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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 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Structural and Electronic Transitions in Liquid FeO Under High Pressure

Abstract FeO represents an important end‐member for planetary interiors mineralogy. However, its properties in the liquid state under high pressure are poorly constrained. Here, in situ high‐pressure and high‐temperature X‐ray diffraction experiments, ab initio simulations, and thermodynamic calculations are combined to study the local structure and density evolution of liquid FeO under extreme conditions. Our results highlight a strong shortening of the Fe‐Fe distance, particularly pronounced between ambient pressure and ∼40 GPa, possibly related with the insulator to metal transition occurring in solid FeO over a similar pressure range. Liquid density is smoothly evolving between 60 and 150 GPa from values calculated for magnetic liquid to those calculated for non‐magnetic liquid, compatibly with a continuous spin crossover in liquid FeO. The present findings support the potential decorrelation between insulator/metal transition and the high‐spin to low‐spin continuous transition, and relate the changes in the microscopic structure with macroscopic properties, such as the closure of the Fe‐FeO miscibility gap. Finally, these results are used to construct a parameterized thermal equation of state for liquid FeO providing densities up to pressure and temperature conditions expected at the Earth's core‐mantle boundary.

Morard, Guillaume↗
DOE OSTI2020

Pronounced Pressure Dependence of Electronic Transitions for Americium Compared to Isomorphous Neodymium and Samarium Mellitates

The mellitate ion is relevant in spent nuclear fuel processing and is utilized as a surrogate for studying the interactions of f elements with humic acids. A wealth of different coordination modes gives the potential for diverse structural chemistry across the actinide series. In this study, an americium mellitate, 243 Am 2 [(C 6 (COO – ) 6 ](H 2 O) 8 ·2H 2 O (1-Am), has been synthesized and characterized using structural analysis and spectroscopy at ambient and elevated pressures. 1-Am was then compared to isomorphous neodymium (1-Nd) and samarium (1-Sm) mellitates via bond-length analysis and pressure dependence of their Laporte-forbidden f → f transitions. Results show that the pressure dependence of the f → f transitions of 1-Am is significantly greater than that observed in 1-Nd and 1-Sm, with average shifts of 21.4, 4.7, and 3.6 cm –1 /GPa, respectively. Finally, this greater shift found in 1-Am shows further evidence that the 5f orbitals are more affected than the 4f orbitals when pressure is applied to isostructural compounds.

Actinides↗
DOE OSTI2025

Coupling of electronic transition to ferroelectric order in a 2D semiconductor

A ferroelectric material often exhibits a soft transverse optical (TO) phonon mode which governs its phase transition. Charge coupling to this ferroelectric soft mode may further mediate emergent physical properties, including superconductivity and defect tolerance in semiconductors. However, direct experimental evidence for such coupling is scarce. Here we show that a photogenerated coherent phonon couples strongly to the electronic transition above the bandgap in the van der Waals (vdW) two-dimensional (2D) ferroelectric semiconductor NbOI 2 . Using terahertz time-domain spectroscopy and first-principles calculations, we identify this mode as the TO phonon responsible for ferroelectric order. This exclusive coupling occurs only with the above-gap electronic transition and is absent in the valence band as revealed by resonant inelastic X-ray scattering. Our findings suggest a new role of the soft TO phonon mode in electronic and optical properties of ferroelectric semiconductors.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Fluctuating Ru trimer precursor to a two-stage electronic transition in RuP

Superconductivity in binary ruthenium pnictides occurs proximal to and upon suppression of a non-magnetic ground state, preceded by a pseudogap phase associated with Fermi surface instability, and its critical temperature, T c , is maximized around the pseudogap quantum critical point. By analogy with isoelectronic iron based counterparts, antiferromagnetic fluctuations became ''usual suspects" as putative mediators of superconducting pairing. Here we report on a high temperature local symmetry breaking in RuP, nonsuperconducting parent of the maximum- T c branch of these novel superconductors, revealed by combined nanostructure-sensitive powder and single crystal X-ray total scattering analyses. Large local distortions of Ru chains associated with orbital-charge trimerization, further assembled into hexamers, exist above the two-stage electronic transition in RuP. In the pseudogap regime the precursors order with distortions retaining their strength, whereas they acquire spin-singlet characteristics which dramatically enhances the distortion as the nonmagnetic ground state establishes. The precursors enable the nonmagnetic ground state and presumed complex oligomerization, and the relevance of pseudogap fluctuations for superconductivity emerges as a distinct prospect. As a transition metal system in which partial d-manifold filling combined with high crystal symmetry promotes electronic instabilities, this represents a further example of local electronic precursors underpinning the macroscopic collective behavior of quantum materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Electron transitions in a Ce(iii)-catecholate metal–organic framework

A rare three-dimensional catecholate-based Ce(III) metal–organic framework (MOF), denoted as NU-1701, has been synthesized and crystallographically characterized. Density functional theory calculations highlight various possible electronic transitions that may present in NU-1701. These transitions are competitive and indicate increased lanthanide character of Ce(III).

