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Results for “SUPERFLUID MODEL”

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 793 records · Page 44
All sourcesNASA NTRSDOE OSTI
DOE OSTI2010

One-dimensional SnF{sub 2} single crystals in the inner channels of single-wall carbon nanotubes: II. Structure and nanocomposite construction modeling

A structural model of the nanocomposite consisting of one-dimensional (1D) {alpha}-SnF{sub 2} single crystals and single-wall carbon nanotubes (SWCNTs) is proposed. The main cationic motif is revealed in the structure of monoclinic modification: two-layer packing of tin cations along the [283] direction. Four theoretical structural projections of a 1D crystal on the plane parallel to the [283] direction are determined and described. A fragment of the {alpha}-SnF{sub 2} structure in an SWCNT (with a channel diameter of 1.02 nm) is calculated. High-resolution electron microscopy (HREM) images are modeled. These images correspond to the actually observed HREM patterns.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Electron-phonon coupling in the undoped cuprate YBa 2 Cu 3 O 6 estimated from Raman and optical conductivity spectra

We study experimentally the Raman response of the undoped high-T c parent compound YBa 2 Cu 3 O 6 , and give a unified theory of the two-magnon Raman peak and optical conductivity based on the Hubbard-Holstein model with electron-phonon coupling (EPC). The Hubbard model without EPC can qualitatively account for the experimentally observed resonance of the Raman response, but only the Hubbard-Holstein model (i) reproduces the asymmetry of the Raman spectrum, (ii) validates the experimental visibility of the two-magnon peak, and (iii) predicts the correct shape and energy of the lower edge of the charge transfer gap in optical conductivity. A comparison of experiments with the theory gives the EPC strength λ = 0.6. Here, this result convincingly indicates the vital role of EPC in high-T c cuprates, providing a clue to the mechanism of high T c .

2-dimensional systems↗
DOE OSTI2010

Competing pairing states for ultracold fermions in optical lattices with an artificial staggered magnetic field

We study fermionic superfluidity in an ultracold Bose-Fermi mixture loaded into a square optical lattice subjected to a staggered flux. While the bosons form a Bose-Einstein condensate at very low temperature and weak interaction, the interacting fermions experience an additional long-ranged attractive interaction mediated by phonons in the bosonic condensate. This leads us to consider a generalized Hubbard model with on-site and nearest-neighbor attractive interactions, which give rise to two competing pairing channels. We use the Bardeen-Cooper-Schrieffer theory to determine the regimes where distinct fermionic superfluids are stabilized and find that the nonlocal pairing channel favors a superfluid state which breaks both the gauge and the lattice symmetries, similar to unconventional superconductivity occurring in some strongly correlated systems. Furthermore, the particular structure of the single-particle spectrum leads to unexpected consequences, for example, a dome-shaped superfluid region in the temperature versus filing fraction phase diagram, with a normal phase that contains much richer physics than a Fermi liquid. Notably, the relevant temperature regime and coupling strength are readily accessible in state of the art experiments with ultracold trapped atoms.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Internal Josephson oscillations for distinct momenta Bose-Einstein condensates

The internal Josephson oscillations between an atomic Bose-Einstein condensate (BEC) and a molecular one are studied for atoms in a square optical lattice subjected to a staggered gauge field. The system is described by a Bose-Hubbard model with complex and anisotropic hopping parameters that are different for each species, i.e., atoms and molecules. When the flux per plaquette for each species is small, the system oscillates between two conventional zero-momentum condensates. However, there is a regime of parameters in which Josephson oscillations between a vortex-carrying atomic condensate (finite momentum BEC) and a conventional zero-momentum molecular condensate may be realized. The experimental observation of the oscillations between these qualitatively distinct BEC's is possible with state-of-the-art Ramsey interference techniques.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2021

The Steel Equivalency Workbook: An X-Ray Transmission Calculator

The Steel Equivalency Spreadsheet was created as a robust, user-friendly method for the following: 1. Determining if it is possible to image an object. 2. Reducing time needed when selecting equipment and preparing for field imaging activities. 3. Reducing the amount of equipment taken for field imaging activities. 4. Determining a starting point for exposure settings prior to imaging an object. 5. Reducing the amount of dose deposited to complete an imaging operation (in the spirit of As Low As Reasonably Achievable, ALARA). The spreadsheet was developed using various physical models to account for different phenomena. Future implementations aim to expand beyond the Digital Radiography and Computed Tomography Single Munition Scanner (DRCT SMS) in standard configuration to include high energy XRGs (Betatrons) and sub-MeV pulsed XRGs (XRS4). Alternative computational methods that compensate for incoherent scattering when calculating relative transmission are also being pursued. Ultimately the spreadsheet exceeded the developmental goal of having less than 10% average error when comparing calculations to real-world images.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Modulational instability of electrostatic acoustic waves in an electron-hole semiconductor quantum plasma

