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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 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Critical Behavior of the Specific Heat of Pr{sub 0.6}Sr{sub 0.4}Mn{sub 1−x}Fe{sub x}O{sub 3} Manganites

The specific heat of Pr{sub 0.6}Sr{sub 0.4}Mn{sub 1−x}Fe{sub x}O{sub 3} (x = 0 and 0.1) manganites was measured in the temperature range 80–350 K. A nature of the ferromagnetic phase transition is studied via the analysis of a specific heat critical behavior near the Curie temperature with account of correction to the scaling. The critical indices calculated with the account of correction to the scaling α = −0.0981 and −0.1208 for x = 0 and 0.10, correspondingly, are close to those of the 3D Heisenberg model.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Two-particle correlation functions in cluster perturbation theory: Hubbard spin susceptibilities

Cluster perturbation theory (CPT) is a computationally economic method commonly used to estimate the momentum- and energy-resolved single-particle Green's function. It has been used extensively in direct comparisons with experiments that effectively measure the single-particle Green's function, e.g., angle-resolved photoemission spectroscopy. However, many experimental observables are given by two-particle correlation functions. CPT can be extended to compute two-particle correlation functions by approximately solving the Bethe-Salpeter equation. We implement this method and focus on the transverse spin susceptibility, measurable via inelastic neutron scattering or with optical probes of atomic gases in optical lattices. Finally, we benchmark the method with the one-dimensional Fermi-Hubbard model by comparing with known results.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Importance of counter-rotating coupling in the superfluid-to-Mott-insulator quantum phase transition of light in the Jaynes-Cummings lattice

The quantum phase transition between Mott insulator and superfluid is studied in the two-dimensional Jaynes-Cummings square lattice in which the counter-rotating coupling (CRC) is included. Both the ground state and the spectra of low-lying excitations are obtained with use of a sophisticated unitary transformation. This CRC is shown not only to induce a long-range interaction between cavities, favoring the long-range superfluid order, but also to break the conservation of local polariton number at each site, leading to the absence of the Mott lobes in the phase diagram, in sharp contrast with the case without the CRC as well as that of the Bose-Hubbard model.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Thermodynamic and Magnetic Properties of the Two-Dimensional Anisotropic Ising Model with Competing Interactions

The two-dimensional anisotropic Ising model with competing interactions is studied on a square lattice by Monte Carlo methods using the Wang–Landau algorithm. The temperature dependences of the main thermodynamic and magnetic parameters are calculated. The distinctive features of temperature dependences of these parameters for different values of the competing interaction are shown. The dependences of the wave number of modulated structures on the ratio of exchange interaction parameters are plotted. The phase diagram of the model is built. The coordinates of the Lifshits point and the localization region of the phases with different wave vector values are calculated by interpolating phase boundaries of the d-iagram.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Broadband terahertz time-domain spectroscopy of ferroelectric LiTaO{sub 3}: Phonon-polariton dispersion

The lowest frequency IR active A{sub 1}(z) and E(x) modes of a ferroelectric congruent lithium tantalate crystal were studied by the broadband polarized Terahertz Time-Domain Spectroscopy using the coherent THz radiation from 0.2 to 6.5 THz. The dispersion relations of the real and imaginary parts of a polariton wave vector were determined from the complex extraordinary and ordinary refractive indices for the A{sub 1}(z) and E(x) symmetry phonon-polaritons, respectively. The observed complex dispersion relations of two symmetries are in agreement within the experimental uncertainty with the calculated dispersion curves by the damped harmonic oscillator model.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Exactly Solvable Model for Strongly Interacting Electrons in a Magnetic Field

States of strongly interacting particles are of fundamental interest in physics and can produce exotic emergent phenomena and topological structures. We consider here two-dimensional electrons in a magnetic field, and, departing from the standard practice of restricting to the lowest LL, introduce a model short-range interaction that is infinitely strong compared to the cyclotron energy. Here, we demonstrate that this model lends itself to an exact solution for the ground as well as excited states at arbitrary filling factors v<1/2p and produces a fractional quantum Hall effect at fractions of the form v=n/(2pn+1), where n and p are integers. The fractional quantum Hall states of our model share many topological properties with the corresponding Coulomb ground states in the lowest Landau level, such as the edge physics and the fractional charge of the excitations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Antiferromagnetism in the kagome-lattice compound α-Cu 3 Mg(OH) 6 Br 2

