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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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All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

The cosine-sine decomposition with different-orbitals-for-different-spins determinants

We report the spin decomposition of a spin-contaminated different-orbitals-for-different-spins (DODS) wave function is formulated in terms of the Krylov space of the $\hat{S}^2$ ||operator. The number of determinants that contribute to the various projected spin eigenfunctions vertical | $\psi ; S, M \rangle$ depends on the sparsity of the orbital overlap matrix X between the α and β spatial orbitals. The cosine-sine decomposition (CSD) procedure may be applied to this overlap matrix, and the resulting redundant orbital transformations may be applied to the α and β spatial orbitals. This produces a sparse X' matrix in the transformed basis in which each row or column has either one or two nonzero elements. This sparse X' matrix simplifies the spin-decomposition procedure in three ways:1) it reduces the number of contributing determinants within the Krylov basis functions and within the projected spin functions, 2) it reduces the effective orbital dimension through elimination of the frozen core and frozen virtual orbitals, and 3) it simplifies the spin-decomposition procedure in both the original and transformed bases by limiting the Krylov space dimension. These simplifications all reduce the computational effort for the spin-decomposition process. This procedure is implemented within a string-based DODS determinant formulation and applied to spin projection of unrestricted Hartree-Fock determinants.

unrestricted Hartree-Fock↗
DOE OSTI2015

Diagrammatic theories of anharmonic molecular vibrations

In this review, we summarise recent developments in our laboratory in the field of many-body quantum-mechanical calculations of the anharmonic vibrational structure of molecules. Our size-extensive vibrational self-consistent field (XVSCF) and size-extensive second-order many-body perturbation (XVMP2) methods are, unlike their parent methods (VSCF and VMP2), defined in diagram- matic formulations of the energies and Dyson self-energies, leading to manifestly size-consistent expressions for zero-point energies and anharmonic vibrational frequencies calculable with much greater efficiency. The effective one-mode potentials of XVSCF are quadratic and hence the Schrödinger equation for each mode can be solved analytically, unlike VSCF, where a basis-set expansion of wave functions on more complex one-mode potentials need to be performed; VSCF potentials and their minima (anharmonic geometry) are shown to reduce to the quadratic potentials and their minima (also given analytically) of XVSCF in the thermodynamic limit. By self-consistently solving the Dyson equation with frequency-dependent self-energies, XVMP2 has the ability to calculate anharmonic frequencies of fundamentals as well as combinations and overtones in the presence of strong anharmonic resonance without a multireference or quasi-degenerate formulation, which tends to be non-size-consistent. To eliminate the computational bottleneck of XVSCF and XVMP2, which is the high-rank force-constant evaluation, we have developed alternative algorithms in which the diagrammatic equations are recast as a small number of high-dimensional integrals and then evaluated stochastically using a Metropolis Monte Carlo (MC) method. These MC-XVSCF and MC-XVMP2 methods not only remove the need for force-constant evaluation or storage, but also take into account force constants of up to infinite order according to their importance. Furthermore, they are a new branch of quantum Monte Carlo which can calculate frequencies (excitation energies) directly without fixed-node errors.

Dyson equation↗
DOE OSTI2013

Ultraviolet luninescence of ScPO 4 , AlPO 4 , and GaPO 4 crystals

The luminescence of self-trapped excitons (STE) was previously observed and described for the case of tetragonal-symmetry ScPO 4 single crystals in 1996 by Trukhin and Boatner. The subject band in this material is situated in the UV spectral range of ~210 nm or ~5.8 eV. In the present work, we are both expanding this earlier luminescence study and seeking to identify similar luminescence phenomena in other orthophosphate crystals i.e., AlPO 4 and GaPO 4 . These efforts have proven to be successful - in spite of the structural differences between these materials and ScPO 4 . Specifically we have found that for AlPO 4 and GaPO 4 , in addition to an -quartz-like STE, there is a UV luminescence that is similar in position and decay properties to that of ScPO 4 crystals. Potentially this represents an STE in AlPO 4 , and GaPO 4 crystals that is analogous to the STE of ScPO 4 and other orthophosphates. The decay kinetics of the UV luminescence of ScPO 4 were studied over a wide temperature range from 8 to 300 K, and they exhibited some unusual decay characteristics when subjected to pulses from an F 2 excimer laser (157 nm). These features could be ascribed to a triplet state of the STE that is split in a zero magnetic field. A fast decay of the STE was detected as well, and therefore, we conclude that, in addition to the slow luminescence corresponding to a transition from the triplet state, there are singlet-singlet transitions of the STE. Furthermore, time-resolved spectra of the slow and fast decay exhibit a small shift (~0.15 eV) indicating that the singlet triplet splitting is small and the corresponding wave function of the STE is widely distributed over the atoms of the ScPO 4 crystal where the STE is created.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Statistical measures of Planck scale signal correlations in interferometers