The modulational instability of quantum electrostatic acoustic waves in electron-hole quantum semiconductor plasmas is investigated using the quantum hydrodynamic model, from which a modified nonlinear Schrödinger equation with damping effects is derived using the reductive perturbation method. Here, we consider the combined effects of quantum recoil, quantum degenerate pressures, as well as the exchange-correlation effect standing for the electrons (holes) spin. The modulational instability for different semiconductors (GaAs, GaSb, and InP) is discussed. The collision between electron (hole) and phonon is also investigated. The permitted maximum time for modulational instability and the damping features of quantum envelope solitary wave are all determined by the collision. The approximate solitary solution with damping effects is presented in weak collision limit. The damping properties were discussed by numerical method.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1993

The Geminga neutron star: Evidence for nucleon superfluidity at very high density

A comparison of the recent age and temperature estimates of the Geminga neutron star with cooling models is presented. This star is already in the photon cooling era and it is shown that its temperature can be understood within both the slow and fast neutrino emission scenarios and consequently will not allow discrimination between these two scenarios. However in both cases agreement of the theoretical cooling curves with the observed temperature depends crucially on the presence of nucleon pairing in most, if not all, of the core.

Page, Dany↗
DOE OSTI2015

Two-band description of resonant superfluidity in atomic Fermi gases

Fermionic superfluidity in atomic Fermi gases across a Feshbach resonance is normally described by the atom-molecule theory, which treats the closed channel as a noninteracting point boson. In this work we present a theoretical description of the resonant superfluidity in analogy to the two-band superconductors. We employ the underlying two-channel scattering model of Feshbach resonance where the closed channel is treated as a composite boson with binding energy ε 0 and the resonance is triggered by the microscopic interchannel coupling U 12 . The binding energy ε 0 naturally serves as an energy scale of the system, which has been sent to infinity in the atom-molecule theory. We show that the atom-molecule theory can be viewed as a leading-order low-energy effective theory of the underlying fermionic theory in the limit ε 0 →∞ and U 12 →0, while keeping the phenomenological atom-molecule coupling finite. The resulting two-band description of the superfluid state is in analogy to the BCS theory of two-band superconductors. In the dilute limit ε 0 →∞, the two-band description recovers precisely the atom-molecule theory. The two-band theory provides a natural approach to study the corrections because of a finite binding energy ε 0 in realistic experimental systems. For broad and moderate resonances, the correction is not important for current experimental densities. However, for extremely narrow resonance, we find that the correction becomes significant. Lastly, the finite binding energy correction could be important for the stability of homogeneous polarized superfluid against phase separation in imbalanced Fermi gases across a narrow Feshbach resonance.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2019

Faithful tight-binding models and fragile topology of magic-angle bilayer graphene

Correlated insulators and superconductivity have been observed in “magic-angle” twisted bilayer graphene, when the nearly flat bands close to neutrality are partially filled. Here, while a momentum-space continuum model accurately describes these flat bands, interaction effects are more conveniently incorporated in tight-binding models. We have previously shown that no fully symmetric tight-binding model can be minimal, in the sense of capturing just the flat bands, so extended models are unavoidable. Here, we introduce a family of tight-binding models that capture the flat bands while simultaneously retaining all symmetries. In particular, we construct three concrete models with five, six, or ten bands per valley and per spin. These models are also faithful, in that the additional degrees of freedom represent energy bands further away from neutrality, and they serve as optimal starting points for a controlled study of interaction effects. Furthermore, our construction demonstrates the “fragile topology” of the nearly flat bands; i.e., the obstruction to constructing exponentially localized Wannier functions can be resolved when a particular set of trivial bands is added to the model.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2010

Neutron diffraction study on the two-dimensional Ising system KEr(MoO{sub 4}){sub 2}

The magnetic properties of the two-dimensional Ising antiferromagnet KEr(MoO{sub 4}){sub 2} have been investigated below and above transition temperature T{sub N}{approx}0.95 K in zero field and in fields up to 6.5 T by means of elastic neutron-diffraction, heat-capacity, and magnetization measurements. The low-temperature signal recorded at 0.34 K by neutron diffraction is explained within a noncollinear magnetic structure model. However, additional contribution is also present when applying the external magnetic field along the c axis even at temperatures well above the magnetic transition temperature T{sub N}. Various explanations are discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Coupled-cluster method: A lattice-path-based subsystem approximation scheme for quantum lattice models

An approximation hierarchy, called the lattice-path-based subsystem (LPSUBm) approximation scheme, is described for the coupled-cluster method (CCM). It is applicable to systems defined on a regular spatial lattice. We then apply it to two well-studied prototypical (spin-(1/2) Heisenberg antiferromagnetic) spin-lattice models, namely, the XXZ and the XY models on the square lattice in two dimensions. Results are obtained in each case for the ground-state energy, the ground-state sublattice magnetization, and the quantum critical point. They are all in good agreement with those from such alternative methods as spin-wave theory, series expansions, quantum Monte Carlo methods, and the CCM using the alternative lattice-animal-based subsystem (LSUBm) and the distance-based subsystem (DSUBm) schemes. Each of the three CCM schemes (LSUBm, DSUBm, and LPSUBm) for use with systems defined on a regular spatial lattice is shown to have its own advantages in particular applications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2010

