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At least 433 records · Page 24

All sources NASA NTRS DOE OSTI

DOE OSTI2020

Metal-insulator and magnetic phase diagram of Ca 2 RuO 4 from auxiliary field quantum Monte Carlo and dynamical mean field theory

Layered perovskite ruthenium oxides exhibit a striking series of metal-insulator and magnetic-nonmagnetic phase transitions easily tuned by temperature, pressure, epitaxy, and nonlinear drive. In this work, we combine results from two complementary state-of-the-art many-body methods, auxiliary field quantum Monte Carlo and dynamical mean field theory, to determine the low-temperature phase diagram of Ca 2 RuO 4 . Both methods predict a low-temperature, pressure-driven metal-insulator transition accompanied by a ferromagnetic-antiferromagnetic transition. Further, the properties of the ferromagnetic state vary nonmonotonically with pressure and are dominated by the ruthenium $d_{xy}$ orbital, while the properties of the antiferromagnetic state are dominated by the $d_{xz}$ and $d_{yz}$ orbitals. Differences in the details of the predictions of the two methods are analyzed. This work is theoretically important as it presents the first application of the auxiliary field quantum Monte Carlo method to an orbitally degenerate system with both Mott and Hunds physics and provides an important comparison of the dynamical mean field and auxiliary field quantum Monte Carlo methods.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2013

Upper critical field of isoelectron substituted SrFe2(As1-xPx)2

The upper critical field H-c2 of optimally doped, iron-based superconductor SrFe2(As1-xPx)(2) (x = 0.35, T-c = 25 K) was measured as a function of temperature down to 1.6K for two principal directions of magnetic field H parallel to c and H parallel to a. Measurements were performed in pulsed magnetic fields up to 65 T using a tunnel-diode resonator technique on as-grown and heavy-ion-irradiated single crystals, with columnar defect density corresponding to a matching field B-phi = 25 T. The H-c2,(c) (T) is close to T linear, while clear saturation is observed for H-c2,(a) (T), leading to a strongly temperature-dependent anisotropy parameter gamma. The linear shape of H-c2,(c) (T) is very similar to that observed in nodal KFe2As2, but very different from full-gap LiFeAs. Irradiation does not introduce any additional features on the H-c2(T) line corresponding to the matching field. Instead, it suppresses uniformly both T-c and H-c2, keeping their ratio constant.

36 MATERIALS SCIENCE↗

DOE OSTI2015

Study of grain boundary transparency in(Yb$_{1-x}$Ca$_x$)Ba$_2$Cu$_3$O bicrystal thin films over a wide temperature, field, and field orientation range

The residual low angle grain boundary (GB) network is still the most important current-limiting mechanism operating in bi-axially textured rare earth barium copper oxide (REBCO) coated conductors. While Ca-doping is well established to improve super-current flow across low angle GBs in weak fields at high temperatures, Ca-doping also depresses T c , making it so far impractical for high temperature applications of REBCO coated conductors. On the other hand, high field magnet applications of REBCO require low temperatures. Here we systematically evaluate the effectiveness of Ca-doping in improving the GB transparency, r GB = J c GB /J c grain , of low angle Yb 1-x Ca x BaCuO [001] tilt bi-crystal films down to 10K and with magnetic fields perpendicular and parallel to the film surfaces, while varying the Ca and oxygen doping level. Using Low Temperature Scanning Laser Microscopy (LTSLM) and Magneto-Optical Imaging (MOI), we found r GB to strongly depend on the angle between magnetic field and the GB plane and clearly identified regimes in which J c GB can exceed J c grain (rGB>1) where the GB pinning is optimized by the field being parallel to the GB dislocations. However, even in this favorable situation, we found that r GB became much smaller at lower temperatures. Calculations of the GB Ca segregation profile predict that the high J c channels between the GB dislocation cores are almost Ca-free. It may be therefore that the positive effects of Ca doping seen by many authors near T c are partly a consequence of the higher Tc of these Ca-free channels.

