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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 1,693 records · Page 94
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Tuning the dead-layer behavior of La{sub 0.67}Sr{sub 0.33}MnO{sub 3}/SrTiO{sub 3} via interfacial engineering

The dead-layer behavior, deterioration of the bulk properties in near-interface layers, restricts the applications of many oxide heterostructures. We present the systematic study of the dead-layer in La{sub 0.67}Sr{sub 0.33}MnO{sub 3}/SrTiO{sub 3} grown by ozone-assisted molecular beam epitaxy. Dead-layer behavior is systematically tuned by varying the interfacial doping, while unchanged with varied doping at any other atomic layers. In situ photoemission and low energy electron diffraction measurements suggest intrinsic oxygen vacancies at the surface of ultra-thin La{sub 0.67}Sr{sub 0.33}MnO{sub 3}, which are more concentrated in thinner films. Our results show correlation between interfacial doping, oxygen vacancies, and the dead-layer, which can be explained by a simplified electrostatic model.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Conditions for spin-gapless semiconducting behavior in Mn{sub 2}CoAl inverse Heusler compound

Employing ab initio electronic structure calculations, we investigate the conditions for spin-gapless semiconducting (SGS) behavior in the inverse Mn{sub 2}CoAl Heusler compound. We show that tetragonalization of the lattice, which can occur during films growth, keeps the SGS character of the perfect cubic compound. On the contrary, atomic swaps even between sites with different local symmetry destroy the SGS character giving rise to a half-metallic state. Furthermore, the occurrence of Co-surplus leads also to half-metallicity. Thus, we propose that in order to achieve SGS behavior during the growth of Mn{sub 2}CoAl (and similar SGS Heusler compounds) thin films, one should minimize the occurrence of defects, while small deformations of the lattice, due to the lattice mismatch with the substrate, do not play a crucial role.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Interfacial charge and strain effects on the ferroelectric behavior of epitaxial (001) PbTiO 3 films on (110) DyScO 3 substrates

In-situ synchrotron x-ray observations reveal that the ferroelectric behavior of epitaxial (001) PbTiO3 thin films grown on (110) DyScO3 substrates depends on both film thickness and interfacial electrical properties. A 92-nm-thick film was found to exhibit an a/c domain structure with a ferroelectric Curie temperature similar to that theoretically predicted based on the strain state. In contrast, 6-nm-thick films contained only c-oriented domains, and the ferroelectric behavior was found to depend strongly on the nature of the electrical boundary condition at the buried interface.

Highland, M. J.↗
DOE OSTI2014

Spin-dependent transport behavior in C{sub 60} and Alq{sub 3} based spin valves with a magnetite electrode (invited)

The spin-dependent transport behavior in organic semiconductors (OSs) is generally observed at low temperatures, which likely results from poor spin injection efficiency at room temperature from the ferromagnetic metal electrodes to the OS layer. Possible reasons for this are the low Curie temperature and/or the small spin polarization efficiency for the ferromagnetic electrodes used in these devices. Magnetite has potential as an advanced candidate for use as the electrode in spintronic devices, because it can achieve 100% spin polarization efficiency in theory, and has a high Curie temperature (850 K). Here, we fabricated two types of organic spin valves using magnetite as a high efficiency electrode. C{sub 60} and 8-hydroxyquinoline aluminum (Alq{sub 3}) were employed as the OS layers. Magnetoresistance ratios of around 8% and over 6% were obtained in C{sub 60} and Alq{sub 3}-based spin valves at room temperature, respectively, which are two of the highest magnetoresistance ratios in organic spin valves reported thus far. The magnetoresistance effect was systemically investigated by varying the thickness of the Alq{sub 3} layer. Moreover, the temperature dependence of the magnetoresistance ratios for C{sub 60} and Alq{sub 3}-based spin valves were evaluated to gain insight into the spin-dependent transport behavior. This study provides a useful method in designing organic spin devices operated at room temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Temperature-dependent Raman and ultraviolet photoelectron spectroscopy studies on phase transition behavior of VO{sub 2} films with M1 and M2 phases

