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1

Ionization By Impact Electrons in Solids: Electron Mean Free Path Fitted Over A Wide Energy Range  

SciTech Connect

We propose a simple formula for fitting the electron mean free paths in solids both at high and at low electron energies. The free-electron-gas approximation used for predicting electron mean free paths is no longer valid at low energies (E < 50 eV), as the band structure effects become significant at those energies. Therefore we include the results of the band structure calculations in our fit. Finally, we apply the fit to 9 elements and 2 compounds.

Ziaja, B; London, R A; Hajdu, J

2005-06-09

2

Evaluation of Calculated and Measured Electron Inelastic Mean Free Paths Near Solid Surfaces  

Microsoft Academic Search

An analysis is given of the consistency of calculated and measured electron inelastic mean free paths (IMFPs) near solid surfaces for electron energies between 50 and 104 eV, the energy range of relevance for surface analysis by Auger electron spectroscopy and x-ray photoelectron spectroscopy. This evaluation is based on IMFPs calculated from experimental optical data and on IMFPs measured by

C. J. Powell; A. Jablonski

1999-01-01

3

Electron inelastic mean free path, electron attenuation length, and low-energy electron-diffraction theory  

NASA Astrophysics Data System (ADS)

Low-energy electron diffraction (LEED) theory is used for describing the electron transport in crystalline solids with the purpose of determining the electron attenuation length. The inelastic scattering of the primary electron in the electron gas of the material is introduced into the LEED theory in terms of the electron inelastic mean free path derived by Tanuma, Powell, and Penn from the Lindhard dielectric function and optical data [Surf. Interface Anal. 17, 911 (1991)]. The theorem of flux reversal for electrons in situations of inward and outward propagation is deduced from local inversion symmetry and specific boundary conditions at the sources. The theory is applied to 50-400 eV electrons incident on the three low-index surfaces of copper, and a fair agreement is found with a previous Monte Carlo simulation of the electron transport in amorphous copper. In addition to the inelastic electron-electron gas scattering, the inelastic electron-phonon scattering has a significant effect on the attenuation length in a crystalline material. The temperature parameter, necessary in a LEED calculation, does not occur in current Monte Carlo simulations. Common scattering potential models, at low energy, for LEED and for Auger electron spectroscopy and x-ray photoemission spectroscopy are discussed.

Rundgren, J.

1999-02-01

4

Inelastic Interactions of Electrons with Polystyrene: Calculations of Mean Free Paths, Stopping Powers, and CSDA Ranges  

Microsoft Academic Search

A theoretical description of the inelastic interactions of electrons with solid polystyrene is presented. The response of the valence electrons to energy and momentum transfers is determined by a model insulator theory; carbon K-shell ionization cross sections are derived from atomic, generalized oscillator strengths. Contributions to the inverse mean free path and stopping power due to these two excitation processes

J. C. Ashley; C. J. Tung; R. H. Ritchie

1978-01-01

5

Arc-Evaporated Carbon Films: Optical Properties and Electron Mean Free Paths.  

National Technical Information Service (NTIS)

This paper describes briefly a method which can be used to calculate inelastic mean free paths for electrons with energies in the range of interest for the interpretation of surface phenomena. This method requires a knowledge of the optical properties of ...

M. W. Williams E. T. Arakawa S. M. Dolfini J. C. Ashley

1984-01-01

6

Mean free path limitation of thermoelectric properties of bismuth nanowire  

NASA Astrophysics Data System (ADS)

A limiting mean free path was considered in order to better understand the temperature and wire diameter dependence of the resistivity and Seebeck coefficient of bismuth microwire and nanowire samples. The mean free path limited mobility was numerically calculated from experimentally measured mobility in a bulk bismuth sample, and the electron and hole mobilities were dramatically decreased to a 10 ?m mean free path. Therefore, the temperature dependence of resistivity in very thin wire was quite different from that of a bulk sample, which had a positive temperature coefficient. The calculations showed that the temperature coefficient decreased gradually with decreasing mean free path, and the coefficient became negative for a mean free path of less than 1 ?m at about 150 K. The Seebeck coefficient was also calculated, but showed only a weak dependence on mean free path compared with the resistivity. Experimental comparisons were made to previous measurements of bismuth microwire or nanowire samples, and the temperature and wire diameter dependencies of the resistivity and Seebeck coefficient were qualitatively and quantitatively in very good agreement. Therefore, the temperature dependencies of nanowire samples over 850 nm in diameter were well described using the mean free path limitation.

Murata, Masayuki; Nakamura, Daiki; Hasegawa, Yasuhiro; Komine, Takashi; Taguchi, Takashi; Nakamura, Shinichiro; Jaworski, Christopher M.; Jovovic, Vladimir; Heremans, Joseph P.

2009-06-01

7

Inverse mean free path, stopping power, csda range, and straggling in polystyrene for electrons of energy < or = 10 keV. Interim technical report  

Microsoft Academic Search

The interaction of electrons with solid polystyrene, -(C⁸H⁸)-, is described, based on a model insulator theory to account for the response of the valence electrons, and carbon K-shell ionization cross sections derived from atomic, generalized oscillator strengths. Contributions to the inverse mean free path and energy loss due to these two excitation processes are tabulated for incident electrons with energies

J. C. Ashley; C. J. Tung; R. H. Ritchie; V. E. Anderson

1978-01-01

8

Electron inelastic mean free path formula and CSDA-range calculation in biological compounds for low and intermediate energies.  

