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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Electrical Control of Magnetic Resonance in Phase Change Materials

Metal–insulator transitions (MITs) in resistive switching materials can be triggered by an electric stimulus that produces significant changes in the electrical response. When these phases have distinct magnetic characteristics, dramatic changes in the spin excitations are also expected. The transition metal oxide La 0.7 Sr 0.3 MnO 3 (LSMO) is a ferromagnetic metal at low temperatures and a paramagnetic insulator above room temperature. When LSMO is in its metallic phase, a critical electrical bias has been shown to lead to an MIT that results in the formation of a paramagnetic resistive barrier transverse to the applied electric field. Using spin-transfer ferromagnetic resonance spectroscopy, we show that even for electrical biases less than the critical value that triggers the MIT, there is magnetic phase separation, with the spin-excitation resonances varying systematically with applied bias. Therefore, voltage-triggered MITs in LSMO can alter magnetic resonance characteristics, offering an effective method for tuning synaptic weights in neuromorphic circuits.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Ultrathin VO 2 Films on Functional Substrates

The metal–insulator transition (MIT) in vanadium dioxide (VO 2 ) thin films is strongly affected by grain size, thickness, and interfacial properties. Typically, a minimum thickness around 50 nm is required for VO 2 to exhibit a significant MIT when functional substrates like sapphire and silicon are used. Several works have shown that thin films below 20 nm, with up to 2–3 decades of change in the resistance across the MIT, can be achieved but require complex pre- or postprocessing of the samples. We show that predeposition substrate condition control facilitates the direct growth of VO 2 ultrathin 15 nm films, exhibiting a resistance change between 3 and 4 decades across the MIT. Our findings indicate that the interface between the film and the substrate is crucial in determining the initial growth layers and the structural evolution. With appropriate substrate surface treatment, the desired VO 2 MIT can be enhanced regardless of the substrate crystallographic orientation. Moreover, we propose a novel approach to obtain large resistance changes across the MIT in ultrathin VO 2 films by incorporating a predeposited 1.5 nm vanadium oxide buffer layer, thereby eliminating the need to use different materials or complex pre- or postprocessing of the samples. Here we also demonstrate that this method improves the transition of 25–50 nm VO 2 thin films on silicon substrates. Our study reveals a simple approach for direct growth of ultrathin VO 2 films exhibiting a significant MIT, which is commonly accepted unattainable over substrates of technological importance, such as sapphire and silicon.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Ion diffusion retarded by diverging chemical susceptibility

Abstract For first-order phase transitions, the second derivatives of Gibbs free energy (specific heat and compressibility) diverge at the transition point, resulting in an effect known as super-elasticity along the pressure axis, or super-thermicity along the temperature axis. Here we report a chemical analogy of these singularity effects along the atomic doping axis, where the second derivative of Gibbs free energy (chemical susceptibility) diverges at the transition point, leading to an anomalously high energy barrier for dopant diffusion in co-existing phases, an effect we coin as super-susceptibility. The effect is realized in hydrogen diffusion in vanadium dioxide (VO 2 ) with a metal-insulator transition (MIT). We show that hydrogen faces three times higher energy barrier and over one order of magnitude lower diffusivity when it diffuses across a metal-insulator domain wall in VO 2 . The additional energy barrier is attributed to a volumetric energy penalty that the diffusers need to pay for the reduction of latent heat. The super-susceptibility and resultant retarded atomic diffusion are expected to exist universally in all phase transformations where the transformation temperature is coupled to chemical composition, and inspires new ways to engineer dopant diffusion in phase-coexisting material systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Local strain inhomogeneities during electrical triggering of a metal–insulator transition revealed by X-ray microscopy

Electrical triggering of a metal–insulator transition (MIT) often results in the formation of characteristic spatial patterns such as a metallic filament percolating through an insulating matrix or an insulating barrier splitting a conducting matrix. When MIT triggering is driven by electrothermal effects, the temperature of the filament or barrier can be substantially higher than the rest of the material. Using X-ray microdiffraction and dark-field X-ray microscopy, we show that electrothermal MIT triggering leads to the development of an inhomogeneous strain profile across the switching device, even when the material does not undergo a pronounced, discontinuous structural transition coinciding with the MIT. Diffraction measurements further reveal evidence of unique features associated with MIT triggering including lattice distortions, tilting, and twinning, which indicate structural nonuniformity of both low- and high-resistance regions inside the switching device. Such lattice deformations do not occur under equilibrium, zero-voltage conditions, highlighting the qualitative difference between states achieved through increasing temperature and applying voltage in nonlinear electrothermal materials. Electrically induced strain, lattice distortions, and twinning could have important contributions in the MIT triggering process and drive the material into nonequilibrium states, providing an unconventional pathway to explore the phase space in strongly correlated electronic systems.

