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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

Mechanisms of Metal Additive-Induced Ordering During SNIPS Membrane Formation

Isoporous membranes can be fabricated by combining self-assembly with nonsolvent induced phase separation (SNIPS) using an amphiphilic block copolymer like polystyrene-b-poly(4-vinylpyridine) (SV). Poly(4-vinylpyridine) (V) is known to complex with metal salts, which are hypothesized to stabilize solution ordering and preserve structure during casting. We explored how the molar ratio of metal additive to the poly(4-vinylpyridine) block affected the final membrane morphology via scanning electron microscopy (SEM). Dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), and in situ grazing-incidence SAXS were used to track changes in solution ordering and chain conformation as a function of the molar ratio of the additive to the V block. Additives induced aggregation, promoted the formation of more compact conformations in solution, and facilitated micelle ordering onto lattices at optimal ratios. Furthermore, these experimental results were supported by random phase approximation calculations, which helped explain how the thermodynamic order–disorder transition shifts with additive binding strength. Stronger additive–polymer interactions reduced the block copolymer volume fraction required for ordering in solution, allowing ordered domains to form at lower polymer concentrations.

Additives↗
DOE OSTI2021

Direct Structural Evidence for Interfacial Gradients in Asymmetric Polymer Nanocomposite Blends

Understanding the complex structure of polymer blends filled with nanoparticles (NPs) is key to design their macroscopic properties. In this work, the spatial distribution of hydrogenated (H) and deuterated (D) polymer chains asymmetric in mass is studied by small-angle neutron scattering. Depending on the chain mass, a qualitatively new large-scale organization of poly(vinyl acetate) chains beyond the random-phase approximation is evidenced in nanocomposites with attractive polymer–silica interactions. The silica is found to systematically induce bulk segregation. Only with long H-chains, a strong scattering signature is observed in the q range of the NP size: it is the sign of interfacial isotopic enrichment, that is, of contrasted polymer shells close to the NP surface. A quantitative model describing both the bulk segregation and the interfacial gradient (over ca. 10–20 nm depending on the NP size) is developed, showing that both are of comparable strength. In all cases, NP surfaces trap the polymer blend in a non-equilibrium state, with preferential adsorption around NPs only if the chain length and isotopic preference toward the surface combine their entropic and enthalpic driving forces. This structural evidence for interfacial polymer gradients will open the road for quantitative understanding of the dynamics of many-chain nanocomposite systems.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Structural phase transition, s ± -wave pairing, and magnetic stripe order in bilayered superconductor La 3 Ni 2 O 7 under pressure

Motivated by the recently discovered high-T c superconductor La 3 Ni 2 O 7 , we comprehensively study this system using density functional theory and random phase approximation calculations. At low pressures, the Amam phase is stable, containing the Y 2- mode distortion from the Fmmm phase, while the Fmmm phase is unstable. Because of small differences in enthalpy and a considerable Y 2- mode amplitude, the two phases may coexist in the range between 10.6 and 14 GPa, beyond which the Fmmm phase dominates. In addition, the magnetic stripe-type spin order with wavevector (π, 0) was stable at the intermediate region. Pairing is induced in the s ± -wave channel due to partial nesting between the M = (π, π) centered pockets and portions of the Fermi surface centered at the X = (π, 0) and Y = (0, π) points. This resembles results for iron-based superconductors but has a fundamental difference with iron pnictides and selenides. Moreover, our present efforts also suggest La 3 Ni 2 O 7 is qualitatively different from infinite-layer nickelates and cuprate superconductors.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Predicting nucleon-nucleus scattering observables using nuclear structure theory

Developing a predictive capability for inelastic scattering will find applications in multiple areas. Experimental data for neutron-nucleus inelastic scattering is limited and thus one needs a robust theoretical framework to complement it. Charged-particle inelastic scattering can be used as a surrogate for ( n, γ ) reactions to predict capture cross sections for unstable nuclei. Our work uses microscopic nuclear structure calculations for spherical nuclei to obtain nucleon-nucleus scattering potentials and calculate cross sections for these processes. We implement the Jeukenne, Lejeune, Mahaux (JLM) semi-microscopic folding approach, where the medium effects on nuclear interaction are parameterized in nuclear matter to obtain the nucleon-nucleon ( NN ) interaction in a medium at positive energies. We solve for the nuclear ground state using the Hartree-Fock-Bogliubov (HFB) many-body method, assuming the nucleons within the nucleus interact via the Gogny-D1M potential. The vibrational excited states of the target nucleus are calculated using the quasi-particle random phase approximation (QRPA). We demonstrate our approach for spherical nuclei in the medium-mass region, showing scattering results for the 90 Zr nucleus.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2025

