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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 325 records · Page 18
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DOE OSTI2018

Extended BCS-Bose Crossover

Applying the generalized Bose-Einstein condensation (GBEC) formalism, we extend the BCS-Bose crossover theory by explicitly including hole Cooper pairs (2hCPs). From this, follows a phase diagram with two pure phases, one with 2hCPs and the other with electron Cooper pairs (2eCPs), plus a mixed phase with arbitrary proportions of 2eCPs and 2hCPs. The special-case phase when there is perfect symmetry, i.e., with ideal 50-50 proportions between 2eCPs and 2hCPs, corresponds to the usual BCS-Bose crossover. Explicitly including 2hCPs yields an extended BCS-Bose crossover which predicts improved T{sub c}/T{sub F} values for some conventional superconductors (i.e., with electron-phonon dynamics) when compared with experiment. To do this, we employ the BCS dimensionless coupling constant λ{sub BCS} via the BCS gap equation and compare with the Bogoliubov et al. upper limit λ{sub BCS} ≤ 1/2. Another phase diagram presented exhibits experimental T{sub c}/T{sub F} values for some conventional superconductors for arbitrary proportions between 2eCPs and 2hCPs as function of Δn = n/n{sub f} − 1, where n is the electron concentration and n{sub f} that of unbound electrons at T = 0. The extended crossover is compared with experimental T{sub c}/T{sub F} values as well as to the gap-to- T{sub c} ratio.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2016

Magnetovolume effects and effect of pressure on the temperature dependence of sublattice magnetization in Ho(Co{sub 0.9}Ga{sub 0.1}){sub 2} compounds

The temperature dependence of the magnetic moments of cobalt and holmium ions in the temperature range of 5–300 K at pressures of P = 0 and 5 kbar was determined based on neutron diffraction data on Ho(Co{sub 0.9}Ga{sub 0.1}){sub 2} intermetallide. The temperature dependence of the lattice parameter was established, and the spontaneous bulk magnetostriction was determined. These quantities were calculated using the exchange striction model of a ferrimagnet, and good agreement with the experiment was obtained. The parameters of the exchange interaction between pairs of atoms of this compound and the magnetoelastic coupling constant were estimated. An original interpretation of the nature of the first-order magnetic phase transition in HoCo{sub 2} is proposed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Ab initio calculations on the magnetic properties of transition metal complexes

We present a protocol for the ab initio determination of the magnetic properties of mono- and polynuclear transition metal compounds. First, we obtain the low lying electronic states by multireference methods. Then, we include spin-orbit coupling and an external magnetic field for the determination of zero-field splitting and g-tensors. For the polynuclear complexes the magnetic exchange coupling constants are determined by a modified complete active space self consistent field method. Based on the results of the ab initio calculations, magnetic data such as magnetic susceptibility or magnetization are simulated and compared to experimental data. The results obtained for the polynuclear complexes are further analysed by calculations on model complexes where part of the magnetic centers are substituted by diamagnetic ions. The methods are applied to different Co and Ni containing transition metal complexes.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Electric field-controlled magnetization in bilayered magnetic films for magnetoelectric memory

Bilayered magnetic films (Co{sub 50}Fe{sub 50} (CoFe)/Metglas) were RF sputtered on both (001)-oriented and (011)-oriented PMN-PT (lead magnesium niobate-lead titanate) substrates. Electric field-controlled magnetization changes were observed in all these samples: 65 nm CoFe/24 nm Metglas/(001) PMN-PT, 65 nm CoFe/24 nm Metglas/(011) PMN-PT, and 30 nm CoFe/12 nm Metglas/(011) PMN-PT. The maximum magnetic remanence ratio change (ΔM{sub r}/M{sub s}) was 46% for CoFe/Metglas/(001) PMN-PT. In this heterostructure, the electric-field created two new non-volatile switchable remanence states and the as-grown remanence state was altered permanently. High-resolution transmission electron microscopy images show a sharp and smooth interface between Metglas and substrate and conversely a rougher interface was observed between Metglas and CoFe films. In the 30 nm CoFe/12 nm Metglas/(011) PMN-PT sample, a large ΔM{sub r}/M{sub s} of 80% along the [100] direction was measured, while the ΔM{sub r}/M{sub s} along the [01-1] direction was 60% at the applied electric field of 5 kV/cm, corresponding to a giant magnetoelectric coupling constant α = μ{sub o}ΔM{sub r}/E = 2.9 × 10{sup −6} s/m.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Contribution of relativistic quantum chemistry to electron’s electric dipole moment for CP violation