42 ENGINEERING↗
DOE OSTI2024

Zero-Strain Metal-Insulator Transition by the Local Fluctuation of Cation Dimerization

The coupled electronic and structural transitions in metal-insulator transition (MIT) hinder ultrafast switching and ultimate endurance. Decoupling these transitions and achieving a zero-strain electronic MIT can overcome the fundamental limitations of MIT in solid materials. Here, this study demonstrates that iso-valent Ti dopants in supercooled VO 2 epitaxial films cause MIT with minimal hysteresis without changing unit-cell volume and crystal symmetry. The Ti dopants in the VO 2 lattice locally alter the configuration of V-V pairs, where the long-range ordering in V-V pairs is disrupted, and the nano-domains of V-V dimers are formed. Strikingly, these local V-V dimers persist even above the electronic transition temperature (T MI ), facilitating the zero-strain electronic MIT with nanoscale structural heterogeneity. The geometrically compatible interface between insulating and metallic phases drastically enhances switching speed and endurance during electrically and optically driven zero-strain MIT. In conclusion, this discovery offers a fresh perspective on the scientific understanding of MIT and the improved functionality in terms of device speed and reliability by decoupling electronic and structural transitions.

36 MATERIALS SCIENCE↗
DOE OSTI2025

High-frequency electrical behavior in V 3 O 5 thin films

Vanadium oxides are known for their metal–insulator transition (MIT), with V 3 O 5 being notable for its transition temperature exceeding room temperature. At about 430 K, this material shows a change in crystal symmetry accompanied with one order of magnitude increase in its electrical conductivity and alterations in its optical properties. Although the property changes during the MIT in V 3 O 5 are less pronounced than those observed in VO 2 , its transition temperature is 90 K higher, making it appealing for applications requiring elevated temperatures. In this article, the high-frequency characteristics were determined in a V 3 O 5 two-terminal device in the range from 5 to 35 GHz. The S-parameters showed that the return loss at room temperature was close to −1.5 dB, and the isolation between ports was approximately −50 dB. At temperatures above the metal–insulator transition, the isolation decreased to around −40 dB at 35 GHz. For S11 and S22, similar behavior was observed at room temperature, with a notable change in the S-parameter phase of the device. This behavior suggests that V 3 O 5 may function well as a capacitor because the considerable change in phase could control the flow of electrical signals in devices. This property also may be used for matching purposes, especially considering its response to temperature changes. Additionally, conductivity calculation from S-parameters shows a decrease of approximately two orders of magnitude at 500 K and one order of magnitude at 300 K compared to DC values. These findings highlight V 3 O 5 potential for integration into radio frequency devices that demand consistent performance in high-temperature environments.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Harnessing the Second-Order Metal−Insulator Transition for Neuromorphic Computing

Vanadium oxides are widely studied phase change materials for brain-inspired computing architectures. Systems like VO 2 and V 2 O 3 exhibit first-order metal−insulator transitions (MITs) with hysteresis and percolative switching, increasing stochasticity and device variability. Here, we focus on the less explored Magnéli phase V 4 O 7 , which undergoes a continuous, non-hysteretic, second-order MIT. This surprisingly enables highly reproducible volatile resistive switching in spiking-neuron-type devices. We synthesize V 4 O 7 films, characterize their structural and transport properties, and demonstrate voltage and current-driven threshold switching with electrothermal feedback. In a Pearson–Anson oscillator, V 4 O 7 devices produce stable, tunable spiking across 20–200 kHz, with consistent operation among multiple devices. We introduce a numerical analog leaky-integrate-and-fire (aLIF) model that captures waveform shapes and their dependence on load resistance, temperature, and voltage. Furthermore, these findings suggest that second-order MIT materials like V 4 O 7 are promising for deterministic, scalable spiking neuron arrays for neuromorphic computing.

