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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 1,279 records · Page 71
All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

Strain-Driven Evolution of Structural and Carrier Recombination Dynamics in Germanium

Strain engineering in group-IV semiconductors, in particular germanium (Ge), offers a promising route for advancing next-generation optoelectronic device performance. High minority carrier lifetime serves as a direct indicator of superior material quality and device efficiency. Several factors govern the minority carrier lifetime in both strained Ge and strained GeSn, such as strain level, epilayer thickness, defects, surface roughness, and Sn alloying. In this study, the impact of biaxial strain on effective minority carrier lifetimes of tensile-strained Ge (0% ..epsilon..-Ge/Al?As, 0.78% ..epsilon..-Ge/In0.115?Ga0.885?As, and 0.93% ..epsilon..-Ge/In0.16?Ga0.84?As) and compressively strained GeSn (-0.53% c-Ge0.97?Sn0.03/Al?As) epilayers, grown by molecular beam epitaxy, was experimentally investigated via the microwave-reflection photoconductive decay (..mu..-PCD) technique as a simultaneous function of thickness and strain. The structural and strain-relaxation properties were analyzed by high-resolution x-ray diffraction and Raman spectroscopy, while surface morphology analysis using atomic force microscopy provided insights into the surface roughness of strained Ge-based epilayers. Defect characterization by cross-section transmission electron microscopy revealed superior crystalline quality of ..epsilon..-Ge in one epilayer, while twin boundary formation was observed in the other. We investigated the carrier recombination dynamics in epitaxially strained Ge by extracting the bulk recombination lifetime and surface recombination velocity using the thickness-dependent minority carrier lifetime. High effective minority carrier lifetimes of approximately 60-340 ns were achieved at high strain levels with minimal dependence on the thickness of the strained Ge-based epilayers by means of the ..mu..-PCD technique. Minority carrier lifetimes in Ge-based epilayers exhibit a quadratic dependence on the biaxial strain, ranging from -0.88% (compressive) to +1.6% (tensile). The analysis of the carrier recombination dynamics in strained Ge-based heterostructures offers a pathway to designing group-IV based optoelectronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Correlated purification for restoring 𝑁-representability in quantum simulation

Experimentally measured reduced density matrices (RDMs) often violate constraints that ensure they represent N-electron states—known as N-representability conditions—because of statistical and hardware noise. In this work, we present a correlated purification framework based on semidefinite programming to restore the accuracy of a noisy, unphysical two-electron RDM (2-RDM). The method performs a bi-objective optimization that minimizes both the many-electron energy and the nuclear norm of the correction to the measured 2-RDM. The nuclear norm, often employed in matrix completion, promotes low-rank corrections, while the energy term acts as a regularization term that can improve the purity of the ground state. While the method is particularly effective for ground states, it can also be applied to excited and nonstationary states by decreasing the weight of the energy relative to the error norm. In an application to fermionic shadow tomography of large hydrogen chains, correlated purification yields substantial reductions in both energy and 2-RDM error, achieving chemical accuracy across dissociation curves. This framework provides a robust strategy for tomography in many-body quantum simulations.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

X-ray-line coincidence photopumping in a potassium-chlorine mixed plasma

Exploiting the multiple long pulse capability and suite of x-ray diagnostics of the Orion laser, we have set out to explore line coincidence photopuming—the enhancement in population of an atomic level brought on by resonant absorption of x rays from a different emitting ion. Unlike previous work, the two ions are in the same plasma and so the experiment is an x-ray analog of the well-known Bowen resonance fluorescence mechanism that operates in astrophysical situations in the optical region. Herein, our measurements have shown enhanced fluorescence in a chlorine plasma, attributable to line coincident photopumping from co-mixed potassium ions. To detect this relatively low signal-to-noise phenomenon, the data from multiple shots are combined, and the statistical method of bootstrapping is used to assign a confidence value to the measured enhancement, resulting in an estimate of the enhancement of 39 ± 18 16 % compared to the null case, where no pumping occurs. The experimental results have been compared to coupled radiation-transport and radiation hydrodynamics simulations using the CRETIN code together with the NYM radiation hydrodynamics model and agreement has been found, with the simulations also predicting modest enhancement.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2024

0D, 1D, 2D, and 3D simulations of an idealized coaxial impedance-matched Marx generator

We have conducted 0D, 1D, 2D, and 3D simulations of an idealized coaxial impedance-matched Marx generator (IMG) []. The 0D calculations were conducted with a four-element circuit model; the 1D, 2D, and 3D calculations were conducted with highly resolved, fully electromagnetic representations. The IMG consists of 30 stages distributed axially and connected electrically in series. Each stage is powered by two bricks separated by 180° and connected electrically in parallel. Each brick comprises two opposite-polarity capacitors in series with a single switch. The bricks drive an internal impedance-matched coaxial transmission line terminated by a resistive load. The simulations neglect effects due to the switch-triggering circuit, the capacitor-charging circuit, external conducting boundaries, and reactive components of the load. We find dimensionality does not significantly affect the electrical power delivered by the IMG to its load: peak load powers estimated by the 0D, 1D, 2D, and 3D simulations agree to within 1%. The 3D calculations demonstrate that electromagnetic power radiated by the bricks, and axial gaps between stages, reduces the peak load power by less than ∼ 1 % . Each simulation assumes the load impedance is 34% above that at which the load power is maximized. Operating an IMG with such an overmatched load offers several advantages while decreasing the peak load power by only 2%. The 0D, 1D, 2D, and 3D models outlined herein could be adapted to assess computationally competing IMG designs, and conduct a variety of numerical IMG experiments, an IMG is constructed. Published by the American Physical Society 2024

