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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 1,387 records · Page 77
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Nuclear structure studies with gamma-ray beams

In stable and weakly bound neutron-rich nuclei, a resonance-like concentration of dipole states has been observed for excitation energies below the neutron-separation energy. This clustering of strong dipole states has been named the Pygmy Dipole Resonance (PDR) in contrast to the Giant Dipole Resonance (GDR) that dominates the E1 response. Understanding the PDR is presently of great interest in nuclear structure and nuclear astrophysics. High-sensitivity studies of E1 and M1 transitions in closed-shell nuclei using monoenergetic and 100% linearly-polarized photon beams are presented.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2016

Si clusters are more metallic than bulk Si

Dipole polarizabilities were computed using density functional theory for silicon clusters over a broad range of sizes up to N = 147 atoms. The calculated total effective polarizabilities, which include contributions from permanent dipole moments of the clusters, are in very good agreement with recently measured values. We show that the permanent dipole contributions are most important for clusters in the intermediate size range and that the measured polarizabilities can be used to distinguish between energetically nearly degenerate cluster isomers at these sizes. We decompose the computed total polarizabilities into the so-called dipole and charge transfer contributions, $α^p$ and $α^q$, using a sitespecific analysis. When the per-atom values of these quantities are plotted against $N^{-1/3}$, clear linear trends emerge that can be extrapolated to the large size limit ($N^{-1/3}$ → 0), resulting in a value for $\frac{α}{N}$ of 30.5 bohrs 3 /atom that is significantly larger than the per-atom polarizability of semiconducting bulk Si, 25.04 bohrs 3 /atom. This indicates that Si clusters possess a higher degree of metallicity than bulk Si, a conclusion that is consistent with the strong electrostatic screening of the cluster interiors made evident by the analysis of the calculated atomic polarizabilities.

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

Theory and modeling of molecular modes in the NMR relaxation of fluids

Traditional theories of the nuclear magnetic resonance (NMR) autocorrelation function for intra-molecular dipole pairs assume a single-exponential decay, yet the calculated autocorrelation of realistic systems displays a rich, multi-exponential behavior, resulting in anomalous NMR relaxation dispersion (i.e., frequency dependence). We develop an approach to model and interpret the multi-exponential intra-molecular autocorrelation using simple, physical models within a rigorous statistical mechanical development that encompasses both rotational diffusion and translational diffusion in the same framework. Here, we recast the problem of evaluating the autocorrelation in terms of averaging over a diffusion propagator whose evolution is described by a Fokker–Planck equation. The time-independent part admits an eigenfunction expansion, allowing us to write the propagator as a sum over modes. Each mode has a spatial part that depends on the specified eigenfunction and a temporal part that depends on the corresponding eigenvalue (i.e., correlation time) with a simple, exponential decay. The spatial part is a probability distribution of the dipole pair, analogous to the stationary states of a quantum harmonic oscillator. Drawing inspiration from the idea of inherent structures in liquids, we interpret each of the spatial contributions as a specific molecular mode. These modes can be used to model and predict the NMR dipole–dipole relaxation dispersion of fluids by incorporating phenomena on the molecular level. We validate our statistical mechanical description of the distribution in molecular modes with molecular dynamics simulations interpreted without any relaxation models or adjustable parameters: the most important poles in the Padé–Laplace transform of the simulated autocorrelation agree with the eigenvalues predicted by the theory

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Designing artificial two-dimensional landscapes via atomic-layer substitution

Significance Manipulating materials with atomic-scale precision is essential for the development of a next-generation material design toolbox. Tremendous efforts have been made to advance the compositional, structural, and spatial accuracy of material deposition and patterning. Here, we presented a new reaction pathway to implement the conversions of two-dimensional materials within the atomic-layer thickness at room temperature for electrical dipole manipulation. Not only could various Janus monolayer transition metal dichalcogenides with vertical dipole be realized, but also some heterostructures, including the dipole-nondipole heterostructures (MoS 2 -MoSSe) and multiheterostructures (MoS 2 -MoSSe-MoSeS-MoSe 2 ) within the same monolayer host structure are developed, in which the dipoles can be selectively patterned to be zero (MoS 2 , MoSe 2 ), positive (MoSSe), and negative (MoSeS).

Guo, Yunfan↗
DOE OSTI2019

Pygmy resonances: what’s in a name?

