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Results for “Methods in superconductivity”

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 991 records · Page 55
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Difficulty of carrier generation in orthorhombic PbO

Polycrystalline β-PbO films were grown by pulsed laser deposition in atmospheres ranging from oxygen-poor (the oxygen pressure of 0.01 Pa) to oxygen-rich (13 Pa) conditions, and the oxygen chemical potential was further enhanced by ozone annealing to examine hole doping. It was found that each of the as-grown β-PbO films showed poor electrical conductivity, σ < 1.4 × 10{sup −7} S cm{sup −1}, regardless of the oxygen pressure. The density functional calculations revealed that native defects including Pb and O vacancies have deep transition levels and extremely high formation enthalpies, which indicates difficulty of carrier generation in β-PbO and explains the experimentally observed poor electrical conductivity. The analysis of the electronic structures showed that the interaction between Pb 6s and O 2p orbitals is weak due to the deep energy level of Pb 6s and does not raise the valence band maximum (VBM) level unlike that observed in SnO, which is also supported by ultraviolet photoemission spectroscopy measurements. The deep acceptor transition levels of the native defects are attributed to the deep VBM of β-PbO. On the other hand, annealing β-PbO films in reactive oxygen-containing atmospheres (i.e., O{sub 3}) led to a significantly enhanced electrical conductivity (i.e., σ > 7.1 × 10{sup 2} S cm{sup −1}) but it is the result of the formation of an n-type PbO{sub 2} phase because oxygen chemical potential exceeded the phase boundary limit. The striking difference in carrier generation between PbO and SnO is discussed based on the electronic structures calculated by density functional theory.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Ab initio study of phase stability of NaZr{sub 2}(PO{sub 4}){sub 3} under pressure

The elastic constants of NaZr{sub 2}(PO{sub 4}){sub 3} were computed as a function of pressure through Density Functional Theory calculations. The behavior of elastic constants show that the rhombohedral (R-3c) NaZr{sub 2}(PO{sub 4}){sub 3} becomes unstable above 8 GPa and is driven by softening of C{sub 44} through one of the Born stability criteria. High pressure equation of state and enthalpy show further that the ambient rhombohedral (R-3c)) NaZr{sub 2}(PO{sub 4}){sub 3} transforms first to another rhombohedral (R3) phase and subsequently to LiZr{sub 2}(PO{sub 4}){sub 3}-type orthorhombic phase at pressures above 6 and 8 GPa respectively which are in agreement with recent X-ray diffraction study.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

A first principles study of structural, electronic mechanical and magnetic properties of rare earth nitride:TmN

The structural, electronic and mechanical properties of rare earth nitride TmN is investigated by the first principles calculations based on density functional theory using the Vienna ab-initio simulation package. At ambient pressure TmN is stable in the ferromagnetic state with NaCl structure. The calculated lattice parameters are in good agreement with the available results. The electronic structure reveals that TmN is metallic at normal pressure. Ferromagnetic to non magnetic phase transition is predicted in TmN at high pressure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Ferro and antiferro orbital ordering in Fe{sub 0.5}Mn{sub 0.5}V{sub 2}O{sub 4}

Using density functional theory calculations, we have investigated the orbital ordering in Fe{sub 0.5}Mn{sub 0.5}V{sub 2}O{sub 4} where Fe and V sites are orbitally active. Our first principles study within GGA+U and GGA+U+SO shows ferro-orbital ordering of d{sub x2−y2} orbital at all Fe sites, whereas A-type antiferro-orbital ordering at V sites where one 3d electron occupies d{sub xy} orbital at every V site and another electron occupies either 1/√2 (d{sub xz} + d{sub yz}) or 1/√2 (d{sub xz} - d{sub yz}) orbital alternatively along c axis. Insulating nature and the orbital ordering of this compound are found to be correlation driven while the effect of spin-orbit interaction on orbital ordering is not significant.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Triphenylamine based organic dyes for dye sensitized solar cells: A theoretical approach

