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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 1,099 records · Page 61
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Study of Dissipative Losses in AC-Biased Mo/Au Bilayer Transition-Edge Sensors

We are developing kilo-pixel arrays of transition-edge sensors (TESs) for the X-ray Integral Field Unit on ESA’s Athena observatory. Previous measurements of AC-biased Mo/Au TESs have highlighted a frequency-dependent loss mechanism that results in broader transitions and worse spectral performance compared to the same devices measured under DC bias. In order to better understand the nature of this loss, we are now studying TES pixels in different geometric configurations. We present measurements on devices of different sizes and with different metal features used for noise mitigation and X-ray absorption. Our results show how the loss mechanism is strongly dependent upon the amount of metal in close proximity to the sensor and can be attributed to induced eddy current coupling to these features. We present a finite element model that successfully reproduces the magnitude and geometry dependence of the losses. Using this model, we present mitigation strategies that should reduce the losses to an acceptable level.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

d{sup 0} Half-Metallic Ferromagnetism in CaN and CaAs Pnictides: an Ab Initio Study

Conventional magnetism occurs in systems which contain transition metals or rare earth ions with partially filled d or f shells. It is theoretically predicted that compounds of groups IA and IIA with IV and V, in some structural phases, are ferromagnetic half-metals which made them new candidates for spintronics applications. Employing density functional theory (DFT), we investigate magnetism in binary compounds CaN and CaAs. Regarding the structure of analogous magnetic materials and experimental results of CaAs synthesis, we have considered two cubic structures: rocksalt (RS) and zincblende (ZB), and four hexagonal structures: NiAs, wurtzite (WZ), anti-NiAs, and NaO. The calculated results show that CaN in cubic, NiAs, and wurtzite structures, and CaAs only in zincblende phase have ferromagnetic ground states with a magnetic moment of 1 μ{sub B}. Electronic structure analysis of these materials indicates that magnetism originates from anion p states. Existence of flat p bands and consequently high density of states at the Fermi level of magnetic structures gives rise to Stoner spin splitting and spontaneous ferromagnetism.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Magnetism in Boron Nitride Monolayer Induced by Cobalt or Nickel Doping

We have investigated the electronic structure and magnetic properties of cobalt (Co)- or nickel (Ni)-doped hexagonal boron nitride (h-BN) monolayer using density functional theory calculations. The h-BN monolayer without any doping is a nonmagnetic insulator. Our studies show that isolated Co or Ni atom can both induce the local magnetic state in h-BN monolayer. And the impurity energy levels will be formed in the band gap. Results of our first-principle calculations reveal that isolated Co atom can result in a magnetic moment of 3.57 μ{sub B}, while the Ni atom is 0.87 μ{sub B}. And the magnetic moments mainly come from d orbitals of the doped atoms. The studies of magnetic coupling show that the two Co atoms at different distances in h-BN monolayer do not always couple ferromagnetically. When the distances are 2.504 and 6.625 Å, the spins induced by the two Co atoms will form antiferromagnetic ground states. While the two Ni atoms at different distances in the monolayer always couple ferromagnetically. So our results imply that the Ni-doped h-BN is more suitable for spintronic material than Co doping. These results are useful for spintronic application.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2018

Elasticity, Anisotropy and Magnetism of Anti-perovskite M{sub 3}AlC (M=Ti, Fe, Sm) from First-Principle Calculations