V4O7↗
DOE OSTI2025

Ligand Hole Driven Metal–Insulator Transition Exemplified in a Layered Transition Metal Oxide

The interplay of cooperative Jahn–Teller (JT) distortions and charge-disproportionation (CD) with a strong electronic correlation in transition metal oxides leads to structural symmetry breaking. Both JT and CD often manifest in the form of significant modifications in electronic and structural properties such as band splitting, metal–insulator transitions (MIT), and enhanced electron lattice interactions. Notably, the charge-disproportionation is a key electronic feature that drives the MIT. Here, we demonstrate and quantify it using first-principles calculations combining density-functional theory, dynamical mean-field theory, and spin–lattice Monte Carlo simulations. Taking Ca 2 FeMnO 6 as a prototypical example of a correlated oxide, our ab initio study shows that MIT in Ca 2 FeMnO 6 arises from the partial localization of oxygen ligand holes at alternate Fe sites that control both charge and magnetic ordering. Interestingly, the band gap was found to be fundamentally controlled by the strength of the charge-transfer energy and not by the Mott–Hubbard interactions. The novel physics and insights presented in this work reveal promising routes for tuning the electronic functionality in transition-metal oxides.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Purely electronic insulator-metal transition in rutile VO 2

Volatile resistive switching in neuromorphic computing can be tuned by external stimuli such as temperature or electric-field. However, this type of switching is generally coupled to structural changes, resulting in slower reaction speed and higher energy consumption when incorporated into an electronic device. The vanadium dioxide (VO 2 ), which has near room temperature metal-insulator transition (MIT), is an archetypical volatile resistive switching system. Here, we demonstrate an isostructural MIT in an ultrathin VO 2 film capped with a photoconductive cadmium sulfide (CdS) layer. Transmission electron microscopy, resistivity experiments, and first-principles calculations show that the hole carriers induced by CdS photovoltaic effect are driving the MIT in rutile VO 2 . The insulating-rutile VO 2 phase has been proved and can remain stable for hours. Our finding provides a new approach to produce purely electronically driven MIT in VO 2 , and widens its applications in fast-response, low-energy neuromorphic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Voltage-induced magnetic domain evolution in a phase-change material

Applying voltage to metal–insulator transition (MIT) materials allows electrical actuation of the local electronic phase state. In MIT systems that have the electronic order coupled with the magnetic order, voltage switching of the electronic phase state can also enable the electrical manipulation of magnetic properties. In this work, we utilized x-ray magnetic circular dichroism photoemission electron microscopy (XMCD-PEEM) to investigate the control of magnetic domain configurations in ferromagnetic MIT electrical switches. For applied voltages above a threshold value, the XMCD-PEEM images show that the magnetic domains separate into two distinct regions: one with a high contrast (white/black), indicating well-defined micrometer-scale magnetic domains with a component of their magnetization aligned parallel/antiparallel to the x-ray helicity, and the other with different shades of intermediate contrast (gray). Significant changes in magnetic domain configurations upon voltage biasing were only observed in these gray regions. Furthermore, the voltage-induced magnetic domain separation was found to be bias polarity-dependent, with the gray regions expanding from the opposite sample edge when the applied voltage polarity was reversed. This polarity-dependent electrical control of magnetic domain configurations during the MIT switching opens alternative opportunities in memory applications for magnetic MIT switching materials.

42 ENGINEERING↗
DOE OSTI2026

Modulating charge transport via 2 MeV He + irradiation in VO 2

Vanadium dioxide (VO 2 ) is of interest for adaptive electronic applications such as neuromorphic neuristor devices and variable emissivity or tunable thermal control materials, thanks to its key property—a metal–insulator transition (MIT) at 68 °C that is accompanied by a dramatic change in electrical and optical properties. To improve performance in these roles, it is critical to develop approaches to engineer transport properties and the MIT behavior. While many documented techniques exist to modulate the MIT and film resistivities via lattice strain and chemical doping, less is known about the effects of ion irradiation on the intrinsic properties of VO 2 , despite the ability to control the spatial distribution of irradiation beams and the prevalence of high energy ion implantation in the semiconductor industry. The impact of irradiation of different acceleration energies on the responses of VO 2 is of specific interest, as charged particle energy generally impacts both the resulting defect profile and corresponding transport behavior. Here, we demonstrate that 2 MeV He ions at equivalent calculated displacements per atom, in two different types of films, can create remarkable changes to the nature of charge transport in VO 2 , especially in the low-temperature insulating phase. Simulation of resulting changes in electrical conductivity reveals that He ion irradiation offers a strategy to increase both oscillation frequency and the signal transmission. These results provide insights into the intentional design of defect populations to modulate transport for neuromorphic VO 2 devices.