The centroid, width and percentage of energy weighted sum rule of dipole resonances can be strongly affected by dynamical fluctuations and static deformations of the nuclear surface, deformations and fluctuations which, in turn, depend on pairing, and thus on Cooper pairs. Because of angular momentum conservation, such insight is restricted, to lowest order, to fluctuations/deformations of quadrupole and monopole type. The latter being closely connected with the neutron (excess) skin and thus with soft dipole modes. From the values ( N - Z)/ A ≈ 0.18, 0.21, and 0.45 for the nuclei 122 Sn, 208 Pb, and 11 Li, it is expected that the latter system, which is weakly bound by pairing effects (spatially extended single Cooper pair and odd proton acting as spectator), constitutes an attractive laboratory to study the properties of soft E 1-modes and thus of isospin nuclear deformation. From the calculation of the full dipole response function in QRPA, discretizing the continuum in a spherical box of radius of 40 fm, one finds a GDR with centroid E x ≈ 24 MeV, width Γ ≈11 MeV and carrying 90% of the EWSR, and a low-lying collective resonance characterized by E X = 0.75 MeV, Γ = 0.5 MeV and 6.2% EWSR. The wave function of the latter resonance is built out of about fifteen components (both protons and neutrons), typical of a collective mode. The transition densities indicate this soft E 1 - mode to be generated by surface density oscillation of the neutron (halo) skin (Δ r np ≈ 1.71 fm) relative to an approximately isospin-saturated core. Through a detailed study of the full dipole response of 11 Li we will draw a comparison between the soft E 1 - mode of this halo nucleus and the PDR of heavy stable nuclei, pointing to the physical similarities and also to the basic differences.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2016

Effect of higher-order multipole moments on the Stark line shape

Spectral line shapes are sensitive to plasma conditions and are often used to diagnose electron density of laboratory plasmas as well as astrophysical plasmas. Stark line-shape models take into account the perturbation of the radiator’s energy structure due to the Coulomb interaction with the surrounding charged particles. Solving this Coulomb interaction is challenging and is commonly approximated via a multipole expansion. Yet, most models include only up to the second term of the expansion (the dipole term). While there have been studies on the higher-order terms due to one of the species (i.e. either ions or electrons), there is no model that includes the terms beyond dipole from both species. Here, we investigate the importance of the higher-order multipole terms from both species on the Hβ line shape. First, we find that it is important to include higher order terms consistently from both ions and electrons to reproduce measured line-shape asymmetry. Next, we study the line shape convergence with the higher-order terms at 1017, 1018, and 1019 e/cc. We discover that the line shape calculated with the dipole-only approximation becomes inaccurate as density increases. It is necessary to include only up to the third (quadrupole) term to compute the line-shape accurately within 2%. Since most existing models include only the dipole terms, the densities inferred with such models are in question. Finally, we investigate the impact of neglecting the quadrupole terms on density diagnostics. We find that the model without the quadrupole term slightly underestimates the density, and the discrepancy becomes as large as 12% at high densities. Although the case of study is limited to Hβ , we expect similar impact on other lines.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2021

Trapping interlayer excitons in van der Waals heterostructures by potential arrays

Transition metal dichalcogenide heterostructures can host interlayer excitons (IXs), which consist of electrons and holes spatially separated in different layers. IXs possess permanent dipoles and have proven to offer a wealth of novel physics. Here we develop a discrete, random-walk model which includes annihilation and repulsion interactions among IXs. Using this model, we simulate the trapping of IXs in traps of different depths, densities, and shapes. Our results show that dipole-dipole interactions play an important role in regulating IX trapping. The effects of dipole interactions can be mitigated with small, deep traps which are realizable with atomic defects and moire potentials.

2-dimensional systems↗
DOE OSTI2023

Fracton superfluid hydrodynamics

We examine the hydrodynamics of systems with spontaneously broken multipolar symmetries using a systematic effective field theory. We focus on the simplest nontrivial setting: a system with charge and dipole symmetry, but without momentum conservation. When no symmetries are broken, our formalism reproduces the quartic subdiffusion (ω ~ –ik 4 ) characteristic of “fracton hydrodynamics” with conserved dipole moment. Our formalism also captures spontaneous breaking of charge and/or dipole symmetry. When charge symmetry is spontaneously broken, the hydrodynamic modes are quadratically propagating and quartically relaxing (ω ~ ±k 2 – ik 4 ). When the dipole symmetry is spontaneously broken but the charge symmetry is preserved, then we find quadratically relaxing (diffusive) transverse modes, plus another mode which, depending on parameters, may be either purely diffusive (ω ~ – ik 2 ) or quadratically propagating and quadratically relaxing (ω ~ ± k 2 – ik 2 ). Furthermore, our work provides concrete predictions that may be tested in near-term cold atom experiments, and also lays out a general framework that may be applied to study systems with spontaneously broken multipolar symmetries.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Origin of the crossover between a freezing and a structural transition at low concentration in the relaxor ferroelectric K 1-x Li x TaO 3

