Stability of β-equilibrated dense matter and core-crust transition in neutron stars
NASA Astrophysics Data System (ADS)
Atta, Debasis; Basu, D. N.
2014-09-01
The stability of the β-equilibrated dense nuclear matter is analyzed with respect to the thermodynamic stability conditions. Based on the density dependent M3Y effective nucleon-nucleon interaction, the effects of the nuclear incompressibility on the proton fraction in neutron stars and the location of the inner edge of their crusts and core-crust transition density and pressure are investigated. The high-density behavior of symmetric and asymmetric nuclear matter satisfies the constraints from the observed flow data of heavy-ion collisions. The neutron star properties studied using β-equilibrated neutron star matter obtained from this effective interaction for a pure hadronic model agree with the recent observations of the massive compact stars. The density, pressure, and proton fraction at the inner edge separating the liquid core from the solid crust of neutron stars are determined to be ρt=0.0938 fm-3, Pt=0.5006 MeV fm-3, and xp (t)=0.0308, respectively.
Ionization transition in low-density plasma
Triger, S. A.; Khomkin, A. L.; Shumikhin, A. S.
2011-09-15
Ionization equilibrium in low-density low-temperature plasma is considered. It is demonstrated using hydrogen and cesium as examples that the Saha equation predicts an almost jump-like change in the electron density on isochors in a narrow temperature range. Thus, in contrast to a smooth rise in the degree of ionization with increasing temperature at high plasma densities, an increase in the temperature in low-density plasma should result in a sharp transition from a neutral state to a fully ionized plasma. This transition is accompanied by a jump-like increase in the electric conductivity. The relation of these effects to the recombination transition in the model of the early Universe is discussed. The possibility of observing such a transition experimentally and the problems concerning the time of plasma relaxation into an equilibrium state at long free path lengths of plasma particles are considered.
Transition density of one-dimensional diffusion with discontinuous drift
NASA Technical Reports Server (NTRS)
Zhang, Weijian
1990-01-01
The transition density of a one-dimensional diffusion process with a discontinuous drift coefficient is studied. A probabilistic representation of the transition density is given, illustrating the close connections between discontinuities of the drift and Brownian local times. In addition, some explicit results are obtained based on the trivariate density of Brownian motion, its occupation, and local times.
The transition to the metallic state in low density hydrogen.
McMinis, Jeremy; Morales, Miguel A; Ceperley, David M; Kim, Jeongnim
2015-11-21
Solid atomic hydrogen is one of the simplest systems to undergo a metal-insulator transition. Near the transition, the electronic degrees of freedom become strongly correlated and their description provides a difficult challenge for theoretical methods. As a result, the order and density of the phase transition are still subject to debate. In this work, we use diffusion quantum Monte Carlo to benchmark the transition between paramagnetic and anti-ferromagnetic body centered cubic atomic hydrogen in its ground state. We locate the density of the transition by computing the equation of state for these two phases and identify the phase transition order by computing the band gap near the phase transition. These benchmark results show that the phase transition is continuous and occurs at a Wigner-Seitz radius of rs = 2.27(3) a0. We compare our results to previously reported density functional theory, Hedin's GW approximation, and dynamical mean field theory results. PMID:26590549
A density-independent rigidity transition in biological tissues
NASA Astrophysics Data System (ADS)
Bi, Dapeng; Lopez, J. H.; Schwarz, J. M.; Manning, M. Lisa
2015-12-01
Cell migration is important in many biological processes, including embryonic development, cancer metastasis and wound healing. In these tissues, a cell’s motion is often strongly constrained by its neighbours, leading to glassy dynamics. Although self-propelled particle models exhibit a density-driven glass transition, this does not explain liquid-to-solid transitions in confluent tissues, where there are no gaps between cells and therefore the density is constant. Here we demonstrate the existence of a new type of rigidity transition that occurs in the well-studied vertex model for confluent tissue monolayers at constant density. We find that the onset of rigidity is governed by a model parameter that encodes single-cell properties such as cell-cell adhesion and cortical tension, providing an explanation for liquid-to-solid transitions in confluent tissues and making testable predictions about how these transitions differ from those in particulate matter.
Transition matrices and orbitals from reduced density matrix theory
Etienne, Thibaud
2015-06-28
In this contribution, we report two different methodologies for characterizing the electronic structure reorganization occurring when a chromophore undergoes an electronic transition. For the first method, we start by setting the theoretical background necessary to the reinterpretation through simple tensor analysis of (i) the transition density matrix and (ii) the natural transition orbitals in the scope of reduced density matrix theory. This novel interpretation is made more clear thanks to a short compendium of the one-particle reduced density matrix theory in a Fock space. The formalism is further applied to two different classes of excited states calculation methods, both requiring a single-determinant reference, that express an excited state as a hole-particle mono-excited configurations expansion, to which particle-hole correlation is coupled (time-dependent Hartree-Fock/time-dependent density functional theory) or not (configuration interaction single/Tamm-Dancoff approximation). For the second methodology presented in this paper, we introduce a novel and complementary concept related to electronic transitions with the canonical transition density matrix and the canonical transition orbitals. Their expression actually reflects the electronic cloud polarisation in the orbital space with a decomposition based on the actual contribution of one-particle excitations from occupied canonical orbitals to virtual ones. This approach validates our novel interpretation of the transition density matrix elements in terms of the Euclidean norm of elementary transition vectors in a linear tensor space. A proper use of these new concepts leads to the conclusion that despite the different principles underlying their construction, they provide two equivalent excited states topological analyses. This connexion is evidenced through simple illustrations of (in)organic dyes electronic transitions analysis.
A Density-Independent Flocking Transition in Confluent Tissues
NASA Astrophysics Data System (ADS)
Czajkowski, Michael; Bi, Dapeng; Manning, M. Lisa; Marchetti, M. Cristina
Some of us recently demonstrated a density-independent solid-liquid transition in confluent tissues controlled by cell motility and a cell shape parameter measuring the interplay of cortical tension and cell-cell adhesion. An important insight of this work is that the rigidity and dynamics of cell layers depends sensitively on cell shape. To explore the influence of cell shape on collective states, we have constructed continuum equations that couple a scalar field describing cell-shape anisotropy to cell polarization. The model displays a density independent transition to a polarized state of elongated cells driven by a cellular ``shape-index'' parameter. We map out the phase diagram using linear stability analysis and numerical solution of the nonlinear hydrodynamic equations. The proposed transition constitutes a density-independent flocking transition. We acknowledge support from The Simons Foundation and NSF-DGE-1068780.
The transition to the metallic state in low density hydrogen
McMinis, Jeremy; Morales, Miguel A.; Ceperley, David M.; Kim, Jeongnim
2015-11-21
Solid atomic hydrogen is one of the simplest systems to undergo a metal-insulator transition. Near the transition, the electronic degrees of freedom become strongly correlated and their description provides a difficult challenge for theoretical methods. As a result, the order and density of the phase transition are still subject to debate. In this work, we use diffusion quantum Monte Carlo to benchmark the transition between paramagnetic and anti-ferromagnetic body centered cubic atomic hydrogen in its ground state. We locate the density of the transition by computing the equation of state for these two phases and identify the phase transition order by computing the band gap near the phase transition. These benchmark results show that the phase transition is continuous and occurs at a Wigner-Seitz radius of r{sub s} = 2.27(3) a{sub 0}. We compare our results to previously reported density functional theory, Hedin’s GW approximation, and dynamical mean field theory results.
The transition to the metallic state in low density hydrogen
McMinis, Jeremy; Morales, Miguel A.; Ceperley, David M.; Kim, Jeongnim
2015-11-18
Solid atomic hydrogen is one of the simplest systems to undergo a metal-insulator transition. Near the transition, the electronic degrees of freedom become strongly correlated and their description provides a difficult challenge for theoretical methods. As a result, the order and density of the phase transition are still subject to debate. In this work we use diffusion quantum Monte Carlo to benchmark the transition between the paramagnetic and anti-ferromagnetic phases of ground state body centered cubic atomic hydrogen. We locate the density of the transition by computing the equation of state for these two phases and identify the phase transition order by computing the band gap near the phase transition. These benchmark results show that the phase transition is continuous and occurs at a Wigner-Seitz radius of r_{s} = 2.27(3)a_{0}. As a result, we compare our results to previously reported density functional theory, Hedin s GW approximation, and dynamical mean field theory results.
The transition to the metallic state in low density hydrogen
McMinis, Jeremy; Morales, Miguel A.; Ceperley, David M.; Kim, Jeongnim
2015-11-18
Solid atomic hydrogen is one of the simplest systems to undergo a metal-insulator transition. Near the transition, the electronic degrees of freedom become strongly correlated and their description provides a difficult challenge for theoretical methods. As a result, the order and density of the phase transition are still subject to debate. In this work we use diffusion quantum Monte Carlo to benchmark the transition between the paramagnetic and anti-ferromagnetic phases of ground state body centered cubic atomic hydrogen. We locate the density of the transition by computing the equation of state for these two phases and identify the phase transitionmore » order by computing the band gap near the phase transition. These benchmark results show that the phase transition is continuous and occurs at a Wigner-Seitz radius of rs = 2.27(3)a0. As a result, we compare our results to previously reported density functional theory, Hedin s GW approximation, and dynamical mean field theory results.« less
The glass transition in high-density amorphous ice
Loerting, Thomas; Fuentes-Landete, Violeta; Handle, Philip H.; Seidl, Markus; Amann-Winkel, Katrin; Gainaru, Catalin; Böhmer, Roland
2015-01-01
There has been a long controversy regarding the glass transition in low-density amorphous ice (LDA). The central question is whether or not it transforms to an ultraviscous liquid state above 136 K at ambient pressure prior to crystallization. Currently, the most widespread interpretation of the experimental findings is in terms of a transformation to a superstrong liquid above 136 K. In the last decade some work has also been devoted to the study of the glass transition in high-density amorphous ice (HDA) which is in the focus of the present review. At ambient pressure HDA is metastable against both ice I and LDA, whereas at > 0.2 GPa HDA is no longer metastable against LDA, but merely against high-pressure forms of crystalline ice. The first experimental observation interpreted as the glass transition of HDA was made using in situ methods by Mishima, who reported a glass transition temperature Tg of 160 K at 0.40 GPa. Soon thereafter Andersson and Inaba reported a much lower glass transition temperature of 122 K at 1.0 GPa. Based on the pressure dependence of HDA's Tg measured in Innsbruck, we suggest that they were in fact probing the distinct glass transition of very high-density amorphous ice (VHDA). Very recently the glass transition in HDA was also observed at ambient pressure at 116 K. That is, LDA and HDA show two distinct glass transitions, clearly separated by about 20 K at ambient pressure. In summary, this suggests that three glass transition lines can be defined in the p–T plane for LDA, HDA, and VHDA. PMID:25641986
Lifshitz transition in two-dimensional spin density wave models.
Lin, J.; Materials Science Division
2010-11-09
We argue that both pocket-disappearing and neck-disrupting types of Lifshitz transitions can be realized in two-dimensional spin-density wave models for underdoped cuprates, and study both types of transitions with impurity scattering treated in the self-consistent Born approximation. We first solve for the electron self-energy from the self-consistent equation, and then study the low-temperature electrical conductivity and thermopower. Close to the Lifshitz transition, the thermopower is strongly enhanced. For the pocket-disappearing type, it has a sharp peak while for the neck-disrupting type, it changes sign at the transition, with its absolute value peaked on both sides of the transition. We discuss possible applications to underdoped cuprates.
Lipid Bilayer Phase Transition: Density Measurements and Theory
Nagle, J. F.
1973-01-01
The overall change of density for dipalmitoyl lecithin bilayers agrees with a general order-disorder theory and yields about seven gauche rotations per molecule for the biologically relevant high-temperature phase. The shape of the curve of density against temperature is similar to the result of an exact calculation on a specific model, which gives a 3/2-order phase transition. PMID:4519637
Fluid hydrogen at high density - The plasma phase transition
NASA Technical Reports Server (NTRS)
Saumon, D.; Chabrier, G.
1989-01-01
A new model equation of state is applied, based on realistic interparticle potentials and a self-consistent treatment of the internal levels, to fluid hydrogen at high density. This model shows a strong connection between molecular dissociation and pressure ionization. The possibility of a first-order plasma phase transition is considered, and for which both the evolution in temperature and the critical point is given.
Moustakidis, Ch. C.; Lalazissis, G. A.; Niksic, T.; Vretenar, D.; Ring, P.
2010-06-15
The transition density n{sub t} and pressure P{sub t} at the inner edge between the liquid core and the solid crust of a neutron star are analyzed using the thermodynamical method and the framework of relativistic nuclear energy density functionals. Starting from a functional that has been carefully adjusted to experimental binding energies of finite nuclei, and varying the density dependence of the corresponding symmetry energy within the limits determined by isovector properties of finite nuclei, we estimate the constraints on the core-crust transition density and pressure of neutron stars: 0.086 fm{sup -3}<=n{sub t}<0.090 fm{sup -3} and 0.3 MeV fm{sup -3}
Metal-insulator transition by holographic charge density waves.
Ling, Yi; Niu, Chao; Wu, Jian-Pin; Xian, Zhuo-Yu; Zhang, Hongbao
2014-08-29
We construct a gravity dual for charge density waves (CDWs) in which the translational symmetry along one spatial direction is spontaneously broken. Our linear perturbation calculation on the gravity side produces the frequency dependence of the optical conductivity, which exhibits the two familiar features of CDWs, namely, the pinned collective mode and gapped single-particle excitation. These two features indicate that our gravity dual also provides a new mechanism to implement the metal to insulator phase transition by CDWs, which is further confirmed by the fact that dc conductivity decreases with the decreased temperature below the critical temperature. PMID:25215974
Multiple charge density wave transitions in Gd2Te5
Shin, K.Y.; Ru, N.; Condron, C.L.; Wu, Y.Q.; Kramer, M.J.; Toney, M.F.; Fisher, I.R.; /Stanford U., Geballe Lab. /Stanford U., Appl. Phys. Dept.
2010-02-15
Diffraction measurements performed via transmission electron microscopy and high resolution X-ray scattering reveal two distinct charge density wave transitions in Gd{sub 2}Te{sub 5} at T{sub c1} = 410(3) and T{sub c2} = 532(3) K, associated with the on-axis incommensurate lattice modulation and off-axis commensurate lattice modulation respectively. Analysis of the temperature dependence of the order parameters indicates a non-vanishing coupling between these two distinct CDW states.
KEPLER-7b: A TRANSITING PLANET WITH UNUSUALLY LOW DENSITY
Latham, David W.; Buchhave, Lars A.; Furesz, Gabor; Geary, John C.; Borucki, William J.; Koch, David G.; Lissauer, Jack J.; Rowe, Jason F.; Brown, Timothy M.; Basri, Gibor; Batalha, Natalie M.; Caldwell, Douglas A.; Jenkins, Jon M.; Cochran, William D.; Dunham, Edward W.; Gautier, Thomas N.; Howell, Steve B.; Marcy, Geoffrey W.; Monet, David G.
2010-04-20
We report on the discovery and confirmation of Kepler-7b, a transiting planet with unusually low density. The mass is less than half that of Jupiter, M {sub P} = 0.43 M {sub J}, but the radius is 50% larger, R {sub P} = 1.48 R {sub J}. The resulting density, {rho}{sub P} = 0.17 g cm{sup -3}, is the second lowest reported so far for an extrasolar planet. The orbital period is fairly long, P = 4.886 days, and the host star is not much hotter than the Sun, T {sub eff} = 6000 K. However, it is more massive and considerably larger than the Sun, M {sub *} = 1.35 M {sub sun} and R {sub *} = 1.84 R {sub sun}, and must be near the end of its life on the main sequence.
Energy boost in laser wakefield accelerators using sharp density transitions
NASA Astrophysics Data System (ADS)
Döpp, A.; Guillaume, E.; Thaury, C.; Lifschitz, A.; Ta Phuoc, K.; Malka, V.
2016-05-01
The energy gain in laser wakefield accelerators is limited by dephasing between the driving laser pulse and the highly relativistic electrons in its wake. Since this phase depends on both the driver and the cavity length, the effects of dephasing can be mitigated with appropriate tailoring of the plasma density along propagation. Preceding studies have discussed the prospects of continuous phase-locking in the linear wakefield regime. However, most experiments are performed in the highly non-linear regime and rely on self-guiding of the laser pulse. Due to the complexity of the driver evolution in this regime, it is much more difficult to achieve phase locking. As an alternative, we study the scenario of rapid rephasing in sharp density transitions, as was recently demonstrated experimentally. Starting from a phenomenological model, we deduce expressions for the electron energy gain in such density profiles. The results are in accordance with particle-in-cell simulations, and we present gain estimations for single and multiple stages of rephasing.
Charge density wave transition in single-layer titanium diselenide
Chen, P.; Chan, Y. -H.; Fang, X. -Y.; Zhang, Y.; Chou, M. Y.; Mo, S. -K.; Hussain, Z.; Fedorov, A. -V.; Chiang, T. -C.
2015-11-16
A single molecular layer of titanium diselenide (TiSe2) is a promising material for advanced electronics beyond graphene--a strong focus of current research. Such molecular layers are at the quantum limit of device miniaturization and can show enhanced electronic effects not realizable in thick films. We show that single-layer TiSe2 exhibits a charge density wave (CDW) transition at critical temperature TC=232±5 K, which is higher than the bulk TC=200±5 K. Angle-resolved photoemission spectroscopy measurements reveal a small absolute bandgap at room temperature, which grows wider with decreasing temperature T below TC in conjunction with the emergence of (2 × 2) ordering.more » The results are rationalized in terms of first-principles calculations, symmetry breaking and phonon entropy effects. The behavior of the Bardeen-Cooper-Schrieffer (BCS) gap implies a mean-field CDW order in the single layer and an anisotropic CDW order in the bulk.« less
Charge density wave transition in single-layer titanium diselenide
Chen, P; Chan, Y. -H.; Fang, X. -Y.; Zhang, Y; Chou, M Y; Mo, S. -K.; Hussain, Z; Fedorov, A. -V.; Chiang, T. -C.
2015-01-01
A single molecular layer of titanium diselenide (TiSe2) is a promising material for advanced electronics beyond graphene—a strong focus of current research. Such molecular layers are at the quantum limit of device miniaturization and can show enhanced electronic effects not realizable in thick films. We show that single-layer TiSe2 exhibits a charge density wave (CDW) transition at critical temperature TC=232±5 K, which is higher than the bulk TC=200±5 K. Angle-resolved photoemission spectroscopy measurements reveal a small absolute bandgap at room temperature, which grows wider with decreasing temperature T below TC in conjunction with the emergence of (2 × 2) ordering. The results are rationalized in terms of first-principles calculations, symmetry breaking and phonon entropy effects. The observed Bardeen-Cooper-Schrieffer (BCS) behaviour of the gap implies a mean-field CDW order in the single layer and an anisotropic CDW order in the bulk. PMID:26568512
Charge density wave transition in single-layer titanium diselenide
Chen, P.; Chan, Y. -H.; Fang, X. -Y.; Zhang, Y.; Chou, M. Y.; Mo, S. -K.; Hussain, Z.; Fedorov, A. -V.; Chiang, T. -C.
2015-11-16
A single molecular layer of titanium diselenide (TiSe_{2}) is a promising material for advanced electronics beyond graphene--a strong focus of current research. Such molecular layers are at the quantum limit of device miniaturization and can show enhanced electronic effects not realizable in thick films. We show that single-layer TiSe_{2} exhibits a charge density wave (CDW) transition at critical temperature T_{C}=232±5 K, which is higher than the bulk T_{C}=200±5 K. Angle-resolved photoemission spectroscopy measurements reveal a small absolute bandgap at room temperature, which grows wider with decreasing temperature T below T_{C} in conjunction with the emergence of (2 × 2) ordering. The results are rationalized in terms of first-principles calculations, symmetry breaking and phonon entropy effects. The behavior of the Bardeen-Cooper-Schrieffer (BCS) gap implies a mean-field CDW order in the single layer and an anisotropic CDW order in the bulk.
Two Transiting Low Density Sub-Saturns from K2
NASA Astrophysics Data System (ADS)
Petigura, Erik A.; Howard, Andrew W.; Lopez, Eric D.; Deck, Katherine M.; Fulton, Benjamin J.; Crossfield, Ian J. M.; Ciardi, David R.; Chiang, Eugene; Lee, Eve J.; Isaacson, Howard; Beichman, Charles A.; Hansen, Brad M. S.; Schlieder, Joshua E.; Sinukoff, Evan
2016-02-01
We report the discovery and confirmation of K2-24 b and c, two sub-Saturn planets orbiting a bright (V = 11.3), metal-rich ([Fe/H] = 0.42 ± 0.04 dex) G3 dwarf in the K2 Campaign 2 field. The planets are 5.68 ± 0.56 {R}\\oplus and 7.82 ± 0.72 {R}\\oplus and have orbital periods of 20.8851 ± 0.0003 days and 42.3633 ± 0.0006 days, near the 2:1 mean-motion resonance. We obtained 32 radial velocities with Keck/HIRES and detected the reflex motion due to K2-24 b and c. These planets have masses of 21.0 ± 5.4 {M}\\oplus and 27.0 ± 6.9 {M}\\oplus , respectively. With low densities of 0.63 ± 0.25 g cm-3 and 0.31 ± 0.12 g cm-3, respectively, the planets require thick envelopes of H/He to explain their large sizes and low masses. Interior structure models predict that the planets have fairly massive cores of 17.6+/- 4.3 {M}\\oplus and 16.1+/- 4.2 {M}\\oplus , respectively. They may have formed exterior to their present locations, accreted their H/He envelopes at large orbital distances, and migrated in as a resonant pair. The proximity to resonance, large transit depths, and host star brightness offers rich opportunities for TTV follow-up. Finally, the low surface gravities of the K2-24 planets make them favorable targets for transmission spectroscopy by Hubble Space Telescope, Spitzer, and James Webb Space Telescope.
Empirical transverse charge densities in the nucleon and the nucleon-to-Delta transition
Carl Carlson; Marc Vanderhaeghen
2008-01-01
Using only the current empirical information on the nucleon electromagnetic form factors we map out the transverse charge density in proton and neutron as viewed from a light front moving towards a transversely polarized nucleon. These charge densities are characterized by a dipole pattern, in addition to the monopole field corresponding with the unpolarized density. Furthermore, we use the latest empirical information on the $N \\to \\Delta$ transition form factors to map out the transition charge density which induces the $N \\to \\Delta$ excitation. This transition charge density in a transversely polarized $N$ and $\\Delta$ contains both monopole, dipole and quadrupole patterns, the latter corresponding with a deformation of the hadron's charge distribution.
E→H mode transition density and power in two types of inductively coupled plasma configuration
Wang, Jian; Du, Yin-chang; Zhang, Xiao; Zheng, Zhe; Liu, Yu; Xu, Liang; Wang, Pi; Cao, Jin-xiang
2014-07-15
E → H transition power and density were investigated at various argon pressures in inductively coupled plasma (ICP) in a cylindrical interlaid chamber. The transition power versus the pressure shows a minimum transition power at 4 Pa (ν/ω=1) for argon. Then the transition density hardly changes at low pressures (ν/ω≪1), but it increases clearly when argon pressure exceeds an appropriate value. In addition, both the transition power and transition density are lower in the re-entrant configuration of ICP compared with that in the cylindrical configuration of ICP. The result may be caused from the decrease of stochastic heating in the re-entrant configuration of ICP. This work is useful to understand E → H mode transition and control the transition points in real plasma processes.
Neutron transition densities for 48Ca from proton scattering at 200 and 318 MeV
NASA Astrophysics Data System (ADS)
Feldman, A. E.; Kelly, J. J.; Flanders, B. S.; Khandaker, M. A.; Seifert, H.; Boberg, P.; Hyman, S. D.; Karen, P. H.; Norum, B. E.; Welch, P.; Chen, Q.; Bacher, A. D.; Berg, G. P.; Stephenson, E. J.; Nanda, S.; Saha, A.; Scott, A.
1994-04-01
Differential cross sections and analyzing powers for scattering of 200 and 318 MeV protons have been measured for states of 48Ca up to 7 MeV of excitation. The data cover c.m. momentum transfers from approximately 0.4 to 3.0 fm-1. Neutron transition densities were extracted for the 2+1,3-1,3-2,4+2, and 5-1 states using density-dependent empirical effective interactions previously calibrated upon elastic and inelastic scattering data for 16O and 40Ca. The corresponding proton transition densities were obtained from electron scattering data and held fixed during the analysis. Fits performed to the data for either energy provide excellent predictions for the other. Neutron densities fitted to data for either energy independently agree very well with each other and with the densities fitted to both data sets simultaneously. These densities are also consistent with earlier data for 500 MeV protons. The energy-independence of the extracted transition densities demonstrates that residual errors in the reaction model are compatible with the error bands estimated by the fitting procedure. Several additional tests of the model dependence of the results were performed also. The proton and neutron transition densities are compared with calculations based upon the extended random phase approximation, which includes 2p2h correlations. These calculations are most successful for densities dominated by 1p1h configurations, whereas densities requiring substantial 2p2h contributions tend to be underestimated.
Koga, K; Tanaka, H; Zeng, X C
2000-11-30
Supercooled water and amorphous ice have a rich metastable phase behaviour. In addition to transitions between high- and low-density amorphous solids, and between high- and low-density liquids, a fragile-to-strong liquid transition has recently been proposed, and supported by evidence from the behaviour of deeply supercooled bilayer water confined in hydrophilic slit pores. Here we report evidence from molecular dynamics simulations for another type of first-order phase transition--a liquid-to-bilayer amorphous transition--above the freezing temperature of bulk water at atmospheric pressure. This transition occurs only when water is confined in a hydrophobic slit pore with a width of less than one nanometre. On cooling, the confined water, which has an imperfect random hydrogen-bonded network, transforms into a bilayer amorphous phase with a perfect network (owing to the formation of various hydrogen-bonded polygons) but no long-range order. The transition shares some characteristics with those observed in tetrahedrally coordinated substances such as liquid silicon, liquid carbon and liquid phosphorus. PMID:11117739
NASA Technical Reports Server (NTRS)
Ingels, F.; Schoggen, W. O.
1981-01-01
The various methods of high bit transition density encoding are presented, their relative performance is compared in so far as error propagation characteristics, transition properties and system constraints are concerned. A computer simulation of the system using the specific PN code recommended, is included.
Phase transition in finite density and temperature lattice QCD
NASA Astrophysics Data System (ADS)
Wang, Rui; Chen, Ying; Gong, Ming; Liu, Chuan; Liu, Yu-Bin; Liu, Zhao-Feng; Ma, Jian-Ping; Meng, Xiang-Fei; Zhang, Jian-Bo
2015-06-01
We investigate the behavior of the chiral condensate in lattice QCD at finite temperature and finite chemical potential. The study was done using two flavors of light quarks and with a series of β and ma at the lattice size 24 × 122 × 6. The calculation was done in the Taylor expansion formalism. We are able to calculate the first and second order derivatives of ≤ft< {\\bar{\\psi} \\psi } \\right> in both isoscalar and isovector channels. With the first derivatives being small, we find that the second derivatives are sizable close to the phase transition and that the magnitude of \\bar{\\psi} \\psi decreases under the influence of finite chemical potential in both channels. Supported by National Natural Science Foundation of China (11335001, 11105153, 11405178), Projects of International Cooperation and Exchanges NSFC (11261130311)
Phase transitions in core-collapse supernova matter at sub-saturation densities
NASA Astrophysics Data System (ADS)
Pais, Helena; Newton, William G.; Stone, Jirina R.
2014-12-01
Phase transitions in hot, dense matter in the collapsing cores of massive stars have an important impact on the core-collapse supernova mechanism as they absorb heat, disrupt homology, and so weaken the developing shock. We perform a three-dimensional, finite temperature Skyrme-Hartree-Fock (SHF) study of inhomogeneous nuclear matter to determine the critical density and temperature for the phase transition between the pasta phase and homogeneous matter and its properties. We employ four different parametrizations of the Skyrme nuclear energy-density functional, SkM*, SLy4, NRAPR, and SQMC700, which span a range of saturation-density symmetry energy behaviors constrained by a variety of nuclear experimental probes. For each of these interactions we calculate free energy, pressure, entropy, and chemical potentials in the range of particle number densities where the nuclear pasta phases are expected to exist, 0.02-0.12 fm-3, temperatures 2-8 MeV, and a proton fraction of 0.3. We find unambiguous evidence for a first-order phase transition to uniform matter, unsoftened by the presence of the pasta phases. No conclusive signs of a first-order phase transition between the pasta phases is observed, and it is argued that the thermodynamic quantities vary continuously right up to the first-order phase transition to uniform matter. We compare our results with thermodynamic spinodals calculated using the same Skyrme parametrizations, finding that the effect of short-range Coulomb correlations and quantum shell effects included in our model leads to the pasta phases existing at densities up to 0.01 fm-3 above the spinodal boundaries, thus increasing the transition density to uniform matter by the same amount. The transition density is otherwise shown to be insensitive to the symmetry energy at saturation density within the range constrained by the concordance of a variety of experimental constraints, and can be taken to be a well determined quantity.
Membrane tension and peripheral protein density mediate membrane shape transitions
Shi, Zheng; Baumgart, Tobias
2015-01-01
Endocytosis is a ubiquitous eukaryotic membrane budding, vesiculation, and internalization process fulfilling numerous roles including compensation of membrane area increase after bursts of exocytosis. The mechanism of the coupling between these two processes to enable homeostasis is not well understood. Recently, an ultrafast endocytosis (UFE) pathway was revealed with a speed significantly exceeding classical clathrin-mediated endocytosis (CME). Membrane tension reduction is a potential mechanism by which endocytosis can be rapidly activated at remote sites. Here we provide experimental evidence for a mechanism whereby membrane tension reduction initiates membrane budding and tubulation mediated by endocytic proteins such as endophilin A1. We find that shape instabilities occur at well-defined membrane tensions and surface densities of endophilin. From our data, we obtain a membrane shape stability diagram that shows remarkable consistency with a quantitative model. This model applies to all laterally diffusive curvature coupling proteins and therefore a wide range of endocytic proteins. PMID:25569184
Membrane tension and peripheral protein density mediate membrane shape transitions.
Shi, Zheng; Baumgart, Tobias
2015-01-01
Endocytosis is a ubiquitous eukaryotic membrane budding, vesiculation and internalization process fulfilling numerous roles including compensation of membrane area increase after bursts of exocytosis. The mechanism of the coupling between these two processes to enable homeostasis is not well understood. Recently, an ultrafast endocytosis (UFE) pathway was revealed with a speed significantly exceeding classical clathrin-mediated endocytosis (CME). Membrane tension reduction is a potential mechanism by which endocytosis can be rapidly activated at remote sites. Here, we provide experimental evidence for a mechanism whereby membrane tension reduction initiates membrane budding and tubulation mediated by endocytic proteins, such as endophilin A1. We find that shape instabilities occur at well-defined membrane tensions and surface densities of endophilin. From our data, we obtain a membrane shape stability diagram that shows remarkable consistency with a quantitative model. This model applies to all laterally diffusive curvature-coupling proteins and therefore a wide range of endocytic proteins. PMID:25569184
Membrane tension and peripheral protein density mediate membrane shape transitions
NASA Astrophysics Data System (ADS)
Shi, Zheng; Baumgart, Tobias
2015-01-01
Endocytosis is a ubiquitous eukaryotic membrane budding, vesiculation and internalization process fulfilling numerous roles including compensation of membrane area increase after bursts of exocytosis. The mechanism of the coupling between these two processes to enable homeostasis is not well understood. Recently, an ultrafast endocytosis (UFE) pathway was revealed with a speed significantly exceeding classical clathrin-mediated endocytosis (CME). Membrane tension reduction is a potential mechanism by which endocytosis can be rapidly activated at remote sites. Here, we provide experimental evidence for a mechanism whereby membrane tension reduction initiates membrane budding and tubulation mediated by endocytic proteins, such as endophilin A1. We find that shape instabilities occur at well-defined membrane tensions and surface densities of endophilin. From our data, we obtain a membrane shape stability diagram that shows remarkable consistency with a quantitative model. This model applies to all laterally diffusive curvature-coupling proteins and therefore a wide range of endocytic proteins.
Molecular dynamics investigation of a density-driven glass transition in a liquid crystal system.
Vieira, Pablo A; Lacks, Daniel J
2004-08-22
Molecular dynamics simulations are carried out to address the density-driven glass transition in a system of rodlike particles that interact with the Gay-Berne potential. Since crystallization occurs in this system on the time scale of the simulations, direct simulation of the glass transition is not possible. Instead, glasses with isotropic orientational order are heated to a temperature T, and the relaxation times by which nematic orientational order develops are determined. These relaxation times appear to diverge at a critical density rho(c); i.e., the system can equilibrate at rho
An analytic model for limiting high density LH transition by the onset of the tertiary instability
NASA Astrophysics Data System (ADS)
Singh, Raghvendra; Jhang, Hogun; Kaang, Helen H.
2016-07-01
We perform an analytic study of the tertiary instability driven by a strong excitation of zonal flows during high density low to high (LH) mode transition. The drift resistive ballooning mode is assumed to be a dominant edge turbulence driver. The analysis reproduces main qualitative features of early computational results [Rogers and Drake, Phys. Rev. Lett. 81, 4396 (1998); Guzdar et al., Phys. Plasmas 14, 020701 (2007)], as well as new characteristics of the maximum edge density due to the onset of the tertiary instability. An analytical scaling indicates that the density scaling of LH transition power may be determined by the onset condition of the tertiary instability when the operating density approaches to the Greenwald density.
The nonlinear transition period of broadband ultrasound attenuation as bone density varies.
Serpe, L; Rho, J Y
1996-07-01
The purpose of this study was to determine whether a transition period occurs between cortical and cancellous bone in the relationship between ultrasound parameters [broadband ultrasound attenuation (BUA) and ultrasonic velocity] and density. Twenty-two cancellous bone discs wee obtained from proximal bovine tibiae. Also included were three samples of human vertebral cancellous bone from an elderly female and four samples of bovine cortical bone. Ultrasonic velocity did not show any transition period as density varied from cancellous to cortical bone. Ultrasonic velocity exhibited a definite linear dependence on density over the entire range examined. However, BUA has shown a transition period as density varied. Although BUA increased linearly with density for a low density cancellous bone tested (below 0.64 g cm-3), the dependence of BUA on density is nonlinear with a downwardly inflected parabola shape when covering a wide density range (0.130-0.913 g cm-3) of cancellous bone. When one includes cortical bone, the parabola tends to level off in a slow exponential decay. This nonlinear dependence may help to understand the characteristics of BUA measurement. PMID:8809627
NASA Astrophysics Data System (ADS)
C, Santosh K.; Zhang, Chenxi; Hong, Suklyun; Wallace, Robert M.; Cho, Kyeongjae
2015-09-01
Transition metal dichalcogenides (TMDs) have been investigated extensively for potential application as device materials in recent years. TMDs are found to be stable in trigonal prismatic (H), octahedral (T), or distorted octahedral (Td) coordination of the transition metal. However, the detailed understanding of stabilities of TMDs in a particular phase is lacking. In this work, the detailed TMD phase stability using first-principles calculations based on density functional theory (DFT) has been investigated to clarify the mechanism of phase stabilities of TMDs, consistent with the experimental observation. Our results indicate that the phase stability of TMDs can be explained considering the relative strength of two competing mechanisms: ligand field stabilization of d-orbitals corresponding to transition metal coordination geometry, and charge density wave (CDW) instability accompanied by a periodic lattice distortion (PLD) causing the phase transition in particular TMDs.
NASA Technical Reports Server (NTRS)
Ingels, F. M.; Schoggen, W. O.
1982-01-01
The design to achieve the required bit transition density for the Space Shuttle high rate multiplexes (HRM) data stream of the Space Laboratory Vehicle is reviewed. It contained a recommended circuit approach, specified the pseudo random (PN) sequence to be used and detailed the properties of the sequence. Calculations showing the probability of failing to meet the required transition density were included. A computer simulation of the data stream and PN cover sequence was provided. All worst case situations were simulated and the bit transition density exceeded that required. The Preliminary Design Review and the critical Design Review are documented. The Cover Sequence Generator (CSG) Encoder/Decoder design was constructed and demonstrated. The demonstrations were successful. All HRM and HRDM units incorporate the CSG encoder or CSG decoder as appropriate.
He,P.; Blaskiewicz, M.; Fischer, W.
2009-01-02
In this report we summarize electron-cloud simulations for the RHIC dipole regions at injection and transition to estimate if scrubbing over practical time scales at injection would reduce the electron cloud density at transition to significantly lower values. The lower electron cloud density at transition will allow for an increase in the ion intensity.
Low-density to high-density transition in Ce75Al23Si2 metallic glass.
Zeng, Q S; Fang, Y Z; Lou, H B; Gong, Y; Wang, X D; Yang, K; Li, A G; Yan, S; Lathe, C; Wu, F M; Yu, X H; Jiang, J Z
2010-09-22
Using in situ high-pressure x-ray diffraction (XRD), we observed a pressure-induced polyamorphic transition from the low-density amorphous (LDA) state to the high-density amorphous (HDA) state in Ce(75)Al(23)Si(2) metallic glass at about 2 GPa and 300 K. The thermal stabilities of both LDA and HDA metallic glasses were further investigated using in situ high-temperature and high-pressure XRD, which revealed different pressure dependences of the onset crystallization temperature (T(x)) between them with a turning point at about 2 GPa. Compared with Ce(75)Al(25) metallic glass, minor Si doping shifts the onset polyamorphic transition pressure from 1.5 to 2 GPa and obviously stabilizes both LDA and HDA metallic glasses with higher T(x) and changes their slopes dT(x)/dP. The results obtained in this work reveal another polyamorphous metallic glass system by minor alloying (e.g. Si), which could modify the transition pressure and also properties of LDA and HDA metallic glasses. The minor alloying effect reported here is valuable for the development of more polyamorphous metallic glasses, even multicomponent bulk metallic glasses with modified properties, which will trigger more investigations in this field and improve our understanding of polyamorphism and metallic glasses. PMID:21403196
Charge and transition densities for the samarium isotopes by electron scattering
NASA Astrophysics Data System (ADS)
Moinester, M. A.; Alster, J.; Azuelos, G.; Bellicard, J. B.; Frois, B.; Huet, M.; Leconte, P.; Ho, Phan Xuan
1981-07-01
We analyzed 251.5 and 401.4 MeV electron scattering data on 144,148,150,152Sm. The momentum transfer ranged from 0.6 to 2.5 fm-1. These isotopes span the transition region from the spherical 144Sm to the deformed 152Sm. Ground state charge distributions and lowest 2+ state transition charge densities were determined via a phase shift analysis for elastic scattering and distorted-wave Born approximation calculations for inelastic scattering. Our analysis used charge densities described as a sum of spherical Bessel functions over a radius interval from zero to a cutoff of R, with densities zero at larger radii. The fitting for the ground and 2+ states included constraints in the form of measured Barrett moments from muonic experiments and measured B(E2) transition rates from muonic and other experiments. Error bands were determined for the densities including statistical and normalization uncertainties, and model dependent uncertainties associated with contributions from higher terms in the spherical Bessel function form. We find that as neutrons are added from isotope to isotope, the charge is displaced from the region of 4.0 fm to the region of 7.5 fm. The rms radii of 144,148,150,152Sm were deduced with uncertainties of about 0.006 to 0.009 fm. [NUCLEAR REACTIONS 144,148,150,152Sm(e,e') analysis. Determination of charge and transition densities via Fourier-Bessel analysis.
Lifshitz transition in two-dimensional spin-density wave models
Lin, Jie
2010-11-09
We argue that both pocket-disappearing and neck-disrupting types of Lifshitz transitions can be realized in two-dimensional spin-density wave models for underdoped cuprates, and study both types of transitions with impurity scattering treated in the self-consistent Born approximation. We first solve for the electron self-energy from the self-consistent equation, and then study the low-temperature electrical conductivity and thermopower. Close to the Lifshitz transition, the thermopower is strongly enhanced. For the pocket-disappearing type, it has a sharp peak while for the neck-disrupting type, it changes sign at the transition, with its absolute value peaked on both sides of the transition. We discuss possible applications to underdoped cuprates.
Confinement transition to density wave order in metallic doped spin liquids
NASA Astrophysics Data System (ADS)
Patel, Aavishkar A.; Chowdhury, Debanjan; Allais, Andrea; Sachdev, Subir
2016-04-01
Insulating quantum spin liquids can undergo a confinement transition to a valence bond solid via the condensation of topological excitations of the associated gauge theory. We extend the theory of such transitions to fractionalized Fermi liquids (FL*): These are metallic doped spin liquids in which the Fermi surfaces only have gauge neutral quasiparticles. Using insights from a duality transform on a doped quantum dimer model for the U(1)-FL* state, we show that projective symmetry group of the theory of the topological excitations remains unmodified, but the Fermi surfaces can lead to additional frustrating interactions. We propose a theory for the confinement transition of Z2-FL* states via the condensation of visons. A variety of confining, incommensurate density wave states are possible, including some that are similar to the incommensurate d -form factor density wave order observed in several recent experiments on the cuprate superconductors.
The dipole moment of the spin density as a local indicator for phase transitions
Schmitz, D.; Schmitz-Antoniak, C.; Warland, A.; Darbandi, M.; Haldar, S.; Bhandary, S.; Eriksson, O.; Sanyal, B.; Wende, H.
2014-01-01
The intra-atomic magnetic dipole moment - frequently called 〈Tz〉 term - plays an important role in the determination of spin magnetic moments by x-ray absorption spectroscopy for systems with nonspherical spin density distributions. In this work, we present the dipole moment as a sensitive monitor to changes in the electronic structure in the vicinity of a phase transiton. In particular, we studied the dipole moment at the Fe2+ and Fe3+ sites of magnetite as an indicator for the Verwey transition by a combination of x-ray magnetic circular dichroism and density functional theory. Our experimental results prove that there exists a local change in the electronic structure at temperatures above the Verwey transition correlated to the known spin reorientation. Furthermore, it is shown that measurement of the dipole moment is a powerful tool to observe this transition in small magnetite nanoparticles for which it is usually screened by blocking effects in classical magnetometry. PMID:25041757
Elastic anomalies at the charge density wave transition in TbTe3
NASA Astrophysics Data System (ADS)
Saint-Paul, M.; Guttin, C.; Lejay, P.; Remenyi, G.; Leynaud, O.; Monceau, P.
2016-05-01
The set of elastic constants of the charge density wave (CDW) rare earth tritelluride TbTe3 has been measured at 15 MHz in the temperature range 300-360 K. Large anomalies in the velocity and ultrasonic attenuation of the longitudinal C11 and C33 modes are observed at the charge density wave phase transition TCDW=333 K. Anisotropic stress dependence ∂TCDW / ∂σ is found, the components ∂TCDW / ∂σ11 and ∂TCDW / ∂σ33 in the (a,c) plane are one order of magnitude larger than the component ∂TCDW / ∂σ22 perpendicular to it. The Landau theory has been used to explain the experimental data. Critical behaviour near the charge density wave phase transition is described in terms of a phenomenological dynamic scaling theory.
NASA Astrophysics Data System (ADS)
Chen, Shaohao; Qing, Bo; Li, Jiaming
2007-10-01
Using the multiconfiguration Dirac-Fock method, including the quantum electrodynamics corrections, especially with the Breit interactions, we calculate the electric quadrupole (E2) and magnetic dipole (M1) transition rates for the two transitions D5/2,3/2o2→S3/2o4 of OII . We show systematically that the correlation effects owing to core electron excitations and the Breit interactions are vitally important for the transition rates. We present a benchmark for the intensity ratio between the two transitions in the limit of high electron density in planetary nebulas, i.e., r(∞)=0.345-0.014+0.028 , which is in good agreement with modern astronomical observations.
Chen Shaohao; Qing Bo; Li Jiaming
2007-10-15
Using the multiconfiguration Dirac-Fock method, including the quantum electrodynamics corrections, especially with the Breit interactions, we calculate the electric quadrupole (E2) and magnetic dipole (M1) transition rates for the two transitions {sup 2}D{sub 5/2,3/2}{sup o}{yields}{sup 4}S{sub 3/2}{sup o} of O II. We show systematically that the correlation effects owing to core electron excitations and the Breit interactions are vitally important for the transition rates. We present a benchmark for the intensity ratio between the two transitions in the limit of high electron density in planetary nebulas, i.e., r({infinity})=0.345{sub -0.014}{sup +0.028}, which is in good agreement with modern astronomical observations.
Carlson, Rebecca K; Li Manni, Giovanni; Sonnenberger, Andrew L; Truhlar, Donald G; Gagliardi, Laura
2015-01-13
Kohn-Sham density functional theory, resting on the representation of the electronic density and kinetic energy by a single Slater determinant, has revolutionized chemistry, but for open-shell systems, the Kohn-Sham Slater determinant has the wrong symmetry properties as compared to an accurate wave function. We have recently proposed a theory, called multiconfiguration pair-density functional theory (MC-PDFT), in which the electronic kinetic energy and classical Coulomb energy are calculated from a multiconfiguration wave function with the correct symmetry properties, and the rest of the energy is calculated from a density functional, called the on-top density functional, that depends on the density and the on-top pair density calculated from this wave function. We also proposed a simple way to approximate the on-top density functional by translation of Kohn-Sham exchange-correlation functionals. The method is much less expensive than other post-SCF methods for calculating the dynamical correlation energy starting with a multiconfiguration self-consistent-field wave function as the reference wave function, and initial tests of the theory were quite encouraging. Here, we provide a broader test of the theory by applying it to bond energies of main-group molecules and transition metal complexes, barrier heights and reaction energies for diverse chemical reactions, proton affinities, and the water dimerization energy. Averaged over 56 data points, the mean unsigned error is 3.2 kcal/mol for MC-PDFT, as compared to 6.9 kcal/mol for Kohn-Sham theory with a comparable density functional. MC-PDFT is more accurate on average than complete active space second-order perturbation theory (CASPT2) for main-group small-molecule bond energies, alkyl bond dissociation energies, transition-metal-ligand bond energies, proton affinities, and the water dimerization energy. PMID:26574206
Multiple charge-density-wave transitions in single-crystalline Lu2Ir3Si5
NASA Astrophysics Data System (ADS)
Sangeetha, N. S.; Thamizhavel, A.; Tomy, C. V.; Basu, Saurabh; Awasthi, A. M.; Rajak, Piu; Bhattacharyya, Somnath; Ramakrishnan, S.; Pal, D.
2015-05-01
The physical properties of the single-crystalline samples of Lu2Ir3Si5 have been investigated by magnetic susceptibility, resistivity, and heat capacity studies. We observed multiple charge-density-wave (CDW) transitions in all the measurements. A strong thermal hysteresis at these transitions suggests a possible first order CDW ordering. In addition, the first order nature is ascertained by a very narrow and a huge cusp (62 J/mol K) in the zero field specific heat data which also suggests strong electron-phonon interchain coupling.
The Hagedorn spectrum, nuclear level densities and first order phase transitions
Moretto, Luciano G.; Larsen, A. C.; Guttormsen, M.; Siem, S.
2015-10-15
An exponential mass spectrum, like the Hagedorn spectrum, with slope 1/T{sub H} was interpreted as fixing an upper limiting temperature T{sub H} that the system can achieve. However, thermodynamically, such spectrum indicates a 1{sup st} order phase transition at a fixed temperature T{sub H}. A much lower energy example is the log linear level nuclear density below the neutron binding energy that prevails throughout the nuclear chart. We show that, for non-magic nuclei, such linearity implies a 1{sup st} order phase transition from the pairing superfluid to an ideal gas of quasi particles.
NASA Astrophysics Data System (ADS)
Zheng, Yongping; Xiao, Wei; Cho, Maenghyo; Cho, Kyeongjae
2013-10-01
Oxygen adsorption and dissociation on a pristine graphene, nitrogen doped graphene (N-graphene), and transition metal doped graphene (M-graphene) are studied with density functional theory calculations coupled with nudged elastic band (NEB) method. Four 3d transition metals (Fe, Co, Ni, and Cu) are selected as the doping atoms. The O binding energies on the Co-graphene and Ni-graphene have intermediate strength. The O2 dissociation barriers for these two types of doped graphenes are also lower than that on the pristine graphene and N-graphene. The Co and Ni doped graphenes are predicted to be promising ORR catalysts.
Density functional study of complete, first-order and critical wedge filling transitions.
Malijevský, Alexandr; Parry, Andrew O
2013-07-31
We present numerical studies of complete, first-order and critical wedge filling transitions, at a right angle corner, using a microscopic fundamental measure density functional theory. We consider systems with short-ranged, cut-off Lennard-Jones, fluid-fluid forces and two types of wall-fluid potential: a purely repulsive hard wall and also a long-ranged potential with three different strengths. For each of these systems we first determine the wetting properties occurring at a planar wall, including any wetting transition and the dependence of the contact angle on temperature. The hard wall corner is completely filled by vapour on approaching bulk coexistence and the numerical results for the growth of the meniscus thickness are in excellent agreement with effective Hamiltonian predictions for the critical exponents and amplitudes, at leading and next-to-leading order. In the presence of the attractive wall-fluid interaction, the corresponding planar wall-fluid interface exhibits a first-order wetting transition for each of the interaction strengths considered. In the right angle wedge geometry the two strongest interactions produce first-order filling transitions while for the weakest interaction strength, for which wetting and filling occur closest to the bulk critical point, the filling transition is second-order. For this continuous transition the critical exponent describing the divergence of the meniscus thickness is found to be in good agreement with effective Hamiltonian predictions. PMID:23836779
NASA Astrophysics Data System (ADS)
D'Escamard, Gabriella; De Rosa, Claudio; Auriemma, Finizia
2016-05-01
Crosslink sulfur density in rubber compounds and interactions in polymer blends are two of the composition elements that affect the rubber compound properties and glass transition temperature (Tg), which is a marker of polymer properties related to its applications. Natural rubber (NR), butadiene rubber (BR) and styrene-butadiene rubber (SBR) compounds were investigated using calorimetry (DSC) and dynamic mechanical analysis (DMA). The results indicate that the Di Marzio's and Schneider's Models predict with accuracy the dependence of Tg on crosslink density and composition in miscible blends, respectively, and that the two model may represent the base to study the relevant "in service" properties of real rubber compounds.
NASA Astrophysics Data System (ADS)
Zu, Mengjie; Liu, Jun; Tong, Hua; Xu, Ning
2016-08-01
We find that both continuous and discontinuous hexatic-liquid transitions can happen in the melting of two-dimensional solids of soft-core disks. For three typical model systems, Hertzian, harmonic, and Gaussian-core models, we observe the same scenarios. These systems exhibit reentrant crystallization (melting) with a maximum melting temperature Tm happening at a crossover density ρm. The hexatic-liquid transition at a density smaller than ρm is discontinuous. Liquid and hexatic phases coexist in a density interval, which becomes narrower with increasing temperature and tends to vanish approximately at Tm. Above ρm, the transition is continuous, in agreement with the Kosterlitz-Thouless-Halperin-Nelson-Young theory. For these soft-core systems, the nature of the hexatic-liquid transition depends on density (pressure), with the melting at ρm being a plausible transition point from discontinuous to continuous hexatic-liquid transition.
Zu, Mengjie; Liu, Jun; Tong, Hua; Xu, Ning
2016-08-19
We find that both continuous and discontinuous hexatic-liquid transitions can happen in the melting of two-dimensional solids of soft-core disks. For three typical model systems, Hertzian, harmonic, and Gaussian-core models, we observe the same scenarios. These systems exhibit reentrant crystallization (melting) with a maximum melting temperature T_{m} happening at a crossover density ρ_{m}. The hexatic-liquid transition at a density smaller than ρ_{m} is discontinuous. Liquid and hexatic phases coexist in a density interval, which becomes narrower with increasing temperature and tends to vanish approximately at T_{m}. Above ρ_{m}, the transition is continuous, in agreement with the Kosterlitz-Thouless-Halperin-Nelson-Young theory. For these soft-core systems, the nature of the hexatic-liquid transition depends on density (pressure), with the melting at ρ_{m} being a plausible transition point from discontinuous to continuous hexatic-liquid transition. PMID:27588868
Coherent structural dynamics of a prototypical charge-density-wave-to-metal transition.
Huber, T; Mariager, S O; Ferrer, A; Schäfer, H; Johnson, J A; Grübel, S; Lübcke, A; Huber, L; Kubacka, T; Dornes, C; Laulhe, C; Ravy, S; Ingold, G; Beaud, P; Demsar, J; Johnson, S L
2014-07-11
Using femtosecond time-resolved x-ray diffraction, we directly monitor the coherent lattice dynamics through an ultrafast charge-density-wave-to-metal transition in the prototypical Peierls system K(0.3)MoO(3) over a wide range of relevant excitation fluences. While in the low fluence regime we directly follow the structural dynamics associated with the collective amplitude mode; for fluences above the melting threshold of the electronic density modulation we observe a transient recovery of the periodic lattice distortion. We can describe these structural dynamics as a motion along the coordinate of the Peierls distortion triggered by the prompt collapse of electronic order after photoexcitation. The results indicate that the dynamics of a structural symmetry-breaking transition are determined by a high-symmetry excited state potential energy surface distinct from that of the initial low-temperature state. PMID:25062214
Fragment transition density method to calculate electronic coupling for excitation energy transfer
Voityuk, Alexander A.
2014-06-28
A general approach, the Fragment Transition Density (FTD) scheme, is introduced to estimate electronic coupling for excitation energy transfer in a molecular system. Within this method, the excitation energies and transition densities of the system are used to derive the coupling matrix element. The scheme allows one to treat systems where exciton donor and acceptor are close together and their exchange interaction and orbital overlap are significant. The FTD method can be applied in combination with any quantum mechanical approach to treat excited states of general nature including single-, double-, and higher excitations. Using FTD approach, we derive excitonic couplings for several systems computed with the CIS, TD DFT and MS-CASPT2 methods. In particular, it is shown that the estimated coupling values in DNA π-stacks are strongly affected by the short-range electronic interaction of adjacent nucleobases.
Coherent Structural Dynamics of a Prototypical Charge-Density-Wave-to-Metal Transition
NASA Astrophysics Data System (ADS)
Huber, T.; Mariager, S. O.; Ferrer, A.; Schäfer, H.; Johnson, J. A.; Grübel, S.; Lübcke, A.; Huber, L.; Kubacka, T.; Dornes, C.; Laulhe, C.; Ravy, S.; Ingold, G.; Beaud, P.; Demsar, J.; Johnson, S. L.
2014-07-01
Using femtosecond time-resolved x-ray diffraction, we directly monitor the coherent lattice dynamics through an ultrafast charge-density-wave-to-metal transition in the prototypical Peierls system K0.3MoO3 over a wide range of relevant excitation fluences. While in the low fluence regime we directly follow the structural dynamics associated with the collective amplitude mode; for fluences above the melting threshold of the electronic density modulation we observe a transient recovery of the periodic lattice distortion. We can describe these structural dynamics as a motion along the coordinate of the Peierls distortion triggered by the prompt collapse of electronic order after photoexcitation. The results indicate that the dynamics of a structural symmetry-breaking transition are determined by a high-symmetry excited state potential energy surface distinct from that of the initial low-temperature state.
Density-functional Monte-Carlo simulation of CuZn order-disorder transition
Khan, Suffian N.; Eisenbach, Markus
2016-01-25
We perform a Wang-Landau Monte Carlo simulation of a Cu0.5Zn0.5 order-disorder transition using 250 atoms and pairwise atom swaps inside a 5 x 5 x 5 BCC supercell. Each time step uses energies calculated from density functional theory (DFT) via the all-electron Korringa-Kohn- Rostoker method and self-consistent potentials. Here we find CuZn undergoes a transition from a disordered A2 to an ordered B2 structure, as observed in experiment. Our calculated transition temperature is near 870 K, comparing favorably to the known experimental peak at 750 K. We also plot the entropy, temperature, specific-heat, and short-range order as a function ofmore » internal energy.« less
Steam Reforming on Transition-metal Carbides from Density-functional Theory
Vojvodic, Aleksandra
2012-05-11
A screening study of the steam reforming reaction on clean and oxygen covered early transition-metal carbides surfaces is performed by means of density-functional theory calculations. It is found that carbides provide a wide spectrum of reactivities, from too reactive via suitable to too inert. Several molybdenum-based systems are identified as possible steam reforming catalysts. The findings suggest that carbides provide a playground for reactivity tuning, comparable to the one for pure metals.
Zhao, Zuofeng; Angell, C Austen
2016-02-12
The striking increases in response functions observed during supercooling of pure water have been the source of much interest and controversy. Imminent divergences of compressibility etc. unfortunately cannot be confirmed due to pre-emption by ice crystallization. Crystallization can be repressed by addition of second components, but these usually destroy the anomalies of interest. Here we study systems in which protic ionic liquid second components dissolve ideally in water, and ice formation is avoided without destroying the anomalies. We observe a major heat capacity spike during cooling, which is reversed during heating, and is apparently of first order. It occurs just before the glassy state is reached and is preceded by water-like density anomalies. We propose that it is the much-discussed liquid-liquid transition previously hidden by crystallization. Fast cooling should allow the important fluctuations/structures to be preserved in the glassy state for leisurely investigation. PMID:26756943
NASA Astrophysics Data System (ADS)
Lim, S. P.; Sheng, D. N.
2016-07-01
A many-body localized (MBL) state is a new state of matter emerging in a disordered interacting system at high-energy densities through a disorder-driven dynamic phase transition. The nature of the phase transition and the evolution of the MBL phase near the transition are the focus of intense theoretical studies with open issues in the field. We develop an entanglement density matrix renormalization group (En-DMRG) algorithm to accurately target highly excited states for MBL systems. By studying the one-dimensional Heisenberg spin chain in a random field, we demonstrate the accuracy of the method in obtaining energy eigenstates and the corresponding statistical results of quantum states in the MBL phase. Based on large system simulations by En-DMRG for excited states, we demonstrate some interesting features in the entanglement entropy distribution function, which is characterized by two peaks: one at zero and another one at the quantized entropy S =ln2 with an exponential decay tail on the S >ln2 side. Combining En-DMRG with exact diagonalization simulations, we demonstrate that the transition from the MBL phase to the delocalized ergodic phase is driven by rare events where the locally entangled spin pairs develop power-law correlations. The corresponding phase diagram contains an intermediate or crossover regime, which has power-law spin-z correlations resulting from contributions of the rare events. We discuss the physical picture for the numerical observations in this regime, where various distribution functions are distinctly different from results deep in the ergodic and MBL phases for finite-size systems. Our results may provide new insights for understanding the phase transition in such systems.
NASA Astrophysics Data System (ADS)
Dang, Hung T.; Ai, Xinyuan; Millis, Andrew J.; Marianetti, Chris A.
2014-09-01
The combination of density functional theory and single-site dynamical mean-field theory, using both Hartree and full continuous-time quantum Monte Carlo impurity solvers, is used to study the metal-insulator phase diagram of perovskite transition-metal oxides of the form ABO3 with a rare-earth ion A =Sr, La, Y and transition metal B =Ti, V, Cr. The correlated subspace is constructed from atomiclike d orbitals defined using maximally localized Wannier functions derived from the full p-d manifold; for comparison, results obtained using a projector method are also given. Paramagnetic DFT + DMFT computations using full charge self-consistency along with the standard "fully localized limit" (FLL) double counting are shown to incorrectly predict that LaTiO3, YTiO3, LaVO3, and SrMnO3 are metals. A more general examination of the dependence of physical properties on the mean p-d energy splitting, the occupancy of the correlated d states, the double-counting correction, and the lattice structure demonstrates the importance of charge-transfer physics even in the early transition-metal oxides and elucidates the factors underlying the failure of the standard approximations. If the double counting is chosen to produce a p-d splitting consistent with experimental spectra, single-site dynamical mean-field theory provides a reasonable account of the materials properties. The relation of the results to those obtained from "d-only" models in which the correlation problem is based on the frontier orbital p-d antibonding bands is determined. It is found that if an effective interaction U is properly chosen the d-only model provides a good account of the physics of the d1 and d2 materials.
Phase transitions and charge ordering in a square spin ice model with conserved monopole density
NASA Astrophysics Data System (ADS)
Xie, Yunlong; Zhou, Xiaohui; Liu, Jun-Ming
2015-03-01
Artificial spin ices represent a class of highly interested frustrated magnetic systems under intensive investigations for fascinating ground states and thermodynamics/dynamics of spin excitations in recent years. As one of these issues, magnetic charge ordering and the corresponding phase transitions in the two-dimensional system are emerging topics in condensed matter physics. In this work, we investigate all the monopole-ordered phases of the square spin ice model using the conserved monopole density algorithm. In low monopole density (ρ ~ 0), the Coulomb potential determines the monopoles' dynamics. We test the Coulomb's law in a two-dimension lattice and justify the monopole dimerization which is quite different from the three-dimensional pyrochlore spin ice. These monopole dimers are charge neutral, and the interactions between them have also been investigated using our algorithm. In the cases of high monopole density (ρ ~ 1), the system is similar to the dipolar kagome spin ice model, and our simulation results show that there exists an intermediate phase between the paramagnetic phase and the ordered magnetic phase. Such intermediate phase can be distinguished by the order of magnetic charges. In a cooling process, the system undergoes a two-stage magnetic phase transition before freezing to the long range magnetic ordered phase via a staggered charge ordering. Furthermore, a liquefaction process of monopole dimers can be justified upon the increasing effective internal pressure in the isothermal condition.
NASA Astrophysics Data System (ADS)
Kang, Joohoon; Seo, Jung-Woo T.; Alducin, Diego; Ponce, Arturo; Yacaman, Miguel Jose; Hersam, Mark C.
2014-11-01
Two-dimensional transition metal dichalcogenides have emerged as leading successors to graphene due to their diverse properties, which depend sensitively on sample thickness. Although solution-based exfoliation methods hold promise for scalable production of these materials, existing techniques introduce irreversible structural defects and/or lack sufficient control over the sample thickness. In contrast, previous work on carbon nanotubes and graphene has shown that isopycnic density gradient ultracentrifugation can produce structurally and electronically monodisperse nanomaterial populations. However, this approach cannot be directly applied to transition metal dichalcogenides due to their high intrinsic buoyant densities when encapsulated with ionic small molecule surfactants. Here, we overcome this limitation and thus demonstrate thickness sorting of pristine molybdenum disulfide (MoS2) by employing a block copolymer dispersant composed of a central hydrophobic unit flanked by hydrophilic chains that effectively reduces the overall buoyant density in aqueous solution. The resulting solution-processed monolayer MoS2 samples exhibit strong photoluminescence without further chemical treatment.