Structural and electronic phase transitions behavior of two polycrystalline VO{sub 2} films, one with pure M1 phase and the other with pure M2 phase at room temperature, were investigated by temperature-controlled Raman spectroscopy and ultraviolet photoelectron spectroscopy (UPS). We observed characteristic transient dynamics in which the Raman modes at 195 cm{sup −1} (V-V vibration) and 616 cm{sup −1} (V-O vibration) showed remarkable hardening along the temperature in M1 phase film, indicating the rearrangements of V-V pairs and VO{sub 6} octahedra. It was also shown that the M1 Raman mode frequency approached those of invariant M2 peaks before entering rutile phase. In UPS spectra with high energy resolution of 0.03 eV for the M2 phase film, narrower V{sub 3d} band was observed together with smaller gap compared to those of M1 phase film, supporting the nature of Mott insulator of M2 phase even in the polycrystalline film. Cooperative behavior of lattice rearrangements and electronic phase transition was suggested for M1 phase film.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Structural and magnetization behavior of highly spin polarized Co{sub 2}CrAl full Heusler alloy

The half metallic ferromagnet Co{sub 2}CrAl full Huesler alloy was successfully prepared by arc melting process. The electrical and magnetic properties of Co{sub 2}CrAl alloy have been studied in the temperature range of 5 – 300 K. The ferromagnetic Curie temperature T{sub c} of the same alloy has been observed at 329.8 K. The alloy shows semiconducting like electronic transport behavior throughout the studied temperature range. The origin of the semiconducting behavior of Co{sub 2}CrAl alloy can be best explained by the localization of conduction electrons and the presence of an energy gap in the electronic spectrum near the Fermi level E{sub F}.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Negative differential resistance and resistance switching behaviors in BaTiO{sub 3} thin films

The polycrystalline BaTiO{sub 3} (BTO) thin films were grown on F-doped SnO{sub 2} substrates by pulsed laser deposition. The devices show a rectification at a small voltage, while bipolar resistive switching (RS) and negative differential resistance (NDR) appear at a large voltage. Furthermore, RS remains and NDR disappears when no positive bias is applied, while both RS and NDR behaviors improve when increasing the positive bias. The electrons trapped/detrapped by interface states at Au/BTO interface are proposed to understand the above behaviors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Modulating resonance behaviors by noise recycling in bistable systems with time delay

In this paper, the impact of noise recycling on resonance behaviors is studied theoretically and numerically in a prototypical bistable system with delayed feedback. According to the interior cooperating and interacting activity of noise recycling, a theory has been proposed by reducing the non-Markovian problem into a two-state model, wherein both the master equation and the transition rates depend on not only the current state but also the earlier two states due to the recycling lag and the feedback delay. By virtue of this theory, the formulae of the power spectrum density and the linear response function have been found analytically. And the theoretical results are well verified by numerical simulations. It has been demonstrated that both the recycling lag and the feedback delay play a crucial role in the resonance behaviors. In addition, the results also suggest an alternative scheme to modulate or control the coherence or stochastic resonance in bistable systems with time delay.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Magnetic and dielectric behavior in YMn{sub 1−x}Fe{sub x}O{sub 3} (x ≤ 0.5)

The role of doping Fe on the structural, magnetic, and dielectric properties of frustrated antiferromagnet YMn{sub 1−x}Fe{sub x}O{sub 3} (x ≤ 0.5) has been investigated. The neutron diffraction analysis shows that the structure of these polycrystalline samples changes from hexagonal phase (space group P6{sub 3}cm) to orthorhombic phase (space group Pnma) for x > 0.2. The frustration parameter decreases with Fe substitution. All the compounds are antiferromagnetic, and the magnetic structure is described as a mixture of Γ3 and Γ4 irreducible representation (IR) in the hexagonal phase, and the ratio of these two IRs is found to vary with Fe doping (x ≤ 0.2). A continuous spin reorientation as a function of temperature is observed in these samples. The magnetic ground state in the orthorhombic phase of the higher doped samples (x ≥ 0.3) is explained by taking Γ1 (G{sub x}C{sub y}A{sub z}) representation of Pnma setting. In YMnO{sub 3} suppression of dielectric constant ε′ is observed below T{sub N} indicative of magnetoelectric coupling. This anomalous behavior reduces in Fe doped samples. The dielectric constant is found to be correlated with the magnetic moment (M) obtained from neutron diffraction experiments and follows a M{sup 2} behavior close to T{sub N} in agreement with Landau theory.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Self-Q-switching behavior of erbium-doped tellurite microstructured fiber lasers