The search for the electric dipole moment of the electron (eEDM) is important because it is a probe of Charge Conjugation-Parity (CP) violation. It can also shed light on new physics beyond the standard model. It is not possible to measure the eEDM directly. However, the interaction energy involving the effective electric field (E{sub eff}) acting on an electron in a molecule and the eEDM can be measured. This quantity can be combined with E{sub eff}, which is calculated by relativistic molecular orbital theory to determine eEDM. Previous calculations of E{sub eff} were not sufficiently accurate in the treatment of relativistic or electron correlation effects. We therefore developed a new method to calculate E{sub eff} based on a four-component relativistic coupled-cluster theory. We demonstrated our method for YbF molecule, one of the promising candidates for the eEDM search. Using very large basis set and without freezing any core orbitals, we obtain a value of 23.1 GV/cm for E{sub eff} in YbF with an estimated error of less than 10%. The error is assessed by comparison of our calculations and experiments for two properties relevant for E{sub eff}, permanent dipole moment and hyperfine coupling constant. Our method paves the way to calculate properties of various kinds of molecules which can be described by a single-reference wave function.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Testing universal relations of neutron stars with a nonlinear matter-gravity coupling theory

Due to our ignorance of the equation of state (EOS) beyond nuclear density, there is still no unique theoretical model for neutron stars (NSs). It is therefore surprising that universal EOS-independent relations connecting different physical quantities of NSs can exist. Lau et al. found that the frequency of the f-mode oscillation, the mass, and the moment of inertia are connected by universal relations. More recently, Yagi and Yunes discovered the I-Love-Q universal relations among the mass, the moment of inertia, the Love number, and the quadrupole moment. In this paper, we study these universal relations in the Eddington-inspired Born-Infeld (EiBI) gravity. This theory differs from general relativity (GR) significantly only at high densities due to the nonlinear coupling between matter and gravity. It thus provides us an ideal case to test how robust the universal relations of NSs are with respect to the change of the gravity theory. Due to the apparent EOS formulation of EiBI gravity developed recently by Delsate and Steinhoff, we are able to study the universal relations in EiBI gravity using the same techniques as those in GR. We find that the universal relations in EiBI gravity are essentially the same as those in GR. Our work shows that, within the currently viable coupling constant, there exists at least one modified gravity theory that is indistinguishable from GR in view of the unexpected universal relations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

Electronic Properties and Lattice Dynamics Studies of the Nickel-Based Superconductor ThNiAsN

We perform theoretical studies of the newly discovered nickel-based pnictide ThNiAsN. The obtained large density of states may explain the normal-state Sommerfeld coefficient observed by former experiments. The obtained band structure and Fermi surfaces are rather two-dimensional with a small hole-like Fermi surface around X point and three electron-like ones around M point, which share great similarities with LaNiAsO. Meanwhile, the lattice dynamics of ThNiAsN are also studied within density functional perturbation theory (DFPT). The electron-phonon coupling constant is 0.67, suggesting that ThNiAsN is a phonon-mediated superconductor. In the end, as a prototype of nickel-based superconductor, we obtain an effective tight-binding model by means of the maximally localized Wannier function (MLWF). Based on this model, the nesting properties have been studied using Lindhard function for nominal doping and hole doping. We find the hole doping will introduce better nesting properties, making this system a potential candidate to develop magnetic and unconventional superconducting instabilities.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Parity violation in the CMB bispectrum by a rolling pseudoscalar