Elasticity, anisotropy, and magnetism of anti-perovskite M{sub 3}AlC (M=Ti, Fe, Sm) have been calculated and investigated via non-relativistic density functional theory. And the elasticity constant, elastic modulus, Poisson ratio, and Cauchy pressure of three structures have been obtained. Based on the Poisson ratio, Pugh modulus ratio, and Cauchy pressure, bonding attributes of the three materials have been analyzed. The result showed that Ti{sub 3}AlC is a brittle material with high degrees of strength, rigidity, and hardness, and Sm{sub 3}AlC and Fe{sub 3}AlC possess excellent toughness. Based on the universal elastic anisotropy index A{sup U}, anisotropy fraction ratios A{sub B}, A{sub G}, and A{sub E},and Zener factor A, it showed that all three M{sub 3}AlC structures demonstrated elasticity and anisotropy. The elasticity and anisotropy of the three M{sub 3}AlC structures has been illustrated in three-dimensional images of the elastic modulus. Compared with Ti{sub 3}AlC and Fe{sub 3}AlC, Sm{sub 3}AlC has a significantly higher level of anisotropy. Moreover, in this paper, the magnetism of transition metal Fe and rare earth element Sm in the M{sub 3}AlC structure has been compared and analyzed from the perspective of the electronic structure and population analysis.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Effect of Core Electrons in Defining the Total Energy, Correlation Energy, and Binding Energy of Graphene, Graphite, and Diamond: a First-Principles Study

Effects of core electrons on total energy, correlation energy, and binding energy of graphene, graphite, and diamond have been investigated along with density functional theory (DFT) calculations at the PBE level of theory using all electron and frozen-core calculations. For these calculations, correlation-consistent basis sets cc-pVXZ and cc-pCVXZ have been used where X is the cardinal number that represents the maximum angular momentum number in the basis set. By taking the difference between all electron and frozen-core calculations, core-electron binding energy contribution for each basis set has been obtained. It has been shown that to reduce the effects of core electrons, large basis sets should be used.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Calculations of the Structural, Elastic, Magnetic, and Electronic Properties of the New Compound BaZr{sub 0.5}Mn{sub 0.5}O{sub 3} with Tetragonal Structure

Based on the density functional theory, we have calculated the structural properties of the BaZrO{sub 3} (BZO) cubic structure with PBE-GGA, PBEsol-GGA, and LDA approximations. The equilibrium lattice constant within GGA-PBEsol (a = 4.1847 Å) is in very good agreement with the experimental data (Holland and Redfern, Miner. Mag. 61: 65, 1997). The calculations of elastic properties of pure and doped BaZrO{sub 3} are calculated with PBEsol-GGA; both BZO and BZM are compressible and show a brittle nature. According to calculated electronic and magnetic properties, it is found that Mn-doped BaZrO{sub 3} system is a half metallic and exhibits a ferromagnetic character. The total magnetic moment of the cell is equal to3.57 µ{sub B}; this value comes from manganese atom (Mn) with a value of 3.176 µ{sub B}. The magnetic moments of barium, oxygen, and zirconium atoms are approximately equal to zero.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Insight into Phase Transition, Electronic, Magnetic, Mechanical, and Thermodynamic Properties of TbTe: a DFT Investigation

Theoretical investigation on TbTe for its structural, electronic, magnetic, and thermodynamic stuffs has been carried within density functional theory (DFT) as implemented in WIEN2K code. TbTe was found stable in ferromagnetic phase. The calculated ground-state parameters were found in a good agreement with the experimental data. The compound was found to have a structural stability in cubic B1 (NaCl-type structure) phase, but under the application of high pressure (at 27 GPa), it undergone to B2 (CsCl-type structure) phase of pressure. The second-order elastic constants and mechanical properties like Young’s modulus, Shear modulus, Poisson ratio, Cauchy pressure (C{sub 12}–C{sub 44}), and Pugh’s ratio (B/G) were calculated. The present calculations confirmed the ductile nature of TbTe. Further, the thermodynamic investigations have been carried using quasi-harmonic Debye approximation. We have calculated the pressure and temperature dependence of Debye temperature (𝜃{sub D}), bulk modulus (B), thermal expansion (α), heat capacities (C{sub V}), and entropy (S) in the temperature range of 0 to 1000 K and pressure range of 0 to 25 GPa.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Electronic Structure and Linhard Function of Iron-Based Superconductor La{sub 0.5−x}Na{sub 0.5 + x}Fe{sub 2}As{sub 2}