The geometry, electronic structure and absorption spectra for newly designed triphenylamine based organic dyes were investigated by density functional theory (DFT) and time dependent density functional theory (TD-DFT) with the Becke 3-Parameter-Lee-Yang-parr(B3LYP) functional, where the 6-31G(d,p) basis set was employed. All calculations were performed using the Gaussian 09 software package. The calculated HOMO and LUMO energies show that charge transfer occurs in the molecule. Ultraviolet–visible (UV–vis) spectrum was simulated by TD-DFT in gas phase. The calculation shows that all of the dyes can potentially be good sensitizers for DSSC. The LUMOs are just above the conduction band of TiO{sub 2} and their HOMOs are under the reduction potential energy of the electrolytes (I{sup −}/I{sub 3}{sup −}) which can facilitate electron transfer from the excited dye to TiO{sub 2} and charge regeneration process after photo oxidation respectively. The simulated absorption spectrum of dyes match with solar spectrum. Frontier molecular orbital results show that among all the three dyes, the “dye 3” can be used as potential sensitizer for DSSC.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

DFT and TD-DFT computation of charge transfer complex between o-phenylenediamine and 3,5-dinitrosalicylic acid

DFT and TD-DFT studies of o-phenylenediamine (PDA), 3,5-dinitrosalicylic acid (DNSA) and their charge transfer complex have been carried out at B3LYP/6-311G(d,p) level of theory. Molecular geometry and various other molecular properties like natural atomic charges, ionization potential, electron affinity, band gap, natural bond orbital (NBO) and frontier molecular analysis have been presented at same level of theory. Frontier molecular orbital and natural bond orbital analysis show the charge delocalization from PDA to DNSA.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

The electronic and magnetic properties of quaternary Heusler alloy CoFeMnGe

We present study of quaternary Heusler alloy CoFeMnGe using density functional theory. The compound is half-metallic with half-metallic gap of 0.13 eV. The total magnetic moment of this compound is 3.96 μ{sub B} which is in close agreement with Slater-Pauling rule. The effect of lattice compression and expansion shows the robustness of half-metallicity. A large value of half-metallic gap and 100% spin-polarization makes this material interesting for spin dependent applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Mössbauer parameters of Fe-related defects in group-IV semiconductors: First principles calculations

We employ a combination of pseudopotential and all-electron density functional calculations, to relate the structure of defects in supercells to the isomer shifts and quadrupole splittings observed in Mössbauer spectroscopy experiments. The methodology is comprehensively reviewed and applied to the technologically relevant case of iron-related defects in silicon, and to other group-IV hosts to a lesser degree. Investigated defects include interstitial and substitutional iron, iron-boron pairs, iron-vacancy, and iron-divacancy. We find that, in general, agreement between the calculations and Mössbauer data is within a 10% error bar. Nonetheless, we show that the methodology can be used to make accurate assignments, including to separate peaks of similar defects in slightly different environments.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Band gap engineering of MoS{sub 2} upon compression

Molybdenum disulfide (MoS{sub 2}) is a promising candidate for 2D nanoelectronic devices, which shows a direct band-gap for monolayer structure. In this work we study the electronic structure of MoS{sub 2} upon both compressive and tensile strains with first-principles density-functional calculations for different number of layers. The results show that the band-gap can be engineered for experimentally attainable strains (i.e., ±0.15). However, compressive strain can result in bucking that can prevent the use of large compressive strain. We then studied the stability of the compression, calculating the critical strain that results in the on-set of buckling for free-standing nanoribbons of different lengths. The results demonstrate that short structures, or few-layer MoS{sub 2}, show semi-conductor to metal transition upon compressive strain without bucking.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Direct optical transitions at K- and H-point of Brillouin zone in bulk MoS{sub 2}, MoSe{sub 2}, WS{sub 2}, and WSe{sub 2}

