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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Challenges for density functional theory in simulating metal–metal singlet bonding: A case study of dimerized VO2

VO2 is renowned for its electric transition from an insulating monoclinic (M1) phase, characterized by V–V dimerized structures, to a metallic rutile (R) phase above 340 K. This transition is accompanied by a magnetic change: the M1 phase exhibits a non-magnetic spin-singlet state, while the R phase exhibits a state with local magnetic moments. Simultaneous simulation of the structural, electric, and magnetic properties of this compound is of fundamental importance, but the M1 phase alone has posed a significant challenge to the density functional theory (DFT). In this study, we show none of the commonly used DFT functionals, including those combined with on-site Hubbard U to treat 3d electrons better, can accurately predict the V–V dimer length. The spin-restricted method tends to overestimate the strength of the V–V bonds, resulting in a small V–V bond length. Conversely, the spin-symmetry-breaking method exhibits the opposite trends. Each of these two bond-calculation methods underscores one of the two contentious mechanisms, i.e., Peierls lattice distortion or Mott localization due to electron–electron repulsion, involved in the metal–insulator transition in VO2. To elucidate the challenges encountered in DFT, we also employ an effective Hamiltonian that integrates one-dimensional magnetic sites, thereby revealing the inherent difficulties linked with the DFT computations.

Chemistry↗
DOE OSTI2022

Orbital-selective Mott phase and non-Fermi liquid in FePS 3

The layered metal phosphorous trisulfide FePS 3 is reported to be a Mott insulator at ambient conditions and to undergo structural and insulator-metal phase transitions under pressure. However, the character of the resulting metallic states has not been understood clearly so far. Here, we theoretically study the phase transitions of FePS 3 using first-principles methods based on density functional theory and embedded dynamical mean field theory. We find that the Mott transition in FePS 3 can be orbital selective, with t 2g states undergoing a correlation-induced insulator-to-metal transition while e g states remain gapped. We show that this orbital-selective Mott phase, which occurs only when nonhydrostatic pressure is used, is a bad metal (or non-Fermi liquid) with large fluctuating moments due to Hund's coupling. Further application of pressure increases the crystal-field splitting and converts the system to a conventional Fermi liquid with low-spin configurations dominant. Finally, our results show that FePS 3 is an example of a system that realizes an orbital-selective Mott phase, allowing tuning between correlated and uncorrelated metallic properties in an accessible pressure range (≤ 18 GPa).

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

The first-order structural transition in NiO at high pressure

Insulator-metal transition in NiO and concomitant collapse of spin and magnetism in the material are important subjects in studies of strong electronic correlations and as such they generate multiple theoretical and experimental efforts aimed at understanding the complicated physics of strongly correlated electronic systems. We report here the iso-structural transition in NiO accompanying the previously observed by us transition to the metallic state. These findings resolve some experimental controversies regarding the transition. We also discuss the latest theoretical results in detail and show that our data confirm some recent theoretical models. Given the simplicity of the material and its fundamental importance to the understanding of strongly correlated electronic systems, our findings should stimulate further experimental and theoretical efforts to understand this prototype Mott insulator.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2022

Dielectric catastrophe at the Wigner-Mott transition in a moiré superlattice

The bandwidth-tuned Wigner-Mott transition is an interaction-driven phase transition from a generalized Wigner crystal to a Fermi liquid. Because the transition is generally accompanied by both magnetic and charge-order instabilities, it remains unclear if a continuous Wigner-Mott transition exists. Here, we demonstrate bandwidth-tuned metal-insulator transitions at fixed fractional fillings of a MoSe 2 /WS 2 moiré superlattice. The bandwidth is controlled by an out-of-plane electric field. The dielectric response is probed optically with the 2s exciton in a remote WSe 2 sensor layer. The exciton spectral weight is negligible for the metallic state with a large negative dielectric constant. It continuously vanishes when the transition is approached from the insulating side, corresponding to a diverging dielectric constant or a ‘dielectric catastrophe’ driven by the critical charge dynamics near the transition. Our results support the scenario of continuous Wigner-Mott transitions in two-dimensional triangular lattices and stimulate future explorations of exotic quantum phases in their vicinities.