Microscopic calculations with noniterative finite amplitude methods and the application to neutron radiative captures and inelastic scatterings

We derive the fully self-consistent quasiparticle random-phase approximation (QRPA) equations with noniterative finite amplitude methods and calculate the transition strengths of giant resonances. Then, we apply the QRPA results to both neutron radiative capture calculations based on the statistical Hauser-Feshbach theory and inelastic scattering calculations based on distorted-wave Born approximation (DWBA). We compare the calculated results with available experimental data and demonstrate how our approach can reproduce giant resonances and various nuclear reactions.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2025

Electric dipole excitations near the neutron separation energies in 96 Mo

Electric dipole strength near the neutron separation energy significantly impacts nuclear structure properties and astrophysical scenarios. These excitations are complex in nature and may involve the so-called pygmy dipole resonance (PDR). Transition densities play a crucial role in understanding the nature of nuclear excited states, including collective excitations, as well as in constructing transition potentials in DWBA or coupled-channels equations. In this work, we focus on electric dipole excitations in spherical molybdenum isotopes, particularly 96 Mo, employing fully consistent Hartree-Fock-Bogoliubov (HFB) and Quasiparticle Random Phase Approximation (QRPA) methods. We analyze the dipole strength near the neutron separation energy, which represents the threshold for neutron capture processes, and examine the isospin characteristics of PDR states through transition density calculations. Examination of proton and neutron transition densities reveals distinctive features of each dipole state, indicating their isoscalar and isovector nature. We observe that the primary component in the enhanced low-energy region exhibits isovector character. The PDR displays a mixture of isoscalar and isovector nature, distinguishing it from the isovector giant dipole resonance (IVGDR). These findings lay the groundwork for future investigations into the role of transition densities in reaction models and for their application to inelastic scattering calculations.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2024

Cooper minima in high-Z atoms: effects of correlation and relativity on np photoionization

Abstract Photoionization dipole transition matrix elements pass through a zero or attain a minimum that leaves imprints on photoionization parameters like the cross-section, angular distribution asymmetry parameter, phase shift, and photoionization time delay. This minimum is commonly known as the ‘Cooper minimum’ (CM). The CM, in general, is strongly affected by relativistic and correlation effects. Previous works investigated CM in the 6pand 5psubshell photoionization up toZ= 100 using the single-particle Dirac-Slater (DS) method. The present work extends the earlier work toZup to 120 using more accurate methods; Dirac–Hartree–Fock (DHF) which includes the relativistic effects and exchange correlations, and the relativistic random phase approximation (RRPA) which includes both initial and final state electron-electron correlations along with relativistic effects. In addition to the study of photoionization from the 6pand 5psubshells, the 4psubshell has also been investigated in the present work. To demonstrate the prominent effects in the high-Z atoms, Rn (Z= 86), Ra (Z= 88), No (Z= 102), Cn (Z = 112), Og (Z= 118), and Ubn (Z= 120) are investigated.

Optics↗
DOE OSTI2022

How effective is the Brink–Axel hypothesis for astrophysical weak rates?

We explore the effectiveness of the Brink–Axel hypothesis (BAH) for the computation of stellar electron capture (EC) and β-decay (BD) rates, namely that the transition strength function depends only upon the transition energy and not upon the details of the initial state. For this purpose, we calculated Gamow–Teller (GT) strength distributions for a selection of sd-shell nuclides, using two different microscopic models, namely the proton–neutron quasiparticle random phase approximation and the full configuration-interaction shell model, taking into account the first 100 states of both the initial and final nuclides. The GT transition strengths among these levels evolve with initial state energy. These transition strength functions we folded into weak-interaction mediated rates in stellar matter, specifically EC and BD rates, for a range of densities 10 g cm –3 ≤ ρ ≤ 10 11 g cm –3 and range of temperatures 1 GK ≤ T ≤ 30 GK. When transitions from excited states were approximated using the BAH, augmented by so-called 'back-resonance' transitions, the rates were affected by up to three orders of magnitude or more at high temperatures and densities. Furthermore the BAH is not a reliable approximation for the calculation of stellar rates, especially in high temperature–density environments.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

Dispersive interactions between standard and Dirac materials and the role of dimensionality