We investigate parity-violating signatures of temperature and polarization bispectra of the cosmic microwave background (CMB) in an inflationary model where a rolling pseudoscalar produces large equilateral tensor non-Gaussianity. By a concrete computation based on full-sky formalism, it is shown that resultant CMB bispectra have nonzero signals in both parity-even (ℓ{sub 1}+ℓ{sub 2}+ℓ{sub 3} = even) and parity-odd (ℓ{sub 1}+ℓ{sub 2}+ℓ{sub 3} = odd) spaces, and are almost uncorrelated with usual scalar-mode equilateral bispectra. These characteristic signatures and polarization information help to detect such tensor non-Gaussianity. Use of both temperature and E-mode bispectra potentially improves of 400% the detectability with respect to an analysis with temperature bispectrum alone. Considering B-mode bispectrum, the signal-to-noise ratio may be able to increase by 3 orders of magnitude. We present the 1σ uncertainties of a parameter depending on a coupling constant and a rolling condition for the pseudoscalar expected in the Planck and the proposed PRISM experiments.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Nonreciprocal Optical and Magnetooptical Effects in Semiconductor Quantum Wells

Abstract—Effects implying violation of the Fresnel light reflection laws in semiconductor structures with quantum wells (QWs) have been investigated. This violation is related to the manifestation of spatial dispersion caused by spin–orbit coupling in structures without inversion centers. The spin–orbit coupling constants characterizing polarization conversion in symmetric and asymmetric structures with QWs have been measured.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Zero-Field NMR and Millitesla-SLIC Spectra for >200 Molecules from Density Functional Theory and Spin Dynamics

NMR is usually performed at magnetic fields of 1 T and above to obtain sufficient sensitivity and spectral dispersion to identify chemicals based on chemical shifts and J couplings. At lower fields, the advent of hyperpolarization technologies and sensitive detectors can address sensitivity concerns. However, it remains disputed whether spectral signatures at zero and ultra-low fields are sufficient for chemical identification. Here, we report an all–electron DFT-based batch calculation of J-coupling constants, which are used to generate J coupling NMR spectra at zero field and 6.5 mT for over 200 small molecules. In the developed computational tool chain, we first used the all-electron FHI-aims code to calculate the molecular J couplings and chemical shifts. We then fed the calculated NMR parameters into the NMR simulation package SPINACH to simulate both heteronuclear J coupling spectra at zero-field, and homonuclear J coupling spectra as spin-lock induced crossing (SLIC) spectra at ultra-low field (6.5 mT). The resulting spectra demonstrate that zero and ultra-low field NMR spectra can represent unique identifiers of chemical structure for small molecules.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Ground-state and dynamical properties of two-dimensional dipolar Fermi liquids

We study the ground-state properties of a two-dimensional spin-polarized fluid of dipolar fermions within the Euler–Lagrange Fermi-hypernetted-chain approximation. Our method is based on the solution of a scattering Schrödinger equation for the “pair amplitude” √(g(r)), where g(r) is the pair distribution function. A key ingredient in our theory is the effective pair potential, which includes a bosonic term from Jastrow–Feenberg correlations and a fermionic contribution from kinetic energy and exchange, which is tailored to reproduce the Hartree–Fock limit at weak coupling. Very good agreement with recent results based on quantum Monte Carlo simulations is achieved over a wide range of coupling constants up to the liquid-to-crystal quantum phase transition. Using the fluctuation–dissipation theorem and a static approximation for the effective inter-particle interactions, we calculate the dynamical density–density response function, and furthermore demonstrate that an undamped zero-sound mode exists for any value of the interaction strength, down to infinitesimally weak couplings. -- Highlights: •We have studied the ground state properties of a strongly correlated two-dimensional fluid of dipolar fermions. •We have calculated the effective inter-particle interaction and the dynamical density–density response function. •We have shown that an undamped zero sound mode exists at any value of the interaction strength.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

A regular version of Smilansky model

We discuss a modification of Smilansky model in which a singular potential “channel” is replaced by a regular, below unbounded potential which shrinks as it becomes deeper. We demonstrate that, similarly to the original model, such a system exhibits a spectral transition with respect to the coupling constant, and determine the critical value above which a new spectral branch opens. The result is generalized to situations with multiple potential “channels.”.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

First principles calculation of lattice thermal conductivity of metals considering phonon-phonon and phonon-electron scattering

