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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 1,441 records · Page 80
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Zeeman spectroscopy as a method for determining the magnetic field distribution in self-magnetic-pinch diodes (invited)

In the self-magnetic-pinch diode, the electron beam, produced through explosive field emission, focuses on the anode surface due to its own magnetic field. This process results in dense plasma formation on the anode surface, consisting primarily of hydrocarbons. Direct measurements of the beam’s current profile are necessary in order to understand the pinch dynamics and to determine x-ray source sizes, which should be minimized in radiographic applications. In this paper, the analysis of the C IV doublet (580.1 and 581.2 nm) line shapes will be discussed. The technique yields estimates of the electron density and electron temperature profiles, and the method can be highly beneficial in providing the current density distribution in such diodes.

Patel, S. G.↗
DOE OSTI2016

Magnetic phase diagram of underdoped YBa 2 Cu 3 O y inferred from torque magnetization and thermal conductivity

We obtain the magnetic phase diagram in the underdoped cuprate YBa2Cu3Oy using torque magnetometry at temperatures 0.3–70 K and magnetic fields up to 45 T. At low fields, vortices (quantized flux tubes) form a vortex solid that is strongly pinned to the lattice. At large fields, melting of the solid to a vortex liquid produces nonzero dissipation. However, the vortex liquid persists to fields above 41 T. We have also mapped out the “transition” fields at which the charge-density–wave state (observed in X-ray diffraction experiments) becomes stable. Our results show that, in intense fields, superconductivity adjusts to coexist with the charge-density wave, but the Cooper pairs, which define the superconducting fluid, survive to fields well above 41 T.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Influence of finite volume and magnetic field effects on the QCD phase diagram

The 2 + 1 SU(3) Polyakov linear sigma model (PLSM) is used to investigate the respective influence of a finite volume and a magnetic field on the quark-hadron phase boundary in the plane of baryon chemical potential (μ B ) vs. temperature (T) of the QCD phase diagram. The calculated results indicate sizable shifts of the quark-hadron phase boundary to lower values of (μ B and T) for increasing magnetic field strength, and an opposite shift to higher values of (μ B and T) for decreasing system volume. Such shifts could have important implications for extraction of the thermodynamic properties of the QCD phase diagram from heavy ion data.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Coil current and vacuum magnetic flux calculation for axisymmetric equilibria

In fixed-boundary axisymmetric equilibrium calculations the plasma shape is assigned from input. In several circumstances, the plasma shape may not be known a priori, or one may desire to also compute the magnetic field in the volume surrounding the plasma through the calculation of a free-boundary equilibrium. This requires either the coil currents or the magnetic poloidal flux on a curve in the vacuum region to be assigned as input for the free-boundary equilibrium calculation. The FREE-FIX code presented in this article is a general tool for calculating coil currents being given a fixed-boundary calculation. Finally, a new formulation is presented, which considerably reduces the computational cost of the calculation. FREE-FIX performs well for different geometries and experiments.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Crystallographic, electronic, thermal, and magnetic properties of single-crystal SrCo 2 As 2

In tetragonal SrCo 2 As 2 single crystals, inelastic neutron scattering measurements demonstrated that strong stripe-type antiferromagnetic (AFM) correlations occur at a temperature T = 5 K [W. Jayasekara et al., arXiv:1306.5174] that are the same as in the isostructural AFe 2 As 2 (A = Ca, Sr, Ba) parent compounds of high-T c superconductors. This surprising discovery suggests that SrCo 2 As 2 may also be a good parent compound for high-T c superconductivity. Here, structural and thermal expansion, electrical resistivity ρ, angle-resolved photoemission spectroscopy (ARPES), heat capacity C p , magnetic susceptibility χ, 75 As NMR and neutron diffraction measurements of SrCo 2 As 2 crystals are reported together with LDA band structure calculations that shed further light on this fascinating material. The c-axis thermal expansion coefficient α c is negative from 7 to 300 K, whereas α a is positive over this T range. The ρ(T) shows metallic character. The ARPES measurements and band theory confirm the metallic character and in addition show the presence of a flat band near the Fermi energy E F . The band calculations exhibit an extremely sharp peak in the density of states D(E F ) arising from a flat d x 2 -y 2 band. A comparison of the Sommerfeld coefficient of the electronic specific heat with χ(T → 0) suggests the presence of strong ferromagnetic itinerant spin correlations which on the basis of the Stoner criterion predicts that SrCo 2 As 2 should be an itinerant ferromagnet, in conflict with the magnetization data. The χ(T) does have a large magnitude, but also exhibits a broad maximum at 115 K suggestive of dynamic short-range AFM spin correlations, in agreement with the neutron scattering data. The measurements show no evidence for any type of phase transition between 1.3 and 300 K and we propose that metallic SrCo 2 As 2 has a gapless quantum spin-liquid ground state.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Gigantic magnetic-field-induced polarization and magnetoelectric coupling in a ferrimagnetic oxide CaBaCo 4 O 7

