At low temperatures, in B -site spinels the occurrence of spin molecules has been reported, strongly coupled spin entities, e.g., hexamers, with accompanying exotic excitations. A spin-driven Jahn–Teller effect is a further possibility to release magnetic frustration. This phenomenon has been tested in detail in a variety of spinel compounds. In addition, in spinels with both cation lattices carrying magnetic moments, competing magnetic exchange interactions become important, yielding ground states like the time-honoured triangular Yafet–Kittel structure. Very recently, it was found that under external magnetic fields this triangular structure evolves into very complex spin orders, which can be mapped on spin super-liquid and spin super-solid phases. In addition, due to magnetic frustration, competing interactions, and coupling to the lattice, very robust magnetization plateaus appear in a variety of spinel compounds as function of an external magnetic field. Furthermore, spinels gained considerable importance in elucidating the complex physics driven by the interplay of spin, charge, orbital, and lattice degrees of freedom in materials with partly filled d shells. This entanglement of the internal degrees of freedom supports an exceptionally rich variety of phase transitions and complex ground states, in many cases with emerging functionalities. It also makes these materials extremely susceptible to temperature, pressure, or external magnetic and electric fields, an important prerequisite to realize technological applications. Finally, yet importantly, there exists a long-standing dispute about the possibility of a polar ground state in spinels, despite their reported overall cubic symmetry. Indeed, recently a number of multiferroic spinels were identified, including multiferroic spin super-liquid and spin super-solid phases. The spinels also belong to the rare examples of multiferroics, where vector chirality alone drives long-range ferroelectric order. In addition, a variety of spinel compounds were investigated up to very high pressures up to 40 GPa and in high magnetic fields up to 100 T, revealing complex (p , T) and (H , T)-phase diagrams. • Ground-breaking concepts in magnetism were developed conceptually using spinel compounds. • Spinels are prototypical examples of strongly frustrated magnets. • Ferroelectricity and multiferroicity in spinels result from very different mechanisms. • Spinels represent outstanding examples of metal-to-insulator transitions and charge order. • Examples of well-established d -derived heavy-fermion​ behaviour. [ABSTRACT FROM AUTHOR]

The science of heterogeneous catalysis is primarily based on surface phenomena, which occur on nanoscale surfaces on active sites with sub-Å dimensions. On these surfaces, only some atomic sites which have a unsaturated valence feature (under-coordinated or coordinately unsaturated) – typically located at steps, edges, kinks, corners, and other discontinuities, are discerned to be the active sites, i.e., active for surface processes, including binding and adsorption, surface reactions, and desorption of products. These sites have a profound effect on surface reactivity, on account of their "under-coordinated" feature, and a highly electronic character, with a high electron density in the valence band, evidenced by its location around the Fermi level. In part I of this Series, we offered new knowledge-based analysis of the geometric effects/properties of the nanoclusters and active sites, which are governed by a specific size (also termed as "quantum size," at nm scale) and shape/morphology. Here, in part II, we elaborate on the electronic properties which are evident as a result of typical metallic to nonmetallic transition of small metallic particles at the quantum size of 2–5 nm. With several practical research examples/case studies from recent literature, we illustrate how the techniques of core-level XPS and its analogs, AP-XPS and NAP-XPS, and STM/STS can be utilized to investigate and discern the electronic structure of catalyst surfaces and interfaces, including electronic band gaps, onset of valence and conduction band transitions, assessment of cluster size effects on band gaps, topographical features with atomic-scale resolution, presence of defects/charge centers at the metal-support interface, and chemical imaging via chemical contrast. We also introduce and discuss new concepts such as "Brønsted–Evans–Polanyi (B.E.P.) relationships" and "volcano curves," and illustrate how the information gathered from surface-sensitive characterization techniques and first-principles calculations can be used to design more effective and efficient catalysts with increased chemical selectivity. We will also illustrate the efficacy of two interrelated techniques, STEM-EELS and STXM-XAS, which can provide insights into the atomic-scale (sub-Å) features of these nanoclusters, and gather corroborative evidences for other studies. This information can provide novel insights into the scientific basis for first-principles design of new catalysts and nanostructured materials and adds to the current understanding about behavior of these nanomaterials under in-situ conditions. [ABSTRACT FROM AUTHOR]