43 PARTICLE ACCELERATORS↗
DOE OSTI2022

Fathoming the anisotropic magnetoelasticity and magnetocaloric effect in GdNi

Intermetallic GdNi adopts a CrB type of crystal structure (space group Cmcm), and it orders ferromagnetically via a second-order phase transition at 70 K, exhibiting unusually strong spontaneous striction along the three independent crystallographic axes in the ferromagnetically ordered state. Here we introduce a microscopic model to describe anisotropic changes of lattice parameters and elastic contribution to magnetocaloric effect of GdNi. In the model, results of density functional theory (DFT) calculations are used as inputs into a Hamiltonian that includes elastic energy of an anisotropic crystal lattice, exchange interactions, and Zeeman effect. The magnetic and elastic Hamiltonians are coupled through an anisotropic Bean-Rodbell model of magnetoelastic interactions. This coupling gives rise to anisotropic changes in the lattice parameters observed experimentally, and the model reveals good to reasonable agreements between the current theoretical results and earlier experimental data, thus validating the model within the limits of assumptions made. We also show that DFT calculations with 4f electrons of Gd treated as core electrons lead to a more adequate estimate of elastic constants of GdNi in comparison with the LDA + U method where 4f electrons are treated as valence electrons.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Designed metal-insulator transition in low-symmetry magnetic intermetallics

Compounds exhibiting half-metallic character and metal-insulator transitions draw considerable attention in condensed-matter and materials physics due to their potential use in spintronics. Here we show that specific electron doping plays a crucial role in tailoring thermodynamic, structural, electronic, and magnetic properties of materials derived from low-symmetry magnetic intermetallics, exemplified on triclinic Mn 4 Al 11 . Upon chemical doping of Al by Ge, we predict improved phase stability and adherence to a generalized “18-n rule” governing closed-shell configurations in intermetallic with narrow bandgaps. Validating experiments include measurements of phase stability and electronic transport properties of electron-doped Mn 4 Al 10 Ge that crystallizes in the Mn 4 Al 11 -type structure, confirming the bandgap opening as predicted by chemical analysis and density-functional theory. Here we also demonstrate theoretically that hydrostatic pressure enhances the predicted half-metallic gap in the up-spin channel, which leads to a ferrimagnetic-to-ferromagnetic transition driven by Mn-Mn charge ordering in the Mn 4 Al 11 parent. We have also discussed the generality of our approach in predicting the design of other classes of intermetallics including half and full Heusler compounds.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Benchmarking the extraction of statistical neutron capture cross sections on short-lived nuclei for applications using the β -Oslo method

Numerous scientific fields including astrophysics, nuclear power, and nuclear forensics require a knowledge of basic nuclear properties for large numbers of short-lived, radioactive isotopes far removed from stable nuclei. Neutron-capture cross sections are one such piece of nuclear data where direct measurements are not possible and theoretical predictions can vary by orders of magnitude. A recently developed indirect technique for inferring neutron capture rates, the β-Oslo method, has been introduced but not compared against a known, directly measured neutron capture cross section. To provide this benchmark, two indirect methods based on β decay and charged-particle reactions were used to extract the nuclear level density and γ-ray strength function of 51 Ti. The nuclear level density and γ-ray strength function from the two data sets were found to be equivalent and were used to extract the neutron capture cross section of 50 Ti which agrees with previous direct measurements at high neutron energies. Here, the results demonstrate the validity of the β-Oslo method for extracting neutron capture cross sections of short-lived nuclei and provide a sufficiently small uncertainty to be used in various applications.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2020

Measurement of neutron-proton capture in the $\mathrmSNO+ water phase

The SNO+ experiment collected data as a low-threshold water Cherenkov detector from September 2017 to July 2019. Measurements of the 2.2-MeV γ 's produced by neutron capture on hydrogen were made using an Am-Be calibration source, for which a large fraction of emitted neutrons are produced simultaneously with a 4.4-MeV γ. Analysis of the delayed coincidence between the 4.4-MeV γ and the 2.2-MeV capture γ revealed a neutron detection efficiency that is centered around 50% and varies at the level of 1% across the inner region of the detector, which to our knowledge is the highest efficiency achieved among pure water Cherenkov detectors. Finally, in addition, the neutron capture time constant was measured and converted to a thermal neutron-proton capture cross section of $336.3^{+1.2}_{-1.5}$ mb .

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