The antiferromagnetism in α-Cu 3 Mg(OH) 6 Br 2 was studied by magnetic-susceptibility, specificheat and neutron-diffraction measurements. The crystal structure consists of Cu 2+ kagome layers with Mg 2+ ions occupying the centers of the hexagons, separated by Br 1- ions. The magnetic system orders antiferromagnetically at 5.4 K with the magnetic moments aligned ferromagnetically within the kagome planes. The ordered moment is 0.94 µ B , suggesting little quantum and geometrical fluctuations. By comparing the magnetic and specific-heat properties with those of the haydeeite, we suggest that α-Cu 3 Mg(OH) 6 Br 2 may be described by the two-dimensional spin-1/2 Heisenberg kagome model and is in the region of the ferromagnetic-order side of the phase diagram.

2-dimensional systems↗
DOE OSTI2010

Superconductivity Generated by Coupling to a Cooperon in a 2-Dimensional Array of 4-Leg Hubbard Ladders

Starting from an array of four-leg Hubbard ladders weakly doped away from half-filling and weakly coupled by interladder tunneling, we derive an effective low-energy model which contains a partially truncated Fermi surface and a well-defined cooperon excitation formed by a bound pair of holes. An attractive interaction in the Cooper channel is generated on the Fermi surface through virtual scattering into the cooperon state. Although the model is derived in the weak coupling limit of a four-leg ladder array, an examination of exact results on finite clusters for the strong coupling t-J model suggests the essential features are also present for a strong coupling Hubbard model on a square lattice near half-filling.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2010

Fermions and disorder in ising and related models in two dimensions

The aspects of phase transitions in the two-dimensional Ising models modified by quenched and annealed site disorder are discussed in the framework of fermionic approach based on the reformulation of the problem in terms of integrals with anticommuting Grassmann variables.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2010

Spontaneous symmetry breaking of Bose-Fermi mixtures in double-well potentials

We study the spontaneous symmetry breaking (SSB) of a superfluid Bose-Fermi (BF) mixture in a double-well potential (DWP). The mixture is described by the Gross-Pitaevskii equation (GPE) for the bosons, coupled to an equation for the order parameter of the Fermi superfluid, which is derived from the respective density functional in the unitarity limit (a similar model applies to the BCS regime, too). Straightforward SSB in the degenerate Fermi gas loaded into a DWP is impossible, as it requires an attractive self-interaction, and the intrinsic nonlinearity in the Fermi gas is repulsive. Nonetheless, we demonstrate that the symmetry breaking is possible in the mixture with attraction between fermions and bosons, like {sup 40}K and {sup 87}Rb. Numerical results are represented by dependencies of asymmetry parameters for both components on particle numbers of the mixture, N{sub F} and N{sub B}, and by phase diagrams in the (N{sub F},N{sub B}) plane, which displays regions of symmetric and asymmetric ground states. The dynamical picture of the SSB, induced by a gradual transformation of the single-well potential into the DWP, is reported too. An analytical approximation is proposed for the case when the GPE for the boson wave function may be treated by means of the Thomas-Fermi (TF) approximation. Under a special linear relationship between N{sub F} and N{sub B}, the TF approximation allows us to reduce the model to a single equation for the fermionic function, which includes competing repulsive and attractive nonlinear terms. The latter one directly displays the mechanism of the generation of the effective attraction in the Fermi superfluid, mediated by the bosonic component of the mixture.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Viscosity and scale invariance in the unitary Fermi gas

We compute the shear viscosity of the unitary Fermi gas above the superfluid transition temperature, using a diagrammatic technique that starts from the exact Kubo formula. The formalism obeys a Ward identity associated with scale invariance which guarantees that the bulk viscosity vanishes identically. For the shear viscosity, vertex corrections and the associated Aslamazov-Larkin contributions are shown to be crucial to reproduce the full Boltzmann equation result in the high-temperature, low fugacity limit. The frequency dependent shear viscosity {eta}({omega}) exhibits a Drude-like transport peak and a power-law tail at large frequencies which is proportional to the Tan contact. The weight in the transport peak is given by the equilibrium pressure, in agreement with a sum rule due to Taylor and Randeria. Near the superfluid transition the peak width is of the order of 0.5T{sub F}, thus invalidating a quasiparticle description. The ratio {eta}/s between the static shear viscosity and the entropy density exhibits a minimum near the superfluid transition temperature whose value is larger than the string theory bound h/(4{pi}k{sub B}) by a factor of about seven.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Wetting state on hydrophilic and hydrophobic micro-textured surfaces: Thermodynamic analysis and X-ray visualization

