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Results for “Ising models”

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 811 records · Page 45
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

He-3 in galactic cosmic rays

Cosmic-ray He-3/He-4 observations, including a new measurement around 65 MeV per nucleon from ISEE-3, are compared with interstellar propagation and solar modulation calculations in an effort to understand the origin of cosmic-ray He nuclei. A survey of spacecraft and balloon observations of the He-3/He-4 ratio shows improved consistency among measurements in the 50-300 MeV per nucleon energy range when a previously neglected contribution from atmospheric secondary He-3 is taken into account. These low-energy observations imply a mean escape length of 6-8 g/sq cm in the standard 'leaky box' model for cosmic-ray propagation in the Galaxy, a value consistent with that derived from studies of heavier nuclei. No evidence is found for an excess of low-energy He-3 such as that reported at high energies.

Mewaldt, R. A.↗
DOE OSTI2019

Crystal fields and magnetic structure of the Ising antiferromagnet Er 3 Ga 5 O 12

Rare-earth garnets are an exciting playground for studying the exotic magnetic properties of the frustrated hyperkagome lattice. In this paper we present a comprehensive study of the single ion and collective magnetic properties of the garnet Er 3 Ga 5 O 12 . Using inelastic neutron scattering, we find a crystal-field ground-state doublet for Er 3 + with strong Ising anisotropy along local [100] axes. Magnetic susceptibility and heat-capacity measurements provide evidence for long-range magnetic ordering with T N = 0.8 K , and no evidence for residual entropy is found when cooling through the ordering transition. Neutron powder diffraction reveals that the ground-state spin configuration corresponds to the six-sublattice, Ising antiferromagnetic state ( Γ 3 ) common to many of the rare-earth garnets. However, we also found that μ SR appears to be insensitive to the ordering transition in this material, in which a low-temperature relaxation plateau was observed with no evidence of spontaneous muon precession. The combined muon and neutron results may be indicative of a dynamical ground state with a relatively long correlation time. Despite this potential complication, our work indicates that Er 3 Ga 5 O 12 is an excellent model system for studying the complex metamagnetism expected for a multiaxis antiferromagnet.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Magnetic order and energy-scale hierarchy in artificial spin-ice structures

In order to explain and predict the properties of many physical systems, it is essential to understand the interplay of different energy scales. Here we present investigations of the magnetic order in thermalized artificial spin-ice structures, with different activation energies of the interacting Ising-like elements. We image the thermally equilibrated magnetic states of the nanostructures using synchrotron-based magnetic microscopy. By comparing results obtained from structures with one or two different activation energies, we demonstrate a clear impact on the resulting magnetic order. The differences are obtained by the analysis of the magnetic spin structure factors, in which the role of the activation energies is manifested by distinct short-range order. Furthermore, these results highlight the potential of artificial spin-ice structures to serve as model systems for designing various energy-scale hierarchies and investigating their impact on the collective dynamics and magnetic order.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
NASA NTRS1986

Hot, diamagnetic cavities upstream from the earth's bow shock

On eight occasions the ISEE 1 and 2 spacecraft registered peculiar plasma structures upstream of the earth's bow shock. The events exhibit a temporary, strong reduction in the magnitude of the magnetic field and strong enhancements of the field strength bordering the reduction zone. The low field strength regions featured temperatures from 1-10 million k and pressure an order of magnitude greater than the solar wind. The pressure gradients exceeded the magnetic tension around the structures, although the field of the cavities may be a closed structure. A model is proposed of hot, expanding diamagnetic plasma cavities with scales on the order of a few earth radii. Speculations on the interaction and origin or impetus for the cavities within the bow shock, foreshock, the magnetosphere and the solar wind are discussed. Similarities between the phenomena detected and signatures obtained with the AMPTE releases of chemicals in the solar wind are noted.