Kang, Joohoon; Seo, Jung-Woo T; Alducin, Diego; Ponce, Arturo; Yacaman, Miguel Jose; Hersam, Mark C
2014-01-01
Two-dimensional transition metal dichalcogenides have emerged as leading successors to graphene due to their diverse properties, which depend sensitively on sample thickness. Although solution-based exfoliation methods hold promise for scalable production of these materials, existing techniques introduce irreversible structural defects and/or lack sufficient control over the sample thickness. In contrast, previous work on carbon nanotubes and graphene has shown that isopycnic density gradient ultracentrifugation can produce structurally and electronically monodisperse nanomaterial populations. However, this approach cannot be directly applied to transition metal dichalcogenides due to their high intrinsic buoyant densities when encapsulated with ionic small molecule surfactants. Here, we overcome this limitation and thus demonstrate thickness sorting of pristine molybdenum disulfide (MoS2) by employing a block copolymer dispersant composed of a central hydrophobic unit flanked by hydrophilic chains that effectively reduces the overall buoyant density in aqueous solution. The resulting solution-processed monolayer MoS2 samples exhibit strong photoluminescence without further chemical treatment. PMID:25391315
Observation and modeling of deflagration-to-detonation (DDT) transition in low-density HMX
NASA Astrophysics Data System (ADS)
Tringe, Joseph; Vandersall, Kevin; Reaugh, Jack; Levie, Harold; Henson, Bryan; Smilowitz, Laura; Parker, Gary
2015-06-01
We employ simultaneous flash x-ray radiography and streak imaging, together with a multi-phase finite element model, to understand deflagration-to-detonation transition (DDT) phenomena in low-density (~ 1.2 gm/cm3) powder of the explosive cyclotetramethylene-tetranitramine (HMX). HMX powder was lightly hand-tamped in a 12.7 mm diameter column, relatively lightly-confined in an optically-transparent polycarbonate cylinder with wall thickness 25.4 mm. We observe apparent compaction of the powder in advance of the detonation transition, both by x-ray contrast and by the motion of small steel spheres pre-emplaced throughout the length of explosive. High-speed imaging along the explosive cylinder length provides a temporally continuous record of the transition that is correlated with the high-resolution x-ray image record. Preliminary simulation of these experiments with the HERMES model implemented in the ALE3D code enables improved understanding of the explosive particle burning, compaction and detonation phenomena which are implied by the observed reaction rate and transition location within the cylinder. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
Luo, Da-Wei; Xu, Jing-Bo
2015-03-15
We use an alternative method to investigate the quantum criticality at zero and finite temperature using trace distance along with the density matrix renormalization group. It is shown that the average correlation measured by the trace distance between the system block and environment block in a DMRG sweep is able to detect the critical points of quantum phase transitions at finite temperature. As illustrative examples, we study spin-1 XXZ chains with uniaxial single-ion-type anisotropy and the Heisenberg spin chain with staggered coupling and external magnetic field. It is found that the trace distance shows discontinuity at the critical points of quantum phase transition and can be used as an indicator of QPTs.
Transition-density-fragment interaction approach for exciton-coupled circular dichroism spectra
NASA Astrophysics Data System (ADS)
Fujimoto, Kazuhiro J.
2010-09-01
A transition-density-fragment interaction (TDFI) method for exciton-coupled circular dichroism (ECCD) spectra is proposed. The TDFI method was previously developed for excitation-energy transfer, which led to the successful estimation of the electronic coupling energy between donor and accepter molecules in xanthorhodopsin [K. J. Fujimoto and S. Hayashi, J. Am. Chem. Soc. 131, 14152 (2009)]. In the present study, the TDFI scheme is extended to the ECCD spectral calculation based on the matrix method and is applied to a dimerized retinal (all-trans N-retinylidene-L-alanine Schiff base) chromophore. Compared with the dipole-dipole and transition charge from ESP methods, TDFI has a much improved description of the electronic coupling. In addition, the matrix method combined with TDFI can reduce the computational costs compared with the full quantum-mechanical calculation. These advantages of the present method make it possible to accurately evaluate the CD Cotton effects observed in experiment.
Possibility of charge density wave transition in a SrPt2Sb2 superconductor
NASA Astrophysics Data System (ADS)
Ibuka, Soshi; Imai, Motoharu
2016-04-01
The first-order transition at T 0 = 270 K for the platinum-based SrPt2Sb2 superconductor was investigated using x-ray diffraction and magnetic susceptibility measurements. When polycrystalline SrPt2Sb2 was cooled down through T 0, the structure was transformed from monoclinic to a modulated orthorhombic structure, and no magnetic order was formed, which illustrates the possibility of a charge density wave (CDW) transition at T 0. SrPt2Sb2 can thus be a new example to examine the interplay of CDW and superconductivity in addition to SrPt2As2, BaPt2As2, and LaPt2Si2. It is unique that the average structure of the low-temperature phase has higher symmetry than that of the high-temperature phase.
Density of states at disorder-induced phase transitions in a multichannel Majorana wire
NASA Astrophysics Data System (ADS)
Rieder, Maria-Theresa; Brouwer, Piet W.
2014-11-01
An N -channel spinless p -wave superconducting wire is known to go through a series of N topological phase transitions upon increasing the disorder strength. Here, we show that at each of those transitions the density of states shows a Dyson singularity ν (ɛ ) ∝ɛ-1|lnɛ| -3 , whereas ν (ɛ ) ∝ɛ|α |-1 has a power-law singularity for small energies ɛ away from the critical points. Using the concept of "superuniversality" [Gruzberg et al., Phys. Rev. B 71, 245124 (2005), 10.1103/PhysRevB.71.245124], we are able to relate the exponent α to the wire's transport properties at zero energy and, hence, to the mean free path l and the superconducting coherence length ξ .
NASA Astrophysics Data System (ADS)
Shim, Hyungjoon; Lee, Geunseop; Hyun, Jung-Min; Kim, Hanchul
2015-09-01
Impurities interact with a charge density wave (CDW) and affect the phase transitions in low-dimensional systems. By using scanning tunneling microscopy, we visualize the interaction between oxygen impurities and the CDW in indium atomic wires on Si(111), a prototypical one-dimensional electronic system, and unveil the microscopic mechanism of the intriguing O-induced increase of the transition temperature (Tc). Driven by the fluctuating CDW, the O atoms adopt an asymmetric structure. By adjusting the asymmetry, a pair of O impurities in close distance can pin the one-dimensional CDW, which develops into the two-dimensional domains. First-principles calculations showed that the asymmetric interstitially-incorporated O defects induce shear strains, which assists the formation of hexagon structure of the CDW phase. The cooperative interplay between the O impurities and the CDW is responsible for the enhancement of the CDW condensation and the consequent increase in Tc.
Gorissen, Mieke; Hooyberghs, Jef; Vanderzande, Carlo
2009-02-01
Cumulants of a fluctuating current can be obtained from a free-energy-like generating function, which for Markov processes equals the largest eigenvalue of a generalized generator. We determine this eigenvalue with the density-matrix renormalization group for stochastic systems. We calculate the variance of the current in the different phases, and at the phase transitions, of the totally asymmetric exclusion process. Our results can be described in the terms of a scaling ansatz that involves the dynamical exponent z . We also calculate the generating function of the dynamical activity (total number of configuration changes) near the absorbing-state transition of the contact process. Its scaling properties can be expressed in terms of known critical exponents. PMID:19391693
Possibility of charge density wave transition in a SrPt2Sb2 superconductor.
Ibuka, Soshi; Imai, Motoharu
2016-04-27
The first-order transition at T 0 = 270 K for the platinum-based SrPt2Sb2 superconductor was investigated using x-ray diffraction and magnetic susceptibility measurements. When polycrystalline SrPt2Sb2 was cooled down through T 0, the structure was transformed from monoclinic to a modulated orthorhombic structure, and no magnetic order was formed, which illustrates the possibility of a charge density wave (CDW) transition at T 0. SrPt2Sb2 can thus be a new example to examine the interplay of CDW and superconductivity in addition to SrPt2As2, BaPt2As2, and LaPt2Si2. It is unique that the average structure of the low-temperature phase has higher symmetry than that of the high-temperature phase. PMID:27023674
NASA Astrophysics Data System (ADS)
Porer, M.; Ménard, J.-M.; Poellmann, C.; Dachraoui, H.; Mouchliadis, L.; Perakis, I. E.; Heinzmann, U.; Demsar, J.; Rossnagel, K.; Galopin, E.; Lemaître, A.; Amo, A.; Bloch, J.; Huber, R.
2016-05-01
Many-body correlation effects in complex quantum systems often lead to phase transitions that bear great technological potential. However, the underlying microscopic driving mechanisms or even the quantum-mechanical properties of the novel ground state often remain elusive. Here we employ phase-locked ultrabroadband terahertz (THz) pulses to disentangle two coexisting orders in the charge density wave phase 1T-TiSe2 via their individual non-equilibrium multi- THz dynamics. Furthermore, we demonstrate that few-cycle THz pulses can project out the matter part of a transient cold exciton-polariton condensate, providing novel insights into the very nature of this macroscopic quantum state.
Stoessel, J.P.; Wolynes, P.G.
1989-01-01
With analogy to the ''highly accurate'' summation of cluster diagrams for hard sphere fluids a la Carnahan-Starling, we present a simple, real space free energy density functional for arbitrary potential systems, based on the generalization of the second virial coefficient to inhomogeneous systems which, when applied to hard sphere, soft-sphere, and Lennard-Jones freezing, yield melting characteristics in remarkable agreement with experiment. Implications for the liquid-glass transition in all three potential systems are also presented. 45 refs., 7 figs., 1 tab.
NASA Technical Reports Server (NTRS)
Ingels, F.; Schoggen, W. O.
1981-01-01
Several methods for increasing bit transition densities in a data stream are summarized, discussed in detail, and compared against constraints imposed by the 2 MHz data link of the space shuttle high rate multiplexer unit. These methods include use of alternate pulse code modulation waveforms, data stream modification by insertion, alternate bit inversion, differential encoding, error encoding, and use of bit scramblers. The psuedo-random cover sequence generator was chosen for application to the 2 MHz data link of the space shuttle high rate multiplexer unit. This method is fully analyzed and a design implementation proposed.
Precise Measurements of the Density and Critical Phenomena Near Phase Transitions in Liquid Helium
NASA Technical Reports Server (NTRS)
Yeh, Nai-Chang
1997-01-01
The first-year progress for the project of precise measurements of the density and critical phenomena of helium near phase transitions is summarized below: (1) completion of a cryogenic sample probe for the proposed measurements, and the rehabilitation of a designated laboratory at Caltech for this project; (2) construction and testing of a superconducting niobium cavity; (3) acquisition of one phase-locked-loop system for high-resolution frequency control and read- out; (4) setting up high-resolution thermometry (HRT) for temperature readout and control; (5) developing new approaches for calibrating the coefficient between the resonant frequency shift (delta f) and the helium density (rho), as well as for measuring the effect of gravity on T(sub lambda) to a much better precision; (6) programming of the interface control of all instruments for automatic data acquisition; and (7) improving data analyses and fitting procedures.
Truncated Transition Densities for Analysis of (Nonlinear) Optical Properties of carbo-Chromophores.
Poidevin, Corentin; Lepetit, Christine; Ben Amor, Nadia; Chauvin, Remi
2016-08-01
The optical properties of several quadrupolar carbo-benzene derivatives are investigated at various levels of calculation (TDDFT and CASPT2) and analyzed using a new theoretical tool here disclosed: The "visualization" of the transition dipole moment from the transition density truncated to the main monoexcitations involved in the electronic transition (TTD). The experimental or calculated one-photon UV-visible absorption spectra of the carbo-benzene derivatives fit with the Gouterman model originally proposed for porphyrins, where the first four excited states involve linear combinations of monoexcitations of the same four frontier molecular orbitals. The relative intensities of the absorption bands are analyzed from the transition dipole moments calculated from the TTDs and an analogy between porphyrins and carbo-benzenes is argued. The two-photon absorption (TPA) cross section related to the third-order nonlinear optical response is calculated for each two-photon-allowed excited state |f⟩ from the contribution of all possible intermediate excited states |i⟩ using the "sum-over-state" (SOS) scheme. The quadrupolar carbo-benzene derivatives fit into the three-level model, as their TPA cross section exhibits a dominant contribution of one of the intermediate excited states. The origin of TPA efficiency (enhancement) upon carbo-merisation of the C-C link to the para-substituents is discussed from the excitation energies of the intermediate and final excited states and from the two corresponding transition dipole moments (μ0i and μif). The latter may be calculated from the TTDs. PMID:27359162
Securing the Extremely Low-Densities of Low-Mass Planets Characterized by Transit Timing Variations
NASA Astrophysics Data System (ADS)
Ford, Eric B.
2015-12-01
Transit timing variations (TTVs) provide an excellent tool to characterize the masses and orbits of dozens of small planets, including many at orbital periods beyond the reach of both Doppler surveys and photoevaporation-induced atmospheric loss. Dynamical modeling of these systems has identified low-mass planets with surprisingly large radii and low densities (e.g., Kepler-79d, Jontof-Hutter et al. 2014; Kepler-51, Masuda 2014; Kepler-87c, Ofir et al. 2014). Additional low-density, low-mass planets will likely become public before ESS III (Jontof-Hutter et al. in prep). Collectively, these results suggest that very low density planets with masses of 2-6 MEarth are not uncommon in compact multiple planet systems. Some astronomers have questioned whether there could be an alternative interpretation of the TTV observations. Indeed, extraordinary claims require extraordinary evidence. While the physics of TTVs is rock solid, the statistical analysis of Kepler observations can be challenging, due to the complex interactions between model parameters and high-dimensional parameter spaces that must be explored. We summarize recent advances in computational statistics that enable robust characterization of planetary systems using TTVs. We present updated analyses of a few particularly interesting systems and discuss the implications for the robustness of extremely low densities for low-mass planets. Such planets pose an interesting challenge for planet formation theory and are motivating detailed theoretical studies (e.g., Lee & Chiang 2015 and associated ESS III abstracts).
Song, Yun S.; Steinrücken, Matthias
2012-01-01
The transition density function of the Wright–Fisher diffusion describes the evolution of population-wide allele frequencies over time. This function has important practical applications in population genetics, but finding an explicit formula under a general diploid selection model has remained a difficult open problem. In this article, we develop a new computational method to tackle this classic problem. Specifically, our method explicitly finds the eigenvalues and eigenfunctions of the diffusion generator associated with the Wright–Fisher diffusion with recurrent mutation and arbitrary diploid selection, thus allowing one to obtain an accurate spectral representation of the transition density function. Simplicity is one of the appealing features of our approach. Although our derivation involves somewhat advanced mathematical concepts, the resulting algorithm is quite simple and efficient, only involving standard linear algebra. Furthermore, unlike previous approaches based on perturbation, which is applicable only when the population-scaled selection coefficient is small, our method is nonperturbative and is valid for a broad range of parameter values. As a by-product of our work, we obtain the rate of convergence to the stationary distribution under mutation–selection balance. PMID:22209899
Gazzadi, Gian Carlo; Frabboni, Stefano
2015-01-01
Suspended nanowires (SNWs) have been deposited from Co-carbonyl precursor (Co2(CO)8) by focused electron beam induced deposition (FEBID). The SNWs dimensions are about 30-50 nm in diameter and 600-850 nm in length. The as-deposited material has a nanogranular structure of mixed face-centered cubic (FCC) and hexagonal close-packed (HCP) Co phases, and a composition of 80 atom % Co, 15 atom % O and 5 atom % C, as revealed by transmission electron microscopy (TEM) analysis and by energy-dispersive X-ray (EDX) spectroscopy, respectively. Current (I)-voltage (V) measurements with current densities up to 10(7) A/cm(2) determine different structural transitions in the SNWs, depending on the I-V history. A single measurement with a sudden current burst leads to a polycrystalline FCC Co structure extended over the whole wire. Repeated measurements at increasing currents produce wires with a split structure: one half is polycrystalline FCC Co and the other half is graphitized C. The breakdown current density is found at 2.1 × 10(7) A/cm(2). The role played by resistive heating and electromigration in these transitions is discussed. PMID:26199833
NASA Astrophysics Data System (ADS)
Dang, Hung
2015-03-01
Recently, the combination of density functional theory (DFT) and dynamical mean-field theory (DMFT) has become a widely-used beyond-mean-field approach for strongly correlated materials. However, not only is the correlation treated in DMFT but also in DFT to some extent, a problem arises as the correlation is counted twice in the DFT+DMFT framework. The correction for this problem is still not well-understood. To gain more understanding of this ``double counting'' problem, I provide a detailed study of the metal-insulator transition in transition metal oxides in the subspace of oxygen p and transition metal correlated d orbitals using DFT+DMFT. I will show that the fully charge self-consistent DFT+DMFT calculations with the standard ``fully-localized limit'' (FLL) double counting correction fail to predict correctly materials such as LaTiO3, LaVO3, YTiO3 and SrMnO3 as insulators. Investigations in a wide range of the p- d splitting, the d occupancy, the lattice structure and the double counting correction itself will be presented to understand the reason behind this failure. I will also show that if the double counting correction is chosen to reproduce the p- d splitting consistent with experimental data, the DFT+DMFT approach can still give reasonable results in comparison with experiments.
Fanarraga, M.L.
2009-02-01
Microglia are highly dynamic cells of the CNS that continuously survey the welfare of the neural parenchyma and play key roles modulating neurogenesis and neuronal cell death. In response to injury or pathogen invasion parenchymal microglia transforms into a more active cell that proliferates, migrates and behaves as a macrophage. The acquisition of these extra skills implicates enormous modifications of the microtubule and actin cytoskeletons. Here we show that tubulin cofactor B (TBCB), which has been found to contribute to various aspects of microtubule dynamics in vivo, is also implicated in microglial cytoskeletal changes. We find that TBCB is upregulated in post-lesion reactive parenchymal microglia/macrophages, in interferon treated BV-2 microglial cells, and in neonate amoeboid microglia where the microtubule densities are remarkably low. Our data demonstrate that upon TBCB downregulation both, after microglia differentiation to the ramified phenotype in vivo and in vitro, or after TBCB gene silencing, microtubule densities are restored in these cells. Taken together these observations support the view that TBCB functions as a microtubule density regulator in microglia during activation, and provide an insight into the understanding of the complex mechanisms controlling microtubule reorganization during microglial transition between the amoeboid, ramified, and reactive phenotypes.
Koperwas, K. Grzybowski, A.; Grzybowska, K.; Wojnarowska, Z.; Paluch, M.
2015-07-14
In this paper, we define and experimentally verify thermodynamic characteristics of the liquid-glass transition, taking into account a kinetic origin of the process. Using the density scaling law and the four-point measure of the dynamic heterogeneity of molecular dynamics of glass forming liquids, we investigate contributions of enthalpy, temperature, and density fluctuations to spatially heterogeneous molecular dynamics at the liquid-glass transition, finding an equation for the pressure coefficient of the glass transition temperature, dTg/dp. This equation combined with our previous formula for dTg/dp, derived solely from the density scaling criterion, implies a relationship among thermodynamic coefficients at Tg. Since this relationship and both the equations for dTg/dp are very well validated using experimental data at Tg, they are promising alternatives to the classical Prigogine-Defay ratio and both the Ehrenfest equations in case of the liquid-glass transition.
Siclari, Francesca; Bernardi, Giulio; Riedner, Brady A.; LaRocque, Joshua J.; Benca, Ruth M.; Tononi, Giulio
2014-01-01
Objectives: To assess how the characteristics of slow waves and spindles change in the falling-asleep process. Design: Participants undergoing overnight high-density electroencephalographic recordings were awakened at 15- to 30-min intervals. One hundred forty-one falling-asleep periods were analyzed at the scalp and source level. Setting: Sleep laboratory. Participants: Six healthy participants. Interventions: Serial awakenings. Results: The number and amplitude of slow waves followed two dissociated, intersecting courses during the transition to sleep: slow wave number increased slowly at the beginning and rapidly at the end of the falling-asleep period, whereas amplitude at first increased rapidly and then decreased linearly. Most slow waves occurring early in the transition to sleep had a large amplitude, a steep slope, involved broad regions of the cortex, predominated over frontomedial regions, and preferentially originated from the sensorimotor and the posteromedial parietal cortex. Most slow waves occurring later had a smaller amplitude and slope, involved more circumscribed parts of the cortex, and had more evenly distributed origins. Spindles were initially sparse, fast, and involved few cortical regions, then became more numerous and slower, and involved more areas. Conclusions: Our results provide evidence for two types of slow waves, which follow dissociated temporal courses in the transition to sleep and have distinct cortical origins and distributions. We hypothesize that these two types of slow waves result from two distinct synchronization processes: (1) a “bottom-up,” subcorticocortical, arousal system-dependent process that predominates in the early phase and leads to type I slow waves, and (2) a “horizontal,” corticocortical synchronization process that predominates in the late phase and leads to type II slow waves. The dissociation between these two synchronization processes in time and space suggests that they may be differentially
HATS-8b: A Low-density Transiting Super-Neptune
NASA Astrophysics Data System (ADS)
Bayliss, D.; Hartman, J. D.; Bakos, G. Á.; Penev, K.; Zhou, G.; Brahm, R.; Rabus, M.; Jordán, A.; Mancini, L.; de Val-Borro, M.; Bhatti, W.; Espinoza, N.; Csubry, Z.; Howard, A. W.; Fulton, B. J.; Buchhave, L. A.; Henning, T.; Schmidt, B.; Ciceri, S.; Noyes, R. W.; Isaacson, H.; Marcy, G. W.; Suc, V.; Lázár, J.; Papp, I.; Sári, P.
2015-08-01
HATS-8b is a low density transiting super-Neptune discovered as part of the HATSouth project. The planet orbits its solar-like G-dwarf host (V = 14.03+/- 0.10, {T}{eff} = 5679+/- 50 K) with a period of 3.5839 days. HATS-8b is the third lowest-mass transiting exoplanet to be discovered from a wide-field ground-based search, and with a mass of 0.138+/- 0.019 {M}{{J}} it is approximately halfway between the masses of Neptune and Saturn. However, HATS-8b has a radius of {0.873}-0.075+0.123 {R}{{J}}, resulting in a bulk density of just 0.259+/- 0.091 {{g}} {{cm}}-3. The metallicity of the host star is super-solar ([{Fe}/{{H}}] = 0.210+/- 0.080), providing evidence against the idea that low-density exoplanets form from metal-poor environments. The low density and large radius of HATS-8b results in an atmospheric scale height of almost 1000 km, and in addition to this there is an excellent reference star of nearly equal magnitude at just 19″ separation in the sky. These factors make HATS-8b an exciting target for future atmospheric characterization studies, particularly for long-slit transmission spectroscopy. The HATSouth network is operated by a collaboration consisting of Princeton University (PU), the Max Planck Institute für Astronomie (MPIA), the Australian National University (ANU), and the Pontificia Universidad Católica de Chile (PUC). The station at Las Campanas Observatory (LCO) of the Carnegie Institute is operated by PU in conjunction with PUC, the station at the High Energy Spectroscopic Survey site is operated in conjunction with MPIA, and the station at Siding Spring Observatory is operated jointly with ANU. This paper includes data gathered with the 6.5 m Magellan Telescopes located in LCO, Chile. The work is based in part on observations made with the MPG 2.2 m Telescope and the ESO 3.6 m Telescope at the ESO Observatory in La Silla. This paper uses observations obtained using the facilities of the Las Cumbres Observatory Global Telescope.
New density functional approach for solid-liquid-vapor transitions in pure materials.
Kocher, Gabriel; Provatas, Nikolas
2015-04-17
A new phase field crystal (PFC) type theory is presented, which accounts for the full spectrum of solid-liquid-vapor phase transitions within the framework of a single density order parameter. Its equilibrium properties show the most quantitative features to date in PFC modeling of pure substances, and full consistency with thermodynamics in pressure-volume-temperature space is demonstrated. A method to control either the volume or the pressure of the system is also introduced. Nonequilibrium simulations show that 2- and 3-phase growth of solid, vapor, and liquid can be achieved, while our formalism also allows for a full range of pressure-induced transformations. This model opens up a new window for the study of pressure driven interactions of condensed phases with vapor, an experimentally relevant paradigm previously missing from phase field crystal theories. PMID:25933321
New Density Functional Approach for Solid-Liquid-Vapor Transitions in Pure Materials
NASA Astrophysics Data System (ADS)
Kocher, Gabriel; Provatas, Nikolas
2015-04-01
A new phase field crystal (PFC) type theory is presented, which accounts for the full spectrum of solid-liquid-vapor phase transitions within the framework of a single density order parameter. Its equilibrium properties show the most quantitative features to date in PFC modeling of pure substances, and full consistency with thermodynamics in pressure-volume-temperature space is demonstrated. A method to control either the volume or the pressure of the system is also introduced. Nonequilibrium simulations show that 2- and 3-phase growth of solid, vapor, and liquid can be achieved, while our formalism also allows for a full range of pressure-induced transformations. This model opens up a new window for the study of pressure driven interactions of condensed phases with vapor, an experimentally relevant paradigm previously missing from phase field crystal theories.
Dense attosecond electron sheets from laser wakefields using an up-ramp density transition.
Li, F Y; Sheng, Z M; Liu, Y; Meyer-ter-Vehn, J; Mori, W B; Lu, W; Zhang, J
2013-03-29
Controlled electron injection into a laser-driven wakefield at a well defined space and time is reported based on particle-in-cell simulations. Key novel ingredients are an underdense plasma target with an up-ramp density profile followed by a plateau and a fairly large laser focus diameter that leads to an essentially one-dimensional (1D) regime of laser wakefield, which is different from the bubble (complete blowout) regime occurring for tightly focused drive beams. The up-ramp profile causes 1D wave breaking to occur sharply at the up-ramp-plateau transition. As a result, it generates an ultrathin (few nanometer, corresponding to attosecond duration), strongly overdense relativistic electron sheet that is injected and accelerated in the wakefield. A peaked electron energy spectrum and high charge (∼nC) distinguish the final sheet. PMID:23581329
Thickness dependence of the charge-density-wave transition temperature in VSe{sub 2}
Yang, Jiyong; Liu, Yan; Du, Haifeng; Ning, Wei; Zheng, Guolin; Jin, Chiming; Han, Yuyan; Wang, Ning; Tian, Mingliang Zhang, Yuheng; Wang, Weike; Yang, Zhaorong
2014-08-11
A set of three-dimensional charge-density-wave (3D CDW) VSe{sub 2} nano-flakes with different thicknesses were obtained by the scotch tape-based micro-mechanical exfoliation method. Resistivity measurements showed that the 3D CDW transition temperature T{sub p} decreases systematically from 105 K in bulk to 81.8 K in the 11.6 nm thick flake. The Hall resistivity ρ{sub xy} of all the flakes showed a linear dependent behavior against the magnetic field with a residual electron concentration of the order of ∼10{sup 21} cm{sup −3} at 5 K. The electron concentration n increases slightly as the thickness d decreases, possibly due to the CDW gap is reduced with the decrease of the thickness.
Weinberger, Christopher R.; Tucker, Garritt J.; Foiles, Stephen M.
2013-02-01
It is well known that screw dislocation motion dominates the plastic deformation in body-centered-cubic metals at low temperatures. The nature of the nonplanar structure of screw dislocations gives rise to high lattice friction, which results in strong temperature and strain rate dependence of plastic flow. Thus the nature of the Peierls potential, which is responsible for the high lattice resistance, is an important physical property of the material. However, current empirical potentials give a complicated picture of the Peierls potential. Here, we investigate the nature of the Peierls potential using density functional theory in the bcc transition metals. The results show that the shape of the Peierls potential is sinusoidal for every material investigated. Furthermore, we show that the magnitude of the potential scales strongly with the energy per unit length of the screw dislocation in the material.
Calle-Vallejo, Federico; Martínez, José Ignacio; Rossmeisl, Jan
2011-09-14
Low-temperature fuel cells are appealing alternatives to the conventional internal combustion engines for transportation applications. However, in order for them to be commercially viable, effective, stable and low-cost electrocatalysts are needed for the Oxygen Reduction Reaction (ORR) at the cathode. In this contribution, on the basis of Density Functional Theory (DFT) calculations, we show that graphitic materials with active sites composed of 4 nitrogen atoms and transition metal atoms belonging to groups 7 to 9 in the periodic table are active towards ORR, and also towards Oxygen Evolution Reaction (OER). Spin analyses suggest that the oxidation state of those elements in the active sites should in general be +2. Moreover, our results verify that the adsorption behavior of transition metals is not intrinsic, since it can be severely altered by changes in the local geometry of the active site, the chemical nature of the nearest neighbors, and the oxidation states. Nonetheless, we find that these catalysts trend-wise behave as oxides and that their catalytic activity is limited by exactly the same universal scaling relations. PMID:21796295
Hybrid Hartree-Fock density functional study of transition-metal doped ZnO
NASA Astrophysics Data System (ADS)
Betancourt, Jesuan; Kalitsov, Alan; Velev, Julian
2012-02-01
Dilute magnetic semiconductors (DMS) obtained by doping semiconductors with transition metals (TM) hold much promise for spintronics. Transition metal doped ZnO (ZnO:TM) has been investigated for a possible room-temperature DMS. Density functional theory gives incorrect prediction for the band gap of ZnO which leads to diverging interpretations for the magnetic behavior of ZnO:TM [1,2]. Here we report Heyd-Scuseria-Ernzerhof (HSE) hybrid functional study of the electronic structure of ZnO:TM (TM=Cu, Ni, Co, Fe, Mn). The hybrid functional corrects for both the bandgap problem on the host and the lack of correlation in the impurity, without the use of ad-hoc intra-atomic potentials. Our results show although the HSE opens the band gap of the host, the Hubbard splitting of the impurity levels makes the empty impurity levels reside in the host conduction band. This leaves open the possibility for spin polarized carriers. We discuss the validity of the results and explore their implications for the magnetic behavior of ZnO:TM. [1] H. Raebiger, S. Lany, and A. Zunger, Physical Review B 79, 165202 (2009). [2] P. Gopal and N. A. Spaldin, Phys.l Review B 74, 094418 (2006).
Multiple charge density wave transitions in the antiferromagnets R NiC2 (R =Gd ,Tb)
NASA Astrophysics Data System (ADS)
Shimomura, S.; Hayashi, C.; Hanasaki, N.; Ohnuma, K.; Kobayashi, Y.; Nakao, H.; Mizumaki, M.; Onodera, H.
2016-04-01
X-ray scattering and electrical resistivity measurements were performed on GdNiC2 and TbNiC2. We found a set of satellite peaks characterized by q1=(0.5 ,η ,0 ) below T1, at which the resistivity shows a sharp inflection, suggesting the charge density wave (CDW) formation. The value of η decreases with decreasing temperature below T1, and then a transition to a commensurate phase with q1 C=(0.5 ,0.5 ,0 ) takes place. The diffuse scattering observed above T1 indicates the presence of soft phonon modes associated with CDW instabilities at q1 and q2=(0.5 ,0.5 ,0.5 ) . The long-range order given by q2 is developed in addition to that given by q1 C in TbNiC2, while the short-range correlation with q2 persists even at 6 K in GdNiC2. The amplitude of the q1 C lattice modulation is anomalously reduced below an antiferromagnetic transition temperature TN in GdNiC2. In contrast, the q2 order vanishes below TN in TbNiC2. We demonstrate that R NiC2 (R = rare earth) compounds exhibit similarities with respect to their CDW phenomena, and discuss the effects of magnetic transitions on CDWs. We offer a possible displacement pattern of the modulated structure characterized by q1 C and q2 in terms of frustration.
NASA Astrophysics Data System (ADS)
Chevrier, V. L.; Ong, S. P.; Armiento, R.; Chan, M. K. Y.; Ceder, G.
2010-08-01
We compare the accuracy of conventional semilocal density functional theory (DFT), the DFT+U method, and the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional for structural parameters, redox reaction energies, and formation energies of transition metal compounds. Conventional DFT functionals significantly underestimate redox potentials for these compounds. Zhou [Phys. Rev. B 70, 235121 (2004)]10.1103/PhysRevB.70.235121 addressed this issue with DFT+U and a linear-response scheme for calculating U values. We show that the Li intercalation potentials of prominent Li-ion intercalation battery materials, such as the layered LixMO2 ( M=Co and Ni), LixTiS2 ; olivine LixMPO4 ( M=Mn , Fe, Co, and Ni); and spinel-like LixMn2O4 , LixTi2O4 , are also well reproduced by HSE06, due to the self-interaction error correction from the partial inclusion of Hartree-Fock exchange. For formation energies, HSE06 performs well for transition metal compounds, which typically are not well reproduced by conventional DFT functionals but does not significantly improve the results of nontransition metal oxides. Hence, we find that hybrid functionals provide a good alternative to DFT+U for transition metal applications when the large extra computational effort is compensated by the benefits of (i) avoiding species-specific adjustable parameters and (ii) a more universal treatment of the self-interaction error that is not exclusive to specific atomic orbital projections on selected ions.
Density slope of the nuclear symmetry energy from the neutron skin thickness of heavy nuclei
Chen Liewen; Ko Che Ming; Xu Jun; Li Baoan
2010-08-15
Expressing explicitly the parameters of the standard Skyrme interaction in terms of the macroscopic properties of asymmetric nuclear matter, we show in the Skyrme-Hartree-Fock approach that unambiguous correlations exist between observables of finite nuclei and nuclear matter properties. We find that existing data on neutron skin thickness {Delta}r{sub np} of Sn isotopes give an important constraint on the symmetry energy E{sub sym}({rho}{sub 0}) and its density slope L at saturation density {rho}{sub 0}. Combining these constraints with those from recent analyses of isospin diffusion and the double neutron/proton ratio in heavy-ion collisions at intermediate energies leads to a more stringent limit on L approximately independent of E{sub sym}({rho}{sub 0}). The implication of these new constraints on the {Delta}r{sub np} of {sup 208}Pb as well as the core-crust transition density and pressure in neutron stars is discussed.
Collins, Patricia A; Agarwal, Ajay
2015-01-01
Public transit ridership offers valuable opportunities for modest amounts of daily physical activity (PA). Transit is a more feasible option for most Canadian commuters who live too far from work to walk or cycle, yet public transit usage in midsized Canadian cities has historically remained low due to inefficient transit service. The objectives of this longitudinal study were threefold: to assess whether the introduction of express transit service in the low-density city of Kingston, Ontario, has translated to greater transit use among a targeted employee group; to document the characteristics of those employees that have shifted to transit; and to examine the PA levels of employees using transit compared to other commute modes. An online survey was administered in October 2013 and October 2014 to all non-student employees at Queen's University. 1356 employees completed the survey in 2013, and 1123 in 2014; 656 of these employees completed the survey both years, constituting our longitudinal sample. Year-round transit ridership increased from 5.5% in 2013 to 8.5% in 2014 (p < 0.001). Employees who shifted to transit had fewer household-level opportunities to drive to work and more positive attitudes toward transit. Transit commuters accrued an average of 80 minutes/week of commute-related PA, and 50 minutes/week more total PA than those that commuted entirely passively. Kingston Transit's express service has stimulated an increase in transit ridership among one of their target employers, Queen's University. The findings from this study suggest that shifting to transit from entirely passive commuting can generate higher overall PA levels. PMID:26844163
Collins, Patricia A.; Agarwal, Ajay
2015-01-01
Public transit ridership offers valuable opportunities for modest amounts of daily physical activity (PA). Transit is a more feasible option for most Canadian commuters who live too far from work to walk or cycle, yet public transit usage in midsized Canadian cities has historically remained low due to inefficient transit service. The objectives of this longitudinal study were threefold: to assess whether the introduction of express transit service in the low-density city of Kingston, Ontario, has translated to greater transit use among a targeted employee group; to document the characteristics of those employees that have shifted to transit; and to examine the PA levels of employees using transit compared to other commute modes. An online survey was administered in October 2013 and October 2014 to all non-student employees at Queen's University. 1356 employees completed the survey in 2013, and 1123 in 2014; 656 of these employees completed the survey both years, constituting our longitudinal sample. Year-round transit ridership increased from 5.5% in 2013 to 8.5% in 2014 (p < 0.001). Employees who shifted to transit had fewer household-level opportunities to drive to work and more positive attitudes toward transit. Transit commuters accrued an average of 80 minutes/week of commute-related PA, and 50 minutes/week more total PA than those that commuted entirely passively. Kingston Transit's express service has stimulated an increase in transit ridership among one of their target employers, Queen's University. The findings from this study suggest that shifting to transit from entirely passive commuting can generate higher overall PA levels. PMID:26844163
Hybrid Density Functional Calculations of Redox Potentials of Transition Metal Compounds
NASA Astrophysics Data System (ADS)
Armiento, Rickard; Chevrier, Vincent; Ong, Shyue Ping; Ceder, Gerbrand
2010-03-01
Prior works have shown that density functional theory (DFT) with the DFT+U method resolves the underestimation of redox potentials calculated by conventional functionals for a number of transition metal compounds relevant for battery applications, including the olivine LixMPO4 (M = Fe, Mn, Co, Ni), layered LixMO2 (M = Co, Ni) and spinel-like LixMn2O4. We show that the redox potentials of these compounds are also well reproduced by the hybrid density functional by Heyd-Scuseria-Ernzerhof (HSE06). Hybrid functionals combine a conventional DFT functional with a part of Hartree-Fock (HF) exchange. While the HF part increases the computational expense by at least one order of magnitude, it provides, in contrast to DFT+U, a correction for the self-interaction error that does not rely on special treatment of the occupancies of the orbital states of ions or species-specific parameters. We compare the accuracy of regular DFT, DFT+U and HSE06 for the redox potentials, lattice constants, and other properties. Examples of electron delocalization problems connected to the self-interaction error in the systems are discussed, and shown to be resolved both by the hybrid functional and DFT+U methods. Comments are made on the possibility to approach the delocalization problem with a semi-local functional.
Superfluid density in He II near the lambda transition: First principles theory
NASA Astrophysics Data System (ADS)
Jackson, H. W.
2015-03-01
A first principles theory of the λ transition in liquid 4He was introduced in a recent paper [H. W. Jackson, J. Low Temp. Phys. 155, 1 (2009)]. In that theory critical fluctuations consisting of isothermal fourth sound waves are treated with quantum statistical mechanics methods in deriving formulas for constant volume conditions for specific heat, correlation length, equal time pair correlation function, and isothermal compressibility. To leading order terms in (Tλ-T) the theory yields exact results α‧=0 and ν‧=2/3 for critical exponents at constant volume. A follow-up study in the present paper demonstrates by a least squares fit that a logarithmic function accurately describes the specific heat at svp when (Tλ-T) is between 10-9 K and 10-5 K. This logarithmic divergent behavior conflicts with previous analyses of experimental data and predictions of renormalization group theory that constant pressure specific heat is finite at Tλ, but Is thermodynamically consistent with logarithmic asymptotic behavior of specific heat at constant volume predicted in the new theory. The first principles theory is extended in this paper to derive formulas for superfluid density and for a relation between superfluid density and correlation length in He II near Tλ. Numerical results based on these formulas are in good agreement with experimental data produced by second sound measurements.
Densities and eccentricities of 139 Kepler planets from transit time variations
Hadden, Sam; Lithwick, Yoram
2014-05-20
We extract densities and eccentricities of 139 sub-Jovian planets by analyzing transit time variations (TTVs) obtained by the Kepler mission through Quarter 12. We partially circumvent the degeneracies that plague TTV inversion with the help of an analytical formula for the TTV. From the observed TTV phases, we find that most of these planets have eccentricities of the order of a few percent. More precisely, the rms eccentricity is 0.018{sub −0.004}{sup +0.005}, and planets smaller than 2.5 R {sub ⊕} are around twice as eccentric as those bigger than 2.5 R {sub ⊕}. We also find a best-fit density-radius relationship ρ ≈ 3 g cm{sup –3} × (R/3 R {sub ⊕}){sup –2.3} for the 56 planets that likely have small eccentricity and hence small statistical correction to their masses. Many planets larger than 2.5 R {sub ⊕} are less dense than water, implying that their radii are largely set by a massive hydrogen atmosphere.
NASA Astrophysics Data System (ADS)
Tabtimsai, Chanukorn; Ruangpornvisuti, Vithaya; Wanno, Banchob
2013-03-01
The binding of VIIIB transition metals i.e. Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, and Pt single atoms to single-walled carbon nanotube (SWCNT) was investigated using the density functional theory method. The B3LYP/LanL2DZ calculation shows that all these transition metal atoms have strong binding abilities to SWCNT. The binding abilities of these transition metals onto SWCNT are in following order: Os>Ru>Ir>Fe>Rh>Pt>Ni>Co>Pd. The Os single atom binding on SWCNT is the strongest binding of which the binding energy is -240.66 kcal/mol. The partial charge transfers from transition metal to SWCNT, density of states and energy gaps of metal atoms deposited on SWCNTs were analyzed and reported.
Tuning of the charge-density wave in the halogen-bridged transition-metal linear-chain compounds
Alouani, M.; Wilkins, J.W. ); Albers, R.C.; Wills, J.M. )
1993-08-30
Local-density-approximation calculations are used to show that the metal-metal distance along the chains controls the charge-density wave (CDW) in halogen-bridged transition-metal linear-chain compounds. The strength of the CDW can be understood in terms of a two-band Su-Schrieffer-Heeger model if a hard-core ion-ion repulsion potential is also added. We predict a second-order phase transition from an insulating to a semimetallic ground state and explain trends in dimerization, bond-length ratios, band gaps, and Raman breathing modes in terms of the metal-metal distance.
Tuning of the charge-density wave in the halogen-bridged transition-metal linear-chain compounds
NASA Astrophysics Data System (ADS)
Alouani, M.; Wilkins, J. W.; Albers, R. C.; Wills, J. M.
1993-08-01
Local-density-approximation calculations are used to show that the metal-metal distance along the chains controls the charge-density wave (CDW) in halogen-bridged transition-metal linear-chain compounds. The strength of the CDW can be understood in terms of a two-band Su-Schrieffer-Heeger model if a hard-core ion-ion repulsion potential is also added. We predict a second-order phase transition from an insulating to a semimetallic ground state and explain trends in dimerization, bond-length ratios, band gaps, and Raman breathing modes in terms of the metal-metal distance.
Order-disorder transition in the surface charge-density-wave phase of Cu(001)-c(4x4)-In
Hatta, S.; Okuyama, H.; Aruga, T.; Sakata, O.
2005-08-15
We have investigated the x-ray critical scattering from In/Cu(001) near the charge-density-wave phase transition. The critical exponents of the temperature dependence of the order parameter, the susceptibility, and the correlation length of the order parameter fluctuations are in agreement with those expected for a two-dimensional Ising-type phase transition. The lattice transition temperature, T{sub cl}=345 K, is 60 K lower than that for the electronic transition reported recently. The surface phase transition in this system is not well described by conventional weak- or strong-coupling theories, which assume a single characteristic energy gap. It is suggested that the electronic and lattice degrees of freedom on the surface are governed separately by two different energy gaps.
Gravitational Effects on Flow Instability and Transition in Low Density Jets
NASA Technical Reports Server (NTRS)
Agrawal A. K.; Parthasarathy, K.; Pasumarthi, K.; Griffin, D. W.
2000-01-01
Recent experiments have shown that low-density gas jets injected into a high-density ambient gas undergo an instability mode, leading to highly-periodic oscillations in the flow-field for certain conditions. The transition from laminar to turbulent flow in these jets is abrupt, without the gradual change in scales. Even the fine scale turbulent structure repeats itself with extreme regularity from cycle to cycle. Similar observations were obtained in buoyancy-dominated and momentum-dominated jets characterized by the Richardson numbers, Ri = [gD(rho(sub a)-rho(sub j))/rho(sub j)U(sub j)(exp 2) ] where g is the gravitational acceleration, D is the jet diameter, rho(sub a) and rho(sub a) are, respectively, the free-stream and jet densities, and U(sub j) is the mean jet exit velocity. At high Richardson numbers, the instability is presumably caused by buoyancy since the flow-oscillation frequency (f) or the Strouhal number, St = [fD/U(sub j)] scales with Ri. In momentum-dominated jets, however, the Strouhal number of the oscillating flow is relatively independent of the Ri. In this case, a local absolute instability is predicted in the potential core of low-density jets with S [= rho(sub j)/rho(sub a)] < 0.7, which agrees qualitatively with experiments. Although the instability in gas jets of high Richardson numbers is attributed to buoyancy, direct physical evidence has not been acquired in experiments. If the instability is indeed caused by buoyancy, the near-field flow structure of the jet will change significantly when the buoyancy is removed, for example, in the microgravity environment. Thus, quantitative data on the spatial and temporal evolutions of the instability, length and time scale of the oscillating mode and its effects on the mean flow and breakdown of the potential core are needed in normal and microgravity to delineate gravitational effects in buoyant jets. In momentum dominated low-density jets, the instability is speculated to originate in the
NASA Astrophysics Data System (ADS)
Chen, Hanghui; Millis, Andrew J.
2016-01-01
Previous work on the physical content of exchange-correlation functionals that depend on both charge and spin densities is extended to elemental transition metals and a wider range of perovskite transition metal oxides. A comparison of spectra and magnetic moments calculated using charge-only and spin-dependent exchange-correlation functionals as well as their +U and +J extensions confirms previous conclusions that the spin-dependent part of the exchange-correlation functional provides an effective Hund's interaction acting on the transition metal d orbitals. For the local spin density approximation and spin-dependent generalized gradient approximation in the Perdew-Burke-Ernzerhof parametrization, the effective Hund's exchange implied by the spin dependence of the exchange correlation functional is found to be larger than 1 eV. The results indicate that at least as far as applications to transition metals and their oxides are concerned, +U , +J , and +dynamical-mean-field-theory extensions of density functional theory should be based on charge-only exchange-correlation functionals.
Zeng, Menglong; Shang, Yuan; Araki, Yoichi; Guo, Tingfeng; Huganir, Richard L; Zhang, Mingjie
2016-08-25
Postsynaptic densities (PSDs) are membrane semi-enclosed, submicron protein-enriched cellular compartments beneath postsynaptic membranes, which constantly exchange their components with bulk aqueous cytoplasm in synaptic spines. Formation and activity-dependent modulation of PSDs is considered as one of the most basic molecular events governing synaptic plasticity in the nervous system. In this study, we discover that SynGAP, one of the most abundant PSD proteins and a Ras/Rap GTPase activator, forms a homo-trimer and binds to multiple copies of PSD-95. Binding of SynGAP to PSD-95 induces phase separation of the complex, forming highly concentrated liquid-like droplets reminiscent of the PSD. The multivalent nature of the SynGAP/PSD-95 complex is critical for the phase separation to occur and for proper activity-dependent SynGAP dispersions from the PSD. In addition to revealing a dynamic anchoring mechanism of SynGAP at the PSD, our results also suggest a model for phase-transition-mediated formation of PSD. PMID:27565345
AC-field-induced quantum phase transitions in density of states
NASA Astrophysics Data System (ADS)
Yang, Kai-Hua; Liu, Kai-Di; Wang, Huai-Yu; Qin, Chang-Dong
2016-02-01
We investigate the joint effects of the intralead electron interaction and an external alternating gate voltage on the time-averaged local density of states (DOSs) of a quantum dot coupled to two Luttinger-liquid leads in the Kondo regime. A rich dependence of the DOSs on the driving amplitude and intralead interaction is demonstrated. We show that the feature is quite different for different interaction strengths in the presence of the ac field. It is shown that the photon-assisted transport processes cause an additional splitting of the Kondo peak or dip, which exhibits photon-assisted single-channel (1CK) or two-channel Kondo (2CK) physics behavior. The phase transition between photon-assisted 1CK and 2CK physics occurs when the interaction strength is moderately strong. The inelastic channels associated with photon-assisted electron tunneling can dominate electron transport for weak interaction when the ac amplitude is greater than the frequency by one order of magnitude. In the limit of strong interaction the DOSs scale as a power-law behavior which is strongly affected by the ac field.
Linking high-pressure structure and density of albite liquid near the glass transition
NASA Astrophysics Data System (ADS)
Gaudio, Sarah J.; Lesher, Charles E.; Maekawa, Hideki; Sen, Sabyasachi
2015-05-01
The pressure-induced densification of NaAlSi3O8 liquid is determined following annealing immediately above the glass transition and upon quenching from superliquidus temperatures. High-field 27Al magic-angle-spinning NMR spectroscopy is used to investigate the corresponding changes in Al coordination environment that accompany the densification. We show that samples synthesized by quenching from superliquidus temperatures record lower fictive pressures (Pf) than annealed samples at the same nominal load and have lower recovered densities and average Al coordination number. Accounting for differences in Pf brings melt-quench and annealed samples into excellent agreement. The proportion of [5]Al increases from ∼3% to 29% and [6]Al from 0% to 8% between 1.8 and 7.2 GPa. The production of high-coordinated Al ([5]Al + [6]Al) with pressure is most dramatic above 3 GPa. Changes in network topology and structural disorder as revealed by the high-field 27Al NMR spectra provide new insights into the structural mechanisms of densification of the albite liquid. We posit that it is an overall weakening of the network structure on compression that is largely responsible for the anomalous pressure dependence of the transport properties observed for this liquid below ∼5 GPa.
Živković, Daniel; Steinrücken, Matthias; Song, Yun S.; Stephan, Wolfgang
2015-01-01
Advances in empirical population genetics have made apparent the need for models that simultaneously account for selection and demography. To address this need, we here study the Wright–Fisher diffusion under selection and variable effective population size. In the case of genic selection and piecewise-constant effective population sizes, we obtain the transition density by extending a recently developed method for computing an accurate spectral representation for a constant population size. Utilizing this extension, we show how to compute the sample frequency spectrum in the presence of genic selection and an arbitrary number of instantaneous changes in the effective population size. We also develop an alternate, efficient algorithm for computing the sample frequency spectrum using a moment-based approach. We apply these methods to answer the following questions: If neutrality is incorrectly assumed when there is selection, what effects does it have on demographic parameter estimation? Can the impact of negative selection be observed in populations that undergo strong exponential growth? PMID:25873633
Gravitational Effects on Flow Instability and Transition in Low Density Jets
NASA Technical Reports Server (NTRS)
Agrawal A. K.; Parthasarathy, K.; Pasumarthi, K.; Griffin, D. W.
2000-01-01
Recent experiments have shown that low-density gas jets injected into a high-density ambient gas undergo an instability mode, leading to highly-periodic oscillations in the flow-field for certain conditions. The transition from laminar to turbulent flow in these jets is abrupt, without the gradual change in scales. Even the fine scale turbulent structure repeats itself with extreme regularity from cycle to cycle. Similar observations were obtained in buoyancy-dominated and momentum-dominated jets characterized by the Richardson numbers, Ri = [gD(rho(sub a)-rho(sub j))/rho(sub j)U(sub j)(exp 2) ] where g is the gravitational acceleration, D is the jet diameter, rho(sub a) and rho(sub a) are, respectively, the free-stream and jet densities, and U(sub j) is the mean jet exit velocity. At high Richardson numbers, the instability is presumably caused by buoyancy since the flow-oscillation frequency (f) or the Strouhal number, St = [fD/U(sub j)] scales with Ri. In momentum-dominated jets, however, the Strouhal number of the oscillating flow is relatively independent of the Ri. In this case, a local absolute instability is predicted in the potential core of low-density jets with S [= rho(sub j)/rho(sub a)] < 0.7, which agrees qualitatively with experiments. Although the instability in gas jets of high Richardson numbers is attributed to buoyancy, direct physical evidence has not been acquired in experiments. If the instability is indeed caused by buoyancy, the near-field flow structure of the jet will change significantly when the buoyancy is removed, for example, in the microgravity environment. Thus, quantitative data on the spatial and temporal evolutions of the instability, length and time scale of the oscillating mode and its effects on the mean flow and breakdown of the potential core are needed in normal and microgravity to delineate gravitational effects in buoyant jets. In momentum dominated low-density jets, the instability is speculated to originate in the
NASA Astrophysics Data System (ADS)
Singh, Ram Chandra
2007-09-01
We have used the density-functional theory to study the effect of varying temperature on the isotropic-nematic transition of a fluid of molecules interacting via the Gay-Berne intermolecular potential. The nematic phase is found to be stable with respect to isotropic phase in the temperature range 0.80<=T*<=1.25. Pair correlation functions needed as input information in density-functional theory is calculated using the Percus-Yevick integral equation theory. We find that the density-functional theory is good for studying the isotropic-nematic transition in molecular fluids if the values of the pair-correlation functions in the isotropic phase are known accurately. We have also compared our results with computer simulation results wherever they are available.
On the role of the edge density profile for the L-H transition power threshold in ASDEX Upgrade
NASA Astrophysics Data System (ADS)
Shao, L. M.; Wolfrum, E.; Ryter, F.; Birkenmeier, G.; Laggner, F. M.; Viezzer, E.; Fischer, R.; Willensdorfer, M.; Kurzan, B.; Lunt, T.; the ASDEX Upgrade Team
2016-02-01
The L-H transition power threshold ({{P}\\text{L-\\text{H}}} ) in full tungsten (W) wall discharges is lower by 25% compared to those with graphite (C) mix tungsten walls in ASDEX Upgrade (Ryter et al 2013 Nucl. Fusion 53 113003). The lower power threshold in the full tungsten wall discharges has been found to correlate with higher edge density as well as steeper edge density gradient. An estimate of the minimum in the neoclassical radial electric field well inside the separatrix yields a constant value for all analyzed L-H transitions at fixed toroidal magnetic field ({{B}\\text{T}} ). The decrease of the threshold power is explained by the steeper edge density gradient in the discharges with full tungsten wall.
The performance of semilocal and hybrid density functionals in 3d transition-metal chemistry
NASA Astrophysics Data System (ADS)
Furche, Filipp; Perdew, John P.
2006-01-01
We investigate the performance of contemporary semilocal and hybrid density functionals for bond energetics, structures, dipole moments, and harmonic frequencies of 3d transition-metal (TM) compounds by comparison with gas-phase experiments. Special attention is given to the nonempirical metageneralized gradient approximation (meta-GGA) of Tao, Perdew, Staroverov, and Scuseria (TPSS) [Phys. Rev. Lett. 91, 146401 (2003)], which has been implemented in TURBOMOLE for the present work. Trends and error patterns for classes of homologous compounds are analyzed, including dimers, monohydrides, mononitrides, monoxides, monofluorides, polyatomic oxides and halogenides, carbonyls, and complexes with organic π ligands such as benzene and cyclopentadienyl. Weakly bound systems such as Ca2, Mn2, and Zn2 are discussed. We propose a reference set of reaction energies for benchmark purposes. Our all-electron results with quadruple zeta valence basis sets validate semilocal density-functional theory as the workhorse of computational TM chemistry. Typical errors in bond energies are substantially larger than in (organic) main group chemistry, however. The Becke-Perdew'86 [Phys. Rev. A 38, 3098 (1988); Phys. Rev. B 33, 8822 (1986)] GGA and the TPSS meta-GGA have the best price/performance ratio, while the TPSS hybrid functional achieves a slightly lower mean absolute error in bond energies. The popular Becke three-parameter hybrid B3LYP underbinds significantly and tends to overestimate bond distances; we give a possible explanation for this. We further show that hybrid mixing does not reduce the width of the error distribution on our reference set. The error of a functional for the s-d transfer energy of a TM atom does not predict its error for TM bond energies and bond lengths. For semilocal functionals, self-interaction error in one- and three-electron bonds appears to be a major source of error in TM reaction energies. Nevertheless, TPSS predicts the correct ground
Phonon density of states of Fe2O3 across high-pressure structural and electronic transitions
Lin, Jung-Fu; Tse, John S.; Alp, Esen E.; Zhao, Jiyong; Lerche, Michael; Sturhahn, Wolfgang; Xiao, Yuming; Chow, Paul
2011-08-24
High-pressure phonon density of states (PDOS) of Fe₂O₃ across structural and electronic transitions has been investigated by nuclear resonant inelastic x-ray scattering (NRIXS) and first-principles calculations together with synchrotron Mössbauer, x-ray diffraction, and x-ray emission spectroscopies. Drastic changes in elastic, thermodynamic, and vibrational properties of Fe₂O₃ occur across the Rh₂O₃(II)-type structural transition at 40–50 GPa, whereas the Mott insulator-metal transition occurring after the structural transition only causes nominal changes in the properties of the Fe₂O₃. The observed anomalous mode-softening behavior of the elastic constants is associated with the structural transition at 40–50 GPa, leading to substantial changes in the Debye-like part of the PDOS in the terahertz acoustic phonons. Our experimental and theoretical studies provide new insights into the effects of the structural and electronic transitions in the transition-metal oxide (TMO) compounds.
NASA Astrophysics Data System (ADS)
Jontof-Hutter, Daniel; Ford, Eric B.; Rowe, Jason F.; Lissauer, Jack J.; Fabrycky, Daniel C.; Van Laerhoven, Christa; Agol, Eric; Deck, Katherine M.; Holczer, Tomer; Mazeh, Tsevi
2016-03-01
We infer dynamical masses in eight multiplanet systems using transit times measured from Kepler's complete data set, including short-cadence data where available. Of the 18 dynamical masses that we infer, 10 pass multiple tests for robustness. These are in systems Kepler-26 (KOI-250), Kepler-29 (KOI-738), Kepler-60 (KOI-2086), Kepler-105 (KOI-115), and Kepler-307 (KOI-1576). Kepler-105 c has a radius of 1.3 R⊕ and a density consistent with an Earth-like composition. Strong transit timing variation (TTV) signals were detected from additional planets, but their inferred masses were sensitive to outliers or consistent solutions could not be found with independently measured transit times, including planets orbiting Kepler-49 (KOI-248), Kepler-57 (KOI-1270), Kepler-105 (KOI-115), and Kepler-177 (KOI-523). Nonetheless, strong upper limits on the mass of Kepler-177 c imply an extremely low density of ˜0.1 g cm-3. In most cases, individual orbital eccentricities were poorly constrained owing to degeneracies in TTV inversion. For five planet pairs in our sample, strong secular interactions imply a moderate to high likelihood of apsidal alignment over a wide range of possible eccentricities. We also find solutions for the three planets known to orbit Kepler-60 in a Laplace-like resonance chain. However, nonlibrating solutions also match the transit timing data. For six systems, we calculate more precise stellar parameters than previously known, enabling useful constraints on planetary densities where we have secure mass measurements. Placing these exoplanets on the mass-radius diagram, we find that a wide range of densities is observed among sub-Neptune-mass planets and that the range in observed densities is anticorrelated with incident flux.
Evolution of the phonon density of states of LaCoO3 over the spin state transition
Golosova, N. O.; Kozlenko, D. P.; Kolesnikov, Alexander I; Kazimirov, V. Yu.; Smirnov, M. B.; Jirak, Z.; Savenko, B. N.
2011-01-01
The phonon spectra of LaCoO3 were studied by inelastic neutron scattering in the temperature range of 4 120 K. The DFT calculations of the lattice dynamics have been made for interpretation of the experimental data. The observed and calculated phonon frequencies were found to be in a reasonable agreement. The evolution of the phonon density of states over the spin state transition was analyzed. In the low-temperature range (T < 50 K), an increase in the energy of resolved breathing, stretching, and bending phonon modes was found, followed by their softening and broadening at higher temperatures due to the spin state transition and relevant orbital-phonon coupling.
NASA Technical Reports Server (NTRS)
Richards, P. G.; Torr, D. G.; Buonsanto, M. J.; Miller, K. L.
1989-01-01
The ionospheric electron density and temperature variations is simulated during the equinox transition study in September 1984 and the results are compared with measurements made at Millstone Hill. The agreement between the modeled and measured electron density and temperature for the quiet day (18 September) is very good but there are large differences on the day of the storm (19 September). On the storm day, the measured electron density decreases by a factor of 1.7 over the previous day, while the model density actually increases slightly. The model failure is attributed to an inadequate increase in the ratio of atomic oxygen to molecular neutral densities in the MSIS neutral atmosphere model, for this particular storm. A factor of 3 to 5 increase in the molecular to atomic oxygen density ratio at 300 km is needed to explain the observed decrease in electron density. The effect of vibrationally excited N sub 2 on the electron density were studied and found to be small.
NASA Astrophysics Data System (ADS)
Pastore, G.; Bernu, B.; Hansen, J. P.; Hiwatari, Y.
1988-07-01
Using molecular-dynamics (MD) data on a binary-alloy model, we have computed the self (incoherent) -part of the density autocorrelation functions of both species in the supercooled liquid and near the glass transition, over an extensive range of wave numbers. Standard theoretical models of liquid-state theory fail to reproduce the data, while the Chudley-Elliott jump diffusion model yields reasonable results in the glass range. With a suitable scaling of the time axis, the data for different temperatures can be brought onto a single master curve, which is well fitted by a Kohlrausch (``stretched-exponential'') function with a wave-number-dependent exponent.
Concomitant charge-density-wave and unit-cell-doubling structural transitions in Dy5Ir4Si10
NASA Astrophysics Data System (ADS)
Lee, M. H.; Chen, C. H.; Tseng, C. M.; Lue, C. S.; Kuo, Y. K.; Yang, H. D.; Chu, M.-W.
2014-05-01
The tetragonal rare-earth transition-metal silicide system with three-dimensional crystallographic structure R5T4Si10, where R is Dy, Ho, Er, Tm, and Lu, and T=Ir and Rh, has been shown to exhibit fascinating charge-density-wave (CDW) phase transitions, a phenomenon largely found in otherwise low-dimensional systems. In this study, we report the investigations of CDW in Dy5Ir4Si10 at different temperatures using transmission electron microscopy techniques including electron diffraction and dark-field imaging. Incommensurate superlattice spots along the c axis were observed in the electron-diffraction patterns when the sample was cooled below the CDW transition temperature at ˜208 K. CDW becomes commensurate with further cooling and configurations of CDW dislocations convincingly show that the CDW phase transition is accompanied by a concomitant cell-doubling crystallographic structural phase transition. Intriguingly, the cell-doubling transition is featured by a broken inversion symmetry along the c axis and a disparity in the CDW-modulation vectors with opposite signs, which gives rise to two sets of CDW domains with reversed contrasts. The profound physics underlining this notable domain-contrast behavior is discussed.
Dynamic density field measurements of an explosively driven α → ϵ phase transition in iron
Hull, L. M.; Gray, G. T.; Warthen, B. J.
2014-07-28
We provide a unique set of observations of the behavior of the α→ϵ phase transition under a complex axially symmetric loading path created by sweeping a detonation wave along the end surface of a cylindrical sample. The primary data sets are the measured mass density distributions acquired at 5 independent times during the sweep of the detonation along the surface. Shocked regions and boundaries are measured, as well as regions and boundaries of elevated density (presumed to be the ϵ−phase iron). The formation and dynamics of these regions were captured and are available for comparisons to material descriptions. We also applied 16 Photon Doppler Velocimetry probes to capture the free surface velocity along a discrete set of radially distributed points in order to compare and correlate the density measurements with previous shock wave studies. The velocimetry data are in nearly exact agreement with previous shock wave studies of the α→ϵ phase transition, the density distributions, while generally in agreement with expectations evolved from the shock wave studies, show that the epsilon phase is generated in regions of high shear stress but at hydrostatic stresses below the typically quoted 13 GPa value. The density field measurements are particularly useful for observing the effects of the forward and reverse transformation kinetics, as well as the reverse transformation hysteresis.
Scanning tunnelling microscopy of charge-density waves in transition metal chalcogenides
NASA Astrophysics Data System (ADS)
Coleman, R. V.; Giambattista, B.; Hansma, P. K.; Johnson, A.; McNairy, W. W.; Slough, C. G.