We reported self-Q-switching behavior of erbium-doped tellurite microstructured fiber (EDTMF) lasers and further demonstrated a self-Q-switched EDTMF laser with a high repetition rate of more than 1 MHz. A 14 cm EDTMF was used as the gain medium. Upon a pump power of ∼705 mW at 1480 nm, output pulses with a lasing wavelength of ∼1558 nm, a repetition rate of ∼1.14 MHz, and a pulse width of ∼282 ns were generated from the fiber by employing a linear cavity. The maximum output power was ∼316 mW and the slope efficiency was about 72.6% before the saturation of the laser power. Moreover, the influence of the fiber length on laser performances was investigated. The results showed that self-Q-switching behavior in our experiments was caused by the re-absorption originated from the ineffectively pumped part of the active fiber.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

High-pressure behavior of amorphous selenium from ultrasonic measurements and Raman spectroscopy

The high-pressure behavior of melt-quenched amorphous selenium (a-Se) has been investigated via ultrasonic measurements and Raman scattering at room temperature. The ultrasonic measurements were conducted on a-Se in a multi-anvil apparatus with two different sample assemblies at pressures of up to 4.5 and 4.8 GPa. We discovered that similar kinks occur in the slopes of the pressure dependence characteristics of the travel time and the sound velocity in both shear and longitudinal waves in the 2.0–2.5 GPa range. These kinks are independent of the sample assemblies, indicating an intrinsic transformation of the a-Se. Additionally, we deduced the pressure-volume relationship of a-Se from the sound velocity characteristics using the Birch–Murnaghan equation of state, and the results agreed well with those of previous reports. In situ high-pressure Raman scattering measurements of a-Se were conducted in a diamond anvil cell with an 830 nm excitation line up to a pressure of 4.3 GPa. We found that the characteristic band of a-Se at ∼250 cm{sup −1} experienced a smooth shift to a lower frequency with pressure, but a sharp slope change in the band intensity versus pressure occurred near 2.5 GPa. The results of X-ray diffraction and differential scanning calorimetry measurements indicate that the samples remain in their amorphous states after decompression. Thus, we proposed that the abnormal compression behavior of a-Se in the 2.0–2.5 GPa range can be attributed to pressure-induced local atomic reconfiguration, implying an amorphous-amorphous transition of the elementary selenium.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

On the mechanism of acceleration behavior of plasma bullet

Two special experiments are designed to study the mechanism of the acceleration behavior of a plasma bullet when it exits a nozzle. First, a T-shape device is used to simulate the air diffusion when a plasma plume exits the nozzle. It is found that adding just 1% of N{sub 2}, O{sub 2}, or air to the main working gas He results in the acceleration of the plasma bullet. Second, materials of different permittivity are added to the left part of the outside of the tube. The experimental results show that the plasma bullet accelerates at the moment when it enters into the right part of the tube where there is no extra material on the outside of the tube. These two experiments confirm that the acceleration behavior of the plasma bullet when it exits the nozzle is due to the air diffusion, hence Penning ionization, and the permittivity change when the bullet exits the nozzle, for example, from a tube with high permittivity to air with low permittivity. Besides, electric field measurements show that the electric field in the bullet head increases when the plasma bullet accelerates. This confirms the electric field driven nature of the plasma bullet propagation.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Platinum dendritic nanoparticles with magnetic behavior

Magnetic nanoparticles have attracted increasing attention for biomedical applications in magnetic resonance imaging, high frequency magnetic field hyperthermia therapies, and magnetic-field-gradient-targeted drug delivery. In this study, three-dimensional (3D) platinum nanostructures with large surface area that features magnetic behavior have been demonstrated. The well-developed 3D nanodendrites consist of plentiful interconnected nano-arms ∼4 nm in size. The magnetic behavior of the 3D dendritic Pt nanoparticles is contributed by the localization of surface electrons due to strongly bonded oxygen/Pluronic F127 and the local magnetic moment induced by oxygen vacancies on the neighboring Pt and O atoms. The magnetization of the nanoparticles exhibits a mixed paramagnetic and ferromagnetic state, originating from the core and surface, respectively. The 3D nanodendrite structure is suitable for surface modification and high amounts of drug loading if the transition temperature was enhanced to room temperature properly.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Spin-glass behavior and anomalous magnetoresistance in ferromagnetic Ge{sub 1-x}Fe{sub x}Te epilayer

We report that the Ge{sub 1-x}Fe{sub x}Te thin film exhibits spin-glass behavior when the Fe concentration increases to 0.08. A large bifurcation between the zero-field cooling and field cooling temperature-dependent magnetization was observed. The hysteresis loops after zero-field cooling and field cooling show an exchange bias effect. A time-dependent thermoremanent magnetization follows power-law decay, which confirms the existence of spin glass. The anomalous magnetotranport properties present a further evidence for spin-glass behavior and give a freezing temperature T{sub g} ∼ 5 K in the Ge{sub 0.92}Fe{sub 0.08}Te thin film.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Anomalous magnetic behavior in nanocomposite materials of reduced graphene oxide-Ni/NiFe{sub 2}O{sub 4}