We perform theoretical studies of the iron-based superconductor La{sub 0.5−x}Na{sub 0.5 + x}Fe{sub 2}As{sub 2}. Using density functional calculation with virtual crystal approximation, band structures, density of states, and Fermi surfaces are obtained for different La/Na composition by varying x value. Furthermore, the nesting property is also investigated using the Linhard function. Our main findings are as follows: For undoped (x = 0.0) case, the magnetic and electronic structure share great similarities with extensively studied BaFe{sub 2}As{sub 2}. Upon hole doping (x = 0.3), the Q = (π,π) nesting property are preserved. However, upon electron doping (x = -0.3), the Fermi surfaces undergo a dramastic change into 3D and complex structure. We find this change is attributed to the comprehensive influence of band filling and chemical pressure of La{sup 3+}. We propose the asymmetric change of Fermi surfaces upon doping may in some sense explain the electron-hole asymmetry in phase diagram.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

The Theoretical Investigation of Electronic, Magnetic, and Thermoelectric Behavior of LiZ{sub 2}O{sub 4} (Z = Mn, Fe, Co, and Ni) by Modified Becke and Johnson Approach

Magnetic spinel oxides LiZ{sub 2}O{sub 4} (Z = Mn, Fe, Co, and Ni) have recently appealed the scientific community due to their interesting magnetic and thermoelectric applications. In the current article, the electronic, magnetic, and thermoelectric properties of LZO have been elaborated using density functional theory-based Wien2k code. The band structures and total density of states ensure the half metallic ferromagnetic (HMF) nature of the studied compounds. Furthermore, the magnetism is discussed in detail using crystal field, John-Teller, and exchange energies involved in the system and spin density. Finally, electrical conductivity, thermal conductivity, power factor, Seebeck coefficient, and thermal efficiency computed by using BoltztraP code suggest these compounds for thermoelectric device fabrications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Investigation of Electronic, Magnetic, Thermodynamic, and Thermoelectric Properties of Half-Metallic XLiSn (X = Ce, Nd) Alloys

We have investigated the electronic, magnetic, thermodynamic, and thermoelectric properties of half-Heusler CeLiSn and NdLiSn using density functional theory. The materials crystallize in F 4-bar 3mstructure with ferromagnetic state at optimized lattice parameter of 6.73 Å for CeLiSn and 6.74 Å for NdLiSn. The modified Becke-Johnson potential (mBJ) and generalized gradient approximation (GGA) have been used for computation of band structure, where mBJ shows a perfect half-metallic nature of the bands in both the alloys. The calculated magnetic moment of 1.0 μ{sub B} for CeLiSn and 3.0 μ{sub B} for NdLiSn follow Slater-Pauling rule, with ferromagnetic interactions. The temperature variation of the Seebeck coefficient shows the p-type behavior with room temperature values of 27 μV/K (43 μV/K) for CeLiSn (NdLiSn). Pressure-dependent variation of specific heat, Debye temperature, and Gruneisen parameter were calculated to infer the nature of lattice vibration and stability at high pressures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Investigation of dynamics of nematicons in liquid crystals by extended sinh-Gordon equation expansion method

This paper secures new nematicons in liquid crystals from its governing equation by extended sinh-Gordon equation expansion method with the aid of symbolic computational package Maple. The two laws of nonlinearity, namely Kerr and parabolic laws are studied. As outcomes, some new explicit complex hyperbolic and complex trigonometric function solutions are derived regarding dark, bright, combined dark–bright, singular, combined singular optical, periodic wave, dipole soliton solutions and others. To show the real physical meaning of the derived results, three dimensional graphs with contours and two dimensional graphs are prepared under the choice of proper values of the arbitrary parameters. The derived solutions have been checked back into their corresponding equations with the symbolic computational package Maple. All the combined solutions are found to be new, which may play an important role in the context of liquid crystals. The applied method is found to provide a powerful tool for extracting exact solitary wave solutions and overcome the difficulties of the solitary wave ansatz method.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Estimation of Crystal Lattice Deformations with Depth of Proton-Exchanged Optical Waveguides on Lithium Niobate Single Crystal Substrates