1988-11-01
We have used scanning tunnelling microscopes (STMs) operating at liquid helium and liquid nitrogen temperatures to image the charge-density waves (CDWs) in transition metal chalcogenides. The layer structure dichalcogenides TaSe2, TaS2, NbSe2, VSe2, TiSe2 and TiS2 have been studied including representative polytype phases such as 1T, 2H and 4Hb. Experimental results are presented for the complete range of CDW amplitudes and structures observed in these materials. In most cases both the CDW and the surface atomic structure have been simultaneously imaged. Results on the trichalcogenide NbSe3 are also included.The formation of the CDW along with the associated periodic lattice distortion gaps the Fermi surface (FS) and modifies the local density-of-states (LDOS) detected by the tunnelling process. The tunnelling microscopes have been operated mostly in the constant current mode which maps the LDOS at the position of the tunnelling tip. The relative amplitudes and profiles of the CDW superlattice and the atomic lattice have been measured and confirm on an atomic scale the CDW structures predicted by X-ray, electron and neutron diffraction. The absolute STM deflections are larger than expected for the CDW induced modifications of the LDOS above the surface and possible enhancement mechanisms are reviewed.In the 2H trigonal prismatic coordination phases the CDWs involve a relatively small charge transfer and the atomic structure dominates the STM images. In the 1T octahedral coordination phases the charge transfer is large and the CDW structure dominates the STM image with an anomalously large enhancement of the STM profile. Systematic comparison of the STM profiles with band structure and FS information is included.In the case of the 4Hb mixed coordination phases at the lowest temperatures two nearly independent CDWs form in alternate sandwiches. STM studies on 4Hb crystals with both octahedral and trigonal prismatic surface sandwiches have been carried out. The STM
ERIC Educational Resources Information Center
Nathanson, Jeanne H., Ed.
1993-01-01
This theme issue on transitions for individuals with disabilities contains nine papers discussing transition programs and issues. "Transition Issues for the 1990s," by Michael J. Ward and William D. Halloran, discusses self-determination, school responsibility for transition, continued educational engagement of at-risk students, and service…
Liquid-solid and solid-solid phase transition of monolayer water: High-density rhombic monolayer ice
NASA Astrophysics Data System (ADS)
Kaneko, Toshihiro; Bai, Jaeil; Yasuoka, Kenji; Mitsutake, Ayori; Zeng, Xiao Cheng
2014-05-01
Liquid-solid and solid-solid phase transitions of a monolayer water confined between two parallel hydrophobic surfaces are studied by molecular dynamics simulations. The solid phase considered is the high-density rhombic monolayer ice. Based on the computed free energy surface, it is found that at a certain width of the slit nanopore, the monolayer water exhibits not only a high freezing point but also a low energy barrier to crystallization. Moreover, through analyzing the oxygen-hydrogen-oxygen angle distribution and oxygen-hydrogen radial distribution, the high-density monolayer ice is classified as either a flat ice or a puckered ice. The transition between a flat ice and a puckered ice reflects a trade-off between the water-wall interactions and the electrostatic interactions among water molecules.
Kaneko, Toshihiro; Bai, Jaeil; Yasuoka, Kenji; Mitsutake, Ayori; Zeng, Xiao Cheng
2014-05-14
Liquid-solid and solid-solid phase transitions of a monolayer water confined between two parallel hydrophobic surfaces are studied by molecular dynamics simulations. The solid phase considered is the high-density rhombic monolayer ice. Based on the computed free energy surface, it is found that at a certain width of the slit nanopore, the monolayer water exhibits not only a high freezing point but also a low energy barrier to crystallization. Moreover, through analyzing the oxygen-hydrogen-oxygen angle distribution and oxygen-hydrogen radial distribution, the high-density monolayer ice is classified as either a flat ice or a puckered ice. The transition between a flat ice and a puckered ice reflects a trade-off between the water-wall interactions and the electrostatic interactions among water molecules. PMID:24832288
NASA Astrophysics Data System (ADS)
Tian, Jun-Fang; Yuan, Zhen-Zhou; Jia, Bin; Fan, Hong-Qiang
2013-03-01
We investigate the phase transitions and the Korteweg-de Vries (KdV) equation in the density difference lattice hydrodynamic (DDLM) model, which shows a close connection with the gas-kinetic-based model and the microscopic car following model. The KdV equation near the neutral stability line is derived and the corresponding soliton solution describing the density waves is obtained. Numerical simulations are conducted in two aspects. On the one hand, under periodic conditions perturbations are applied to demonstrate the nonlinear analysis result. On the other hand, the open boundary condition with random fluctuations is designed to explore the empirical congested traffic patterns. The phase transitions among the free traffic (FT), widening synchronized flow pattern (WSP), moving localized cluster (MLC), oscillatory congested traffic (OCT) and homogeneous congested traffic (HCT) occur by varying the amplitude of the fluctuations. To our knowledge, it is the first research showing that the lattice hydrodynamic model could reproduce so many congested traffic patterns.
Pressure-induced changes in the electron density distribution in α-Ge near the α-β transition
NASA Astrophysics Data System (ADS)
Li, Rui; Liu, Jing; Bai, Ligang; Tse, John S.; Shen, Guoyin
2015-08-01
Electron density distributions in α-Ge have been determined under high pressure using maximum entropy method with structure factors obtained from single crystal synchrotron x-ray diffraction in a diamond anvil cell. The results show that the sp3 bonding is enhanced with increasing pressure up to 7.7(1) GPa. At higher pressures but below the α-β transition pressure of 11.0(1) GPa, the sp3-like electron distribution progressively weakens with a concomitant increase of d-orbitals hybridization. The participation of d-orbitals in the electronic structure is supported by Ge Kβ2 (4p-1s) x-ray emission spectroscopy measurements showing the reduction of 4s character in the valence band at pressures far below the α-β transition. The gradual increase of d-orbitals in the valence level in the stability field of α-Ge is directly related to the eventual structural transition.
Electronic band structure and charge density wave transition in quasi-2D KMo6O17 purple bronze
NASA Astrophysics Data System (ADS)
Valbuena, M. A.; Avila, J.; Vyalikh, D. V.; Guyot, H.; Laubschat, C.; Molodtsov, S. L.; Asensio, M. C.
2008-03-01
High resolution angle-resolved photoemission of quasi-2D KMo6O17 purple bronze has been performed in the range from room temperature to 130 K, slightly above the charge density wave (CDW) transition (Tc = 110 K), and down to 35 K (well below Tc). In this paper we report a detailed study of how electronic band structure is affected by this transition driven by the hidden nesting scenario. The expected spectroscopic fingerprints of the CDW phase transition have been found and discussed according to the hidden one dimension and the development of a quasi-commensurate CDW. The excellent agreement between theory and our experimental results makes of potassium purple bronze a reference system for studying this type of instabilities.
First-order density-wave-like transitions in surface-doped Na2IrO3
NASA Astrophysics Data System (ADS)
Mehlawat, Kavita; Singh, Yogesh
2016-07-01
We demonstrate that the surface of the honeycomb lattice iridate Na2IrO3 is extremely tunable by plasma etching. We have succeeded in turning the surface of Na2IrO3 metallic by argon plasma etching which leads to the removal of Na from the surface. The surface structure does not change in this process as revealed by grazing incidence small-angle x-ray scattering. The sheet resistance Rs can be reduced by several orders of magnitude by varying the etching duration. Temperature-dependent Rs(T ) for the metallic samples shows signatures of spin- or charge-density-wave transitions with abrupt changes in Rs. Thermal hysteresis between cooling and warming measurements across the transition indicates a first-order transition. For the most metallic sample Rs(T ) data at low temperatures follow a T2 behavior suggesting normal Fermi-liquid behavior.
Schönherr, Mandes; Slater, Ben; Hutter, Jürg; VandeVondele, Joost
2014-01-16
The dielectric properties of the hydrogen disordered hexagonal phase (Ih) of water ice have been computed using density functional theory (DFT) based Monte Carlo simulations in the isobaric-isothermal ensemble. Temperature dependent data yield a fit for the Curie-Weiss law of the system and hence a prediction of the temperature of the phase transition from the Ih phase to the hydrogen ordered ice XI phase. Direct simulations around the phase transition temperature confirm and refine the predicted phase transition temperatures and provide data for further properties, such as the linear thermal expansion coefficient. Results have been obtained with both hybrid and semilocal density functionals, which yields insight in the performance of the electronic structure method. In particular, the hybrid functional yields significantly more realistic dielectric constants than the semilocal variant, namely ε ≈ 116 as opposed to ε ≈ 151 at 273 K (εexperiment = 95). This can be attributed to the tendency of semilocal functionals to be biased to configurations with a large dipole moment, and their overestimation of the dipole moments of these configurations. This is also reflected in the estimates of the Ih/XI transition temperature, which is 70-80 and 90-100 K for the hybrid and semilocal functional respectively. DFT based sampling of the millions of configurations necessary for this work has been enabled by a Tree Monte Carlo algorithm, designed for massively parallel computers. PMID:24392971
NASA Astrophysics Data System (ADS)
Hirano, Y.; Kiyama, S.; Fujiwara, Y.; Koguchi, H.; Sakakita, H.
2015-11-01
A high current density (≈3 mA/cm2) hydrogen ion beam source operating in an extremely low-energy region (Eib ≈ 150-200 eV) has been realized by using a transition to a highly focused state, where the beam is extracted from the ion source chamber through three concave electrodes with nominal focal lengths of ≈350 mm. The transition occurs when the beam energy exceeds a threshold value between 145 and 170 eV. Low-level hysteresis is observed in the transition when Eib is being reduced. The radial profiles of the ion beam current density and the low temperature ion current density can be obtained separately using a Faraday cup with a grid in front. The measured profiles confirm that more than a half of the extracted beam ions reaches the target plate with a good focusing profile with a full width at half maximum of ≈3 cm. Estimation of the particle balances in beam ions, the slow ions, and the electrons indicates the possibility that the secondary electron emission from the target plate and electron impact ionization of hydrogen may play roles as particle sources in this extremely low-energy beam after the compensation of beam ion space charge.
Hirano, Y; Kiyama, S; Fujiwara, Y; Koguchi, H; Sakakita, H
2015-11-01
A high current density (≈3 mA/cm(2)) hydrogen ion beam source operating in an extremely low-energy region (E(ib) ≈ 150-200 eV) has been realized by using a transition to a highly focused state, where the beam is extracted from the ion source chamber through three concave electrodes with nominal focal lengths of ≈350 mm. The transition occurs when the beam energy exceeds a threshold value between 145 and 170 eV. Low-level hysteresis is observed in the transition when E(ib) is being reduced. The radial profiles of the ion beam current density and the low temperature ion current density can be obtained separately using a Faraday cup with a grid in front. The measured profiles confirm that more than a half of the extracted beam ions reaches the target plate with a good focusing profile with a full width at half maximum of ≈3 cm. Estimation of the particle balances in beam ions, the slow ions, and the electrons indicates the possibility that the secondary electron emission from the target plate and electron impact ionization of hydrogen may play roles as particle sources in this extremely low-energy beam after the compensation of beam ion space charge. PMID:26628125
Hirano, Y. E-mail: hirano.yoichi@phys.cst.nihon-u.ac.jp; Kiyama, S.; Koguchi, H.; Fujiwara, Y.; Sakakita, H.
2015-11-15
A high current density (≈3 mA/cm{sup 2}) hydrogen ion beam source operating in an extremely low-energy region (E{sub ib} ≈ 150–200 eV) has been realized by using a transition to a highly focused state, where the beam is extracted from the ion source chamber through three concave electrodes with nominal focal lengths of ≈350 mm. The transition occurs when the beam energy exceeds a threshold value between 145 and 170 eV. Low-level hysteresis is observed in the transition when E{sub ib} is being reduced. The radial profiles of the ion beam current density and the low temperature ion current density can be obtained separately using a Faraday cup with a grid in front. The measured profiles confirm that more than a half of the extracted beam ions reaches the target plate with a good focusing profile with a full width at half maximum of ≈3 cm. Estimation of the particle balances in beam ions, the slow ions, and the electrons indicates the possibility that the secondary electron emission from the target plate and electron impact ionization of hydrogen may play roles as particle sources in this extremely low-energy beam after the compensation of beam ion space charge.
Evidence for a Peierls phase-transition in a three-dimensional multiple charge-density waves solid
Mansart, Barbara; Cottet, Mathieu J. G.; Penfold, Thomas J.; Dugdale, Stephen B.; Tediosi, Riccardo; Chergui, Majed; Carbone, Fabrizio
2012-01-01
The effect of dimensionality on materials properties has become strikingly evident with the recent discovery of graphene. Charge ordering phenomena can be induced in one dimension by periodic distortions of a material’s crystal structure, termed Peierls ordering transition. Charge-density waves can also be induced in solids by strong coulomb repulsion between carriers, and at the extreme limit, Wigner predicted that crystallization itself can be induced in an electrons gas in free space close to the absolute zero of temperature. Similar phenomena are observed also in higher dimensions, but the microscopic description of the corresponding phase transition is often controversial, and remains an open field of research for fundamental physics. Here, we photoinduce the melting of the charge ordering in a complex three-dimensional solid and monitor the consequent charge redistribution by probing the optical response over a broad spectral range with ultrashort laser pulses. Although the photoinduced electronic temperature far exceeds the critical value, the charge-density wave is preserved until the lattice is sufficiently distorted to induce the phase transition. Combining this result with ab initio electronic structure calculations, we identified the Peierls origin of multiple charge-density waves in a three-dimensional system for the first time. PMID:22451898
Buoyancy Effects on Flow Transition in Low-Density Inertial Gas Jets
NASA Technical Reports Server (NTRS)
Pasumarthi, Kasyap S.; Agrawal, Ajay K.
2005-01-01
Effects of buoyancy on transition from laminar to turbulent flow are presented for momentum-dominated helium jet injected into ambient air. The buoyancy was varied in a 2.2-sec drop tower facility without affecting the remaining operating parameters. The jet flow in Earth gravity and microgravity was visualized using the rainbow schlieren deflectometry apparatus. Results show significant changes in the flow structure and transition behavior in the absence of buoyancy.
A closely packed system of low-mass, low-density planets transiting Kepler-11.
Lissauer, Jack J; Fabrycky, Daniel C; Ford, Eric B; Borucki, William J; Fressin, Francois; Marcy, Geoffrey W; Orosz, Jerome A; Rowe, Jason F; Torres, Guillermo; Welsh, William F; Batalha, Natalie M; Bryson, Stephen T; Buchhave, Lars A; Caldwell, Douglas A; Carter, Joshua A; Charbonneau, David; Christiansen, Jessie L; Cochran, William D; Desert, Jean-Michel; Dunham, Edward W; Fanelli, Michael N; Fortney, Jonathan J; Gautier, Thomas N; Geary, John C; Gilliland, Ronald L; Haas, Michael R; Hall, Jennifer R; Holman, Matthew J; Koch, David G; Latham, David W; Lopez, Eric; McCauliff, Sean; Miller, Neil; Morehead, Robert C; Quintana, Elisa V; Ragozzine, Darin; Sasselov, Dimitar; Short, Donald R; Steffen, Jason H
2011-02-01
When an extrasolar planet passes in front of (transits) its star, its radius can be measured from the decrease in starlight and its orbital period from the time between transits. Multiple planets transiting the same star reveal much more: period ratios determine stability and dynamics, mutual gravitational interactions reflect planet masses and orbital shapes, and the fraction of transiting planets observed as multiples has implications for the planarity of planetary systems. But few stars have more than one known transiting planet, and none has more than three. Here we report Kepler spacecraft observations of a single Sun-like star, which we call Kepler-11, that reveal six transiting planets, five with orbital periods between 10 and 47 days and a sixth planet with a longer period. The five inner planets are among the smallest for which mass and size have both been measured, and these measurements imply substantial envelopes of light gases. The degree of coplanarity and proximity of the planetary orbits imply energy dissipation near the end of planet formation. PMID:21293371
Sandusky, H W; Granholm, R H; Bohl, D G; Vandersall, K S; Hare, D E; Garcia, F
2006-06-20
The potential for deflagration-to-detonation transition (DDT) in LX-04 (85/15 HMX/Viton) is being evaluated as a function of loading density, temperature, and confinement. In the high confinement arrangement, a matrix of tests is nearly completed with the LX-04 loaded at {approx} 51, 70, 90, and {approx} 99% of theoretical maximum density (TMD); and temperatures of ambient, 160 C, and 190 C at each loading density. A more limited set of tests with {approx}99 %TMD loadings at medium confinement were conducted at temperatures of ambient and 186 C. LX-04 does not undergo DDT at near TMD loadings in both medium and high confinement, although the latter still results in significant fragmentation. Most porous beds in high confinement undergo DDT, with the minimum run distance to detonation (l) for a 70 %TMD loading at ambient temperature. LX-04 does not transit to detonation for a pour density (51.3 %TMD) loading at 160 C, but does at 190 C with a longer l than at ambient. The limited ambient temperature measurements for l in high confinement are similar to previous data for 91/9 HMX/wax, which has nearly the same %volume of HMX as LX-04.
Observation of a possible charge-density-wave transition in cubic Ce3Co4Sn13
NASA Astrophysics Data System (ADS)
Lue, C. S.; Liu, H. F.; Hsu, S.-L.; Chu, M. W.; Liao, H. Y.; Kuo, Y. K.
2012-05-01
We report an observation of a first-order phase transition in Ce3Co4Sn13 by means of the specific heat, electrical resistivity, Seebeck coefficient, and thermal conductivity, as well as 59Co nuclear magnetic resonance (NMR) measurements. The phase transition has been evidenced by marked features near To≃155 K in all measured physical quantities except for magnetic susceptibility. This excludes a magnetic origin for the observed phase transition. In addition, x-ray diffraction results below and above To confirm the absence of a structural change, suggesting that the peculiar phase transition is possibly related to an electronic origin and/or electron-lattice coupling such as the formation of a charge density wave (CDW). As a matter of fact, the disappearance of the double-peak feature of 59Co NMR central lines below To can be realized as the spatial modulation of the electric field gradient due to incommensurate CDW superlattices. Also, a distinct peak found in the spin-lattice relaxation rate near To manifests a phase transition and its feature can be accounted for by the thermally driven normal modes of the CDW. From the NMR analyses, we obtained a consistent picture that the change of electronic structures below To is mainly due to the weakening of p-d hybridization. Such an effect could result in possible electron-lattice instability and, thus, the formation of a CDW state in Ce3Co4Sn13.
ERIC Educational Resources Information Center
Field, David; And Others
1992-01-01
Includes four articles: "Career Aspirations" (Field); "Making the Transition to a New Curriculum" (Baker, Householder); "How about a 'Work to School' Transition?" (Glasberg); and "Technological Improvisation: Bringing CNC to Woodworking" (Charles, McDuffie). (SK)
ERIC Educational Resources Information Center
Thompson, Sandy, Ed.; And Others
1990-01-01
This "feature issue" focuses on transition from school to adult life for persons with disabilities. Included are "success stories," brief program descriptions, and a list of resources. Individual articles include the following titles and authors: "Transition: An Energizing Concept" (Paul Bates); "Transition Issues for the 1990s" (William Halloran…
An introduction to inhomogeneous liquids, density functional theory, and the wetting transition
NASA Astrophysics Data System (ADS)
Hughes, Adam P.; Thiele, Uwe; Archer, Andrew J.
2014-12-01
Classical density functional theory (DFT) is a statistical mechanical theory for calculating the density profiles of the molecules in a liquid. It is widely used, for example, to study the density distribution of the molecules near a confining wall, the interfacial tension, wetting behavior, and many other properties of nonuniform liquids. DFT can, however, be somewhat daunting to students entering the field because of the many connections to other areas of liquid-state science that are required and used to develop the theories. Here, we give an introduction to some of the key ideas, based on a lattice-gas (Ising) model fluid. This approach builds on knowledge covered in most undergraduate statistical mechanics and thermodynamics courses, so students can quickly get to the stage of calculating density profiles, etc., for themselves. We derive a simple DFT for the lattice gas and present some typical results that can readily be calculated using the theory.
NASA Astrophysics Data System (ADS)
Singh, Ram Chandra; Ram, Jokhan
2011-11-01
The effects of quadrupole moments on the isotropic-nematic (IN) phase transitions are studied using the density-functional theory (DFT) for a Gay-Berne (GB) fluid for a range of length-to-breadth parameters ? in the reduced temperature range ? . The pair-correlation functions of the isotropic phase, which enter into the DFT as input parameters are found by solving the Percus-Yevick integral equation theory. The method used involves an expansion of angle-dependent functions appearing in the integral equations in terms of spherical harmonics and the harmonic coefficients are obtained by an iterative algorithm. All the terms of harmonic coefficients which involve l indices up to less than or equal to 6 are considered. The numerical accuracy of the results depends on the number of spherical harmonic coefficients considered for each orientation-dependent function. As the length-to-breadth ratio of quadrupolar GB molecules is increased, the IN transition is seen to move to lower density (and pressure) at a given temperature. It has been observed that the DFT is good to study the IN transitions in such fluids. The theoretical results have also been compared with the computer simulation results wherever they are available.
NASA Astrophysics Data System (ADS)
Verma, Ashok K.; Modak, P.; Sharma, Surinder M.; Svane, A.; Christensen, N. E.; Sikka, S. K.
2013-07-01
First-principles calculations have been performed for americium (Am) metal using the generalized gradient approximation + orbital-dependent onsite Coulomb repulsion via Hubbard interaction (GGA+U) and hybrid density functional theory (HYB-DFT) methods to investigate various ground state properties and pressure-induced structural transitions. Both methods yield equilibrium volume and bulk modulus in good agreement with the experimental results. The GGA+spin orbit coupling+U method reproduced all structural transitions under pressure correctly, but the HYB-DFT method failed to reproduce the observed Am-I to Am-II transition. Good agreement was found between calculated and experimental equations of states for all phases, but the first three phases need larger U (α) parameters (where α represents the fraction of Hartree-Fock exchange energy replacing the DFT exchange energy) than the fourth phase in order to match the experimental data. Thus, neither the GGA+U nor the HYB-DFT methods are able to describe the energetics of Am metal properly in the entire pressure range from 0 GPa to 50 GPa with a single choice of their respective U and α parameters. Low binding-energy peaks in the experimental photoemission spectrum at ambient pressure relate, for some parameter choices, well to peak positions in the calculated density of states function of Am-I.
Schlieren Measurements of Buoyancy Effects on Flow Transition in Low-Density Gas Jets
NASA Technical Reports Server (NTRS)
Pasumarthi, Kasyap S.; Agrawal, Ajay K.
2005-01-01
The transition from laminar to turbulent flow in helium jets discharged into air was studied using Rainbow Schlieren Deflectometry technique. In particular, the effects of buoyancy on jet oscillations and flow transition length were considered. Experiments to simulate microgravity were conducted in the 2.2s drop tower at NASA Glenn Research Center. The jet Reynolds numbers varied from 800 to1200 and the jet Richardson numbers ranged between 0.01 and 0.004. Schlieren images revealed substantial variations in the flow structure during the drop. Fast Fourier Transform (FFT) analysis of the data obtained in Earth gravity experiments revealed the existence of a discrete oscillating frequency in the transition region, which matched the frequency in the upstream laminar regime. In microgravity, the transition occurred farther downstream indicating laminarization of the jet in the absence of buoyancy. The amplitude of jet oscillations was reduced by up to an order of magnitude in microgravity. Results suggest that jet oscillations were buoyancy induced and that the brief microgravity period may not be sufficient for the oscillations to completely subside.
NASA Astrophysics Data System (ADS)
Yadav, Amarjeet; Mishra, P. C.
2014-04-01
Polycyclic aromatic hydrocarbons (PAHs) form an important class of molecules as they are ubiquitous, pollute air and cause severe health problems. Lowest vertical π-π* singlet-singlet or triplet-triplet excitation energies and corresponding oscillator strengths were studied for several linear and two-dimensional PAHs employing time-dependent density functional theory. Excited-state electron density, molecular electrostatic potential (MEP) and spin density distributions in the PAHs, along with ground-state chemical hardness, were also studied. It has been found that, generally, excitation energies and oscillator strengths decrease with increase in PAH size, and excitation energies and chemical hardness are strongly linearly correlated. Enhanced electron density edge effect, which was found to occur in the ground states of the molecules, continues to hold in their excited states also. A strong similarity between the ground and π-π* excited-state MEP maps suggests that σ electrons are the main contributors to the enhanced electron density at the edges. Due to their strong electronic absorption transitions in the visible and infrared regions, the PAHs can be used for harnessing solar energy efficiently.
Sandusky, H W; Granholm, R H; Bohl, D G; Hare, D E; Vandersall, K S; Garcia, F
2005-06-01
The potential for deflagration-to-detonation transition (DDT) in LX-04 (85/15 HMX/Viton) is being evaluated as a function of loading density, temperature, and confinement. In the high confinement arrangement, a matrix of tests will be performed with the LX-04 loaded at {approx}50, 70, 90, and {approx}99 %TMD; and temperatures of ambient, 160 C, and 190 C, at each loading density. A more limited set of tests at medium confinement will be conducted. As expected, LX-04 does not undergo DDT at near TMD loadings in both medium and high confinement, although the later still results in significant fragmentation. In high confinement at pour density (50.3 %TMD), LX-04 does not transit to detonation at 160 C, but does at ambient and 190 C with the shortest run distance to detonation (l) at ambient temperature. With a 70% TMD loading at ambient temperature, l was even less. The limited ambient temperature measurements for l in high confinement are similar to previous data for 91/9 HMX/wax, which has nearly the same %volume of HMX as LX-04.
NASA Astrophysics Data System (ADS)
Fan, Liangyou; Ziegler, Tom
1990-03-01
We present here the first determination of transition state structures based on Hartree-Fock-Slater (HFS) density functional calculations. The optimization of the transition state structures utilized the analytical energy gradients evaluated by a recently developed method. The transition state structures obtained by the HFS method are similar to those determined by ab initio methods in which electron correlation is partly taken into account. The energy barrier for the CH3NC→CH3CN isomerization process, corrected for the zero-point energy, is 40.1 kcal mol-1 which is in good agreement with the experimental value of 38.4 kcal mol-1. Experimental barriers are not available for the HNC→HCN and N2H2(trans)→N2H2(cis) isomerization processes. However, the barriers calculated by the HFS method are in good accordance with the barriers obtained from electron-correlation calculations. Furthermore, the vibrational frequencies calculated by the HFS method are in good agreement either with experiments in the case of stable molecules or with electron-correlation calculations in the case of transition states. The HFS method seems in general to provide better estimates of vibrational frequencies and activation barriers than the Hartree-Fock method in which electron correlation is lacking. The later method generally afford too high values for both frequencies and activation barriers.
Structural Transition in a Fluid of Spheroids: A Low-Density Vestige of Jamming
NASA Astrophysics Data System (ADS)
Cohen, A. P.; Dorosz, S.; Schofield, A. B.; Schilling, T.; Sloutskin, E.
2016-03-01
A thermodynamically equilibrated fluid of hard spheroids is a simple model of liquid matter. In this model, the coupling between the rotational degrees of freedom of the constituent particles and their translations may be switched off by a continuous deformation of a spheroid of aspect ratio t into a sphere (t =1 ). We demonstrate, by experiments, theory, and computer simulations, that dramatic nonanalytic changes in structure and thermodynamics of the fluids take place, as the coupling between rotations and translations is made to vanish. This nonanalyticity, reminiscent of a second-order liquid-liquid phase transition, is not a trivial consequence of the shape of an individual particle. Rather, free volume considerations relate the observed transition to a similar nonanalyticity at t =1 in structural properties of jammed granular ellipsoids. This observation suggests a deep connection to exist between the physics of jamming and the thermodynamics of simple fluids.
Structural Transition in a Fluid of Spheroids: A Low-Density Vestige of Jamming.
Cohen, A P; Dorosz, S; Schofield, A B; Schilling, T; Sloutskin, E
2016-03-01
A thermodynamically equilibrated fluid of hard spheroids is a simple model of liquid matter. In this model, the coupling between the rotational degrees of freedom of the constituent particles and their translations may be switched off by a continuous deformation of a spheroid of aspect ratio t into a sphere (t=1). We demonstrate, by experiments, theory, and computer simulations, that dramatic nonanalytic changes in structure and thermodynamics of the fluids take place, as the coupling between rotations and translations is made to vanish. This nonanalyticity, reminiscent of a second-order liquid-liquid phase transition, is not a trivial consequence of the shape of an individual particle. Rather, free volume considerations relate the observed transition to a similar nonanalyticity at t=1 in structural properties of jammed granular ellipsoids. This observation suggests a deep connection to exist between the physics of jamming and the thermodynamics of simple fluids. PMID:26991202
Fluid-solid transition in simple systems using density functional theory
NASA Astrophysics Data System (ADS)
Bharadwaj, Atul S.; Singh, Yashwant
2015-09-01
A free energy functional for a crystal which contains both the symmetry-conserved and symmetry-broken parts of the direct pair correlation function has been used to investigate the fluid-solid transition in systems interacting via purely repulsive Weeks-Chandler-Anderson Lennard-Jones potential and the full Lennard-Jones potential. The results found for freezing parameters for the fluid-face centred cubic crystal transition are in very good agreement with simulation results. It is shown that although the contribution made by the symmetry broken part to the grand thermodynamic potential at the freezing point is small compared to that of the symmetry conserving part, its role is crucial in stabilizing the crystalline structure and on values of the freezing parameters.
Fluid-solid transition in simple systems using density functional theory
Bharadwaj, Atul S.; Singh, Yashwant
2015-09-28
A free energy functional for a crystal which contains both the symmetry-conserved and symmetry-broken parts of the direct pair correlation function has been used to investigate the fluid-solid transition in systems interacting via purely repulsive Weeks-Chandler-Anderson Lennard–Jones potential and the full Lennard–Jones potential. The results found for freezing parameters for the fluid-face centred cubic crystal transition are in very good agreement with simulation results. It is shown that although the contribution made by the symmetry broken part to the grand thermodynamic potential at the freezing point is small compared to that of the symmetry conserving part, its role is crucial in stabilizing the crystalline structure and on values of the freezing parameters.
Nanda, Vikas; Kant, Niti
2014-04-15
Enhanced and early relativistic self-focusing of Hermite-cosh-Gaussian (HChG) beam in the plasmas under density transition has been investigated theoretically using Wentzel-Kramers-Brillouin and paraxial ray approximation for mode indices m=0, 1, and 2. The variation of beam width parameter with normalized propagation distance for m=0, 1, and 2 is reported, and it is observed that strong self-focusing occurs as the HChG beam propagates deeper inside the nonlinear medium as spot size shrinks due to highly dense plasmas and the results are presented graphically. A comparative study between self-focusing of HChG beam in the presence and absence of plasmas density transition is reported. The dependency of beam width parameter on the normalized propagation distance for different values of decentered parameter “b” has also been presented graphically. For m=0 and 1, strong self-focusing is reported for b=1.8, and for m=2 and b=1.8, beam gets diffracted. The results obtained indicate the dependency of the self-focusing of the HChG beam on the selected values of decentered parameter. Moreover, proper selection of decentered parameter results strong self-focusing of HChG beam. Stronger self-focusing of laser beam is observed due to the presence of plasma density transition which might be very useful in the applications like the generation of inertial fusion energy driven by lasers, laser driven accelerators, etc.
Yi, M.
2010-06-02
Through a systematic high-resolution angle-resolved photoemission study of the iron pnictide compounds (Ba,Sr)Fe{sub 2}As{sub 2}, we show that the electronic structures of these compounds are significantly reconstructed across the spin density wave transition, which cannot be described by a simple folding scenario of conventional density wave ordering. Moreover, we find that LDA calculations with an incorporated suppressed magnetic moment of 0.5{mu}{sub B} can match well the details in the reconstructed electronic structure, suggesting that the nature of magnetism in the pnictides is more itinerant than local, while the origin of suppressed magnetic moment remains an important issue for future investigations.
NASA Astrophysics Data System (ADS)
Singh, R. C.
2009-07-01
The effects of quadrupole moments on the phase behaviour of isotropic-nematic transition are studied by using density functional theory for a system of molecules which interact via the Gay-Berne pair potential. The pair correlation functions of isotropic phase, which enter in the theory as input information, are found from the Percus-Yevick integral equation theory. The method used involves an expansion of angle-dependent functions appearing in the integral equations in terms of spherical harmonics and the harmonic coefficients are obtained by an iterative algorithm. All the terms of harmonic coefficients which involve l indices up to less than or equal to six have been considered. The dependence of the accuracy of the results on the number of terms taken in the basis set is explored for both fluids at different densities, temperatures and quadrupole moments. The results have been compared with the available computer simulation results.
Density Transition Based Self-Focusing of cosh-Gaussian Laser Beam in Plasma with Linear Absorption
NASA Astrophysics Data System (ADS)
Niti, Kant; Manzoor, Ahmad Wani
2015-07-01
Density transition based self-focusing of cosh-Gaussian laser beam in plasma with linear absorption has been studied. The field distribution in the plasma is expressed in terms of beam width parameter, decentered parameter, and linear absorption coefficient. The differential equation for the beam width parameter is solved by following Wentzel-Kramers-Brillouin (WKB) and paraxial approximation through parabolic wave equation approach. The behaviour of beam width parameter with dimensionless distance of propagation is studied at optimum values of plasma density, decentered parameter and with different absorption levels in the medium. The results reveal that these parameters can affect the self-focusing significantly. Supported by a Financial Grant from CSIR, New Delhi, India, under Project No. 03(1277)/13/EMR-II
LETTER TO THE EDITOR: Density minimum and liquid liquid phase transition
NASA Astrophysics Data System (ADS)
Poole, Peter H.; Saika-Voivod, Ivan; Sciortino, Francesco
2005-11-01
We present a high-resolution computer simulation study of the equation of state of ST2 water, evaluating the liquid-state properties at 2718 state points, and precisely locating the liquid-liquid critical point (LLCP) occurring in this model. We are thereby able to reveal the interconnected set of density anomalies, spinodal instabilities and response function extrema that occur in the vicinity of an LLCP for the case of a realistic, off-lattice model of a liquid with local tetrahedral order. In particular, we unambiguously identify a density minimum in the liquid state, define its relationship to other anomalies, and show that it arises due to the approach of the liquid structure to a defect-free random tetrahedral network of hydrogen bonds.
A summary of transition probabilities for atomic absorption lines formed in low-density clouds
NASA Technical Reports Server (NTRS)
Morton, D. C.; Smith, W. H.
1973-01-01
A table of wavelengths, statistical weights, and excitation energies is given for 944 atomic spectral lines in 221 multiplets whose lower energy levels lie below 0.275 eV. Oscillator strengths were adopted for 635 lines in 155 multiplets from the available experimental and theoretical determinations. Radiation damping constants also were derived for most of these lines. This table contains the lines most likely to be observed in absorption in interstellar clouds, circumstellar shells, and the clouds in the direction of quasars where neither the particle density nor the radiation density is high enough to populate the higher levels. All ions of all elements from hydrogen to zinc are included which have resonance lines longward of 912 A, although a number of weaker lines of neutrals and first ions have been omitted.
Squeezed states of electrons and transitions of the density of states
NASA Technical Reports Server (NTRS)
Lee, Seung Joo; Um, Chung IN
1993-01-01
Electron systems which have low dimensional properties have been constructed by squeezing the motion in zero, one, or two-directions. An isolated quantum dot is modeled by a potential box with delta-profiled, penetrable potential walls embedded in a large outer box with infinitely high potential walls which represent the world function with respect to vacuum. We show the smooth crossover of the density of states from the three-dimensional to the quasi-zero dimensional electron gas.
A density spike on astrophysical scales from an N-field waterfall transition
NASA Astrophysics Data System (ADS)
Halpern, Illan F.; Hertzberg, Mark P.; Joss, Matthew A.; Sfakianakis, Evangelos I.
2015-09-01
Hybrid inflation models are especially interesting as they lead to a spike in the density power spectrum on small scales, compared to the CMB, while also satisfying current bounds on tensor modes. Here we study hybrid inflation with N waterfall fields sharing a global SO (N) symmetry. The inclusion of many waterfall fields has the obvious advantage of avoiding topologically stable defects for N > 3. We find that it also has another advantage: it is easier to engineer models that can simultaneously (i) be compatible with constraints on the primordial spectral index, which tends to otherwise disfavor hybrid models, and (ii) produce a spike on astrophysically large length scales. The latter may have significant consequences, possibly seeding the formation of astrophysically large black holes. We calculate correlation functions of the time-delay, a measure of density perturbations, produced by the waterfall fields, as a convergent power series in both 1 / N and the field's correlation function Δ (x). We show that for large N, the two-point function is < δt (x) δt (0) > ∝Δ2 (| x |) / N and the three-point function is < δt (x) δt (y) δt (0) > ∝ Δ (| x - y |) Δ (| x |) Δ (| y |) /N2. In accordance with the central limit theorem, the density perturbations on the scale of the spike are Gaussian for large N and non-Gaussian for small N.
NASA Astrophysics Data System (ADS)
Burmistrov, I. S.; Gornyi, I. V.; Mirlin, A. D.
2016-05-01
We develop a theory of the local density of states (LDOS) of disordered superconductors, employing the nonlinear sigma-model formalism and the renormalization-group framework. The theory takes into account the interplay of disorder and interaction couplings in all channels, treating the systems with short-range and Coulomb interactions on equal footing. We explore two-dimensional systems that would be Anderson insulators in the absence of interaction and two- or three-dimensional systems that undergo an Anderson transition in the absence of interaction. We evaluate both the average tunneling density of states and its mesoscopic fluctuations which are related to the LDOS multifractality in normal disordered systems. The obtained average LDOS shows a pronounced depletion around the Fermi energy, both in the metallic phase (i.e., above the superconducting critical temperature Tc) and in the insulating phase near the superconductor-insulator transition (SIT). The fluctuations of the LDOS are found to be particularly strong for the case of short-range interactions, especially, in the regime when Tc is enhanced by Anderson localization. On the other hand, the long-range Coulomb repulsion reduces the mesoscopic LDOS fluctuations. However, also in a model with Coulomb interaction, the fluctuations become strong when the systems approach the SIT.
Pressure-induced changes in the electron density distribution in α-Ge near the α-β transition
Li, Rui; Liu, Jing; Bai, Ligang; Shen, Guoyin; Tse, John S.
2015-08-17
Electron density distributions in α-Ge have been determined under high pressure using maximum entropy method with structure factors obtained from single crystal synchrotron x-ray diffraction in a diamond anvil cell. The results show that the sp{sup 3} bonding is enhanced with increasing pressure up to 7.7(1) GPa. At higher pressures but below the α-β transition pressure of 11.0(1) GPa, the sp{sup 3}-like electron distribution progressively weakens with a concomitant increase of d-orbitals hybridization. The participation of d-orbitals in the electronic structure is supported by Ge Kβ{sub 2} (4p-1s) x-ray emission spectroscopy measurements showing the reduction of 4s character in the valence band at pressures far below the α-β transition. The gradual increase of d-orbitals in the valence level in the stability field of α-Ge is directly related to the eventual structural transition.
NASA Astrophysics Data System (ADS)
Yi, Jinqiao; Zhang, Ling; Xie, Bing; Jiang, Shenglin
2015-09-01
Anti-ferroelectric (AFE) composite ceramics of (Pb0.858Ba0.1La0.02Y0.008)(Zr0.65Sn0.3Ti0.05)O3-(Pb0.97La0.02)(Zr0.9Sn0.05Ti0.05)O3 (PBLYZST-PLZST) were fabricated by the conventional solid-state sintering process (CS), the glass-aided sintering (GAS), and the spark plasma sintering (SPS), respectively. The influence of the temperature induced phase transition on the phase structure, hysteresis loops, and energy storage properties of the composite ceramics were investigated in detail. The measured results of X-ray diffraction demonstrate that the composite ceramics exhibit the perovskite phases and small amounts of non-functional pyrochlore phases. Compared with the CS process, the GAS and SPS processes are proven more helpful to suppress the diffusion behaviors between the PBLYZST and PLZST phases according to the field emission scanning electron microscopy, thereby being able to improve the contribution of PBLYZST phase to the temperature stability of the orthogonal AFE phase. When the ambient temperature rises from 25 °C to 125 °C, CS and GAS samples have undergone a phase transition from orthorhombic AFE phase to tetragonal AFE phase, which results in a sharp decline in the energy storage density. However, the phase transition temperature of SPS samples is higher than 125 °C, and the energy storage density only slightly decreases due to the disorder of material microstructure caused by the high temperature. As a result, the SPS composite ceramics obtain a recoverable high energy storage density of 6.46 J/cm3 and the excellent temperature stability of the energy storage density of 1.16 × 10-2 J/°C.cm3, which is 1.29 × 10-2 J/°C.cm3 lower than that of CS samples and about 0.43 times as that of GAS samples.
Yi, Jinqiao; Zhang, Ling; Xie, Bing; Jiang, Shenglin
2015-09-28
Anti-ferroelectric (AFE) composite ceramics of (Pb{sub 0.858}Ba{sub 0.1}La{sub 0.02}Y{sub 0.008})(Zr{sub 0.65}Sn{sub 0.3}Ti{sub 0.05})O{sub 3}-(Pb{sub 0.97}La{sub 0.02})(Zr{sub 0.9}Sn{sub 0.05} Ti{sub 0.05})O{sub 3} (PBLYZST-PLZST) were fabricated by the conventional solid-state sintering process (CS), the glass-aided sintering (GAS), and the spark plasma sintering (SPS), respectively. The influence of the temperature induced phase transition on the phase structure, hysteresis loops, and energy storage properties of the composite ceramics were investigated in detail. The measured results of X-ray diffraction demonstrate that the composite ceramics exhibit the perovskite phases and small amounts of non-functional pyrochlore phases. Compared with the CS process, the GAS and SPS processes are proven more helpful to suppress the diffusion behaviors between the PBLYZST and PLZST phases according to the field emission scanning electron microscopy, thereby being able to improve the contribution of PBLYZST phase to the temperature stability of the orthogonal AFE phase. When the ambient temperature rises from 25 °C to 125 °C, CS and GAS samples have undergone a phase transition from orthorhombic AFE phase to tetragonal AFE phase, which results in a sharp decline in the energy storage density. However, the phase transition temperature of SPS samples is higher than 125 °C, and the energy storage density only slightly decreases due to the disorder of material microstructure caused by the high temperature. As a result, the SPS composite ceramics obtain a recoverable high energy storage density of 6.46 J/cm{sup 3} and the excellent temperature stability of the energy storage density of 1.16 × 10{sup −2} J/°C·cm{sup 3}, which is 1.29 × 10{sup −2} J/°C·cm{sup 3} lower than that of CS samples and about 0.43 times as that of GAS samples.
Comparison of transition densities in the DDHMS model of pre-equilibrium emission
Brito, L.; Carlson, B. V.
2014-11-11
The DDHMS (double differential hybrid Monte Carlo simulation) model treats nucleon-induced pre-equilibrium reactions as a series of particle-particle and particle-hole interactions in the space of energy and angle. This work compares spectra obtained within the model using diferent approximations to the density of accessible states. The calculations are performed with the EMPIRE reaction model code, a modular system containing several nuclear reaction models that permits a fairly complete descritpion of the reaction, from elastic scattering and absorption through the pre-equilbrium stage to the final decay by statistical emission.
Comparison of transition densities in the DDHMS model of pre-equilibrium emission
NASA Astrophysics Data System (ADS)
Brito, L.; Carlson, B. V.
2014-11-01
The DDHMS (double differential hybrid Monte Carlo simulation) model treats nucleon-induced pre-equilibrium reactions as a series of particle-particle and particle-hole interactions in the space of energy and angle. This work compares spectra obtained within the model using diferent approximations to the density of accessible states. The calculations are performed with the EMPIRE reaction model code, a modular system containing several nuclear reaction models that permits a fairly complete descritpion of the reaction, from elastic scattering and absorption through the pre-equilbrium stage to the final decay by statistical emission.
Transition from order to chaos, and density limit, in magnetized plasmas
Carati, A.; Maiocchi, A.; Marino, M.; Galgani, L.; Zuin, M.; Martines, E.
2012-09-15
It is known that a plasma in a magnetic field, conceived microscopically as a system of point charges, can exist in a magnetized state, and thus remain confined, inasmuch as it is in an ordered state of motion, with the charged particles performing gyrational motions transverse to the field. Here, we give an estimate of a threshold, beyond which transverse motions become chaotic, the electrons being unable to perform even one gyration, so that a breakdown should occur, with complete loss of confinement. The estimate is obtained by the methods of perturbation theory, taking as perturbing force acting on each electron that due to the so-called microfield, i.e., the electric field produced by all the other charges. We first obtain a general relation for the threshold, which involves the fluctuations of the microfield. Then, taking for such fluctuations, the formula given by Iglesias, Lebowitz, and MacGowan for the model of a one component plasma with neutralizing background, we obtain a definite formula for the threshold, which corresponds to a density limit increasing as the square of the imposed magnetic field. Such a theoretical density limit is found to fit pretty well the empirical data for collapses of fusion machines.
Fermi Surface Evolution Across Multiple Charge Density Wave Transitions in ErTe3
Moore, R.G.; Brouet, V.; He, R.; Lu, D.H.; Ru, N.; Chu, J.-H.; Fisher, I.R.; Shen, Z.-X.; /SLAC, SSRL /Stanford U., Geballe Lab.
2010-02-15
The Fermi surface (FS) of ErTe{sub 3} is investigated using angle-resolved photoemission spectroscopy (ARPES). Low temperature measurements reveal two incommensurate charge density wave (CDW) gaps created by perpendicular FS nesting vectors. A large {Delta}{sub 1} = 175 meV gap arising from a CDW with c* - q{sub CDW1} {approx} 0.70(0)c* is in good agreement with the expected value. A second, smaller {Delta}{sub 2} = 50 meV gap is due to a second CDW with a* - q{sub CDW2} {approx} 0.68(5)a*. The temperature dependence of the FS, the two gaps and possible interaction between the CDWs are examined.
Averkiev, Boris B; Zhao, Yan; Truhlar, Donald G
2010-06-01
The structures of Pd(PH₃)₂ and Pt(PH₃)₂ complexes with ethene and conjugated CnH_{n+2} systems (n=4, 6, 8, and 10) were studied. Their binding energies were calculated using both wave function theory (WFT) and density functional theory (DFT). Previously it was reported that the binding energy of the alkene to the transition metal does not depend strongly on the size of the conjugated C_{n}H_{n+2} ligand, but that DFT methods systematically underestimate the binding energy more and more significantly as the size of the conjugated system is increased. Our results show that recently developed density functionals predict the binding energy for these systems much more accurately. New benchmark calculations carried out by the coupled cluster method based on Brueckner orbitals with double excitations and a quasiperturbative treatment of connected triple excitations (BCCD(T)) with a very large basis set agree even better with the DFT predictions than do the previous best estimates. The mean unsigned error in absolute and relative binding energies of the alkene ligands to Pd(PH₃)₂ is 2.5 kcal/mol for the ωB97 and M06 density functionals and 2.9 kcal/mol for the M06-L functional. Adding molecular mechanical damped dispersion yields even smaller mean unsigned errors: 1.3 kcal/mol for the M06-D functional, 1.5 kcal/mol for M06- L-D, and 1.8 kcal/mol for B97-D and ωB97X-D. The new functionals also lead to improved accuracy for the analogous Pt complexes. These results show that recently developed density functionals may be very useful for studying catalytic systems involving Pd d¹º centers and alkenes.
Herron, Jeffrey A.; Scaranto, Jessica; Ferrin, Peter A.; Li, Sha; Mavrikakis, Manos
2014-12-05
We present a first-principles, self-consistent periodic density functional theory (PW91-GGA) study of formic acid (HCOOH) decomposition on model (111) and (100) facets of eight fcc metals (Au, Ag, Cu, Pt, Pd, Ni, Ir, and Rh) and (0001) facets of four hcp (Co, Os, Ru, and Re) metals. The calculated binding energies of key formic acid decomposition intermediates including formate (HCOO), carboxyl (COOH), carbon monoxide (CO), water (H2O), carbon dioxide (CO2), hydroxyl (OH), carbon (C), oxygen (O), and hydrogen (H; H2) are presented. Using these energetics, we develop thermochemical potential energy diagrams for both the carboxyl-mediated and the formate-mediated dehydrogenation mechanisms on each surface. We evaluate the relative stability of COOH, HCOO, and other isomeric intermediates (i.e., CO + OH, CO2 + H, CO + O + H) on these surfaces. These results provide insights into formic acid decomposition selectivity (dehydrogenation versus dehydration), and in conjunction with calculated vibrational frequency modes, the results can assist with the experimental search for the elusive carboxyl (COOH) surface intermediate. Results are compared against experimental reports in the literature.
NASA Astrophysics Data System (ADS)
Muñoz, M. C.; Sacedón, J. L.; Soria, F.; Martinez, V.
1986-07-01
Auger and electron loss spectroscopies have been used to study the local chemical bond between Si and Al, in the first stages of growth of Si deposited at room temperature on Al(111) surfaces. Si follows a layer-by-layer mechanism up to 2 monolayers with the formation of an Al(111)-3 × 3-Si structure at about 0.44 monolayers. A detailed analysis of the L 2,3VV Auger spectra for this structure allows to interpret the Si and Al Auger transition density of states (TDOS) in terms of the actual p-like partial DOS centered on the Si and Al sites. The experimental results indicate a strong SiAl interaction with the formation of a p-type local covalent bond between the Si and Al surface atoms.
NASA Astrophysics Data System (ADS)
Das, Tanmoy
2016-07-01
We study directional dependent band gap evolutions and metal–insulator transitions (MITs) in model quantum wire systems within the spin–orbit density wave (SODW) model. The evolution of MIT is studied as a function of varying anisotropy between the intra-wire hopping ({{t}\\parallel} ) and inter-wire hopping ({{t}\\bot} ) with Rashba spin–orbit coupling. We find that as long as the anisotropy ratio (β ={{t}\\bot}/{{t}\\parallel} ) remains below 0.5, and the Fermi surface nesting is tuned to {{\\mathbf{Q}}1}=≤ft(π,0\\right) , an exotic SODW induced MIT easily develops, with its critical interaction strength increasing with increasing anisotropy. As β \\to 1 (2D system), the nesting vector switches to {{\\mathbf{Q}}2}=≤ft(π,π \\right) , making this state again suitable for an isotropic MIT. Finally, we discuss various physical consequences and possible applications of the directional dependent MIT.
Liu, Bin; Greeley, Jeffrey P.
2013-05-07
We describe an accelerated density functional theory (DFT)-based computational strategy to determine trends in the decomposition of glycerol via elementary dehydrogenation, C–C, and C–O bond scission reactions on close-packed transition metal surfaces. Beginning with periodic DFT calculations on Pt(111), the thermochemistry of glycerol dehydrogenation on Pd(111), Rh(111), Cu(111) and Ni(111) is determined using a parameter-free, bond order-based scaling relationship. By combining the results with Brønsted–Evans–Polanyi (BEP) relationships to estimate elementary reaction barriers, free energy diagrams are developed on the respective metal surfaces, and trends concerning the relative selectivity and activity for C–C and C–O bond scission in glycerol on the various metals are obtained. The results are consistent with available theoretical and experimental literature and demonstrate that scaling relationships are capable of providing powerful insights into the catalytic chemistry of complex biomolecules.
Das, Tanmoy
2016-07-27
We study directional dependent band gap evolutions and metal-insulator transitions (MITs) in model quantum wire systems within the spin-orbit density wave (SODW) model. The evolution of MIT is studied as a function of varying anisotropy between the intra-wire hopping ([Formula: see text]) and inter-wire hopping ([Formula: see text]) with Rashba spin-orbit coupling. We find that as long as the anisotropy ratio ([Formula: see text]) remains below 0.5, and the Fermi surface nesting is tuned to [Formula: see text], an exotic SODW induced MIT easily develops, with its critical interaction strength increasing with increasing anisotropy. As [Formula: see text] (2D system), the nesting vector switches to [Formula: see text], making this state again suitable for an isotropic MIT. Finally, we discuss various physical consequences and possible applications of the directional dependent MIT. PMID:27248294
A Framework for Characterizing the Atmospheres of Low-mass Low-density Transiting Planets
NASA Astrophysics Data System (ADS)
Fortney, Jonathan J.; Mordasini, Christoph; Nettelmann, Nadine; Kempton, Eliza M.-R.; Greene, Thomas P.; Zahnle, Kevin
2013-09-01
We perform modeling investigations to aid in understanding the atmospheres and composition of small planets of ~2-4 Earth radii, which are now known to be common in our Galaxy. GJ 1214b is a well-studied example whose atmospheric transmission spectrum has been observed by many investigators. Here we take a step back from GJ 1214b to investigate the role that planetary mass, composition, and temperature play in impacting the transmission spectra of these low-mass low-density (LMLD) planets. Under the assumption that these planets accrete modest hydrogen-dominated atmospheres and planetesimals, we use population synthesis models to show that predicted metal enrichments of the H/He envelope are high, with metal mass fraction Z env values commonly 0.6-0.9, or ~100-400+ times solar. The high mean molecular weight of such atmospheres (μ ≈ 5-12) would naturally help to flatten the transmission spectrum of most LMLD planets. The high metal abundance would also provide significant condensible material for cloud formation. It is known that the H/He abundance in Uranus and Neptune decreases with depth, and we show that atmospheric evaporation of LMLD planets could expose atmospheric layers with gradually higher Z env. However, values of Z env close to solar composition can also arise, so diversity should be expected. Photochemically produced hazes, potentially due to methane photolysis, are another possibility for obscuring transmission spectra. Such hazes may not form above T eq of ~800-1100 K, which is testable if such warm, otherwise low mean molecular weight atmospheres are stable against atmospheric evaporation. We find that available transmission data are consistent with relatively high mean molecular weight atmospheres for GJ 1214b and "warm Neptune" GJ 436b. We examine future prospects for characterizing GJ 1214b with Hubble and the James Webb Space Telescope.
Liang, Binyong; Wang, Xuefeng; Andrews, Lester
2009-04-01
Laser-ablated Ni, Pd, and Pt atoms were reacted with sulfur molecules emerging from a microwave discharge in argon during condensation at 7 K. Reaction products were identified from matrix infrared spectra, sulfur isotopic shifts, spectra of sulfur isotopic mixtures, and frequencies from density functional calculations. The strongest absorptions are observed at 597.9, 596.1, and 583.6 cm(-1), respectively, for the group 10 metals. These absorptions show large sulfur-34 shifts and 32/34 isotopic frequency ratios (1.0282, 1.0285, 1.0298) that are appropriate for S-S stretching modes. Of most importance, mixed 32/34 isotopic 1/4/4/2/4/1 sextets identify this product with two equivalent S(2) molecules containing equivalent atomic positions as the bisdisulfur pi complexes M(S(2))(2). Our DFT calculations find stable D(2h) structures with B(1u) ground states and intense b(1u) infrared active modes a few wavenumbers higher than the observed values. A minor Ni product at 505.8, 502.7 cm(-1) shows the proper sulfur-34 shift for assignment to (58)NiS, (60)NiS. Another major product with Pt at 512.2 cm(-1) reveals an asymmetric triplet absorption with mixed sulfur 32/34, which is appropriate for assignment to the SPtS disulfide molecule. A weak 491.7 cm(-1) peak exhibits the sulfur-34 shift expected for PtS, and this assignment follows. PMID:19281209
A FRAMEWORK FOR CHARACTERIZING THE ATMOSPHERES OF LOW-MASS LOW-DENSITY TRANSITING PLANETS
Fortney, Jonathan J.; Nettelmann, Nadine; Mordasini, Christoph; Kempton, Eliza M.-R.; Greene, Thomas P.; Zahnle, Kevin
2013-09-20
We perform modeling investigations to aid in understanding the atmospheres and composition of small planets of ∼2-4 Earth radii, which are now known to be common in our Galaxy. GJ 1214b is a well-studied example whose atmospheric transmission spectrum has been observed by many investigators. Here we take a step back from GJ 1214b to investigate the role that planetary mass, composition, and temperature play in impacting the transmission spectra of these low-mass low-density (LMLD) planets. Under the assumption that these planets accrete modest hydrogen-dominated atmospheres and planetesimals, we use population synthesis models to show that predicted metal enrichments of the H/He envelope are high, with metal mass fraction Z{sub env} values commonly 0.6-0.9, or ∼100-400+ times solar. The high mean molecular weight of such atmospheres (μ ≈ 5-12) would naturally help to flatten the transmission spectrum of most LMLD planets. The high metal abundance would also provide significant condensible material for cloud formation. It is known that the H/He abundance in Uranus and Neptune decreases with depth, and we show that atmospheric evaporation of LMLD planets could expose atmospheric layers with gradually higher Z{sub env}. However, values of Z{sub env} close to solar composition can also arise, so diversity should be expected. Photochemically produced hazes, potentially due to methane photolysis, are another possibility for obscuring transmission spectra. Such hazes may not form above T{sub eq} of ∼800-1100 K, which is testable if such warm, otherwise low mean molecular weight atmospheres are stable against atmospheric evaporation. We find that available transmission data are consistent with relatively high mean molecular weight atmospheres for GJ 1214b and 'warm Neptune' GJ 436b. We examine future prospects for characterizing GJ 1214b with Hubble and the James Webb Space Telescope.
NASA Astrophysics Data System (ADS)
Nawa, Kenji; Nakamura, Kohji; Akiyama, Toru; Ito, Tomonori; Weinert, Michael
Effective on-site Coulomb interactions (Ueff) and electron configurations in the localized d and f orbitals of metal complexes in transition-metal oxides and organometallic molecules, play a key role in the first-principles search for the true ground-state. However, wide ranges of values in the Ueff parameter of a material, even in the same ionic state, are often reported. Here, we revisit this issue from constraint density functional theory (DFT) by using the full-potential linearized augmented plane wave method. The Ueff parameters for prototypical transition-metal oxides, TMO (TM =Mn, Fe, Co, Ni), were calculated by the second derivative of the total energy functional with respect to the d occupation numbers inside the muffin-tin (MT) spheres as a function of the sphere radius. We find that the calculated Ueff values depend significantly on the MT radius, with a variation of more than 3 eV when the MT radius changes from 2.0 to 2.7 a.u., but importantly an identical valence band structure can be produced in all the cases, with an approximate scaling of Ueff. This indicates that a simple transferability of the Ueff value among different calculation methods is not allowed. We further extend the constraint DFT to treat various electron configurations of the localized d-orbitals in organometallic molecules, TMCp2 (TM =Cr, Mn, Fe, Co, Ni), and find that the calculated Ueff values can reproduce the experimentally determined ground-state electron configurations.
NASA Astrophysics Data System (ADS)
Truhlik, Vladimir; Triskova, Ludmila; Benson, Robert; Bilitza, Dieter; Chu, Philip; Richards, Phil G.; Wang, Yongli
The upper transition height (Ht) (the altitude of the transition from heavy atomic ions to light ions or in the simplest form the transition from O+ to H+) is an important parameter, representing the boundary between the ionosphere and the plasmasphere. Ht is very sensitive to various geophysical parameters, like solar and magnetic activity and strongly depends on latitude and local time. There were numerous studies of this parameter in past decades. In spite of these efforts, no model satisfactorily represents this parameter so far. Moreover, surprising evidence of very low transition heights during the last prolonged solar minimum, of a level never obtained before, have been reported. We investigate the upper transition height on the global scale. We made progress in processing large data sets of Ht deduced from the Alouette/ISIS topside sounder and from the Formosat-3/COSMIC vertical electron-density profiles Ne(h) using the theoretical Global Plasma Ionosphere Density (GPID) model (Webb and Essex, 2004) and a revised non-linear function describing the scale height vs. altitude (Titheridge, 1976) to fit the vertical density profiles to the observed profiles and to determine the upper transition height. Since both methods require the plasma temperatures and their gradients as input, these are calculated using the IRI2012 model. Both methods are verified using a large amount of electron and ion density profiles simulated by the FLIP theoretical model and their accuracy is discussed. We compare the results from Alouette/ISIS and Formosat-3/COSMIC and present a global distribution of the calculated Ht and its dependence on geophysical parameters. Finally we compare it with Ht calculated using the IRI ion composition model. Titheridge, J.E., 1976. Ion Transition Heights from Topside Electron-Density Profiles. Planetary and Space Science 24 (3), 229-245. Webb, P.A., Essex, E.A., 2004. A dynamic global model of the plasmasphere. Journal of Atmospheric and Solar
Cao, Ye; Liu, Xia -Ji; He, Lianyi; Long, Gui -Lu; Hu, Hui
2015-02-09
We theoretically investigate the superfluid density and Berezinskii-Kosterlitz-Thouless (BKT) transition of a two-dimensional Rashba spin-orbit-coupled atomic Fermi gas with both in-plane and out-of-plane Zeeman fields. It was recently predicted that, by tuning the two Zeeman fields, the system may exhibit different exotic Fulde-Ferrell (FF) superfluid phases, including the gapped FF, gapless FF, gapless topological FF, and gapped topological FF states. Due to the FF paring, we show that the superfluid density (tensor) of the system becomes anisotropic. When an in-plane Zeeman field is applied along the x direction, the tensor component along the y direction n_{s},yy is generally larger than n_{s},xx in most parameter space. At zero temperature, there is always a discontinuity jump in n_{s},xx as the system evolves from a gapped FF into a gapless FF state. With increasing temperature, such a jump is gradually washed out. The critical BKT temperature has been calculated as functions of the spin-orbit-coupling strength, interatomic interaction strength, and in-plane and out-of-plane Zeeman fields. We predict that the novel FF superfluid phases have a significant critical BKT temperature, typically at the order of 0.1T_{F}, where T_{F} is the Fermi degenerate temperature. Furthermore, their observation is within the reach of current experimental techniques in cold-atom laboratories.
Cao, Ye; Liu, Xia -Ji; He, Lianyi; Long, Gui -Lu; Hu, Hui
2015-02-09
We theoretically investigate the superfluid density and Berezinskii-Kosterlitz-Thouless (BKT) transition of a two-dimensional Rashba spin-orbit-coupled atomic Fermi gas with both in-plane and out-of-plane Zeeman fields. It was recently predicted that, by tuning the two Zeeman fields, the system may exhibit different exotic Fulde-Ferrell (FF) superfluid phases, including the gapped FF, gapless FF, gapless topological FF, and gapped topological FF states. Due to the FF paring, we show that the superfluid density (tensor) of the system becomes anisotropic. When an in-plane Zeeman field is applied along the x direction, the tensor component along the y direction ns,yy is generally larger thanmore » ns,xx in most parameter space. At zero temperature, there is always a discontinuity jump in ns,xx as the system evolves from a gapped FF into a gapless FF state. With increasing temperature, such a jump is gradually washed out. The critical BKT temperature has been calculated as functions of the spin-orbit-coupling strength, interatomic interaction strength, and in-plane and out-of-plane Zeeman fields. We predict that the novel FF superfluid phases have a significant critical BKT temperature, typically at the order of 0.1TF, where TF is the Fermi degenerate temperature. Furthermore, their observation is within the reach of current experimental techniques in cold-atom laboratories.« less
Stránský, Pavel; Macek, Michal; Cejnar, Pavel
2014-06-15
Quantum systems with a finite number of freedom degrees f develop robust singularities in the energy spectrum of excited states as the system’s size increases to infinity. We analyze the general form of these singularities for low f, particularly f=2, clarifying the relation to classical stationary points of the corresponding potential. Signatures in the smoothed energy dependence of the quantum state density and in the flow of energy levels with an arbitrary control parameter are described along with the relevant thermodynamical consequences. The general analysis is illustrated with specific examples of excited-state singularities accompanying the first-order quantum phase transition. -- Highlights: •ESQPTs found in infinite-size limit of systems with low numbers of freedom degrees f. •ESQPTs related to non-analytical evolutions of classical phase–space properties. •ESQPT signatures analyzed for general f, particularly f=2, extending known case f=1. •ESQPT signatures identified in smoothened density and flow of energy spectrum. •ESQPTs shown to induce a new type of thermodynamic anomalies.