Benefitting from exceptional energy storage performance, dielectric-based capacitors are playing increasingly important roles in advanced electronics and high-power electrical systems. Nevertheless, a series of unresolved structural puzzles represent obstacles to further improving the energy storage performance. Compared with ferroelectrics and linear dielectrics, antiferroelectric materials have unique advantages in unlocking these puzzles due to the inherent coupling of structural transitions with the energy storage process. In this review, we summarize the most recent studies about in-situ structural phase transitions in PbZrO3-based and NaNbO3-based systems. In the context of the ultrahigh energy storage density of SrTiO3-based capacitors, we highlight the necessity of extending the concept of antiferroelectric-to-ferroelectric (AFE-to-FE) transition to broader antiferrodistortive-to-ferrodistortive (AFD-to-FD) transition for materials that are simultaneously ferroelastic. Combining discussion of the factors driving ferroelectricity, electric-field-driven metal-to-insulator transition in a (La1−xSrx)MnO3 electrode is emphasized to determine the role of ionic migration in improving the storage performance. We believe that this review, aiming at depicting a clearer structure–property relationship, will be of benefit for researchers who wish to carry out cutting-edge structure and energy storage exploration.

We demonstrate a practical way to identify the presence of a perovskite phase in rare-earth nickelates (R NiO 3) using X-ray photoelectron spectroscopy (XPS). By varying the calcination temperature, we prepared R NiO 3 powders with different degrees of chemical reaction. We found that perovskite R NiO 3 becomes predominant after high-temperature calcination (≥1,000 °C) in X-ray diffraction and XPS (at Ni 3 p and O 1 s edges) measurements. While the observed spectra at the Ni 3 p edge are similar for all powders, a sizable difference was observed in the O 1 s -edge spectra depending on the calcination temperature. With the formation of a perovskite phase with a trivalent Ni3+ state, an XPS peak corresponding to oxygen ions in the perovskite lattice distinctly emerges. Our work shows that the Ni3+ state cannot be determined by analyzing the Ni 3 p edge solely and rather, the O 1 s edge should be simultaneously monitored for explicit identification. • We report a way to identify a perovskite phase in rare-earth nickelates using XPS. • We prepared R NiO 3 powders by varying their calcination temperatures. • We observed a perovskite R NiO 3 phase in high-temperature calcined powders. • We monitored Ni 3 p - and O 1 s -edge XPS spectra in the as-calcined R NiO 3 powders. • XPS peaks of oxygen ions in perovskite lattices emerge after high-temperature calcination. [ABSTRACT FROM AUTHOR]

We theoretically investigate a metal-to-insulator transition in artificial two-dimensional (2D) crystals (i.e., metasurfaces) of tightly coupled closed-ring resonators. Strong interaction between unit resonators in the metasurfaces yields the effective permittivity highly dependent on the lattice spacing of unit resonators. Through our rigorous theory, we provide a closed form of effective permittivity of the metasurface and reveal that the permittivity possesses a Lorentzian-type resonant behavior, implying that the transition of the effective permittivity can arise when the lattice spacing passes a critical value.

Abstract While controlling the cation contents in perovskite rare-earth nickelate thin films, a metal-to-insulator phase transition is reported. Systematic control of cation stoichiometry has been achieved by manipulating the irradiation of excimer laser in pulsed laser deposition. Two rare-earth nickelate bilayer thin-film heterostructures with the controlled cation stoichiometry (i.e. stoichiometric and Ni-excessive) have been fabricated. It is found that the Ni-excessive nickelate film is structurally less dense than the stoichiometric film, albeit both of them are epitaxial and coherent with respect to the underlying substrate. More interestingly, as a temperature decreases, a metal-to-insulator transition is only observed in the Ni-excessive nickelate films, which can be associated with the enhanced disproportionation of the Ni charge valence. Based on our theoretical results, possible origins (e.g. anti-site defects) of the low-temperature insulating state are discussed with the need of future work for deeper understanding. Our work can be utilized to realize unusual physical phenomena (e.g. metal-to-insulator phase transitions) in complex oxide films by manipulating the chemical stoichiometry in pulsed laser deposition. Highlights • Cation stoichiometry in nickelate ceramic targets is controlled by laser irradiation treatment. • A Ni-excessive bilayer nickelate film exhibits the metal-to-insulator transition. • Anti-site Ni defects can induce the observed metal-to-insulator transition. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Size dependent metal-to-insulator transition in MoO 2 was unveiled by systematic study with various spectroscopic methods. • Electronic transport measurements showed similar metal-to-insulator transition by film thickness. • Surface oxidation in epitaxial thin films would be the main driver to trigger this transition. Abstract Interfaces in transition metal oxides play critical roles for tuning physical properties. In thin film form, multiple interfaces can be created in between a film and a substrate, in between a film and air, and within a thin film. The role of each interface has been rarely studied. In this research, we used MoO 2 as a model system to study the role of the oxidized layer at film-air interface in thickness-driven metal-insulator transition. The oxidized layer at the surface is likely to be the main cause in positive temperature coefficient of resistivity in MoO 2 thin films thinner than about 20 nm. To find the origin of this insulating behavior in electronic transport measurements, we used x-ray diffraction, density functional theory and various spectroscopic methods. We observed the formation of oxidized MoO 2+ x at the film-air interface and its thickness explain the peculiar insulating behavior in the thinner films and even nanoparticles from the literature. [ABSTRACT FROM AUTHOR]

