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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 487 records · Page 27
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Universal corrections to the superfluid gap in a cold Fermi gas

A framework for computing the superfluid gap in an effective field theory (EFT) of fermions interacting via momentum-independent contact forces is developed. The leading universal corrections in the EFT are one-loop in-medium effects at the Fermi surface, and reproduce the well-known Gor'kov-Melik-Barkhudarov result. Here, the complete subleading universal corrections are presented here, and include one-loop effects away from the Fermi surface, two-loop in-medium effects, as well as modifications to the fermion propagator. Together, these effects are found to reduce the gap at low densities. Applications to neutron superfluidity in neutron stars are also discussed.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2016

Density fluctuations due to Raman forward scattering in quantum plasma

Density fluctuations due Raman forward scattering (RFS) is analysed in the interaction of a high intensity laser pulse with high density quantum plasma. The interaction model is developed using the quantum hydrodynamic (QHD) model which consist of a set of equations describing the transport of charge, density, momentum and energy of a charged particle system interacting through a self-consistent electrostatic potential. The nonlinear source current has been obtained incorporating the effects of quantum Bohm potential, Fermi pressure and electron spin. The laser spectrum is strongly modulated by the interaction, showing sidebands at the plasma frequency. Furthermore, as the quiver velocity of the electrons in the high electric field of the laser beam is quit large, various quantum effects are observed which can be attributed to the variation of electron mass with laser intensity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2026

Pathways from a chiral superconductor to a composite Fermi liquid

Recent experiments have reported chiral time-reversal broken superconductivity in n-layer rhombohedral graphene for n=4,5,6. Introducing a moiré potential by alignement with a hexagonal boron nitride substrate suppresses the superconductivity but leads instead to various fractional quantum anomalous Hall phenomena. Motivated by these observations, we consider the fate of the phase transition between (a chiral) Landau Fermi liquid (LFL) metal and a Composite Fermi Liquid (CFL) metal in the presence of attractive interactions. These are parent states, respectively, for the superconductor and the fractional quantum Hall states. For weak attractive interactions, the LFL is usually unstable to superconductivity while the CFL is stable. This raises the possibility of a direct continuous phase transition between the chiral superconductor and the CFL. However, we show that generically the LFL close to the transition to the CFL is stable against superconductivity. Furthermore the evolution between the CFL and chiral superconductor goes through an intermediate stable LFL phase for weak attractive interactions. With stronger interactions, the evolution can instead go through a non-Abelian paired quantum Hall state.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

What causes high resistivity in CdTe

Shallow donors are often introduced into semiconductor materials to enhance n-type conductivity. However, they can sometimes also be used to obtain compensation between donors and acceptors, resulting in high resistivity in semiconductors. For example, CdTe can be made semi-insulating by shallow donor doping. This is routinely done to obtain high resistivity in CdTe-based radiation detectors. However, it is widely believed that the shallow donor alone cannot be responsible for the high resistivity in CdTe. This is based on the argument that it is practically impossible to control the shallow donor doping level so precisely that the free carrier density can be brought below the desired value suitable for radiation detection applications. Therefore, a deep native donor is usually assumed to exist in CdTe and pin the Fermi level near midgap. In this paper, we present our calculations on carrier statistics and energetics of shallow donors and native defects in CdTe and illustrate different donor-specific mechanisms for achieving carrier compensation. Our results show that the shallow donor can be used to reliably obtain high resistivity in CdTe without requiring additional deep donors. Since radiation detection applications require both high resistivity and good carrier transport, one should generally use shallow donors and shallow acceptors for carrier compensation and avoid deep centers that are effective carrier traps. This study highlights how the interaction between impurities and native defects intricately affects the Fermi level pinning in the semiconductor band gap and the associated resistivity of the material.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Quantum field theory for condensation of bosons and fermions