NASA Astrophysics Data System (ADS)
Goodpaster, Jason D.; Barnes, Taylor A.; Manby, Frederick R.; Miller, Thomas F.
2012-12-01
Density functional theory (DFT) embedding provides a formally exact framework for interfacing correlated wave-function theory (WFT) methods with lower-level descriptions of electronic structure. Here, we report techniques to improve the accuracy and stability of WFT-in-DFT embedding calculations. In particular, we develop spin-dependent embedding potentials in both restricted and unrestricted orbital formulations to enable WFT-in-DFT embedding for open-shell systems, and develop an orbital-occupation-freezing technique to improve the convergence of optimized effective potential calculations that arise in the evaluation of the embedding potential. The new techniques are demonstrated in applications to the van-der-Waals-bound ethylene-propylene dimer and to the hexa-aquairon(II) transition-metal cation. Calculation of the dissociation curve for the ethylene-propylene dimer reveals that WFT-in-DFT embedding reproduces full CCSD(T) energies to within 0.1 kcal/mol at all distances, eliminating errors in the dispersion interactions due to conventional exchange-correlation (XC) functionals while simultaneously avoiding errors due to subsystem partitioning across covalent bonds. Application of WFT-in-DFT embedding to the calculation of the low-spin/high-spin splitting energy in the hexaaquairon(II) cation reveals that the majority of the dependence on the DFT XC functional can be eliminated by treating only the single transition-metal atom at the WFT level; furthermore, these calculations demonstrate the substantial effects of open-shell contributions to the embedding potential, and they suggest that restricted open-shell WFT-in-DFT embedding provides better accuracy than unrestricted open-shell WFT-in-DFT embedding due to the removal of spin contamination.
NASA Astrophysics Data System (ADS)
Fujimoto, Kazuhiro J.
2012-07-01
A transition-density-fragment interaction (TDFI) combined with a transfer integral (TI) method is proposed. The TDFI method was previously developed for describing electronic Coulomb interaction, which was applied to excitation-energy transfer (EET) [K. J. Fujimoto and S. Hayashi, J. Am. Chem. Soc. 131, 14152 (2009)] and exciton-coupled circular dichroism spectra [K. J. Fujimoto, J. Chem. Phys. 133, 124101 (2010)]. In the present study, the TDFI method is extended to the exchange interaction, and hence it is combined with the TI method for applying to the EET via charge-transfer (CT) states. In this scheme, the overlap correction is also taken into account. To check the TDFI-TI accuracy, several test calculations are performed to an ethylene dimer. As a result, the TDFI-TI method gives a much improved description of the electronic coupling, compared with the previous TDFI method. Based on the successful description of the electronic coupling, the decomposition analysis is also performed with the TDFI-TI method. The present analysis clearly shows a large contribution from the Coulomb interaction in most of the cases, and a significant influence of the CT states at the small separation. In addition, the exchange interaction is found to be small in this system. The present approach is useful for analyzing and understanding the mechanism of EET.
Escudero, Daniel E-mail: thiel@kofo.mpg.de; Thiel, Walter E-mail: thiel@kofo.mpg.de
2014-05-21
We report an assessment of the performance of density functional theory-based multireference configuration interaction (DFT/MRCI) calculations for a set of 3d- and 4d-transition metal (TM) complexes. The DFT/MRCI results are compared to published reference data from reliable high-level multi-configurational ab initio studies. The assessment covers the relative energies of different ground-state minima of the highly correlated CrF{sub 6} complex, the singlet and triplet electronically excited states of seven typical TM complexes (MnO{sub 4}{sup −}, Cr(CO){sub 6}, [Fe(CN){sub 6}]{sup 4−}, four larger Fe and Ru complexes), and the corresponding electronic spectra (vertical excitation energies and oscillator strengths). It includes comparisons with results from different flavors of time-dependent DFT (TD-DFT) calculations using pure, hybrid, and long-range corrected functionals. The DFT/MRCI method is found to be superior to the tested TD-DFT approaches and is thus recommended for exploring the excited-state properties of TM complexes.
NASA Technical Reports Server (NTRS)
Ricca, Alessandra; Bauschlicher, Charles W.; Langhoff, Stephen R. (Technical Monitor)
1994-01-01
Density functional theory (DFT) is found to give a better description of the geometries and vibrational frequencies of FeL and FeL(sup +) systems than second order Moller Plesset perturbation theory (MP2). Namely, the DFT correctly predicts the shift in the CO vibrational frequency between free CO and the Sigma(sup -) state of FeCO and yields a good result for the Fe-C distance in the quartet states of FeCH4(+) 4 These are properties where the MP2 results are unsatisfactory. Thus DFT appears to be an excellent approach for optimizing the geometries and computing the zero-point energies of systems containing first transition row atoms. Because the DFT approach is biased in favor of the 3d(exp 7) occupation, whereas the more traditional approaches are biased in favor of the 3d(exp 6) occupation, differences are found in the relative ordering of states. It is shown that if the dissociation is computed to the most appropriate atomic asymptote and corrected to the ground state asymptote using the experimental separations, the DFT results are in good agreement with high levels of theory. The energetics at the DFT level are much superior to the MP2 and in most cases in good agreement with high levels of theory.
NASA Astrophysics Data System (ADS)
Bagchi, Sabyasachi; Mandal, Debasish; Ghosh, Deepanwita; Das, Abhijit K.
2012-05-01
The structure, bonding, and energetics of the complexes obtained from the interaction between the most stable tautomeric forms of free DNA and RNA bases and Zn2+, Cd2+ and Hg2+ cations have been studied using density functional B3LYP method. The 6-311+G (2df, 2p) basis set along with LANL2DZ pseudopotentials for the cations are used in the calculations. The tautomerization paths of the nucleobases are investigated and transition states between the tautomeric forms of the free bases are located. The relative stability of the complexes and the tautomers of the free nucleobases are discussed referring to MIA and relative energy values. For uracil, thymine and adenine, interaction of the metal cations with the most stable tautomers form the least stable molecular complexes. For cytosine and guanine, the stability of the metalated complexes differs significantly. The enthalpy (ΔH), entropy (TΔS) and free energy (ΔG) of the complexes at 298 K have also been calculated.
NASA Astrophysics Data System (ADS)
Mattsson, Ann E.; Armiento, Rickard; Hao, Feng
2011-03-01
The transition metal oxides (TMO) are a class of compounds that are difficult to treat in density functional theory (DFT) with simple local and semi-local functionals. Especially for CuO, they failed to give the correct equilibrium monoclinic structure. The major source of the deficiency is attributed to the imperfect cancellation of the electronic self-interaction (SI) in the approximated exchange energy. Previous studies show that a large part of the SI error is connected to the confinement error that can be modeled by harmonic-oscillator (HO) systems. We discuss recent advances towards a simple methodology to quantify the confinement errors in real TMO systems. Our results show that these confinement errors may account for the deficiencies of DFT functionals in obtaining the correct equilibrium structure of the TMO. Sandia is a multiprogram laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the U.S. Department of Energy's National Nuclear Security Administration under Contract No. DE-AC04-94AL85000.
High-density lipoprotein contribute to G0-G1/S transition in Swiss NIH/3T3 fibroblasts
Angius, Fabrizio; Spolitu, Stefano; Uda, Sabrina; Deligia, Stefania; Frau, Alessandra; Banni, Sebastiano; Collu, Maria; Accossu, Simonetta; Madeddu, Clelia; Serpe, Roberto; Batetta, Barbara
2015-01-01
High density lipoproteins (HDLs) play a crucial role in removing excess cholesterol from peripheral tissues. Although their concentration is lower during conditions of high cell growth rate (cancer and infections), their involvement during cell proliferation is not known. To this aim, we investigated the replicative cycles in synchronised Swiss 3T3 fibroblasts in different experimental conditions: i) contact-inhibited fibroblasts re-entering cell cycle after dilution; ii) scratch-wound assay; iii) serum-deprived cells induced to re-enter G1 by FCS, HDL or PDGF. Analyses were performed during each cell cycle up to quiescence. Cholesterol synthesis increased remarkably during the replicative cycles, decreasing only after cells reached confluence. In contrast, cholesteryl ester (CE) synthesis and content were high at 24 h after dilution and then decreased steeply in the successive cycles. Flow cytometry analysis of DiO-HDL, as well as radiolabeled HDL pulse, demonstrated a significant uptake of CE-HDL in 24 h. DiI-HDL uptake, lipid droplets (LDs) and SR-BI immunostaining and expression followed the same trend. Addition of HDL or PDGF partially restore the proliferation rate and significantly increase SR-BI and pAKT expression in serum-deprived cells. In conclusion, cell transition from G0 to G1/S requires CE-HDL uptake, leading to CE-HDL/SR-BI pathway activation and CEs increase into LDs. PMID:26640042
High-density lipoprotein contribute to G0-G1/S transition in Swiss NIH/3T3 fibroblasts.
Angius, Fabrizio; Spolitu, Stefano; Uda, Sabrina; Deligia, Stefania; Frau, Alessandra; Banni, Sebastiano; Collu, Maria; Accossu, Simonetta; Madeddu, Clelia; Serpe, Roberto; Batetta, Barbara
2015-01-01
High density lipoproteins (HDLs) play a crucial role in removing excess cholesterol from peripheral tissues. Although their concentration is lower during conditions of high cell growth rate (cancer and infections), their involvement during cell proliferation is not known. To this aim, we investigated the replicative cycles in synchronised Swiss 3T3 fibroblasts in different experimental conditions: i) contact-inhibited fibroblasts re-entering cell cycle after dilution; ii) scratch-wound assay; iii) serum-deprived cells induced to re-enter G1 by FCS, HDL or PDGF. Analyses were performed during each cell cycle up to quiescence. Cholesterol synthesis increased remarkably during the replicative cycles, decreasing only after cells reached confluence. In contrast, cholesteryl ester (CE) synthesis and content were high at 24 h after dilution and then decreased steeply in the successive cycles. Flow cytometry analysis of DiO-HDL, as well as radiolabeled HDL pulse, demonstrated a significant uptake of CE-HDL in 24 h. DiI-HDL uptake, lipid droplets (LDs) and SR-BI immunostaining and expression followed the same trend. Addition of HDL or PDGF partially restore the proliferation rate and significantly increase SR-BI and pAKT expression in serum-deprived cells. In conclusion, cell transition from G0 to G1/S requires CE-HDL uptake, leading to CE-HDL/SR-BI pathway activation and CEs increase into LDs. PMID:26640042
Piestrup, M.A. ); Moran, M.J. ); Boyers, D.G.; Pincus, C.I. ); Kephart, J.O. ); Gearhart, R.A. ); Maruyama, X.K. )
1991-03-01
In experiments using targets consisting of many thin metal foils, we have demonstrated that a narrow, forward-directed cone of transition radiation in the 8- to 60-keV spectral range can be generated by electron beams with moderate energies (between 100 and 500 MeV). The theory suggests that high-density, moderate-atomic-number metals are the optimum foil materials and that the foil thickness can be chosen to maximize photon production within a desired spectral range. The three targets used in the experiments consisted of 10 foils of 1-{mu}m-thick gold, 40 foils of 8.5-{mu}m stainless steel, and 20 foils of 7.9-{mu}m copper. The efficiency with which hard x rays are generated, and the fact that the requisite electron-beam energies are lower by a factor of 5 to 10, make such a radiation source an attractive alternative to synchrotron radiation for applications such as medical imaging, spectroscopy, and microscopy.
NASA Astrophysics Data System (ADS)
Kolincio, Kamil; Pérez, Olivier; Hébert, Sylvie; Fertey, Pierre; Pautrat, Alain
2016-06-01
Detailed structural and magnetotransport properties of monophosphate tungsten bronze Kx(PO2)4(WO3)8 single crystals are reported. Both galvanomagnetic and thermal properties are shown to be consistent with a charge density wave electronic transition due to hidden nesting of the quasi-1D portion of the Fermi surface. We also observe the enhancement of electronic anisotropy due to reconstruction of the Fermi surface at the Peierls transition. The resistivity presents a thermal hysteresis suggesting a first-order nature characteristic of a strong-coupling scenario. However, other measurements such as the change of carrier density demonstrate a second-order Peierls scenario with weak-coupling features. We suggest that the structural transition driven by the residual strain in the K-P-O environment is responsible for the resistivity hysteresis and modifies the Fermi surface which then helps the rise to the second-order Peierls instability.
NASA Astrophysics Data System (ADS)
Thakur, Vishal; Kant, Niti
2016-08-01
The resonant third-harmonic generation of a self-focusing laser in plasma with a density transition was investigated. Because of self-focusing of the fundamental laser pulse, a transverse intensity gradient was created, which generated a plasma wave at the fundamental wave frequency. Phase matching was satisfied by using a Wiggler magnetic field, which provided additional angular momentum to the third-harmonic photon to make the process resonant. An enhancement was observed in the resonant third-harmonic generation of an intense short-pulse laser in plasma embedded with a magnetic Wiggler with a density transition. A plasma density ramp played an important role in the self-focusing, enhancing the third-harmonic generation in plasma. We also examined the effect of the Wiggler magnetic field on the pulse slippage of the third-harmonic pulse in plasma. The pulse slippage was due to the group-velocity mismatch between the fundamental and third-harmonic pulses.
NASA Astrophysics Data System (ADS)
Mohanta, S. K.; Mishra, S. N.; Srivastava, S. K.
2014-04-01
We present first principles calculations of electronic structure and magnetic properties of dilute transition metal (3d, 4d and 5d) impurities in a Gd host. The calculations have been performed within the density functional theory using the full potential linearized augmented plane wave technique and the GGA+U method. The spin and orbital contributions to the magnetic moment and the hyperfine fields have been computed. We find large magnetic moments for 3d (Ti-Co), 4d (Nb-Ru) and 5d (Ta-Os) impurities with magnitudes significantly different from the values estimated from earlier mean field calculation [J. Magn. Magn. Mater. 320 (2008) e446-e449]. The exchange interaction between the impurity and host Gd moments is found to be positive for early 3d elements (Sc-V) while in all other cases an anti-ferromagnetic coupling is observed. The trends for the magnetic moment and hyperfine field of d-impurities in Gd show qualitative difference with respect to their behavior in Fe, Co and Ni. The calculated total hyperfine field, in most cases, shows excellent agreement with the experimental results. A detailed analysis of the Fermi contact hyperfine field has been made, revealing striking differences for impurities having less or more than half filled d-shell. The impurity induced perturbations in host moments and the change in the global magnetization of the unit cell have also been computed. The variation within each of the d-series is found to correlate with the d-d hybridization strength between the impurity and host atoms.
Fang, Zongtang; Both, Johan; Li, Shenggang; Yue, Shuwen; Aprà, Edoardo; Keçeli, Murat; Wagner, Albert F; Dixon, David A
2016-08-01
The heats of formation and the normalized clustering energies (NCEs) for the group 4 and group 6 transition metal oxide (TMO) trimers and tetramers have been calculated by the Feller-Peterson-Dixon (FPD) method. The heats of formation predicted by the FPD method do not differ much from those previously derived from the NCEs at the CCSD(T)/aT level except for the CrO3 nanoclusters. New and improved heats of formation for Cr3O9 and Cr4O12 were obtained using PW91 orbitals instead of Hartree-Fock (HF) orbitals. Diffuse functions are necessary to predict accurate heats of formation. The fluoride affinities (FAs) are calculated with the CCSD(T) method. The relative energies (REs) of different isomers, NCEs, electron affinities (EAs), and FAs of (MO2)n (M = Ti, Zr, Hf, n = 1-4) and (MO3)n (M = Cr, Mo, W, n = 1-3) clusters have been benchmarked with 55 exchange-correlation density functional theory (DFT) functionals including both pure and hybrid types. The absolute errors of the DFT results are mostly less than ±10 kcal/mol for the NCEs and the EAs and less than ±15 kcal/mol for the FAs. Hybrid functionals usually perform better than the pure functionals for the REs and NCEs. The performance of the two types of functionals in predicting EAs and FAs is comparable. The B1B95 and PBE1PBE functionals provide reliable energetic properties for most isomers. Long range corrected pure functionals usually give poor FAs. The standard deviation of the absolute error is always close to the mean errors, and the probability distributions of the DFT errors are often not Gaussian (normal). The breadth of the distribution of errors and the maximum probability are dependent on the energy property and the isomer. PMID:27384926
NASA Astrophysics Data System (ADS)
Fortney, Jonathan J.; Nettelmann, N.; Kempton, E.; Mordasini, C.; Zahnle, K.; Lopez, E.; Morley, C. V.; Marley, M. S.
2012-10-01
The atmosphere of the low-mass low-density transiting planet GJ 1214b has been extensively characterized via transmission spectroscopy. Observations include spectra and photometric points from blue to mid-infrared wavelengths. The transmission spectrum appears relatively featureless, indicating an atmosphere that does not show strong molecular absorption features. It has been suggested that this ``flat" spectrum could be due to an obscuring grey cloud/haze layer, or due to a high mean molecular weight (MMW) atmosphere. If the planet is similar to a scaled down version of Uranus or Neptune, as suggested by Nettelmann et al. (2011), both explanations could well be viable. To lift the degeneracy of these explanations, one can imagine characterizing a range of similar planets, which are now being found. Here we examine the structure and atmospheres of volatile-rich planets from 5-20 Earth masses and T_eq from 100 - 1500 K. Based on population synthesis models of core-accretion planet formation, we examine the expected Z_atmosphere and MMW these low mass planets. We examine how atmospheric escape of the outermost layers of such planets may expose deeper atmospheric layers with less hydrogen and a higher Z_atmosphere and MMW. We note that the hottest variants of these planets should feature atmospheres rich in CO, rather than CH4, potentially eliminating a pathway to photochemical haze formation. We provide a synthesis of these physical effects over a range of mass, temperature, and metallicity parameters. We highlight where in parameter space these GJ 1214b and Neptune-like planets are likely to have atmospheres that are most amenable to characterization from transmission spectroscopy.
NASA Astrophysics Data System (ADS)
Franzese, G.; Malescio, G.; Skibinsky, A.; Buldyrev, S. V.; Stanley, H. E.
2002-11-01
We investigate the phase behavior of a single-component system in three dimensions with spherically-symmetric, pairwise-additive, soft-core interactions with an attractive well at a long distance, a repulsive soft-core shoulder at an intermediate distance, and a hard-core repulsion at a short distance, similar to potentials used to describe liquid systems such as colloids, protein solutions, or liquid metals. We showed [Nature (London) 409, 692 (2001)] that, even with no evidence of the density anomaly, the phase diagram has two first-order fluid-fluid phase transitions, one ending in a gas-low-density-liquid (LDL) critical point, and the other in a gas-high-density-liquid (HDL) critical point, with a LDL-HDL phase transition at low temperatures. Here we use integral equation calculations to explore the three-parameter space of the soft-core potential and perform molecular dynamics simulations in the interesting region of parameters. For the equilibrium phase diagram, we analyze the structure of the crystal phase and find that, within the considered range of densities, the structure is independent of the density. Then, we analyze in detail the fluid metastable phases and, by explicit thermodynamic calculation in the supercooled phase, we show the absence of the density anomaly. We suggest that this absence is related to the presence of only one stable crystal structure.
Li Anyi; Alexandru, Andrei; Liu, Keh-Fei; Meng Xiangfei
2010-09-01
In a progress toward searching for the QCD critical point, we study the finite density phase transition of N{sub f}=4 and 2 lattice QCD at finite temperature with the canonical ensemble approach. We develop a winding number expansion method to accurately project out the particle number from the fermion determinant which greatly extends the applicable range of baryon number sectors to make the study feasible. Our lattice simulation was carried out with the clover fermions and improved gauge action. For a given temperature, we calculate the baryon-chemical potential from the canonical approach to look for the mixed phase as a signal for the first-order phase transition. In the case of N{sub f}=4, we observe an 'S-shape' structure in the chemical potential-density plane due to the surface tension of the mixed phase in a finite volume which is a signal for the first-order phase transition. We use the Maxwell construction to determine the phase boundaries for three temperatures below T{sub c}. The intersecting point of the two extrapolated boundaries turns out to be at the expected first-order transition point at T{sub c} with {mu}=0. This serves as a check for our method of identifying the critical point. We also studied the N{sub f}=2 case, but do not see a signal of the mixed phase for temperature as low as 0.83T{sub c}.
McKay, D. C.; Jervis, D.; Fine, D. J.; Simpson-Porco, J. W.; Edge, G. J. A.; Thywissen, J. H.
2011-12-15
We report the laser cooling and trapping of neutral potassium on an open transition. Fermionic {sup 40}K is captured using a magneto-optical trap (MOT) on the closed 4S{sub 1/2}{yields}4P{sub 3/2} transition at 767 nm and then transferred, with high efficiency, to a MOT on the open 4S{sub 1/2}{yields}5P{sub 3/2} transition at 405 nm. Because the 5P{sub 3/2} state has a smaller linewidth than the 4P{sub 3/2} state, the Doppler limit is reduced from 145 {mu}K to 24 {mu}K, and we observe temperatures as low as 63(6) {mu}K. The density of trapped atoms also increases, due to reduced temperature and reduced expulsive light forces. We measure a two-body loss coefficient of {beta}=1.4(1)x10{sup -10} cm{sup 3}/s near saturation intensity, and estimate an upper bound of 8x10{sup -18} cm{sup 2} for the ionization cross section of the 5P state at 405 nm. The combined temperature and density improvement in the 405 nm MOT is a twenty-fold increase in phase-space density over our 767 nm MOT, showing enhanced precooling for quantum gas experiments. A qualitatively similar enhancement is observed in a 405 nm MOT of bosonic {sup 41}K.
NASA Astrophysics Data System (ADS)
Buchhave, Lars A.
2015-08-01
The majority of exoplanets discovered by the Kepler Mission have sizes that range between 1-4 Earth radii, populating a regime of planets with no Solar System analogues. This regime is critical for understanding the frequency of potentially habitable worlds and to help inform planet formation theories, because it contains the transition from lower-density planets with extended H/He envelopes to higher-density rocky planets with compact atmospheres. HARPS-N is an ultra-stable high-resolution spectrograph optimized for the measurement of precise radial velocities, yielding precise planetary masses and thus densities of small transiting exoplanets. In this talk, I will review the progress to populate the mass-radius parameter space with precisely measured densities of small planets. I will in particular focus on the latest HARPS-N results and their implication for our understanding of these super-Earth and small Neptune type planets.Additionally, I will discuss our progress to measure the masses of longer period sub-Neptune sized planets. In Buchhave el al. 2014, we found suggestive observational evidence that the transition from rocky to gaseous planets might depend on the orbital period, such that larger planets further away from their host star could be massive planets without a large gaseous envelope. To test this hypothesis, we have used HARPS-N to observe longer period planet candidates to determine whether they are in fact massive rocky planets or if they have extended H/He envelopes and thus lower bulk densities.HARPS-N at the Telescopio Nazionale Galileo, La Palma is an international collaboration and was funded by the Swiss Space Office, the Harvard Origin of Life Initiative, the Scottish Universities Physics Alliance, the University of Geneva, the Smithsonian Astrophysical Observatory, and the Italian National Astrophysical Institute, University of St. Andrews, Queens University Belfast, and University of Edinburgh.
Bučko, Tomáš; Hafner, Jürgen; Lebègue, Sébastien; Ángyán, János G
2012-04-28
Periodic dispersion corrected DFT calculations have been performed to study the spin-crossover transition of Fe(phen)(2)(NCS)(2) in the molecular and in the crystalline state. We show that London dispersion interactions play a crucial role in the cohesion of the crystals. Based on calculations of vibrational eigenstates of the isolated molecule and of the crystalline phase in both the low- and high-spin states, the transition entropies and enthalpies have been calculated. We demonstrate that, due to the stabilization of the low-spin state by intermolecular dispersion forces, the transition enthalpy at the transition temperature is larger for the crystalline phase in comparison with an isolated molecule. The effective coordination number of the nitrogen atoms of the ligands around the iron atom has been identified as the order parameter driving the quasi-reversible low-spin to high-spin transition in the crystal. Finally, using constrained geometry relaxations at fixed values of the coordination number, we computed the energy barrier of the LS to HS transition and found it to be in a reasonable agreement with the experimental value. PMID:22415338
Masuda, Kento
2014-03-01
We present an analysis of the transit timing variations (TTVs) in the multi-transiting planetary system around Kepler-51 (KOI-620). This system consists of two confirmed transiting planets, Kepler-51b (P {sub b} = 45.2 days) and Kepler-51c (P {sub c} = 85.3 days), and one transiting planet candidate KOI-620.02 (P {sub 02} = 130.2 days), which lie close to a 1: 2: 3 resonance chain. Our analysis shows that their TTVs are consistently explained by the three-planet model, and constrains their masses as M{sub b}=2.1{sub −0.8}{sup +1.5} M{sub ⊕} (Kepler-51b), M {sub c} = 4.0 ± 0.4 M {sub ⊕} (Kepler-51c), and M {sub 02} = 7.6 ± 1.1 M {sub ⊕} (KOI-620.02), thus confirming KOI-620.02 as a planet in this system. The masses inferred from the TTVs are rather small compared to the planetary radii based on the stellar density and planet-to-star radius ratios determined from the transit light curves. Combining these estimates, we find that all three planets in this system have densities among the lowest determined, ρ {sub p} ≲ 0.05 g cm{sup –3}. With this feature, the Kepler-51 system serves as another example of low-density compact multi-transiting planetary systems. We also identify a curious feature in the archived Kepler light curve during the double transit of Kepler-51b and KOI-620.02, which could be explained by their overlapping on the stellar disk (a planet-planet eclipse). If this is really the case, the sky-plane inclination of KOI-620.02's orbit relative to that of Kepler-51b is given by ΔΩ=−25.3{sub −6.8}{sup +6.2} deg, implying significant misalignment of their orbital planes. This interpretation, however, seems unlikely because such an event that is consistent with all of the observations is found to be exceedingly rare.
Delezie, E; Swennen, Q; Buyse, J; Decuypere, E
2007-07-01
Commercial broilers are exposed to a number of stressors prior to slaughter, including feed deprivation, crating density (high vs. low), and transportation. Hence, the individual and additive or overruling effects of these stressors on welfare and energy metabolism were examined. Live weight gain, rectal temperature, physiological responses, and meat quality of broilers were determined. The fasting of broilers before being transported resulted in a decrease of triglycerides, uric acid, and triiodothyronine concentrations, indicating a negative energy balance. Feed withdrawal was also associated with a reduction in body weight, and highest body weight losses were observed after being fasted for 13 h. For some parameters there was a combined effect of feed withdrawal and crating density, whereas for others the crating density overruled the effect of previous feed withdrawal: broilers that had no access to feed before being transported had higher thyroxine and lower lactate concentrations (only at high crating density) compared with their fed counterparts before the transport process, indicating the combined effect of both actions. The distinction due to the feeding pattern could no longer be observed for the plasma uric acid, nonesterified fatty acids, triglycerides, and triiodothyronine concentrations because it was overruled by the transport effect, especially if broilers were transported at high crating density. Plasma corticosterone concentrations increased as a consequence of the procedure of transportation and peaked if broilers were crated at high density. In our study, no significant effect of preslaughter stressors on meat quality, plasma creatine kinase activity, or lipid peroxidation levels were noticed. It can be concluded that transportation at high stocking densities should be avoided to reduce economic losses and stress to broilers. Plasma hormone as well as metabolites, rectal temperature, and heat shock protein 70 mRNA all indicated the high stress
Electron density determinations of stellar coronae using Fe XXI ndarrow2p (n=3,4,5,6) transitions
NASA Astrophysics Data System (ADS)
Chen, H.; Beiersdorfer, P.; Liedahl, D. A.; Bitter, M.; Brown, G. V.; Kahn, S. M.
2002-04-01
The idea of using L-shell iron spectroscopic information to infer the electron density has become very attractive following the availability of high-resolution data measured by current x-ray observatories. Following suggestions in [1], we developed the n darrow2p line (n=3,4,5,6) intensities in Fe XXI as a diagnostic that is sensitive to densities near 10^13 cm-3 and applied it to Chandra observations of HR1099. HULLAC code predictions of the density sensitivity were checked with the Livermore EBIT-II ion trap for low densities. Checks at high density are in progress at the Princeton NSTX tokamak. For HR1099 we find a density of about 2 to 8 x 10^13 cm-3. This is in the range of 10^12 - 10^13 cm-3 that was determined using the Fe XXI EUV line ratios (102/128 ÅÅobserved with EUVE [2]. This work was performed under the auspice of DOE by UC-LLNL under contract W-7405-Eng-48 and supported by DOE OFES and by NASA SARA grants to LLNL, GSFC, PPPL and Columbia University.\\$[1]. Wargelin, et al, APJ 496 (1998). [2]. Ayres,et al, APJ, 549, (2001) 554)
NASA Astrophysics Data System (ADS)
Parisi, Filippo; Sciascia, Luciana; Princivalle, Francesco; Merli, Marcello
2012-02-01
In order to characterize the pressure-induced decomposition of ringwoodite (γ-Mg2SiO4), the topological analysis of the electron density ρ( r), based upon the theory of atoms in molecules (AIM) developed by Bader in the framework of the catastrophe theory, has been performed. Calculations have been carried out by means of the ab initio CRYSTAL09 code at the HF/DFT level, using Hamiltonians based on the Becke- LYP scheme containing hybrid Hartree-Fock/density functional exchange-correlation terms. The equation of state at 0 K has been constructed for the three phases involved in the post-spinel phase transition (ringwoodite → Mg-perovskite + periclase) occurring at the transition zone-lower mantel boundary. The topological results show that the decomposition of the ringwoodite at high pressures is caused by a conflict catastrophe. Furthermore, topological evidences of the central role played by the oxygen atoms to facilitate the pressure-induced ringwoodite decomposition and the subsequent phase transition have been noticed.
NASA Astrophysics Data System (ADS)
Yu, Haoyu; He, Xiao; Truhlar, Donald G.; Donald G. Truhlar Team
The accuracy of Kohn-Sham density functional theory depends on the exchange-correlation functional. Local functionals depending on only the density (ρ) , density gradient (grad), and possibly kinetic energy density (τ) have been popular because of their low cost and simplicity, but the most successful functionals for chemistry have involved nonlocal Hartree-Fock exchange (hybrid functionals). We have designed a new meta gradient approximation called MN15-L and a new hybrid meta gradient approximation called MN15 and tested them systematically for 17 absolute atomic energies, 51 noncovalent interaction energies, 56 data on transition metal atoms and molecules, and for 298 other atomic and molecular energetic data, including main-group and transition metal bond energies, ionization potentials, proton affinities, reaction barrier heights, hydrocarbon thermochemistry, excitation energies, and isomerization energies. When compared with 84 previous density MN15 and MN15-L give respectively the smallest and second smallest mean unsigned errors (MUEs, in kcal/mol) on all 422 data with errors for the 4 subsets above being: MN15: 6, 0.26, 4.4, 1.6; MN15-L: 7, 0.45, 4.3, 2.0. Third best: M06: 4, 0.35, 7.7, 2.2. Best previous local functional: M06-L: 7, 0.42, 6.0, 3.5. Other popular functionals: B3LYP: 18, 0.82, 8.2, 4.3; HSE06: 33, 0.58, 8.8, 3.6; TPSS: 18, 0.89, 7.25, 5.0; PBE, 47, 0.88, 9.1, 6.0. MN15-L also performs well for solid-state cohesive energies. This research is supported by the U.S. Department of Energy and inorganic catalyst design center from university of Minnesota.
NASA Astrophysics Data System (ADS)
Khidzir, S. M.; Ibrahim, K. N.; Wan Abdullah, W. A. T.
2016-05-01
Momentum density studies are the key tool in Fermiology in which electronic structure calculations have proven to be the integral underlying methodology. Agreements between experimental techniques such as Compton scattering experiments and conventional density functional calculations for late transition metal oxides (TMOs) prove elusive. In this work, we report improved momentum densities of late TMOs using the GW approximation (GWA) which appears to smear the momentum density creating occupancy above the Fermi break. The smearing is found to be largest for NiO and we will show that it is due to more spectra surrounding the NiO Fermi energy compared to the spectra around the Fermi energies of FeO and CoO. This highlights the importance of the positioning of the Fermi energy and the role played by the self-energy term to broaden the spectra and we elaborate on this point by comparing the GWA momentum densities to their LDA counterparts and conclude that the larger difference at the intermediate level shows that the self-energy has its largest effect in this region. We finally analyzed the quasiparticle renormalization factor and conclude that an increase of electrons in the d-orbital from FeO to NiO plays a vital role in changing the magnitude of electron correlation via the self-energy.
NASA Astrophysics Data System (ADS)
Wang, Y. Q.; Lu, P. C.; Wu, J. J.; Liu, J.; Wang, X. C.; Zhao, J. Y.; Bi, W.; Alp, E. E.; Park, C. Y.; Popov, D.; Jin, C. Q.; Sun, J.; Lin, J. F.
2016-07-01
To help our understanding of the structural and superconducting transitions in ferropnictides, partial phonon density of states (PDOS) of iron in a single-crystal SrF e2A s2 pnictide have been investigated from both out-of-plane and in-plane polarizations with respect to the basal plane of the crystal structure using nuclear resonant inelastic x-ray scattering in a high-pressure diamond anvil cell at ambient temperature. The partial PDOS of iron in the pnictide crystal changes dramatically at approximately 8 GPa, which can be associated with the tetragonal (T) to collapsed tetragonal (CT) isostructural transition as evidenced in high-pressure x-ray diffraction measurements and theoretical calculations. Across the T-CT phase transition, analysis of the PDOS spectra shows a rapid stiffening of the optical phonon modes and a dramatic increase of the Lamb-Mössbauer factor (fLM) and mean force constant which can be associated with the rapid decrease of the c axis and the anomalous expansion of the a axis. Theoretically calculated Fe partial PDOS and lattice parameters of SrF e2A s2 further reveal the strong correlation between the lattice parameters and phonons. Our results show that the T-CT transition can induce significant changes in the vibrational, elastic, and thermodynamic properties of SrF e2A s2 single crystal at high pressure.
NASA Astrophysics Data System (ADS)
Chernenkaya, Alisa; Medjanik, Katerina; Nagel, Peter; Merz, Michael; Schuppler, Stefan; Canadell, Enric; Pouget, Jean-Paul; Schönhense, Gerd
2015-01-01
The electronic structure of TTF-TCNQ was studied by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy in order to detect a spectroscopic signature of the phase transitions, especially that occurring at 54 K, which is related to a Peierls gap opening on the TCNQ stacks. All unoccupied TCNQ orbitals predicted by first-principles calculations and, in particular the pair σ ∗( π( a g , b 3 u )), located in the cyano groups, are clearly resolved in our experimental data. The latter orbital was observed for the first time in our NEXAFS spectra. The temperature dependence of NEXAFS peak intensities gives evidence of a subtle modification of the electronic structure when the charge density wave (CDW) fluctuations develop as the Peierls transition of the TCNQ stacks is approached from higher temperatures. These changes are explained on the basis of the charge transfer, the shape of the lower empty TCNQ molecular orbitals and the deformation of TCNQ during the pre-transitional CDW fluctuations. Finally the data suggest that the internal stack deformation consisting in a substantial out of plane displacement of the central ring with respect to the cyano-groups allows to gain C α -C α bonding energy which helps the stabilization of the Peierls transition on the TCNQ stack.
NASA Astrophysics Data System (ADS)
Crauste, O.; Couedo, F.; Bergé, L.; Marrache, C.; Dumoulin, L.
2012-12-01
We report on the study of the Disordered-induced Superconductor-Insulator Transition (D-SIT) in NbxSi1-x thin films. These films, synthesized by electron-beam co-deposition, are continuous, amorphous, homogeneously disordered and structurally stable for a wide range of compositions, thicknesses and annealing temperature and thus particularly well suited for the study of D-SIT. We present an analysis of the D-SIT induced by three different parameters: the thickness, the Nb composition that changes the electronic density of states and the annealing temperature that changes the microscopic disorder. The annealing changes quantum interference patterns that decreases the local conductance. Our results show that the effect of the thickness on the destruction of superconductivity is very distinct from those of the composition or the annealing. We point out this material is particularly interesting to disentangle the effect of the parameters driving this quantum phase transition.
Zhang, Yu; Mukamel, Shaul; Khalil, Munira; Govind, Niranjan
2015-11-09
Valence-to-core (VtC) X-ray emission spectroscopy (XES) has emerged as a power- ful technique for the structural characterization of complex organometallic compounds in realistic environments. Since the spectrum represents electronic transitions from the ligand molecular orbitals to the core holes of the metal centers, the approach is more chemically sensitive to the metal-ligand bonding character compared with con- ventional X-ray absorption techniques. In this paper we study how linear-response time-dependent density functional theory (LR-TDDFT) can be harnessed to simulate K-edge VtC X-ray emission spectra reliably. LR-TDDFT allows one to go beyond the single-particle picture that has been extensively used to simulate VtC-XES. We con- sider seven low- and high-spin model complexes involving chromium, manganese and iron transition metal centers. Our results are in good agreement with experiment.
Gibbs, G V; Spackman, M A; Jayatilaka, D; Rosso, K M; Cox, D F
2006-11-01
For a variety of molecules and earth materials, the theoretical local kinetic energy density, G(r(c)), increases and the local potential energy density, V(r(c)), decreases as the M-O bond lengths (M = first- and second-row metal atoms bonded to O) decrease and the electron density, rho(r(c)), accumulates at the bond critical points, r(c). Despite the claim that the local kinetic energy density per electronic charge, G(r(c))/rho(r(c)), classifies bonded interactions as shared interactions when less than unity and closed-shell when greater, the ratio was found to increase from 0.5 to 2.5 au as the local electronic energy density, H(r(c)) = G(r(c)) + V(r(c)), decreases and becomes progressively more negative. The ratio appears to be a measure of the character of a given M-O bonded interaction, the greater the ratio, the larger the value of rho(r(c)), the smaller the coordination number of the M atom and the more shared the bonded interaction. H(r(c))/rho(r(c)) versus G(r(c))/rho(r(c)) scatter diagrams categorize the M-O bonded interactions into domains with the local electronic energy density per electron charge, H(r(c))/rho(r(c)), tending to decrease as the electronegativity differences for the bonded pairs of atoms decrease. The values of G(r(c)) and V(r(c)), estimated with a gradient-corrected electron gas theory expression and the local virial theorem, are in good agreement with theoretical values, particularly for the bonded interactions involving second-row M atoms. The agreement is poorer for shared C-O and N-O bonded interactions. PMID:17078623
NASA Astrophysics Data System (ADS)
Bao, S. S.; Shen, H.
2016-02-01
We study the liquid-gas phase transition of stellar matter with the inclusion of the finite-size effect from surface and Coulomb energies. The equilibrium conditions for two coexisting phases are determined by minimizing the total free energy including the surface and Coulomb contributions, which are different from the Gibbs conditions used in the bulk calculations. The finite-size effect can significantly reduce the region of the liquid-gas mixed phase. The influence of the symmetry energy on the liquid-gas phase transition is investigated with the inclusion of finite-size effects. It is found that the slope of the symmetry energy plays an important role in determining the boundary and properties of the mixed phase.
Egorov, Sergei A; Milchev, Andrey; Virnau, Peter; Binder, Kurt
2016-06-14
Semiflexible polymers in solution are studied for a wide range of both contour length L and persistence length lp as a function of monomer concentration under good solvent conditions. Both density-functional theory (DFT) and molecular dynamics (MD) simulation methods are used, and a very good agreement between both techniques is observed for rather stiff polymers. Evidence for a new mechanism of order parameter fluctuations in the nematic phase is presented, namely collective deformations of bundles of wormlike chains twisted around each other, and the typical wavelengths and amplitudes of these modes are estimated. These long wavelength fluctuations cause a reduction of the order parameter in comparison with the DFT prediction. It is also found that DFT becomes unreliable for rather flexible polymers in predicting that the transition from the isotropic (I)-phase to the nematic (N)-phase still exists at very high monomer concentrations (which in reality does not occur). However, under conditions when DFT is accurate, it provides reliable predictions also for the width of the I-N two-phase coexistence region, which are difficult to obtain from MD in spite of the use of very large systems (up to 500 000 monomers) by means of graphics processing units (GPU). For short and not very stiff chains, a pre-transitional chain stretching is found in the isotropic phase near the I-N-transition, not predicted by theories. A comparison with theoretical predictions by Khokhlov-Semenov, Odijk, and Chen reveals that the scaled transition densities are not simply functions of L/lp only, as these theories predict, but depend on d/lp (where d is the chain diameter) as well. Chain properties in the nematically ordered phase are compared to those of chains confined in tubes, and the deflection length concept is tested. Eventually, some consequences for the interpretation of experiments are spelled out. PMID:27249320
Vinod, K. Sharma, Shilpam; Sundar, C. S.; Bharathi, A.
2015-06-24
Heat capacity measurements were done on sub-micron sized BaFe{sub 2−x}Ru{sub x}As{sub 2} single crystals using thin film membrane based the AC steady state calorimetry technique. Noticeable thermal hysteresis is observed in the heat capacity of the BaFe{sub 2−x}Ru{sub x}As{sub 2} during cooling and warming cycles, indicating first order nature of the SDW transition.
HAT-P-18b AND HAT-P-19b: TWO LOW-DENSITY SATURN-MASS PLANETS TRANSITING METAL-RICH K STARS
Hartman, J. D.; Bakos, G. A.; Torres, G.; Noyes, R. W.; Latham, D. W.; Buchhave, L. A.; Fueresz, G.; Perumpilly, G.; Beky, B.; Stefanik, R. P.; Sasselov, D. D.; Esquerdo, G. A.; Everett, M.; Csubry, Z.; Sato, B.; Kovacs, G.; Fischer, D. A.; Howard, A. W.; Marcy, G. W.; Johnson, J. A.
2011-01-01
We report the discovery of two new transiting extrasolar planets. HAT-P-18b orbits the V = 12.759 K2 dwarf star GSC 2594-00646, with a period P = 5.508023 {+-} 0.000006 days, transit epoch T{sub c} = 2454715.02174 {+-} 0.00020 (BJD), and transit duration 0.1131 {+-} 0.0009 days. The host star has a mass of 0.77 {+-} 0.03 M{sub sun}, radius of 0.75 {+-} 0.04 R{sub sun}, effective temperature 4803 {+-} 80 K, and metallicity [Fe/H] = +0.10 {+-} 0.08. The planetary companion has a mass of 0.197 {+-} 0.013 M{sub J} and radius of 0.995 {+-} 0.052 R{sub J}, yielding a mean density of 0.25 {+-} 0.04 g cm{sup -3}. HAT-P-19b orbits the V = 12.901 K1 dwarf star GSC 2283-00589, with a period P = 4.008778 {+-} 0.000006 days, transit epoch T{sub c} = 2455091.53417 {+-} 0.00034 (BJD), and transit duration 0.1182 {+-} 0.0014 days. The host star has a mass of 0.84 {+-} 0.04 M{sub sun}, radius of 0.82 {+-} 0.05 R{sub sun}, effective temperature 4990 {+-} 130 K, and metallicity [Fe/H] = +0.23 {+-} 0.08. The planetary companion has a mass of 0.292 {+-} 0.018 M{sub J} and radius of 1.132 {+-} 0.072 R{sub J}, yielding a mean density of 0.25 {+-} 0.04 g cm{sup -3}. The radial velocity residuals for HAT-P-19 exhibit a linear trend in time, which indicates the presence of a third body in the system. Comparing these observations with theoretical models, we find that HAT-P-18b and HAT-P-19b are each consistent with a hydrogen-helium-dominated gas giant planet with negligible core mass. HAT-P-18b and HAT-P-19b join HAT-P-12b and WASP-21b in an emerging group of low-density Saturn-mass planets, with negligible inferred core masses. However, unlike HAT-P-12b and WASP-21b, both HAT-P-18b and HAT-P-19b orbit stars with super-solar metallicity. This calls into question the heretofore suggestive correlation between the inferred core mass and host star metallicity for Saturn-mass planets.
Crespo, V.; Rodrigo, J. G.; Suderow, H.; Vieira, S.; Hinks, D.; Schuller, I. K.; Materials Science Division; Univ. Autonoma de Madrid; Univ. of California at San Diego
2009-01-01
We present local tunneling spectroscopy experiments in the superconducting and ferromagnetic phases of the reentrant superconductor ErRh4B4. The tunneling conductance curves jump from showing normal to superconducting features within a few mK close to the ferromagnetic transition temperature, with a clear hysteretic behavior. Within the ferromagnetic phase, we do not detect any superconducting correlations. Within the superconducting phase we find a peculiar V-shaped density of states at low energies, which is produced by the magnetically modulated phase that coexists with superconductivity just before ferromagnetism sets in.
Krykunov, Mykhaylo; Seth, Mike; Ziegler, Tom
2014-05-14
We have applied the relaxed and self-consistent extension of constricted variational density functional theory (RSCF-CV-DFT) for the calculation of the lowest charge transfer transitions in the molecular complex X-TCNE between X = benzene and TCNE = tetracyanoethylene. Use was made of functionals with a fixed fraction (α) of Hartree-Fock exchange ranging from α = 0 to α = 0.5 as well as functionals with a long range correction (LC) that introduces Hartree-Fock exchange for longer inter-electronic distances. A detailed comparison and analysis is given for each functional between the performance of RSCF-CV-DFT and adiabatic time-dependent density functional theory (TDDFT) within the Tamm-Dancoff approximation. It is shown that in this particular case, all functionals afford the same reasonable agreement with experiment for RSCF-CV-DFT whereas only the LC-functionals afford a fair agreement with experiment using TDDFT. We have in addition calculated the CT transition energy for X-TCNE with X = toluene, o-xylene, and naphthalene employing the same functionals as for X = benzene. It is shown that the calculated charge transfer excitation energies are in as good agreement with experiment as those obtained from highly optimized LC-functionals using adiabatic TDDFT. We finally discuss the relation between the optimization of length separation parameters and orbital relaxation in the RSCF-CV-DFT scheme.
Zhu, Siyuan; Ye, Mao; Shirai, Kaito; Taniguchi, Masaki; Ueda, Shigenori; Miura, Yoshio; Shirai, Masafumi; Umetsu, Rie Yamauchi; Kainuma, Ryosuke; Kanomata, Takeshi; Kimura, Akio
2015-09-16
We have unravelled the electronic structure of a class of metamagnetic shape memory alloy Ni2Mn1+x In1-x by combining bulk-sensitive hard x-ray photoelectron spectroscopy and first-principles density-functional calculations. A sharp drop in the Ni 3d e(g) density of states forming a pseudogap in the martensitic phase transition (MPT) for x = 0.36 has been observed near the Fermi level. As a feature of MPT, hysteretic behaviour of this drop has been confirmed in both cooling and warming. This pseudogap is responsible for the giant negative magnetoresistance. The experimental result is well reproduced by the first principle calculation. We have also clarified theoretically that the MPT is linked to a competition of ferromagnetic and anti-ferromagnetic coupling between ordinary and anti-site Mn atoms. PMID:26289060
NASA Astrophysics Data System (ADS)
Chen, H.; Beiersdorfer, P.; Brown, G. V.; Liedahl, D. A.
2001-10-01
The idea of using L-shell iron spectroscopic information as electron density diagnostics is not only of interest for fusion plasmas, where the iron maybe indigenous to the plasma, it also has become very attractive for diagnosing astrophysical plasmas following the availability of high-resolution data measured by current x-ray observatories. Using the Livermore electron beam ion trap EBIT-II, we measured density dependent ndarrow2p line (n=3 to 6) intensities in Fe XXII and Fe XXI. We compared the experimental data with detailed atomic models using the HULLAC code. We present the analysis of these transitions and a discussion of their diagnostic applications to fusion and astrophysical plasmas. This work was performed under the auspices of DOE by UC-LLNL under contract W-7405-Eng-48 and supported by NASA SARA grants to LLNL, GSFC, and Columbia University.
Yi, Ming
2011-08-19
Nematicity, defined as broken rotational symmetry, has recently been observed in competing phases proximate to the superconducting phase in the cuprate high temperature superconductors. Similarly, the new iron-based high temperature superconductors exhibit a tetragonal to orthorhombic structural transition (i.e. a broken C{sub 4} symmetry) that either precedes or is coincident with a collinear spin density wave (SDW) transition in undoped parent compounds, and superconductivity arises when both transitions are suppressed via doping. Evidence for strong in-plane anisotropy in the SDW state in this family of compounds has been reported by neutron scattering, scanning tunneling microscopy, and transport measurements. Here we present an angle resolved photoemission spectroscopy study of detwinned single crystals of a representative family of electron-doped iron-arsenide superconductors, Ba(Fe{sub 1-x}Co{sub x}){sub 2}As{sub 2} in the underdoped region. The crystals were detwinned via application of in-plane uniaxial stress, enabling measurements of single domain electronic structure in the orthorhombic state. At low temperatures, our results clearly demonstrate an in-plane electronic anisotropy characterized by a large energy splitting of two orthogonal bands with dominant d{sub xz} and d{sub yz} character, which is consistent with anisotropy observed by other probes. For compositions x > 0, for which the structural transition (T{sub S}) precedes the magnetic transition (T{sub SDW}), an anisotropic splitting is observed to develop above T{sub SDW}, indicating that it is specifically associated with T{sub S}. For unstressed crystals, the band splitting is observed close to T{sub S}, whereas for stressed crystals the splitting is observed to considerably higher temperatures, revealing the presence of a surprisingly large in-plane nematic susceptibility in the electronic structure.
Bao, Junwei Lucas; Truhlar, Donald G
2016-04-21
The growth of anionic silicon hydride clusters is a critically important process in nanodusty plasmas. In the current study, we focus on the formation of homologs of silylene (Sin+1H2n+2(-), n = 3, 4) and silyl (SinH2n+1(-), n = 4, 5) anions via anion-neutral reaction pathways. Species like silyl or silylene anions and their related elementary reactions, which are involved in the formation of silicon hydride clusters, were not used in developing exchange-correlation (xc) density functionals (i.e., they were not included in the training set of semiempirical density functionals); therefore, we explored the accuracy of various widely used xc density functionals based on reaction energies and barrier heights. Among the 21 density functionals we tested, M06-2X has the best performance for a hybrid functional, and MN15-L has the best performance for a local functional. Thermal rate constants of the elementary reactions involved in the reaction mechanism are calculated using M06-2X and multistructural canonical variational transition state theory with the small-curvature tunneling approximation (MS-CVT/SCT). The pressure dependence of unimolecular isomerization reactions is treated with system-specific quantum RRK theory (SS-QRRK) and the Lindemann-Hinshelwood mechanism. PMID:27009479
Cai, Weihua; Lai, Ting; Lai, Jianwei; Xie, Haoting; Ouyang, Liuzhang; Ye, Jianshan; Yu, Chengzhong
2016-01-01
Fiber shaped supercapacitors are promising candidates for wearable electronics because they are flexible and light-weight. However, a critical challenge of the widespread application of these energy storage devices is their low cell voltages and low energy densities, resulting in limited run-time of the electronics. Here, we demonstrate a 1.5 V high cell voltage and high volumetric energy density asymmetric fiber supercapacitor in aqueous electrolyte. The lightweight (0.24 g cm−3), highly conductive (39 S cm−1), and mechanically robust (221 MPa) graphene fibers were firstly fabricated and then coated by NiCo2S4 nanoparticles (GF/NiCo2S4) via the solvothermal deposition method. The GF/NiCo2S4 display high volumetric capacitance up to 388 F cm−3 at 2 mV s−1 in a three-electrode cell and 300 F cm−3 at 175.7 mA cm−3 (568 mF cm−2 at 0.5 mA cm−2) in a two-electrode cell. The electrochemical characterizations show 1000% higher capacitance of the GF/NiCo2S4 as compared to that of neat graphene fibers. The fabricated device achieves high energy density up to 12.3 mWh cm−3 with a maximum power density of 1600 mW cm−3, outperforming the thin-film lithium battery. Therefore, these supercapacitors are promising for the next generation flexible and wearable electronic devices. PMID:27248510
NASA Astrophysics Data System (ADS)
Cai, Weihua; Lai, Ting; Lai, Jianwei; Xie, Haoting; Ouyang, Liuzhang; Ye, Jianshan; Yu, Chengzhong
2016-06-01
Fiber shaped supercapacitors are promising candidates for wearable electronics because they are flexible and light-weight. However, a critical challenge of the widespread application of these energy storage devices is their low cell voltages and low energy densities, resulting in limited run-time of the electronics. Here, we demonstrate a 1.5 V high cell voltage and high volumetric energy density asymmetric fiber supercapacitor in aqueous electrolyte. The lightweight (0.24 g cm‑3), highly conductive (39 S cm‑1), and mechanically robust (221 MPa) graphene fibers were firstly fabricated and then coated by NiCo2S4 nanoparticles (GF/NiCo2S4) via the solvothermal deposition method. The GF/NiCo2S4 display high volumetric capacitance up to 388 F cm‑3 at 2 mV s‑1 in a three-electrode cell and 300 F cm‑3 at 175.7 mA cm‑3 (568 mF cm‑2 at 0.5 mA cm‑2) in a two-electrode cell. The electrochemical characterizations show 1000% higher capacitance of the GF/NiCo2S4 as compared to that of neat graphene fibers. The fabricated device achieves high energy density up to 12.3 mWh cm‑3 with a maximum power density of 1600 mW cm‑3, outperforming the thin-film lithium battery. Therefore, these supercapacitors are promising for the next generation flexible and wearable electronic devices.
Cai, Weihua; Lai, Ting; Lai, Jianwei; Xie, Haoting; Ouyang, Liuzhang; Ye, Jianshan; Yu, Chengzhong
2016-01-01
Fiber shaped supercapacitors are promising candidates for wearable electronics because they are flexible and light-weight. However, a critical challenge of the widespread application of these energy storage devices is their low cell voltages and low energy densities, resulting in limited run-time of the electronics. Here, we demonstrate a 1.5 V high cell voltage and high volumetric energy density asymmetric fiber supercapacitor in aqueous electrolyte. The lightweight (0.24 g cm(-3)), highly conductive (39 S cm(-1)), and mechanically robust (221 MPa) graphene fibers were firstly fabricated and then coated by NiCo2S4 nanoparticles (GF/NiCo2S4) via the solvothermal deposition method. The GF/NiCo2S4 display high volumetric capacitance up to 388 F cm(-3) at 2 mV s(-1) in a three-electrode cell and 300 F cm(-3) at 175.7 mA cm(-3) (568 mF cm(-2) at 0.5 mA cm(-2)) in a two-electrode cell. The electrochemical characterizations show 1000% higher capacitance of the GF/NiCo2S4 as compared to that of neat graphene fibers. The fabricated device achieves high energy density up to 12.3 mWh cm(-3) with a maximum power density of 1600 mW cm(-3), outperforming the thin-film lithium battery. Therefore, these supercapacitors are promising for the next generation flexible and wearable electronic devices. PMID:27248510
NASA Astrophysics Data System (ADS)
Deleuil, M.; Bonomo, A. S.; Ferraz-Mello, S.; Erikson, A.; Bouchy, F.; Havel, M.; Aigrain, S.; Almenara, J.-M.; Alonso, R.; Auvergne, M.; Baglin, A.; Barge, P.; Bordé, P.; Bruntt, H.; Cabrera, J.; Carpano, S.; Cavarroc, C.; Csizmadia, Sz.; Damiani, C.; Deeg, H. J.; Dvorak, R.; Fridlund, M.; Hébrard, G.; Gandolfi, D.; Gillon, M.; Guenther, E.; Guillot, T.; Hatzes, A.; Jorda, L.; Léger, A.; Lammer, H.; Mazeh, T.; Moutou, C.; Ollivier, M.; Ofir, A.; Parviainen, H.; Queloz, D.; Rauer, H.; Rodríguez, A.; Rouan, D.; Santerne, A.; Schneider, J.; Tal-Or, L.; Tingley, B.; Weingrill, J.; Wuchterl, G.
2012-02-01
We report the discovery by the CoRoT space mission of a new giant planet, CoRoT-20b. The planet has a mass of 4.24 ± 0.23 MJup and a radius of 0.84 ± 0.04 RJup. With a mean density of 8.87 ± 1.10 g cm-3, it is among the most compact planets known so far. Evolutionary models for the planet suggest a mass of heavy elements of the order of 800 M⊕ if embedded in a central core, requiring a revision either of the planet formation models or both planet evolution and structure models. We note however that smaller amounts of heavy elements are expected by more realistic models in which they are mixed throughout the envelope. The planet orbits a G-type star with an orbital period of 9.24 days and an eccentricity of 0.56.The star's projected rotational velocity is vsini = 4.5 ± 1.0 km s-1, corresponding to a spin period of 11.5 ± 3.1 days if its axis of rotation is perpendicular to the orbital plane. In the framework of Darwinian theories and neglecting stellar magnetic breaking, we calculate the tidal evolution of the system and show that CoRoT-20b is presently one of the very few Darwin-stable planets that is evolving toward a triple synchronous state with equality of the orbital, planetary and stellar spin periods. The CoRoT space mission, launched on December 27th 2006, has been developed and is operated by CNES, with the contribution of Austria, Belgium, Brazil, ESA (RSSD and Science Programme), Germany, and Spain.
Zobač, Vladimír; Lewis, James P; Abad, Enrique; Mendieta-Moreno, Jesús I; Hapala, Prokop; Jelínek, Pavel; Ortega, José
2015-05-01
The computational simulation of photo-induced processes in large molecular systems is a very challenging problem. Firstly, to properly simulate photo-induced reactions the potential energy surfaces corresponding to excited states must be appropriately accessed; secondly, understanding the mechanisms of these processes requires the exploration of complex configurational spaces and the localization of conical intersections; finally, photo-induced reactions are probability events, that require the simulation of hundreds of trajectories to obtain the statistical information for the analysis of the reaction profiles. Here, we present a detailed description of our implementation of a molecular dynamics with electronic transitions algorithm within the local-orbital density functional theory code FIREBALL, suitable for the computational study of these problems. As an example of the application of this approach, we also report results on the [2 + 2] cycloaddition of ethylene with maleic anhydride and on the [2 + 2] photo-induced polymerization reaction of two C60 molecules. We identify different deactivation channels of the initial electron excitation, depending on the time of the electronic transition from LUMO to HOMO, and the character of the HOMO after the transition. PMID:25791682
NASA Astrophysics Data System (ADS)
Zobač, Vladimír; Lewis, James P.; Abad, Enrique; Mendieta-Moreno, Jesús I.; Hapala, Prokop; Jelínek, Pavel; Ortega, José
2015-05-01
The computational simulation of photo-induced processes in large molecular systems is a very challenging problem. Firstly, to properly simulate photo-induced reactions the potential energy surfaces corresponding to excited states must be appropriately accessed; secondly, understanding the mechanisms of these processes requires the exploration of complex configurational spaces and the localization of conical intersections; finally, photo-induced reactions are probability events, that require the simulation of hundreds of trajectories to obtain the statistical information for the analysis of the reaction profiles. Here, we present a detailed description of our implementation of a molecular dynamics with electronic transitions algorithm within the local-orbital density functional theory code FIREBALL, suitable for the computational study of these problems. As an example of the application of this approach, we also report results on the [2 + 2] cycloaddition of ethylene with maleic anhydride and on the [2 + 2] photo-induced polymerization reaction of two C60 molecules. We identify different deactivation channels of the initial electron excitation, depending on the time of the electronic transition from LUMO to HOMO, and the character of the HOMO after the transition.
Nakata, Ayako; Tsuneda, Takao; Hirao, Kimihiko
2011-12-14
A long-range corrected (LC) time-dependent density functional theory (TDDFT) incorporating relativistic effects with spin-orbit couplings is presented. The relativistic effects are based on the two-component zeroth-order regular approximation Hamiltonian. Before calculating the electronic excitations, we calculated the ionization potentials (IPs) of alkaline metal, alkaline-earth metal, group 12 transition metal, and rare gas atoms as the minus orbital (spinor) energies on the basis of Koopmans' theorem. We found that both long-range exchange and spin-orbit coupling effects are required to obtain Koopmans' IPs, i.e., the orbital (spinor) energies, quantitatively in DFT calculations even for first-row transition metals and systems containing large short-range exchange effects. We then calculated the valence excitations of group 12 transition metal atoms and the Rydberg excitations of rare gas atoms using spin-orbit relativistic LC-TDDFT. We found that the long-range exchange and spin-orbit coupling effects significantly contribute to the electronic spectra of even light atoms if the atoms have low-lying excitations between orbital spinors of quite different electron distributions.
2015-01-01
Insights into the growth of high edge density carbon nanostructures were achieved by a systematic parametric study of plasma-enhanced chemical vapor deposition (PECVD). Such structures are important for electrode performance in a variety of applications such as supercapacitors, neural stimulation, and electrocatalysis. A morphological trend was observed as a function of temperature whereby graphenated carbon nanotubes (g-CNTs) emerged as an intermediate structure between carbon nanotubes (CNTs) at lower temperatures and vertically oriented carbon nanosheets (CNS), composed of few-layered graphene, at higher temperatures. This is the first time that three distinct morphologies and dimensionalities of carbon nanostructures (i.e., 1D CNTs, 2D CNSs, and 3D g-CNTs) have been synthesized in the same reaction chamber by varying only a single parameter (temperature). A design of experiments (DOE) approach was utilized to understand the range of growth permitted in a microwave PECVD reactor, with a focus on identifying graphenated carbon nanotube growth within the process space. Factors studied in the experimental design included temperature, gas ratio, catalyst thickness, pretreatment time, and deposition time. This procedure facilitates predicting and modeling high edge density carbon nanostructure characteristics under a complete range of growth conditions that yields various morphologies of nanoscale carbon. Aside from the morphological trends influenced by temperature, a relationship between deposition temperature and specific capacitance emerged from the DOE study. Transmission electron microscopy was also used to understand the morphology and microstructure of the various high edge density structures. From these results, a new graphene foliate formation mechanism is proposed for synthesis of g-CNTs in a single deposition process. PMID:25089165
Ubnoske, Stephen M; Raut, Akshay S; Brown, Billyde; Parker, Charles B; Stoner, Brian R; Glass, Jeffrey T
2014-07-24
Insights into the growth of high edge density carbon nanostructures were achieved by a systematic parametric study of plasma-enhanced chemical vapor deposition (PECVD). Such structures are important for electrode performance in a variety of applications such as supercapacitors, neural stimulation, and electrocatalysis. A morphological trend was observed as a function of temperature whereby graphenated carbon nanotubes (g-CNTs) emerged as an intermediate structure between carbon nanotubes (CNTs) at lower temperatures and vertically oriented carbon nanosheets (CNS), composed of few-layered graphene, at higher temperatures. This is the first time that three distinct morphologies and dimensionalities of carbon nanostructures (i.e., 1D CNTs, 2D CNSs, and 3D g-CNTs) have been synthesized in the same reaction chamber by varying only a single parameter (temperature). A design of experiments (DOE) approach was utilized to understand the range of growth permitted in a microwave PECVD reactor, with a focus on identifying graphenated carbon nanotube growth within the process space. Factors studied in the experimental design included temperature, gas ratio, catalyst thickness, pretreatment time, and deposition time. This procedure facilitates predicting and modeling high edge density carbon nanostructure characteristics under a complete range of growth conditions that yields various morphologies of nanoscale carbon. Aside from the morphological trends influenced by temperature, a relationship between deposition temperature and specific capacitance emerged from the DOE study. Transmission electron microscopy was also used to understand the morphology and microstructure of the various high edge density structures. From these results, a new graphene foliate formation mechanism is proposed for synthesis of g-CNTs in a single deposition process. PMID:25089165
NASA Astrophysics Data System (ADS)
Lin, Wei-Ting; Lai, Ching-Yau; Chang, Chi-Chung; Chen, Yih-Yuh; Tsai, Jih-Chiang; Inst. of Physics, Academia Sinica Collaboration; Dept. of Physics, Nat'l Taiwan University Collaboration
2011-11-01
We study experimentally an oil-water interface maintained in a cylindrical container with its upper boundary rotating at constant speeds. The interface exhibits intriguing morphology as the rotation speeds up, making transitions from a smooth hump that presumably compensates the centrifugal-force induced pressure dip, to more fascinating geometries such as a spinning flap top (a plateau) or a mound with distinct spatial steps. The available scenarios can be controlled by varying the depths of two fluids as well. Increasing the rotation rates also tends to induce wavy patterns that break the axial symmetry of those base shapes, while a violent collapse of the oil-water interface occurs at sufficiently fast rotations. We attempt to give partial explanations for the different scenarios. Supported by Academia Sinica and NSC, Taiwan.