Thomsen, M. F.↗
NASA NTRS1987

Properties of a large-scale interplanetary loop structure as deduced from low-energy proton anisotropy and magnetic field measurements

Correlated particle and magnetic field measurements by the ISEE 3 spacecraft are presented for the loop structure behind the interplanetary traveling shock event of Nov. 12, 1978. Following the passage of the turbulent shock region, strong bidirectional streaming of low-energy protons is observed for approximately 6 hours, corresponding to a loop thickness of about 0.07 AU. This region is also characterized by a low relative variance of the magnetic field, a depressed proton intensity, and a reduction in the magnetic power spectral density. Using quasi-linear theory applied to a slab model, a value of 3 AU is derived for the mean free path during the passage of the closed loop. It is inferred from this observation that the proton regime associated with the loop structure is experiencing scatter-free transport and that either the length of the loop is approximately 3 AU between the sun and the earth or else the protons are being reflected at both ends of a smaller loop.

Tranquille, C.↗
DOE OSTI2023

Continuous symmetry breaking in a two-dimensional Rydberg array

Spontaneous symmetry breaking underlies much of our classification of phases of matter and their associated transitions. The nature of the underlying symmetry being broken determines many of the qualitative properties of the phase; this is illustrated by the case of discrete versus continuous symmetry breaking. Indeed, in contrast to the discrete case, the breaking of a continuous symmetry leads to the emergence of gapless Goldstone modes controlling, for instance, the thermodynamic stability of the ordered phase. Here, we realize a two-dimensional dipolar XY model that shows a continuous spin-rotational symmetry using a programmable Rydberg quantum simulator. We demonstrate the adiabatic preparation of correlated low-temperature states of both the XY ferromagnet and the XY antiferromagnet. In the ferromagnetic case, we characterize the presence of a long-range XY order, a feature prohibited in the absence of long-range dipolar interaction. In conclusion, our exploration of the many-body physics of XY interactions complements recent works using the Rydberg-blockade mechanism to realize Ising-type interactions showing discrete spin rotation symmetry.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1981

First-order Fermi acceleration of the diffuse ion population near the earth's bow shock

The flux of 30-65 keV particles observed by the ISEE-3 200 earth radii upstream is shown to be an upstream escape of the energetic ions in the earth's bow shock. A formal solution to the transport equation for the distribution function of energetic particles upstream from an isotropic monoenergetic source of particles/sq cm at a plane shock where the plasma changes speed is found, and escape conditions are defined. The efficiency of the acceleration is calculated to depend on the charge/particle, and fluxes near and far upstream of the shock are described analytically. Any model which takes into account shock acceleration by diffusive scattering with significant escape losses produces the observed spectrum close to the shock. The escape loss upstream is demonstrated to control the spectrum and the variation of flux and anisotropy with distance from the shock.

Forman, M. A.↗
NASA NTRS1983

A statistical study of large electric field events in the earth's magnetotail

A statistical study of large electric field events, measured on ISEE 1 at geocentric distances of 3 to 23 earth radii, has been performed in order to determine the occurrence frequency and spatial distribution of such events. Large electric fields were seen at all altitudes, with the lowest altitude events occurring on auroral field lines and the higher altitude events occurring on the high latitude boundary of the plasma sheet. The largest events had amplitudes that were approximately proportional to the square root of the local magnetic field strength, in agreement with expectations of a simple mapping model. The apparent concentration of big field events near local midnight is caused, at least in part, by a nonuniform frequency of satellite crossings through the plasma sheet boundary as a function of local time. Thus, the spatial distribution and amplitudes of the large field events are not inconsistent with the mapping of low-altitude electrostatic shocks to the plasma sheet boundary along equipotential magnetic field lines.

Levin, S.↗
NASA NTRS1984

Magnetospheric ions and electrons in the distant magnetosheath at about 50 and 180 earth radii - ISEE-3 observations

Energetic particle data were obtained in the dawn magnetosheath at about 50 earth radii (ER), analyzed, and compared with data taken at 180 ER. Both data sets were acquired during the ISEE-3 geotail mission. The results of analyses of the 16 sec time profiles of 32 and 130 keV protons, 75-115 keV electrons, and some of the 30-36 keV proton angular distributions (PAD) are presented. No temporal dispersions were detected in the particle bursts, implying instead a spatial dispersion. The particles filled flux tubes of 6 ER, with the energetic electrons being beamed along magnetic field lines. PADs displayed anisotropy symmetric relative to the magnetic field. The electrons were not detected at 180 ER. The phenomena are modeled as flux tubes connected to the magnetotail magnetic field in the near-earth magnetopause. The particles leak out along connected field lines. Lion roars scatter the electrons, which escape into interplanetary space. Ions, however, may carry their intensity out to 200 ER, a factor which awaits verification by further data.