Li, Pei↗

DOE OSTI2017

In-plane electric fields and the 𝜈 = $\frac{5}{2}$ fractional quantum Hall effect in a disk geometry

The 𝜈 = $\frac{5}{2}$ fractional quantum Hall effect is of experimental and theoretical interest due to the possible non-Abelian statistics of the excitations in the electron liquid. A small voltage difference across a sample applied in experiments to probe the system is often ignored in theoretical studies due to the Galilean invariance in the thermodynamic limit. No experimental sample, however, is Galilean invariant. In this Rapid Communication, we explore the effects of the probe electric fields in a disk geometry with finite thickness. We find that weak probe fields enhance the Moore-Read Pfaffian state, but sufficiently strong electric fields destroy the incompressible state. In a disk geometry, the behavior of the system depends on the polarity of the applied radial field, which can potentially be observed in experiments using a Corbino disk configuration. Finally, our simulation also shows that the application of such a field enhances the coherence length of quasiholes propagating through the edge channels.

exact diagonalization↗

DOE OSTI2017

Characterization of switching field distributions in Ising-like magnetic arrays

Not Available

Fraleigh, Robert D.↗

DOE OSTI2017

Magnetic field dependent dynamics and field-driven metal-to-insulator transition of the half-filled Hubbard model: A DMFT+DMRG study

Here, we study the magnetic field-driven metal-to-insulator transition in half-filled Hubbard model on the Bethe lattice, using the dynamical mean-field theory by solving the quantum impurity problem with density-matrix renormalization group algorithm. The method enables us to obtain a high-resolution spectral densities in the presence of a magnetic field. It is found that the Kondo resonance at the Fermi level splits at relatively high magnetic field: the spin-up and -down components move away from the Fermi level and finally form a spin-polarized band insulator. By calculating the magnetization and spin susceptibility, we clarify that an applied magnetic field drives a transition from a paramagnetic metallic phase to a band insulating phase. In the weak interaction regime, the nature of the transition is continuous and captured by the Stoner's description, while in the strong interaction regime the transition is very likely to be metamagnetic, evidenced by the hysteresis curve. Furthermore, we determine the phase boundary by tracking the kink in the magnetic susceptibility, and the steplike change of the entanglement entropy and the entanglement gap closing. Interestingly, the phase boundaries determined from these two different ways are largely consistent with each other.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Single classical field description of interacting scalar fields

In this report we test the degree to which interacting Bosonic systems can be approximated by a classical field as total occupation number is increased. This is done with our publicly available code repository, QIBS, a new massively parallel solver for these systems. We use a number of toy models well studied in the literature and track when the classical field description admits quantum corrections, called the quantum breaktime. This allows us to test claims in the literature regarding the rate of convergence of these systems to the classical evolution. We test a number of initial conditions, including coherent states, number eigenstates, and field number states. We find that of these initial conditions, only number eigenstates do not converge to the classical evolution as occupation number is increased. We find that systems most similar to scalar field dark matter exhibit a logarithmic enhancement in the quantum breaktime with total occupation number. Systems with contact interactions or with field number state initial conditions, and linear dispersions, exhibit a power law enhancement. Finally, we find that the breaktime scaling depends on both model interactions and initial conditions.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2018

Effects of an axial magnetic field on the current distribution in a Z-pinch implosion with preembedded magnetic field

In this study, the fundamental physics of the magnetic field distribution in a plasma implosion with a pre-embedded magnetic field is investigated within a gas puff Z-pinch. Time and space resolved spectroscopy of the polarized Zeeman effect, applied for the first time, reveals the impact of a pre-embedded axial field on the evolution of the current distribution driven by a pulsed-power generator. The measurements show that the azimuthal magnetic field in the imploding plasma, even in the presence of a weak axial magnetic field, is substantially smaller than expected from the ratio of the driving current to the plasma radius. Much of the current flows at large radii through a slowly-imploding, low-density plasma. The development of a force-free current configuration is suggested to explain this phenomenon. Finally, previously unpredicted observations in higher-power imploding-magnetized-plasma experiments, including recent unexplained structures observed in the Magnetized Liner Inertial Fusion experiment, may be explained by the present discovery.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2026