HAT-P-12b: A LOW-DENSITY SUB-SATURN MASS PLANET TRANSITING A METAL-POOR K DWARF
Hartman, J. D.; Bakos, G. A.; Torres, G.; Noyes, R. W.; Pal, A.; Latham, D. W.; Sipocz, B.; Esquerdo, G. A.; Sasselov, D. D.; Kovacs, Gabor; Stefanik, R. P.; Fernandez, J. M.; Kovacs, Geza; Fischer, D. A.; Johnson, J. A.; Marcy, G. W.; Howard, A. W.; Butler, R. P.; Lazar, J.; Papp, I.
2009-11-20
We report on the discovery of HAT-P-12b, a transiting extrasolar planet orbiting the moderately bright V approx 12.8 K4 dwarf GSC 03033 - 00706, with a period P = 3.2130598 +- 0.0000021 d, transit epoch T{sub c} = 2454419.19556 +- 0.00020 (BJD), and transit duration 0.0974 +- 0.0006 d. The host star has a mass of 0.73 +- 0.02 M{sub sun}, radius of 0.70{sup +0.02}{sub -0.01} R{sub sun}, effective temperature 4650 +- 60 K, and metallicity [Fe/H] = -0.29 +- 0.05. We find a slight correlation between the observed spectral line bisector spans and the radial velocity, so we consider, and rule out, various blend configurations including a blend with a background eclipsing binary, and hierarchical triple systems where the eclipsing body is a star or a planet. We conclude that a model consisting of a single star with a transiting planet best fits the observations, and show that a likely explanation for the apparent correlation is contamination from scattered moonlight. Based on this model, the planetary companion has a mass of 0.211 +- 0.012 M{sub J} and radius of 0.959{sup +0.029}{sub -0.021} R{sub J} yielding a mean density of 0.295 +- 0.025 g cm{sup -3}. Comparing these observations with recent theoretical models, we find that HAT-P-12b is consistent with a approx1-4.5 Gyr, mildly irradiated, H/He-dominated planet with a core mass M{sub C} approx< 10 M {sub +}. HAT-P-12b is thus the least massive H/He-dominated gas giant planet found to date. This record was previously held by Saturn.
2012-01-01
Background Oxygen-base (O-base) oxidation in protein backbone is important in the protein backbone fragmentation due to the attack from reactive oxygen species (ROS). In this study, an alanine peptide was used model system to investigate this O-base oxidation by employing density functional theory (DFT) calculations combining with continuum solvent model. Detailed reaction steps were analyzed along with their reaction rate constants. Results Most of the O-base oxidation reactions for this alanine peptide are exothermic except for the bond-breakage of the Cα-N bond to form hydroperoxy alanine radical. Among the reactions investigated in this study, the activated energy of OH α-H abstraction is the lowest one, while the generation of alkylperoxy peptide radical must overcome the highest energy barrier. The aqueous situation facilitates the oxidation reactions to generate hydroxyl alanine peptide derivatives except for the fragmentations of alkoxyl alanine peptide radical. The Cα-Cβ bond of the alkoxyl alanine peptide radical is more labile than the peptide bond. Conclusion the rate-determining step of oxidation in protein backbone is the generation of hydroperoxy peptide radical via the reaction of alkylperoxy peptide radical with HO2. The stabilities of alkylperoxy peptide radical and complex of alkylperoxy peptide radical with HO2 are crucial in this O-base oxidation reaction. PMID:22524792
NASA Astrophysics Data System (ADS)
Ruggieri, M.; Peng, G. X.
2016-05-01
In this article, we study spontaneous chiral symmetry breaking for quark matter in the background of static and homogeneous parallel electric field E and magnetic field B . We use a Nambu-Jona-Lasinio model with a local kernel interaction to compute the relevant quantities to describe chiral symmetry breaking at a finite temperature for a wide range of E and B . We study the effect of this background on the inverse catalysis of chiral symmetry breaking for E and B of the same order of magnitude. We then focus on the effect of the equilibration of chiral density n5 , produced dynamically by an axial anomaly on the critical temperature. The equilibration of n5 , a consequence of chirality-flipping processes in the thermal bath, allows for the introduction of the chiral chemical potential μ5, which is computed self-consistently as a function of the temperature and field strength by coupling the number equation to the gap equation and solving the two within an expansion in E /T2 , B /T2 , and μ52/T2 . We find that even if chirality is produced and equilibrates within a relaxation time τM , it does not change drastically the thermodynamics, with particular reference to the inverse catalysis induced by the external fields, as long as the average μ5 at equilibrium is not too large.
NASA Astrophysics Data System (ADS)
Odkhuu, D.; Rhim, S. H.; Park, N.; Hong, S. C.
2013-11-01
Significant enhancement of the magnetocrystalline anisotropy (MCA) of an Fe(001) surface capped by 4d and 5d transition metal monolayers is presented in this study using first principles density functional calculations. In particular, an extremely large perpendicular MCA of +10 meV/Ir was found in Ir-capped Fe(001), which originates not from the Fe but from the large spin-orbit coupling of the Ir atoms. From the spin-channel decomposition of the MCA matrix and electronic structure analyses, we find that strong 3d-5d band hybridization in the minority spin state is responsible for the sign changes of the MCA from parallel to perpendicular.
NASA Astrophysics Data System (ADS)
Zong, Jing; Wang, Qiang
2013-03-01
How fluctuations change the order-disorder transition (ODT) of symmetric diblock copolymers (DBC) is a classic yet unsolved problem in polymer physics.[1] Taking a model system of discrete Gaussian chains interacting with soft, finite-range repulsions as commonly used in dissipative-particle dynamics simulations we formulate a density-functional theory (DFT) based on the polymer integral equation theories,[2] which includes the system fluctuations and correlations neglected by the mean-field theory (i.e., the widely applied self-consistent field theory) and can be reduced to the latter under the mean-spherical approximation. We then unambiguously reveal the fluctuation/correlation effects on the ODT of symmetric DBC by direct comparisons among the mean-field theory, DFT, and fast off-lattice Monte Carlo simulations,[3] all using exactly the same model system (Hamiltonian) and thus without any parameter-fitting.
Wang, Jiaqi; Liu, Lily; Wilson, Angela K
2016-02-11
The catalytic degradation of lignin is of considerable interest because the depolymerization of lignin to small molecules is the initial step for the conversion of lignin to biofuels and other useful chemicals. Because of the complex structure of lignin, methoxyethane was used in this study as a representative model of the most common linkage within lignin, the β-O-4 linkage. The completely renormalized coupled cluster with singles, doubles, and perturbative triples [CR-CCSD(T)] method was used to calculate the energetics of the C-O bond cleavage in methoxyethane by late 3d, 4d, and 5d transition metal atoms and to evaluate the performance of a set of density functionals (BLYP, B97D, TPSS, M06L, B3LYP, PBE0, M06, TPSSh, and B2PLYP) in predicting the reaction energetics. PMID:26735613
NASA Astrophysics Data System (ADS)
Shirayama, Masaki; Kadowaki, Hideyuki; Miyadera, Tetsuhiko; Sugita, Takeshi; Tamakoshi, Masato; Kato, Masato; Fujiseki, Takemasa; Murata, Daisuke; Hara, Shota; Murakami, Takurou N.; Fujimoto, Shohei; Chikamatsu, Masayuki; Fujiwara, Hiroyuki
2016-01-01
Light-induced photocarrier generation is an essential process in all solar cells, including organic-inorganic hybrid (CH3NH3PbI3 ) solar cells, which exhibit a high short-circuit current density (Jsc ) of approximately 20 mA /cm2 . Although the high Jsc observed in the hybrid solar cells relies on strong electron-photon interaction, the optical transitions in the perovskite material remain unclear. Here, we report artifact-free CH3NH3PbI3 optical constants extracted from ultrasmooth perovskite layers without air exposure and assign all of the optical transitions in the visible and ultraviolet region unambiguously, based on density-functional theory (DFT) analysis that assumes a simple pseudocubic crystal structure. From the self-consistent spectroscopic ellipsometry analysis of the ultrasmooth CH3NH3PbI3 layers, we find that the absorption coefficients of CH3NH3PbI3 (α =3.8 ×104 cm-1 at 2.0 eV) are comparable to those of CuInGaSe2 and CdTe, and high α values reported in earlier studies are overestimated seriously by the extensive surface roughness of CH3NH3PbI3 layers. The polarization-dependent DFT calculations show that CH3NH3 + interacts strongly with the PbI3 - cage, modifying the CH3NH3PbI3 dielectric function in the visible region rather significantly. In particular, the transition matrix element of CH3NH3PbI3 varies, depending on the position of CH3NH3 + within the Pb—I network. When the effect of CH3NH3 + on the optical transition is eliminated in the DFT calculation, the CH3NH3PbI3 dielectric function deduced from DFT shows an excellent agreement with the experimental result. As a result, distinct optical transitions observed at E0(Eg)=1.61 eV , E1=2.53 eV , and E2=3.24 eV in CH3NH3PbI3 are attributed to the direct semiconductor-type transitions at the R , M , and X points in the pseudocubic Brillouin zone, respectively. We further perform the quantum efficiency (QE) analysis for a standard hybrid-perovskite solar cell incorporating a mesoporous TiO2
Li, Y.; Noh, D.; Gallois, B. ); Tompa, G.S.; Norris, P.E.; Zawadzki, P.A. )
1990-10-01
A technique for the determination of the critical current of superconducting thin films by a current-dependent ac susceptibility measurement has been developed. This method has been used to characterize superconducting YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}} films grown {ital in situ} at 1073 K by metalorganic chemical vapor deposition. Two superconducting phases with transition temperatures of 91 and 84 K have been detected by the measurement of ac susceptibility as a function of temperature even though the variation of resistance with temperature indicated a sharp transition. The critical current densities of the two superconducting phases have been determined from the variations of ac susceptibility with current at constant temperature and found to be equal to 1.14{times}10{sup 4} A/cm{sup 2} and 3.6{times}10{sup 3} A/cm{sup 2} at 75 K. The advantages of the technique in comparison to current methods of measurement of critical current are discussed.
Singh, Priya; Sarkar, Subir K.; Bandyopadhyay, Pradipta
2014-07-07
We present the results of a high-statistics equilibrium study of the folding/unfolding transition for the 20-residue mini-protein Trp-cage (TC5b) in water. The ECEPP/3 force field is used and the interaction with water is treated by a solvent-accessible surface area method. A Wang-Landau type simulation is used to calculate the density of states and the conditional probabilities for the various values of the radius of gyration and the number of native contacts at fixed values of energy—along with a systematic check on their convergence. All thermodynamic quantities of interest are calculated from this information. The folding-unfolding transition corresponds to a peak in the temperature dependence of the computed specific heat. This is corroborated further by the structural signatures of folding in the distributions for radius of gyration and the number of native contacts as a function of temperature. The potentials of mean force are also calculated for these variables, both separately and jointly. A local free energy minimum, in addition to the global minimum, is found in a temperature range substantially below the folding temperature. The free energy at this second minimum is approximately 5 k{sub B}T higher than the value at the global minimum.
Ramanantoanina, Harry; Urland, Werner; Cimpoesu, Fanica; Daul, Claude
2013-09-01
Herein we present a Ligand Field Density Functional Theory (LFDFT) based methodology for the analysis of the 4f(n)→ 4f(n-1)5d(1) transitions in rare earth compounds and apply it for the characterization of the 4f(2)→ 4f(1)5d(1) transitions in the quantum cutter Cs2KYF6:Pr(3+) with the elpasolite structure type. The methodological advances are relevant for the analysis and prospection of materials acting as phosphors in light-emitting diodes. The positions of the zero-phonon energy corresponding to the states of the electron configurations 4f(2) and 4f(1)5d(1) are calculated, where the praseodymium ion may occupy either the Cs(+)-, K(+)- or the Y(3+)-site, and are compared with available experimental data. The theoretical results show that the occupation of the three undistorted sites allows a quantum-cutting process. However size effects due to the difference between the ionic radii of Pr(3+) and K(+) as well as Cs(+) lead to the distortion of the K(+)- and the Cs(+)-site, which finally exclude these sites for quantum-cutting. A detailed discussion about the origin of this distortion is also described. PMID:23846586
NASA Astrophysics Data System (ADS)
Singh, Priya; Sarkar, Subir K.; Bandyopadhyay, Pradipta
2014-07-01
We present the results of a high-statistics equilibrium study of the folding/unfolding transition for the 20-residue mini-protein Trp-cage (TC5b) in water. The ECEPP/3 force field is used and the interaction with water is treated by a solvent-accessible surface area method. A Wang-Landau type simulation is used to calculate the density of states and the conditional probabilities for the various values of the radius of gyration and the number of native contacts at fixed values of energy—along with a systematic check on their convergence. All thermodynamic quantities of interest are calculated from this information. The folding-unfolding transition corresponds to a peak in the temperature dependence of the computed specific heat. This is corroborated further by the structural signatures of folding in the distributions for radius of gyration and the number of native contacts as a function of temperature. The potentials of mean force are also calculated for these variables, both separately and jointly. A local free energy minimum, in addition to the global minimum, is found in a temperature range substantially below the folding temperature. The free energy at this second minimum is approximately 5 kBT higher than the value at the global minimum.
Ziegler, Tom; Seth, Michael; Krykunov, Mykhaylo; Autschbach, Jochen; Wang, Fan
2009-04-21
It is shown that it is possible to derive the basic eigenvalue equation of adiabatic time-dependent density functional theory within the Tamm-Dancoff approximation (TD-DFT/TD) from a variational principle. The variational principle is applied to the regular Kohn-Sham formulation of DFT energy expression for a single Slater determinant and leads to the same energy spectrum as TD-DFT/TD. It is further shown that this variational approach affords the same electric and magnetic transition moments as TD-DFT/TD. The variational scheme can also be applied without the Tamm-Dancoff approximation. Practical implementations of TD-DFT are limited to second order response theory which introduces errors in transition energies for charge transfer and Rydberg excitations. It is indicated that higher order terms can be incorporated into the variational approach. It is also discussed how the current variational method is related to traditional DFT schemes based on variational principles such as DeltaSCF-DFT, and how they can be combined. PMID:19388731
Sesé, Luis M; Bailey, Lorna E
2007-04-28
The structural features of the quantum hard-sphere system in the region of the fluid-face-centered-cubic-solid transition, for reduced number densities 0.45
Phung, Quan Manh; Wouters, Sebastian; Pierloot, Kristine
2016-09-13
The complete active space second order perturbation theory (CASPT2) can be extended to larger active spaces by using the density matrix renormalization group (DMRG) as solver. Two variants are commonly used: the costly DMRG-CASPT2 with exact 4-particle reduced density matrix (4-RDM) and the cheaper DMRG-cu(4)-CASPT2 in which the 4-cumulant is discarded. To assess the accuracy and limitations of the latter variant DMRG-cu(4)-CASPT2 we study the spin state energetics of iron porphyrin Fe(P) and its model compound FeL2, a model for the active center of NiFe hydrogenase, and manganese-oxo porphyrin MnO(P)(+); a series of excited states of chromium hexacarbonyl Cr(CO)6; and the interconversion of two Cu2O2(2+) isomers. Our results clearly show that PT2 on top of DMRG is essential in order to obtain quantitative results for transition metal complexes. Good results were obtained with DMRG-cu(4)-CASPT2 as compared to full CASPT2 and DMRG-CASPT2 in calculations with small- and medium-sized active spaces. In calculations with large-sized active spaces (∼30 active orbitals), the performance of DMRG-cu(4)-CASPT2 is less impressive due to the errors originating from both the finite number of renormalized states m and the 4-RDM approximation. PMID:27547847
Basu, Banasri; Bandyopadhyay, Pratul; Majumdar, Priyadarshi
2011-03-15
We have studied quantum phase transition induced by a quench in different one-dimensional spin systems. Our analysis is based on the dynamical mechanism which envisages nonadiabaticity in the vicinity of the critical point. This causes spin fluctuation which leads to the random fluctuation of the Berry phase factor acquired by a spin state when the ground state of the system evolves in a closed path. The two-point correlation of this phase factor is associated with the probability of the formation of defects. In this framework, we have estimated the density of defects produced in several one-dimensional spin chains. At the critical region, the entanglement entropy of a block of L spins with the rest of the system is also estimated which is found to increase logarithmically with L. The dependence on the quench time puts a constraint on the block size L. It is also pointed out that the Lipkin-Meshkov-Glick model in point-splitting regularized form appears as a combination of the XXX model and Ising model with magnetic field in the negative z axis. This unveils the underlying conformal symmetry at criticality which is lost in the sharp point limit. Our analysis shows that the density of defects as well as the scaling behavior of the entanglement entropy follows a universal behavior in all these systems.
Jung, Jae Won; Kim, Jong Oh; Yeo, Inhwan Jason; Cho, Young-Bin; Kim, Sun Mo; DiBiase, Steven
2012-12-15
Purpose: Fast and accurate transit portal dosimetry was investigated by developing a density-scaled layer model of electronic portal imaging device (EPID) and applying it to a clinical environment. Methods: The model was developed for fast Monte Carlo dose calculation. The model was validated through comparison with measurements of dose on EPID using first open beams of varying field sizes under a 20-cm-thick flat phantom. After this basic validation, the model was further tested by applying it to transit dosimetry and dose reconstruction that employed our predetermined dose-response-based algorithm developed earlier. The application employed clinical intensity-modulated beams irradiated on a Rando phantom. The clinical beams were obtained through planning on pelvic regions of the Rando phantom simulating prostate and large pelvis intensity modulated radiation therapy. To enhance agreement between calculations and measurements of dose near penumbral regions, convolution conversion of acquired EPID images was alternatively used. In addition, thickness-dependent image-to-dose calibration factors were generated through measurements of image and calculations of dose in EPID through flat phantoms of various thicknesses. The factors were used to convert acquired images in EPID into dose. Results: For open beam measurements, the model showed agreement with measurements in dose difference better than 2% across open fields. For tests with a Rando phantom, the transit dosimetry measurements were compared with forwardly calculated doses in EPID showing gamma pass rates between 90.8% and 98.8% given 4.5 mm distance-to-agreement (DTA) and 3% dose difference (DD) for all individual beams tried in this study. The reconstructed dose in the phantom was compared with forwardly calculated doses showing pass rates between 93.3% and 100% in isocentric perpendicular planes to the beam direction given 3 mm DTA and 3% DD for all beams. On isocentric axial planes, the pass rates varied
NASA Astrophysics Data System (ADS)
Shirakawa, Tomonori; Yunoki, Seiji
2016-05-01
The density matrix renormalization group method is introduced in energy space to study Anderson impurity models. The method allows for calculations in the thermodynamic limit and is advantageous for studying not only the dynamical properties, but also the quantum entanglement of the ground state at the vicinity of an impurity quantum phase transition. This method is applied to obtain numerically exactly the ground-state phase diagram of the single-impurity Anderson model on the honeycomb lattice at half-filling. The calculation of local static quantities shows that the phase diagram contains two distinct phases, the local moment (LM) phase and the asymmetric strong coupling (ASC) phase, but no Kondo screening phase. These results are supported by the local spin and charge excitation spectra, which exhibit qualitatively different behavior in these two phases and also reveal the existence of the valence fluctuating point at the phase boundary. For comparison, we also study the low-energy effective pseudogap Anderson model using the method introduced here. Although the high-energy excitations are obviously different, we find that the ground-state phase diagram and the asymptotically low-energy excitations are in good quantitative agreement with those for the single-impurity Anderson model on the honeycomb lattice, thus providing a quantitative justification for the previous studies based on low-energy approximate approaches. Furthermore, we find that the lowest entanglement level is doubly degenerate for the LM phase, whereas it is singlet for the ASC phase and is accidentally threefold degenerate at the valence fluctuating point. This should be contrasted with the degeneracy of the energy spectrum because the ground state is found to be always singlet. Our results therefore clearly demonstrate that the low-lying entanglement spectrum can be used to determine with high accuracy the phase boundary of the impurity quantum phase transition.
Kanner, G.S.; Gammel, J.T.; Love, S.P.; Johnson, S.R.; Scott, B.; Swanson, B.I. )
1994-12-15
We report the pressure dependence up to 95 kbar of Raman-active stretching modes in the quasi-one-dimensional [ital MX] chain solid Pt[sub 2]Br[sub 6](NH[sub 3])[sub 4]. The data indicate that a predicted pressure-induced insulator-to-metal transition does not occur, but are consistent with the solid undergoing either a three-dimensional structural distortion or a transition from a charge-density wave to another broken-symmetry ground state. We show that such a transition can be well modeled within a Peierls-Hubbard Hamiltonian.
NASA Astrophysics Data System (ADS)
Vojvodic, A.; Ruberto, C.; Lundqvist, B. I.
2010-09-01
This study explores atomic and molecular adsorption on a number of early transition-metal carbides (TMCs) in NaCl structure by means of density-functional theory calculations. The investigated substrates are the TM-terminated TMC(111) surfaces, of interest because of the presence of different types of surface resonances (SRs) on them and because of their technological importance in growth processes. Also, TM compounds have shown potential in catalysis applications. Trend studies are conducted with respect to both period and group in the periodic table, choosing the substrates ScC, TiC, VC, ZrC, NbC, δ-MoC, TaC, and WC (in NaCl structure) and the adsorbates H, B, C, N, O, F, NH, NH2, and NH3. Trends in adsorption strength are explained in terms of surface electronic factors, by correlating the calculated adsorption-energy values with the calculated surface electronic structures. The results are rationalized by use of a concerted-coupling model (CCM), which has previously been applied successfully to the description of adsorption on TiC(111) and TiN(111) surfaces (Ruberto et al 2007 Solid State Commun. 141 48). First, the clean TMC(111) surfaces are characterized by calculating surface energies, surface relaxations, Bader charges, and surface-localized densities of states (DOSs). Detailed comparisons between surface and bulk DOSs reveal the existence of transition-metal localized SRs (TMSRs) in the pseudogap and of several C-localized SRs (CSRs) in the upper valence band on all considered TMC(111) surfaces. The spatial extent and the dangling bond nature of these SRs are supported by real-space analyses of the calculated Kohn-Sham wavefunctions. Then, atomic and molecular adsorption energies, geometries, and charge transfers are presented. An analysis of the adsorbate-induced changes in surface DOSs reveals a presence of both adsorbate-TMSR and adsorbate-CSRs interactions, of varying strengths depending on the surface and the adsorbate. These variations are
Joshi, K.B.; Paliwal, U.; Galav, K.L.; Trivedi, D.K.; Bredow, T.
2013-08-15
Stability of B1 and B2 phases of Mg{sub x}Cd{sub 1−x}O is studied by calculating the formation energy within the framework of density functional theory applying the crystalline-orbital program package. Structural and electronic properties of the two polymorphs are reported for x=0.25, 0.50 and 0.75. The equilibrium lattice constants and bulk moduli are computed. Enthalpy calculations show pressure induced B1→B2 phase transitions at 92 GPa, 138 GPa and 212 GPa, respectively, for Mg{sub 0.25}Cd{sub 0.75}O, Mg{sub 0.50}Cd{sub 0.50}O and Mg{sub 0.75}Cd{sub 0.25}O compositions. Formation energy of ternary oxides in the B1 phase is negative with respect to mixing of B2-MgO with B1-CdO. Mixing B1-MgO with B2-CdO also leads to negative formation energy in Cd rich B1 phase ternary oxides (0≤x≤0.5). Band structure calculations predict direct band gaps in the B1 phase and indirect band gaps in the B2 phase ternary oxides. Mulliken population analysis is performed for the two polymorphs to study the charge transfer. - Graphical abstract: Diagram reveals trends in formation energy while mixing B2-MgO with B1-CdO to form B1-Mg{sub x}Cd{sub 1−x}O. Formation energies obtained from mixing isostructural and nonisostructural components are also shown. Display Omitted - Highlights: • Lattice constants and bulk moduli are computed for Mg{sub 0.25}Cd{sub 0.75}O, Mg{sub 0.50}Cd{sub 0.50}O and Mg{sub 0.75}Cd{sub 0.25}O compositions. • Enthalpy calculations signify pressure induced B1→B2 phase transitions at 92 GPa, 138 GPa and 212 GPa, respectively, in Mg{sub 0.25}Cd{sub 0.75}O, Mg{sub 0.50}Cd{sub 0.50}O and Mg{sub 0.75}Cd{sub 0.25}O. • Band structure calculations predict direct band gaps in the B1 phase ternary oxides. • In the B2 phase ternary oxides band structure calculations show valence band maximum along the Γ–X direction and the conduction band minimum at the Γ point of symmetry.
Zibordi-Besse, Larissa; Tereshchuk, Polina; Chaves, Anderson S; Da Silva, Juarez L F
2016-06-23
Transition-metal (TM) nanoparticles supported on oxides or carbon black have attracted much attention as potential catalysts for ethanol steam reforming reactions for hydrogen production. To improve the performance of nanocatalysts, a fundamental understanding of the interaction mechanism between water and ethanol with finite TM particles is required. In this article, we employed first-principles density functional theory with van der Waals (vdW) corrections to investigate the interaction of ethanol and water with TM13 clusters, where TM = Ni, Cu, Pd, Ag, Pt, and Au. We found that both water and ethanol bind via the anionic O atom to onefold TM sites, while at higher-energy structures, ethanol binds also via the H atom from the CH2 group to the TM sites, which can play an important role at real catalysts. The putative global minimum TM13 configurations are only slightly affected upon the adsorption of water or ethanol; however, for few systems, the compact higher-energy icosahedron structure changes its configuration upon ethanol or water adsorption. That is, those configurations are only shallow local minimums in the phase space. Except few deviations, we found similar trends for the magnitude of the adsorption energies of water and ethanol, that is, Ni13 > Pt13 > Pd13 and Cu13 > Au13 > Ag13, which is enhanced by the addition of the vdW correction (i.e., from 4% to 62%); however, the trend is the same. We found that the magnitude of the adsorption energy increases by shifting the center of gravity of the d-states toward the highest occupied molecular orbital. On the basis of the Mulliken and Hirshfeld charge analysis, as well as electron density differences, we identified the location of the charge redistribution and a tiny charge transfer (from 0.01 e to 0.19 e) from the molecules to the TM13 clusters. Our vibrational analysis indicates the red shifts in the OH modes upon binding of both water and ethanol molecules to the TM13 clusters, suggesting a weakening of
NASA Astrophysics Data System (ADS)
Yang, Yue; Weaver, Michael N.; Merz, Kenneth M.
2009-08-01
Computational chemists have long demonstrated great interest in finding ways to reliably and accurately predict the molecular properties for transition-metal-containing complexes. This study is a continuation of our validation efforts of density functional theory (DFT) methods when applied to transition-metal-containing systems (Riley, K.E.; Merz, K. M., Jr. J. Phys. Chem. 2007, 111, 6044-6053). In our previous work we examined DFT using all-electron basis sets, but approaches incorporating effective core potentials (ECPs) are effective in reducing computational expense. With this in mind, our efforts were expanded to include evaluation of the performance of the basis set derived to approximate such an approach as well on the same set of density functionals. Indeed, employing an ECP basis such as LANL2DZ (Los Alamos National Laboratory 2 double ζ) for transition metals, while using all-electron basis sets for all other non-transition-metal atoms, has become more and more popular in computations on transition-metal-containing systems. In this study, we assess the performance of 12 different DFT functionals, from the GGA (generalized gradient approximation), hybrid-GGA, meta-GGA, and hybrid-meta-GGA classes, respectively, along with the 6-31+G** + LANL2DZ (on the transition metal) mixed basis set in predicting two important molecular properties, heats of formation and ionization potentials, for 94 and 58 systems containing first-row transition metals from Ti to Zn, which are all in the third row of the periodic table. An interesting note is that the inclusion of the exact exchange term in density functional methods generally increases the accuracy of ionization potential prediction for the hybrid-GGA methods but decreases the reliability of determining the heats of formation for transition-metal-containing complexes for all hybrid density functional methods. The hybrid-GGA functional B3LYP gives the best performance in predicting the ionization potentials, while the
O'Keefe, Christopher A; Johnston, Karen E; Sutter, Kiplangat; Autschbach, Jochen; Gauvin, Régis; Trébosc, Julien; Delevoye, Laurent; Popoff, Nicolas; Taoufik, Mostafa; Oudatchin, Konstantin; Schurko, Robert W
2014-09-15
Chlorine ligands in a variety of diamagnetic transition-metal (TM) complexes in common structural motifs were studied using (35)Cl solid-state NMR (SSNMR), and insight into the origin of the observed (35)Cl NMR parameters was gained through first-principles density functional theory (DFT) calculations. The WURST-CPMG pulse sequence and the variable-offset cumulative spectrum (VOCS) methods were used to acquire static (35)Cl SSNMR powder patterns at both standard (9.4 T) and ultrahigh (21.1 T) magnetic field strengths, with the latter affording higher signal-to-noise ratios (S/N) and reduced experimental times (i.e., <1 h). Analytical simulations were performed to extract the (35)Cl electric field gradient (EFG) tensor and chemical shift (CS) tensor parameters. It was found that the chlorine ligands in various bonding environments (i.e., bridging, terminal-axial, and terminal-equatorial) have drastically different (35)Cl EFG tensor parameters, suggesting that (35)Cl SSNMR is ideal for characterizing chlorine ligands in TM complexes. A detailed localized molecular orbital (LMO) analysis was completed for NbCl5. It was found that the contributions of individual molecular orbitals must be considered to fully explain the observed EFG parameters, thereby negating simple arguments based on comparison of bond lengths and angles. Finally, we discuss the application of (35)Cl SSNMR for the structural characterization of WCl6 that has been grafted onto a silica support material. The resulting tungsten-chloride surface species is shown to be structurally distinct from the parent compound. PMID:25162702
Sokolowsky, Kathleen P; Bailey, Heather E; Hoffman, David J; Andersen, Hans C; Fayer, Michael D
2016-07-21
Two-dimensional infrared (2D IR) data are presented for a vibrational probe in three nematogens: 4-cyano-4'-pentylbiphenyl, 4-cyano-4'-octylbiphenyl, and 4-(trans-4-amylcyclohexyl)-benzonitrile. The spectral diffusion time constants in all three liquids in the isotropic phase are proportional to [T*/(T - T*)](1/2), where T* is 0.5-1 K below the isotropic-nematic phase transition temperature (TNI). Rescaling to a reduced temperature shows that the decays of the frequency-frequency correlation function (FFCF) for all three nematogens fall on the same curve, suggesting a universal dynamic behavior of nematogens above TNI. Spectral diffusion is complete before significant orientational relaxation in the liquid, as measured by optically heterodyne detected-optical Kerr effect (OHD-OKE) spectroscopy, and before any significant orientational randomization of the probe measured by polarization selective IR pump-probe experiments. To interpret the OHD-OKE and FFCF data, we constructed a mode coupling theory (MCT) schematic model for the relationships among three correlation functions: ϕ1, a correlator for large wave vector density fluctuations; ϕ2, the orientational correlation function whose time derivative is the observable in the OHD-OKE experiment; and ϕ3, the FFCF for the 2D IR experiment. The equations for ϕ1 and ϕ2 match those in the previous MCT schematic model for nematogens, and ϕ3 is coupled to the first two correlators in a straightforward manner. Resulting models fit the data very well. Across liquid crystals, the temperature dependences of the coupling constants show consistent, nonmonotonic behavior. A remarkable change in coupling occurs at ∼5 K above TNI, precisely where the rate of spectral diffusion in 5CB was observed to deviate from that of a similar nonmesogenic liquid. PMID:27363680
Conradie, Jeanet; Patra, Ashis K; Harrop, Todd C; Ghosh, Abhik
2015-02-16
Density functional theory (in the form of the PW91, BP86, OLYP, and B3LYP exchange-correlation functionals) has been used to map out the low-energy states of a series of eight-coordinate square-antiprismatic (D2d) first-row transition metal complexes, involving Mn(II), Fe(II), Co(II), Ni(II), and Cu(II), along with a pair of tetradentate N4 ligands. Of the five complexes, the Mn(II) and Fe(II) complexes have been synthesized and characterized structurally and spectroscopically, whereas the other three are as yet unknown. Each N4 ligand consists of a pair of terminal imidazole units linked by an o-phenylenediimine unit. The imidazole units are the strongest ligands in these complexes and dictate the spatial disposition of the metal three-dimensional orbitals. Thus, the dx(2)-y(2) orbital, whose lobes point directly at the coordinating imidazole nitrogens, has the highest orbital energy among the five d orbitals, whereas the dxy orbital has the lowest orbital energy. In general, the following orbital ordering (in order of increasing orbital energy) was found to be operative: dxy < dxz = dyz ≤ dz(2) < dx(2)-y(2). The square-antiprism geometry does not lead to large energy gaps between the d orbitals, which leads to an S = 2 ground state for the Fe(II) complex. Nevertheless, the dxy orbital has significantly lower energy relative to that of the dxz and dyz orbitals. Accordingly, the ground state of the Fe(II) complex corresponds unambiguously to a dxy(2)dxz(1)dyz(1)dz(2)(1)dx(2)-y(2)(1) electronic configuration. Unsurprisingly, the Mn(II) complex has an S = 5/2 ground state and no low-energy d-d excited states within 1.0 eV of the ground state. The Co(II) complex, on the other hand, has both a low-lying S = 1/2 state and multiple low-energy S = 3/2 states. Very long metal-nitrogen bonds are predicted for the Ni(II) and Cu(II) complexes; these bonds may be too fragile to survive in solution or in the solid state, and the complexes may therefore not be isolable
Javaid, Saqib; Javed Akhtar, M.
2014-07-14
We have investigated the behavior of orthoferrite LaFeO{sub 3} at ambient conditions and under pressure using DFT (generalized gradient approximation (GGA)) + U approach. Ground state electronic (band gap) and magnetic properties are considerably improved due to the Hubbard correction. Moreover, the experimentally observed pressure-driven phase transition, namely, the simultaneous occurrence of spin crossover, isostructural volume collapse, and drastic reduction in electrical resistance (electronic phase transition) is nicely described by GGA + U calculations. In particular, despite a sharp drop in resistance, a small band gap still remains in the low spin state indicating an insulator to semiconductor phase transition, in good agreement with the experiments but in contrast to GGA, which predicts metallic behavior in low spin state. We discuss the origin of variation in electronic structure of LaFeO{sub 3} in low spin state as obtained from GGA to GGA + U methods. These results emphasize the importance of correlation effects in describing the pressure-driven phase transition in LaFeO{sub 3} and other rare-earth orthoferrites.
Zhou, Y
2006-08-21
The Euler similarity criteria for laboratory experiments and time-dependent mixing transition are important concepts introduced recently for application to prediction and analysis of astrophysical phenomena. However Euler scaling by itself provides no information on the distinctive spectral range of high Reynolds number turbulent flows found in astrophysics situations. On the other hand, time-dependent mixing transition gives no indication on whether a flow that just passed the mixing transition is sufficient to capture all of the significant dynamics of the complete astrophysical spectral range. In this paper, a new approach, based on additional insight gained from review of Navier-Stokes turbulence theory, is developed. It allows for revelations about the distinctive spectral scale dynamics associated with high Reynolds number astrophysical flows. From this perspective, we caution that the energy containing range of the turbulent flow measured in a laboratory setting must not be unintentionally contaminated in such a way that the interactive influences of this spectral scale range in the corresponding astrophysical situation cannot be faithfully represented. In this paper we introduce the concept of a minimum state as the lowest Reynolds number turbulent flow that a time-dependent mixing transition must achieve to fulfill this objective. Later in the paper we show that the Reynolds number of the minimum state may be determined as 1.6 x 10{sup 5}. Our efforts here can be viewed as a unification and extension of the concepts of both similarity scaling and transient mixing transition concepts. At the last the implications of our approach in planning future intensive laser experiments or massively parallel numerical simulations are discussed. A systematic procedure is outlined so that as the capabilities of the laser interaction experiments and supporting results from detailed numerical simulations performed in recently advanced supercomputing facilities increase
He, Xiao; Ryu, Shinsei; Hirata, So
2014-01-14
Finite-temperature extensions of ab initio Gaussian-basis-set spin-restricted Hartree–Fock (HF) and second-order many-body perturbation (MP2) theories are implemented for infinitely extended, periodic, one-dimensional solids and applied to the Peierls and charge-density-wave (CDW) transitions in polyyne and all-trans polyacetylene. The HF theory predicts insulating CDW ground states for both systems in their equidistant structures at low temperatures. In the same structures, they turn metallic at high temperatures. Starting from the “dimerized” low-temperature equilibrium structures, the systems need even higher temperatures to undergo a Peierls transition, which is accompanied by geometric as well as electronic distortions from dimerized to non-dimerized forms. The conventional finite-temperature MP2 theory shows a sign of divergence in any phase at any nonzero temperature and is useless. The renormalized finite-temperature MP2 (MP2R) theory is divergent only near metallic electronic structures, but is well behaved elsewhere. MP2R also predicts CDW and Peierls transitions occurring at two different temperatures. The effect of electron correlation is primarily to lower the Peierls transition temperature.
NASA Astrophysics Data System (ADS)
Guenther, E. W.; Díaz, R. F.; Gazzano, J.-C.; Mazeh, T.; Rouan, D.; Gibson, N.; Csizmadia, Sz.; Aigrain, S.; Alonso, R.; Almenara, J. M.; Auvergne, M.; Baglin, A.; Barge, P.; Bonomo, A. S.; Bordé, P.; Bouchy, F.; Bruntt, H.; Cabrera, J.; Carone, L.; Carpano, S.; Cavarroc, C.; Deeg, H. J.; Deleuil, M.; Dreizler, S.; Dvorak, R.; Erikson, A.; Ferraz-Mello, S.; Fridlund, M.; Gandolfi, D.; Gillon, M.; Guillot, T.; Hatzes, A.; Havel, M.; Hébrard, G.; Jehin, E.; Jorda, L.; Lammer, H.; Léger, A.; Moutou, C.; Nortmann, L.; Ollivier, M.; Ofir, A.; Pasternacki, Th.; Pätzold, M.; Parviainen, H.; Queloz, D.; Rauer, H.; Samuel, B.; Santerne, A.; Schneider, J.; Tal-Or, L.; Tingley, B.; Weingrill, J.; Wuchterl, G.
2012-01-01
Context. Observations of transiting extrasolar planets are of key importance to our understanding of planets because their mass, radius, and mass density can be determined. These measurements indicate that planets of similar mass can have very different radii. For low-density planets, it is generally assumed that they are inflated owing to their proximity to the host-star. To determine the causes of this inflation, it is necessary to obtain a statistically significant sample of planets with precisely measured masses and radii. Aims: The CoRoT space mission allows us to achieve a very high photometric accuracy. By combining CoRoT data with high-precision radial velocity measurements, we derive precise planetary radii and masses. We report the discovery of CoRoT-19b, a gas-giant planet transiting an old, inactive F9V-type star with a period of four days. Methods: After excluding alternative physical configurations mimicking a planetary transit signal, we determine the radius and mass of the planet by combining CoRoT photometry with high-resolution spectroscopy obtained with the echelle spectrographs SOPHIE, HARPS, FIES, and SANDIFORD. To improve the precision of its ephemeris and the epoch, we observed additional transits with the TRAPPIST and Euler telescopes. Using HARPS spectra obtained during the transit, we then determine the projected angle between the spin of the star and the orbit of the planet. Results: We find that the host star of CoRoT-19b is an inactive F9V-type star close to the end of its main-sequence life. The host star has a mass M∗ = 1.21 ± 0.05 M⊙ and radius R∗ = 1.65 ± 0.04 R⊙. The planet has a mass of MP = 1.11 ± 0.06 MJup and radius of RP = 1.29 ± 0.03 RJup. The resulting bulk density is only ρ = 0.71 ± 0.06 g cm-3, which is much lower than that for Jupiter. Conclusions: The exoplanet CoRoT-19b is an example of a giant planet of almost the same mass as Jupiter but a ≈30% larger radius. The CoRoT space mission, launched on
NASA Astrophysics Data System (ADS)
Kumar, Avinash; Eckel, Stephen; Jendrzejewski, Fred; Campbell, Gretchen
We study the decay of a persistent, quantized current state in a toroidal geometry. Our experiment involves trapping neutral 23Na atoms in an all optical ``target trap'' shaped potential. This potential consists of a disc surrounded by an annular potential. A current in a superfluid can be sustained only below a critical current. This critical current can be tuned by introducing a density perturbation which depletes the local density. The decay time of a persistent current state can also be controlled by enhancing fluctuations of the system thermally. We study the decay at four different temperatures between 30 nK and 190 nK. For each temperature we record the decay at four different perturbation strengths. We find that increasing the magnitude of the density depletion or the temperature leads to a faster decay, and have seen the decay constant change by over two orders of magnitude. We also studied the size of hysteresis loop between different current states as a function of temperature, allowing us to extract a critical velocity. We find that the discrepancies between the experimentally extracted critical velocity and theoretically calculated critical velocity (using local-density approximation) decreases as the temperature is decreased. Now at University of Heidelberg.
Jiang, Jiang; DeAngelis, Donald L.; Zhang, B.; Cohen, J.E.
2014-01-01
Taylor's power law describes an empirical relationship between the mean and variance of population densities in field data, in which the variance varies as a power, b, of the mean. Most studies report values of b varying between 1 and 2. However, Cohen (2014a) showed recently that smooth changes in environmental conditions in a model can lead to an abrupt, infinite change in b. To understand what factors can influence the occurrence of an abrupt change in b, we used both mathematical analysis and Monte Carlo samples from a model in which populations of the same species settled on patches, and each population followed independently a stochastic linear birth-and-death process. We investigated how the power relationship responds to a smooth change of population growth rate, under different sampling strategies, initial population density, and population age. We showed analytically that, if the initial populations differ only in density, and samples are taken from all patches after the same time period following a major invasion event, Taylor's law holds with exponent b=1, regardless of the population growth rate. If samples are taken at different times from patches that have the same initial population densities, we calculate an abrupt shift of b, as predicted by Cohen (2014a). The loss of linearity between log variance and log mean is a leading indicator of the abrupt shift. If both initial population densities and population ages vary among patches, estimates of b lie between 1 and 2, as in most empirical studies. But the value of b declines to ~1 as the system approaches a critical point. Our results can inform empirical studies that might be designed to demonstrate an abrupt shift in Taylor's law.
NASA Technical Reports Server (NTRS)
DeSoto, G. E.; Frey, H. V.
2005-01-01
Understanding the fundamental age relationships of the different parts of the Mars Crustal Dichotomy is essential to fully understanding the events that shaped the early history and formation of the surface of Mars. A dominant question is what are the true relative ages of the Northern Lowlands and the Southern Highlands? Using MOLA data from the Mars Global Surveyor and Viking visual images, a dataset of both buried and visible crater diameters was created over a nine million sq km study area of a section of the dichotomy boundary stretching from Arabia Terra to Utopia Planitia. Cumulative frequency plots on a log-log scale were used to determine the relative ages for the Highlands, the Lowlands, and the Transition Zone, separately for the visible, the buried and the combined total (visible+ buried) populations. We find the overall Highland crater population in this area is slightly older than the Lowlands, consistent with previous global studies, but the Lowlands and Transition Zone are also very old and formed at roughly the same time. It appears that the formation of the Lowlands in this region formed contemporaneously with a large-scale resurfacing event in the Highlands, perhaps caused by the process responsible for the Lowland formation.
Matsui, Toru; Kitagawa, Yasutaka; Shigeta, Yasuteru; Okumura, Mitsutaka
2013-07-01
We propose an accurate scheme to evaluate the redox potential of a wide variety of transition metal complexes by adding a charge-dependent correction term for a counterion around the charged complexes, which is based on Generalized Born theory, to the solvation energy. The mean absolute error (MAE) toward experimental redox potentials of charged complexes is considerably reduced from 0.81 V (maximum error 1.22 V) to 0.22 V (maximum error 0.50 V). We found a remarkable exchange-correlation functional dependence on the results rather than the basis set ones. The combination of Wachters+f (for metal) and 6-31++G(d,p) (for other atoms) with the B3LYP functional gives the least MAE 0.15 V for the test complexes. This scheme is applicable to other solvents, and heavier transition metal complexes such as M1(CO)5(pycn) (M1 = Cr, Mo, W), M2(mnt)2 (M2 = Ni, Pd, Pt), and M3(bpy)3 (M3 = Fe, Ru, Os) with the same quality. PMID:26583980
Hermes, Matthew R; Hirata, So
2015-09-14
One-dimensional (1D) solids exhibit a number of striking electronic structures including charge-density wave (CDW) and spin-density wave (SDW). Also, the Peierls theorem states that at zero temperature, a 1D system predicted by simple band theory to be a metal will spontaneously dimerize and open a finite fundamental bandgap, while at higher temperatures, it will assume the equidistant geometry with zero bandgap (a Peierls transition). We computationally study these unique electronic structures and transition in polyyne and all-trans polyacetylene using finite-temperature generalizations of ab initio spin-unrestricted Hartree-Fock (UHF) and spin-restricted coupled-cluster doubles (CCD) theories, extending upon previous work [He et al., J. Chem. Phys. 140, 024702 (2014)] that is based on spin-restricted Hartree-Fock (RHF) and second-order many-body perturbation (MP2) theories. Unlike RHF, UHF can predict SDW as well as CDW and metallic states, and unlike MP2, CCD does not diverge even if the underlying RHF reference wave function is metallic. UHF predicts a gapped SDW state with no dimerization at low temperatures, which gradually becomes metallic as the temperature is raised. CCD, meanwhile, confirms that electron correlation lowers the Peierls transition temperature. Furthermore, we show that the results from all theories for both polymers are subject to a unified interpretation in terms of the UHF solutions to the Hubbard-Peierls model using different values of the electron-electron interaction strength, U/t, in its Hamiltonian. The CCD wave function is shown to encompass the form of the exact solution of the Tomonaga-Luttinger model and is thus expected to describe accurately the electronic structure of Luttinger liquids. PMID:26374011
Hermes, Matthew R.; Hirata, So
2015-09-14
One-dimensional (1D) solids exhibit a number of striking electronic structures including charge-density wave (CDW) and spin-density wave (SDW). Also, the Peierls theorem states that at zero temperature, a 1D system predicted by simple band theory to be a metal will spontaneously dimerize and open a finite fundamental bandgap, while at higher temperatures, it will assume the equidistant geometry with zero bandgap (a Peierls transition). We computationally study these unique electronic structures and transition in polyyne and all-trans polyacetylene using finite-temperature generalizations of ab initio spin-unrestricted Hartree–Fock (UHF) and spin-restricted coupled-cluster doubles (CCD) theories, extending upon previous work [He et al., J. Chem. Phys. 140, 024702 (2014)] that is based on spin-restricted Hartree–Fock (RHF) and second-order many-body perturbation (MP2) theories. Unlike RHF, UHF can predict SDW as well as CDW and metallic states, and unlike MP2, CCD does not diverge even if the underlying RHF reference wave function is metallic. UHF predicts a gapped SDW state with no dimerization at low temperatures, which gradually becomes metallic as the temperature is raised. CCD, meanwhile, confirms that electron correlation lowers the Peierls transition temperature. Furthermore, we show that the results from all theories for both polymers are subject to a unified interpretation in terms of the UHF solutions to the Hubbard–Peierls model using different values of the electron-electron interaction strength, U/t, in its Hamiltonian. The CCD wave function is shown to encompass the form of the exact solution of the Tomonaga–Luttinger model and is thus expected to describe accurately the electronic structure of Luttinger liquids.
Sorkin, Anastassia; Iron, Mark A.; Truhlar, Donald G.
2008-02-01
The research described in this product was performed in part in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. The ground and lower excited states of Fe2, Fe2 -, and FeO+ were studied using a number of density functional theory (DFT) methods. Specific attention was paid to the relative state energies, the internuclear distances (re), and the harmonic vibrational frequencies (öe). A number of factors influencing the calculated values of these properties were examined. These include basis sets, the nature of the density functional chosen, the percentage of Hartree- Fock exchange in the density functional, and constraints on orbital symmetry. A number of different types of generalized gradient approximation (GGA) density functionals (straight GGA, hybrid GGA, meta-GGA, and hybrid meta-GGA) were examined, and it was found that the best results were obtained with hybrid GGA or hybrid meta-GGA functionals that contain nonzero fractions of HF exchange; specifically, the best overall results were obtained with B3LYP, M05, and M06, closely followed by B1LYP. One significant observation was the effect of enforcing symmetry on the orbitals. When a degenerate orbital (ð or ä) is partially occupied in the 4¼ excited state of FeO+, reducing the enforced symmetry (from C6v to C4v to C2v) results in a lower energy since these degenerate orbitals are split in the lower symmetries. The results obtained were compared to higher level ab initio results from the literature and to recent PBE+U plane wave results by Kulik et al. (Phys. Rev. Lett. 2006, 97, 103001). It was found that some of the improvements that were afforded by the semiempirical +U correction can also be accomplished by improving the form of the DFT functional and, in one case, by not enforcing high symmetry on the orbitals.
NASA Astrophysics Data System (ADS)
Ramirez-Torres, Alfredo; Turkowski, Volodymyr; Rahman, Talat S.
2014-08-01
To examine optically excited bound states, excitons and trions, in monolayer MoS2,MoSe2, and WSe2, we have formulated and applied a generalized time-dependent density-matrix functional theory approach. Three different types of exchange-correlation (XC) kernels were used and their validity was evaluated through comparison with available experimental data. For excitons, we find that the local kernels, from the local density approximation and its gradient-corrected form, lead to much smaller binding energy than that extracted from experimental data, while those based on long-range (LR) interactions fare much better. The same is the case for the trion binding energy once screening effects are taken into account. Our results suggest that for both excitons and trions, the LR form of the XC kernel is necessary to describe bound states. These results confirm information from experimental data on single-layer dichalcogenides that their exciton and trion binding energies are of the order of hundreds (excitons) and tens (trions) of milli-electron volts, a result that may suggest technological application of these materials at room temperature. The proposed methodology can be straightforwardly extended to bound states with a larger number of electrons and holes than considered here.
Li, Zhendong; Liu, Wenjian
2016-06-14
Compared with closed-shell systems, open-shell systems place three additional challenges to time-dependent density functional theory (TD-DFT) for electronically excited states: (a) the spin-contamination problem is a serious issue; (b) the exchange-correlation (XC) kernel may be numerically instable; and (c) the single-determinant description of open-shell ground states readily becomes energetically instable. Confined to flip-up single excitations, the spin-contamination problem can largely be avoided by using the spin-flip TD-DFT (SF-TD-DFT) formalism, provided that a noncollinear XC kernel is employed. As for the numerical instabilities associated with such a kernel, only an ad hoc scheme has been proposed so far, viz., the ALDA0 kernel, which amounts to setting the divergent components (arising from density gradients and kinetic energy density) simply to zero. The ground-state instability problem can effectively be avoided by introducing the Tamm-Dancoff approximation (TDA) to TD-DFT. Therefore, on a general basis, the SF-TDA/ALDA0 Ansatz is so far the only promising means within the TD-DFT framework for flip-up single excitations of open-shell systems. To assess systematically the performance of SF-TDA/ALDA0, in total 61 low-lying quartet excited states of the benchmark set of 11 small radicals [J. Chem. Theory Comput. 2016, 12, 238] are investigated with various XC functionals. Taking the MRCISD+Q (multireference configuration interaction with singles and doubles plus the Davidson correction) results as benchmark, it is found that the mean absolute errors of SF-TDA/ALDA0 with the SAOP (statistical averaging of model orbital potentials), global hybrid, and range-separated hybrid functionals are in the range of 0.2-0.4 eV. This is in line not only with the typical accuracy of TD-DFT for singlet and triplet excited states of closed-shell systems but also with the gross accuracy of spin-adapted TD-DFT for spin-conserving excited states of open-shell systems. PMID
Zu, Ningning; Wang, Jing; Wang, Ying; Wu, Zhijian
2014-06-21
La2VMnO6 is an insulating ferrimagnet experimentally. By substituting La with Sr, La2-xSrxVMnO6 (x = 0.5, 1.0, 1.5, 2.0) was investigated using the density functional theory. Our results indicated that half metallic properties are obtained for x = 0.5, 1.5, 2.0. For x = 1.0, it is insulating. With the increase of hole doping, the holes initially go to V 3d orbitals for x = 0.5, 1.0, and after that, the holes go to the Mn 3d orbitals for x = 1.5, 2.0. Ferrimagnetic coupling between V and Mn is found to be the ground state for x = 0.5, 1.5, while for x = 1.0 and 2.0, ferromagnetic and antiferromagnetic couplings between Mn and Mn are competitive for the ground state. PMID:24769726
NASA Astrophysics Data System (ADS)
Liu, Bin; Cheng, Lei; Curtiss, Larry; Greeley, Jeffrey
2014-04-01
The hydrogenation of furfural to furfuryl alcohol on Pd(111), Cu(111) and Pt(111) is studied with both standard Density Functional Theory (DFT)-GGA functionals and with van der Waals-corrected density functionals. VdW-DF functionals, including optPBE, optB88, optB86b, and Grimme's method, are used to optimize the adsorption configurations of furfural, furfuryl alcohol, and related intermediates resulting from hydrogenation of furfural, and the results are compared to corresponding values determined with GGA functionals, including PW91 and PBE. On Pd(111) and Pt(111), the adsorption geometries of the intermediates are not noticeably different between the two classes of functionals, while on Cu(111), modest changes are seen in both the perpendicular distance and the orientation of the aromatic ring with respect to the planar surface. In general, the binding energies increase substantially in magnitude as a result of van der Waals contributions on all metals. In contrast, however, dispersion effects on the kinetics of hydrogenation are relatively small. It is found that activation barriers are not significantly affected by the inclusion of dispersion effects, and a Brønsted-Evans-Polanyi relationship developed solely from PW91 calculations on Pd(111) is capable of describing corresponding results on Cu(111) and Pt(111), even when the dispersion effects are included. Finally, the reaction energies and barriers derived from the dispersion-corrected and pure GGA calculations are used to plot simple potential energy profiles for furfural hydrogenation to furfuryl alcohol on the three considered metals, and an approximately constant downshift of the energetics due to the dispersion corrections is observed.
Liu, Bin; Cheng, Lei; Curtiss, Larry A.; Greeley, Jeffrey P.
2014-04-01
The hydrogenation of furfural to furfuryl alcohol on Pd(111), Cu(111) and Pt(111) is studied with both standard Density Functional Theory (DFT)-GGA functionals and with van der Waals-corrected density functionals. VdWDF functionals, including optPBE, optB88, optB86b, and Grimme's method, are used to optimize the adsorption configurations of furfural, furfuryl alcohol, and related intermediates resulting from hydrogenation of furfural, and the results are compared to corresponding values determined with GGA functionals, including PW91 and PBE. On Pd(111) and Pt(111), the adsorption geometries of the intermediates are not noticeably different between the two classes of functionals, while on Cu(111), modest changes are seen in both the erpendicular distance and the orientation of the aromatic ringwith respect to the planar surface. In general, the binding energies increase substantially in magnitude as a result of van derWaals contributions on all metals. In contrast, however, dispersion effects on the kinetics of hydrogenation are relatively small. It is found that activation barriers are not significantly affected by the inclusion of dispersion effects, and a Brønsted–Evans–Polanyi relationship developed solely fromPW91 calculations on Pd(111) is capable of describing corresponding results on Cu(111) and Pt(111), even when the dispersion effects are included. Finally, the reaction energies and barriers derived from the dispersion-corrected and pure GGA calculations are used to plot simple potential energy profiles for furfural hydrogenation to furfuryl alcohol on the three considered metals, and an approximately constant downshift of the energetics due to the dispersion corrections is observed.
Greendale, Gail A.; Tseng, Chi-hong; Han, Weijuan; Huang, Mei-Hua; Leung, Katherine; Crawford, Sybil; Gold, Ellen B.; Waetjen, L. Elaine; Karlamangla, Arun S.
2014-01-01
Objective To examine the cross-sectional and longitudinal relations between dietary intake of isoflavones and BMD of the lumbar spine (LS) and femoral neck (FN) in Black, White, Chinese and Japanese women during the menopause transition (MT). Methods We tested whether tertiles of isoflavone intake were associated with baseline BMD, when all women were pre- or early perimenopausal. To analyze whether isoflavone intake was associated with longitudinal BMD, we fit piece-wise linear models to repeated measurements of baseline-normalized LS or FN BMD, as functions of time before or after the final menstrual period (FMP) date. Results Multiply-adjusted mean FN BMD values of premenopausal, Japanese women were monotonically, positively related to isoflavone consumption (p for trend = 0.0003). Otherwise, no statistically significant baseline associations were observed. During the time period of 1 year prior to through 5 years after the FMP, all participants lost LS and FN BMD. Loss was unrelated to isoflavone intake, with the exception of Japanese women during 1 year prior to 2 years after the FMP: higher tertiles of isoflavone intake were associated with greater annual LS BMD loss rates (p for trend = 0.01) and FN loss rates (p for trend = 0.04). Conclusions In Japanese women, higher isoflavone intake was associated with higher peak FN BMD but also with greater rates of LS and FN BMD loss during the MT. Results in the other racial/ethnic groups did not support a relation between dietary intake of isoflavones and either peak BMD or BMD loss during the MT. PMID:25116050
NASA Technical Reports Server (NTRS)
Cummings, J.
1976-01-01
Model information and data obtained from wind tunnel tests performed on a 0.006 scale model of the Rockwell International space shuttle orbiter and external tank in the 18 inch Variable Density Hypersonic Wind Tunnel (VDHT) at NASA Langley Research Center are presented. Tests were performed at a Mach number of 8.0 over a Reynolds Number range from 0.1 to 10.0 million per foot at 0 deg and -5 deg angle of attack and 0 deg sideslip angle. Transition heating rates were determined using thin skin thermocouples located at various locations on the orbiter and ET. The test was conducted in three stages: orbiter plus external tank (mated configuration); orbiter alone, and external tank alone. The effects of boundary layer trips were also included in the test sequence. The plotted results presented show the effect of configuration interference on the orbiter lower surface and on the ET. Tabulated data are given.
Ramanantoanina, Harry; Kuri, Goutam; Daul, Claude; Bertsch, Johannes
2016-07-28
Ligand field density functional theory (LFDFT) calculations have been used to model the uranium M4,5, N4,5 and O4,5-edge X-ray absorption near edge structure (XANES) in UO2, characterized by the promotion of one electron from the core and the semi-core 3d, 4d and 5d orbitals of U(4+) to the valence 5f. The model describes the procedure to resolve non-empirically the multiplet energy levels originating from the two-open-shell system with d and f electrons and to calculate the oscillator strengths corresponding to the dipole allowed d(10)f(2)→ d(9)f(3) transitions appropriate to represent the d electron excitation process. In the first step, the energy and UO2 unit-cell volume corresponding to the minimum structures are determined using the Hubbard model (DFT+U) approach. The model of the optical properties due to the uranium nd(10)5f(2)→nd(9)5f(3) transitions, with n = 3, 4 and 5, has been tackled by means of electronic structure calculations based on the ligand field concept emulating the Slater-Condon integrals, the spin-orbit coupling constants and the parameters of the ligand field potential needed by the ligand field Hamiltonian from Density Functional Theory. A deep-rooted theoretical procedure using the LFDFT approach has been established for actinide-bearing systems that can be valuable to compute targeted results, such as spectroscopic details at the electronic scale. As a case study, uranium dioxide has been considered because it is a nuclear fuel material, and both atomic and electronic structure calculations are indispensable for a deeper understanding of irradiation driven microstructural changes occurring in this material. PMID:27356168
Roper, Ian P E; Besley, Nicholas A
2016-03-21
The simulation of X-ray emission spectra of transition metal complexes with time-dependent density functional theory (TDDFT) is investigated. X-ray emission spectra can be computed within TDDFT in conjunction with the Tamm-Dancoff approximation by using a reference determinant with a vacancy in the relevant core orbital, and these calculations can be performed using the frozen orbital approximation or with the relaxation of the orbitals of the intermediate core-ionised state included. Both standard exchange-correlation functionals and functionals specifically designed for X-ray emission spectroscopy are studied, and it is shown that the computed spectral band profiles are sensitive to the exchange-correlation functional used. The computed intensities of the spectral bands can be rationalised by considering the metal p orbital character of the valence molecular orbitals. To compute X-ray emission spectra with the correct energy scale allowing a direct comparison with experiment requires the relaxation of the core-ionised state to be included and the use of specifically designed functionals with increased amounts of Hartree-Fock exchange in conjunction with high quality basis sets. A range-corrected functional with increased Hartree-Fock exchange in the short range provides transition energies close to experiment and spectral band profiles that have a similar accuracy to those from standard functionals. PMID:27004859
NASA Astrophysics Data System (ADS)
Almenara, J. M.; Bouchy, F.; Gaulme, P.; Deleuil, M.; Havel, M.; Gandolfi, D.; Deeg, H. J.; Wuchterl, G.; Guillot, T.; Gardes, B.; Pasternacki, T.; Aigrain, S.; Alonso, R.; Auvergne, M.; Baglin, A.; Bonomo, A. S.; Bordé, P.; Cabrera, J.; Carpano, S.; Cochran, W. D.; Csizmadia, Sz.; Damiani, C.; Diaz, R. F.; Dvorak, R.; Endl, M.; Erikson, A.; Ferraz-Mello, S.; Fridlund, M.; Hébrard, G.; Gillon, M.; Guenther, E.; Hatzes, A.; Léger, A.; Lammer, H.; MacQueen, P. J.; Mazeh, T.; Moutou, C.; Ollivier, M.; Ofir, A.; Pätzold, M.; Parviainen, H.; Queloz, D.; Rauer, H.; Rouan, D.; Santerne, A.; Samuel, B.; Schneider, J.; Tal-Or, L.; Tingley, B.; Weingrill, J.