Modulated reflectance (contactless electroreflectance (CER), photoreflectance (PR), and piezoreflectance (PzR)) has been applied to study direct optical transitions in bulk MoS{sub 2}, MoSe{sub 2}, WS{sub 2}, and WSe{sub 2}. In order to interpret optical transitions observed in CER, PR, and PzR spectra, the electronic band structure for the four crystals has been calculated from the first principles within the density functional theory for various points of Brillouin zone including K and H points. It is clearly shown that the electronic band structure at H point of Brillouin zone is very symmetric and similar to the electronic band structure at K point, and therefore, direct optical transitions at H point should be expected in modulated reflectance spectra besides the direct optical transitions at the K point of Brillouin zone. This prediction is confirmed by experimental studies of the electronic band structure of MoS{sub 2}, MoSe{sub 2}, WS{sub 2}, and WSe{sub 2} crystals by CER, PR, and PzR spectroscopy, i.e., techniques which are very sensitive to critical points of Brillouin zone. For the four crystals besides the A transition at K point, an A{sub H} transition at H point has been observed in CER, PR, and PzR spectra a few tens of meV above the A transition. The spectral difference between A and A{sub H} transition has been found to be in a very good agreement with theoretical predictions. The second transition at the H point of Brillouin zone (B{sub H} transition) overlaps spectrally with the B transition at K point because of small energy differences in the valence (conduction) band positions at H and K points. Therefore, an extra resonance which could be related to the B{sub H} transition is not resolved in modulated reflectance spectra at room temperature for the four crystals.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Degradation and annealing effects caused by oxygen in AlGaN/GaN high electron mobility transistors

Hot-carrier degradation and room-temperature annealing effects are investigated in unpassivated ammonia-rich AlGaN/GaN high electron mobility transistors. Devices exhibit a fast recovery when annealed after hot carrier stress with all pins grounded. The recovered peak transconductance can exceed the original value, an effect that is not observed in control passivated samples. Density functional theory calculations suggest that dehydrogenation of pre-existing O{sub N}-H defects in AlGaN plays a significant role in the observed hot carrier degradation, and the resulting bare O{sub N} can naturally account for the “super-recovery” in the peak transconductance.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Static electric field enhancement in nanoscale structures

We study the effect of local atomic- and nano-scale protrusions on field emission and, in particular, on the local field enhancement which plays a key role as known from the Fowler-Nordheim model of electronic emission. We study atomic size defects which consist of right angle steps forming an infinite length staircase on a tungsten surface. This structure is embedded in a 1 GV/m ambient electrostatic field. We perform calculations based upon density functional theory in order to characterize the total and induced electronic densities as well as the local electrostatic fields taking into account the detailed atomic structure of the metal. We show how the results must be processed to become comparable with those of a simple homogeneous tungsten sheet electrostatic model. We also describe an innovative procedure to extrapolate our results to nanoscale defects of larger sizes, which relies on the microscopic findings to guide, tune, and improve the homogeneous metal model, thus gaining predictive power. Furthermore, we evidence analytical power laws for the field enhancement characterization. The main physics-wise outcome of this analysis is that limited field enhancement is to be expected from atomic- and nano-scale defects.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

First-principles study of twin grain boundaries in epitaxial BaSi{sub 2} on Si(111)

Epitaxial films of BaSi{sub 2} on Si(111) for solar cell applications possess three epitaxial variants and exhibit a minority carrier diffusion length (ca. 9.4 μm) much larger than the domain size (ca. 0.2 μm); thus, the domain boundaries (DBs) between the variants do not act as carrier recombination centers. In this work, transmission electron microscopy (TEM) was used to observe the atomic arrangements around the DBs in BaSi{sub 2} epitaxial films on Si(111), and the most stable atomic configuration was determined by first-principles calculations based on density functional theory to provide possible interface models. Bright-field TEM along the a-axis of BaSi{sub 2} revealed that each DB was a twin boundary between two different epitaxial variants, and that Ba{sup (II)} atoms form hexagons containing central Ba{sup (I)} atoms in both the bulk and DB regions. Four possible interface models containing Ba{sup (I)}-atom interface layers were constructed, each consistent with TEM observations and distinguished by the relationship between the Si tetrahedron arrays in the two domains adjacent across the interface. This study assessed the structural relaxation of initial interface models constructed from surface slabs terminated by Ba{sup (I)} atoms or from zigzag surface slabs terminated by Si tetrahedra and Ba{sup (II)} atoms. In these models, the interactions or relative positions between Si tetrahedra appear to dominate the relaxation behavior and DB energies. One of the four interface models whose relationship between first-neighboring Si tetrahedra across the interface was the same as that in the bulk was particularly stable, with a DB energy of 95 mJ/m{sup 2}. There were no significant differences in the partial densities of states and band gaps between the bulk and DB regions, and it was therefore concluded that such DBs do not affect the minority carrier properties of BaSi{sub 2}.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Impact of incomplete metal coverage on the electrical properties of metal-CNT contacts: A large-scale ab initio study