Scholer, M.↗
NASA NTRS1982

Spatial structure of greater than 100 keV X-ray sources in solar flares

Recent ISEE 3 and Pioneer Venus Orbiter observations relevant to the altitude structure of the hard X-ray source down to 3000 km or less above the photosphere are reported. About 90 percent of the impulsive X-ray emission and about 70 percent of the gradual emission originates at altitudes 2500 km or less above the photosphere. In the 100-500 keV range, this altitude dependence is essentially independent of photon energy. Brightness of the impulsive X-ray source decreases rapidly with increase in altitude. As far as models of the impulsive phase are concerned, the thermal model with adiabatic compression and expansion of a magnetically confined plasma and the thin target model are not consistent with these observations. The thick target model and the dissipative thermal model are consistent with the observed low altitude of the hard X-ray source.

Kane, S. R.↗
NASA NTRS1984

ISEE 1 and 2 observations of an oscillating outward moving current sheet near midnight

The ISEE 1 and 2 magnetometers are used to examine a current sheet which is moving outward with a velocity of about 17 km/sec, and whose current is flowing into the ionosphere. A wave is traveling along the current sheet, from midnight toward the east, at 400 km/sec. The plasma oscillations normal to the current sheet associated with this wave are sufficient for explaining the amplitude of the electric field oscillations in the plane of the current sheet observed by the University of California's electric field detector. The present oscillating current sheet model explains both magnetic and electric field observations.

Kelly, T. J.↗
DOE OSTI2019

Resonating quantum three-coloring wave functions for the kagome quantum antiferromagnet

Motivated by the recent discovery of a macroscopically degenerate exactly solvable point of the spin-1/2 XXZ model for J z /J = –1/2 on the kagome lattice —a result that holds for arbitrary magnetization—we develop an exact mapping between its exact ”quantum three-coloring” wave functions and the characteristic localized and topological magnons. This map, involving ”resonating two-color loops,” is developed to represent exact many-body ground state wave functions for special high magnetizations. Using this map we show that these exact ground state solutions are valid for any J z /J ≥ –1/2. This demonstrates the equivalence of the ground-state wave function of the Ising, Heisenberg, and XY regimes all the way to the J z /J = –1/2 point for these high magnetization sectors. In the hardcore bosonic language, this means that a certain class of exact many-body solutions, previously argued to hold for purely repulsive interactions (J z ≥ 0), actually hold for attractive interactions as well, up to a critical interaction strength. For the case of zero magnetization, where the ground state is not exactly known, we perform density matrix renormalization group calculations. Based on the calculation of the ground state energy and measurement of order parameters, we provide evidence for a lack of any qualitative change in the ground state on finite clusters in the Ising (J z >> J), Heisenberg (J z = J), and XY (J z = 0) regimes, continuing adiabatically to the vicinity of the macroscopically degenerate J z /J = –1/2 point. Furthermore, these findings offer a framework for recent results in the literature and also suggest that the J z /J = –1/2 point is an unconventional quantum critical point whose vicinity may contain the key to resolving the spin-1/2 kagome problem.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Theories of glass formation and glass transition

This key-issues review is a plea for a new focus on simpler and more realistic models of glass-forming fluids. It seems to me that we have too often been led astray by sophisticated mathematical models that beautifully capture some of the most intriguing features of glassy behavior, but are too unrealistic to provide bases for predictive theories. As illustrations of what I mean, the first part of this article is devoted to brief summaries of imaginative, sensible, but disparate and often contradictory ideas for solving glass problems. Almost all of these ideas remain alive today, with their own enthusiastic advocates. I then describe numerical simulations, mostly by H Tanaka and coworkers, in which it appears that very simple, polydisperse systems of hard disks and spheres develop long range, Ising-like, bond-orientational order as they approach glass transitions. Finally, a summary of my recent proposal that topologically ordered clusters of particles, in disordered environments, tend to become aligned with each other as if they were two-state systems, and thus produce the observed Ising-like behavior. Neither Tanaka’s results nor my proposed interpretation of them fit comfortably within any of the currently popular glass theories.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Theory of quantum kagome ice and vison zero modes