Magnetic Field Mapping of a 2.5 T Fixed-Field HTS Gantry Magnet for Proton Therapy

We present results from testing a high-temperature superconducting (HTS) magnet prototype for proton therapy. This magnet is specifically designed for a novel rotating gantry capable of delivering the entire proton beam energy range (70225 MeV) while maintaining a fixed magnetic field in the superconducting magnets. The gantry layout simplifies the magnet design, enabling the use of straight, flat racetrack Bi-2223 (DI-BSCCO) coil technology and operation at higher temperatures (1015 K). The magnet has a non-linear field distribution for bending and focusing the proton beams. To validate this feature, we developed a system for measuring the magnetic field distribution in the magnet aperture. We present the design of this hall probe array and experimental results from two different magnet tests at 4.2 K in a liquid helium bath. These results are compared with the simulated field distribution and discussed in the context of the required field quality for the application.

Mosat, M↗

DOE OSTI2012

Relationship between x-ray illumination field size and flat field intensity and its impacts on x-ray imaging

Purpose: X-ray cone-beam CT (CBCT) is being increasingly used for various clinical applications, while its performance is still hindered by image artifacts. This work investigates a new source of reconstruction error, which is often overlooked in the current CBCT imaging. The authors find that the x-ray flat field intensity (I{sub 0}) varies significantly as the illumination volume size changes at different collimator settings. A wrong I{sub 0} value leads to inaccurate CT numbers of reconstructed images as well as wrong scatter measurements in the CBCT research. Methods: The authors argue that the finite size of x-ray focal spot together with the detector glare effect cause the I{sub 0} variation at different illumination sizes. Although the focal spot of commercial x-ray tubes typically has a nominal size of less than 1 mm, the off-focal-spot radiation covers an area of several millimeters on the tungsten target. Due to the large magnification factor from the field collimator to the detector, the penumbra effects of the collimator blades result in different I{sub 0} values for different illumination field sizes. Detector glare further increases the variation, since one pencil beam of incident x-ray is scattered into an area of several centimeters on the detector. In this paper, the authors study these two effects by measuring the focal spot distribution with a pinhole assembly and the detector point spread function (PSF) with an edge-spread function method. The authors then derive a formula to estimate the I{sub 0} value for different illumination field sizes, using the measured focal spot distribution and the detector PSF. Phantom studies are carried out to investigate the accuracy of scatter measurements and CT images with and without considering the I{sub 0} variation effects. Results: On our tabletop system with a Varian Paxscan 4030CB flat-panel detector and a Varian RAD-94 x-ray tube as used on a clinical CBCT system, the focal spot distribution has a measured full-width-at-half-maximum (FWHM) of around 0.4 mm, while non-negligible off-focal-spot radiation is observed at a distance of over 2 mm from the center. The measured detector PSF has an FWHM of 0.510 mm, with a shape close to Gaussian. From these two distributions, the author calculate the estimated I{sub 0} values at different collimator settings. The I{sub 0} variation mainly comes from the focal spot effect. The estimation matches well with the measurements at different collimator widths in both horizontal and vertical directions, with an average error of less than 3%. Our method improves the accuracy of conventional scatter measurements, where the scatter is measured as the difference between fan-beam and cone-beam projections. On a uniform water cylinder phantom, more accurate I{sub 0} suppresses the unfaithful high-frequency signals at the object boundaries of the measured scatter, and the SPR estimation error is reduced from 0.158 to 0.014. The proposed I{sub 0} estimation also reduces the reconstruction error from about 20 HU on the Catphan Copyright-Sign 600 phantom in the selected regions of interest to less than 4 HU. Conclusions: The I{sub 0} variation is identified as one additional error source in x-ray imaging. By measuring the focal-spot distribution and detector PSF, the authors propose an accurate method of estimating the I{sub 0} value for different illumination field sizes. The method obtains more accurate scatter measurements and therefore facilitates scatter correction algorithm designs. As correction methods for other CBCT artifacts become more successful, our research is significant in further improving the CBCT imaging accuracy.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