42 ENGINEERING↗
DOE OSTI2023

Deep learning Hamiltonians from disordered image data in quantum materials

The capabilities of image probe experiments are rapidly expanding, providing new information about quantum materials on unprecedented length- and timescales. Many such materials feature inhomogeneous electronic properties with intricate pattern formation on the observable surface. This rich spatial structure contains information about interactions, dimensionality, and disorder—a spatial encoding of the Hamiltonian driving the pattern formation. Image recognition techniques from machine learning are an excellent tool for interpreting information encoded in the spatial relationships in such images. Here, we develop a deep learning framework for using the rich information available in these spatial correlations in order to discover the underlying Hamiltonian driving the patterns. We first vet the method on a known case, scanning near-field optical microscopy on a thin film of V⁢O 2 . We then apply our trained convolutional neural network architecture to new optical microscope images of a different V⁢O 2 film as it goes through the metal-insulator transition. We find that a two-dimensional Hamiltonian with both interactions and random field disorder is required to explain the intricate, fractal intertwining of metal and insulator domains during the transition. This detailed knowledge about the underlying Hamiltonian paves the way for using the model to control the pattern formation via, e.g., tailored hysteresis protocols. Finally, we also introduce a distribution-based confidence measure on the results of a multilabel classifier, which does not rely on adversarial training. In addition, we propose a machine-learning-based criterion for diagnosing a physical system's proximity to criticality.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Ab Initio Electron-Phonon Interactions in Correlated Electron Systems

Electron-phonon ( e - ph ) interactions are pervasive in condensed matter, governing phenomena such as transport, superconductivity, charge-density waves, polarons, and metal-insulator transitions. First-principles approaches enable accurate calculations of e - ph interactions in a wide range of solids. However, they remain an open challenge in correlated electron systems (CES), where density functional theory often fails to describe the ground state. Therefore reliable e - ph calculations remain out of reach for many transition metal oxides, high-temperature superconductors, Mott insulators, planetary materials, and multiferroics. Here we show first-principles calculations of e - ph interactions in CES, using the framework of Hubbard-corrected density functional theory ( DFT + U ) and its linear response extension ( DFPT + U ), which can describe the electronic structure and lattice dynamics of many CES. We showcase the accuracy of this approach for a prototypical Mott system, CoO, carrying out a detailed investigation of its e - ph interactions and electron spectral functions. While standard DFPT gives unphysically divergent and short-ranged e - ph interactions, DFPT + U is shown to remove the divergences and properly account for the long-range Fröhlich interaction, allowing us to model polaron effects in a Mott insulator. Our work establishes a broadly applicable and affordable approach for quantitative studies of e - ph interactions in CES, a novel theoretical tool to interpret experiments in this broad class of materials.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2020

Featureless adaptive optimization accelerates functional electronic materials design

Electronic materials that exhibit phase transitions between metastable states (e.g., metal-insulator transition materials with abrupt electrical resistivity transformations) are challenging to decode. For these materials, conventional machine learning methods display limited predictive capability due to data scarcity and the absence of features that impede model training. In this article, we demonstrate a discovery strategy based on multi-objective Bayesian optimization to directly circumvent these bottlenecks by utilizing latent variable Gaussian processes combined with high-fidelity electronic structure calculations for validation in the chalcogenide lacunar spinel family. We directly and simultaneously learn phase stability and bandgap tunability from chemical composition alone to efficiently discover all superior compositions on the design Pareto front. Previously unidentified electronic transitions also emerge from our featureless adaptive optimization engine. Our methodology readily generalizes to optimization of multiple properties, enabling co-design of complex multifunctional materials, especially where prior data is sparse.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Correlation control of the Mott transition in LaTiO 3 /SrTiO 3 heterostructures

The Mott metal-insulator transition arises from electron-electron interactions determined by the ratio of Coulomb to kinetic energy scales (U/t). While temperature, pressure, and doping can induce Mott transitions, direct control of U in solid-state systems remains largely unexplored, particularly in the inhomogeneous environments of emerging neuromorphic devices, where conductive filaments create complex gradients of local properties. Here we show that interface-induced screening can continuously tune the electron-electron interaction strength U to drive an isothermal Mott transition. Using thickness-graded LaTiO3/SrTiO3 heterostructures, we used diffraction and photoemission spectroscopy to reveal a continuous, isothermal quantum phase transition from a Fermi liquid quasiparticle with incoherent excitations to a Mott insulator with Hubbard bands. The primary determinant of the transition is the enhanced local screening environment, which directly influences the interaction strength U, driving the system metallic. This demonstrates that interface engineering of the local screening environment provides a promising approach to manipulate Mott physics through correlation control, beyond traditional bandwidth or filling approaches.