2013-07-01
We report the discovery of two transiting exoplanets, CoRoT-25b and CoRoT-26b, both of low density, one of which is in the Saturn mass-regime. For each star, ground-based complementary observations through optical photometry and radial velocity measurements secured the planetary nature of the transiting body and allowed us to fully characterize them. For CoRoT-25b we found a planetary mass of 0.27 ± 0.04 MJup, a radius of 1.08-0.10+0.3 RJup and hence a mean density of 0.15-0.06+0.15 g cm-3. The planet orbits an F9 main-sequence star in a 4.86-day period, that has a V magnitude of 15.0, solar metallicity, and an age of 4.5-2.0+1.8-Gyr. CoRoT-26b orbits a slightly evolved G5 star of 9.06 ± 1.5-Gyr age in a 4.20-day period that hassolar metallicity and a V magnitude of 15.8. With a mass of 0.52 ± 0.05 MJup, a radius of 1.26-0.07+0.13 RJup, and a mean density of 0.28-0.07+0.09 g cm-3, it belongs to the low-mass hot-Jupiter population. Planetary evolution models allowed us to estimate a core mass of a few tens of Earth mass for the two planets with heavy-element mass fractions of 0.52-0.15+0.08 and 0.26-0.08+0.05, respectively, assuming that a small fraction of the incoming flux is dissipated at the center of the planet. In addition, these models indicate that CoRoT-26b is anomalously large compared with what standard models could account for, indicating that dissipation from stellar heating could cause this size. The CoRoT space mission, launched on December 27th 2006, has been developed and is operated by CNES, with the contribution of Austria, Belgium, Brazil, ESA (RSSD and Science Programme), Germany and Spain. Partly based on observations obtained at the European Southern Observatory at Paranal and La Silla, Chile in programs 083.C-0690(A), 184.C-0639.
NASA Astrophysics Data System (ADS)
Mikhailov, Ivan A.; Tafur, Sergio; Masunov, Artëm E.
2008-01-01
The effect of static and dynamic electron correlation on the nature of excited states and state-to-state transition dipole moments is studied with a multideterminant wave function approach on the example of all-trans linear polyenes ( C4H6 , C6H8 , and C8H10 ). Symmetry-forbidden singlet nAg states were found to separate into three groups: purely single, mostly single, and mostly double excitations. The excited-state absorption spectrum is dominated by two bright transitions: 1Bu-2Ag and 1Bu-mAg , where mAg is the state, corresponding to two-electron excitation from the highest occupied to lowest unoccupied molecular orbital. The richness of the excited-state absorption spectra and strong mixing of the doubly excited determinants into lower- nAg states, reported previously at the complete active space self-consistent field level of theory, were found to be an artifact of the smaller active space, limited to π orbitals. When dynamic σ-π correlation is taken into account, single- and double-excited states become relatively well separated at least at the equilibrium geometry of the ground state. This electronic structure is closely reproduced within time-dependent density-functional theory (TD DFT), where double excitations appear in a second-order coupled electronic oscillator formalism and do not mix with the single excitations obtained within the linear response. An extension of TD DFT is proposed, where the Tamm-Dancoff approximation (TDA) is invoked after the linear response equations are solved (a posteriori TDA). The numerical performance of this extension is validated against multideterminant-wave-function and quadratic-response TD DFT results. It is recommended for use with a sum-over-states approach to predict the nonlinear optical properties of conjugated molecules.
ERIC Educational Resources Information Center
Statfeld, Jenna L.
2011-01-01
Post-school transition is the movement of a child with disabilities from school to activities that occur after the completion of school. This paper provides information about: (1) post-school transition; (2) transition plan; (3) transition services; (4) transition planning; (5) vocational rehabilitation services; (6) services that are available…
ERIC Educational Resources Information Center
Keiter, Richard L.; Puzey, Whitney L.; Blitz, Erin A.
2006-01-01
Metal rods of high purity for many elements are now commercially available and may be used to construct a display of relative densities. We have constructed a display with nine metal rods (Mg, Al, Ti, V, Fe, Cu, Ag, Pb, and W) of equal mass whose densities vary from 1.74 to 19.3 g cm[superscript -3]. The relative densities of the metals may be…
NASA Astrophysics Data System (ADS)
Cameron, Andrew Collier
An extrasolar planet will transit the visible hemisphere of its host star if its orbital plane lies sufficiently close to the observer's line of sight. The resulting periodic dips in stellar flux reveal key system parameters, including the density of the host star and, if radial-velocity observations are available, the surface gravitational acceleration of the planet. In this chapter I present the essential methodology for modelling the time-dependent flux variation during a transit, and its use in determining the posterior probability distribution for the physical parameters of the system. Large-scale searches for transiting systems are an efficient way of discovering planets whose bulk densities, and hence compositions, can be accessed if their masses can also be determined. I present algorithms for detrending large ensembles of light curves, for searching for transit-like signals among them. I also discuss methods for identifying diluted stellar eclipsing binaries mimicking planetary transit signals, and validation of transit candidates too faint for radial-velocity follow-up. I review the use of time-resolved spectrophotometry and high-resolution spectroscopy during transits to identify the molecular constituents of exoplanetary atmospheres.
... bone health. It compares your bone density, or mass, to that of a healthy person who is ... Whether your osteoporosis treatment is working Low bone mass that is not low enough to be osteoporosis ...
... Density Exam/Testing › Low Bone Density Low Bone Density Low bone density is when your bone density ... people with normal bone density. Detecting Low Bone Density A bone density test will determine whether you ...
Young, Bruce Kai Fong
1988-09-01
The determination of level populations and detailed population mechanisms in dense plasmas has become an increasingly important problem in atomic physics. In this work, the density variation of line intensities and level populations in aluminum K-shell and molybdenum and silver L-shell emission spectra have been measured from high-powered, laser-produced plasmas. For each case, the density dependence of the observed line emission is due to the effect of high frequency electron-ion collisions on metastable levels. The density dependent line intensities vary greatly in laser-produced plasmas and can be used to extract detailed information concerning the population kinetics and level populations of the ions. The laser-plasmas had to be fully characterized in order to clearly compare the observed density dependence with atomic theory predictions. This has been achieved through the combined use of new diagnostic instruments and microdot targets which provided simultaneously space, time, and spectrally resolved data. The plasma temperatures were determined from the slope of the hydrogen-like recombination continuum. The time resolved electron density profiles were measured using multiple frame holographic interferometry. Thus, the density dependence of K-shell spectral lines could be clearly examined, independent of assumptions concerning the dynamics of the plasma. In aluminum, the electron density dependence of various helium-like line intensity ratios were measured. Standard collisional radiative equilibrium models fail to account for the observed density dependence measured for the ''He/sub ..cap alpha..//IC'' ratio. Instead, a quasi-steady state atomic model based on a purely recombining plasma is shown to accurately predict the measured density dependence. This same recombining plasma calculation successfully models the density dependence of the high-n ''He/sub ..gamma..//He/sub ..beta../'' and ''He/sub delta//He/sub ..beta../'' helium-like resonance line intensity
ERIC Educational Resources Information Center
Baird, Matthew David
2012-01-01
I study three separate questions in this dissertation. In Chapter 1, I develop and estimate a structural dynamic model of occupation and job choice to test hypotheses of the importance of wages and non-wages and learning in occupational transitions, and find that wages are approximately 3 times as important as non-wage benefits in decisions and…
Transition and laminar instability
NASA Technical Reports Server (NTRS)
Mack, L. M.
1977-01-01
The linear stability theory was applied to the problem of boundary layer transition in incompressible flow. The theory was put into a form suitable for three-dimensional boundary layers; both the temporal and spatial theories were examined; and a generalized Gaster relation for three-dimensional boundary layers was derived. Numerical examples include the stability characteristics of Falkner-Skan boundary layers, the accuracy of the two-dimensional Gaster relation for these boundary layers, and the magnitude and direction of the group velocity for oblique waves in the Blasius boundary layer. Available experiments which bear on the validity of stability theory and its relation to transition are reviewed and the stability theory is applied to transition prediction. The amplitude method is described in which the wide band disturbance amplitude in the boundary layer is estimated from stability theory and an interaction relation for the initial amplitude density of the most unstable frequency.
Tong, Glenna So Ming; Law, Yuen-Chi; Kui, Steven C F; Zhu, Nianyong; Leung, King Hong; Phillips, David Lee; Che, Chi-Ming
2010-06-11
The complexes [Pt(tBu(3)tpy){C[triple bond]C(C(6)H(4)C[triple bond]C)(n-1)R}](+) (n = 1: R = alkyl and aryl (Ar); n = 1-3: R = phenyl (Ph) or Ph-N(CH(3))(2)-4; n = 1 and 2, R = Ph-NH(2)-4; tBu(3)tpy = 4,4',4''-tri-tert-butyl-2,2':6',2''-terpyridine) and [Pt(Cl(3)tpy)(C[triple bond]CR)](+) (R = tert-butyl (tBu), Ph, 9,9'-dibutylfluorene, 9,9'-dibutyl-7-dimethyl-amine-fluorene; Cl(3)tpy = 4,4',4''-trichloro-2,2':6',2''-terpyridine) were prepared. The effects of substituent(s) on the terpyridine (tpy) and acetylide ligands and chain length of arylacetylide ligands on the absorption and emission spectra were examined. Resonance Raman (RR) spectra of [Pt(tBu(3)tpy)(C[triple bond]CR)](+) (R = n-butyl, Ph, and C(6)H(4)-OCH(3)-4) obtained in acetonitrile at 298 K reveal that the structural distortion of the C[triple bond]C bond in the electronic excited state obtained by 502.9 nm excitation is substantially larger than that obtained by 416 nm excitation. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations on [Pt(H(3)tpy)(C[triple bond]CR)](+) (R = n-propyl (nPr), 2-pyridyl (Py)), [Pt(H(3)tpy){C[triple bond]C(C(6)H(4)C[triple bond]C)(n-1)Ph}](+) (n = 1-3), and [Pt(H(3)tpy){C[triple bond]C(C(6)H(4)C[triple bond]C)(n-1)C(6)H(4)-N(CH(3))(2)-4}](+)/+H(+) (n = 1-3; H(3)tpy = nonsubstituted terpyridine) at two different conformations were performed, namely, with the phenyl rings of the arylacetylide ligands coplanar ("cop") with and perpendicular ("per") to the H(3)tpy ligand. Combining the experimental data and calculated results, the two lowest energy absorption peak maxima, lambda(1) and lambda(2), of [Pt(Y(3)tpy)(C[triple bond]CR)](+) (Y = tBu or Cl, R = aryl) are attributed to (1)[pi(C[triple bond]CR)-->pi*(Y(3)tpy)] in the "cop" conformation and mixed (1)[d(pi)(Pt)-->pi*(Y(3)tpy)]/(1)[pi(C[triple bond]CR)-->pi*(Y(3)tpy)] transitions in the "per" conformation. The lowest energy absorption peak lambda(1) for [Pt(tBu(3)tpy){C[triple bond]C(C(6)H(4)C
Density limits investigation and high density operation in EAST tokamak
NASA Astrophysics Data System (ADS)
Zheng, Xingwei; Li, Jiangang; Hu, Jiansheng; Liu, Haiqing; Jie, Yinxian; Wang, Shouxin; Li, Jiahong; Duan, Yanming; Li, Miaohui; Li, Yongchun; Zhang, Ling; Ye, Yang; Yang, Qingquan; Zhang, Tao; Cheng, Yingjie; Xu, Jichan; Wang, Liang; Xu, Liqing; Zhao, Hailin; Wang, Fudi; Lin, Shiyao; Wu, Bin; Lyu, Bo; Xu, Guosheng; Gao, Xiang; Shi, Tonghui; He, Kaiyang; Lan, Heng; Chu, Nan; Cao, Bin; Sun, Zhen; Zuo, Guizhong; Ren, Jun; Zhuang, Huidong; Li, Changzheng; Yuan, Xiaolin; Yu, Yaowei; Wang, Houyin; Chen, Yue; Wu, Jinhua; EAST Team
2016-05-01
Increasing the density in a tokamak is limited by the so-called density limit, which is generally performed as an appearance of disruption causing loss of plasma confinement, or a degradation of high confinement mode which could further lead to a H → L transition. The L-mode and H-mode density limit has been investigated in EAST tokamak. Experimental results suggest that density limits could be triggered by either edge cooling or excessive central radiation. The L-mode density limit disruption is generally triggered by edge cooling, which leads to the current profile shrinkage and then destabilizes a 2/1 tearing mode, ultimately resulting in a disruption. The L-mode density limit scaling agrees well with the Greenwald limit in EAST. The observed H-mode density limit in EAST is an operational-space limit with a value of 0.8∼ 0.9{{n}\\text{GW}} . High density H-mode heated by neutral beam injection (NBI) and lower hybrid current drive (LHCD) are analyzed, respectively. The constancy of the edge density gradients in H-mode indicates a critical limit caused perhaps by e.g. ballooning induced transport. The maximum density is accessed at the H → L transition which is generally caused by the excessive core radiation due to high Z impurities (Fe, Cu). Operating at a high density (>2.8× {{10}19} {{\\text{m}}-3} ) is favorable for suppressing the beam shine through NBI. High density H-mode up to 5.3× {{10}19}{{\\text{m}}-3}~≤ft(∼ 0.8{{n}\\text{GW}}\\right) could be sustained by 2 MW 4.6 GHz LHCD alone, and its current drive efficiency is studied. Statistics show that good control of impurities and recycling facilitate high density operation. With careful control of these factors, high density up to 0.93{{n}\\text{GW}} stable H-mode operation was carried out heated by 1.7 MW LHCD and 1.9 MW ion cyclotron resonance heating with supersonic molecular beam injection fueling.
Density distribution in Earth.
Press, F
1968-06-14
Earth models selected by a Monte Carlo procedure were tested against geophysical data; 5 million models were examined and six have passed all tests. Common features of successful models are an increased core radius and a chemically inhomogeneous core consistent with Fe-Ni alloy (20 to 50 percent Fe) for the solid portion and Fe-Si alloy (15 to 25 percent Fe) for the fluid core. The inhomogeneous mantle is consistent with an increase in the FeO:FeO + MgO ratio by a factor of 2 in the deep mantle. The transition zone is a region of not only phase change but also composition change; this condition would inhibit mantlewide convection. The upper-mantle solutions show large fluctuations in density; this state implies insufficient constraint on solutions for this region, or lateral variations in mantle composition ranging from pyrolite to eclogite. PMID:17818740
Phase transitions in nuclear matter
Glendenning, N.K.
1984-11-01
The rather general circumstances under which a phase transition in hadronic matter at finite temperature to an abnormal phase in which baryon effective masses become small and in which copious baryon-antibaryon pairs appear is emphasized. A preview is also given of a soliton model of dense matter, in which at a density of about seven times nuclear density, matter ceases to be a color insulator and becomes increasingly color conducting. 22 references.
NASA Technical Reports Server (NTRS)
1992-01-01
This report describes NASA's metric transition in terms of seven major program elements. Six are technical areas involving research, technology development, and operations; they are managed by specific Program Offices at NASA Headquarters. The final program element, Institutional Management, covers both NASA-wide functional management under control of NASA Headquarters and metric capability development at the individual NASA Field Installations. This area addresses issues common to all NASA program elements, including: Federal, state, and local coordination; standards; private industry initiatives; public-awareness initiatives; and employee training. The concluding section identifies current barriers and impediments to metric transition; NASA has no specific recommendations for consideration by the Congress.
Properties of atoms in molecules: Transition probabilities
NASA Astrophysics Data System (ADS)
Bader, R. F. W.; Bayles, D.; Heard, G. L.
2000-06-01
The transition probability for electric dipole transitions is a measurable property of a system and is therefore, partitionable into atomic contributions using the physics of a proper open system. The derivation of the dressed property density, whose averaging over an atomic basin yields the atomic contribution to a given oscillator strength, is achieved through the development of perturbation theory for an open system. A dressed density describes the local contribution resulting from the interaction of a single electron at some position r, as determined by the relevant observable, averaged over the motions of all of the remaining particles in the system. In the present work, the transition probability density expressed in terms of the relevant transition density, yields a local measure of the associated oscillator strength resulting from the interaction of the entire molecule with a radiation field. The definition of the atomic contributions to the oscillator strength enables one to determine the extent to which a given electronic or vibrational transition is spatially localized to a given atom or functional group. The concepts introduced in this article are applied to the Rydberg-type transitions observed in the electronic excitation of a nonbonding electron in formaldehyde and ammonia. The atomic partitioning of the molecular density distribution and of the molecular properties by surfaces of zero flux in the gradient vector field of the electron density, the boundary condition defining the physics of a proper open system, is found to apply to the density distributions of the excited, Rydberg states.
Dynamics of a Quantum Phase Transition
Zurek, Wojciech H.; Dorner, Uwe; Zoller, Peter
2005-09-02
We present two approaches to the dynamics of a quench-induced phase transition in the quantum Ising model. One follows the standard treatment of thermodynamic second order phase transitions but applies it to the quantum phase transitions. The other approach is quantum, and uses Landau-Zener formula for transition probabilities in avoided level crossings. We show that predictions of the two approaches of how the density of defects scales with the quench rate are compatible, and discuss the ensuing insights into the dynamics of quantum phase transitions.
NASA Astrophysics Data System (ADS)
Kettemann, S.; Mucciolo, E. R.; Varga, I.; Slevin, K.
2012-03-01
Dilute magnetic impurities in a disordered Fermi liquid are considered close to the Anderson metal-insulator transition (AMIT). Critical power-law correlations between electron wave functions at different energies in the vicinity of the AMIT result in the formation of pseudogaps of the local density of states. Magnetic impurities can remain unscreened at such sites. We determine the density of the resulting free magnetic moments in the zero-temperature limit. While it is finite on the insulating side of the AMIT, it vanishes at the AMIT, and decays with a power law as function of the distance to the AMIT. Since the fluctuating spins of these free magnetic moments break the time-reversal symmetry of the conduction electrons, we find a shift of the AMIT, and the appearance of a semimetal phase. The distribution function of the Kondo temperature TK is derived at the AMIT, in the metallic phase, and in the insulator phase. This allows us to find the quantum phase diagram in an external magnetic field B and at finite temperature T. We calculate the resulting magnetic susceptibility, the specific heat, and the spin relaxation rate as a function of temperature. We find a phase diagram with finite-temperature transitions among insulator, critical semimetal, and metal phases. These new types of phase transitions are caused by the interplay between Kondo screening and Anderson localization, with the latter being shifted by the appearance of the temperature-dependent spin-flip scattering rate. Accordingly, we name them Kondo-Anderson transitions.
Isoconversion Analysis of the Glass Transition
NASA Astrophysics Data System (ADS)
Badrinarayanan, Prashanth; Zheng, Wei; Simon, Sindee
2007-03-01
At temperatures below their glass transition temperatures (Tgs), glass forming materials deviate from equilibrium density and form a glass. The kinetic nature of the glass transition process is manifested in the cooling rate dependence of the glass transition temperature and by structural relaxation below Tg. Various facets of the glass transition kinetics have been well described by phenomenological models of the glass transition, such as the TNM and KAHR model. An important yet frequently questioned assumption in these models is that the apparent activation energy, which describes the temperature dependence of the relaxation time, does not vary during the glass transition process. Some recent reports suggest that the activation energy varies significantly during the glass transition process. In this work we apply an isoconversion analysis to data in the glass transition region which was obtained on cooling from capillary dilatometry and differential scanning calorimetry (DSC) in order to determine whether the apparent activation energy increases as the glassy state is approached.
Mattei, Jean-François; Troit, Virginie
2016-02-01
In two centuries, modern humanitarian action has experienced several fractures often linked to crises. Although its professionalism and intervention force remain indisputable, it faces, since the 2000s, a new context that limits its ability to act and confronts it with new dilemmas, even though it must deal with needs for aid of unprecedented scale. These difficulties reveal a humanitarian transition period that was not anticipated. This transition period reflects the change from a dominant paradigm of North-South solidarity of Western origin to a much more complex model. This article provides a summary of the current mutations that are dominated by the States' assertion of sovereignty. Among the possible solutions, it argues for an ethical approach and a better integration of the research carried out in the Global South, prerequisites for building a true partnership and placing the victims at the heart of the operations which involve them. PMID:26936180
ERIC Educational Resources Information Center
Gallick, Barb; Lee, Lisa
2010-01-01
Adults often find themselves transitioning from one activity to another in a short time span. Most of the time, they do not feel they have a lot of control over their schedules, but wish that they could carve out extended time to relax and focus on one project. Picture a group of children in the block area who have spent 15 or 20 minutes building…
Analysis of Nuclear Quantum Phase Transitions
Li, Z. P.; Meng, J.; Niksic, T.; Vretenar, D.; Lalazissis, G. A.; Ring, P.
2009-08-26
A microscopic analysis, based on nuclear energy density functionals, is presented for shape phase transitions in Nd isotopes. Low-lying excitation spectra and transition probabilities are calculated starting from a five-dimensional Hamiltonian, with parameters determined by constrained relativistic mean-field calculations for triaxial shapes. The results reproduce available data, and show that there is an abrupt change of structure at N = 90, that corresponds to a first-order quantum phase transition between spherical and axially deformed shapes.
Lifshitz transition in d-wave superconductors
Botelho, S.S.; Sa de Melo, C.A.R.
2005-04-01
The BCS-to-BEC evolution has been recently the focus of studies in superconductors and cold atomic gases. For a d-wave system, we show that a Lifshitz transition occurs at a critical particle density which separates two topologically distinct phases according to their quasiparticle excitation energies: a BCS-like gapless superconductor in the higher-density limit and a BEC-like fully gapped superconductor in the lower-density limit. This transition is second-order according to Ehrenfest's classification, but it occurs without a change in the symmetry of the order parameter and thus cannot be classified under Landau's scheme. To illustrate the nature of the transition, we compute the compressibility and the superfluid density as functions of particle density.
Bernstein, P. Harnois, C.; Mc Loughlin, C.; Noudem, J.; Thimont, Y.; Ferro, G.; Osorio, M. R.; Veira, J. A.; Vidal, D.; Vidal, F.
2014-02-07
The influence of surface defects, in particular of a-axis grains, on the transition to the normal state induced by high current densities in YBa{sub 2}Cu{sub 3}O{sub 7−δ} (YBCO) thin films and in a commercial 2G-coated conductor is investigated. For that purpose, the surface of the samples is observed by scanning electron microscopy and isothermal current-voltage curves are measured at different temperatures with pulsed currents up to the quenching value I*. The results show that the ratio of I* to the critical current is large if a-axis grains are not visible at the surface of the YBCO films, while it is much lower if the surface includes a-axis grains as this is the case for the coated conductor. The connection between the transition onset and the vortex dynamics, as well as the role of the a-axis grains in this process are discussed. The relation between the I* values obtained from thermal calculations and those resulting from vortex dynamics considerations is also discussed, as well as the possible consequences suggested by this work for the different applications of the coated conductors.
De Angelis, Filippo; Fantacci, Simona; Sgamelotti, Antonio; Cariati, Franco; Roberto, Dominique; Tessore, Francesca; Ugo, Renato
2006-02-14
We report a theoretical study based on density functional theory (DFT) and time-dependent DFT (TDDFT) calculations on the nature and role of the absorption bands involved in the nonlinear optical response of the complexes [Ru(CF3CO2)3T] (T = T1, T2; T1 = 4'-(C6H4-p-NBu2)-2,2':6',2''-terpyridine, T2 = 4'-(C6H4-p-NMe2)-2,2':6',2''-terpyridine). Geometry optimizations, performed without any symmetry constraints, confirm a twisting of the -C6H4-p-NBu2 moiety with respect to the plane of the chelated terpyridine. Despite this lack of strong pi interaction, TDDFT excited states calculations of the electronic spectrum in solution provide evidence of a relevant role of the NBu2 donor group in the low-energy LMCT band at 911 nm. Calculations also show that the two bands at higher energy (508 and 455 nm) are not attributable only to LMCT and ILCT transitions but to a mixing of ILCT/MLCT and ILCT/pi-pi* transitions, respectively. The 911 nm LMCT band, appearing at lower wavelength of the second harmonic (670 nm) of the EFISH experiment, controls the negative value of the second-order NLO response. This is confirmed by our calculations of the static component beta0(zzz) of the quadratic hyperpolarizability tensor, showing a large positive value. In addition we have found that the increase of the dipole moment upon excitation occurs, in all the characterized transitions, along the dipole moment axis, thus explaining why the EFISH and solvatochromic experimental values of the quadratic hyperpolarizability agree as sign and value. PMID:16437181
Verma, M. K.; Yadav, R.; Chandra, M.; Paul, S.; Wahi, P.
2010-11-23
In this article we review the experimental and numerical results related to the dynamo transitions. Recent experiments of Von Karman Sodium (VKS) exhibit various dynamo states including constant, time-periodic, and chaotic magnetic fields. Similarly pseudospectral simulations of dynamo show constant, time-periodic, quasiperiodic, and chaotic magnetic field configurations. One of the windows for the magnetic Prandtl number of unity shows period doubling route to chaos. Quasiperiodic route to chaos has been reported for the Prandtl number of 0.5. The dynamo simulations also reveal coexisting multiple attractors that were obtained for different initial conditions.
NASA Astrophysics Data System (ADS)
Miao, Maosheng
2005-07-01
We show that the previously proposed transition paths for high pressure phase transitions for semiconductor from zinc blende (ZB) to rocksalt (RS) and from wurtzite (WZ) to rocksalt can be unified and can be extended to transitions from various tetrahedrally bonded polytypes to rocksalt [1]. Our first principle pseudopotential calculations with density functional and constrained relaxation methods on SiC showed that the ZB to RS transition has the lowest transition barrier. Our calculations on ZB to RS transition path for other semiconductors including II-VI, III-V and group IV semiconductors, show that the position and the geometry of the transition state, the state that correspond to the transition barrier, are universal and do not depend on the chemical components of the system [2]. We also extended the Landau phase transition model to a ZB to RS transition by using a cosine function of the atom displacement as order parameter. The model shows that the position of the transition state does not depend on the coupling between the atom displacement and the strains of the lattice, which is the key point that the transition state is independent of the chemical components. [1] M. S. Miao and Walter R. L. Lambrecht, Phys. Rev. B 68, 092103 (2003). [2] M. S. Miao and Walter R. L. Lambrecht, Phys. Rev. Lett., accepted
Universal transition state scaling relations for (de)hydrogenation over transition metals.
Wang, S; Petzold, V; Tripkovic, V; Kleis, J; Howalt, J G; Skúlason, E; Fernández, E M; Hvolbæk, B; Jones, G; Toftelund, A; Falsig, H; Björketun, M; Studt, F; Abild-Pedersen, F; Rossmeisl, J; Nørskov, J K; Bligaard, T
2011-12-14
We analyse the transition state energies for 249 hydrogenation/dehydrogenation reactions of atoms and simple molecules over close-packed and stepped surfaces and nanoparticles of transition metals using Density Functional Theory. Linear energy scaling relations are observed for the transition state structures leading to transition state scaling relations for all the investigated reactions. With a suitable choice of reference systems the transition state scaling relations form a universality class that can be approximated with one single linear relation describing the entire range of reactions over all types of surfaces and nanoclusters. PMID:21996683
LATTICE QCD AT FINITE DENSITY.
SCHMIDT, C.
2006-07-23
I discuss different approaches to finite density lattice QCD. In particular, I focus on the structure of the phase diagram and discuss attempts to determine the location of the critical end-point. Recent results on the transition line as function of the chemical potential (T{sub c}({mu}{sub q})) are reviewed. Along the transition line, hadronic fluctuations have been calculated; which can be used to characterize properties of the Quark Gluon plasma and eventually can also help to identify the location of the critical end-point in the QCD phase diagram on the lattice and in heavy ion experiments. Furthermore, I comment on the structure of the phase diagram at large {mu}{sub q}.
Negative refraction using Raman transitions and chirality
Sikes, D. E.; Yavuz, D. D.
2011-11-15
We present a scheme that achieves negative refraction with low absorption in far-off resonant atomic systems. The scheme utilizes Raman resonances and does not require the simultaneous presence of an electric-dipole transition and a magnetic-dipole transition near the same wavelength. We show that two interfering Raman tran-sitions coupled to a magnetic-dipole transition can achieve a negative index of refraction with low absorption through magnetoelectric cross-coupling. We confirm the validity of the analytical results with exact numerical simulations of the density matrix. We also discuss possible experimental implementations of the scheme in rare-earth metal atomic systems.
Negative refraction using Raman transitions and chirality
NASA Astrophysics Data System (ADS)
Sikes, D. E.; Yavuz, D. D.
2011-11-01
We present a scheme that achieves negative refraction with low absorption in far-off resonant atomic systems. The scheme utilizes Raman resonances and does not require the simultaneous presence of an electric-dipole transition and a magnetic-dipole transition near the same wavelength. We show that two interfering Raman tran-sitions coupled to a magnetic-dipole transition can achieve a negative index of refraction with low absorption through magnetoelectric cross-coupling. We confirm the validity of the analytical results with exact numerical simulations of the density matrix. We also discuss possible experimental implementations of the scheme in rare-earth metal atomic systems.
Electronic doping of transition metal oxide perovskites
NASA Astrophysics Data System (ADS)
Cammarata, Antonio; Rondinelli, James M.
2016-05-01
CaFeO3 is a prototypical negative charge transfer oxide that undergoes electronic metal-insulator transition concomitant with a dilation and contraction of nearly rigid octahedra. Altering the charge neutrality of the bulk system destroys the electronic transition, while the structure is significantly modified at high charge content. Using density functional theory simulations, we predict an alternative avenue to modulate the structure and the electronic transition in CaFeO3. Charge distribution can be modulated using strain-rotation coupling and thin film engineering strategies, proposing themselves as a promising avenue for fine tuning electronic features in transition metal-oxide perovskites.
Kepler-79's low density planets
Jontof-Hutter, Daniel; Lissauer, Jack J.; Rowe, Jason F.; Fabrycky, Daniel C.
2014-04-10
Kepler-79 (KOI-152) has four planetary candidates ranging in size from 3.5 to 7 times the size of the Earth, in a compact configuration with orbital periods near a 1:2:4:6 chain of commensurability, from 13.5 to 81.1 days. All four planets exhibit transit timing variations with periods that are consistent with the distance of each planet to resonance with its neighbors. We perform a dynamical analysis of the system based on transit timing measurements over 1282 days of Kepler photometry. Stellar parameters are obtained using a combination of spectral classification and the stellar density constraints provided by light curve analysis and orbital eccentricity solutions from our dynamical study. Our models provide tight bounds on the masses of all four transiting bodies, demonstrating that they are planets and that they orbit the same star. All four of Kepler-79's transiting planets have low densities given their sizes, which is consistent with other studies of compact multiplanet transiting systems. The largest of the four, Kepler-79 d (KOI-152.01), has the lowest bulk density yet determined among sub-Saturn mass planets.
More holographic Berezinskii-Kosterlitz-Thouless transitions
Jensen, Kristan
2010-08-15
We find two systems via holography that exhibit quantum Berezinskii-Kosterlitz-Thouless (BKT) phase transitions. The first is the ABJM theory with flavor and the second is a flavored (1, 1) little string theory. In each case the transition occurs at nonzero density and magnetic field. The BKT transition in the little string theory is the first example of a quantum BKT transition in (3+1) dimensions. As in the 'original' holographic BKT transition in the D3/D5 system, the exponential scaling is destroyed at any nonzero temperature and the transition becomes second order. Along the way we construct holographic renormalization for probe branes in the ABJM theory and propose a scheme for the little string theory. Finally, we obtain the embeddings and (half of) the meson spectrum in the ABJM theory with massive flavor.
Trapped resonant fermions above the superfluid transition temperature
Cheng, C.-H.; Yip, S.-K.
2007-01-01
We investigate trapped resonant fermions with unequal populations within the local density approximation above the superfluid transition temperature. By tuning the attractive interaction between fermions via Feshbach resonance, the system evolves from weakly interacting fermi gas to strongly interacting fermi gas, and finally becomes a Bose-Fermi mixture. The density profiles of fermions are examined and compared with experiments. We also point out the simple relationships between the local density, the axial density, and the gas pressure within the local density approximation.
Interplay between micelle formation and waterlike phase transitions
NASA Astrophysics Data System (ADS)
Heinzelmann, G.; Figueiredo, W.; Girardi, M.
2010-02-01
A lattice model for amphiphilic aggregation in the presence of a structured waterlike solvent is studied through Monte Carlo simulations. We investigate the interplay between the micelle formation and the solvent phase transition in two different regions of temperature-density phase diagram of pure water. A second order phase transition between the gaseous (G) and high density liquid (HDL) phases that occurs at very high temperatures, and a first order phase transition between the low density liquid (LDL) and (HDL) phases that takes place at lower temperatures. In both cases, we find the aggregate size distribution curve and the critical micellar concentration as a function of the solvent density across the transitions. We show that micelle formation drives the LDL-HDL first order phase transition to lower solvent densities, while the transition G-HDL is driven to higher densities, which can be explained by the markedly different degrees of micellization in both cases. The diffusion coefficient of surfactants was also calculated in the LDL and HDL phases, changing abruptly its behavior due to the restructuring of waterlike solvent when we cross the first order LDL-HDL phase transition. To understand such behavior, we calculate the solvent density and the number of hydrogen bonds per water molecule close to micelles. The curves of the interfacial solvent density and the number of hydrogen bonds per water molecule in the first hydration signal a local phase change of the interfacial water, clarifying the diffusion mechanism of free surfactants in the solvent.
Density functional theory for carbon dioxide crystal
Chang, Yiwen; Mi, Jianguo Zhong, Chongli
2014-05-28
We present a density functional approach to describe the solid−liquid phase transition, interfacial and crystal structure, and properties of polyatomic CO{sub 2}. Unlike previous phase field crystal model or density functional theory, which are derived from the second order direct correlation function, the present density functional approach is based on the fundamental measure theory for hard-sphere repulsion in solid. More importantly, the contributions of enthalpic interactions due to the dispersive attractions and of entropic interactions arising from the molecular architecture are integrated in the density functional model. Using the theoretical model, the predicted liquid and solid densities of CO{sub 2} at equilibrium triple point are in good agreement with the experimental values. Based on the structure of crystal-liquid interfaces in different planes, the corresponding interfacial tensions are predicted. Their respective accuracies need to be tested.
Density induced transition in a school of fish
NASA Astrophysics Data System (ADS)
Cambuí, Dorílson Silva; Rosas, Alexandre
2012-08-01
Collective behaviour has been studied in various fields of science. As an example, we may consider the patterns observed in living systems, whose aggregates form organized groups such as flocks of birds, herds of mammals and schools of fish. These aggregates may be formed as a consequence of an external stimulus or due to the local interaction among nearby specimens. As an example of the latter case one may think about a school of fish, where each individual bases its behaviour on its perception of the position and velocity of its nearest neighbours. As a result of these interactions, global collective behaviour may emerge, originating and maintaining the cohesion of the aggregate. In this work, we model the collective movement of a school of fish using an agent-based model which follows biologically motivated behavioural rules previously proposed. The distributions of nearest neighbour distance and relative orientations between neighbouring fishes are measured and the results are found to be in good agreement with previous experimental measurements.
Palenik, Mark C.; Dunlap, Brett I.
2015-07-28
Despite the fundamental importance of electron density in density functional theory, perturbations are still usually dealt with using Hartree-Fock-like orbital equations known as coupled-perturbed Kohn-Sham (CPKS). As an alternative, we develop a perturbation theory that solves for the perturbed density directly, removing the need for CPKS. This replaces CPKS with a true Hohenberg-Kohn density perturbation theory. In CPKS, the perturbed density is found in the basis of products of occupied and virtual orbitals, which becomes ever more over-complete as the size of the orbital basis set increases. In our method, the perturbation to the density is expanded in terms of a series of density basis functions and found directly. It is possible to solve for the density in such a way that it makes the total energy stationary even if the density basis is incomplete.
Water’s second glass transition
Amann-Winkel, Katrin; Gainaru, Catalin; Handle, Philip H.; Seidl, Markus; Nelson, Helge; Böhmer, Roland
2013-01-01
The glassy states of water are of common interest as the majority of H2O in space is in the glassy state and especially because a proper description of this phenomenon is considered to be the key to our understanding why liquid water shows exceptional properties, different from all other liquids. The occurrence of water’s calorimetric glass transition of low-density amorphous ice at 136 K has been discussed controversially for many years because its calorimetric signature is very feeble. Here, we report that high-density amorphous ice at ambient pressure shows a distinct calorimetric glass transitions at 116 K and present evidence that this second glass transition involves liquid-like translational mobility of water molecules. This “double Tg scenario” is related to the coexistence of two liquid phases. The calorimetric signature of the second glass transition is much less feeble, with a heat capacity increase at Tg,2 about five times as large as at Tg,1. By using broadband-dielectric spectroscopy we resolve loss peaks yielding relaxation times near 100 s at 126 K for low-density amorphous ice and at 110 K for high-density amorphous ice as signatures of these two distinct glass transitions. Temperature-dependent dielectric data and heating-rate–dependent calorimetric data allow us to construct the relaxation map for the two distinct phases of water and to extract fragility indices m = 14 for the low-density and m = 20–25 for the high-density liquid. Thus, low-density liquid is classified as the strongest of all liquids known (“superstrong”), and also high-density liquid is classified as a strong liquid. PMID:24101518
Transitions: A Personal Perspective.
ERIC Educational Resources Information Center
Wood, Ann Stace
1995-01-01
Distinguishes between unchosen transitions (children maturing and leaving, parents aging, companies downsizing) and chosen ones (moving, divorce, marriage, career changes). Describes the steps one goes through: uneasiness, renewed energy, complaining, exploration, partial transition, and the completed transition. (JOW)
Quantum phase transitions with dynamical flavors
NASA Astrophysics Data System (ADS)
Bea, Yago; Jokela, Niko; Ramallo, Alfonso V.
2016-07-01
We study the properties of a D6-brane probe in the Aharony-Bergman-Jafferis-Maldacena (ABJM) background with smeared massless dynamical quarks in the Veneziano limit. Working at zero temperature and nonvanishing charge density, we show that the system undergoes a quantum phase transition in which the topology of the brane embedding changes from a black hole to a Minkowski embedding. In the unflavored background the phase transition is of second order and takes place when the charge density vanishes. We determine the corresponding critical exponents and show that the scaling behavior near the quantum critical point has multiplicative logarithmic corrections. In the background with dynamical quarks the phase transition is of first order and occurs at nonzero charge density. In this case we compute the discontinuity of several physical quantities as functions of the number Nf of unquenched quarks of the background.
Thermomechanical properties of 3d transition metals
Karaoglu, B.; Rahman, S.M.M. . Dept. of Physics)
1994-05-15
The authors have investigated the density variation of the Einstein temperatures and elastic constants of the 3d transition metals. In this respect they have employed the transition metal (TM) pair potentials involving the sp contribution with an appropriate exchange and correlation function, the d-band broadening contribution and the d-band hybridization term. These calculations are aimed at testing the TM pair potentials in generating the quasilocal and local thermomechanical properties.
ERIC Educational Resources Information Center
Shaw, Mike
2003-01-01
Integrates story telling into a science activity on the density of liquids in order to increase student interest. Shows the relationship between mass and volume ratio and how they determine density. Includes teacher notes. (YDS)
Direct Density Derivative Estimation.
Sasaki, Hiroaki; Noh, Yung-Kyun; Niu, Gang; Sugiyama, Masashi
2016-06-01
Estimating the derivatives of probability density functions is an essential step in statistical data analysis. A naive approach to estimate the derivatives is to first perform density estimation and then compute its derivatives. However, this approach can be unreliable because a good density estimator does not necessarily mean a good density derivative estimator. To cope with this problem, in this letter, we propose a novel method that directly estimates density derivatives without going through density estimation. The proposed method provides computationally efficient estimation for the derivatives of any order on multidimensional data with a hyperparameter tuning method and achieves the optimal parametric convergence rate. We further discuss an extension of the proposed method by applying regularized multitask learning and a general framework for density derivative estimation based on Bregman divergences. Applications of the proposed method to nonparametric Kullback-Leibler divergence approximation and bandwidth matrix selection in kernel density estimation are also explored. PMID:27140943
PyTransit: Transit light curve modeling
NASA Astrophysics Data System (ADS)
Parviainen, Hannu
2015-05-01
PyTransit implements optimized versions of the Giménez and Mandel & Agol transit models for exoplanet transit light-curves. The two models are implemented natively in Fortran with OpenMP parallelization, and are accessed by an object-oriented python interface. PyTransit facilitates the analysis of photometric time series of exoplanet transits consisting of hundreds of thousands of data points, and of multipassband transit light curves from spectrophotometric observations. It offers efficient model evaluation for multicolour observations and transmission spectroscopy, built-in supersampling to account for extended exposure times, and routines to calculate the projected planet-to-star distance for circular and eccentric orbits, transit durations, and more.
Information geometric density estimation
NASA Astrophysics Data System (ADS)
Sun, Ke; Marchand-Maillet, Stéphane
2015-01-01
We investigate kernel density estimation where the kernel function varies from point to point. Density estimation in the input space means to find a set of coordinates on a statistical manifold. This novel perspective helps to combine efforts from information geometry and machine learning to spawn a family of density estimators. We present example models with simulations. We discuss the principle and theory of such density estimation.
ERIC Educational Resources Information Center
Design and Environment, 1972
1972-01-01
Three-part report pinpointing problems and uncovering solutions for the dual concepts of density (ratio of people to space) and crowding (psychological response to density). Section one, A Primer on Crowding,'' reviews new psychological and social findings; section two, Density in the Suburbs,'' shows conflict between status quo and increased…
Pressure-induced phase transition in pentacene
NASA Astrophysics Data System (ADS)
Farina, L.; Brillante, A.; Della Valle, R. G.; Venuti, E.; Amboage, M.; Syassen, K.
2003-07-01
We have recently studied two solid phases of bulk pentacene (polymorphs H and C) by means of lattice phonon Raman spectroscopy. The assignment, previously based on lattice dynamics calculations alone, is now verified by X-ray diffraction measurements, conclusively confirming the existence of both polymorphs. Furthermore, Raman phonon spectra indicate a pressure-induced phase transition where the polymorph C (lower density phase) transforms to the H form (higher density phase). The onset pressure for the phase transition is only 0.2 GPa. The phase change is irreversible.
Intersystem transitions of interstellar carbon monoxide toward zeta Ophiuchi
NASA Technical Reports Server (NTRS)
Federman, S. R.; Cardelli, Jason A.; Sheffer, Yaron; Lambert, David L.; Morton, D. C.
1994-01-01
Absorption from seven intersystem (triplet-singlet) transitions of interstellar (12)CO were detected in ultraviolet spectra of zeta Oph. The observed equivalent widths are approximately consistent with the transitions' predicted f-values and the (12) CO column density derived from the weakest of the observed A-X bands. These unsaturated intersystem transitions provide the opportunity to measure the (12)CO column density for heavily reddened (dense) sight lines. Laboratory measurements of oscillator strengths more precise than available ones will be needed to derive accurate column densities.
Applied uses of level density models
Arthur, E.D.; Guenther, P.T.; Smith, A.B.; Smith, D.L.; Argonne National Lab., IL )
1989-01-01
This paper addresses issues associated with the use of nuclear level density models in calculations made for data applications. The two most commonly used models, the Gilbert Cameron and the Back-Shifted Fermi Gas, are briefly summarized and examples are provided of recent efforts to improve their parameterization. Calculated particle emission spectra are compared with recent experimental data in order to assess performance and sensitivity to these models. Extrapolation of nuclear level densities for calculations involving nuclei away from stability poses special problems and examples of recent efforts to improve such extrapolations are cited. The sensitivity of current schemes in the modeling of fission transition state densities are explored through calculated (n,f) cross sections. Two newer phenomenological models, those of Ignatyuk and Schmidt, provide a more physically realistic description of level densities. Calculations concerning {sup 207}Pb(n,xn) cross sections compare results using the Ignatyuk formalism with the Gilbert Cameron results. 31 refs., 10 figs.
Oxides having high energy densities
Ceder, Gerbrand; Kang, Kisuk
2013-09-10
Certain disclosed embodiments generally relate to oxide materials having relatively high energy and/or power densities. Various aspects of the embodiments are directed to oxide materials having a structure B.sub.i(M.sub.jY.sub.k)O.sub.2, for example, a structure Li.sub.j(Ni.sub.jY.sub.k)O.sub.2 such as Li(Ni.sub.0.5Mn.sub.0.5)O.sub.2. In this structure, Y represents one or more atoms, each independently selected from the group consisting of alkaline earth metals, transition metals, Group 14 elements, Group 15, or Group 16 elements. In some embodiments, such an oxide material may have an O3 crystal structure, and/or a layered structure such that the oxide comprises a plurality of first, repeating atomic planes comprising Li, and a plurality of second, repeating atomic planes comprising Ni and/or Y.
Transiting Exoplanet Survey Satellite (TESS)
NASA Technical Reports Server (NTRS)
Ricker, G. R.; Clampin, M.; Latham, D. W.; Seager, S.; Vanderspek, R. K.; Villasenor, J. S.; Winn, J. N.
2012-01-01
The Transiting Exoplanet Survey Satellite (TESS) will discover thousands of exoplanets in orbit around the brightest stars in the sky. In a two-year survey, TESS will monitor more than 500,000 stars for temporary drops in brightness caused by planetary transits. This first-ever spaceborne all-sky transit survey will identify planets ranging from Earth-sized to gas giants, around a wide range of stellar types and orbital distances. No ground-based survey can achieve this feat. A large fraction of TESS target stars will be 30-100 times brighter than those observed by Kepler satellite, and therefore TESS . planets will be far easier to characterize with follow-up observations. TESS will make it possible to study the masses, sizes, densities, orbits, and atmospheres of a large cohort of small planets, including a sample of rocky worlds in the habitable zones of their host stars. TESS will provide prime targets for observation with the James Webb Space Telescope (JWST), as well as other large ground-based and space-based telescopes of the future. TESS data will be released with minimal delay (no proprietary period), inviting immediate community-wide efforts to study the new planets. The TESS legacy will be a catalog of the very nearest and brightest main-sequence stars hosting transiting exoplanets, thus providing future observers with the most favorable targets for detailed investigations.
Densities of stratospheric micrometeorites
NASA Technical Reports Server (NTRS)
Love, Stanley G.; Joswiak, David J.; Brownlee, Donald E.
1994-01-01
We have measured the densities of roughly 150 5- to 15-microns interplanetary dust particles (IDPs) harvested in the stratosphere. Care was taken to minimize selection bias in the sample population. Masses were determined using an absolute X-ray analysis technique with a transmission electron microscope, and volumes were found using scanning electron microscope imagery. Unmelted chondritic particles have densities ranging between 0.3 and 6.2 g/cu cm, averaging 2.0 g/cu cm. The low medium densities indicates appreciable porosity, suggesting primitive, uncompacted parent bodies for these particles. Porosities greater than 70% are rare. IDPs with densities above 3.5 g/cu cm usually contain large sulfide grains. We find no evidence of bimodality in the unmelted particle density distribution. Chondritic spherules (melted particles) have densities near 3.4 g/cu cm, consistent with previous results for stony spheurles culled from deep-sea sediments.
Atomic Transitions in Dense Plasmas
NASA Astrophysics Data System (ADS)
Murillo, Michael Sean
Motivation for the study of hot, dense ( ~solid density) plasmas has historically been in connection with stellar interiors. In recent years, however, there has been a growing interest in such plasmas due to their relevance to short wavelength (EUV and x-ray) lasers, inertial confinement fusion, and optical harmonic generation. In constrast to the stellar plasmas, these laboratory plasmas are typically composed of high-z elements and are not in thermal equilibrium. Descriptions of nonthermal plasma experiments must necessarily involve the consideration of the various atomic processes and the rates at which they occur. Traditionally, the rates of collisional atomic processes are calculated by considering a binary collision picture. For example, a single electron may be taken to collisionally excite an ion. A cross section may be defined for this process and, multiplying by a flux, the rate may be obtained. In a high density plasma this binary picture clearly breaks down as the electrons no longer act independently of each other. The cross section is ill-defined in this regime and another approach is needed to obtain rates. In this thesis an approach based on computing rates without recourse to a cross section is presented. In this approach, binary collisions are replaced by stochastic density fluctuations. It is then these density fluctuations which drive transitions in the ions. Furthermore, the oscillator strengths for the transitions are computed in screened Coulomb potentials which reflect the average polarization of the plasma near the ion. Numerical computations are presented for the collisional ionization rate. The effects of screening in the plasma -ion interaction are investigated for He^+ ions in a plasma near solid density. It is shown that dynamic screening plays an important role in this process. Then, density effects in the oscillator strength are explored for both He^+ and Ar^{+17}. Approximations which introduce a nonorthogonality between the initial
Bonnor, W.B.
1987-05-01
The Einstein-Straus (1945) vacuole is here used to represent a bound cluster of galaxies embedded in a standard pressure-free cosmological model, and the average density of the cluster is compared with the density of the surrounding cosmic fluid. The two are nearly but not quite equal, and the more condensed the cluster, the greater the difference. A theoretical consequence of the discrepancy between the two densities is discussed. 25 references.
Grimes, S.M.
2005-05-24
Recent research in the area of nuclear level densities is reviewed. The current interest in nuclear astrophysics and in structure of nuclei off of the line of stability has led to the development of radioactive beam facilities with larger machines currently being planned. Nuclear level densities for the systems used to produce the radioactive beams influence substantially the production rates of these beams. The modification of level-density parameters near the drip lines would also affect nucleosynthesis rates and abundances.
Modeling thermospheric neutral density
NASA Astrophysics Data System (ADS)
Qian, Liying
Satellite drag prediction requires determination of thermospheric neutral density. The NCAR Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIEGCM) and the global-mean Thermosphere-Ionosphere-Mesosphere-Electrodynamics General Circulation Model (TIMEGCM) were used to quantify thermospheric neutral density and its variations, focusing on annual/semiannual variation, the effect of using measured solar irradiance on model calculations of solar-cycle variation, and global change in the thermosphere. Satellite drag data and the MSIS00 empirical model were utilized to compare to the TIEGCM simulations. The TIEGCM simulations indicated that eddy diffusion and its annual/semiannual variation is a mechanism for annual/semiannual density variation in the thermosphere. It was found that eddy diffusion near the turbopause can effectively influence thermospheric neutral density. Eddy diffusion, together with annual insolation variation and large-scale circulation, generated global annual/semiannual density variation observed by satellite drag. Using measured solar irradiance as solar input for the TIEGCM improved the solar-cycle dependency of the density calculation shown in F10.7 -based thermospheric empirical models. It has been found that the empirical models overestimate density at low solar activity. The TIEGCM simulations did not show such solar-cycle dependency. Using historic measurements of CO2 and F 10.7, simulations of the global-mean TIMEGCM showed that thermospheric neutral density at 400 km had an average long-term decrease of 1.7% per decade from 1970 to 2000. A forecast of density decrease for solar cycle 24 suggested that thermospheric density will decrease at 400 km from present to the end of solar cycle 24 at a rate of 2.7% per decade. Reduction in thermospheric density causes less atmospheric drag on earth-orbiting space objects. The implication of this long-term decrease of thermospheric neutral density is that it will increase the
Visualization of electronic density
Grosso, Bastien; Cooper, Valentino R.; Pine, Polina; Hashibon, Adham; Yaish, Yuval; Adler, Joan
2015-04-22
An atom’s volume depends on its electronic density. Although this density can only be evaluated exactly for hydrogen-like atoms, there are many excellent numerical algorithms and packages to calculate it for other materials. 3D visualization of charge density is challenging, especially when several molecular/atomic levels are intertwined in space. We explore several approaches to 3D charge density visualization, including the extension of an anaglyphic stereo visualization application based on the AViz package to larger structures such as nanotubes. We will describe motivations and potential applications of these tools for answering interesting questions about nanotube properties.
Twisted transition of one bit written by trapezoidal single pole
NASA Astrophysics Data System (ADS)
Hu, Shengbin; Yuan, Zhimin; Taslim, Sumitro Joyo; Yu, Shengkai; Liu, Bo
This paper investigates bits transition shift written by a trapezoid single pole at a large skew angle. This work uses the even harmonic ripple effect to modulate the demagnetization field and observe the transition shape clearly. Experiment results indicate that the trapezoid single pole can produce a twisted transition at a large skew angle. This phenomenon is mainly due to the low-frequency data overwriting on the media. The twisted transition will limit the recording density in the perpendicular recording. Simulation work was completed to validate our experiment results. It suggests that the structure of single pole should be further improved to solve the twisted transition generated by the trapezoid single pole.
Energy density fluctuations in early universe
Guardo, G. L.; Ruggieri, M.; Greco, V.
2014-05-09
The primordial nucleosinthesys of the element can be influenced by the transitions of phase that take place after the Big Bang, such as the QCD transition. In order to study the effect of this phase transition, in this work we compute the time evolution of thermodynamical quantities of the early universe, focusing on temperature and energy density fluctuations, by solving the relevant equations of motion using as input the lattice QCD equation of state to describe the strongly interacting matter in the early universe plasma. We also study the effect of a primordial strong magnetic field by means of a phenomenological equation of state. Our results show that small inhomogeneities of strongly interacting matter in the early Universe are moderately damped during the crossover.
Density fluctuations of polymers in disordered media
Deutsch, Joshua M.; Olvera de la Cruz, Monica
2011-03-02
We study self-avoiding random walks in an environment where sites are excluded randomly, in two and three dimensions. For a single polymer chain, we study the statistics of the time averaged monomer density and show that these are well described by multifractal statistics. This is true even far from the percolation transition of the disordered medium. We investigate solutions of chains in a disordered environment and show that the statistics cease to be multifractal beyond the screening length of the solution.
Bakosi, Jozsef; Ristorcelli, Raymond J
2010-01-01
Probability density function (PDF) methods are extended to variable-density pressure-gradient-driven turbulence. We apply the new method to compute the joint PDF of density and velocity in a non-premixed binary mixture of different-density molecularly mixing fluids under gravity. The full time-evolution of the joint PDF is captured in the highly non-equilibrium flow: starting from a quiescent state, transitioning to fully developed turbulence and finally dissipated by molecular diffusion. High-Atwood-number effects (as distinguished from the Boussinesq case) are accounted for: both hydrodynamic turbulence and material mixing are treated at arbitrary density ratios, with the specific volume, mass flux and all their correlations in closed form. An extension of the generalized Langevin model, originally developed for the Lagrangian fluid particle velocity in constant-density shear-driven turbulence, is constructed for variable-density pressure-gradient-driven flows. The persistent small-scale anisotropy, a fundamentally 'non-Kolmogorovian' feature of flows under external acceleration forces, is captured by a tensorial diffusion term based on the external body force. The material mixing model for the fluid density, an active scalar, is developed based on the beta distribution. The beta-PDF is shown to be capable of capturing the mixing asymmetry and that it can accurately represent the density through transition, in fully developed turbulence and in the decay process. The joint model for hydrodynamics and active material mixing yields a time-accurate evolution of the turbulent kinetic energy and Reynolds stress anisotropy without resorting to gradient diffusion hypotheses, and represents the mixing state by the density PDF itself, eliminating the need for dubious mixing measures. Direct numerical simulations of the homogeneous Rayleigh-Taylor instability are used for model validation.
Density-dependent covariant energy density functionals
Lalazissis, G. A.
2012-10-20
Relativistic nuclear energy density functionals are applied to the description of a variety of nuclear structure phenomena at and away fromstability line. Isoscalar monopole, isovector dipole and isoscalar quadrupole giant resonances are calculated using fully self-consistent relativistic quasiparticle randomphase approximation, based on the relativistic Hartree-Bogoliubovmodel. The impact of pairing correlations on the fission barriers in heavy and superheavy nuclei is examined. The role of pion in constructing desnity functionals is also investigated.
Local spin analyses using density functional theory
NASA Astrophysics Data System (ADS)
Abate, Bayileyegn; Peralta, Juan
Local spin analysis is a valuable technique in computational investigations magnetic interactions on mono- and polynuclear transition metal complexes, which play vital roles in catalysis, molecular magnetism, artificial photosynthesis, and several other commercially important materials. The relative size and complex electronic structure of transition metal complexes often prohibits the use of multi-determinant approaches, and hence, practical calculations are often limited to single-determinant methods. Density functional theory (DFT) has become one of the most successful and widely used computational tools for the electronic structure study of complex chemical systems; transition metal complexes in particular. Within the DFT formalism, a more flexible and complete theoretical modeling of transition metal complexes can be achieved by considering noncollinear spins, in which the spin density is 'allowed to' adopt noncollinear structures in stead of being constrained to align parallel/antiparallel to a universal axis of magnetization. In this meeting, I will present local spin analyses results obtained using different DFT functionals. Local projection operators are used to decompose the expectation value
Gas turbine combustor transition
Coslow, Billy Joe; Whidden, Graydon Lane
1999-01-01
A method of converting a steam cooled transition to an air cooled transition in a gas turbine having a compressor in fluid communication with a combustor, a turbine section in fluid communication with the combustor, the transition disposed in a combustor shell and having a cooling circuit connecting a steam outlet and a steam inlet and wherein hot gas flows from the combustor through the transition and to the turbine section, includes forming an air outlet in the transition in fluid communication with the cooling circuit and providing for an air inlet in the transition in fluid communication with the cooling circuit.
Gas turbine combustor transition
Coslow, B.J.; Whidden, G.L.
1999-05-25
A method is described for converting a steam cooled transition to an air cooled transition in a gas turbine having a compressor in fluid communication with a combustor, a turbine section in fluid communication with the combustor, the transition disposed in a combustor shell and having a cooling circuit connecting a steam outlet and a steam inlet and wherein hot gas flows from the combustor through the transition and to the turbine section, includes forming an air outlet in the transition in fluid communication with the cooling circuit and providing for an air inlet in the transition in fluid communication with the cooling circuit. 7 figs.
How to Calculate Molecular Column Density
NASA Astrophysics Data System (ADS)
Mangum, Jeffrey G.; Shirley, Yancy L.
2015-03-01
The calculation of the molecular column density from molecular spectral (rotational or ro-vibrational) transition measurements is one of the most basic quantities derived from molecular spectroscopy. Starting from first principles where we describe the basic physics behind the radiative and collisional excitation of molecules and the radiative transfer of their emission, we derive a general expression for the molecular column density. As the calculation of the molecular column density involves a knowledge of the molecular energy level degeneracies, rotational partition functions, dipole moment matrix elements, and line strengths, we include generalized derivations of these molecule-specific quantities. Given that approximations to the column density equation are often useful, we explore the optically thin, optically thick, and low-frequency limits to our derived general molecular column density relation. We also evaluate the limitations of the common assumption that the molecular excitation temperature is constant and address the distinction between beam-averaged and source-averaged column densities. As non-LTE approaches to the calculation of molecular spectral line column density have become quite common, we summarize non-LTE models that calculate molecular cloud volume densities, kinetic temperatures, and molecular column densities. We conclude our discussion of the molecular column density with worked examples for C18O, C17O, N2H+, NH3, and H2CO. Ancillary information on some subtleties involving line profile functions, conversion between integrated flux and brightness temperature, the calculation of the uncertainty associated with an integrated intensity measurement, the calculation of spectral line optical depth using hyperfine or isotopologue measurements, the calculation of the kinetic temperature from a symmetric molecule excitation temperature measurement, and relative hyperfine intensity calculations for NH3 are presented in appendices. The intent of
NASA Astrophysics Data System (ADS)
Yong, Jie; Il'in, K.; Siegel, M.; Lemberger, Thomas
2013-03-01
We report temperature dependent superfluid densities λ -2(T) in ultrathin NbN films near thickness-tuned superconductor-insulator transition (SIT). Superfluid densities in these films are measured by two-coil mutual inductance apparatus. For thick films, dirty limit BCS theory fits experimental data well and this verifies the correctness of this technique. As films get thinner and closer to SIT, sharp downturns near transition temperatures (Tc), signature of Berezinsky-Kosterlitz-Thouless transition, are observed. This downturn occurs much earlier than what 2-D XY theory predicts. This might due to smaller vortex core energy than expected in 2-D XY model. The superconducting gap, deduced from fitting low temperature λ -2(T), is linear with Tc for most films but remain finite across SIT. This is consistent with the scenario that superconductivity is destroyed by phase fluctuations. Zero temperature sheet superfluid density also shows correlation with Tc, further proving the importance of fluctuations near SIT.
NASA Technical Reports Server (NTRS)
1931-01-01
Variable Density Tunnel in operation. Man at far right is probably Harold J. 'Cannonball' Tuner, longtime safety officer, who started with Curtiss in the teens. This view of the Variable Density Tunnel clearly shows the layout of the Tunnel's surroundings, as well as the plumbing and power needs of the this innovative research tool.
ERIC Educational Resources Information Center
Roser, Charles E.; McCluskey, Catherine L.
1998-01-01
Explains how the Canadian soft drink Orbitz can be used for explorations of density in the classroom. The drink has colored spheres suspended throughout that have a density close to that of the liquid. Presents a hands-on activity that can be easily done in two parts. (DDR)
BMD test; Bone density test; Bone densitometry; DEXA scan; DXA; Dual-energy x-ray absorptiometry; p-DEXA; Osteoporosis-BMD ... Bone density testing can be done several ways. The most common and accurate way uses a dual-energy x- ...
The OCD phase transition and supernova core collapse
Gentile, N.A.; Mathews, G.J.; Wilson, J.R.
1993-10-01
We examine the implications for stellar core collapse of a phase transition occurring at densities of a few times nuclear matter density. We use an equation of state that describes a phase transition between bulk nuclear matter and a phase consisting of unbound quarks and gluons. We analyze the effect on the prompt shock, the production of strange matter, and the effect on the neutrino signal and the delayed mechanism.
Clogging and Jamming Transitions in Granular Matter Flowing Through Obstacles
NASA Astrophysics Data System (ADS)
Reichhardt, Cynthia Olson; Reichhardt, Charles
2015-03-01
We consider a two-dimensional system of bidisperse disks driven through a landscape of fixed obstacles. In the limit of a single obstacle, the disks cease moving when the disk density is increased to the jamming density. The threshold density value decreases as the number of obstacles increases, but we also observe a change in the nature of the frozen state. At low obstacle density we find a homogeneous jammed state, but for higher obstacle density we instead find a heterogeneous clogged state containing void areas and possessing a memory of the driving direction. The transition to the clogged state is strongly stochastic and we observe large fluctuations in clogging time both for clogging in the original driving direction and for transverse clogging when the drive is suddenly rotated by 90 degrees. We find evidence for a diverging clogging transition time at a critical disk density well below the jamming density in a clean system.