DOE OSTI2014

SU-E-T-299: Small Fields Profiles Correction Through Detectors Spatial Response Functions and Field Size Dependence Analysis

Purpose: To calculate the spatial response function of various radiation detectors, to evaluate the dependence on the field size and to analyze the small fields profiles corrections by deconvolution techniques. Methods: Crossline profiles were measured on a Novalis Tx 6MV beam with a HDMLC. The configuration setup was SSD=100cm and depth=5cm. Five fields were studied (200×200mm2,100×100mm2, 20×20mm2, 10×10mm2and 5×5mm2) and measured were made with passive detectors (EBT3 radiochromic films and TLD700 thermoluminescent detectors), ionization chambers (PTW30013, PTW31003, CC04 and PTW31016) and diodes (PTW60012 and IBA SFD). The results of passive detectors were adopted as the actual beam profile. To calculate the detectors kernels, modeled by Gaussian functions, an iterative process based on a least squares criterion was used. The deconvolutions of the measured profiles were calculated with the Richardson-Lucy method. Results: The profiles of the passive detectors corresponded with a difference in the penumbra less than 0.1mm. Both diodes resolve the profiles with an overestimation of the penumbra smaller than 0.2mm. For the other detectors, response functions were calculated and resulted in Gaussian functions with a standard deviation approximate to the radius of the detector in study (with a variation less than 3%). The corrected profiles resolve the penumbra with less than 1% error. Major discrepancies were observed for cases in extreme conditions (PTW31003 and 5×5mm2 field size). Conclusion: This work concludes that the response function of a radiation detector is independent on the field size, even for small radiation beams. The profiles correction, using deconvolution techniques and response functions of standard deviation equal to the radius of the detector, gives penumbra values with less than 1% difference to the real profile. The implementation of this technique allows estimating the real profile, freeing from the effects of the detector used for the acquisition.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

DOE OSTI2022

Imaging elliptically polarized infrared near-fields on nanoparticles by strong-field dissociation of functional surface groups

Abstract We investigate the strong-field ion emission from the surface of isolated silica nanoparticles aerosolized from an alcoholic solution, and demonstrate the applicability of the recently reported near-field imaging at 720 nm [Rupp et al., Nat. Comm., 10(1):4655, 2019] to longer wavelength (2 $$\mu $$ μ m) and polarizations with arbitrary ellipticity. Based on the experimental observations, we discuss the validity of a previously introduced semi-classical model, which is based on near-field driven charge generation by a Monte-Carlo approach and classical propagation. We furthermore clarify the role of the solvent in the surface composition of the nanoparticles in the interaction region. We find that upon injection of the nanoparticles into the vacuum, the alcoholic solvent evaporates on millisecond time scales, and that the generated ions originate predominantly from covalent bonds with the silica surface rather than from physisorbed solvent molecules. These findings have important implications for the development of future theoretical models of the strong-field ion emission from silica nanoparticles, and the application of near-field imaging and reaction dynamics of functional groups on isolated nanoparticles. Graphical abstract

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2026

AmeriFlux FLUXNET-1F US-HRC Humnoke Farm Rice Field – Field C

This is the AmeriFlux Management Project (AMP) created FLUXNET-1F version of the carbon flux data for the site US-HRC Humnoke Farm Rice Field – Field C. This is the FLUXNET version of the carbon flux data for the site US-HRC Humnoke Farm Rice Field – Field C produced by applying the standard ONEFlux (1F) software. Site Description - Conventional flood irrigation method on a rice field under continuous rice operation for 50 years