Here, a model-independent statistical framework is presented to interpret data from systems where the mean time derivative of positional cross correlation between world lines, a measure of spreading in a quantum geometrical wave function, is measured with a precision smaller than the Planck time. The framework provides a general way to constrain possible departures from perfect independence of classical world lines, associated with Planck scale bounds on positional information. A parametrized candidate set of possible correlation functions is shown to be consistent with the known causal structure of the classical geometry measured by an apparatus, and the holographic scaling of information suggested by gravity. Frequency-domain power spectra are derived that can be compared with interferometer data. As a result, simple projections of sensitivity for specific experimental set-ups suggests that measurements will directly yield constraints on a universal time derivative of the correlation function, and thereby confirm or rule out a class of Planck scale departures from classical geometry.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2021

Non-relativistic geometry and the equivalence principle

Abstract We describe a geometric and symmetry-based formulation of the equivalence principle in non-relativistic physics. It applies both on the classical and quantum levels and states that the Newtonian potential can be eliminated in favor of a curved and time-dependent spatial metric. It is this requirement that forces the gravitational mass to be equal to the inertial mass. We identify the symmetry responsible for the equivalence principle as the remnant of time-reparameterization symmetry of the relativistic theory. We also clarify the transformation properties of the Schrödinger wave-function under arbitrary frame changes.

Astronomy & Astrophysics↗
DOE OSTI2019

Balmer emission induced by proton impact on atomic hydrogen

The semiclassical two-centre convergent-close-coupling approach is applied to study Balmer emission in proton-hydrogen scattering at the incident proton energies from 5–1 MeV. The approach uses wave-packet pseudostates for the discretization of the continuous spectrum of the hydrogen atom, constructed from the Coulomb wave function. All cross sections for target excitation into the final states with principal quantum numbers n = 3 and 4 required for obtaining the Balmer emission cross sections, polarisation fraction and Balmer decrement are calculated. Corresponding electron-capture cross sections are also given. A substantial variation in the cross sections for population of magnetic sublevels obtained in different theoretical approaches is found. The present cross section for excitation of the n = 3 shell as a whole does not agree with experiment, but supports earlier calculations. At the same time, the individual cross section for excitation of the 3p state displays excellent agreement with available experimental measurements. The results for polarisation fraction of the Balmer-α emission significantly disagree with experimental measurements at high energies. The calculated Balmer decrement plateaus at about 100 keV and can be used in astrophysical applications.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2020

One-center close-coupling approach to two-center rearrangement collisions

Calculations of ionization and electron-capture cross sections in ion-atom collisions usually require solving the Schrödinger equation governing the collision system by expanding the total scattering wave function in a basis of target- and projectile-centered pseudostates. This approach leads to the two-center close-coupling equations which in some cases may become ill-conditioned due to the non-orthogonality of the underlying combined basis. Here we develop a technique which allows to accurately extract the necessary collision information, including that for the rearrangement channels, from the computationally more convenient one-center close-coupling equations which are built from only target-centered pseudostates. The robustness of the method is demonstrated by considering the proton-hydrogen scattering problem across a wide incident energy range. The developed method is then applied to study proton scattering on multielectron target of lithium. Here, the obtained results for the 2s → 2p excitation and the total electron-capture cross sections are in good agreement with corresponding experimental data. It is concluded that the presented technique could be a simpler alternative when integrated cross sections are required.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2024