A method for determining the crystal phase and estimating the crystal lattice deformations in H{sub x}Li{sub 1-x}NbO{sub 3} proton-exchanged optical waveguides from an analysis of the Raman light scattering spectra measured with high spatial resolution (the so-called method of micro-RS spectroscopy) is suggested.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Characterization of ZnO Nanoparticles using Superconducting Tunnel Junction Cryodetection Mass Spectrometry

Zinc oxide (ZnO) nanoparticles coated with either n-octylamine (OA) or α-amino poly(styrene-co-acrylonitrile) (PSAN) ligands (L) have been analyzed using laser desorption/ionization and matrix assisted laser desorption/ionization (MALDI) time-of-flight (TOF) superconducting tunnel junction (STJ) cryodetection mass spectrometry. STJ cryodetection has the advantage of high m/z detection and allows for the determination of average molecular weights and dispersities for 500–600 kDa ZnO-L nanoparticles. The ability to detect the relative energies deposited into the STJs has allowed for investigation of ZnO-L metastable fragmentation. ZnO-L precursor ions gain enough internal energy during the MALDI process to undergo metastable fragmentation in the flight tube. These fragments produced a lower energy peak, which was assigned as ligand-stripped ZnO cores whereas the individual ligands were at too low of an energy to be observed. From these STJ energy resolved peaks, the average weight percentage of inorganic material making up the nanoparticle was determined, where ZnO-OA and ZnO-PSAN nanoparticles are comprised of ~62% and ~68% wt ZnO, respectively. In one example, grafting densities were calculated based on the metastable fragmentation of ligands from the core to be 16 and 1.1 nm{sup –2} for ZnO-OA and ZnO-PSAN, respectively, and compared with values determined by thermogravimetric analysis (TGA) and transmission electron microscopy (TEM). .

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Analysis of the structural, electronic, elastic and thermodynamic properties of CuAl2X4 (X = O, S) spinel structure

Highlights: • Fundamental properties of CuAl{sub 2}X{sub 4} (X = O, S) aluminate spinels have been investigated. • The pressure dependence of the elastic constants C{sub ij} can be successfully fit by a straight line. • CuAl{sub 2}X{sub 4} (X = O, S) compounds are mechanically stable at ambient conditions. • Temperature and pressure dependences of some macroscopic parameters are achieved. - Abstract: The structural, electronic, elastic and thermodynamic properties of oxo- and thio-spinels have been predicted based on the ultrasoft pseudo-potential scheme as implemented in the CASTEP code. The exchange-correlation potential was treated within the generalized gradient approximation (GGA-PBE). The calculated structural parameters such as lattice constants and internal parameters are in good agreement with the available experimental data. The ground state electronic band structure revealed that CuAl{sub 2}X{sub 4} (XO, S) spinels are direct band gap semiconductors with a transition along Γ-Γ symmetry points. The calculated values of elastic constants satisfy the Born criteria at ambient conditions and confirm that the spinel structures of CuAl{sub 2}O{sub 4} and CuAl{sub 2}S{sub 4} are mechanically stable. The thiospinel structure may have a phase transition under pressure, due to the negative value of C{sub 44}. The influence of temperature and pressure on macroscopic characteristics of CuAl{sub 2}O{sub 4} and CuAl{sub 2}S{sub 4} compounds was estimated using the quasi-harmonic Debye model.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Energy band alignment and electronic states of amorphous carbon surfaces in vacuo and in aqueous environment

In this paper, we obtain the energy band positions of amorphous carbon (a–C) surfaces in vacuum and in aqueous environment. The calculations are performed using a combination of (i) classical molecular dynamics (MD), (ii) Kohn-Sham density functional theory with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional, and (iii) the screened-exchange hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE). PBE allows an accurate generation of a-C and the evaluation of the local electrostatic potential in the a-C/water system, HSE yields an improved description of energetic positions which is critical in this case, and classical MD enables a computationally affordable description of water. Our explicit calculation shows that, both in vacuo and in aqueous environment, the a-C electronic states available in the region comprised between the H{sub 2}/H{sub 2}O and O{sub 2}/H{sub 2}O levels of water correspond to both occupied and unoccupied states within the a-C pseudogap region. These are localized states associated to sp{sup 2} sites in a-C. The band realignment induces a shift of approximately 300 meV of the a-C energy band positions with respect to the redox levels of water.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Metal cluster structures and properties from Born-Oppenheimer molecular dynamics