We examined the structural and electrical properties of uniformly-oriented VO 2 /ZnO nanostructures. VO 2 was deposited on ZnO templates by using a direct current-sputtering deposition. Scanning electron microscope and transmission electron microscope measurements indicated that b -oriented VO 2 were uniformly crystallized on ZnO templates with different lengths. VO 2 /ZnO formed nanorods on ZnO nanorods with length longer than 250 nm. X-ray absorption fine structure at the V K edge of VO 2 /ZnO showed M 1 and R phases of VO 2 at 30 and 100 °C, respectively, suggesting structural-phase transition occurring between the two temperatures. Temperature-dependent resistance measurements of VO 2 /ZnO nanostructures revealed metal-to-insulator transition at 65 °C and 55 °C during a heating and a cooling, respectively, regardless of ZnO length. Asymmetry behavior of resistance curves from VO 2 /ZnO nanostructure during a heating and a cooling was attributed from a strong bond of VO 2 and ZnO. [ABSTRACT FROM AUTHOR]

Determination of mobility edge in presence of metal-to-insulator transition Recombination dynamics of excitons was studied in multiple narrow quantum well GaAs/AlGaAs heterostructures. Disorder generated by interface roughness considerably affects transport of the conduction band electrons and at appropriate quantum well width results in a metal-to insulator transition. Localization of the electrons was found to be responsible for the exciton recombination time measured in the vicinity of the metal-to-insulator transition. Measurement of the exciton recombination time as a function of the energy allowed for determination of the critical energy of the mobility edge attributed to the conduction band electrons. The mobility edge energy obtained in this way demonstrates intersection with the Fermi level energy at the critical disorder corresponding to the metal-to-insulator transition. [ABSTRACT FROM AUTHOR]

Metal-insulator transitions in strongly correlated oxides induced by electrochemical charging have been attributed to formation of vacancy defects. However, the role of native defects in affecting these transitions is not clear. Here, we report a new type of phase transition in p-type, nonstoichiometric nickel oxide involving a semiconductor-to-insulator-to-metal transition along with the complete reversal of conductivity from p- to n-type at room temperature induced by electrochemical charging in a Li+-containing electrolyte. Direct observation of vacancy-ion interactions using in situ near-infrared photoluminescence spectroscopy show that the transition is a result of passivation of native nickel (cationic) vacancy defects and subsequent formation of oxygen (anionic) vacancy defects driven by Li+ insertion into the lattice. Changes in the oxidation states of nickel due to defect interactions probed by X-ray photoemission spectroscopy support the above conclusions. In contrast, n-type, nonstoichiometric tungsten oxide shows only insulator-to-metal transition, which is a result of oxygen vacancy formation. The defect-property correlations shown here in these model systems can be extended to other oxides. [ABSTRACT FROM AUTHOR]

Vanadium dioxide (VO2) undergoes significant optical, electronic, and structural changes as it transforms between the low-temperature monoclinic and high-temperature rutile phases. Recently, alternative stimuli have been utilized to trigger insulator-to-metal transformations in VO2, including electrochemical gating. Here, we prepare and electrochemically reduce mesoporous films of VO2 nanocrystals, prepared from colloidally synthesized V2O3 nanocrystals that have been oxidatively annealed, in a three-electrode electrochemical cell. We observe a reversible transition between infrared transparent insulating phases and a darkened metallic phase by in situ visible-near-infrared spectroelectrochemistry and correlate these observations with structural and electronic changes monitored by X-ray absorption spectroscopy, X-ray diffraction, Raman spectroscopy, and conductivity measurements. An unexpected reversible transition from conductive, reduced monoclinic VO2 to an infrared-transparent insulating phase upon progressive electrochemical reduction is observed. This insulator-metal-insulator transition has not been reported in previous studies of electrochemically gated epitaxial VO2 films and is attributed to improved oxygen vacancy formation kinetics and diffusion due to the mesoporous nanocrystal film structure. [ABSTRACT FROM AUTHOR]