The van der Waals (vdW) interaction plays a prominent role between neutral objects at separations where short ranged chemical forces are negligible. This type of dispersive coupling is determined by the interplay between geometry and response properties of the materials making up the objects. Here, we investigate the vdW interaction between 1D, 2D, and 3D standard and Dirac materials within the Random Phase Approximation, which takes into account collective excitations originating from the electronic Coulomb potential. A comprehensive understanding of characteristic functionalities and scaling laws are obtained for systems with parabolic energy dispersion (standard materials) and crossing linear bands (Dirac materials). By comparing the quantum mechanical and thermal limits the onset of thermal fluctuations in the vdW interaction is discussed showing that thermal effects are significantly pronounced at smaller scales in reduced dimensions.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Electronic structure, and magnetic and superconducting pairing tendencies of the alternating single layer–bilayer stacking nickelate La 5 ⁢Ni 3 ⁢O 11 under pressure

Nickelates have continued to surprise since their unconventional superconductivity was discovered. Recently, the layered nickelate La 5 ⁢Ni 3 ⁢O 11 with hybrid single-layer and bilayer stacking showed superconductivity under high pressure. This compound combines features of single-layer La 2 ⁢NiO 4 and bilayer La 3 ⁢Ni 2 ⁢O 7 , but its pairing mechanism remains to be understood. Motivated by this finding, here we report a comprehensive theoretical study of this system. Our density functional theory calculations reveal that the undistorted P4/mmm phase without pressure is unstable due to three distortion modes. As pressure increases, these modes are suppressed, leading to the high-symmetry P4/mmm phase without NiO 6 octahedron tilting. Moreover, the “charge transfer” between the single-layer and bilayer sublattices was observed, leading to hole doping in the single-layer blocks. Our random-phase approximation calculations indicate a leading 𝑑 𝑥 2 −𝑦 2 -wave pairing state that arises from spin-fluctuation scattering between Fermi surface states mainly originating from the single-layer blocks and additional weaker contributions from the bilayer blocks. These spin fluctuations could be detected by inelastic neutron scattering as a strong peak at 𝐪 = (𝜋,𝜋). Our findings distinguish La 5 ⁢Ni 3 ⁢O 11 from other nickelate superconductors discovered so far and the high-𝑇 𝑐 cuprates. We also discuss both similarities and differences between La 5⁢ Ni 3 ⁢O 11 and other hybrid stacking nickelates.

Zhang, Yang [Univ. of Tennessee, Knoxville, TN (Un↗
DOE OSTI2026

Beta-Decay Half-Lives beyond 54 Ca: A Systematic Survey of Decay Properties Approaching the Neutron Dripline

In an experiment performed at the Facility for Rare Isotope Beams (FRIB) using the FRIB Decay Station initiator, 15 new half-lives of isotopes near 54 Ca were measured. A new method of extracting lifetimes from experimental data, taking into account the unknown 𝛽-delayed neutron emission branches of very neutron-rich nuclei, was developed to enable systematic uncertainty analysis. The experiment observed a dramatic change in the half-life systematics for the isotopes with neutron number 𝑁 = 34. Beyond 𝑁 = 34, the decline of nuclear lifetime is much slower, leading to longer than anticipated lifetimes for near-dripline nuclei. State-of-the-art shell-model calculations can explain the experimental results, revealing the imprint of shell effects and the need for modification of single-particle neutron states. In conclusion, the results from a newly developed quasiparticle random phase approximation model with potential for making global predictions were also tested against the experimental results and good agreement was found.

Physics - Nuclear physics and radiation physics↗
DOE OSTI2026

Compressive strain turns 𝑠 ± - into 𝑑-wave pairing in a one-unit-cell La 3 ⁢Ni 2 ⁢O 7 thin film via substrate-induced hole doping

Motivated by recent reports of ambient-pressure superconductivity in La 3 ⁢Ni 2⁢ O 7 films grown on LaSrAlO 4 , we investigate the superconducting instability in a one-unit-cell (1UC) thin film using ab initio and random-phase approximation techniques. Compared to the high-pressure bulk system, the ratio of interlayer 𝑑 3⁢𝑧 2 −𝑟 2 hopping to intralayer 𝑑 𝑥 2 −𝑦 2 hopping is suppressed in the 1UC thin film, and the crystal-field splitting of the 𝑒 𝑔 orbitals is increased. Here, our calculation indicates that spin-fluctuation-driven pairing correlations are weak for the stoichiometric case at ambient pressure, but increase significantly under hole doping. The leading pairing symmetry is also found to change by hole doping. Specifically, we obtain a leading 𝑑 𝑥 2 −𝑦 2 pairing state at moderate hole doping, followed by a 𝑑 𝑥⁢𝑦 state at higher doping. These states are driven by intraband spin-fluctuation scattering within the 𝛾 hole pocket centered around the 𝑀 point, and arise primarily from states in the Ni layer farther from the substrate. These results strongly suggest that the thin-film superconducting samples are hole-doped and that pairing in this system predominantly arises in the layer, as opposed to the interlayer pairing in the pressurized bulk system.