In this brief review, we describe the formalism of the quantum field theory for the analysis of the condensation phenomenon in bosonic systems, by considering the cases widely verified in laboratory of trapped gases as condensate states, either with attractive or with repulsive two-body interactions. We review the mathematical formulation of the quantum field theory for many particles in the mean-field approximation, by adopting contact interaction potential. We also describe the phenomenon of condensation in the case of fermions or the degenerate Fermi gas, also verified in laboratory in the crossover BEC-BCS limit. We explain that such a phenomenon, equivalent to the bosonic condensation, can only occur if we consider the coupling of particles in pairs behaving like bosons, as occurs in the case of Cooper's pairs in superconductivity.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2010

Effective 3-Body Interaction for Mean-Field and Density-Functional Theory

Density functionals for nuclei usually include an effective 3-body interaction that depends on a fractional power of the density. Using insights from the many-body theory of the low-density two-component Fermi gas, we consider a new, nonlocal, form for the energy functional that is consistent with the Fock-space representation of interaction operators. In particular, there is a unique spatially nonlocal generalization of the contact form of the interaction that preserves the {rho}{sup 7/3} density dependence required by the many-body theory. We calculate the ground-state energies for particles in a harmonic trap by using the nonlocal induced 3-body interaction and compare them to numerically accurate Green's function Monte Carlo calculations. Using no free parameters, we find that a nonlocality in the space domain provides a better description of the weak-coupling regime than the local-density approximation.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Exploring Quantum Degenerate Bose-Fermi Mixtures Toward Cooper Pairing of Fermionic Atoms

We have been exploring interaction dynamics in an ultracold, trapped gas of bosonic and fermionic atoms. Investigation of this new class of quantum degenerate gases concentrates on interaction dominated phenomena such as sympathetic cooling, phase separation, excitations, Feshbach resonances, and the effects of quantum degeneracy. In addition to exploring these new phenomena, we seek to understand and ultimately control the interactions in the gas. In particular, effective interactions between the fermionic atoms will be explored in the context of the longer term goal of realizing Cooper pairing of atoms.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

FFLO order in ultra-cold atoms in three-dimensional optical lattices

We explore different ground-state phases of attractive spin-imbalanced populations of fermions in three-dimensional optical lattices. Detailed numerical calculations are performed using Hartree–Fock–Bogoliubov theory to determine the ground-state properties systematically for different values of density, spin polarization and interaction strength. To begin, we consider the high density and low polarization regime, in which the effect of the optical lattice is most evident. Then, we proceed to the low density and high polarization regime where the effects of the underlying lattice are less significant and the system begins to resemble a continuum Fermi gas. We discover the effects of density, polarization and interaction on the character of the phases in each regime and highlight the qualitative differences between the two regimes. In the high-density regime, the order is found to be of Larkin–Ovchinnikov type, linearly oriented with one characteristic wave vector but varying in its direction with the parameters. At lower densities the order parameter develops more structures involving multiple wave vectors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Excitation spectra and rf response near the polaron-to-molecule transition from the functional renormalization group

A light impurity in a Fermi sea undergoes a transition from a polaron to a molecule for increasing interaction. We develop a method to compute the spectral functions of the polaron and molecule in a unified framework based on the functional renormalization group with full self-energy feedback. We discuss the energy spectra and decay widths of the attractive and repulsive polaron branches as well as the molecular bound state, and confirm the scaling of the excited-state decay rate near the transition. The quasiparticle weight of the polaron shifts from the attractive to the repulsive branch across the transition, while the molecular bound state has a very small residue characteristic for a composite particle. We propose an experimental procedure to measure the repulsive branch in a {sup 6}Li Fermi gas using rf spectroscopy and calculate the corresponding spectra.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Recombining plasma in the gamma-ray-emitting mixed-morphology supernova remnant 3C 391