R-matrix calculations for opacities: I. Methodology and computations

Abstract An extended version of the R -matrix methodology is presented for calculation of radiative parameters for improved plasma opacities. Contrast and comparisons with existing methods primarily relying on the distorted wave approximation are discussed to verify accuracy and resolve outstanding issues, particularly with reference to the opacity project (OP). Among the improvements incorporated are: (i) large-scale Breit–Pauli R -matrix calculations for complex atomic systems including fine structure, (ii) convergent close coupling wave function expansions for the ( e + ion) system to compute oscillator strengths and photoionization cross sections, (iii) open and closed shell iron ions of interest in astrophysics and experiments, (iv) a treatment for plasma broadening of autoionizing resonances as function of energy-temperature-density dependent cross sections, (v) a ‘top-up’ procedure to compare convergence with R -matrix calculations for highly excited levels, and (vi) spectroscopic identification of resonances and bound ( e + ion) levels. The present R -matrix monochromatic opacity spectra are fundamentally different from OP and lead to enhanced Rosseland and Planck mean opacities. An outline of the work reported in other papers in this series and those in progress is presented. Based on the present re-examination of the OP work, opacities of heavy elements might require revisions in high temperature-density plasma sources.

Pradhan, A. K. (ORCID:0000000187753643)↗
DOE OSTI2020

S -factor and scattering-parameter extractions from ${}^{3}\mathrm{He}+{}^{4}\mathrm{He}{ \rightarrow }^{7}\mathrm{Be}+\gamma $

Previous studies of the reaction ${}^{3}\mathrm{He}+{}^{4}\mathrm{He}\to {}^{7}\mathrm{Be}+\gamma $ have mainly focused on providing the best central value and error bar for the S factor at solar energies. Experimental measurements of this capture reaction at higher energies, the ${}^{3}\mathrm{He}$–${}^{4}\mathrm{He}$ scattering phase shifts, as well as properties of ${}^{7}\mathrm{Be}$ and its excited state, have been used to constrain the theoretical models employed for this purpose. Here we show that much more information than was previously appreciated can be extracted from angle-integrated capture data alone. We use the next-to-leading-order (NLO) amplitude in an effective field theory (EFT) for ${}^{3}\mathrm{He}+{}^{4}\mathrm{He}\to {}^{7}\mathrm{Be}+\gamma $ to perform the extrapolation. At this order the EFT describes the capture process using an s-wave scattering length and effective range, the asymptotic properties of ${}^{7}\mathrm{Be}$ and its excited state, and short-distance contributions to the E1 capture amplitude. We extract the multi-dimensional posterior of all these parameters via a Bayesian analysis that uses capture data below 2 MeV. We find that properties of the ${}^{7}\mathrm{Be}$ ground and excited states are well constrained. The total S factor $S(0)\,={0.577}_{-0.016}^{+0.015}$ keV b, while the branching ratio for excited- to ground-state capture at zero energy, ${Br}(0)={0.406}_{-0.011}^{+0.013}$, both at 68% degree of belief. This S(0) is broadly consistent with other recent evaluations, and agrees with the previously recommended value $S(0)=0.56\pm 0.03\,\,\mathrm{keV}$ b, but has a smaller error bar. We also find significant constraints on ${}^{3}\mathrm{He}$–${}^{4}\mathrm{He}$ scattering parameters, and we obtain constraints on the angular distribution of capture gamma rays, which is important for interpreting experiments. The path forward for this reaction seems to lie with better measurements of the scattering phase shifts and S(E)'s angular dependence away from zero energy, together with better understanding of the asymptotic normalization coefficients of the ${}^{7}$Be bound states' wave functions. Lastly, data on these could further reduce the uncertainty on S(0).