Connection between slow and fast dynamics of molecular liquids around the glass transition

The mean-square displacement (MSD) was measured by neutron scattering at various temperatures and pressures for a number of molecular glass-forming liquids. The MSD is invariant along the glass-transition line at the pressure studied, thus establishing an 'intrinsic' Lindemann criterion for any given liquid. A one-to-one connection between the MSD's temperature dependence and the liquid's fragility is found when the MSD is evaluated on a time scale of {approx}4 ns, but does not hold when the MSD is evaluated at shorter times. The findings are discussed in terms of the elastic model and the role of relaxations, and the correlations between slow and fast dynamics are addressed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Plasmon modes of metallic nanowires including quantum nonlocal effects

The properties of electrostatic surface and bulk plasmon modes of cylindrical metallic nanowires are investigated, using the quantum hydrodynamic theory of plasmon excitation which allows an analytical study of quantum tunneling effects through the Bohm potential term. New dispersion relations are obtained for each type of mode and their differences with previous treatments based on the standard hydrodynamic model are analyzed in detail. Numerical results show by considering the quantum effects, as the value of wave number increases, the surface modes are slightly red-shifted first and then blue-shifted while the bulk modes are blue-shifted.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Magneto-induced tunability of thermo-spin current in deformed zigzag graphene nanoribbons

The aim of this report is to unfold how the thermo-electric spin-polarized current in a transverse-biased zigzag graphene nanoribbon changes in the presence of uniaxial deformations and uniform perpendicular magnetic field. Employing the two-parameter Hubbard model along with the non-equilibrium Green's function formalism, we found that both uniaxial strain and magnetic field can significantly modulate the bandgap, local distribution of edge states, and the critical transverse electric field needed to achieve the half-metallic phase in the ribbon. Our analysis shows a significant enhancement of the maximum attainable spin-polarized current as functions of both source temperature and contacts temperature difference, with increasing the magnetic field or applying any magnitude of compressive strain. Furthermore, it is shown that the magneto-resistance ratio of the device, can be drastically tuned via strain engineering, reaching values as high as 2 × 10{sup 4}% for compressive strains of 5% magnitude.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Superconductivity in the lightly doped Hubbard model on the cylindrical honeycomb lattice

Here, we have performed large-scale density-matrix renormalization group studies of the lightly doped Hubbard model on the honeycomb lattice on long three- and four-leg cylinders. We find that the ground state of the system upon lightly doping is consistent with that of a superconducting state with coexisting quasi-long-range superconducting and charge density wave orders. Both the superconducting and charge density wave correlations decay as a power law at long distances with corresponding exponents 𝐾 𝑠⁢𝑐 < 2 and 𝐾 𝑐 < 2. On the contrary, the spin-spin and single-particle correlations decay exponentially, although with relatively long correlation lengths.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Electron-electron interactions in low-dimensional Si:P delta layers

Scientists have long studied the physics of highly disordered conducting systems, seeking to understand the multitude of quantum phenomena that govern how electrons move through material systems. Recently, research into silicon-based quantum computing has made disordered conducting systems, such as Si:P monolayers embedded in isotopically pure Si, technically relevant. Consequently, applying and advancing the theoretical frameworks developed to describe electron behavior in disordered systems is a necessary objective in this field of research. This study investigates key components of dopant-based Si quantum computing devices: embedded regions of highly doped delta layers (δ layers). We examine the transport behavior and the electron-electron interaction (EEI) physics in embedded Si:P δ layers by means of self-consistent magnetotransport measurements. Parameters associated with the electronic transport offer a meaningful quantitative characterization of δ-layer quality and dopant diffusion. In addition, by examining EEI behaviors in a set of samples with embedded Si:P δ layers produced with different PH3 exposure procedures prior to Si encapsulation, we show how details of material synthesis affect the dimensionality of charge carrier interactions in embedded Si:P δ layers. Furthermore, the relationship between δ-layer confinement and EEI screening lengths is established here. This understanding will help validate important models used for device simulation and design and lead to improvements in the control of electrostatic gating of and tunneling transport through Si:P single atom transistors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Fluids, superfluids and supersolids: dynamics and cosmology of self-gravitating media

We compute cosmological perturbations for a generic self-gravitating media described by four derivatively-coupled scalar fields. Depending on the internal symmetries of the action for the scalar fields, one can describe perfect fluids, superfluids, solids and supersolids media. Symmetries dictate both dynamical and thermodynamical properties of the media. Generically, scalar perturbations include, besides the gravitational potential, an additional non-adiabatic mode associated with the entropy per particle σ. While perfect fluids and solids are adiabatic with σ constant in time, superfluids and supersolids feature a non-trivial dynamics for σ. Special classes of isentropic media with zero σ can also be found. Tensor modes become massive for solids and supersolids. Such an effective approach can be used to give a very general and symmetry driven modelling of the dark sector.

79 ASTRONOMY AND ASTROPHYSICS↗