The effect of phonon-electron (p-e) scattering on lattice thermal conductivity is investigated for Cu, Ag, Au, Al, Pt, and Ni. We evaluate both phonon-phonon (p-p) and p-e scattering rates from first principles and calculate the lattice thermal conductivity (κ{sub L}). It is found that p-e scattering plays an important role in determining the κ{sub L} of Pt and Ni at room temperature, while it has negligible effect on the κ{sub L} of Cu, Ag, Au, and Al. Specifically, the room temperature κ{sub L}s of Cu, Ag, Au, and Al predicted from density-functional theory calculations with the local density approximation are 16.9, 5.2, 2.6, and 5.8 W/m K, respectively, when only p-p scattering is considered, while it is almost unchanged when p-e scattering is also taken into account. However, the κ{sub L} of Pt and Ni is reduced from 7.1 and 33.2 W/m K to 5.8 and 23.2 W/m K by p-e scattering. Even though Al has quite high electron-phonon coupling constant, a quantity that characterizes the rate of heat transfer from hot electrons to cold phonons in the two-temperature model, p-e scattering is not effective in reducing κ{sub L} owing to the relatively low p-e scattering rates in Al. The difference in the strength of p-e scattering in different metals can be qualitatively understood by checking the amount of electron density of states that is overlapped with the Fermi window. Moreover, κ{sub L} is found to be comparable to the electronic thermal conductivity in Ni.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

An asteroseismic constraint on the mass of the axion from the period drift of the pulsating DA white dwarf star L19-2

We employ an asteroseismic model of L19-2, a relatively massive ( M {sub *} ∼ 0.75 M {sub ⊙}) and hot ( T {sub eff} ∼ 12 100 K) pulsating DA (H-rich atmosphere) white dwarf star (DAV or ZZ Ceti variable), and use the observed values of the temporal rates of period change of its dominant pulsation modes (Π ∼ 113 s and Π ∼ 192 s), to derive a new constraint on the mass of the axion, the hypothetical non-barionic particle considered as a possible component of the dark matter of the Universe. If the asteroseismic model employed is an accurate representation of L19-2, then our results indicate hints of extra cooling in this star, compatible with emission of axions of mass m {sub a} cos{sup 2}β ∼< 25 meV or an axion-electron coupling constant of g {sub ae} ∼< 7 × 10{sup −13}.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Nonstandard interactions in plasmon decay to a neutrino pair in a strongly magnetized medium

The neutrino luminosity of a stellar medium because of plasmon decay to a neutrino pair via nonstandard tensor interaction in a degenerate electron gas subjected to the effect of a magnetic field such strong that the electrons of the gas are in the lowest Landau level is calculated. Relative limits on nonstandard coupling constants are obtained from a comparison of the results of this calculation with the neutrino luminosity generated in the respective standard process proceeding under the same conditions.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Stationary bound states of Dirac particles in collapsar fields

It is the first time stationary bound states of elementary spin 1/2 particles that do not decay with time are obtained for a Schwarzschild gravitational field using a self-conjugate Hamiltonian with a flat scalar product in a wide range of gravitational coupling constant. In order to obtain a discrete energy spectrum, we introduce a boundary condition such that the current density of Dirac particles near the “event horizon” is zero.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Reionization during the dark ages from a cosmic axion background

Recently it has been pointed out that a cosmic background of relativistic axion-like particles (ALPs) would be produced by the primordial decays of heavy fields in the post-inflation epoch, contributing to the extra-radiation content in the Universe today. Primordial magnetic fields would trigger conversions of these ALPs into sub-MeV photons during the dark ages. This photon flux would produce an early reionization of the Universe, leaving a significant imprint on the total optical depth to recombination τ. Using the current measurement of τ and the limit on the extra-radiation content Δ N {sub eff} by the Planck experiment we put a strong bound on the ALP-photon conversions. Namely we obtain upper limits on the product of the photon-ALP coupling constant g {sub a} {sub γ} times the magnetic field strength B down to g {sub a} {sub γ} B ∼> 6 × 10{sup −18} GeV{sup −1} nG for ultralight ALPs.

79 ASTRONOMY AND ASTROPHYSICS↗