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2021

Rotational bands beyond the Elliott model

Rotational bands are commonplace in the spectra of atomic nuclei. Inspired by early descriptions of these bands by quadrupole deformations of a liquid drop, Elliott constructed discrete nucleon representations of SU(3) from fermionic creation and annihilation operators. Ever since, Elliott's model has been foundational to descriptions of rotation in nuclei. Later work, however, suggested the symplectic extension Sp(3, R) provides a more unified picture. We decompose no-core shell-model nuclear wave functions into symmetry-defined subspaces for several beryllium isotopes, as well as 20 Ne, using the quadratic Casimirs of both Elliott's SU(3) and Sp(3, R). The band structure, delineated by strong B(E2) values, has a more consistent description in Sp(3, R) rather than SU(3). In particular, we confirm previous work finding in some nuclides strongly connected upper and lower bands with the same underlying symplectic structure.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2024

The charge and mass symmetry breaking in the KK K¯ system

Abstract In the framework of the Faddeev equations in configuration space, we investigate theK(1460) meson as a resonant state of the KK K ¯ kaonic system. We perform calculations for the particle configurationsK 0 K + K − and K 0 K + K 0 ¯ within two models: theABCmodel, in which all three particles are distinguishable, and theAACmodel when two particles are identical. The models differ in their treatment of the kaon mass difference and the attractive Coulomb force between theK + K − pair. We found that the Coulomb shift adds over 1 MeV to the three-body binding energy. The expected correction to the binding energy due to mass redistribution fromAAtoABis found to be negligible, up to a maximum of 6% of the relative mass correction. At the same time, the symmetry of the wave function is distorted depending on the mass ratio value. We found that the repulsiveKKinteraction plays an essential role in the binding energy of the KK K ¯ system and reports the mass of 1461.8 or 1464.1 MeV for the neutralK 0 (1460) and 1466.5 or 1468.8 MeV for the chargedK + (1460) resonances, respectively, depending on the parameter sets forKKand K K ¯ interactions.

Physics↗
DOE OSTI2018

Spectroscopic evidence for two-gap superconductivity in the quasi-1D chalcogenide Nb 2 Pd 0.81 S 5

Low-dimensional electronic systems with confined electronic wave functions have attracted interest due to their propensity toward novel quantum phases and their use in wide range of nanotechnologies. The newly discovered chalcogenide Nb 2 PdS 5 possesses a quasi-one-dimensional electronic structure and becomes superconducting. Here, we report spectroscopic evidence for two-band superconductivity, where soft point-contact spectroscopic measurements in the superconducting (SC) state reveal Andreev reflection in the differential conductance G. Multiple peaks in G are observed at 1.8 K and explained by the two-band Blonder–Tinkham–Klapwijk model with two gaps Δ 1 =0.61 meV and Δ 2 =1.20 meV. Finally, the progressive evolution of G with temperature and magnetic field corroborates the multiple nature of the SC gaps.

36 MATERIALS SCIENCE↗
DOE OSTI2018

QMCPACK : an open source ab initio quantum Monte Carlo package for the electronic structure of atoms, molecules and solids

QMCPACK is an open source quantum Monte Carlo package for ab-initio electronic structure calculations. It supports calculations of metallic and insulating solids, molecules, atoms, and some model Hamiltonians. Implemented real space quantum Monte Carlo algorithms include variational, diffusion, and reptation Monte Carlo. QMCPACK uses Slater-Jastrow type trial wave functions in conjunction with a sophisticated optimizer capable of optimizing tens of thousands of parameters. The orbital space auxiliary field quantum Monte Carlo method is also implemented, enabling cross validation between different highly accurate methods. The code is specifically optimized for calculations with large numbers of electrons on the latest high performance computing architectures, including multicore central processing unit (CPU) and graphical processing unit (GPU) systems. We detail the program’s capabilities, outline its structure, and give examples of its use in current research calculations. The package is available at http://www.qmcpack.org.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Excited state calculations in solids by auxiliary-field quantum Monte Carlo

Here, we present an approach for ab initio many-body calculations of excited states in solids. Using auxiliary-field quantum Monte Carlo, we introduce an orthogonalization constraint with virtual orbitals to prevent collapse of the stochastic Slater determinants in the imaginary-time propagation. Trial wave functions from density-functional calculations are used for the constraints. Detailed band structures can be calculated. Results for standard semiconductors are in good agreement with experiments; comparisons are also made with GW calculations and the connections and differences are discussed. For the challenging ZnO wurtzite structure, we obtain a fundamental band gap of 3.26(16) eV, consistent with experiments.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Full configuration interaction simulations of exchange-coupled donors in silicon using multi-valley effective mass theory