Electronic and spintronic devices↗
DOE OSTI2026

Electronic coherence evolution at the nearly commensurate-incommensurate CDW boundary of 1⁢T−TaS 2

Transition-metal dichalcogenides host a variety of charge-density-wave phases that couple lattice, charge, and correlation effects. In 1⁢𝑇−TaS 2 , the commensurate and nearly commensurate states are well characterized, yet the transition near 350 K into the incommensurate phase has lacked direct momentum-resolved insight. In this work, we use temperature-dependent angle-resolved photoemission spectroscopy to track the electronic structure across this transition. We observe a suppression of quasiparticle spectral weight at the Brillouin-zone center, coincident with the transport anomaly, but without clear evidence of a full band-gap opening. The transition appears to involve momentum-dependent redistribution of spectral weight, consistent with a loss of coherence that reshapes the Fermi surface while leaving conduction dispersions largely intact. These results suggest that the nearly commensurate–incommensurate transition may not align with a conventional metal-insulator transition picture, but rather as an electronic reconstruction driven by loss of coherence. Our work provides new microscopic insight into the resistivity anomaly near room temperature and may guide design principles for collective electronic switching in transition-metal dichalcogenides.

36 MATERIALS SCIENCE↗
DOE OSTI2023

VO 2 phase change electrodes in Li-ion batteries

Use of electrode materials that show phase change behavior and hence drastic changes in electrochemical activity during operation has not been explored for Li-ion batteries. Here we demonstrate the vanadium oxide (VO 2 ) cathode that undergoes a metal–insulator transition due to the first-order structural phase transition at an accessible temperature of 68 °C for battery operation. Using a suitable electrolyte operable across the phase transition range and compatible with vanadium oxide cathodes, we studied the effect of cathode active material structural changes on lithium insertion followed by the electrochemical characteristics above and below the phase transition temperature. The high-temperature VO 2 phase shows significantly improved capacitance, enhanced current rate capabilities, improved electrical conductivity and lithium-ion diffusivity compared to the insulating low temperature phase. In conclusion, this opens up new avenues for electrode design, allowing manipulation of electrochemical reactions around phase transition temperatures, and in particular enhancing electrochemical properties at elevated temperatures contrary to the existing class of battery chemistries that lead to performance deterioration at elevated temperatures.

25 ENERGY STORAGE↗
DOE OSTI2020

Attractive Effect of a Strong Electronic Repulsion: The Physics of Vertex Divergences

While the breakdown of the perturbation expansion for the many-electron problem has several formal consequences, here we unveil its physical effect: flipping the sign of the effective electronic interaction in specific scattering channels. By decomposing local and uniform susceptibilities of the Hubbard model via their spectral representations, we prove how entering the nonperturbative regime causes an enhancement of the charge response, ultimately responsible for the phase-separation instabilities close to the Mott metal-insulator transition. Furthermore, our analysis opens a new route for understanding phase transitions in the nonperturbative regime and clarifies why attractive effects emerging from a strong repulsion can induce phase separations but not s-wave pairing or charge-density wave instabilities.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

A theory for colors of strongly correlated electronic systems

Many strongly correlated transition metal insulators are colored, even though they have band gaps much larger than the highest energy photons from the visible light. An adequate explanation for the color requires a theoretical approach able to compute subgap excitons in periodic crystals, reliably and without free parameters—a formidable challenge. The literature often fails to disentangle two important factors: what makes excitons form and what makes them optically bright. We pick two archetypal cases as examples: NiO with green color and MnF 2 with pink color, and employ two kinds of ab initio many body Green’s function theories; the first, a perturbative theory based on low-order extensions of the $GW$ approximation, is able to explain the color in NiO, while the same theory is unable to explain why MnF 2 is pink. We show its color originates from higher order spin-flip transitions that modify the optical response, which is contained in dynamical mean-field theory (DMFT). We show that symmetry lowering mechanisms may determine how ‘bright’ these excitons are, but they are not fundamental to their existence.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
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 OSTI2022