Gurrola, Rebeca M. [Texas A & M Univ., College Sta↗
DOE OSTI2024

Low cost switching circuit for van der Pauw resistivity and Hall measurements at various temperatures

The electrical characterization of materials, particularly superconductors, semiconductors, and those undergoing metal-insulator transitions (MITs), relies significantly on resistivity and Hall measurements as a function of temperature. The van der Pauw four-point probe method is commonly used for such measurements, which involves resistance measurements for different circuit configurations connected to sample contacts. However, repeating these measurements at different temperatures is challenging and time consuming. Here, this study introduces a novel approach utilizing a switching circuit controlled by an Arduino device and a LabVIEW program to automate resistance measurements for different electrical configurations. The effectiveness of this setup was demonstrated by testing V 2 O 3 thin film samples deposited on Al 2 O 3 (001) substrates using DC magnetron sputtering. The MIT temperature of the sample was 115 K during heating and 100 K during cooling. The sample exhibited p-type charge carriers with a Hall coefficient of 1.3 ± 0.1 x 10 -4 cm 3 /C and Hall mobility of 1.7 x 10 -1 cm 2 /V∙s, which is consistent with findings from other studies employing commercial equipment.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Lattice distortions and metal-insulator transition in hexagonal FeS

Using total x-ray scattering and density functional theory calculations based on experimental data, we study the metal-insulator transition (MIT) in the strongly correlated system FeS with hexagonal symmetry. We find that it arises from the combined effect of static lattice distortions and antiferromagnetic ordering, while electron-electron correlations appear to play a minor role. The presence of local lattice distortions above the MIT explains well the evolution of transport and magnetic properties with changing temperature or pressure. Intrinsically, lattice distortions in hexagonal FeS act as lattice degrees of freedom that bridge competing electronic phases, thus facilitating transitions between them. The mechanism of MIT operating in h-FeS appears to be common to strongly correlated binary systems involving 3d transition metals, warranting further investigations on the lattice distortions-property relationship in this broad class of materials using the approach adopted here.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Electrically‐Driven Metal‐Insulator Transitions Emerging from Localizing Current Density and Temperature

Negative differential resistance (NDR) is a key electronic response enabling two‐terminal artificial neurons that can be achieved through different physical phenomena, including phase‐homogeneous current density and temperature (electro‐thermal) localizations and spatially‐localized metal‐insulator phase transitions (MITs). These two effects have been observed to occur sequentially in select electrically‐biased transition metal oxides. However, it is unknown why and under what conditions localizing behaviors precede MITs, particularly as a function of device length scale. To this end, the interplay between phase‐homogeneous electro‐thermal localizations and MITs is investigated in a 3D multiphysics simulation of a lateral thin film device, using the material properties of the prototype MIT material VO 2 . These findings demonstrate that the MIT is nucleated through dynamically localizing current density and temperature. A critical device width (≈0.7 µm in this study) is identified, below which both the electrically‐induced electro‐thermal and phase inhomogeneities cease to appear. It is demonstrated that the formation of spatial inhomogeneities directly relates to device dimensions, and demonstrate the decoupling of NDR from the MIT through device scaling relationships. These results provide insight into the material phenomena underlying the material's electrical responses, clarifying conditions under which spatial inhomogeneities form in electrically‐biased MIT materials.

artificial neuron↗
DOE OSTI2024

Suppression of metal-to-insulator transition and stabilization of superconductivity by pressure in Re 3 Ge 7