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

Effect of electronic transitions on near edge optical properties of off-stoichiometric boron carbide thin films

In the present study, soft x-ray optical properties of off-stoichiometric boron carbide thin films are investigated, and the structure and chemical composition of the film is analyzed using angle dependent x-ray reflectivity and x-ray photoelectron spectroscopy techniques. Energy dependent soft x-ray reflectivity measured at a fixed grazing angle of 1.5° is used to determine the optical constants in the boron K edge region by applying the Kramers–Kronig technique. The measured optical constants show near edge fine features corresponding to σ* and π* resonances. Furthermore, the electronic transitions corresponding to σ* resonance cause a 40%–75% increase in the delta value in the above boron K edge region. The π* transitions corresponding to off-stoichiometric nature of the boron carbide are observed in the absorption spectra near ~ 192.7 eV. Details of the measured soft x-ray optical properties of the off-stoichiometric boron carbide thin film are discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2024

La 0.6 Sr 0.4 CoO 3−δ Films Under Deoxygenation: Magnetic And Electronic Transitions Are Apart from The Structural Phase Transition

Abstract Topotactic phase transitions induced by changes in the oxygen vacancy concentration can largely alter the physical properties of complex oxides, including electronic and magnetic phases, while maintaining the structural integrity of the crystal lattice. An oxygen‐vacancy‐induced topotactic phase transition from perovskite (PV) to brownmillerite (BM) is achieved in epitaxial La 0.6 Sr 0.4 CoO 3−δ (LSCO) thin films. Two novel intermediate states with different oxygen content are identified by X‐ray diffraction, which involves a single‐phase reduced PV state and a mixed state of co‐existing PV and BM. The combination of depth‐sensitive polarized neutron reflectometry (PNR) and Rutherford backscattering (RBS) allows a quantitative determination of magnetization and the mean oxygen content in all states, revealing a continuous transition from La 0.6 Sr 0.4 CoO 2.97 to La 0.6 Sr 0.4 CoO 2.5 . BM formation is observed for an LSCO layer with an oxygen content of 2.67, while the magnetic and electronic transition already occurs for a layer with a higher oxygen content of 2.77 (and above) and in the absence of a BM signature. These results demonstrate that the physics of electronic metal‐to‐insulator transition (MIT), magnetic ferromagnet‐to‐non‐ferromagnet transition (FM‐to‐non‐FM), and structural PV‐to‐BM phase transition should be considered within the framework of separate but interrelated processes.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Isostructural electronic transition in MoS 2 probed by solid-state high-harmonic generation spectroscopy

Studying materials under extreme pressure in diamond anvil cells (DACs) is key to discovering emergent states of matter, yet no method currently allows the direct measurement of the electronic structure in this environment. Solid-state high-harmonic generation (sHHG) offers a unique all-optical window into the electronic structure of materials. We demonstrate sHHG spectroscopy inside a DAC by probing 2H-MoS 2 , up to 30 GPa, revealing a pressure-induced crossover of the lowest direct bandgap from the K-point to the Γ-point. This transition manifests as a sharp minimum in harmonic intensity and a 30° rotation of the sHHG polarization anisotropy, despite the absence of a structural phase change. First-principles simulations attribute these features to interference between competing excitation pathways at distinct points in the Brillouin zone. Our results establish sHHG as a sensitive probe of electronic transitions at high pressure, enabling access to quantum phenomena that evade detection by conventional techniques.