Zhang, Yang [Oak Ridge National Laboratory (ORNL),↗
DOE OSTI2025

Production of neutron-rich heavy nuclei in deep-inelastic 208 Pb + 208 Pb collisions within the stochastic mean-field theory

In deep-inelastic collisions of heavy nuclei, reaction products with a wide range of mass and charge are produced. Such collisions been considered as a possible way to produce superheavy nuclei, as an alternative to fusion reactions. To provide reliable theoretical predictions, it is desired to develop microscopic approaches that correctly and accurately describe nucleon transfer processes in dissipative collisions of heavy nuclei. The purpose of the present work is (1) to investigate the mechanism of nucleon transfers in dissipative collisions of two heavy nuclei, and (2) to explore possible pathways to produce neutron-rich heavy nuclei, through detailed theoretical analyses of fluctuations and correlations in nucleon transfers in 208 Pb + 208 Pb reactions. Three-dimensional time-dependent Hartree-Fock (TDHF) calculations are performed for the collisions of 208 Pb + 208 Pb at 𝐸 c.m. = 832, 936, and 1040 MeV, using the Skyrme SLy4d energy density functional. To calculate fluctuations and correlations in nucleon transfers, we employ the stochastic mean-field (SMF) theory, and the results are compared with another theoretical framework currently available, the time-dependent random phase approximation (TDRPA). Primary and secondary production cross sections are calculated with the SMF theory combined with a statistical model, GEMINI ++ . Using information of nucleon flow across a neck of colliding nuclei in TDHF calculations, we solve quantal diffusion equations for fluctuations and correlations in nucleon transfers based on the SMF theory. From the SMF calculations, we obtain the time evolution of diffusion coefficients as well as fluctuations and correlations in nucleon transfers for a range of initial orbital angular momenta. We compare the results of the SMF calculations with those of TDRPA, showing that TDRPA tends to predict substantially larger fluctuations and correlations in strongly damped collisions of heavy nuclei, which exhibit complex initial angular momentum dependence, while the SMF results provide almost constant (stable) values. Using the obtained fluctuations and correlations, we calculate primary and secondary production cross sections for the 208 Pb + 208 Pb collisions. From the results, we find that both lighter and heavier reaction products as compared to 208 Pb are produced for a wide region in the 𝑁−𝑍 plane as primary products, thanks to the quantal diffusion mechanism in the dissipative collisions. However, we show that cross sections for production of heavy nuclei with 𝑍 ≳ 90 or 𝑁 ≳ 135 are washed out due to secondary particle evaporation and/or fission processes. On the other hand, we find that there remain sizable cross sections for production of neutron-rich nuclei along 𝑁 = 126 with 𝑍< 82, even after secondary disintegration processes. We demonstrate that the secondary production cross sections depend weakly on incident energies, but lower (higher) energy is slightly preferred for production of nuclei with smaller (larger) atomic numbers as compared to 𝑍 = 82. Here, based on the microscopic SMF calculations, it has been shown that deep-inelastic collisions of heavy nuclei, such as 208 Pb + 208 Pb examined in this study, can be a promising means to produce neutron-rich heavy nuclei along 𝑁=126. Discrepancies between the SMF and TDRPA approaches are left unsolved for future investigations.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2026

Ab initio calculations of monopole sum rules: From finite nuclei to infinite nuclear matter

We compute moments of the isoscalar monopole response of 𝑁 = 𝑍 closed-shell nuclei based on chiral nucleon-nucleon plus three-nucleon interactions. We employ the random-phase approximation (RPA) and two ab initio many-body approaches, the in-medium similarity renormalization group (IMSRG) and coupled-cluster theory (CC). In the IMSRG framework, the moments are obtained as ground-state expectation values, whereas in the CC approach, they are evaluated through excited-state calculations. We find good agreement between the IMSRG and CC results across all nuclei studied. RPA provides a reasonable approximation to the correlated methods if the interaction is soft. From the calculated moments, we extract average energies of the monopole response, compute finite-nucleus incompressibilities, and estimate the incompressibility of symmetric nuclear matter by a fit to a leptodermous expansion. Our extrapolated values are lower than those obtained in nuclear-matter calculations with the same interactions, but the values are consistent with phenomenological ranges.