Density functional theory (DFT) Born-Oppenheimer molecular dynamics (BOMD) simulations of metal clusters are presented. The calculations have been performed with the deMon2k [1] code employing all-electron basis sets and local and non-local functionals. The capability to perform reasonable long (∼ 100 ps) first-principle BOMD simulations in order to explore potential energy landscape of metallic clusters will be presented [2,3]. The evolution of the cluster structures and properties, such as polarizability and heat capacity, with temperature is discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Magnetic properties of Fe-Mn-Pt for heat assisted magnetic recording applications

We calculate the electronic structures of FePt and Fe{sub 0.5}Mn{sub 0.5}Pt using first-principles calculations based on density functional theory within the local-spin-density approximation. The Curie temperature (T{sub c}) was calculated by mean field approximation. Composition dependence of the Cure temperature (T{sub c}(x)) of Fe{sub 1−x}Mn{sub x}Pt was used to identify a composition to meet the desired T{sub c} in the range of 600–650 K. The identified composition (0.0294 ≤ x ≤ 0.0713) gives saturation magnetization (M{sub s}) in the range of 1041–919 emu/cm{sup 3} and magnetocrystalline anisotropy constant (K) in the range of 9.96–8.36 × 10{sup 6 }J/m{sup 3} at 0 K. Temperature dependent M(T) and K(T) of Fe{sub 1−x}Mn{sub x}Pt (0.0294 ≤ x ≤ 0.0713) were calculated using the Brillouin function and Callen-Callen experimental relation, respectively. Fe{sub 1−x}Mn{sub x}Pt (0.0294 ≤ x ≤ 0.0713) shows 930–800 emu/cm{sup 3} of M{sub s} and 7.18–5.61 × 10{sup 6 }J/m{sup 3} of K at 300 K, thereby satisfying desired magnetic properties for heat-assisted magnetic recording media to achieve 4 Tb/in.{sup 2} areal density.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Influence of the van der Waals interaction in the dissociation dynamics of N{sub 2} on W(110) from first principles

Using ab initio molecular dynamics (AIMD) calculations, we investigate the role of the van der Waals (vdW) interaction in the dissociative adsorption of N{sub 2} on W(110). Hitherto, existing classical dynamics calculations performed on six-dimensional potential energy surfaces based on density functional theory (DFT), and the semi-local PW91 and RPBE [Hammer et al. Phys. Rev. B 59, 7413 (1999)] exchange-correlation functionals were unable to fully describe the dependence of the initial sticking coefficient on the molecular beam incidence conditions as found in experiments. N{sub 2} dissociation on W(110) was shown to be very sensitive not only to short molecule-surface distances but also to large distances where the vdW interaction, not included in semilocal-DFT, should dominate. In this work, we perform a systematic study on the dissociative adsorption using a selection of existing non-local functionals that include the vdW interaction (vdW-functionals). Clearly, the inclusion of the non-local correlation term contributes in all cases to correct the unrealistic energy barriers that were identified in the RPBE at large molecule-surface distances. Among the tested vdW-functionals, the original vdW-DF by Dion et al. [Phys. Rev. Lett. 92, 246401 (2004)] and the ulterior vdW-DF2 give also an adequate description of the N{sub 2} adsorption energy and energy barrier at the transition state, i.e., of the properties that are commonly used to verify the quality of any exchange-correlation functional. However, the results of our AIMD calculations, which are performed at different incidence conditions and hence extensively probe the multi-configurational potential energy surface of the system, do not seem as satisfactory as the preliminary static analysis suggested. When comparing the obtained dissociation probabilities with existing experimental data, none of the used vdW-functionals seems to provide altogether an adequate description of the N{sub 2}/W(110) interaction at short and large distances.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