We derive an effective Z 2 gauge theory to describe the quantum kagome ice (QKI) state that has been observed by Carrasquilla et al. [Nat. Commun. 6, 7421 (2015)] in Monte Carlo studies of the S = $\frac{1}{2}$ kagome XYZ model in a Zeeman field. The numerical results on QKI are consistent with, but do not confirm or rule out, the hypothesis that it is a Z 2 spin liquid. Our effective theory allows us to explore this hypothesis and make a striking prediction for future numerical studies, namely, that symmetry-protected vison zero modes arise at lattice disclination defects, leading to a Curie defect term in the spin susceptibility, and a characteristic (N dis - 1) ln 2 contribution to the entropy, where N dis is the number of disclinations. Only the Z 2 Ising symmetry is required to protect the vison zero modes. This is remarkable because a unitary Z 2 symmetry cannot be responsible for symmetry-protected degeneracies of local degrees of freedom. We also discuss other signatures of symmetry fractionalization in the Z 2 spin liquid, and phase transitions out of the Z 2 spin liquid to nearby ordered phases.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1984

The near Earth current sheet: ISEE-1 and -2 case studies

Two crossings of the near Earth current sheet are examined using magnetic field and plasma data from ISEE-1 and 2. The current sheet thickness is measured to be 0.9 Re using a 2 satellite technique which greatly reduces the ambiguity of such measurements. Approximate pressure balance is found to exist between the diamagnetically dominated current sheet and surrounding, already high beta, plasma sheet. The accurate calculation of the sheet normal velocity with the two satellite technique allows the calculation of the current density distribution in the sheet. The crosstail current density is found to peak about an ion-gyroradius on both sides of the center of the sheet. A simple model consistent with these results is discussed.

Mccomas, D. J.↗
NASA NTRS1989

ISEE 3 observations during the CDAW 8 intervals - Case studies of the distant geomagnetic tail covering a wide range of geomagnetic activity

Observations made by the ISEE 3 spacecraft in the distant geomagnetic tail during the eight CDAW 8 intervals are discussed, along with their relation to concurrent geomagnetic activity. This extensive multiinstrument case study of distant tail data covers a wide range of geomagnetic conditions from extended intervals of magnetic quiet with isolated substorms to prolonged periods of intense disturbance. Plasmoids are observed in the distant tail following disturbance enhancements, the time of their appearance being generally consistent with disconnection from the near-earth region at the time of the enhancement. Their structure is entirely consistent with the neutral line model. However, not all enhancements in geomagnetic activity result in the observation of plasmoids. In particular, the CDAW 8 data suggest that, during extended intervals of strong activity, a continuous neutral line may reside in the near-earth tail and some disturbance enhancements may then relate to an increase in the reconnection rate at a preexisting neutral line, rather than to new neutral line and plasmoid formation.

Richardson, I. G.↗
NASA NTRS1990

Ballooning instability of the earth's plasma sheet region in the presence of parallel flow

Stability of the plasma sheet and plasma sheet boundary layer against the ballooning mode instability is investigated. The equilibrium state of a two-dimensional plasma sheet configuration with parallel sheared flow is modeled. This equilibrium is shown to be ballooning unstable when delta-W is not positive definite, where delta-W is the potential energy. The eigenmode structure of the ballooning mode is found by imposing the boundary conditions that the waves are totally reflected from the ionosphere, and that no waves are coming in from infinity. The eigenmode structure of the unstable balloning modes is highly oscillatory, extending beyond about 100 R(E). The ballooning modes are thus a possible candidate for explaining the MHD waves and other dynamical events observed in the magnetotail by ISEE 3 and other spacecraft.

Lakhina, G. S.↗