Reba, Michele L. [USDA, ARS, DELTA WATER MANAGEMEN↗

DOE OSTI2026

AmeriFlux FLUXNET-1F US-HRA Humnoke Farm Rice Field – Field A

This is the AmeriFlux Management Project (AMP) created FLUXNET-1F version of the carbon flux data for the site US-HRA Humnoke Farm Rice Field – Field A. This is the FLUXNET version of the carbon flux data for the site US-HRA Humnoke Farm Rice Field – Field A produced by applying the standard ONEFlux (1F) software. Site Description - Zero grade rice field (25 ha) under continuous rice operation for > 10 years

Runkle, Benjamin R. K. [University of Arkansas]↗

DOE OSTI2020

Field-Emission Electron Source Embedded in a Field-Enhanced Conduction-Cooled Superconducting RF Cavity

We present simulations and experimental progress toward the development of a high-current electron source with the potential to deliver high charge electron bunches at GHz-level repetition rates. To achieve these goals electrons are generated through field-emission and the cathode is immersed in a conduction-cooled superconducting 650-MHz RF cavity. The field-emitters consist of microscopic silicon pyramids and have a typical enhancement factor of about 500. To trigger field-emission, the peak field inside the RF cavity of about 6 MV/m is further enhanced by placing the field-emitters on the top of a superconducting Nb rod inserted in the RF cavity. So far, we cannot control the duration of the electron bunches which is of the order of RF period. Also, the present cryo-cooler power of about 2 W limits the beam current to microamp level.

43 PARTICLE ACCELERATORS↗

DOE OSTI2026

Multi-Year Electric Field Study at the North Slope of Alaska (OYESNSA) Field Campaign Report

The Global Electric Circuit (GEC) of the atmosphere provides a unique perspective on Earth’s changing climate. Monitoring this global electrical signature provides details of the global nature of electrified clouds and thunderstorms. The North Slope of Alaska (NSA) is a unique location for collecting these electric field measurements. Besides being at the opposite pole from many previous electric field measurements taken at Russia’s Vostok station in Antarctica, this site provides a rare opportunity to use other instruments at the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) User Facility’s NSA observatory at Utqiaġvik (formerly known as Barrow), such as the Ka-band ARM Zenith Radar (KAZR), upward-facing micropulse lidar (MPL), vertical profile of meteorological measures, and other aerosol measurements. Since 2017, Texas A&M University at Corpus Christi (TAMUCC) has established an observational instrument package, including CS110 electric fields and anemometer, at ARM’s NSA observatory at Utqiaġvik. This enabled the unique opportunity to not only provide information about the global signature of the GEC, but also the physical inputs to the local electric field, by analyzing the physical properties of the simultaneous cloud, wind, and aerosol properties occurring with the vertical electric field.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2026

Multi-Year Electric Field Study at the North Slope of Alaska (OYESNSA) Field Campaign Report

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2016

A new test statistic for climate models that includes field and spatial dependencies using Gaussian Markov random fields

Abstract. A new test statistic for climate model evaluation has been developed that potentially mitigates some of the limitations that exist for observing and representing field and space dependencies of climate phenomena. Traditionally such dependencies have been ignored when climate models have been evaluated against observational data, which makes it difficult to assess whether any given model is simulating observed climate for the right reasons. The new statistic uses Gaussian Markov random fields for estimating field and space dependencies within a first-order grid point neighborhood structure. We illustrate the ability of Gaussian Markov random fields to represent empirical estimates of field and space covariances using "witch hat" graphs. We further use the new statistic to evaluate the tropical response of a climate model (CAM3.1) to changes in two parameters important to its representation of cloud and precipitation physics. Overall, the inclusion of dependency information did not alter significantly the recognition of those regions of parameter space that best approximated observations. However, there were some qualitative differences in the shape of the response surface that suggest how such a measure could affect estimates of model uncertainty.

54 ENVIRONMENTAL SCIENCES↗