Nebgen, Bailey R. [University of California, Berke↗
DOE OSTI2021

Quantized electronic transitions in electrodeposited copper indium selenide nanocrystalline homojunctions

Pairing semiconductors with electrochemical processing offers an untapped opportunity to create novel nanostructures for practical devices. Here we report the results of one such pairing: the in-situ formation of highly-doped, interface-matched, sharp nanocrystalline homojunctions (NHJs) with single step electrodeposition of two copper-indium-selenide (CISe) compounds on flexible foil. It produces a homogenous film, comprising inherently ordered, 3-dimensional interconnected network of pn -CISe NHJs. These CISe NHJs exhibit surprising non-linear emissions, quantized transitions, large carrier mobility, low trap-state-density, long carrier lifetime and possible up-conversion. They facilitate efficient separation of minority carriers, reduce recombination and essentially function like quantum materials. This approach mitigates the material issues and complex fabrication of incumbent nanoscale heterojunctions; it also overcomes the flexibility and scale-up challenges of conventional planar pn junctions. The self-stabilized CISe NHJ film can be roll-to-roll processed in ambient atmosphere, thus providing a promising platform for a range of optoelectronic technologies. This concept exemplified by CISe compounds can be adapted to create nano-scale pn junctions with other inorganic semiconductors.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Searching for Muon to Electron Transition [Slides]

Summary : Searches for violations of fundamental symmetries and conservation laws - Powerful Probes of physics beyond the Standard Model (BSM); Charged Lepton Flavor violation : Mu to E conversion; 4-orders of magnitude improvements in BR; SD process can dominate the conversion process in ALP models; fa > O(100)TeV can be achieved, complementary role.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2020

Spinodal electronic phase separation during insulator-metal transitions

Electronic phase transitions such as insulator-metal transitions are common in strongly correlated systems. Here, using a combination of thermodynamic linear-stability analysis and phase-field simulations and employing VO 2 as a prototypical example, we predict that an insulator-metal transition driven by photoexcitation may involve an intermediate, modulated charge density state with a temperature-dependent characteristic wavelength. It is shown that such an intermediate two-phase electronic state is formed through a spinodal mechanism and that its formation can be generic for insulator-metal transitions driven by fast stimuli. This transient electronic phase separation is expected to stimulate future experimental and computational efforts.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Electronic Topological Transition as a Route to Improve Thermoelectric Performance in Bi 0.5 Sb 1.5 Te 3

The electronic structure near the Fermi surface determines the electrical properties of the materials, which can be effectively tuned by external pressure. Bi0.5Sb1.5Te3 is a p-type thermoelectric material which holds the record high figure of merit at room temperature. Here it is examined whether the figure of merit of this model system can be further enhanced through some external parameter. With the application of pressure, it is surprisingly found that the power factor of this material exhibits λ behavior with a high value of 4.8 mW m –1 K –2 at pressure of 1.8 GPa. Such an enhancement is found to be driven by pressure-induced electronic topological transition, which is revealed by multiple techniques. Together with a low thermal conductivity of about 0.89 W m –1 K –1 at the same pressure, a figure of merit of 1.6 is achieved at room temperature. The results and findings highlight the electronic topological transition as a new route for improving the thermoelectric properties.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Electronic topological transitions in cadmium under pressure studied via theoretical and experimental x-ray absorption spectroscopy

Here, an electronic topological transition (ETT) in cadmium below 1 GPa is investigated in situ with experimental x-ray absorption spectroscopy and projecting calculated core-valence excitons onto the band structure. These projections are a useful application of the Bethe-Salpeter equation approach that considers many-body effects. The method described herein can be used for systems that are otherwise difficult to probe in situ; therefore, it provides a generalizable approach to identifying and understanding ETTs under high pressure. Although pressure-induced ETTs are often probed using indirect structural responses, our own x-ray diffraction and Raman studies suggest a second-order structural transition around 3 GPa but are largely insensitive to or inconclusive for the previously studied ETT in this region.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Electronic bonding transitions in oxide glass above two megabar pressures

Inelastic x-ray scattering (IXS) of B 2 O 3 glass up to ∼2.2 Mbar reveals electronic bonding transitions in oxide glasses. B -edge IXS identifies the high-energy feature above ∼1.4 Mbar and a gradual increase in its intensity toward ∼2.2 Mbar, indicating the formation of hypervalent boron via electron polarization to oxygen atoms. The pressure-driven high energy shifts in O -edge IXS indicate pronounced electronic dispersion that increases upon densification of amorphous oxides above ∼2 Mbar. The extent of the energy shifts and enhanced polarization correlate with increasing atomic radius of cation in oxide glass, establishing the role of cation radius in electronic structures of amorphous oxides under compression. The results elucidate the electronic mechanisms behind the structural transformation in low- oxide glasses, where transitions to highly coordinated cations are hindered well above 1 Mbar, providing the origin of incompressibility of low- amorphous oxide under multi-Mbar compression. Published by the American Physical Society 2024

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