A group of middle-aged mixed-morphology (MM) supernova remnants (SNRs) interacting with molecular clouds (MCs) has been discovered to be strong GeV gamma-ray emitters by the Large Area Telescope (LAT) on board the Fermi Gamma-Ray Space Telescope (Fermi-LAT). The recent observations of the Suzaku X-ray satellite have revealed that some of these interacting gamma-ray-emitting SNRs, such as IC443, W49B, W44, and G359.1-0.5, have overionized plasmas. 3C 391 (G31.9+0.0) is another Galactic MM SNR interacting with MCs. It was observed in GeV gamma rays by Fermi-LAT as well as in the 0.3-10.0 keV X-ray band by Suzaku. In this work, 3C 391 was detected in GeV gamma rays with a significance of ∼18σ and we showed that the GeV emission is point-like in nature. The GeV gamma-ray spectrum was shown to be best explained by the decay of neutral pions assuming that the protons follow a broken power-law distribution. We revealed radiative recombination structures of silicon and sulfur from 3C 391 using Suzaku data. In this paper, we discuss the possible origin of this type of radiative plasma and hadronic gamma rays.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

Exotic paired phases in ladders with spin-dependent hopping

Fermions in two dimensions, when subject to anisotropic spin-dependent hopping, can potentially give rise to unusual paired states in unpolarized mixtures that can behave as non-Fermi liquids. One possibility is a fully paired state with a gap for fermion excitations in which the Cooper pairs remain uncondensed. Such a ''Cooper-pair Bose-metal'' phase would be expected to have a singular Bose surface in momentum space. As demonstrated in the context of two-dimensional bosons hopping with a frustrating ring-exchange interaction, an analogous Bose-metal phase has a set of quasi-one-dimensional descendant states when put on a ladder geometry. Here we present a density matrix renormalization group study of the attractive Hubbard model with spin-dependent hopping on a two-leg ladder geometry. In our setup, one spin species moves preferentially along the leg direction, while the other does so along the rung direction. We find compelling evidence for the existence of a novel Cooper-pair Bose-metal phase in a region of the phase diagram at intermediate coupling. We further explore the phase diagram of this model as a function of hopping anisotropy, density, and interaction strength, finding a conventional superfluid phase as well as a phase of paired Cooper pairs with d-wave symmetry, similar to the one found in models of hard-core bosons with ring exchange. We argue that simulating this model with cold Fermi gases on spin-dependent optical lattices is a promising direction for realizing exotic quantum states.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Relativistic description of finite nuclei based on realistic NN interactions

A set of relativistic mean-field models is constructed, which includes the Hartree and Hartree-Fock (HF) approximations accounting for the exchange of isoscalar and isovector mesons as well as the pion. Density-dependent coupling functions are determined to reproduce the components of the nucleon self-energy at the Fermi surface, obtained within the Dirac-Brueckner-HF (DBHF) approach by using a realistic nucleon-nucleon interaction. It is investigated to which extent the various mean-field models can reproduce the DBHF results for the momentum dependence of the self-energies and the total energy of infinite matter. Also, the mean-field models are used to evaluate the bulk properties of spherical closed-shell nuclei. We find that the HF model, which allows for the exchange of {sigma}, {omega}, {rho}, and {delta} mesons and pions, yields the best reproduction of the DBHF results in infinite matter and also provides a good description of the properties of finite nuclei without any adjustment of parameters.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2011

Evolution of symmetry and structure of the gap in iron-based superconductors with doping and interactions

We present a detailed study of the symmetry and structure of the pairing gap in Fe-based superconductors (FeSCs). We treat FeSCs as quasi-2D, decompose the pairing interaction in the XY plane in s-wave and d-wave channels into contributions from scattering between different Fermi surfaces, and analyze how each scattering evolves with doping and input parameters. We verify that each interaction is well approximated by the lowest angular harmonics. We use this simplification to analyze the interplay between the interaction with and without spin-fluctuation components, the origin of the attraction in the s ± and d x 2 –y 2 channels, the competition between them, the angular dependence of the s ± gaps along the electron Fermi surface, the conditions under which the s ± gap develops nodes, and the origin of superconductivity in heavily electron- or hole-doped systems, when only Fermi surfaces of one type are present. We also discuss the relation between RPA and RG approaches for FeSCs.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Revisiting SNR Puppis A with Seven Years of Fermi Large Area Telescope Observations