Materials exhibiting electronic phase transitions have attracted widespread attention. By switching between metallic and insulating states under external stimuli, the accompanying changes in the electrical and optical properties can be harnessed in novel electronic and optical applications. In this work, a laterally confined conductive pattern is inscribed into an otherwise insulating VO2 thin film using ultraviolet light, inducing an almost four orders of magnitude decrease in electrical resistivity of the exposed area. The metallic imprint remains in VO2 after ultraviolet light exposure and can be completely erased by a short low temperature anneal. The ability to optically pattern confined metallic structures provides new opportunities for reconfigurable photonic and plasmonic structures, as well as re-writable electric circuitry. [ABSTRACT FROM AUTHOR]

In order to investigate the temperature dependence of thermophysical properties in crystalline V2O5 thin film, we measure the Seebeck coefficient and the electrical conductivity, and examine the thermoelectric behavior of the film near the transition temperature (Tc). Based on X-ray diffraction (XRD) results, lattice distortion induced by oxygen vacancy formation is observed above 598 K, and the phase transformation develops at 773 K. The crystalline V2O5 film reveals n-type conduction, and the decreasing trend in the Seebeck coefficient underwent a change above 513 K. The film's resistivity and Seebeck coefficient exhibit an insulating or semiconducting state below 513 K, and these behaviors change above 513 K. The power factor (PF) changed significantly with a striking peak at 573 K. This result is due to the changes in resistivity and the Seebeck coefficient near Tc. Our results suggest that the metal-insulator transition (MIT) in V2O5 film is not caused by phase transformation due to the excessive emission of oxygen. Based on our results, we suggest that the MIT in V2O5 film occurs due to lattice distortion induced by the formation of vanadyl-oxygen vacancies. [ABSTRACT FROM AUTHOR]

This work reports on the characteristics of the insulator-to-metal transition (IMT) of reactive pulsed laser deposited vanadium dioxide (VO 2 ) films in the terahertz (THz) frequency range, namely the transition temperature T IMT , the amplitude contrast of the THz transmission over the IMT Δ A , the transition sharpness Δ T and the hysteresis width Δ H . XRD analysis shows the sole formation of VO 2 monoclinic structure with an enhancement of (011) preferential orientation when varying the O 2 pressure ( P O2 ) during the deposition process from 2 to 25 mTorr. THz transmission measurements as a function of temperature reveal that VO 2 films obtained at low P O2 exhibit low T IMT , large Δ A , and narrow Δ H . Increasing P O2 results in VO 2 films with higher T IMT , smaller Δ A , broader Δ H and asymmetric hysteresis loop. The good control of the VO 2 IMT features in the THz domain could be further exploited for the development of advanced smart devices, such as ultrafast switches, modulators, memories and sensors. [ABSTRACT FROM AUTHOR]

Single-crystalline vanadium dioxide (VO2) nanostructures are of great interest because of their single-domain metal-to-insulator transition. In this paper, single-crystalline W-doped VO2 nanobeams are synthesized for optical and electrical applications. As a result of differences in the polarization of the beams along their transverse and longitudinal axes, dual-surface plasmon resonance peaks at 1344 and 619 nm are generated, resulting in an increase in the solar modulating abilities of the VO2 nanobeams. The conductivity of the single-crystalline W-doped VO2 nanobeams changes by three to four orders of magnitude at the transition temperature, which is of great importance for electrical applications. [ABSTRACT FROM AUTHOR]

Undoped VO 2 was fabricated by RF sputtering from metallic target on low emissivity glass substrates at a record low growth temperature. The structure of the films was examined by X-Ray Diffraction whereas temperature dependence transmittance was employed for monitoring the film's thermochromic properties. Thermochromic VO 2 could be achieved at the substrate temperature of 300 °C which is the lowest growth temperature of VO 2 phase by sputtering reported in literature without any additional post-deposition annealing treatment of the films. The thermochromic properties obtained for undoped VO 2 were: luminous transmittance of 36.2%, metal-to-insulator transition temperature at 55.7 °C and infrared transmittance modulation, ΔT IR, (transmittance difference in the infrared, at λ = 2000 nm, when heated from 25 °C to 90 °C) by 20.5% and solar transmittance modulation ΔT sol = 5.2%. Subsequently, magnesium (Mg) was introduced in the structure of thermochromic VO 2 up to 2.4 at.%, and the Mg-induced changes in thermochromisity were examined. By increasing the amount of Mg in VO 2 structure the visible transmittance increased up to 70%. Improvement in thermochromic characteristics was observed for atomic percentage of Mg up to 0.3% above which the films tend to be non-thermochromic. The optimum atomic percentage of Mg (0.3 at.%) leads to thermochromic VO 2 with the characteristic lowest phase transition temperature at 49.2 °C, luminous transmittance Τ Ium = 46.6% and solar transmittance modulation ΔΤ sol = 2.8% and ΔΤ IR = 13%. Overall, 70 nm thick thermochromic VO 2 films deposited at the low substrate temperature of 300 °C and low deposition rate 0.75 nm/min when doped with Mg up to 0.3 at.% resulted in vanadium dioxide with improved thermochromic properties is regarded as particularly promising for smart windows applications on flexible substrates. [ABSTRACT FROM AUTHOR]