Room-temperature valence transition in a strain-tuned perovskite oxide

Abstract Cobalt oxides have long been understood to display intriguing phenomena known as spin-state crossovers, where the cobalt ion spin changes vs. temperature, pressure, etc. A very different situation was recently uncovered in praseodymium-containing cobalt oxides, where a first-order coupled spin-state/structural/metal-insulator transition occurs, driven by a remarkable praseodymium valence transition. Such valence transitions, particularly when triggering spin-state and metal-insulator transitions, offer highly appealing functionality, but have thus far been confined to cryogenic temperatures in bulk materials (e.g., 90 K in Pr 1- x Ca x CoO 3 ). Here, we show that in thin films of the complex perovskite (Pr 1- y Y y ) 1- x Ca x CoO 3-δ , heteroepitaxial strain tuning enables stabilization of valence-driven spin-state/structural/metal-insulator transitions to at least 291 K, i.e., around room temperature. The technological implications of this result are accompanied by fundamental prospects, as complete strain control of the electronic ground state is demonstrated, from ferromagnetic metal under tension to nonmagnetic insulator under compression, thereby exposing a potential novel quantum critical point.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Tunable spin and valley excitations of correlated insulators in Γ-valley moiré bands

Moiré superlattices formed from transition metal dichalcogenides (TMDs) have been shown to support a variety of quantum electronic phases that are highly tunable using applied electromagnetic fields. While the valley character of the low-energy states dramatically affects optoelectronic properties in the constituent TMDs, this degree of freedom has yet to be fully explored in moiré systems. Here, we establish twisted double bilayer WSe 2 as an experimental platform to study electronic correlations within Γ-valley moiré bands. Through a combination of local and global electronic compressibility measurements, we identify charge-ordered phases at multiple integer and fractional moiré band fillings ν. By measuring the magnetic field dependence of their energy gaps and the chemical potential upon doping, we reveal spin-polarized ground states with novel spin polaron quasiparticle excitations. In addition, an applied displacement field allows us to realize a new mechanism of metal-insulator transition at ν=–1 driven by tuning between Γ- and K-valley moiré bands. Together, our results demonstrate control over both the spin and valley character of the correlated ground and excited states in this system.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Transition metals of Pt and Pd on the surface of topological insulator Bi2Se3

Transition metal catalysts supported on topological insulators are predicted to show improved catalytic properties due to the presence of topological surface states, which may float up to the catalysts and provide robust electron transfer. However, experimental studies of surface structures and corresponding catalytic properties of transition metal/topological insulator heterostructures have not been demonstrated so far. Here, we report the structures, chemical states, and adsorption behaviors of two conventional transition metal catalysts, Pt and Pd, on the surface of Bi2Se3, a common topological insulator material. We reveal that Pt forms nanoparticles on the Bi2Se3 surface. Moreover, the interaction between Pt and surface Se is observed. Furthermore, thermal dosing of O2 onto the Pt/Bi2Se3 heterostructure leads to no oxygen adsorption. Detailed scanning tunneling microscopy study indicates that Pt transforms into PtSe2 after the thermal process, thus preventing O2 from adsorption. For another transition metal Pd, it exhibits approximate layer-island growth on Bi2Se3, and Pd–Se interaction is also observed. Our work provides significant insights into the behaviors of transition metals on top of a common topological insulator material and will assist in the future design of catalysts built with topological materials.

Chemistry↗
DOE OSTI2020

Emergence of a competing stripe phase near the Mott transition in Ti-doped bilayer calcium ruthenates

We report the nanoscale imaging of Ti-doped bilayer calcium ruthenates during the Mott metal-insulator transition by microwave impedance microscopy. Different from a typical first-order phase transition where coexistence of the two terminal phases takes place, a new metallic stripe phase oriented along the in-plane crystalline axes emerges inside both the G-type antiferromagnetic insulating state and the paramagnetic metallic state. The effect of this electronic state can be observed in macroscopic measurements, allowing us to construct a phase diagram that takes into account the energetically competing phases. Our work provides a model approach to correlate the macroscopic properties and mesoscopic phase separation in complex oxide materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