Using a dedicated combination of the non-equilibrium Green function formalism and large-scale density functional theory calculations, we investigated how incomplete metal coverage influences two of the most important electrical properties of carbon nanotube (CNT)-based transistors: contact resistance and its scaling with contact length, and maximum current. These quantities have been derived from parameter-free simulations of atomic systems that are as close as possible to experimental geometries. Physical mechanisms that govern these dependences have been identified for various metals, representing different CNT-metal interaction strengths from chemisorption to physisorption. Our results pave the way for an application-oriented design of CNT-metal contacts.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

The boundary density profile of a Coulomb droplet. Freezing at the edge

In this paper, we revisit the problem of computing the boundary density profile of a droplet of two-dimensional one-component plasma (2D OCP) with logarithmic inter- action between particles in a confining harmonic potential. At a sufficiently low temperature, but still in the liquid phase, the density exhibits oscillations as a function of the distance to the boundary of the droplet. We obtain the density profile numerically using Monte-Carlo simulations of the 2D OCP. We argue that the decay and period of those oscillations can be explained within a picture of the Wigner crystallization near the boundary, where the crystal is gradually melted with the increasing distance to the boundary.

2D OCP↗
DOE OSTI2019

Resistivity anisotropy of quantum Hall stripe phases

In this work, quantum Hall stripe phases near half-integer filling factors $\nu$ $\geqslant$ 9/2 were predicted by Hartree-Fock (HF) theory and confirmed by discoveries of giant resistance anisotropies in high-mobility two dimensional electron gases. A theory of such anisotropy was proposed by MacDonald and Fisher, although they used parameters whose dependencies on the filling factor, electron density, and mobility remained unspecified. Here, we fill this void by calculating the hard-to-easy resistivity ratio as a function of these three variables. Quantitative comparison with experiment yields very good agreement, which we view as evidence for the “plain vanilla” smectic stripe HF phases

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Magnon damping in the zigzag phase of the Kitaev-Heisenberg- Γ model on a honeycomb lattice

We calculate dispersions and damping of the magnetic excitations in the model that is relevant to a description of the magnetic properties of iridium oxides α-Li 2 IrO 3 and Na 2 IrO 3 , and Ru-based materials such as α-RuCl 3 . Focusing on the parameter regime with the zigzag spin pattern in the ground state that is consistent with experiments, we are able to obtain explicit analytical expressions for magnon energies and eigenstates and go beyond the standard linear spin-wave theory approximation by calculating magnon damping and demonstrating its role in the dynamical structure factor. Here, we show that the magnon damping effects in both Born and self-consistent approximations are very significant, underscoring the importance of nonlinear magnon coupling in interpreting broad features in the neutron-scattering spectra.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Effects of exchange distortion and spin rotation in the magnetic Kagome Lattice

In this study, we examine the effect of distorted triangular magnetic interactions in the kagome lattice. Using a Holstein-Primakoff expansion, we determine the analytical solutions for classical energies and the spin-wave modes for various magnetic configurations. By understanding the magnetic phase diagram, we characterize the changes in the spin waves and examine the spin distortions of the ferromagnetic, antiferrimagnetic, and 120° phases that are produced by variable exchange interactions and lead to various noncollinear phases, which provides a deeper understanding of the magnetic fingerprints of these configurations for experimental characterization and identification.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