Abstract Donor spins in silicon have achieved record values of coherence times and single-qubit gate fidelities. The next stage of development involves demonstrating high-fidelity two-qubit logic gates, where the most natural coupling is the exchange interaction. To aid the efficient design of scalable donor-based quantum processors, we model the two-electron wave function using a full configuration interaction method within a multi-valley effective mass theory. We exploit the high computational efficiency of our code to investigate the exchange interaction, valley population, and electron densities for two phosphorus donors in a wide range of lattice positions, orientations, and as a function of applied electric fields. The outcomes are visualized with interactive images where donor positions can be swept while watching the valley and orbital components evolve accordingly. Our results provide a physically intuitive and quantitatively accurate understanding of the placement and tuning criteria necessary to achieve high-fidelity two-qubit gates with donors in silicon.

Joecker, Benjamin (ORCID:0000000302635440)↗
DOE OSTI2023

Quantum simulation of bosons with the contracted quantum eigensolver

Abstract Quantum computers are promising tools for simulating many-body quantum systems due to their potential scaling advantage over classical computers. While significant effort has been expended on many-fermion systems, here we simulate a model entangled many-boson system with the contracted quantum eigensolver (CQE). We generalize the CQE to many-boson systems by encoding the bosonic wavefunction on qubits. The CQE provides a compact ansatz for the bosonic wave function whose gradient is proportional to the residual of a contracted Schrödinger equation. We apply the CQE to a bosonic system, where N quantum harmonic oscillators are coupled through a pairwise quadratic repulsion. The model is relevant to the study of coupled vibrations in molecular systems on quantum devices. Results demonstrate the potential efficiency of the CQE in simulating bosonic processes such as molecular vibrations with good accuracy and convergence even in the presence of noise.

Physics↗
DOE OSTI2016

Quantum decoherence during inflation from gravitational nonlinearities

We study the inflationary quantum-to-classical transition for the adiabatic curvature perturbation ζ due to quantum decoherence, focusing on the role played by squeezed-limit mode couplings. We evolve the quantum state Ψ in the Schrödinger picture, for a generic cubic coupling to additional environment degrees of freedom. Focusing on the case of minimal gravitational interactions, we find the evolution of the reduced density matrix for a given long-wavelength fluctuation by tracing out the other (mostly shorter-wavelength) modes of ζ as an environment. We show that inflation produces phase oscillations in the wave functional Ψ[ζ(x)], which suppress off-diagonal components of the reduced density matrix, leaving a diagonal mixture of different classical configurations. Gravitational nonlinearities thus provide a minimal mechanism for generating classical stochastic perturbations from inflation. We identify the time when decoherence occurs, which is delayed after horizon crossing due to the weak coupling, and find that Hubble-scale modes act as the decohering environment. We also comment on the observational relevance of decoherence and its relation to the squeezing of the quantum state.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Investigating color screening in proton-nucleus collisions with complex potentials*

Abstract Color screening and parton inelastic scattering modify the heavy-quark antiquark potential in mediums consisting of particles from quantum chromodynamics (QCD), leading to the suppression of quarkonium production in relativistic heavy-ion collisions. Owing to the small charm/anti-charm ( ) pair production number in proton-nucleus ( pA ) collisions, the correlation between different pairs is negligible, which makes the Schrödinger equation viable for tracking the evolution of only one pair. We employ the time-dependent Schrödinger equation with an in-medium potential to study the evolution of charmonium wave functions in a hydrodynamic-like QCD medium produced in pA collisions. We explore different parametrizations of real and imaginary parts of the potential and calculate the nuclear modification factors ( ) of and in TeV energy p -Pb collisions at the Large Hadron Collider (LHC). Comparing strong and weak screening scenarios with experimental data in this approach, we arrive at the conclusion that color screening is weak at temperatures close to the deconfined phase transition. Moreover, the imaginary part of the potential is crucial in describing the experimental data, which is consistent with widely studied semi-classical approaches, where dissociation rates are essential.

Physics↗

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