The origin of the relaxor behavior in K 1-x Li x TaO 3 (KLT) and other disordered perovskites is now recognized to be due to the reorientation of the polar nanodomains formed by the correlated dipoles of off-center ions. The collective dynamics of these systems evolve through several temperature stages. On decreasing temperature below the so-called Burns temperature T B , individual dipoles become correlated within nanosized regions. On further cooling, the slow dynamics of these polar regions allows local lattice distortions to take place and the formation of polar nanodomains at T* < T B . At still lower temperature, some relaxors undergo a phase transition while others do not. In KLT, there is a critical Li concentration x c = 0.022 above which the system undergoes a structural transition at T c , and below which it freezes in a dipole glass state at T f . To better understand the nature of this critical concentration, the changes that occur upon crossing it and the nature of the dipole glass state, the collective dynamics of KLT have been studied by dielectric spectroscopy and neutron diffraction for two Li concentrations (x = 0.026 and 0.018), close to but straddling the critical concentration x c . Two very different transitional behaviors are observed. Just below this critical concentration, KLT displays critical slowing down and the onset of freezing as seen in hydrogen-bonded molecular ferroelectrics, while just above this concentration, KLT undergoes a first-order structural transition.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Interaction-induced long-time tail of a nonlinear ac absorption in a localized system: A relay-race mechanism

In conventional solid-state electron systems with localized states the ac absorption is linear since the inelastic widths of the energy levels exceed the drive amplitude. The situation is different in the systems of cold atoms in which phonons are absent. Then even a weak drive leads to saturation of the ac absorption within resonant pairs, so that the population of levels oscillates with the Rabi frequency. We demonstrate that, in the presence of weak dipole-dipole interactions, the response of the system acquires a long-time component which oscillates with frequency much smaller than the Rabi frequency. The underlying mechanism of this long-time behavior is that the fields created in the course of the Rabi oscillations serve as resonant drive for the second-generation Rabi oscillations in pairs with level spacings close to the Rabi frequency. Here, the frequency of the second-generation oscillations is of the order of interaction strength. As these oscillations develop, they can initiate the next-generation Rabi oscillations, and so on. Formation of the second-generation oscillations is facilitated by the nondiagonal component of the dipole-dipole interaction tensor.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Magnetic moments of A = 3 nuclei obtained from chiral effective field theory operators

Chiral effective field theory (χEFT) provides a framework for obtaining internucleon interactions in a systematically improvable fashion from first principles, while also providing for the derivation of consistent electroweak current operators. In this study, we apply consistently derived interactions and currents towards calculating the magnetic dipole moments of the A = 3 systems 3 H and 3 He. We focus here on LENPIC interactions obtained using semilocal coordinate-space (SCS) regularization. Starting from the momentum-space representation of the LENPIC χEFT vector current, we derive the SCS-regularized magnetic dipole operator up through next-to-next-to-leading order (N 2 LO). We then carry out no-core shell-model calculations for 3 H and 3 He systems using the SCS LENPIC interaction at N 2 LO in χEFT and evaluate the magnetic dipole moments obtained using the consistently derived one-nucleon and two-nucleon electromagnetic currents. As anticipated by prior results with χEFT currents, the current corrections through N 2 LO provide improved, but not yet complete, agreement with experiment for the 3 H and 3 He magnetic dipole moments.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2024

Electromagnetic observables of open-shell nuclei from coupled-cluster theory

Here we develop a new method to describe electromagnetic observables of open-shell nuclei with two nucleons outside a closed shell. This approach combines the equation-of-motion coupled-cluster method for such systems and the Lorentz integral transform technique, expanding the applicability of coupled-cluster theory for electromagnetic observables beyond closed-(sub)shell nuclei. To validate this new approach, we compute the non-energy-weighted dipole sum rule and the dipole polarizability of 16,24 ⁢O in both the closed-(sub)shell and the new equation-of-motion coupled-cluster frameworks, finding agreement within error bars. We then analyze the evolution of the dipole polarizability along the oxygen and calcium isotopic chains. Our predictions agree well with available experimental data and other available theoretical calculations for the closed-(sub)shell 16,22 ⁢O and the open-shell 18 O. In the calcium isotopes, we observe that our dipole polarizability predictions for open-shell nuclei are lower than those of closed-(sub)shell nuclei. We expect that our predictions for 24 O and 54,56 ⁢Ca will motivate future experimental studies at the drip line.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2016