In this study, the wetting state on hydrophobic and hydrophilic micro-textured surfaces was investigated. High spatial resolution synchrotron X-ray radiography was used to overcome the limitations in visualization in previous research and clearly visualize the wetting state for each droplet under quantified surface conditions. Based on thermodynamic characteristics, a theoretical model for wetting state depending on the chemical composition (intrinsic contact angle) and geometrical morphology (roughness ratio) of the surfaces was developed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Quantum gates controlled by spin chain soliton excitations

Propagation of soliton-like excitations along spin chains has been proposed as a possible way for transmitting both classical and quantum information between two distant parties with negligible dispersion and dissipation. In this work, a somewhat different use of solitons is considered. Solitons propagating along a spin chain realize an effective magnetic field, well localized in space and time, which can be exploited as a means to manipulate the state of an external spin (i.e., a qubit) that is weakly coupled to the chain. We have investigated different couplings between the qubit and the chain, as well as different soliton shapes, according to a Heisenberg chain model. It is found that symmetry properties strongly affect the effectiveness of the proposed scheme, and the most suitable setups for implementing single qubit quantum gates are singled out.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Many-site coherence revivals in the extended Bose-Hubbard model and the Gutzwiller approximation

We investigate the collapse and revival of first-order coherence in deep optical lattices when long-range interactions are turned on and find that the first few revival peaks are strongly attenuated already for moderate values of the nearest-neighbor interaction coupling. It is shown that the conventionally employed Gutzwiller wave function, with only on-site number dependence of the variational amplitudes, leads to incorrect predictions for the collapse and revival oscillations within the extended Bose-Hubbard model. We provide a modified variant of the Gutzwiller ansatz, reproducing the analytically calculated time dependence of first-order coherence in the limit of zero tunneling.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Critical Properties in the Ising Model on a Triangular Lattice with the Variable Interlayer Exchange Interaction

Phase transitions and critical and thermodynamic properties of the three-dimensional antiferromagnetic Ising model on a layered triangular lattice with variable interlayer exchange interaction are studied by the replica algorithm of the Monte Carlo method. The studies are carried out for the ratios of the intralayer J{sub 1} and interlayer J{sub 2} exchange interactions in the range of r = J{sub 2}/J{sub 1} = 0.01–1.0. It is established that a second-order phase transition is observed in the considered r interval. Using the finite size scaling theory, the static critical exponents of the heat capacity α, susceptibility γ, order parameter β, correlation radius ν, and Fisher index η are calculated. It is shown that the universality class of the critical behavior of this model is preserved in the interval of 0.05 < r ≤ 1.0. It was found that with a further decrease in the r value, a crossover from the three-dimensional critical behavior to the quasi-two-dimensional one is observed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

On choosing the order parameter of modulated magnetic structures

Long-period modulated structures in the anisotropic Ising model with competing interactions (the ANNNI model) are studied by Monte Carlo methods. A new order parameter in the form of modulated phase amplitude is proposed for describing modulated structures. It is shown that the properties of the modulated phase-paramagnetic phase transition can be investigated by using the amplitude as the order parameter.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

A reactive molecular dynamics model for uranium/hydrogen containing systems

Uranium-based materials are valuable assets in the energy, medical, and military industries. However, understanding their sensitivity to hydrogen embrittlement is particularly challenging due to the toxicity of uranium and the computationally expensive nature of quantum-based methods generally required to study such processes. In this regard, we have developed a Chebyshev Interaction Model for Efficient Simulation (ChIMES) that can be employed to compute energies and forces of U and UH3 bulk structures with vacancies and hydrogen interstitials with accuracy similar to that of Density Functional Theory (DFT) while yielding linear scaling and orders of magnitude improvement in computational efficiency. Here, we show that the bulk structural parameters, uranium and hydrogen vacancy formation energies, and diffusion barriers predicted by the ChIMES potential are in strong agreement with the reference DFT data. We then use ChIMES to conduct molecular dynamics simulations of the temperature-dependent diffusion of a hydrogen interstitial and determine the corresponding diffusion activation energy. Our model has particular significance in studies of actinides and other high-Z materials, where there is a strong need for computationally efficient methods to bridge length and time scales between experiments and quantum theory.

36 MATERIALS SCIENCE↗