The effect of pressure on the low-temperature states of the Re 3 ⁡Ge 7 is investigated by both electrical and Hall resistance and magnetization measurements. At ambient pressure, the temperature-dependent resistance of Re 3 ⁡Ge 7 behaves quasilinearly from room temperature down to 60 K, then undergoes a two-step metal-to-insulator transition (MIT) at temperatures T 1 =59.4K and T 2 =58.7K, which may be related to a structural phase transition or occurrence of charge-density wave ordering. Upon applying pressure, the two-step (T 1 ,T 2 ) MIT splits into three steps (T 1 ,T 2 . and T 3 ) above 1 GPa, and all traces of MITs are fully suppressed by ~8 GPa. Subsequently, the onset of bulk superconductivity (SC) occurs between 10.8 and 12.2 GPa and persists to our highest pressure of 26.8 GPa. At 12.2 GPa the superconducting transition temperature, T c , and upper critical field, H c ⁢2 reach the maximum of T c (onset) ~5.9 K and H c ⁢2 (1.8 K) ~14 kOe. Finally, our results not only present the observation of SC under high pressure in Re 3 ⁡Ge 7 but also delineate the interplay between SC and other competing electronic states by creating a T-p phase diagram for this potentially topologically nontrivial system Re 3 ⁡Ge 7 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Origin of metal-insulator transitions in the parent compounds of ruthenium-pnictide superconductors

Here we study the interplay of the structural phase transition, flat electronic band dispersion, and metal-to-insulator transition (MIT) in the parent compounds of the Ru-pnictide superconductors by using first-principles calculations. Our electron and phonon calculations reveal that Ru(P,As) undergo MIT accompanied by orthorhombic to monoclinic distortion at low temperature, but RuSb stays orthorhombic and metallic in agreement with the experimental findings. It is demonstrated that electronic correlation, as treated in DFT + U, DFT + Gutzwiller, and dynamical mean-field theory (DMFT), cannot induce MIT in the undistorted crystal structure. We find that, although small monoclinic distortion can remove the van Hove singularity at the Fermi level, it does not immediately gap out the Fermi surface and a large value of monoclinic distortion is necessary for a clear MIT, suggesting the possibility of an intermediate pseudogapped monoclinic metallic phase. Furthermore, we predict a light-induced two-step insulator-to-metal and structural transitions in the monoclinic phases of RuP and RuAs, which can be tested in future ultrafast pump-probe experiments as an alternative ideal playground to VO 2 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Gate control of electron correlations towards Mott field effect transistors (MottFET)

The modulation of channel conductance in field-effect transistors (FETs) via metal-insulator-semiconductor structures has revolutionized information processing and storage. However, the limitations of silicon-based FETs in electrical switching have driven the search for new materials capable of overcoming these constraints. Electrostatic gating of competing electronic phases in a Mott material near its metal-to-insulator transition (MIT) offers the prospects of substantial modulation of the conducting electrons and electrical resistivity through small changes in band filling. While electrostatic control of the MIT has been previously reported, the advancement of Mott materials toward novel Mott-based transistors, MottFETs requires the realization of their unique physical properties in a solid-state device. In this study, we present gate control of electron correlation using a solid-state device utilizing the oxide Mott system La 1-x Sr x VO 3 as a correlated FET channel. We report on a gate resistance response that cannot be explained in a purely electrostatic framework. This behavior suggests an enhancement in effective mass and a reduction in effective carrier density as the system approaches its insulating state, consistent with theoretical predictions for Mott systems, suggesting at least 100× charge gain originating from the correlated behavior. These preliminary results pave the way toward the development of highly efficient, low-power electronic devices that could surpass the performance bottlenecks of conventional FETs by leveraging the electronic phase transitions of correlated electron systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Dynamical correlations leading to site and orbital selective Mott insulator transition in hydrogen doped SmNiO 3

Electron doping induces metal -to -insulator transition (MIT) in SmNiO 3 as realized by experiments. While earlier density functional theory (DFT) studies with static correlations fell short of explaining the recent MIT observations at lower hydrogen concentrations, we present a comprehensive computational investigation employing an advanced approach. We combine DFT with dynamical mean field theory (DFT + DMFT) to efficiently analyze the insulating behavior of hydrogen -doped SmNiO 3 . In contrast to previous theoretical works, our calculations predict an insulator transition occurring at a reduced doping level of H:Ni = 0.5:1. Specifically, while the DFT + U method reveals a gap opening between p-to-d orbitals, the DMFT approach highlights a gap opening between d-to-d orbitals. Our findings uncover a selective Mott transition in site and orbital characteristics, with the Ni ions proximate to the doped hydrogen exhibiting Mott -like traits. Notably, DMFT calculations highlight a pronounced dependence on Hund's parameter J, implying the presence of Hundness in the Mott insulator. Finally, this study underscores the necessity of accounting for dynamical correlations to accurately describe the electronic structure of strongly correlated electron-doped rare-earth nickelates.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