Bonaiti, Francesca [Michigan State Univ., East Lan↗
DOE OSTI2026

Enhanced antineutrino emission from 𝛽 decay in core-collapse supernovae with self-consistent weak decay rates

Nuclear weak-interaction rates are known to exert a prominent effect in the late stages of stellar collapse. Despite their importance, most studies to date on core-collapse supernovae (CCSNe) have focused primarily on the effects of electron captures, neglecting 𝛽 decay contributions. In this work, we present the first CCSNe simulation incorporating global 𝛽 decay rates from a microscopic theory. These are enabled by a large-scale evaluation of both electron capture and 𝛽 decay rates, obtained self-consistently utilizing the relativistic energy density functional theory and finite-temperature quasiparticle random-phase approximation. Including 𝛽 decay leads to a dramatic enhancement of the pre-bounce antineutrino signal as the antineutrino emissivity increases by more than two orders of magnitude and the luminosity by a factor of 50 relative to thermal emission alone, while the average antineutrino energy increases by over 1 MeV. Furthermore, it is expected that these new rates could help us constrain the model uncertainties related to weak-interaction processes, improving the prediction of antineutrino signal during the final stages of stellar death.

Beta decay↗
DOE OSTI2022

Unusual behavior of Cooper minima of n s subshells in high- Z atoms

Here, a study of Cooper minima (CM) arising from the photoionization of 6$\textit{s}$, 5$\textit{s}$, and 4$\textit{s}$ subshells of high-$\textit{Z}$ atoms has been performed using Dirac-Fock (DF), two-channel relativistic-random-phase approximation (RRPA), and fully coupled RRPA. The results show huge splittings between $ns → εp_{3/2}$ and $ns → εp_{1/2}$ CM which increase with $\textit{Z}$ owing primarily to the relativistic interactions (spin orbit) that are attractive for the $εp_{1/2}$ final state but repulsive for the corresponding $εp_{3/2}$. In addition, it was found that correlation in the form of interchannel coupling (essentially configuration interaction in the final continuum states) plays a huge role in determining the location of the CM. For 6$\textit{s}$ photoionization, the 6$s → εp_{3/2}$ and 6$s → εp_{1/2}$ CM behave completely different as a function of $\textit{Z}$ . It was also found that for 5$\textit{s}$ and 4$\textit{s}$ photoionization, the CM move below the threshold, with increasing $\textit{Z}$ , and, at high enough $\textit{Z}$ , the 5s → εp3/2 and 4s → εp3/2 CM re-emerge into the continuum. The calculations have been carried out for the $\textit{ns}$ subshells of Hg ($\textit{Z}$ = 80), Rn ($\textit{Z}$ = 86), Ra ($\textit{Z}$ = 88), No ($\textit{Z}$ = 102), Cn ($\textit{Z}$ = 112), and Og ($\textit{Z}$ = 118).

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2023

Attosecond time delay in atomic photoionization: Angular-dependent transition from dipole to quadrupole and spin-flip dynamics

The angular distribution of attosecond Wigner time delay has been investigated including both quadrupole and relativistic effects using the relativistic-random-phase approximation that is based on the Dirac equation and includes significant aspects of many-body correlations. Here the results show a dramatic evolution of the time delay from (essentially nonrelativistic) dipole to quadrupole and spin-flip dynamics as a function of angle, thereby providing a venue for studying these interactions at the attosecond level.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2021

Control of plasmons in doped topological insulators via basis atoms

Collective excitations in topologically nontrivial systems have attracted considerable attention in recent years. Here we study plasmons in the Su-Schrieffer-Heeger model whose low-energy electronic band is only partially filled, such that the system is metallic. Using the random phase approximation, we calculate the intra- and interband polarization functions and determine the bulk plasmonic dispersion from the dielectric function. In this work, we find that the sublattice basis states strongly affect the polarization functions and therefore control the system’s plasmonic excitations. By varying the real-space separation of these local orbitals, one can thus selectively enhance or suppress the plasmonic energies via a tunable trade-off between intraband and interband screening processes. Specifically, this mechanism can be used to stabilize undamped high energy plasmons that have already been reported in related models. We propose scenarios on how to control and observe these effects in experiments.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