Below Tc, the single-crystal study of CaBaCo40,, a noncollinear ferrimagnet (Tc = 64 K) with a polar orthorhombic space group (Pbn21) between 4 and 293 K, shows a large change inthe electric polarization along its c axis reaching !!iP = 17 mC/m2 at 10 K. At 62.5 K, a magnetic-field-driven giant variation inpolarization P(9 T) - P(O T) = 8 mC/m2 is observed. Moreover, the present magnetoelectric measurements are fully consistent withthe m'm2' magnetic point group. This ferrimagnetic oxide, which belongs to the "114" structural family, opens an avenue for the search for new magnetoelectrics.

Caignaert, V.↗
DOE OSTI2014

Magnetic structure and spin excitations in BaMn 2 Bi 2

We present a single crystal neutron scattering study of BaMn 2 Bi 2 , a recently synthesized material with the same ThCr 2 Si 2 type structure found in several Fe-based unconventional superconducting materials. We show long range magnetic order, in the form of a G-type antiferromagnetic structure, to exist up to 390 K with an indication of a structural transition at 100 K. Utilizing inelastic neutron scattering we observe a spin-gap of 16 meV, with spin-waves extending up to 55 meV. We find these magnetic excitations are well fit to a J 1 -J 2 -J c Heisenberg model and present values for the exchange interactions. The spin wave spectrum appears to be unchanged by the 100 K structural phase transition.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Unexpected magnetism, Griffiths phase, and exchange bias in the mixed lanthanide Pr 0.6 Er 0.4 Al 2

We report an unusual coexistence of ferromagnetism and ferrimagnetism, and metamagnetism in Pr 0.6 Er 0.4 Al 2 . In addition, this compound retains a clear Griffiths phase behavior even at 1 kOe magnetic field and shows a large exchange bias after field cooling from the paramagnetic state. The crystal-field excitations and opposite exchange interactions between nearest-neighbor and next-nearest-neighbor rare earth sites explain these behaviors.

Pathak, Arjun K.↗
DOE OSTI2016

Magnetic scattering and spin-orbit coupling induced magnetoresistance in nonmagnetic heavy metal and magnetic insulator bilayer systems

Here, we report on the experimental study of the angular dependent magnetoresistance (MR) of heavy metal/ferromagnetic insulator bilayer structures. Through altering the relative composition in heavy metal Pt δ Ta 1–δ alloy, we continuously tune its spin Hall angle from positive, crossing zero, and to negative and study its impact on the MR. Most notably, both spin Hall effect and MR disappear simultaneously in Pt 0.32 Ta 0.68 (3 nm)/YIG when the effective spin Hall angle vanishes, evidencing the essential role of spin-orbit coupling in heavy metal for the MR. By introducing Fe impurities, we further identify that magnetic scattering is also essential to induce the MR in Pt/Fe-doped SiO 2 at large magnetic field, where the MR ratio increases monotonically with doping level.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Unified band-theoretic description of structural, electronic, and magnetic properties of vanadium dioxide phases

The debate concerning whether the insulating phases of vanadium dioxide (VO 2 ) can be described by band theory or whether it requires a theory of strong electron correlations remains unresolved even after decades of research. Energy-band calculations using hybrid exchange functionals or including self-energy corrections account for the insulating or metallic nature of different phases but have not yet successfully accounted for the observed magnetic orderings. Strongly correlated theories have had limited quantitative success. Here we illustrate that by using hard pseudopotentials and an optimized hybrid exchange functional, the energy gaps and magnetic orderings of both monoclinic VO 2 phases and the metallic nature of the high-temperature rutile phase are consistent with available experimental data, obviating an explicit role for strong correlations. We also identify a potential candidate for the newly found metallic monoclinic phase.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Electronic and magnetic properties of epitaxial SrRh O 3 films

The strong interplay of fundamental order parameters in complex oxides is known to give rise to exotic physical phenomena. The 4$d$ transition-metal oxide SrRh O 3 has generated much interest, but advances have been hindered by difficulties in preparing single-crystalline phases. Here we epitaxially stabilize high-quality single-crystalline SrRh O 3 films and investigate their structural, electronic, and magnetic properties. Lastly, we determine that their properties significantly differ from the paramagnetic metallic ground state that governs bulk samples and are strongly related to rotations of Rh O 6 octahedra.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