In this study a large-area phase change was performed using optical laser pulsing to demonstrate the formation of metastable face-centered cubic GeSbTe blocks in the reset state of iPCM resulting from the Ge atom rearrangement to the pre-existing vacancy layers, which will be the basis for the phase-transition mechanism of next generation non-volatile memory devices. [Display omitted] • Vacancy ordering leads to the transition from the stable trigonal phase (SET state) to the metastable cubic phase (RESET state) • Chemical bond characteristics around the Ge atoms change from metavalent to covalent bonding through the Ge atom order–disorder vacancies. • Transition is diven by Ge atom rearrangement to pre-existing vacancy layers. • DFT modeling shows that the resistance difference is a consequence of intercalation of Ge atoms into vacancy layer. • We determine switching mechanism of the iPCM via Ge displacement in a non-thermal phase-transition. Interfacial phase-change memory (iPCM), comprising alternating layers of two chalcogenide-based phase-change materials—Sb 2 Te 3 (ST) and GeTe (GT)—has demonstrated outstanding performance in resistive memories. However, its comprehensive understanding is controversial. Herein, the phase-change characteristic of iPCM is identified using atomic scale imaging, X-ray diffraction, and chemical analysis with first-principles density functional theory (DFT) calculations. By inducing laser pulsing, the ST/GT superlattice structure in the low-resistance state tends to reversibly convert into the modified metastable face-centered cubic (fcc) GeSbTe structure in the high-resistance state. This transition is driven by Ge atom rearrangement to pre-existing vacancy layers and ordered vacancy-layer formation. DFT atomistic modeling shows that the resistance difference of 102 orders between low- and high-resistance states is a direct consequence of the intercalation of Ge atoms into the vacancy layer. These results provide insights into iPCM phase-change mechanisms and phase-change random access memory design with low energy and high speed. [ABSTRACT FROM AUTHOR]

Orbital occupancy control in correlated oxides allows the realization of new electronic phases and collective state switching under external stimuli. The resultant structural and electronic phase transitions provide an elegant way to encode, store, and process information. In this review, we examine the utilization of Mott metal-to-insulator transitions, for memory and neuromorphic devices. We emphasize the overarching electron–phonon coupling and electron–electron interaction-driven transition mechanisms and kinetics, which renders a general description of Mott memories from aspects such as nonvolatility, sensing scheme, read/write speed, and switching energy. Various memory and neuromorphic device architectures incorporating phase transition elements are reviewed, focusing on their operational principles. The role of Peierls distortions and crystal symmetry changes during phase change is discussed. Prospects for such orbitronic devices as hardware components for information technologies are summarized. [ABSTRACT FROM PUBLISHER]

We report on the magnetic property of 0.67–WO 3 +0.33–VO x mixture film deposit on the corning glass substrate using the chemical sol–gel and atmospheric pressure chemical vapor deposition (APCVD) methods. The XRD and Raman spectroscopy confirm species of both materials, and the morphological studies with FIB-SEM and TEM reveal segregation of W and V atoms. XPS reveals that V 4+ from VO 2 forms only 11% of the film; V 3+ in the form of V 2 O 3 form 1% of the film, 21% is V 5+ from V 2 O 5 and 67% is given to W 6+ from WO 3 . The analysis of the ESR data shows some sharp changes in the magnetism near the metal-to-insulator (MIT), which could be theoretically interpreted as the ordering or alignment of electron spins from net moment nature to parallel alignment of magnetic moment. The derivatives of magnetic susceptibility established the thermally induced magnetic property: two distinct transitions of 339 K for heating data and 338 K for cooling data for 151.2 mT field were obtained. Similar results were also obtained for 308.7 mT field, 336 K for heating data and 335 K for cooling data. VSM results confirm a paramagnetic phase with a small amount of magnetically ordered phase. [ABSTRACT FROM AUTHOR]