Conceptualizing Transitions to Adulthood
ERIC Educational Resources Information Center
Wyn, Johanna
2014-01-01
This chapter provides an overview of theories of the transition to young adulthood. It sets out the argument for conceptual renewal and discusses some implications of new patterns of transition for adult education.
ERIC Educational Resources Information Center
Kneeland, Steven J.
1980-01-01
Having identified the problem of managerial transition in a previous article (CE 510 277), the author outlines a strategy for change which includes performance appraisal, definition of the management structure, and counselling for the individual in transition. (SK)
The QCD phase transitions: From mechanism to observables
Shuryak, E.V.
1997-09-22
This paper contains viewgraphs on quantum chromodynamic phase transformations during heavy ion collisions. Some topics briefly described are: finite T transitions of I molecule pairs; finite density transitions of diquarks polymers; and the softtest point of the equation of state as a source of discontinuous behavior as a function of collision energy or centrality.
Glass transition(s) of ionomers
Weiss, R.A.
1994-09-01
Ionomers are predominantly nonpolar polymers that contain a small amount of bonded salt groups. Microphase separation of ion-rich microdomains occurs as a consequence of the thermodynamic incompatibility of the salt groups and the polymer matrix and associative interactions between salt groups. Associations of the salt groups usually increase the glass transition of the continuous matrix phase, presumably as a consequence of the inhibition of chain mobility that accompanies physical crosslinking. The central question raised in this paper is whether the dispersed ion-rich microphase exhibits a glass transition. Although no glass transition for the microphase is detected by calorimetry, a dynamic mechanical relaxation is commonly observed above the T{sub g} of the matrix phase. This transition has some of the attributes of a glass transition, but it is not clear what is the actual relaxation process that is measured. This paper discusses the effect of the ionic groups on the matrix glass transition, the origin of the high-temperature dynamic mechanical transition, and the effects of the addition of plasticizers on the T{sub g} of the matrix and the higher temperature mechanical relaxation.
NASA Technical Reports Server (NTRS)
1985-01-01
The concept of a large disturbance bypass mechanism for the initiation of transition is reviewed and studied. This mechanism, or some manifestation thereof, is suspected to be at work in the boundary layers present in a turbine flow passage. Discussion is presented on four relevant subtopics: (1) the effect of upstream disturbances and wakes on transition; (2) transition prediction models, code development, and verification; (3) transition and turbulence measurement techniques; and (4) the hydrodynamic condition of low Reynolds number boundary layers.
ERIC Educational Resources Information Center
Regenwetter, Michel; Dana, Jason; Davis-Stober, Clintin P.
2011-01-01
Transitivity of preferences is a fundamental principle shared by most major contemporary rational, prescriptive, and descriptive models of decision making. To have transitive preferences, a person, group, or society that prefers choice option "x" to "y" and "y" to "z" must prefer "x" to "z". Any claim of empirical violations of transitivity by…
Transition: Terms and Concepts.
ERIC Educational Resources Information Center
O'Leary, Ed
This paper provides explanations and case examples of some terms and concepts related to transition of students with disabilities under 1997 amendments to the Individuals with Disabilities Education Act. Explanations and examples focus on the concepts of "statement of transition service needs" and "statement of needed transition services". The…
Visualization of electronic density
NASA Astrophysics Data System (ADS)
Grosso, Bastien; Cooper, Valentino R.; Pine, Polina; Hashibon, Adham; Yaish, Yuval; Adler, Joan
2015-10-01
The spatial volume occupied by an atom depends on its electronic density. Although this density can only be evaluated exactly for hydrogen-like atoms, there are many excellent algorithms and packages to calculate it numerically for other materials. Three-dimensional visualization of charge density is challenging, especially when several molecular/atomic levels are intertwined in space. In this paper, we explore several approaches to this, including the extension of an anaglyphic stereo visualization application based on the AViz package for hydrogen atoms and simple molecules to larger structures such as nanotubes. We will describe motivations and potential applications of these tools for answering interesting physical questions about nanotube properties.
Strongly Interacting Matter at High Energy Density
McLerran,L.
2008-09-07
This lecture concerns the properties of strongly interacting matter (which is described by Quantum Chromodynamics) at very high energy density. I review the properties of matter at high temperature, discussing the deconfinement phase transition. At high baryon density and low temperature, large N{sub c} arguments are developed which suggest that high baryonic density matter is a third form of matter, Quarkyonic Matter, that is distinct from confined hadronic matter and deconfined matter. I finally discuss the Color Glass Condensate which controls the high energy limit of QCD, and forms the low x part of a hadron wavefunction. The Glasma is introduced as matter formed by the Color Glass Condensate which eventually thermalizes into a Quark Gluon Plasma.
Detection by Transit Photometry
NASA Technical Reports Server (NTRS)
Borucki, William J.; Koch, David G.; Jenkins, Jon M.; DeVincenzi, D. (Technical Monitor)
2002-01-01
A periodic sequence of planetary transits provides a valid detection of an orbiting planet and provides the relative size of the planet and its orbital period. Ancillary measurements of the stellar spectrum and the variations of the star's radial velocity or position combined with stellar models allow the absolute size of the planet and its mass to be obtained. The results of this approach have already shown that the planet orbiting HD209458 has only 70% of the mass of Jupiter, but is nearly 50% larger in radius. Based on models of planetary structure, these results imply that the planet must have spent most of its lifetime so close to the star that it has not been able to cool and contract as have the giant planets in our Solar System. Thus its density is much less than Jupiter and Saturn and is actually less than that of water; i.e., about 0.4 gr/cu cm. If more sensitive measurements of the light curve of stars with closely orbiting planets can be made that provide the varying amplitude of the light reflected by the planet at various phases in its orbit, then characteristics of the planetary atmosphere can be obtained. Potentially, these data can identify major molecular species present in the atmosphere and tell us if clouds are present and yield the phase function of the aerosols. Although such detail cannot be obtained for Earth-size planets because their signal amplitudes are too small, it is possible to get data critical to the determination of the structure of extrasolar planetary systems. In particular, the size distributions and their orbital distributions can be measured by the transit photometry missions now in development. The COROT mission should be able to find large terrestrial planets in short-period orbits while the more ambitious Kepler and Eddington missions should be able to detect planets even smaller than the Earth and at orbital distances that place them in the habitable zone of their stars.
BMD test; Bone density test; Bone densitometry; DEXA scan; DXA; Dual-energy x-ray absorptiometry; p-DEXA; Osteoporosis-BMD ... need to undress. This scan is the best test to predict your risk of fractures. Peripheral DEXA ( ...
Histograms and Frequency Density.
ERIC Educational Resources Information Center
Micromath, 2003
2003-01-01
Introduces exercises on histograms and frequency density. Guides pupils to Discovering Important Statistical Concepts Using Spreadsheets (DISCUSS), created at the University of Coventry. Includes curriculum points, teaching tips, activities, and internet address (http://www.coventry.ac.uk/discuss/). (KHR)
... bone the higher the risk of fractures. A bone scan, along with a patient's medical history, is a ... and whether any preventative treatment is needed. A bone density scan has the advantage of being painless and exposing ...
... study linked 32 novel genetic regions to bone mineral density. The findings may help researchers understand why ... or treating osteoporosis. Bones are made of a mineral and protein scaffold filled with bone cells. Bone ...
ERIC Educational Resources Information Center
Dickinson, Annette; Thompson, William T.
1979-01-01
Announces a nutrient density food scoring system called the Index of Nutritional Quality (INQ). It expresses the ratio between the percent RDA of a nutrient and the percent daily allowance of calories in a food. (Author/SA)
ERIC Educational Resources Information Center
Libarkin, Julie C.; Crockett, Cynthia D.; Sadler, Philip M.
2003-01-01
Presents activities to dispel student misconceptions about density, particularly as it applies to buoyancy. Finds that misconceptions fall under three categories: (1) size; (2) shape; and (3) material. (NB)
Critical Density Interaction Studies
Young, P; Baldis, H A; Cheung, P; Rozmus, W; Kruer, W; Wilks, S; Crowley, S; Mori, W; Hansen, C
2001-02-14
Experiments have been performed to study the propagation of intense laser pulses to high plasma densities. The issue of self-focusing and filamentation of the laser pulse as well as developing predictive capability of absorption processes and x-ray conversion efficiencies is important for numerous programs at the Laboratory, particularly Laser Program (Fast Ignitor and direct-drive ICF) and D&NT (radiography, high energy backlighters and laser cutting). Processes such as resonance absorption, profile modification, linear mode conversion, filamentation and stimulated Brillouin scattering can occur near the critical density and can have important effects on the coupling of laser light to solid targets. A combination of experiments have been used to study the propagation of laser light to high plasma densities and the interaction physics of intense laser pulses with solid targets. Nonparaxial fluid codes to study nonstationary behavior of filamentation and stimulated Brillouin scattering at high densities have also been developed as part of this project.
Density Functionals with Broad Applicability in Chemistry
Zhao, Yan; Truhlar, Donald G.
2008-02-01
The research described in this product was performed in part in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. Although density functional theory is widely used in the computational chemistry community, the most popular density functional, B3LYP, has some serious shortcomings: (i) it is better for main-group chemistry than for transition metals; (ii) it systematically underestimates reaction barrier heights; (iii) it is inaccurate for interactions dominated by mediumrange correlation energy, such as van der Waals attraction, aromatic-aromatic stacking, and alkane isomerization energies. We have developed a variety of databases for testing and designing new density functionals. We used these data to design new density functionals, called M06-class (and, earlier, M05-class) functionals, for which we enforced some fundamental exact constraints such as the uniform-electron-gas limit and the absence of self-correlation energy. Our M06-class functionals depend on spin-up and spin-down electron densities (i.e., spin densities), spin density gradients, spin kinetic energy densities, and, for nonlocal (also called hybrid) functionals, Hartree-Fock exchange. We have developed four new functionals that overcome the above-mentioned difficulties: (a) M06, a hybrid meta functional, is a functional with good accuracy “across-theboard” for transition metals, main group thermochemistry, medium-range correlation energy, and barrier heights; (b) M06- 2X, another hybrid meta functional, is not good for transition metals but has excellent performance for main group chemistry, predicts accurate valence and Rydberg electronic excitation energies, and is an excellent functional for aromatic-aromatic stacking interactions; (c) M06-L is not as accurate as M06 for barrier heights but is the most accurate
Igor Kaganovich
2000-12-18
Negative ions tend to stratify in electronegative plasmas with hot electrons (electron temperature Te much larger than ion temperature Ti, Te > Ti ). The boundary separating a plasma containing negative ions, and a plasma, without negative ions, is usually thin, so that the negative ion density falls rapidly to zero-forming a negative ion density front. We review theoretical, experimental and numerical results giving the spatio-temporal evolution of negative ion density fronts during plasma ignition, the steady state, and extinction (afterglow). During plasma ignition, negative ion fronts are the result of the break of smooth plasma density profiles during nonlinear convection. In a steady-state plasma, the fronts are boundary layers with steepening of ion density profiles due to nonlinear convection also. But during plasma extinction, the ion fronts are of a completely different nature. Negative ions diffuse freely in the plasma core (no convection), whereas the negative ion front propagates towards the chamber walls with a nearly constant velocity. The concept of fronts turns out to be very effective in analysis of plasma density profile evolution in strongly non-isothermal plasmas.
Magnetically modulated critical current densities of Co/Nb hybrid
Li, Zhigang; Wang, Weike; Zhang, Li; Yang, Zhaorong; Tian, Mingliang; Zhang, Yuheng
2015-01-01
By tuning morphology and size of magnetic subsystem, ferromagnet-superconductor (F/S) hybrid system provides an effective way to modulate superconductivity due to the interaction between superconducting and magnetic-order parameters at the mesoscopic length scale. In this work, we report on investigations of critical current density in a large-area Co/Nb hybrid via facile colloidal lithography. Here, Co hexagon shell array as a magnetic template build on Nb film to modulate the critical current density. A novel superconducting transition has been observed in I-V curve with two metastable transition states: double-transition and binary-oscillation-transition states. Importantly, such unusual behavior can be adjusted by temperature, magnetic field and contact area of F/S. Such hybrid film has important implications for understanding the role of magnetic subsystem modulating superconductivity, as well as applied to low-energy electronic devices such as superconducting current fault limiters. PMID:26678595
Magnetically modulated critical current densities of Co/Nb hybrid
NASA Astrophysics Data System (ADS)
Li, Zhigang; Wang, Weike; Zhang, Li; Yang, Zhaorong; Tian, Mingliang; Zhang, Yuheng
2015-12-01
By tuning morphology and size of magnetic subsystem, ferromagnet-superconductor (F/S) hybrid system provides an effective way to modulate superconductivity due to the interaction between superconducting and magnetic-order parameters at the mesoscopic length scale. In this work, we report on investigations of critical current density in a large-area Co/Nb hybrid via facile colloidal lithography. Here, Co hexagon shell array as a magnetic template build on Nb film to modulate the critical current density. A novel superconducting transition has been observed in I-V curve with two metastable transition states: double-transition and binary-oscillation-transition states. Importantly, such unusual behavior can be adjusted by temperature, magnetic field and contact area of F/S. Such hybrid film has important implications for understanding the role of magnetic subsystem modulating superconductivity, as well as applied to low-energy electronic devices such as superconducting current fault limiters.
The transition regions of Capella
NASA Technical Reports Server (NTRS)
Linsky, Jeffrey L.; Wood, Brian E.; Judge, Philip; Brown, Alexander; Andrulis, Catherine; Ayers, Thomas R.
1995-01-01
We have used the Goddard High Resolution Spectrometer (GHRS) to observe the spectoscopic binary system Capella (G8 III + G1 III). Exposures with the G140L, G140M, G160M, G200M, and echelle gratings provide emission line profiles with unprecedented signal-to-noise and spectral resolving power (lambda/Delta-lambda) up to 92,000. Multi-Gaussin fits to the line profiles show that the hotter star contributes 60%-70% of the total flux in the chromospheric O I and Mg II resonance lines, but about 90% of the flux in the Si III, Si IV, and C IV lines formed in the transition region at T less than or = 10(exp 5) K. We find clear evidence that the emission lines from the hotter star are systemtically redshifted relative to the photosphere with Doppler shifts of 5 +/- 1 km/s for the +9 +/- 3 km/s in the chromospheric Mg II and O I lines, respectively, increasing to +24 +/- 5 km/s for the transition region Si IV 1393.8A line. The multi-Gaussian fits to permitted transition region lines of SI III, Si IV, C IV, and N V indicate the presence of three components: moderately broad lines formed in the transition region of the hotter star (component H), narrow lines formed in the transition region of the cooler star (component C), and very broad lines that we think are formed in microflares on the hotter star (component B). The He II 1640.4 A feature has an broad profile, which indicates that it is formed by collisional excitation primarily from the hotter star, and a weak narrow component that we interpret as due to radiative recombination on the cooler star. We observed spin-forbidden emission lines of C III), O III), Si III), O IV), O V), and S IV) that are sensitive to electron density. Fainter members of the O IV) multiplet and all of the S IV) lines have never before been seen in any star than the Sun. We determine electron densities in the transition regions of the Capella stars using lines ratios of O IV) lines and emission measure analysis. The emission measures are self
Big bang nucleosynthesis and the quark-hadron transition
NASA Technical Reports Server (NTRS)
Kurki-Suonio, Hannu; Matzner, Richard A.; Olive, Keith A.; Schramm, David N.
1990-01-01
An examination and brief review is made of the effects of quark-hadron transition induced fluctuations on Big Bang nucleosynthesis. It is shown that cosmologically critical densities in baryons are difficult to reconcile with observation, but the traditional baryon density constraints from homogeneous calculations might be loosened by as much as 50 percent, to 0.3 of critical density, and the limit on the number of neutrino flavors remains about N(sub nu) is less than or approximately 4. To achieve baryon densities of greater than or approximately 0.3 of critical density would require initial density contrasts R is much greater the 10(exp e), whereas the simplest models for the transition seem to restrict R to less than of approximately 10(exp 2).
Discontinuous phase transition in a dimer lattice gas
NASA Astrophysics Data System (ADS)
Dickman, Ronald
2012-05-01
I study a dimer model on the square lattice with nearest neighbor exclusion as the only interaction. Detailed simulations using tomographic entropic sampling show that as the chemical potential is varied, there is a strongly discontinuous phase transition, at which the particle density jumps by about 18% of its maximum value, 1/4. The transition is accompanied by the onset of orientational order, to an arrangement corresponding to the {1/2, 0, 1/2} structure identified by Phares et al. [Physica B 409, 1096 (2011)] in a dimer model with finite repulsion at fixed density. Using finite-size scaling and Binder's cumulant, the expected scaling behavior at a discontinuous transition is verified in detail. The discontinuous transition can be understood qualitatively given that the model possesses eight equivalent maximum-density configurations, so that its coarse-grained description corresponds to that of the q = 8 Potts model.
Modelling ionospheric density structures
NASA Technical Reports Server (NTRS)
Schunk, R. W.; Sojka, J. J.
1989-01-01
Large-scale density structures are a common feature in the high-latitude ionsphere. The structures were observed in the dayside cusp, polar cap, and nocturnal auroral region over a range of altitudes, including the E-region, F-region and topside ionosphere. The origins, lifetimes and transport characteristics of large-scale density structures were studied with the aid of a three-dimensional, time-dependent ionospheric model. Blob creation due to particle precipitation, the effect that structured electric fields have on the ionosphere, and the lifetimes and transport characteristics of density structures for different seasonal, solar cycle, and interplanetary magnetic field (IMF) conditions were studied. The main conclusions drawn are: (1) the observed precipitation energy fluxes are sufficient for blob creation if the plasma is exposed to the precipitation for 5 to 10 minutes; (2) structured electric fields produce structured electron densities, ion temperatures, and ion composition; (3) the lifetime of an F-region density structure depends on several factors, including the initial location where it was formed, the magnitude of the perturbation, season, solar cycle and IMF; and (4) depending on the IMF, horizontal plasma convection can cause an initial structure to break up into multiple structures of various sizes, remain as a single distorted structure, or become stretched into elongated segments.
DIFFUSE MOLECULAR CLOUD DENSITIES FROM UV MEASUREMENTS OF CO ABSORPTION
Goldsmith, Paul F.
2013-09-10
We use UV measurements of interstellar CO toward nearby stars to calculate the density in the diffuse molecular clouds containing the molecules responsible for the observed absorption. Chemical models and recent calculations of the excitation rate coefficients indicate that the regions in which CO is found have hydrogen predominantly in molecular form and that collisional excitation is by collisions with H{sub 2} molecules. We carry out statistical equilibrium calculations using CO-H{sub 2} collision rates to solve for the H{sub 2} density in the observed sources without including effects of radiative trapping. We have assumed kinetic temperatures of 50 K and 100 K, finding this choice to make relatively little difference to the lowest transition. For the sources having T{sup ex}{sub 10} only for which we could determine upper and lower density limits, we find (n(H{sub 2})) = 49 cm{sup -3}. While we can find a consistent density range for a good fraction of the sources having either two or three values of the excitation temperature, there is a suggestion that the higher-J transitions are sampling clouds or regions within diffuse molecular cloud material that have higher densities than the material sampled by the J = 1-0 transition. The assumed kinetic temperature and derived H{sub 2} density are anticorrelated when the J = 2-1 transition data, the J = 3-2 transition data, or both are included. For sources with either two or three values of the excitation temperature, we find average values of the midpoint of the density range that is consistent with all of the observations equal to 68 cm{sup -3} for T{sup k} = 100 K and 92 cm{sup -3} for T{sup k} = 50 K. The data for this set of sources imply that diffuse molecular clouds are characterized by an average thermal pressure between 4600 and 6800 K cm{sup -3}.
Characterization of the transition from defect to phase turbulence
Egolf, D.A.; Greenside, H.S. )
1995-03-06
For the complex Ginzburg-Landau equation on a large periodic interval, we show that the transition from defect to phase turbulence is more accurately described as a smooth crossover rather than as a sharp continuous transition. We obtain this conclusion by using a parallel computer to calculate various order parameters, especially the density of space-time defects, the Lyapunov dimension density, and correlation lengths. Remarkably, the correlation length of the field amplitude fluctuations is, within a constant factor, equal to the length scale defined by the dimension density.
Transit of Exoplanet WASP 24-b
NASA Astrophysics Data System (ADS)
Thompson, Robert; Turner, J.; Hardegree-Ullman, K.; Raphael, B.; Smith, C.; Towner, A. P.; Walker-LaFollette, A.; Wallace, S.; Berkson, E.; Greenwood, N.
2013-01-01
We observed two primary transits of exoplanet WASP-24b with the Steward Observatory 1.55 meter Kuiper Telescope in the R photometric band. With our results, we have been able to produce a more complete light curve and refine previously published values for the planet’s mass, radius, density, surface gravity, Safronov number, equilibrium temperature, orbital distance, orbital inclination. One of the goals of this project is to give undergraduates opportunity to learn astronomical observing techniques, get practical experience using a research-class telescope, and perform data reduction using IRAF and Transit Analysis Package (TAP).
Solid-liquid phase transition in argon
NASA Technical Reports Server (NTRS)
Tsang, T.; Tang, H. T.
1978-01-01
Starting from the Lennard-Jones interatomic potential, a modified cell theory has been used to describe the solid-liquid phase transition in argon. The cell-size variations may be evaluated by a self-consistent condition. With the inclusion of cell-size variations, the transition temperature, the solid and liquid densities, and the liquid-phase radial-distribution functions have been calculated. These ab initio results are in satisfactory agreement with molecular-dynamics calculations as well as experimental data on argon.
Grimes, S. M.; Voinov, A.
2009-01-28
A summary of some recent level density research is presented. Although the subject is an old one, it is argued that a number of unanswered questions remain. These include uncertainties in related quantities such as the parity ratio and the spin cutoff parameter, which are needed to deduce level density parameters from resonance counting for low energy neutrons. Additional points of interest are the extent to which the low energy region shows constant temperature rather than Fermi gas energy dependence, whether the region below the neutron binding energy shows significant structure and whether the level density for fixed A shows a drop for neutron-rich and proton-rich nuclei compared to nuclei on the valley of stability.
Dynamics and diffusion mechanism of low-density liquid silicon
Shen, B.; Wang, Z. Y.; Dong, F.; Guo, Y. R.; Zhang, R. J.; Zheng, Y. X.; Wang, S. Y.; Wang, C. Z.; Ho, K. M.; Chen, L. Y.
2015-11-05
A first-order phase transition from a high-density liquid to a low-density liquid has been proposed to explain the various thermodynamic anomies of water. It also has been proposed that such liquid–liquid phase transition would exist in supercooled silicon. Computer simulation studies show that, across the transition, the diffusivity drops roughly 2 orders of magnitude, and the structures exhibit considerable tetrahedral ordering. The resulting phase is a highly viscous, low-density liquid silicon. Investigations on the atomic diffusion of such a novel form of liquid silicon are of high interest. Here we report such diffusion results from molecular dynamics simulations using the classical Stillinger–Weber (SW) potential of silicon. We show that the atomic diffusion of the low-density liquid is highly correlated with local tetrahedral geometries. We also show that atoms diffuse through hopping processes within short ranges, which gradually accumulate to an overall random motion for long ranges as in normal liquids. There is a close relationship between dynamical heterogeneity and hopping process. We point out that the above diffusion mechanism is closely related to the strong directional bonding nature of the distorted tetrahedral network. Here, our work offers new insights into the complex behavior of the highly viscous low density liquid silicon, suggesting similar diffusion behaviors in other tetrahedral coordinated liquids that exhibit liquid–liquid phase transition such as carbon and germanium.
Dynamics and diffusion mechanism of low-density liquid silicon
Shen, B.; Wang, Z. Y.; Dong, F.; Guo, Y. R.; Zhang, R. J.; Zheng, Y. X.; Wang, S. Y.; Wang, C. Z.; Ho, K. M.; Chen, L. Y.
2015-11-05
A first-order phase transition from a high-density liquid to a low-density liquid has been proposed to explain the various thermodynamic anomies of water. It also has been proposed that such liquid–liquid phase transition would exist in supercooled silicon. Computer simulation studies show that, across the transition, the diffusivity drops roughly 2 orders of magnitude, and the structures exhibit considerable tetrahedral ordering. The resulting phase is a highly viscous, low-density liquid silicon. Investigations on the atomic diffusion of such a novel form of liquid silicon are of high interest. Here we report such diffusion results from molecular dynamics simulations using themore » classical Stillinger–Weber (SW) potential of silicon. We show that the atomic diffusion of the low-density liquid is highly correlated with local tetrahedral geometries. We also show that atoms diffuse through hopping processes within short ranges, which gradually accumulate to an overall random motion for long ranges as in normal liquids. There is a close relationship between dynamical heterogeneity and hopping process. We point out that the above diffusion mechanism is closely related to the strong directional bonding nature of the distorted tetrahedral network. Here, our work offers new insights into the complex behavior of the highly viscous low density liquid silicon, suggesting similar diffusion behaviors in other tetrahedral coordinated liquids that exhibit liquid–liquid phase transition such as carbon and germanium.« less
Gravitationally induced quantum transitions
NASA Astrophysics Data System (ADS)
Landry, A.; Paranjape, M. B.
2016-06-01
In this paper, we calculate the probability for resonantly inducing transitions in quantum states due to time-dependent gravitational perturbations. Contrary to common wisdom, the probability of inducing transitions is not infinitesimally small. We consider a system of ultracold neutrons, which are organized according to the energy levels of the Schrödinger equation in the presence of the Earth's gravitational field. Transitions between energy levels are induced by an oscillating driving force of frequency ω . The driving force is created by oscillating a macroscopic mass in the neighborhood of the system of neutrons. The neutron lifetime is approximately 880 sec while the probability of transitions increases as t2. Hence, the optimal strategy is to drive the system for two lifetimes. The transition amplitude then is of the order of 1.06 ×10-5, and hence with a million ultracold neutrons, one should be able to observe transitions.
NASA Astrophysics Data System (ADS)
McCall, G. H.
During the past few years techniques have been developed for producing pinches in solid deuterium. The conditions which exist in these plasmas are different from those produced earlier. The pinch is formed from a fiber of solid deuterium rather than from a low density gas, and the current is driven by a low impedance, high voltage pulse generator. Because of the high initial density, it is not necessary to compress the pinch to reach thermonuclear conditions, and the confinement time required for energy production is much shorter than for a gas. Results, which have been verified by experiments performed at higher current were quite surprising and encouraging. The pinch appeared to be stable for a time much longer than the Alfven radial transit time. It is argued that the pinch is not strictly stable, but it does not appear to disassemble in a catastrophic fashion. It appears that there may be a distinction between stability and confinement in the high density pinch. In the discussion below the status of the high density Z-pinch experiments at laboratories around the world is presented, and some of the calculational and experimental results described. Remarks are confined to recent work on the high density pinch.
Density Shock Waves in Confined Microswimmers
NASA Astrophysics Data System (ADS)
Tsang, Alan Cheng Hou; Kanso, Eva
2016-01-01
Motile and driven particles confined in microfluidic channels exhibit interesting emergent behavior, from propagating density bands to density shock waves. A deeper understanding of the physical mechanisms responsible for these emergent structures is relevant to a number of physical and biomedical applications. Here, we study the formation of density shock waves in the context of an idealized model of microswimmers confined in a narrow channel and subject to a uniform external flow. Interestingly, these density shock waves exhibit a transition from "subsonic" with compression at the back to "supersonic" with compression at the front of the population as the intensity of the external flow increases. This behavior is the result of a nontrivial interplay between hydrodynamic interactions and geometric confinement, and it is confirmed by a novel quasilinear wave model that properly captures the dependence of the shock formation on the external flow. These findings can be used to guide the development of novel mechanisms for controlling the emergent density distribution and the average population speed, with potentially profound implications on various processes in industry and biotechnology, such as the transport and sorting of cells in flow channels.
Production Density Diffusion Equation Propagation and Production
NASA Astrophysics Data System (ADS)
Shirai, Kenji; Amano, Yoshinori
When we call the production flow to transition elements in the next step in the process of product manufactured one, the production flow is considered to be displaced in the direction of the unit production density. Density and production, as captured from different perspectives, also said production costs per unit of production. However, it is assumed that contributed to the production cost of manufacturing 100 percent. They may not correspond to the physical propagation conditions after each step of the production density, the equations governing the manufacturing process, which is intended to be represented by a single diffusion equation. We can also apply the concept of energy levels in statistical mechanics, production density function, in other words, in statistical mechanics “place” that if you use the world of manufacturing and production term. If the free energy in this production (potential) that are consuming the substance is nothing but the entropy production. That is, productivity is defined as the entropy production has to be. Normally, when we increase the number of production units, the product nears completion at year-end number of units completed and will aim to be delivered to the contractor from the turnover order. However, if you stop at any number of units, that will increase production density over time. Thus, the diffusion does not proceed from that would be irreversible. In other words, the congestion will occur in production. This fact and to report the results of analysis based on real data.
Density Shock Waves in Confined Microswimmers.
Tsang, Alan Cheng Hou; Kanso, Eva
2016-01-29
Motile and driven particles confined in microfluidic channels exhibit interesting emergent behavior, from propagating density bands to density shock waves. A deeper understanding of the physical mechanisms responsible for these emergent structures is relevant to a number of physical and biomedical applications. Here, we study the formation of density shock waves in the context of an idealized model of microswimmers confined in a narrow channel and subject to a uniform external flow. Interestingly, these density shock waves exhibit a transition from "subsonic" with compression at the back to "supersonic" with compression at the front of the population as the intensity of the external flow increases. This behavior is the result of a nontrivial interplay between hydrodynamic interactions and geometric confinement, and it is confirmed by a novel quasilinear wave model that properly captures the dependence of the shock formation on the external flow. These findings can be used to guide the development of novel mechanisms for controlling the emergent density distribution and the average population speed, with potentially profound implications on various processes in industry and biotechnology, such as the transport and sorting of cells in flow channels. PMID:26871357
Density Equalizing Map Projections
Close, E. R.; Merrill, D. W.; Holmes, H. H.
1995-07-01
A geographic map is mathematically transformed so that the subareas of the map are proportional to a given quantity such as population. In other words, population density is equalized over the entire map. The transformed map can be used as a display tool, or it can be statistically analyzed. For example, cases of disease plotted on the transformed map should be uniformly distributed at random, if disease rates are everywhere equal. Geographic clusters of disease can be readily identified, and their statistical significance determined, on a density equalized map.
Density Equalizing Map Projections
1995-07-01
A geographic map is mathematically transformed so that the subareas of the map are proportional to a given quantity such as population. In other words, population density is equalized over the entire map. The transformed map can be used as a display tool, or it can be statistically analyzed. For example, cases of disease plotted on the transformed map should be uniformly distributed at random, if disease rates are everywhere equal. Geographic clusters of diseasemore » can be readily identified, and their statistical significance determined, on a density equalized map.« less
Teolis, B. D.; Fama, M.; Baragiola, R. A.
2007-08-21
We report a very low density ({approx}0.5 g/cm{sup 3}) structure of solid ozone. It is produced by irradiation of solid oxygen with 100 keV protons at 20 K followed by heating to sublime unconverted oxygen. Upon heating to 47 K the porous ozone compacts to a density of {approx}1.6 g/cm{sup 3} and crystallizes. We use a detailed analysis of the main infrared absorption band of the porous ozone to interpret previous research, where solid oxygen was irradiated by UV light and keV electrons.
Moons over Jupiter: transits and shadow transits
NASA Astrophysics Data System (ADS)
Rogers, J. H.; et al.
2003-06-01
There is no more beautiful illustration of orbital motions than the movements of Jupiter's satellites. Every six years, their movements are most strikingly displayed, when the jovian system is presented edge-on to Earth. This means that there is a higher frequency of multiple transits over the face of the planet, as all the moons transit across the equatorial zone, whereas in other years Ganymede and Callisto transit near the poles or not at all. Also, for a few months, the satellites pass in front of each other, displaying mutual eclipses and occultations. In 2002/2003 we have been able to observe a fine series of these multiple and mutual events. On the cover, and on these pages, are some of the highest-resolution images received.
Dynamical Transition in polypeptides
NASA Astrophysics Data System (ADS)
He, Yunfen; Markelz, Andrea
2008-03-01
Two of the possible causes for the so called dynamical transition (the rapid increase in flexibility for biomolecules at ˜ 200 K) are: thermally activated side chain diffusive motions with hydration dependent activation energies; or a glass transition in the biological water directly adjacent to the biomolecule. If the transition is strictly due to side chain activation, it should not depend on protein structure. Previously we demonstrated that the dynamical transition remains after tertiary structure was removed using THz time domain dielectric spectroscopy (0.2 -2.0 THz, 0.5-5ps). Here measurements on polyalanine as a function of chain length show that the dynamical transition does not occur for peptide length shorter than 5. However, the transition is observed for 5 mer and higher. Structural and simulation studies indicate that the 5 mer transiently occupies structured forms [1,2]. These results suggest that A) the dynamical transition is not due to thermally activated side chain motion and B) secondary structure is necessary for the dynamical transition. Secondary structure possibly induces sufficient ordering in the adjacent water to result in a fragile to strong glass transition resulting in increased protein flexibility [3]. [1] KAH Wildman et al. Solid State Nucl. Magn. Reson. 24 (2003) 94-109. [2] Yuguang Mu,et al. Proteins 58, (2005) 45-52. [3] S.H. Chen et al. PNAS (2006) 9012--9016.
Photoinduced phase transitions.
Bennemann, K H
2011-02-23
Optically induced ultrafast electronic excitations with sufficiently long lifetimes may cause strong effects on phase transitions like structural and nonmetal→metal ones and on supercooling, supersaturation, etc. Examples are the transitions diamond→graphite, graphite→graphene, non-metal→metal, solid→liquid and vapor→liquid, solid. Photoinduced formation of graphene and water condensation of saturated or supersaturated vapor due to increased bonding amongst water molecules are of particular interest. These nonequilibrium transitions are an ultrafast response, on a few hundred fs time scale, to the fast low to large energy electronic excitations. The energy of the photons is converted into electronic energy via electronic excitations changing the cohesive energy. This changes the chemical potential controlling the phase transition. In view of the advances in laser optics photon induced transitions are expected to become an active area in nonequilibrium physics and phase transition dynamics. Conservation laws like energy or angular momentum conservation control the time during which the transitions occur. Since the photon induced effects result from weakening or strengthening of the bonding between the atoms or molecules transitions like solid/liquid, etc can be shifted in both directions. Photoinduced transitions will be discussed from a unified point of view. PMID:21411879
Demixing of a binary symmetric mixture studied with transition path sampling.
Schöll-Paschinger, Elisabeth; Dellago, Christoph
2010-09-14
We present transition path sampling simulations of the nucleation of the demixing transition in a binary symmetric Lennard-Jones fluid. In this system the demixing transition takes place between two phases of the same density but different compositions. The appropriateness of the reaction coordinate of classical nucleation theory is examined. Using paths harvested with transition path sampling, we investigate the nucleation mechanism and analyze the properties of critical nuclei obtained by determining the transition state ensemble. Our simulations show that despite the fact that the densities of the coexisting phases are equal, the density of the growing cluster plays a crucial role in the nucleation process: nucleation tends to proceed either via small, compact clusters with densities below that of the metastable fluid or via large clusters with even lower densities. PMID:20849176
Suppression of Density Fluctuations in a Quantum Degenerate Fermi Gas
Sanner, Christian; Su, Edward J.; Keshet, Aviv; Gommers, Ralf; Shin, Yong-il; Huang Wujie; Ketterle, Wolfgang
2010-07-23
We study density profiles of an ideal Fermi gas and observe Pauli suppression of density fluctuations (atom shot noise) for cold clouds deep in the quantum degenerate regime. Strong suppression is observed for probe volumes containing more than 10 000 atoms. Measuring the level of suppression provides sensitive thermometry at low temperatures. After this method of sensitive noise measurements has been validated with an ideal Fermi gas, it can now be applied to characterize phase transitions in strongly correlated many-body systems.
Simulations of liquid ribidium expanded to the critical density
Ross, M; Yang, L H; Pilgrim, W
2006-05-16
Quantum molecular dynamic simulations were used to examine the change in atomic and electronic structure in liquid rubidium along its liquid-vapor coexistence curve. Starting from the liquid at the triple point, with increasing expansion we observe a continuous increase in the electron localization leading to ion clustering near the metal-nonmetal transition at about twice the critical density, in agreement with electrical measurements, and to the presence of dimers near and below the critical density.
Models for a liquid-liquid phase transition
NASA Astrophysics Data System (ADS)
Buldyrev, S. V.; Franzese, G.; Giovambattista, N.; Malescio, G.; Sadr-Lahijany, M. R.; Scala, A.; Skibinsky, A.; Stanley, H. E.
2002-02-01
We use molecular dynamics simulations to study two- and three-dimensional models with the isotropic double-step potential which in addition to the hard core has a repulsive soft core of larger radius. Our results indicate that the presence of two characteristic repulsive distances (hard core and soft core) is sufficient to explain liquid anomalies and a liquid-liquid phase transition, but these two phenomena may occur independently. Thus liquid-liquid transitions may exist in systems like liquid metals, regardless of the presence of the density anomaly. For 2D, we propose a model with a specific set of hard core and soft core parameters, that qualitatively reproduces the phase diagram and anomalies of liquid water. We identify two solid phases: a square crystal (high density phase), and a triangular crystal (low density phase) and discuss the relation between the anomalies of liquid and the polymorphism of the solid. Similarly to real water, our 2D system may have the second critical point in the metastable liquid phase beyond the freezing line. In 3D, we find several sets of parameters for which two fluid-fluid phase transition lines exist: the first line between gas and liquid and the second line between high-density liquid (HDL) and low-density liquid (LDL). In all cases, the LDL phase shows no density anomaly in 3D. We relate the absence of the density anomaly with the positive slope of the LDL-HDL phase transition line.
ERIC Educational Resources Information Center
Nesin, Gert; Barrow, Lloyd H.
1984-01-01
Describes a fourth-grade unit on density which introduces a concept useful in the study of chemistry and procedures appropriate to the chemistry laboratory. The hands-on activities, which use simple equipment and household substances, are at the level of thinking Piaget describes as concrete operational. (BC)
Multiple density layered insulator
Alger, T.W.
1994-09-06
A multiple density layered insulator for use with a laser is disclosed which provides at least two different insulation materials for a laser discharge tube, where the two insulation materials have different thermoconductivities. The multiple layer insulation materials provide for improved thermoconductivity capability for improved laser operation. 4 figs.
Multiple density layered insulator
Alger, Terry W.
1994-01-01
A multiple density layered insulator for use with a laser is disclosed wh provides at least two different insulation materials for a laser discharge tube, where the two insulation materials have different thermoconductivities. The multiple layer insulation materials provide for improved thermoconductivity capability for improved laser operation.
Material and Optical Densities
ERIC Educational Resources Information Center
Gluck, Paul
2007-01-01
The bending of a laser beam in a medium with a density and refractive index gradient in the same direction has been described previously. When a transparent container is half filled with a salt or sugar solution and an equal amount of water is floated on top of it, then diffusion will create a concentration gradient from top to bottom. A laser…
Partition density functional theory
NASA Astrophysics Data System (ADS)
Nafziger, Jonathan
Partition density functional theory (PDFT) is a method for dividing a molecular electronic structure calculation into fragment calculations. The molecular density and energy corresponding to Kohn Sham density-functional theory (KS-DFT) may be exactly recovered from these fragments. Each fragment acts as an isolated system except for the influence of a global one-body 'partition' potential which deforms the fragment densities. In this work, the developments of PDFT are put into the context of other fragment-based density functional methods. We developed three numerical implementations of PDFT: One within the NWChem computational chemistry package using basis sets, and the other two developed from scratch using real-space grids. It is shown that all three of these programs can exactly reproduce a KS-DFT calculation via fragment calculations. The first of our in-house codes handles non-interacting electrons in arbitrary one-dimensional potentials with any number of fragments. This code is used to explore how the exact partition potential changes for different partitionings of the same system and also to study features which determine which systems yield non-integer PDFT occupations and which systems are locked into integer PDFT occupations. The second in-house code, CADMium, performs real-space calculations of diatomic molecules. Features of the exact partition potential are studied for a variety of cases and an analytical formula determining singularities in the partition potential is derived. We introduce an approximation for the non-additive kinetic energy and show how this quantity can be computed exactly. Finally a PDFT functional is developed to address the issues of static correlation and delocalization errors in approximations within DFT. The functional is applied to the dissociation of H2 + and H2.
Transition to Old Age (Transition to Retirement).
ERIC Educational Resources Information Center
Bergman, Simon
Several conceptualizations and definitions of retirement have been proposed. One of them--the three-stage transition process--can be illustrated from studies in Israel: (1) leaving the old role; (2) going through the act of formal separation; and (3) adjusting to the new situation and role. Today's higher rate of survival into later years means…
Density fluctuations of polymers in disordered media.
Deutsch, J M; de la Cruz, M Olvera
2011-03-01
We study self-avoiding random walks in an environment where sites are excluded randomly, in two and three dimensions. For a single polymer chain, we study the statistics of the time averaged monomer density and show that these are well described by multifractal statistics. This is true even far from the percolation transition of the disordered medium. We investigate solutions of chains in a disordered environment and show that the statistics cease to be multifractal beyond the screening length of the solution. PMID:21517516
Transitions in Spousal Caregiving.
ERIC Educational Resources Information Center
Burton, Lynda C.; Zdaniuk, Bozena; Schulz, Richard; Jackson, Sharon; Hirsch, Calvin
2003-01-01
Describes transitions over 5 years among community-dwelling elderly spouses into and within caregiving roles and associated health outcomes. The trajectory of health outcomes associated with caregiving was generally downward. Those who transitioned to heavy caregiving had more symptoms of depression, and poorer self-reported health and health…
Good Transitions = Great Starts!
ERIC Educational Resources Information Center
Our Children: The National PTA Magazine, 2012
2012-01-01
The smooth transition of outgoing and incoming board members and officers is of vital importance and can determine the PTA's success for years to come. The transition process is the responsibility of both incoming and outgoing officers and board members. It gives closure to those leaving their positions and allows those coming in to be properly…
ERIC Educational Resources Information Center
Baker, Betty Ruth
Daily transitions in early childhood centers and classrooms include periods when children are completing one activity, preparing to begin a new activity, and moving from place to place in a room or building. Transition activities involve teaching techniques that prepare learners to listen, relax, sit down, move between locations or activities, and…
NASA Astrophysics Data System (ADS)
Anderson, Lara B.; Gray, James; Raghuram, Nikhil; Taylor, Washington
2016-04-01
We explore a novel type of transition in certain 6D and 4D quantum field theories, in which the matter content of the theory changes while the gauge group and other parts of the spectrum remain invariant. Such transitions can occur, for example, for SU(6) and SU(7) gauge groups, where matter fields in a three-index antisymmetric representation and the fundamental representation are exchanged in the transition for matter in the two-index antisymmetric representation. These matter transitions are realized by passing through superconformal theories at the transition point. We explore these transitions in dual F-theory and heterotic descriptions, where a number of novel features arise. For example, in the heterotic description the relevant 6D SU(7) theories are described by bundles on K3 surfaces where the geometry of the K3 is constrained in addition to the bundle structure. On the F-theory side, non-standard representations such as the three-index antisymmetric representation of SU( N) require Weierstrass models that cannot be realized from the standard SU( N) Tate form. We also briefly describe some other situations, with groups such as Sp(3), SO(12), and SU(3), where analogous matter transitions can occur between different representations. For SU(3), in particular, we find a matter transition between adjoint matter and matter in the symmetric representation, giving an explicit Weierstrass model for the F-theory description of the symmetric representation that complements another recent analogous construction.
Transitioning between Clerkship Directors
ERIC Educational Resources Information Center
Soltys, Stephen M.; Pary, Robert J.; Robinson, Stephen W.; Markwell, Stephen J.
2011-01-01
Objective: The authors report on succession-planning for mid-level academic positions. Method: The authors describe the process of succession-planning between clerkship directors and the smooth transition resulting in one case. Results: Gradually transitioning allowed a new faculty person to assume the clerkship-director position with minimal…
Researching Student Transitions
ERIC Educational Resources Information Center
Voorhees, Richard A.; Smith, Gregory P.; Luan, Jing
2006-01-01
This article sketches a research agenda for the further study of community college student transitions. Specific techniques are depicted as are potential data sources that can be used to pursue that agenda. The role of student tracking systems in transition research is discussed as well as the applicability of national surveys to the study of…
ERIC Educational Resources Information Center
Ilfeld, Ellen M., Ed.; Hanssen, Elizabeth, Ed.
1997-01-01
If children are to benefit from a healthy, supportive early childhood experience, it is important to strengthen transitions between early childhood experiences in educational and care settings and the more formal educational system. This issue of Coordinator's Notebook focuses on strengthening linkages and transitions between home, preschool, and…
Metabolic Constraints on the Eukaryotic Transition
NASA Astrophysics Data System (ADS)
Wallace, Rodrick
2009-04-01
Mutualism, obligate mutualism, symbiosis, and the eukaryotic ‘fusion’ of Serial Endosymbiosis Theory represent progressively more rapid and less distorted real-time communication between biological structures instantiating information sources. Such progression in accurate information transmission requires, in turn, progressively greater channel capacity that, through the homology between information source uncertainty and free energy density, requires ever more energetic metabolism. The eukaryotic transition, according to this model, may have been entrained by an ecosystem resilience shift from anaerobic to aerobic metabolism.
Pressure-induced phase transition in CrO2.
Alptekin, Sebahaddin
2015-12-01
The ab initio constant pressure molecular dynamics technique and density functional theory with generalized gradient approximation (GGA) was used to study the pressure-induced phase transition of CrO2. The phase transition of the rutile (P42/mnm) to the orthorhombic CaCl2 (Pnnm) structure at 30 GPa was determined successfully in a constant pressure simulation. This phase transition was analyzed from total energy calculations and, from the enthalpy calculation, occurred at around 17 GPa. Structural properties such as bulk modules, lattice parameters and phase transition were compared with experimental results. The phase transition at 12 ± 3 GPa was in good agreement with experimental results, as was the phase transition from the orthorhombic CaCl2 (Pnnm) to the monoclinic (P21/c) structure also found at 35 GPa. PMID:26541468
[Low density lipoprotein apheresis].
Zaliūnas, Remigijus; Slapikas, Rimvydas; Gustiene, Olivija; Siurkus, Jonas; Vaitkus, Eduardas
2003-01-01
Increased blood cholesterol concentration is one of the main factors in ischemic heart disease, development of which is determined by atherosclerotic changes in coronary vessels. Diet and treatment with 3-hydroxi-3-metilglutaril coenzyme A (HMG-CoA) reductase inhibitors helps to reduce low density lipoprotein cholesterol (LDL-Ch) blood concentration up to recommended level of 3.0 mmol/l in most patients but in some patients particularly with familial dyslipidemias cholesterol concentration remains increased even after treatment with maximal doses of lipid-regulating agents or their combinations. The most frequently used mechanical methods of cholesterol removal from blood include the procedures of extracorporeal apheresis. Low density lipoprotein (LDL) apheresis not only significantly reduces the blood concentrations of total cholesterol (TCh), and LDL-Ch, lipoprotein (a) (Lp(a) and fibrinogen but also stops the progression of atherosclerosis in coronary vessels. PMID:14704503
Extracting primordial density fluctuations
Gawiser; Silk
1998-05-29
The combination of detections of anisotropy in cosmic microwave background radiation and observations of the large-scale distribution of galaxies probes the primordial density fluctuations of the universe on spatial scales varying by three orders of magnitude. These data are found to be inconsistent with the predictions of several popular cosmological models. Agreement between the data and the cold + hot dark matter model, however, suggests that a significant fraction of the matter in the universe may consist of massive neutrinos. PMID:9603724
Dynamic transitions in molecular dynamics simulations of supercooled silicon
NASA Astrophysics Data System (ADS)
Mei, Xiaojun; Eapen, Jacob
2013-04-01
Two dynamic transitions or crossovers, one at a low temperature (T* ≈ 1006 K) and the other at a high temperature (T0 ≈ 1384 K), are shown to emerge in supercooled liquid silicon using molecular dynamics simulations. The high-temperature transition (T0) marks the decoupling of stress, density, and energy relaxation mechanisms. At the low-temperature transition (T*), depending on the cooling rate, supercooled silicon can either undergo a high-density-liquid to low-density-liquid (HDL-LDL) phase transition or experience an HDL-HDL crossover. Dynamically heterogeneous domains that emerge with supercooling become prominent across the HDL-HDL transition at 1006 K, with well-separated mobile and immobile regions. Interestingly, across the HDL-LDL transition, the most mobile atoms form large prominent aggregates while the least mobile atoms get spatially dispersed akin to that in a crystalline state. The attendant partial return to spatial uniformity with the HDL-LDL phase transition indicates a dynamic mechanism for relieving the frustration in supercooled states.
NASA Astrophysics Data System (ADS)
Pellemoine, Frederique
2013-12-01
In the context of new generation rare isotope beam facilities based on high-power heavy-ion accelerators and in-flight separation of the reaction products, the design of the rare isotope production targets is a major challenge. In order to provide high-purity beams for science, high resolution is required in the rare isotope separation. This demands a small beam spot on the production target which, together with the short range of heavy ions in matter, leads to very high power densities inside the target material. This paper gives an overview of the challenges associated with this high power density, discusses radiation damage issues in targets exposed to heavy ion beams, and presents recent developments to meet some of these challenges through different projects: FAIR, RIBF and FRIB which is the most challenging. Extensive use of Finite Element Analysis (FEA) has been made at all facilities to specify critical target parameters and R&D work at FRIB successfully retired two major risks related to high-power density and heavy-ion induced radiation damage.
Fast radial flows in transition disk holes
Rosenfeld, Katherine A.; Andrews, Sean M.; Chiang, Eugene
2014-02-20
Protoplanetary 'transition' disks have large, mass-depleted central cavities, yet also deliver gas onto their host stars at rates comparable to disks without holes. The paradox of simultaneous transparency and accretion can be explained if gas flows inward at much higher radial speeds inside the cavity than outside the cavity, since surface density (and by extension optical depth) varies inversely with inflow velocity at fixed accretion rate. Radial speeds within the cavity might even have to approach free-fall values to explain the huge surface density contrasts inferred for transition disks. We identify observational diagnostics of fast radial inflow in channel maps made in optically thick spectral lines. Signatures include (1) twisted isophotes in maps made at low systemic velocities and (2) rotation of structures observed between maps made in high-velocity line wings. As a test case, we apply our new diagnostic tools to archival Atacama Large Millimeter Array data on the transition disk HD 142527 and uncover evidence for free-fall radial velocities inside its cavity. Although the observed kinematics are also consistent with a disk warp, the radial inflow scenario is preferred because it predicts low surface densities that appear consistent with recent observations of optically thin CO isotopologues in this disk. How material in the disk cavity sheds its angular momentum wholesale to fall freely onto the star is an unsolved problem; gravitational torques exerted by giant planets or brown dwarfs are briefly discussed as a candidate mechanism.
A Posteriori Transit Probabilities
NASA Astrophysics Data System (ADS)
Stevens, Daniel J.; Gaudi, B. Scott
2013-08-01
Given the radial velocity (RV) detection of an unseen companion, it is often of interest to estimate the probability that the companion also transits the primary star. Typically, one assumes a uniform distribution for the cosine of the inclination angle i of the companion's orbit. This yields the familiar estimate for the prior transit probability of ~Rlowast/a, given the primary radius Rlowast and orbital semimajor axis a, and assuming small companions and a circular orbit. However, the posterior transit probability depends not only on the prior probability distribution of i but also on the prior probability distribution of the companion mass Mc, given a measurement of the product of the two (the minimum mass Mc sin i) from an RV signal. In general, the posterior can be larger or smaller than the prior transit probability. We derive analytic expressions for the posterior transit probability assuming a power-law form for the distribution of true masses, dΓ/dMcvpropMcα, for integer values -3 <= α <= 3. We show that for low transit probabilities, these probabilities reduce to a constant multiplicative factor fα of the corresponding prior transit probability, where fα in general depends on α and an assumed upper limit on the true mass. The prior and posterior probabilities are equal for α = -1. The posterior transit probability is ~1.5 times larger than the prior for α = -3 and is ~4/π times larger for α = -2, but is less than the prior for α>=0, and can be arbitrarily small for α > 1. We also calculate the posterior transit probability in different mass regimes for two physically-motivated mass distributions of companions around Sun-like stars. We find that for Jupiter-mass planets, the posterior transit probability is roughly equal to the prior probability, whereas the posterior is likely higher for Super-Earths and Neptunes (10 M⊕ - 30 M⊕) and Super-Jupiters (3 MJup - 10 MJup), owing to the predicted steep rise in the mass function toward smaller
Method to reduce dislocation density in silicon using stress
Buonassisi, Anthony; Bertoni, Mariana; Argon, Ali; Castellanos, Sergio; Fecych, Alexandria; Powell, Douglas; Vogl, Michelle
2013-03-05
A crystalline material structure with reduced dislocation density and method of producing same is provided. The crystalline material structure is annealed at temperatures above the brittle-to-ductile transition temperature of the crystalline material structure. One or more stress elements are formed on the crystalline material structure so as to annihilate dislocations or to move them into less harmful locations.
Integrated data analysis at TJ-II: The density profile
Milligen, B. Ph. van; Estrada, T.; Ascasibar, E.; Tafalla, D.; Lopez-Bruna, D.; Fraguas, A. Lopez; Jimenez, J. A.; Garcia-Cortes, I.; Dinklage, A.; Fischer, R.
2011-07-15
An integrated data analysis system based on Bayesian inference has been developed for the TJ-II stellarator. It reconstructs the electron density profile at a single time point, using data from interferometry, reflectometry, Thomson scattering, and the Helium beam, while providing a detailed error analysis. In this work, we present a novel analysis of the ambiguity inherent in profile reconstruction from reflectometry and show how the integrated data analysis approach elegantly resolves it. Several examples of the application of the technique are provided, in both low-density discharges with and without electrode biasing, and in high-density discharges with an (L-H) confinement transition.
Gedanken densities and exact constraints in density functional theory
Perdew, John P.; Ruzsinszky, Adrienn; Sun, Jianwei; Burke, Kieron
2014-05-14
Approximations to the exact density functional for the exchange-correlation energy of a many-electron ground state can be constructed by satisfying constraints that are universal, i.e., valid for all electron densities. Gedanken densities are designed for the purpose of this construction, but need not be realistic. The uniform electron gas is an old gedanken density. Here, we propose a spherical two-electron gedanken density in which the dimensionless density gradient can be an arbitrary positive constant wherever the density is non-zero. The Lieb-Oxford lower bound on the exchange energy can be satisfied within a generalized gradient approximation (GGA) by bounding its enhancement factor or simplest GGA exchange-energy density. This enhancement-factor bound is well known to be sufficient, but our gedanken density shows that it is also necessary. The conventional exact exchange-energy density satisfies no such local bound, but energy densities are not unique, and the simplest GGA exchange-energy density is not an approximation to it. We further derive a strongly and optimally tightened bound on the exchange enhancement factor of a two-electron density, which is satisfied by the local density approximation but is violated by all published GGA's or meta-GGA’s. Finally, some consequences of the non-uniform density-scaling behavior for the asymptotics of the exchange enhancement factor of a GGA or meta-GGA are given.
Predictability of critical transitions.
Zhang, Xiaozhu; Kuehn, Christian; Hallerberg, Sarah
2015-11-01
Critical transitions in multistable systems have been discussed as models for a variety of phenomena ranging from the extinctions of species to socioeconomic changes and climate transitions between ice ages and warm ages. From bifurcation theory we can expect certain critical transitions to be preceded by a decreased recovery from external perturbations. The consequences of this critical slowing down have been observed as an increase in variance and autocorrelation prior to the transition. However, especially in the presence of noise, it is not clear whether these changes in observation variables are statistically relevant such that they could be used as indicators for critical transitions. In this contribution we investigate the predictability of critical transitions in conceptual models. We study the quadratic integrate-and-fire model and the van der Pol model under the influence of external noise. We focus especially on the statistical analysis of the success of predictions and the overall predictability of the system. The performance of different indicator variables turns out to be dependent on the specific model under study and the conditions of accessing it. Furthermore, we study the influence of the magnitude of transitions on the predictive performance. PMID:26651760
Predictability of critical transitions
NASA Astrophysics Data System (ADS)
Zhang, Xiaozhu; Kuehn, Christian; Hallerberg, Sarah
2015-11-01
Critical transitions in multistable systems have been discussed as models for a variety of phenomena ranging from the extinctions of species to socioeconomic changes and climate transitions between ice ages and warm ages. From bifurcation theory we can expect certain critical transitions to be preceded by a decreased recovery from external perturbations. The consequences of this critical slowing down have been observed as an increase in variance and autocorrelation prior to the transition. However, especially in the presence of noise, it is not clear whether these changes in observation variables are statistically relevant such that they could be used as indicators for critical transitions. In this contribution we investigate the predictability of critical transitions in conceptual models. We study the quadratic integrate-and-fire model and the van der Pol model under the influence of external noise. We focus especially on the statistical analysis of the success of predictions and the overall predictability of the system. The performance of different indicator variables turns out to be dependent on the specific model under study and the conditions of accessing it. Furthermore, we study the influence of the magnitude of transitions on the predictive performance.
Density Gradients in Chemistry Teaching
ERIC Educational Resources Information Center
Miller, P. J.
1972-01-01
Outlines experiments in which a density gradient might be used to advantage. A density gradient consists of a column of liquid, the composition and density of which varies along its length. The procedure can be used in analysis of solutions and mixtures and in density measures of solids. (Author/TS)
Holographic magnetic phase transition
Lifschytz, Gilad; Lippert, Matthew
2009-09-15
We study four-dimensional interacting fermions in a strong magnetic field, using the holographic Sakai-Sugimoto model of intersecting D4- and D8-branes in the deconfined, chiral-symmetric parallel phase. We find that as the magnetic field is varied, while staying in the parallel phase, the fermions exhibit a first-order phase transition in which their magnetization jumps discontinuously. Properties of this transition are consistent with a picture in which some of the fermions jump to the lowest Landau level. Similarities to known magnetic phase transitions are discussed.
Shock-wave-based density down ramp for electron injection
NASA Astrophysics Data System (ADS)
Wang, Chunmei; Li, Ji; Sun, Jun; Luo, Xisheng
2012-02-01
We demonstrate a sharp density transition for electron injection in laser wakefield acceleration through numerical study. This density transition is generated by a detached shock wave induced by a cylinder inserted into a supersonic helium gas flow. In a Mach 1.5 flow, the scale length of the density transition Lgrad can approximately equal to plasma wavelength λp at the shock front, and can be further reduced with an increase of the flow Mach number. A density down ramp with Lgrad≥λp can reduce the phase velocity of the wakefield and lower the energy threshold for the electrons to be trapped. Moreover, the quality of the accelerated beam may be greatly improved by precisely controlling of Lgrad to be one λp. For an even sharper density down ramp with Lgrad≪λp, the oscillating electrons in the plasma wave will up shift their phase when crossing the ramp, therefore a fraction of the electrons are injected into the accelerating field. For this injection mechanism, there is no threshold requirement for the pump laser intensity to reach wave breaking, which is a big advantage as compared with other injection mechanisms.
High Energy Density Capacitors
2010-07-01
BEEST Project: Recapping is developing a capacitor that could rival the energy storage potential and price of today’s best EV batteries. When power is needed, the capacitor rapidly releases its stored energy, similar to lightning being discharged from a cloud. Capacitors are an ideal substitute for batteries if their energy storage capacity can be improved. Recapping is addressing storage capacity by experimenting with the material that separates the positive and negative electrodes of its capacitors. These separators could significantly improve the energy density of electrochemical devices.
Geomagnetic polarity transitions
NASA Astrophysics Data System (ADS)
Merrill, Ronald T.; McFadden, Phillip L.
1999-05-01
The top of Earth's liquid outer core is nearly 2900 km beneath Earth's surface, so we will never be able to observe it directly. This hot, dense, molten iron-rich body is continuously in motion and is the source of Earth's magnetic field. One of the most dynamic manifestations at Earth's surface of this fluid body is, perhaps, a reversal of the geomagnetic field. Unfortunately, the most recent polarity transition occurred at about 780 ka, so we have never observed a transition directly. It seems that a polarity transition spans many human lifetimes, so no human will ever witness the phenomenon in its entirety. Thus we are left with the tantalizing prospect that paleomagnetic records of polarity transitions may betray some of the secrets of the deep Earth. Certainly, if there are systematics in the reversal process and they can be documented, then this will reveal substantial information about the nature of the lowermost mantle and of the outer core. Despite their slowness on a human timescale, polarity transitions occur almost instantaneously on a geological timescale. This rapidity, together with limitations in the paleomagnetic recording process, prohibits a comprehensive description of any reversal transition both now and into the foreseeable future, which limits the questions that may at this stage be sensibly asked. The natural model for the geomagnetic field is a set of spherical harmonic components, and we are not able to obtain a reliable model for even the first few harmonic terms during a transition. Nevertheless, it is possible, in principle, to make statements about the harmonic character of a geomagnetic polarity transition without having a rigorous spherical harmonic description of one. For example, harmonic descriptions of recent geomagnetic polarity transitions that are purely zonal can be ruled out (a zonal harmonic does not change along a line of latitude). Gleaning information about transitions has proven to be difficult, but it does seem
Alternative fuel transit buses
Motta, R.; Norton, P.; Kelly, K.
1996-10-01
The National Renewable Energy Laboratory (NREL) is a U.S. Department of Energy (DOE) national laboratory; this project was funded by DOE. One of NREL`s missions is to objectively evaluate the performance, emissions, and operating costs of alternative fuel vehicles so fleet managers can make informed decisions when purchasing them. Alternative fuels have made greater inroads into the transit bus market than into any other. Each year, the American Public Transit Association (APTA) surveys its members on their inventory and buying plans. The latest APTA data show that about 4% of the 50,000 transit buses in its survey run on an alternative fuel. Furthermore, 1 in 5 of the new transit buses that members have on order are alternative fuel buses. This program was designed to comprehensively and objectively evaluate the alternative fuels in use in the industry.
Superconductivity in transition metals.
Slocombe, Daniel R; Kuznetsov, Vladimir L; Grochala, Wojciech; Williams, Robert J P; Edwards, Peter P
2015-03-13
A qualitative account of the occurrence and magnitude of superconductivity in the transition metals is presented, with a primary emphasis on elements of the first row. Correlations of the important parameters of the Bardeen-Cooper-Schrieffer theory of superconductivity are highlighted with respect to the number of d-shell electrons per atom of the transition elements. The relation between the systematics of superconductivity in the transition metals and the periodic table high-lights the importance of short-range or chemical bonding on the remarkable natural phenomenon of superconductivity in the chemical elements. A relationship between superconductivity and lattice instability appears naturally as a balance and competition between localized covalent bonding and so-called broken covalency, which favours d-electron delocalization and superconductivity. In this manner, the systematics of superconductivity and various other physical properties of the transition elements are related and unified. PMID:25666075
NASA Astrophysics Data System (ADS)
Kasper, Justin
2015-04-01
The heliosphere consists of the connective tissue of particles, fields and photons that mediate our interaction with the Sun and with interstellar space. Exploration of the heliosphere yields clues to the nature of environments we cannot reach ourselves, illuminating the composition of the solar interior, or the acceleration of cosmic rays in the galaxy. The heliosphere is also a laboratory for us to understand the fundamental physics of magnetized plasma, from heating and instabilities to coupling with neutral gas and dust. This talk will review some of the most exciting recent results in the heliosphere with a focus on transitions: what we can learn by exploring transitions within the heliosphere, how the heliosphere is responding to the long term transition in solar activity, and how our very view of the heliosphere is in transition with upcoming missions such as Solar Probe Plus, Solar Orbiter and IMAP.