Triaxial-band structures, chirality, and magnetic rotation in La 133

The structure of 133 La has been investigated using the 116 Cd( 22 Ne,4pn) reaction and the Gammasphere array. Three new bands of quadrupole transitions and one band of dipole transitions are identified and the previously reported level scheme is revised and extended to higher spins. The observed structures are discussed using the cranked Nilsson-Strutinsky formalism, covariant density functional theory, and the particle-rotor model. Triaxial configurations are assigned to all observed bands. For the high-spin bands it is found that rotations around different axes can occur, depending on the configuration. The orientation of the angular momenta of the core and of the active particles is investigated, suggesting chiral rotation for two nearly degenerate dipole bands and magnetic rotation for one dipole band. As a result, it is shown that the h 11/2 neutron holes present in the configuration of the nearly degenerate dipole bands have significant angular momentum components not only along the long axis but also along the short axis, contributing to the balance of the angular momentum components along the short and long axes and thus giving rise to a chiral geometry.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2021

Controlling Interactions between Quantum Emitters Using Atom Arrays

We investigate the potential for two-dimensional atom arrays to modify the radiation and interaction of individual quantum emitters. Specifically, we demonstrate that control over the emission linewidths, resonant frequency shifts, and local driving field enhancement in impurity atoms is possible due to strong dipole-dipole interactions within ordered, subwavelength atom array configurations. We demonstrate that these effects can be used to dramatically enhance coherent dipole-dipole interactions between distant impurity atoms within an atom array. Possible experimental realizations and potential applications are discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Synthetic tensor gauge fields

Synthetic gauge fields have provided physicists with a unique tool to explore a wide range of fundamentally important phenomena. However, most experiments have focused on synthetic vector gauge fields. The very rich physics brought about by coupling tensor gauge fields to fracton phases of matter remains unexplored in laboratories. Here, we propose schemes to realize synthetic tensor gauge fields that address dipoles instead of single particles. A lattice tilted by a strong linear potential and a weak quadratic potential yields a rank-2 electric field for a dipole formed by a particle-hole pair. Such a rank-2 electric field leads to a new type of Bloch oscillations, which modulates the quadrupole moment and preserves the dipole moment of the system. In higher dimensions, the interplay between interactions and vector gauge potentials imprints a phase to the ring-exchange interaction and thus generates synthetic tensor gauge fields. Such tensor gauge fields make it possible to realize a dipolar Harper-Hofstadter model in laboratories. The resultant dipolar Chern insulators feature chiral edge currents of dipoles in the absence of net charge currents. Published by the American Physical Society 2025

Zhang, Shaoliang (ORCID:000000016635044X)↗
DOE OSTI2024

Observation of millihertz-level cooperative Lamb shifts in an optical atomic clock

Collective couplings of atomic dipoles to a shared electromagnetic environment produce a wide range of many-body phenomena. We report on the direct observation of resonant electric dipole-dipole interactions in a cubic array of atoms in the many-excitation limit. The interactions produce spatially dependent cooperative Lamb shifts when spectroscopically interrogating the millihertz-wide optical clock transition in strontium-87. We show that the ensemble-averaged shifts can be suppressed below the level of evaluated systematic uncertainties for optical atomic clocks. Additionally, we demonstrate that excitation of the atomic dipoles near a Bragg angle can enhance these effects by nearly an order of magnitude compared with nonresonant geometries. Our work demonstrates a platform for precise studies of the quantum many-body physics of spins with long-range interactions mediated by propagating photons.

Science & Technology - Other Topics↗
DOE OSTI2017

Micromagnetism in a planar system with a random magnetic anisotropy and two-dimensional magnetic correlations

The hysteresis loops and the micromagnetic structure of a ferromagnetic nanolayer with a randomly oriented local easy magnetization axis and two-dimensional magnetization correlations are studied using a micromagnetic simulation. The properties and the micromagnetic structure of the nanolayer are determined by the competition between the anisotropy and exchange energies and by the dipole–dipole interaction energy. The magnetic microstructure can be described as an ensemble of stochastic magnetic domains and topological magnetization defects. Dipole–dipole interaction suppresses the formation of topological magnetization defects. The topological defects in the magnetic microstructure can cause a sharper change in the coercive force with the crystallite size than that predicted by the random magnetic anisotropy model.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

The Second Moments of the Line Shapes of Multiple Quantum NMR Coherences in One-Dimensional Systems

The second moments of the line shapes of the zeroth- and second-order multiple quantum NMR coherences determined by the dipole–dipole interactions of nuclear spins in crystals have been calculated. The second moments determined by the zz part of the dipole–dipole interaction have been derived both by a direct calculation and based on the exact solution (in terms of the zz model) for the decay of the multiple quantum coherence intensities on the evolution period of a multi-quantum NMR experiment. The decay of the multiple quantum NMR coherence intensities in a single crystal of calcium fluorapatite is well described by the Gaussian curves with the calculated second moments.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