Magnetic Reduced Graphene Oxide-Nickel/NiFe{sub 2}O{sub 4} (RGO-Ni/NF) nanocomposite has been synthesized by one pot solvothermal method. Respective phase formations and their purities in the composite are confirmed by High Resolution Transmission Electron Microscope and X Ray Diffraction, respectively. For the RGO-Ni/NF composite material finite-size effects lead to the anomalous magnetic behavior, which is corroborated in temperature and field dependent magnetization curves. Here, we are reporting the behavior of higher magnetization values for Zero Field Cooled condition to that of Field Cooled for the RGO-Ni/NF nanocomposite. Also, the observed negative and positive moments in Hysteresis loops at relatively smaller applied fields (100 Oe and 200 Oe) are explained on the basis of surface spin disorder.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Behavior of supercooled aqueous solutions stemming from hidden liquid–liquid transition in water

A popular hypothesis that explains the anomalies of supercooled water is the existence of a metastable liquid–liquid transition hidden below the line of homogeneous nucleation. If this transition exists and if it is terminated by a critical point, the addition of a solute should generate a line of liquid–liquid critical points emanating from the critical point of pure metastable water. We have analyzed thermodynamic consequences of this scenario. In particular, we consider the behavior of two systems, H{sub 2}O-NaCl and H{sub 2}O-glycerol. We find the behavior of the heat capacity in supercooled aqueous solutions of NaCl, as reported by Archer and Carter [J. Phys. Chem. B 104, 8563 (2000)], to be consistent with the presence of the metastable liquid–liquid transition. We elucidate the non-conserved nature of the order parameter (extent of “reaction” between two alternative structures of water) and the consequences of its coupling with conserved properties (density and concentration). We also show how the shape of the critical line in a solution controls the difference in concentration of the coexisting liquid phases.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Beam-wave interaction behavior of a 35 GHz metal PBG cavity gyrotron

The RF behavior of a 35 GHz photonic band gap (PBG) cavity gyrotron operating in TE{sub 041}-like mode has been presented to demonstrate its single mode operation capability. In this PBG cavity gyrotron, the conventional tapered cylindrical cavity is replaced by a metal PBG cavity as its RF interaction structure. The beam-wave interaction behavior has been explored using time dependent multimode nonlinear analysis as well as through 3D PIC simulation. Metal PBG cavity is treated here similar to that of a conventional cylindrical cavity for the desired mode confinement. The applied DC magnetic field profile has been considered uniform along the PBG cavity length both in analysis as well as in simulation. Electrons energy and phase along the interaction length of the PBG cavity facilitates bunching mechanism as well as energy transfer phenomena from the electron beam to the RF field. The RF output power for the TE{sub 041}-like design mode as well as nearby competing modes have been estimated and found above to 100 kW in TE{sub 041}-like mode with ∼15% efficiency. Results obtained from the analysis and the PIC simulation are found in agreement within 8% variation, and also it supports the single mode operation, as the PBG cavity does not switch into other parasitic modes in considerably large range of varying DC magnetic field, contrary to the conventional cylindrical cavity interaction structure.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Nanocrystalline Si pathway induced unipolar resistive switching behavior from annealed Si-rich SiN{sub x}/SiN{sub y} multilayers

Adding a resistive switching functionality to a silicon microelectronic chip is a new challenge in materials research. Here, we demonstrate that unipolar and electrode-independent resistive switching effects can be realized in the annealed Si-rich SiN{sub x}/SiN{sub y} multilayers with high on/off ratio of 10{sup 9}. High resolution transmission electron microscopy reveals that for the high resistance state broken pathways composed of discrete nanocrystalline silicon (nc-Si) exist in the Si nitride multilayers. While for the low resistance state the discrete nc-Si regions is connected, forming continuous nc-Si pathways. Based on the analysis of the temperature dependent I-V characteristics and HRTEM photos, we found that the break-and-bridge evolution of nc-Si pathway is the origin of resistive switching memory behavior. Our findings provide insights into the mechanism of the resistive switching behavior in nc-Si films, opening a way for it to be utilized as a material in Si-based memories.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