NASA Technical Reports Server (NTRS)
Doody, Dave
1993-01-01
Aerobraking Magellan would provide the possibility of obtaining gravity field data for Venus all the way to the poles. Attempts to accomplish aerobraking, which began on May 25, 1993 in the Magellan Transition Experiment, are discussed.
Oligocyclopentadienyl transition metal complexes
de Azevedo, Cristina G.; Vollhardt, K. Peter C.
2002-01-18
Synthesis, characterization, and reactivity studies of oligocyclopentadienyl transition metal complexes, namely those of fulvalene, tercyclopentadienyl, quatercyclopentadienyl, and pentacyclopentadienyl(cyclopentadienyl) are the subject of this account. Thermal-, photo-, and redox chemistries of homo- and heteropolynuclear complexes are described.
The animation shows the difference between planet transit timing of single and multiple planet system. In tightly packed planetary systems, the gravitational pull of the planets among themselves ca...
Supercooling and phase coexistence in cosmological phase transitions
Megevand, Ariel; Sanchez, Alejandro D.
2008-03-15
Cosmological phase transitions are predicted by particle physics models, and have a variety of important cosmological consequences, which depend strongly on the dynamics of the transition. In this work we investigate in detail the general features of the development of a first-order phase transition. We find thermodynamical constraints on some quantities that determine the dynamics, namely, the latent heat, the radiation energy density, and the false-vacuum energy density. Using a simple model with a Higgs field, we study numerically the amount and duration of supercooling and the subsequent reheating and phase coexistence. We analyze the dependence of the dynamics on the different parameters of the model, namely, the energy scale, the number of degrees of freedom, and the couplings of the scalar field with bosons and fermions. We also inspect the implications for the cosmological outcomes of the phase transition.
NASA Astrophysics Data System (ADS)
Du, Yanjun; Peng, Zhimin; Ding, Yanjun; Sadeghi, Nader; Bruggeman, Peter J.
2016-08-01
The measurement of absolute densities of reactive species and radicals such as OH is of growing interest for many plasma applications. In this paper, we extend the use of a self-absorption model for atomic emission spectroscopy to molecular emission spectroscopy. The proposed analysis of self-absorbed molecular emission spectra is a simple and inexpensive method to determine OH(X) densities and rotational temperatures compared to laser induced fluorescence. We compare the recorded absolute OH density in a non-equilibrium diffuse atmospheric-pressure RF glow discharge by this method with broadband UV absorption considering a number of rotational lines with J‧ ⩽ 6.5, the detection limit of the line integrated OH(X) density with this method is of the order of 2 × 1019 m‑2. The accuracy of the density is sensitive to the rotational temperature of the OH(A) state and the non-equilibrium rotational population distribution.
47 CFR 61.48 - Transition rules for price cap formula calculations.
Code of Federal Regulations, 2013 CFR
2013-10-01
... price cap formula calculations. (a)-(h) (i) Transport and Special Access Density Pricing Zone Transition.... Price cap local exchange carriers that have established density pricing zones pursuant to § 69.123 of... initially establish density pricing zone SBIs and bands pursuant to the methodology in §§ 61.47(e)...
Stability, transition and turbulence
NASA Technical Reports Server (NTRS)
Hussaini, M. Y.
1987-01-01
A glimpse is provided of the research program in stability, transition and turbulence based on numerical simulations. This program includes both the so-called abrupt and the restrained transition processes. Attention is confined to the prototype problems of channel flow and the parallel boundary layer in the former category and the Taylor-Couette flow in the latter category. It covers both incompressible flows and supersonic flows. Some representative results are presented.
Stability, transition and turbulence
NASA Technical Reports Server (NTRS)
Hussaini, M. Y.
1987-01-01
A glimpse is provided of the research program in stability, transition, and turbulence based on numerical simulations. This program includes both the so-called abrupt and the restrained transition processes. Attention is confined to the prototype problems of channel flow and the parallel boundary layer in the former category and the Taylor-Couette flow in the latter category. It covers both incompressible flows and supersonic flows. Some representative results are presented.
Anderson, Lara B.; Gray, James; Raghuram, Nikhil; Taylor, Washington
2016-04-13
In this study, we explore a novel type of transition in certain 6D and 4D quantum field theories, in which the matter content of the theory changes while the gauge group and other parts of the spectrum remain invariant. Such transitions can occur, for example, for SU(6) and SU(7) gauge groups, where matter fields in a three-index antisymmetric representation and the fundamental representation are exchanged in the transition for matter in the two-index antisymmetric representation. These matter transitions are realized by passing through superconformal theories at the transition point. We explore these transitions in dual F-theory and heterotic descriptions, wheremore » a number of novel features arise. For example, in the heterotic description the relevant 6D SU(7) theories are described by bundles on K3 surfaces where the geometry of the K3 is constrained in addition to the bundle structure. On the F-theory side, non-standard representations such as the three-index antisymmetric representation of SU(N) require Weierstrass models that cannot be realized from the standard SU(N) Tate form. We also briefly describe some other situations, with groups such as Sp(3), SO(12), and SU(3), where analogous matter transitions can occur between different representations. For SU(3), in particular, we find a matter transition between adjoint matter and matter in the symmetric representation, giving an explicit Weierstrass model for the F-theory description of the symmetric representation that complements another recent analogous construction.« less
ON THE ORIGIN OF THE HIGH COLUMN DENSITY TURNOVER IN THE H I COLUMN DENSITY DISTRIBUTION
Erkal, Denis; Gnedin, Nickolay Y.; Kravtsov, Andrey V.
2012-12-10
We study the high column density regime of the H I column density distribution function and argue that there are two distinct features: a turnover at N{sub H{sub I}} Almost-Equal-To 10{sup 21} cm{sup -2}, which is present at both z = 0 and z Almost-Equal-To 3, and a lack of systems above N{sub H{sub I}} Almost-Equal-To 10{sup 22} cm{sup -2} at z = 0. Using observations of the column density distribution, we argue that the H I-H{sub 2} transition does not cause the turnover at N{sub H{sub I}} Almost-Equal-To 10{sup 21} cm{sup -2} but can plausibly explain the turnover at N{sub H{sub I}} {approx}> 10{sup 22} cm{sup -2}. We compute the H I column density distribution of individual galaxies in the THINGS sample and show that the turnover column density depends only weakly on metallicity. Furthermore, we show that the column density distribution of galaxies, corrected for inclination, is insensitive to the resolution of the H I map or to averaging in radial shells. Our results indicate that the similarity of H I column density distributions at z = 3 and 0 is due to the similarity of the maximum H I surface densities of high-z and low-z disks, set presumably by universal processes that shape properties of the gaseous disks of galaxies. Using fully cosmological simulations, we explore other candidate physical mechanisms that could produce a turnover in the column density distribution. We show that while turbulence within giant molecular clouds cannot affect the damped Ly{alpha} column density distribution, stellar feedback can affect it significantly if the feedback is sufficiently effective in removing gas from the central 2-3 kpc of high-redshift galaxies. Finally, we argue that it is meaningful to compare column densities averaged over {approx} kpc scales with those estimated from quasar spectra that probe sub-pc scales due to the steep power spectrum of H I column density fluctuations observed in nearby galaxies.
Ayala, A.L.; Ducati, M.B.G.; Levin, E.M.
1996-10-01
In this talk we present our detailed study (theory and numbers) on the shadowing corrections to the gluon structure functions for nuclei. Starting from rather controversial information on the nucleon structure function which is originated by the recent HERA data, we develop the Glauber approach for the gluon density in a nucleus based on Mueller formula and estimate the value of the shadowing corrections in this case. Then we calculate the first corrections to the Glauber approach and show that these corrections are big. Based on this practical observation we suggest the new evolution equation which takes into account the shadowing corrections and solve it. We hope to convince you that the new evolution equation gives a good theoretical tool to treat the shadowing corrections for the gluons density in a nucleus and, therefore, it is able to provide the theoretically reliable initial conditions for the time evolution of the nucleus-nucleus cascade. The initial conditions should be fixed both theoretically and phenomenologically before to attack such complicated problems as the mixture of hard and soft processes in nucleus-nucleus interactions at high energy or the theoretically reliable approach to hadron or/and parton cascades for high energy nucleus-nucleus interaction. 35 refs., 24 figs., 1 tab.
NASA Astrophysics Data System (ADS)
Piel, Alexander; Arp, Oliver; Menzel, Kristoffer; Klindworth, Markus
2007-11-01
We report on experimental observations of dust density waves in a complex (dusty) plasma under microgravity. The plasma is produced in a radio-frequency parallel-plate discharge (argon, p=15Pa, U=65Vpp). Different sizes of dust particles were used (3.4 μm and 6.4μm diameter). The low-frequency (f 11Hz) dust density waves are naturally unstable modes, which are driven by the ion flow in the plasma. Surprisingly, the wave propagation direction is aligned with the ion flow direction in the bulk plasma but becomes oblique at the boundary of the dust cloud with an inclination of 60^o with respect to the plasma boundary. The experimental results are compared with a kinetic model in the electrostatic approximation [1] and a fluid model [2]. Moreover, the role of dust surface waves is discussed. [1] M. Rosenberg, J. Vac. Sci. Technol. A 14, 631 (1996) [2] A. Piel et al, Phys. Rev. Lett. 97, 205009 (2006)
Energy level transitions of gas in a 2D nanopore
Grinyaev, Yurii V.; Chertova, Nadezhda V.; Psakhie, Sergei G.
2015-10-27
An analytical study of gas behavior in a 2D nanopore was performed. It is shown that the temperature dependence of gas energy can be stepwise due to transitions from one size-quantized subband to another. Taking into account quantum size effects results in energy level transitions governed by the nanopore size, temperature and gas density. This effect leads to an abrupt change of gas heat capacity in the nanopore at the above varying system parameters.
Experimental First Order Pairing Phase Transition in Atomic Nuclei
NASA Astrophysics Data System (ADS)
Moretto, L. G.; Larsen, A. C.; Giacoppo, F.; Guttormsen, M.; Siem, S.
2015-02-01
The natural log of experimental nuclear level densities at low energy is linear with energy. This can be interpreted in terms of a nearly 1st order phase transition from a superfluid to an ideal gas of quasi particles. The transition temperature coincides with the BCS critical temperature and yields gap parameters in good agreement with the values extracted from even- odd mass differences from rotational states. This converging evidence supports the relevance of the BCS theory to atomic nuclei.
NASA Astrophysics Data System (ADS)
Yong, Jie; Lemberger, T. R.; Benfatto, L.; Ilin, K.; Siegel, M.
2013-05-01
Occurrence of the Berezinskii-Kosterlitz-Thouless (BKT) transition is investigated by superfluid density measurements for two-dimensional (2D) disordered NbN films with disorder level very close to a superconductor-insulator transition (SIT). Our data show a robust BKT transition even near this 2D disorder-tuned quantum critical point. This observation is in direct contrast with previous data on deeply underdoped quasi-2D cuprates near the SIT. As our NbN films approach the quantum critical point, the vortex core energy, an important energy scale in the BKT transition, scales with the superconducting gap, not with the superfluid density, as expected within the standard 2D-XY model description of BKT physics.
Fujimoto, Kazuhiro J
2014-12-01
A transition charge, dipole, and quadrupole from electrostatic potential (TrESP-CDQ) method for electronic coupling calculations is proposed. The TrESP method is based on the classical description of electronic Coulomb interaction between transition densities for individual molecules. In the original TrESP method, only the transition charge interactions were considered as the electronic coupling. In the present study, the TrESP method is extended to include the contributions from the transition dipoles and quadrupoles as well as the transition charges. Hence, the self-consistent transition density is employed in the ESP fitting procedure. To check the accuracy of the present approach, several test calculations are performed to a helium dimer, a methane dimer, and an ethylene dimer. As a result, the TrESP-CDQ method gives a much improved description of the electronic coupling, compared with the original TrESP method. The calculated results also show that the self-consistent treatment to the transition densities contributes significantly to the accuracy of the electronic coupling calculations. Based on the successful description of the electronic coupling, the contributions to the electronic coupling are also analyzed. This analysis clearly shows a negligible contribution of the transition charge interaction to the electronic coupling. Hence, the distribution of the transition density is found to strongly influence the magnitudes of the transition charges, dipoles, and quadrupoles. The present approach is useful for analyzing and understanding the mechanism of excitation-energy transfer. PMID:25481127
Fujimoto, Kazuhiro J.
2014-12-07
A transition charge, dipole, and quadrupole from electrostatic potential (TrESP-CDQ) method for electronic coupling calculations is proposed. The TrESP method is based on the classical description of electronic Coulomb interaction between transition densities for individual molecules. In the original TrESP method, only the transition charge interactions were considered as the electronic coupling. In the present study, the TrESP method is extended to include the contributions from the transition dipoles and quadrupoles as well as the transition charges. Hence, the self-consistent transition density is employed in the ESP fitting procedure. To check the accuracy of the present approach, several test calculations are performed to a helium dimer, a methane dimer, and an ethylene dimer. As a result, the TrESP-CDQ method gives a much improved description of the electronic coupling, compared with the original TrESP method. The calculated results also show that the self-consistent treatment to the transition densities contributes significantly to the accuracy of the electronic coupling calculations. Based on the successful description of the electronic coupling, the contributions to the electronic coupling are also analyzed. This analysis clearly shows a negligible contribution of the transition charge interaction to the electronic coupling. Hence, the distribution of the transition density is found to strongly influence the magnitudes of the transition charges, dipoles, and quadrupoles. The present approach is useful for analyzing and understanding the mechanism of excitation-energy transfer.
Liquid-Liquid phase transition in a single component system
NASA Astrophysics Data System (ADS)
Franzese, Giancarlo; Skibinsky, Anna; Buldyrev, Sergey; Stanley, H. Eugene
2001-06-01
Recent experimental results indicate that phosphorus, a single-component system, can have a high-density liquid (HDL) and a low-density liquid (LDL) phase. A first-order LDL-HDL transition line ending in a critical point is consistent with experimental data and Molecular Dynamics (MD) simulations for a variety of single-component systems such as water, silica and carbon, but a coherent and general interpretation of the LDL-HDL transition is lacking. By means of MD, we show that the LDL-HDL transition can be directly related to an interaction potential with an attractive part and with not one but `two' preferred short-range repulsive distances. This kind of interaction is common to other single-component materials in the liquid state, in particular liquid metals. For the fisrt time, we show that the LDL-HDL transition can occur in systems with no density anomaly, opening an experimental challenge to uncover a liquid-liquid transition in systems like liquid metals, regardless of the presence of the density anomaly.
Low density microcellular foams
LeMay, J.D.
1991-11-19
Disclosed is a process of producing microcellular foam which comprises the steps of: (a) selecting a multifunctional epoxy oligomer resin; (b) mixing said epoxy resin with a non-reactive diluent to form a resin-diluent mixture; (c) forming a diluent containing cross-linked epoxy gel from said resin-diluent mixture; (d) replacing said diluent with a solvent therefore; (e) replacing said solvent with liquid carbon dioxide; and (f) vaporizing off said liquid carbon dioxide under supercritical conditions, whereby a foam having a density in the range of 35-150 mg/cc and cell diameters less than about 1 [mu]m is produced. Also disclosed are the foams produced by the process. 8 figures.
Low density microcellular foams
LeMay, James D.
1992-01-01
Disclosed is a process of producing microcellular from which comprises the steps of: (a) selecting a multifunctional epoxy oligomer resin; (b) mixing said epoxy resin with a non-reactive diluent to form a resin-diluent mixture; (c) forming a diluent containing cross-linked epoxy gel from said resin-diluent mixture; (d) replacing said diluent with a solvent therefore; (e) replacing said solvent with liquid carbon dioxide; and (f) vaporizing off said liquid carbon dioxide under supercritical conditions, whereby a foam having a density in the range of 35-150 mg/cc and cell diameters less than about 1 .mu.m is produced. Also disclosed are the foams produced by the process.
Low density microcellular foams
LeMay, James D.
1991-01-01
Disclosed is a process of producing microcellular foam which comprises the steps of: (a) selecting a multifunctional epoxy oligomer resin; (b) mixing said epoxy resin with a non-reactive diluent to form a resin-diluent mixture; (c) forming a diluent containing cross-linked epoxy gel from said resin-diluent mixture; (d) replacing said diluent with a solvent therefore; (e) replacing said solvent with liquid carbon dioxide; and (f) vaporizing off said liquid carbon dioxide under supercritical conditions, whereby a foam having a density in the range of 35-150 mg/cc and cell diameters less than about 1 .mu.m is produced. Also disclosed are the foams produced by the process.
Nuclear Energy Density Optimization
Kortelainen, Erno M; Lesinski, Thomas; More, J.; Nazarewicz, W.; Sarich, J.; Schunck, N.; Stoitsov, M. V.; Wild, S.
2010-01-01
We carry out state-of-the-art optimization of a nuclear energy density of Skyrme type in the framework of the Hartree-Fock-Bogoliubov (HFB) theory. The particle-hole and particle-particle channels are optimized simultaneously, and the experimental data set includes both spherical and deformed nuclei. The new model-based, derivative-free optimization algorithm used in this work has been found to be significantly better than standard optimization methods in terms of reliability, speed, accuracy, and precision. The resulting parameter set UNEDFpre results in good agreement with experimental masses, radii, and deformations and seems to be free of finite-size instabilities. An estimate of the reliability of the obtained parameterization is given, based on standard statistical methods. We discuss new physics insights offered by the advanced covariance analysis.
NASA Astrophysics Data System (ADS)
Mayo, L. A.; Odenwald, S. F.
2002-09-01
December 6th, 1882 was the last transit of the planet Venus across the disk of the sun. It was heralded as an event of immense interest and importance to the astronomical community as well as the public at large. There have been only six such occurrences since Galileo first trained his telescope on the heavens in 1609 and on Venus in 1610 where he concluded that Venus had phases like the moon and appeared to get larger and smaller over time. Many historians consider this the final nail in the coffin of the Ptolemaic, Earth centered solar system. In addition, each transit has provided unique opportunities for discovery such as measurement and refinement of the astronomical unit, calculation of longitudes on the earth, and detection of Venus' atmosphere. The NASA Sun Earth Connection Education Forum in partnership with the Solar System Exploration Forum, DPS, and a number of NASA space missions is developing plans for an international education program centered around the June 8, 2004 Venus transit. The transit will be visible in its entirety from Europe and partially from the East Coast of the United States. We will use a series of robotic observatories including the Telescopes In Education network distributed in latitude to provide observations of the transit that will allow middle and high school students to calculate the A.U. through application of parallax. We will also use Venus transit as a probe of episodes in American history (e.g. 1769: revolutionary era, 1882: post civil war era, and 2004: modern era). Museums and planetariums in the US and Europe will offer real time viewing of the transit and conduct educational programs through professional development seminars, public lectures, and planetarium shows. We are interested in soliciting advice from the research community to coordinate professional research interests with this program.
Interstellar Electron Density Spectra
NASA Astrophysics Data System (ADS)
Lambert, Hendrick Clark
This study concerns the investigation of the form of the wavenumber spectrum of the Galactic electron density fluctuations through an examination of the scattering of the radio pulses emitted by pulsars as they propagate through the diffuse ionized interstellar gas. A widely used model for the electron density spectrum is based on the simple power-law: Pne(q)∝ q-β, where β = 11/3 is usually assumed, corresponding to Kolmogorov's turbulence spectrum. The simple Kolmogorov model provides satisfactory agreement for observations along many lines of sight; however, major inconsistencies remain. The inconsistencies suggest that an increase in the ratio of the power between the high (10-8[ m]-1≤ q<=10-7[ m]-1) and low (10-13[ m]-1≤ q<=10-12[ m]-1) wavenumbers is needed. This enhancement in the ratio can in turn be achieved by either including an inner scale, corresponding to a dissipation scale for the turbulent cascade, in the Kolmogorov spectrum or by considering steeper spectra. Spectra with spectral exponents β > 4 have been in general rejected based on observations of pulsar refractive scintillations. The special case of β = 4 has been given little attention and is analyzed in detail. Physically, this 'β = 4' model corresponds to the random distribution, both in location and orientation, of discrete objects with relatively sharp boundaries across the line of sight. An outer scale is included in the model to account for the average size of such objects. We compare the predictions of the inner-scale and β = 4 models both with published observations and observations we made as part of this investigation. We conclude that the form of the wavenumber spectrum is dependent on the line of sight. We propose a composite spectrum featuring a uniform background turbulence in presence of randomly distributed discrete objects, as modeled by the β = model.
Low density microcellular foams
Aubert, James H.; Clough, Roger L.; Curro, John G.; Quintana, Carlos A.; Russick, Edward M.; Shaw, Montgomery T.
1987-01-01
Low density, microporous polymer foams are provided by a process which comprises forming a solution of polymer and a suitable solvent followed by rapid cooling of the solution to form a phase-separated system and freeze the phase-separated system. The phase-separated system comprises a polymer phase and a solvent phase, each of which is substantially continuous within the other. The morphology of the polymer phase prior to and subsequent to freezing determine the morphology of the resultant foam. Both isotropic and anisotropic foams can be produced. If isotropic foams are produced, the polymer and solvent are tailored such that the solution spontaneously phase-separates prior to the point at which any component freezes. The morphology of the resultant polymer phase determines the morphology of the resultant foam and the morphology of the polymer phase is retained by cooling the system at a rate sufficient to freeze one or both components of the system before a change in morphology can occur. Anisotropic foams are produced by forming a solution of polymer and solvent that will not phase separate prior to freezing of one or both components of the solution. In such a process, the solvent typically freezes before phase separation occurs. The morphology of the resultant frozen two-phase system determines the morphology of the resultant foam. The process involves subjecting the solution to essentially one-dimensional cooling. Means for subjecting such a solvent to one-dimensional cooling are also provided. Foams having a density of less than 0.1 g/cc and a uniform cell size of less than 10 .mu.m and a volume such that the foams have a length greater than 1 cm are provided.
Low density microcellular foams
Aubert, J.H.; Clough, R.L.; Curro, J.G.; Quintana, C.A.; Russick, E.M.; Shaw, M.T.
1985-10-02
Low density, microporous polymer foams are provided by a process which comprises forming a solution of polymer and a suitable solvent followed by rapid cooling of the solution to form a phase-separated system and freeze the phase-separated system. The phase-separated system comprises a polymer phase and a solvent phase, each of which is substantially continuous within the other. The morphology of the polymer phase prior to and subsequent to freezing determine the morphology of the resultant foam. Both isotropic and anisotropic foams can be produced. If isotropic foams are produced, the polymer and solvent are tailored such that the solution spontaneously phase-separates prior to the point at which any component freezes. The morphology of the resultant polymer phase determines the morphology of the reusltant foam and the morphology of the polymer phase is retained by cooling the system at a rate sufficient to freeze one or both components of the system before a change in morphology can occur. Anisotropic foams are produced by forming a solution of polymer and solvent that will not phase separate prior to freezing of one or both components of the solution. In such a process, the solvent typically freezes before phase separation occurs. The morphology of the resultant frozen two-phase system determines the morphology of the resultant foam. The process involves subjecting the solution to essentially one-dimensional cooling. Foams having a density of less than 0.1 g/cc and a uniform cell size of less than 10 ..mu..m and a volume such that the foams have a length greater than 1 cm are provided.
Transition radiation as a source of cosmic X-rays.
NASA Technical Reports Server (NTRS)
Ramaty, R.; Bleach, R. D.
1972-01-01
It is shown that transition radiation generated during the passage of relativistic charged particles through interstellar grains can be an important source of cosmic X-rays. In order to account for recent X-ray observations below 300 eV by transition radiation, an energy density in interstellar space of about 10 eV per cu cm in 10 MeV electrons is required. This seems to rule out transition radiation as an important source of diffuse cosmic X-rays in any energy region.
NASA Astrophysics Data System (ADS)
Dassoulas, John; McNutt, Ralph L.
2007-01-01
Transit, the US Navy's Navigation Satellite System was conceived at the Applied Physics Laboratory in 1957 by observing the Doppler shift while tracking Sputnik I. As spacecraft development proceeded there was concern about the ability of batteries to maintain the hermetic seal over a 5-year operational life requirement; therefore, alternate energy sources were investigated. The radioisotope thermoelectric generator (RTG) concept was pursued and resulted in the launch of SNAP 3s, providing partial power on both Transit 4A and 4B. SNAP 9s provided full power on three Transit 5BNs. All launches occurred in the early 1960s. When the U.S. conducted the high altitude nuclear test from Johnson Island, several spacecraft were lost due to artificial enhancement of charged particles in the Earth's magnetosphere resulting in rapid degradation of solar cell power production. This led to the decision to have both an RTG and Solar cell/battery design for Transit power systems; hence, a new RTG design, with a separable heat source and radiative coupling to the thermoelectric elements, was flown on TRIAD. This pioneering effort provided the impetus for future RTGs on interplanetary spacecraft. This paper describes the origin and purpose of the Transit program and provides details on the five satellites in that program that were powered by the first American RTGs used in space. The rationale and some of the challenges inherent in that use are also described.
Examining hydrogen transitions.
Plotkin, S. E.; Energy Systems
2007-03-01
This report describes the results of an effort to identify key analytic issues associated with modeling a transition to hydrogen as a fuel for light duty vehicles, and using insights gained from this effort to suggest ways to improve ongoing modeling efforts. The study reported on here examined multiple hydrogen scenarios reported in the literature, identified modeling issues associated with those scenario analyses, and examined three DOE-sponsored hydrogen transition models in the context of those modeling issues. The three hydrogen transition models are HyTrans (contractor: Oak Ridge National Laboratory), MARKAL/DOE* (Brookhaven National Laboratory), and NEMS-H2 (OnLocation, Inc). The goals of these models are (1) to help DOE improve its R&D effort by identifying key technology and other roadblocks to a transition and testing its technical program goals to determine whether they are likely to lead to the market success of hydrogen technologies, (2) to evaluate alternative policies to promote a transition, and (3) to estimate the costs and benefits of alternative pathways to hydrogen development.
First-order phase transitions in the real microcanonical ensemble
NASA Astrophysics Data System (ADS)
Schierz, Philipp; Zierenberg, Johannes; Janke, Wolfhard
2016-08-01
We present a simulation and data analysis technique to investigate first-order phase transitions and the associated transition barriers. The simulation technique is based on the real microcanonical ensemble where the sum of kinetic and potential energy is kept constant. The method is tested for the droplet condensation-evaporation transition in a Lennard-Jones system with up to 2048 particles at fixed density, using simple Metropolis-like sampling combined with a replica-exchange scheme. Our investigation of the microcanonical ensemble properties reveals that the associated transition barrier is significantly lower than in the canonical counterpart. Along the line of investigating the microcanonical ensemble behavior, we develop a framework for general ensemble evaluations. This framework is based on a clear separation between system-related and ensemble-related properties, which can be exploited to specifically tailor artificial ensembles suitable for first-order phase transitions.
The quark-hadron transition in cosmology and astrophysics.
Olive, K A
1991-03-01
A transition from normal hadronic matter (such as protons and neutrons) to quark-gluon matter is expected at both high temperatures and densities. In physical situations, this transition may occur in heavy ion collisions, the early universe, and in the cores of neutron stars. Astrophysics and cosmology can be greatly affected by such a phase transition. With regard to the early universe, big bang nucleosynthesis, the theory describing the primordial origin of the light elements, can be affected by inhomogeneities produced during the transition. A transition to quark matter in the interior by neutron stars further enhances our uncertainties regarding the equation of state of dense nuclear matter and neutron star properties such as the maximum mass and rotation frequencies. PMID:17799279
First-order phase transitions in the real microcanonical ensemble.
Schierz, Philipp; Zierenberg, Johannes; Janke, Wolfhard
2016-08-01
We present a simulation and data analysis technique to investigate first-order phase transitions and the associated transition barriers. The simulation technique is based on the real microcanonical ensemble where the sum of kinetic and potential energy is kept constant. The method is tested for the droplet condensation-evaporation transition in a Lennard-Jones system with up to 2048 particles at fixed density, using simple Metropolis-like sampling combined with a replica-exchange scheme. Our investigation of the microcanonical ensemble properties reveals that the associated transition barrier is significantly lower than in the canonical counterpart. Along the line of investigating the microcanonical ensemble behavior, we develop a framework for general ensemble evaluations. This framework is based on a clear separation between system-related and ensemble-related properties, which can be exploited to specifically tailor artificial ensembles suitable for first-order phase transitions. PMID:27627238
The Evolution of Transition Region Loops Using IRIS and AIA
NASA Technical Reports Server (NTRS)
Winebarger, Amy R.; DePontieu, Bart
2014-01-01
Over the past 50 years, the model for the structure of the solar transition region has evolved from a simple transition layer between the cooler chromosphere to the hotter corona to a complex and diverse region that is dominated by complete loops that never reach coronal temperatures. The IRIS slitjaw images show many complete transition region loops. Several of the "coronal" channels in the SDO AIA instrument include contributions from weak transition region lines. In this work, we combine slitjaw images from IRIS with these channels to determine the evolution of the loops. We develop a simple model for the temperature and density evolution of the loops that can explain the simultaneous observations. Finally, we estimate the percentage of AIA emission that originates in the transition region.
Slope-reversed Mott transition in multiorbital systems
NASA Astrophysics Data System (ADS)
Kim, Aaram J.; Choi, MooYoung; Jeon, Gun Sang
2015-09-01
We examine finite-temperature phase transitions in the two-orbital Hubbard model with different bandwidths by means of the dynamical mean-field theory combined with the continuous-time quantum Monte Carlo method. It is found that there emerges a peculiar slope-reversed first-order Mott transition between the orbital-selective Mott phase and the Mott insulator phase in the presence of Ising-type Hund's coupling. The origin of the slope-reversed phase transition is clarified by the analysis of the temperature dependence of the energy density. It turns out that the increase of Hund's coupling lowers the critical temperature of the slope-reversed Mott transition. Beyond a certain critical value of Hund's coupling the first-order transition turns into a finite-temperature crossover. We also reveal that the orbital-selective Mott phase exhibits frozen local moments in the wide orbital, which is demonstrated by the spin-spin correlation functions.
The electroweak phase transition in the Inert Doublet Model
Blinov, Nikita; Profumo, Stefano; Stefaniak, Tim
2015-07-21
We study the strength of a first-order electroweak phase transition in the Inert Doublet Model (IDM), where particle dark matter (DM) is comprised of the lightest neutral inert Higgs boson. We improve over previous studies in the description and treatment of the finite-temperature effective potential and of the electroweak phase transition. We focus on a set of benchmark models inspired by the key mechanisms in the IDM leading to a viable dark matter particle candidate, and illustrate how to enhance the strength of the electroweak phase transition by adjusting the masses of the yet undiscovered IDM Higgs states. We argue that across a variety of DM masses, obtaining a strong enough first-order phase transition is a generic possibility in the IDM. We find that due to direct dark matter searches and collider constraints, a sufficiently strong transition and a thermal relic density matching the universal DM abundance is possible only in the Higgs funnel regime.
Effect of Transition Aerodynamics on Aeroassist Flight Experiment Trajectories
NASA Technical Reports Server (NTRS)
Minier, Elizabeth A.; Suit, William T.
1988-01-01
Various transition methods are used here to study the viscous effects encountered in low density, hypersonic flight, through the transition from free molecular to continuum flow. Methods utilizing Viking data, Shuttle Orbiter data, a Potter number parameter, and a Shock Reynolds number were implemented in the Program to Optimize Simulated Trajectories (POST). Simulations of the Aeroassist Flight Experiment (AFE) using open loop guidance were used to assess the aerodynamic performance of the vehicle. A bank angle was found for each transition method that would result in a 200 nautical-mile apogee. Once this was done, the open loop guidance was replaced by the proposed guidance algorithm for the AFE. Simulations were again conducted using that guidance and the different transitions for comparison. For the gains used, the guidance system showed some sensitivity in apogee altitude to the transition method assumed, but the guidance was able to successfully complete the mission.
The variable density aircraft concept
NASA Technical Reports Server (NTRS)
Davenport, A. C.
1975-01-01
In the variable density aircraft concept the aircraft's density is varied by varying its volume. This is accomplished by combining a variable volume hull, which is called the dynapod, with intrinsic means for the controlled variation of a mass of working fluid or substance within the aircraft. The dynapod is a hinged structure and follows the volumetric variations of the working fluid. The result is a variable density hull, which with the attachment of power plants, etc., becomes a variable density aircraft.
Hydrogen crystallization in low-density aerogels.
Kucheyev, S O; Van Cleve, E; Johnston, L T; Gammon, S A; Worsley, M A
2015-04-01
Crystallization of liquids confined in disordered low-density nanoporous scaffolds is poorly understood. Here, we use relaxation calorimetry to study the liquid-solid phase transition of H2 in a series of silica and carbon (nanotube- and graphene-based) aerogels with porosities ≳94%. Results show that freezing temperatures of H2 inside all the aerogels studied are depressed but do not follow predictions of the Gibbs-Thomson theory based on average pore diameters measured by conventional gas sorption techniques. Instead, we find that, for each material family investigated, the depression of average freezing temperatures scales linearly with the ratio of the internal surface area (measured by gas sorption) and the total pore volume derived from the density of aerogel monoliths. The slope of such linear dependences is, however, different for silica and carbon aerogels, which we attribute to microporosity of carbons and the presence of macropores in silica aerogels. Our results have important implications for the analysis of pore size distributions of low-density nanoporous materials and for controlling crystallization of fuel layers in targets for thermonuclear fusion energy applications. PMID:25781182
Radiance Measurement for Low Density Mars Entry
NASA Technical Reports Server (NTRS)
Cruden, Brett A.
2012-01-01
We report measurements of radiance behind a shock wave in Martian simulant (96% CO2, 4% N2) atmosphere at conditions relevant for aerodynamic decelerators. Shock waves are generated in the NASA Ames Electric Arc Shock Tube (EAST) facility at velocities from 6-8 km/s and freestream densities from 1.2-5.9 x 10(exp -4) kilograms per cubic meter (0.05-0.25 Torr, corresponding to 35-50 km altitude). Absolute radiance is measured as a function of wavelength and position in the shock. Radiance measurements extend from the vacuum ultraviolet to near infrared (120-1650 nm). As at higher density/velocity, radiation is dominate by CO 4th positive radiation in the vacuum ultraviolet, though CN contribution is also significant. At most low density conditions, the shock does not relax to equilibrium over several centimeters. A small number of measurements in the mid-infrared were performed to quantify radiation from the fundamental vibrational transition in CO, and this is found to be a minor contributor to the overall radiance at these speeds. Efforts to extend test time and reliability in the 60 cm (24) shock tube will be discussed in the full paper.
Slowdown of Interhelical Motions Induces a Glass Transition in RNA
Frank, Aaron T.; Zhang, Qi; Al-Hashimi, Hashim M.; Andricioaei, Ioan
2015-01-01
RNA function depends crucially on the details of its dynamics. The simplest RNA dynamical unit is a two-way interhelical junction. Here, for such a unit—the transactivation response RNA element—we present evidence from molecular dynamics simulations, supported by nuclear magnetic resonance relaxation experiments, for a dynamical transition near 230 K. This glass transition arises from the freezing out of collective interhelical motional modes. The motions, resolved with site-specificity, are dynamically heterogeneous and exhibit non-Arrhenius relaxation. The microscopic origin of the glass transition is a low-dimensional, slow manifold consisting largely of the Euler angles describing interhelical reorientation. Principal component analysis over a range of temperatures covering the glass transition shows that the abrupt slowdown of motion finds its explanation in a localization transition that traps probability density into several disconnected conformational pools over the low-dimensional energy landscape. Upon temperature increase, the probability density pools then flood a larger basin, akin to a lakes-to-sea transition. Simulations on transactivation response RNA are also used to backcalculate inelastic neutron scattering data that match previous inelastic neutron scattering measurements on larger and more complex RNA structures and which, upon normalization, give temperature-dependent fluctuation profiles that overlap onto a glass transition curve that is quasi-universal over a range of systems and techniques. PMID:26083927
Slowdown of Interhelical Motions Induces a Glass Transition in RNA.
Frank, Aaron T; Zhang, Qi; Al-Hashimi, Hashim M; Andricioaei, Ioan
2015-06-16
RNA function depends crucially on the details of its dynamics. The simplest RNA dynamical unit is a two-way interhelical junction. Here, for such a unit--the transactivation response RNA element--we present evidence from molecular dynamics simulations, supported by nuclear magnetic resonance relaxation experiments, for a dynamical transition near 230 K. This glass transition arises from the freezing out of collective interhelical motional modes. The motions, resolved with site-specificity, are dynamically heterogeneous and exhibit non-Arrhenius relaxation. The microscopic origin of the glass transition is a low-dimensional, slow manifold consisting largely of the Euler angles describing interhelical reorientation. Principal component analysis over a range of temperatures covering the glass transition shows that the abrupt slowdown of motion finds its explanation in a localization transition that traps probability density into several disconnected conformational pools over the low-dimensional energy landscape. Upon temperature increase, the probability density pools then flood a larger basin, akin to a lakes-to-sea transition. Simulations on transactivation response RNA are also used to backcalculate inelastic neutron scattering data that match previous inelastic neutron scattering measurements on larger and more complex RNA structures and which, upon normalization, give temperature-dependent fluctuation profiles that overlap onto a glass transition curve that is quasi-universal over a range of systems and techniques. PMID:26083927
Tension density as counter force to the Lorentz force density
NASA Astrophysics Data System (ADS)
Nozaki, Hiroo; Senami, Masato; Ichikawa, Kazuhide; Tachibana, Akitomo
2016-08-01
It is confirmed numerically that the tension density defined in quantum field theory is the counter force to the Lorentz force density. We take benzenedithiol in a nonequilibrium steady state as an example for the numerical demonstration of the balance between these densities. While we use simply a nonequilibrium Green’s function method for a quantum conduction state instead of computations based on quantum field theory, the balance between the tension density and the Lorentz force density can be confirmed. The tension density is free from the relaxation time ansatz and defined as a local quantity. The tension density may give a novel viewpoint to the understanding of the physics of electrical conduction.
Scaling and universality in glass transition.
de Candia, Antonio; Fierro, Annalisa; Coniglio, Antonio
2016-01-01
Kinetic facilitated models and the Mode Coupling Theory (MCT) model B are within those systems known to exhibit a discontinuous dynamical transition with a two step relaxation. We consider a general scaling approach, within mean field theory, for such systems by considering the behavior of the density correlator 〈q(t)〉 and the dynamical susceptibility 〈q(2)(t)〉 - 〈q(t)〉(2). Focusing on the Fredrickson and Andersen (FA) facilitated spin model on the Bethe lattice, we extend a cluster approach that was previously developed for continuous glass transitions by Arenzon et al. (Phys. Rev. E 90, 020301(R) (2014)) to describe the decay to the plateau, and consider a damage spreading mechanism to describe the departure from the plateau. We predict scaling laws, which relate dynamical exponents to the static exponents of mean field bootstrap percolation. The dynamical behavior and the scaling laws for both density correlator and dynamical susceptibility coincide with those predicted by MCT. These results explain the origin of scaling laws and the universal behavior associated with the glass transition in mean field, which is characterized by the divergence of the static length of the bootstrap percolation model with an upper critical dimension dc = 8. PMID:27221056
Scaling and universality in glass transition
de Candia, Antonio; Fierro, Annalisa; Coniglio, Antonio
2016-01-01
Kinetic facilitated models and the Mode Coupling Theory (MCT) model B are within those systems known to exhibit a discontinuous dynamical transition with a two step relaxation. We consider a general scaling approach, within mean field theory, for such systems by considering the behavior of the density correlator 〈q(t)〉 and the dynamical susceptibility 〈q2(t)〉 − 〈q(t)〉2. Focusing on the Fredrickson and Andersen (FA) facilitated spin model on the Bethe lattice, we extend a cluster approach that was previously developed for continuous glass transitions by Arenzon et al. (Phys. Rev. E 90, 020301(R) (2014)) to describe the decay to the plateau, and consider a damage spreading mechanism to describe the departure from the plateau. We predict scaling laws, which relate dynamical exponents to the static exponents of mean field bootstrap percolation. The dynamical behavior and the scaling laws for both density correlator and dynamical susceptibility coincide with those predicted by MCT. These results explain the origin of scaling laws and the universal behavior associated with the glass transition in mean field, which is characterized by the divergence of the static length of the bootstrap percolation model with an upper critical dimension dc = 8. PMID:27221056
Scaling and universality in glass transition
NASA Astrophysics Data System (ADS)
de Candia, Antonio; Fierro, Annalisa; Coniglio, Antonio
2016-05-01
Kinetic facilitated models and the Mode Coupling Theory (MCT) model B are within those systems known to exhibit a discontinuous dynamical transition with a two step relaxation. We consider a general scaling approach, within mean field theory, for such systems by considering the behavior of the density correlator and the dynamical susceptibility
2. Focusing on the Fredrickson and Andersen (FA) facilitated spin model on the Bethe lattice, we extend a cluster approach that was previously developed for continuous glass transitions by Arenzon et al. (Phys. Rev. E 90, 020301(R) (2014)) to describe the decay to the plateau, and consider a damage spreading mechanism to describe the departure from the plateau. We predict scaling laws, which relate dynamical exponents to the static exponents of mean field bootstrap percolation. The dynamical behavior and the scaling laws for both density correlator and dynamical susceptibility coincide with those predicted by MCT. These results explain the origin of scaling laws and the universal behavior associated with the glass transition in mean field, which is characterized by the divergence of the static length of the bootstrap percolation model with an upper critical dimension dc = 8.
New melting transition in Quantum Hall systems
NASA Astrophysics Data System (ADS)
Simion, George; Lin, Tsuging; Watson, John D.; Manfra, Michael J.; Csathy, Gabor; Rokhinson, Leonid; Lyanda-Geller, Yuli
2014-03-01
We discover a new melting transition caused by topological excitations of two dimensional electrons in the quantum Hall regime. Experimentally, strain dependence of resistivity changes sign upon crossing filling-factor-specified boundaries of reentrant integer quantum Hall effect (RIQHE) states. This observation violates the symmetry of electron bubble crystal, whose melting was thought to be responsible for insulator to metal transition in the range of RIQHE filling factors. We demonstrate theoretically that electron bubbles become elongated in the vicinity of charge defects and form textures of finite size. Textures lower the energy of excitations. In the two-electron bubble crystal these textures form hedgehogs (vortices) around defects having (lacking) one extra electron. At low density these textures form an insulating Abrikosov lattice. At densities sufficient to cause the textures to overlap, their interactions are described by the XY-model and the defect lattice melts. This explains the sharp metal-insulator transition observed in finite temperature conductivity measurements. In this regime, melting is a function of several variables and forms a continuous phase boundary in the field-temperature (B - T) plane. Research was partially supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Awards DE-SC0010544 (Y.L-G), DE-SC0008630 (L.P.R.), DE-SC0006671 (G.S. and M.M.).
NASA Astrophysics Data System (ADS)
Anderson, Gregory Wayne
An analytic treatment of the one Higgs doublet, electroweak phase transition is given. The phase transition is first order, occurs by the nucleation of thin walled bubbles, and completes at a temperature where the order parameter,
Anderson, G.W.
1991-09-16
An analytic treatment of the one Higgs doublet, electroweak phase transition is given. The phase transition is first order, occurs by the nucleation of thin walled bubbles and completes at a temperature where the order parameter, {l angle}{phi}{r angle}{sub T} is significantly smaller than it is when the origin becomes absolutely unstable. The rate of anomalous baryon number violation is an exponentially function of {l angle}{phi}{r angle}{sub T}. In very minimal extensions of the standard model it is quite easy to increase {l angle}{phi}{r angle}{sub T} so that anomalous baryon number violation is suppressed after completion of the phase transition. Hence baryogenesis at the electroweak phase transition is tenable in minimal of the standard model. In some cases additional phase transitions are possible. For a light Higgs boson, when the top quark mass is sufficiently large, the state where the Higgs field has a vacuum expectation value {l angle}{phi}{r angle} = 246 GeV is not the true minimum of the Higgs potential. When this is the case, and when the top quark mass exceeds some critical value, thermal fluctuations in the early universe would have rendered the state {l angle}{phi}{r angle} = 246 GeV unstable. The requirement that the state {l angle}{phi}{r angle} = 246 GeV is sufficiently long lived constrains the masses of the Higgs boson and the top quark. Finally, we consider whether local phase transitions can be induced by heavy particles which act as seeds for deformations in the scalar field.
Anderson, G.W.
1991-09-16
An analytic treatment of the one Higgs doublet, electroweak phase transition is given. The phase transition is first order, occurs by the nucleation of thin walled bubbles and completes at a temperature where the order parameter, {l_angle}{phi}{r_angle}{sub T} is significantly smaller than it is when the origin becomes absolutely unstable. The rate of anomalous baryon number violation is an exponentially function of {l_angle}{phi}{r_angle}{sub T}. In very minimal extensions of the standard model it is quite easy to increase {l_angle}{phi}{r_angle}{sub T} so that anomalous baryon number violation is suppressed after completion of the phase transition. Hence baryogenesis at the electroweak phase transition is tenable in minimal of the standard model. In some cases additional phase transitions are possible. For a light Higgs boson, when the top quark mass is sufficiently large, the state where the Higgs field has a vacuum expectation value {l_angle}{phi}{r_angle} = 246 GeV is not the true minimum of the Higgs potential. When this is the case, and when the top quark mass exceeds some critical value, thermal fluctuations in the early universe would have rendered the state {l_angle}{phi}{r_angle} = 246 GeV unstable. The requirement that the state {l_angle}{phi}{r_angle} = 246 GeV is sufficiently long lived constrains the masses of the Higgs boson and the top quark. Finally, we consider whether local phase transitions can be induced by heavy particles which act as seeds for deformations in the scalar field.
Big Bang nucleosynthesis and the Quark-Hadron transition
NASA Technical Reports Server (NTRS)
Kurki-Suonio, Hannu; Matzner, Richard A.; Olive, Keith A.; Schramm, David N.
1989-01-01
An examination and brief review is made of the effects of quark-hadron transistion induced fluctuations on Big Bang nucleosynthesis. It is shown that cosmologically critical densities in baryons are difficult to reconcile with observation, but the traditional baryon density constraints from homogeneous calculations might be loosened by as much as 50 percent, to 0.3 of critical density, and the limit on the number of neutrino flavors remains about N(sub nu) is less than or approximately 4. To achieve baryon densities of greater than or approximately 0.3 of critical density would require initial density contrasts R is much greater the 10(exp 3), whereas the simplest models for the transition seem to restrict R to less than of approximately 10(exp 2).
The Quantum Hall Liquid to Insulator Transitions
NASA Astrophysics Data System (ADS)
Shahar, Dan
1996-03-01
We have conducted a systematic study of the quantum phase transitions between insulating and quantum Hall effect (QHE) phases of two dimensional electron system (2DES) at high magnetic fields (B<15.5 T) and low temperatures (T>20 mK). More than 100 samples were studied, with mobilities and densities (μ =1.2\\cdot 10^4-5\\cdot 10^6 cm^2/Vsec and n=8\\cdot 10^9-2.26\\cdot 10^11 cm-2, respectively) that effectively cover the whole range of 2DES samples that are currently available. We observed a remarkable similarity among the various transitions which transcends not only the vast differences in the parameters of our samples, but also the differences between the integer and the fractional QHE states. This similarity can be quantitatively expressed in terms of two parameters describing the transitions, the T scaling parameter, 1/ν z =0.45± 0.05 and the the critical resistivity at the transition, ρ_xxc=25.3± 6 kΩ, both of which are independent of sample parameters and transitions, within the errors specified. In the vicinity of the transitions, the I-V_xx traces are strongly non-linear, and exhibit a marked reflection symmetry between the traces in the QHE and those in the insulator, which we take as evidence for the existence of charge-flux duality symmetry near the transitions. These results support the predictions of the bosonic Chern-Simons theory recently developed by Kivelson, Lee and Zhang (KLZ),^1 to describe the interplay between the various phases of 2DES at high B. Finally, our study included ultra-high mobility samples that exhibit reentrant insulating phases near 1/5 and 1/3 fractional QHE state, which are explicitly forbidden in the framework developed by KLZ. Transitions involving these insulating phases show surprising similarities, and intriguing differences, to the allowed transitions. ^*In collaboration with D. C. Tsui, M. Shayegan, J. E. Cunningham, R. N. Bhatt, E. Shimshoni, S. L. Sondhi. [1] S. A. Kivelson, D. H. Lee, and S. C. Zhang, Phys
Hauschild, M; Elowe-Gruau, E; Dwyer, A; Aquarone, M-P; Unal, S; Jornayvaz, F R; Perrenoud, L; Gastaldi, G; Castellsague, M; Dirlewanger, M; Schwitzgebel, V M
2015-02-18
For patients with type I diabetes, transition from pediatric to adult care is a challenge due to complex treatment requirements and the physical, psychological and social changes of adolescence. Members of the care team must recognize that while these emerging adults need to develop self-management skills, this may conflict at times with the developmentally appropriate desire for increasing autonomy. The role of nursing in coordinating a successful transition is critical for maintaining continuity of patient-centered care that responds to the specific needs of these young adults. PMID:25915986
Network Observability Transitions
NASA Astrophysics Data System (ADS)
Yang, Yang; Wang, Jianhui; Motter, Adilson E.
2012-12-01
In the modeling, monitoring, and control of complex networks, a fundamental problem concerns the comprehensive determination of the state of the system from limited measurements. Using power grids as example networks, we show that this problem leads to a new type of percolation transition, here termed a network observability transition, which we solve analytically for the configuration model. We also demonstrate a dual role of the network’s community structure, which both facilitates optimal measurement placement and renders the networks substantially more sensitive to “observability attacks.” Aside from their immediate implications for the development of smart grids, these results provide insights into decentralized biological, social, and technological networks.
UTM: Universal Transit Modeller
NASA Astrophysics Data System (ADS)
Deeg, Hans J.
2014-12-01
The Universal Transit Modeller (UTM) is a light-curve simulator for all kinds of transiting or eclipsing configurations between arbitrary numbers of several types of objects, which may be stars, planets, planetary moons, and planetary rings. A separate fitting program, UFIT (Universal Fitter) is part of the UTM distribution and may be used to derive best fits to light-curves for any set of continuously variable parameters. UTM/UFIT is written in IDL code and its source is released in the public domain under the GNU General Public License.
NASA Technical Reports Server (NTRS)
2003-01-01
ProVision Technologies, a NASA research partnership center at Sternis Space Center in Mississippi, has developed a new hyperspectral imaging (HSI) system that is much smaller than the original large units used aboard remote sensing aircraft and satellites. The new apparatus is about the size of a breadbox. HSI may be useful to ophthalmologists to study and diagnose eye health, both on Earth and in space, by examining the back of the eye to determine oxygen and blood flow quickly and without any invasion. ProVision's hyperspectral imaging system can scan the human eye and produce a graph showing optical density or light absorption, which can then be compared to a graph from a normal eye. Scans of the macula, optic disk or optic nerve head, and blood vessels can be used to detect anomalies and identify diseases in this delicate and important organ. ProVision has already developed a relationship with the University of Alabama at Birmingham, but is still on the lookout for a commercial partner in this application.
High Energy Density Microwaves
Phillips, R.M.
1999-04-01
These proceedings represent papers presented at the RF98 Workshop entitled `High Energy Density Microwaves` held in California in October, 1998. The topics discussed were predominantly accelerator{minus}related. The Workshop dealt, for the most part, with the generation and control of electron beams, the amplification of RF signals, the design of mode converters, and the effect of very high RF field gradients. This Workshop was designed to address the concerns of the microwave tube industry worldwide, the plasma physicists who deal with very high beam currents and gigawatts of RF power, and researchers in accelerator centers around the world. Papers were presented on multibeam klystrons, gyrotron development, plasmas in microwave tubes, RF breakdown, and alternatives to conventional linear coliders at 1 TeV and above. The Workshop was partially sponsored by the US Department of Energy. There were 46 papers presented at the conference,out of which 19 have been abstracted for the Energy,Science and Technology database.(AIP)
Simulated Ionian Column Densities
NASA Astrophysics Data System (ADS)
Walker, Andrew C.; Goldstein, D. B.; Varghese, P. L.; Trafton, L. M.; Moore, C. H.
2010-10-01
The sublimation atmosphere of Io is modeled using the direct simulation Monte Carlo (DSMC) method. These three-dimensional simulations improve upon previous work by implementing a more accurate two-component surface temperature model. This surface temperature model solves the one-dimensional heat conduction equation with depth for every 1° by 1° surface element. It also includes the following physics: Jovian eclipse, reflected sunlight from Jupiter, latent heat of sublimation/condensation, hot spots, endogenic heating, and independent thermal inertias and albedos for the frost and non-frost surfaces. These simulations model only the dominant dayside atmospheric species, SO2. The non-equilibrium rotational and vibrational energy states of SO2 are treated as well as photo-emission from those states. Plasma heating of the atmosphere by high energy ions and electrons from the Jovian plasma torus is also modeled via a plasma energy flux. Resulting column densities are compared to recent observations in an attempt to constrain the thermal parameters for the frost and non-frost surfaces.
Density functional theory calculations of Rh-β-diketonato complexes.
Conradie, J
2015-01-28
Density functional theory (DFT) results on the geometry, energies and charges of selected Rh-β-diketonato reactants, products and transition states are discussed. Various DFT techniques are used to increase our understanding of the orientation of ligands coordinated to Rh, to identify the lowest energy geometry of possible geometrical isomers and to get a molecular orbital understanding of ground and transition states. Trends and relationships obtained between DFT calculated energies and charges, experimentally measured values and electronic parameters describing the electron donating power of groups and ligands, enable the design of ligands and complexes of specific reactivity. PMID:25429658
Carbon nanotube growth density control
NASA Technical Reports Server (NTRS)
Delzeit, Lance D. (Inventor); Schipper, John F. (Inventor)
2010-01-01
Method and system for combined coarse scale control and fine scale control of growth density of a carbon nanotube (CNT) array on a substrate, using a selected electrical field adjacent to a substrate surface for coarse scale density control (by one or more orders of magnitude) and a selected CNT growth temperature range for fine scale density control (by multiplicative factors of less than an order of magnitude) of CNT growth density. Two spaced apart regions on a substrate may have different CNT growth densities and/or may use different feed gases for CNT growth.
Generic mechanism for generating a liquid-liquid phase transition
NASA Astrophysics Data System (ADS)
Franzese, Giancarlo; Malescio, Gianpietro; Skibinsky, Anna; Buldyrev, Sergey V.; Stanley, H. Eugene
2001-02-01
Recent experimental results indicate that phosphorus-a single-component system-can have a high-density liquid (HDL) and a low-density liquid (LDL) phase. A first-order transition between two liquids of different densities is consistent with experimental data for a variety of materials, including single-component systems such as water, silica and carbon. Molecular dynamics simulations of very specific models for supercooled water, liquid carbon and supercooled silica predict a LDL-HDL critical point, but a coherent and general interpretation of the LDL-HDL transition is lacking. Here we show that the presence of a LDL and a HDL can be directly related to an interaction potential with an attractive part and two characteristic short-range repulsive distances. This kind of interaction is common to other single-component materials in the liquid state (in particular, liquid metals), and such potentials are often used to describe systems that exhibit a density anomaly. However, our results show that the LDL and HDL phases can occur in systems with no density anomaly. Our results therefore present an experimental challenge to uncover a liquid-liquid transition in systems like liquid metals, regardless of the presence of a density anomaly.
Transition Metal Nitrides: A First Principles Study
NASA Astrophysics Data System (ADS)
Pathak, Ashish; Singh, A. K.
2016-04-01
The present work describes the structural stability and electronic and mechanical properties of transition metal nitrides (TmNs: B1 cubic structure (cF8, Fm ‾ overline 3 m)) using first principles density functional theory (DFT) within generalized gradient approximation (GGA). The lattice constant of TmNs increases with increasing the atomic radii of the transition metals. Stability of the TmNs decreases from IVB to VIB groups due to increase in formation energy/atom. The bonding characteristics of these nitrides have been explained based on electronic density of states and charge density. All the TmNs satisfy Born stability criteria in terms of elastic constants except CrN and MoN that do not exist in equilibrium binary phase diagrams. The groups IVB and V-VIB nitrides are associated with brittle and ductile behaviour based on G/B ratios, respectively. The estimated melting temperatures of these nitrides exhibit reasonably good agreement with calculated with B than those of the C11 for all nitrides.
Dynamical phase transitions in quantum mechanics
NASA Astrophysics Data System (ADS)
Rotter, Ingrid
2012-02-01
The nucleus is described as an open many-body quantum system with a non-Hermitian Hamilton operator the eigenvalues of which are complex, in general. The eigenvalues may cross in the complex plane (exceptional points), the phases of the eigenfunctions are not rigid in approaching the crossing points and the widths bifurcate. By varying only one parameter, the eigenvalue trajectories usually avoid crossing and width bifurcation occurs at the critical value of avoided crossing. An analog spectroscopic redistribution takes place for discrete states below the particle decay threshold. By this means, a dynamical phase transition occurs in the many-level system starting at a critical value of the level density. Hence the properties of the low-lying nuclear states (described well by the shell model) and those of highly excited nuclear states (described by random ensembles) differ fundamentally from one another. The statement of Niels Bohr on the collective features of compound nucleus states at high level density is therefore not in contradiction to the shell-model description of nuclear (and atomic) states at low level density. Dynamical phase transitions are observed experimentally in different quantum mechanical systems by varying one or two parameters.
Paramagnetic ionic liquids for measurements of density using magnetic levitation.
Bwambok, David K; Thuo, Martin M; Atkinson, Manza B J; Mirica, Katherine A; Shapiro, Nathan D; Whitesides, George M
2013-09-01
Paramagnetic ionic liquids (PILs) provide new capabilities to measurements of density using magnetic levitation (MagLev). In a typical measurement, a diamagnetic object of unknown density is placed in a container containing a PIL. The container is placed between two magnets (typically NdFeB, oriented with like poles facing). The density of the diamagnetic object can be determined by measuring its position in the magnetic field along the vertical axis (levitation height, h), either as an absolute value or relative to internal standards of known density. For density measurements by MagLev, PILs have three advantages over solutions of paramagnetic salts in aqueous or organic solutions: (i) negligible vapor pressures; (ii) low melting points; (iii) high thermal stabilities. In addition, the densities, magnetic susceptibilities, glass transition temperatures, thermal decomposition temperatures, viscosities, and hydrophobicities of PILs can be tuned over broad ranges by choosing the cation-anion pair. The low melting points and high thermal stabilities of PILs provide large liquidus windows for density measurements. This paper demonstrates applications and advantages of PILs in density-based analyses using MagLev. PMID:23972068
Gravitational waves from the cosmological QCD transition
NASA Astrophysics Data System (ADS)
Mourão Roque, V. R. C.; Roque, G. Lugones o.; Lugones, G.
2014-09-01
We determine the minimum fluctuations in the cosmological QCD phase transition that could be detectable by the eLISA/NGO gravitational wave observatory. To this end, we performed several hydrodynamical simulations using a state-of-the-art equation of state derived from lattice QCD simulations. Based on the fact that the viscosity per entropy density of the quark gluon plasma obtained from heavy-ion collision experiments at the RHIC and the LHC is extremely small, we considered a non-viscous fluid in our simulations. Several previous works about this transition considered a first order transition that generates turbulence which follows a Kolmogorov power law. We show that for the QCD crossover transition the turbulent spectrum must be very different because there is no viscosity and no source of continuous energy injection. As a consequence, a large amount of kinetic energy accumulates at the smallest scales. From the hydrodynamic simulations, we have obtained the spectrum of the gravitational radiation emitted by the motion of the fluid, finding that, if typical velocity and temperature fluctuations have an amplitude Δ v /c ≳ 10-2 and/or Δ T/T_c ≳ 10-3, they would be detected by eLISA/NGO at frequencies larger than ˜ 10-4 Hz.
Shape transition during nest digging in ants
Toffin, Etienne; Di Paolo, David; Campo, Alexandre; Detrain, Claire; Deneubourg, Jean-Louis
2009-01-01
Nest building in social insects is among the collective processes that show highly conservative features such as basic modules (chambers and galleries) or homeostatic properties. Although ant nests share common characteristics, they exhibit a high structural variability, of which morphogenesis and underlying mechanisms remain largely unknown. We conducted two-dimensional nest-digging experiments under homogeneous laboratory conditions to investigate the shape diversity that emerges only from digging dynamics and without the influence of any environmental heterogeneity. These experiments revealed that, during the excavation, a morphological transition occurs because the primary circular cavity evolves into a ramified structure through a branching process. Such a transition is observed, whatever the number of ants involved, but occurs more frequently for a larger number of workers. A stochastic model highlights the central role of density effects in shape transition. These results indicate that nest digging shares similar properties with various physical, chemical, and biological systems. Moreover, our model of morphogenesis provides an explanatory framework for shape transitions in decentralized growing structures in group-living animals. PMID:19846774
Entrainment across density interfaces
NASA Astrophysics Data System (ADS)
Sanchez, M. A.; Carrillo, A.; Mahjoub, O. B.
2010-05-01
The structure of non-homogeneous turbulence affected by stratification and rotation is investigated both by means of laboratory and numerical experiments. The experiments investigate zero mean flow across a stably stratified density interface and are used to quantify the entrainment, the mixing efficiency and different types of dominant instability and the topological aspects of the turbulent cascades detected both horizontally and vertically [1,2]. Grid turbulence in a rotating stratified two layer system is measured with PIV as well as with sonic velocimetry. Observations of the horizontal and vertical velocity energy spectra as well as the structure functions are used to estimate local mixedness, entrainment and intermittency [3,4]. The method of estimation of the average eddy diffusivity from the time series images of a sharp density interface marked by fluoresceine also take anisotropy into account. but on the long run, horizontal ( and 2D type flow such as [5]) flow directions will average out so using a single integral length scale defined in Sanchez and Redondo(1998) varying in height will be enough together with the internal frequency. The method of calculating vertical fluxes in time allows to estimate different intermittency parameters as a function of local instability e.g. Kelvin/Helmholtz, Rayleigh-Taylor or Holbmoe[6-8]. Different concentration interfaces show different fractal dimensions, that are also a power function of the local Richardson number, this may be due to different levels of intermittency and thus different spectra, which are not necessarily inertial nor in equilibrium [8,9]. [1] Sanchez M.A. and Redondo J.M.Observations from Grid Stirred Turbulence. Applied Scientific Research 59, 191-204. 1998. [2] Redondo, J.M. and Cantalapiedra I.R. Mixing in Horizontally Heterogeneous Flows . Jour. Flow Turbulence and Combustion. 51, 217-222. 1993. [3] Castilla R, Redondo J.M., Gamez P.J., Babiano A. Coherent vortices and Lagrangian Dynamics in 2D