Puppis A is a very famous and extensively studied supernova remnant that shows strong evidence of shock-cloud interaction. We reanalyze its GeV γ -ray emission using seven years of Pass 8 data recorded by the Fermi Large Area Telescope. The morphology of the γ -ray emission is more compatible with that of the thermal X-ray and IR emissions than the radio image, which suggests a possible correlation between the gamma-ray-emitting region and dense clouds. The γ -ray spectrum in the energy range of 1–500 GeV shows a break at 7.92 ± 1.91 GeV, with photon indices of 1.81 ± 0.08 below the break and 2.53 ± 0.12 above the break, which can naturally explain the lack of TeV γ -ray emission from Puppis A. The multi-wavelength observations favor a hadronic origin for the γ -ray emission.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

The effect of self-interaction error on electrostatic dipoles calculated using density functional theory

Spurious electron self-interaction in density functional approximations (DFAs) can lead to inaccurate predictions of charge transfer in heteronuclear molecules that manifest as errors in calculated electrostatic dipoles. Here we show the magnitude of these errors on dipoles computed for a diverse set of 47 molecules taken from the recent benchmark study of Hait and Head-Gordon. We compare results of Perdew-Wang local spin density approximation (PW92), Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA), and strongly constrained and appropriatedly normed (SCAN) meta-GGA dipole calculations, along with those of their respective self-interaction-corrected (SIC) counterparts, to reference values from accurate wave function-based methods. The SIC calculations were carried out using the Fermi-Löwdin orbital (FLO-SIC) approach. We find that correcting for self-interaction generally increases the degree of charge transfer, thereby increasing the size of calculated dipole moments. The FLO-SIC-PW92 and FLO-SIC-PBE dipoles are in better agreement with reference values than their uncorrected DFA counterparts, particularly for strongly ionic molecules where significant improvement is seen. Applying FLO-SIC to SCAN does not improve dipole values overall. In addition, we show that removing self-interaction improves the description of the dipole for stretched-bond geometries and recovers the physically correct separated atom limit of zero dipole. Finally, we find that the best agreement between the FLO-SIC-DFA and reference dipoles occurs when the molecular geometries are optimized using the FLO-SIC-DFA.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2020

Interplanetary Protons versus Interacting Protons in the 2017 September 10 Solar Eruptive Event

We analyze the relativistic proton emission from the Sun during the eruptive event on 2017 September 10, which caused a ground-level enhancement (GLE 72) registered by the worldwide network of neutron monitors. Using the neutron monitor data and interplanetary transport modeling both along and across interplanetary magnetic field (IMF) lines, we deduce parameters of the proton injection into the interplanetary medium. The inferred injection profile of the interplanetary protons is compared with the profile of the >100 MeV γ -ray emission observed by the Fermi Large Area Telescope, attributed to pion production from the interaction of >300 MeV protons at the Sun. GLE 72 started with a prompt component that arrived along the IMF lines. This was followed by a more prolonged enhancement caused by protons arriving at the Earth across the IMF lines from the southwest. The interplanetary proton event is modeled using two sources—one source at the root of the Earth-connected IMF line and another source situated near the solar western limb. The maximum phase of the second injection of interplanetary protons coincides with the maximum phase of the prolonged >100 MeV γ -ray emission that originated from a small area at the solar western limb, below the current sheet trailing the associated coronal mass ejection (CME). A possible common source of interacting protons and interplanetary protons is discussed in terms of proton acceleration at the CME bow shock versus coronal (re-)acceleration in the wake of the CME.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

Anisotropic superfluidity in the two-species polar Fermi gas

We study the superfluid pairing in a two-species gas of heteronuclear fermionic molecules with equal density. The interplay of the isotropic s-wave interaction and anisotropic long-range dipolar interaction reveals rich physics. We find that the single-particle momentum distribution has a characteristic ellipsoidal shape that can be reasonably represented by a deformation parameter {alpha} defined similarly to the normal phase. Interesting momentum-dependent features of the order parameter are identified. We calculate the critical temperatures of both the singlet and triplet superfluids, suggesting a possible pairing symmetry transition by tuning the s-wave or dipolar interaction strength.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