PubMed

In this study, for low atomic number targets and biological compounds, an inelastic mean free path (IMFP) formula and energy straggling parameter formula are presented, being valid for low and high electron energies. In addition, calculation of the continuous slowing down approximation-range (CSDA-range) from the stopping power is also made. The IMFP and the energy straggling parameter formulae are evaluated using the generalized oscillator strength (GOS) model and the exchange correction to the inelastic differential cross section (IDCS) given by Inokuti, M., [1978. Inelastic collisions of fast charged particles with atoms and molecules--the Bethe theory revisited. Rev. Mod. Phys. 50, 23-35]. The IMFP and CSDA-range for the biological compounds C5H5N5 (adenine), C5H5N5O (guanine), C4H5N3O (cytosine), C5H6N2O2 (thymine), C20H27N7O13P2 (cytosine-guanine) and C19H26N8O13P2 (thymine-adenine) have been introduced for incident electrons in the energy range 20 eV-1 MeV. The calculated results are compared with semi-empirical results and other theoretical results, good agreement being found with experimental data and Monte Carlo (PENELOPE code) predictions. All the IMFP versus energy curves exhibit minima around 80 eV. PMID:16388951

Akar, Ay?egl; Gm?, Hasan; Okumu?o?lu, Nazmi T

2006-01-04

9

Evaluation of the inelastic mean free path (IMFP) of electrons in polyaniline and polyacetylene samples obtained from elastic peak electron spectroscopy (EPES)  

Microsoft Academic Search

The inelastic mean free path (IMFP) of electrons was determined experimentally for selected polyaniline and polyacetylene\\u000a samples with Ag and Ni references using elastic peak electron spectroscopy (EPES). The surface composition was determined\\u000a by XPS and density by helium pycnometry. The high resolution hemispherical ESA-31 and ADES-400 spectrometers were used for\\u000a measurements in the energy range E = 0.53.0 keV

Gyrgy Gergely; Mikls Menyhard; Attila Sulyok; Sndor Gurban; Beata Lesiak; Aleksander Jablonski; Andrzej Kosinski; Jzsef Toth; Dezso Varga

2007-01-01

10

Mean free paths of charge carriers in CZT crystal  

NASA Astrophysics Data System (ADS)

The asymmetrical distortion of the Cadmium Zinc Telluride ((CZT) energy spectrum is mainly caused by the hole trapping in the CZT crystal, and it can be characterized by the mean free path of hole. The mean free paths of the charge carriers in the CZT crystal can be extracted from fitting the peak shape of the measured energy spectrum. The energy spectra of ?-rays from 241Am, and that of ? particles from 238Pu were measured with a CZT with 555 mm3. The mean free path of the electron was determined from the bias dependence of ?-particle response. The energy spectra of ?-ray were simulated with EGSnrc code, in which Hecht equation was included, and the mean free path of the hole was determined by comparing the measured spectrum with the simulated one. The energy spectrum of 662 keV ?-ray was measured with the CZT detector, and it was compared with the simulated spectrum, in which newly determined mean free paths of the electron and the hole were used.

Park, Se-Hwan; Kim, Yong-Kyun; Jeon, Sung-Dae; Ha, Jang-Ho; Hong, Duk-Geun

2007-08-01

11

Inverse mean free path, stopping power, CSDA range, and straggling in aluminum and aluminum oxide for electrons of energy 10 keV  

Microsoft Academic Search

The interaction of electrons with the solids Al and Al2O3 is described based on the electron gas model for the conduction band electrons in Al, a model insulator theory for the valence electrons in Al2O3, and inner shell ionization derived from atomic, generalized oscillator strengths. Contributions to the inverse mean free path and stopping power from the various interaction processes

J. C. Ashley; C. J. Tung; W. E. Anderson; R. H. Ritchie

1975-01-01

12

Inverse mean free path, stopping power, CSDA range, and straggling in silicon and silicon dioxide for electrons of energy less than or equal to 10 keV  

Microsoft Academic Search

The interaction of electrons with the solids Si and SiO2 is described based on a 'modified electron gas model' for the valence band electrons in Si, a model insulator theory for the valence band electrons in SiO2, and inner shell ionization derived from atomic, generalized oscillator strengths. Contributions to the inverse mean free path and stopping power from the various

C. J. Tung; J. C. Ashley; V. E. Anderson; R. H. Ritchie

1976-01-01

13

Mean free path of inelastic electron scattering in elemental solids and oxides using transmission electron microscopy: Atomic number dependent oscillatory behavior  

SciTech Connect

Mean free path of inelastic electron scattering {lambda} has been measured with a 200 keV transmission electron microscope for the majority of stable elemental solids and their oxides. An oscillating behavior vs atomic number Z has been revealed, such that within one row of the Periodic Table, the minimum (maximum) of {lambda} is observed for elements with completed (empty) outer d shells. A significantly weaker {lambda}(Z) dependence is observed for the oxides. The {lambda}(Z) variation is ascribed to the three major factors: atomic density, number of 'free' electrons per atom, and contribution of atomic core-loss transitions.

Iakoubovskii, Konstantin; Mitsuishi, Kazutaka; Nakayama, Yoshiko; Furuya, Kazuo [Quantum Dot Research Center, National Institute for Materials Science, 3-13 Sakura, Tsukuba 305-0005 (Japan); High Voltage Microscopy Station, National Institute for Materials Science, 3-13 Sakura, Tsukuba 305-0005 (Japan)

2008-03-01

14

Inverse mean free path, stopping power, CSDA range, and straggling in Ge and GaAs for electrons of energy 10 keV  

Microsoft Academic Search

The interaction of electrons with the solids Ge and GaAs is described, based on a model insulator theory for the valence band electrons and inner shell ionization derived from classical-binary-collision cross sections. Contributions to the inverse mean free path and stopping power from the various interaction processes are tabulated for electron energies from threshold, approximately 2 eV for Ge and

J. C. Ashley; C. J. Tung; R. H. Ritchie; V. E. Anderson

1976-01-01

15

Determination of mass attenuation coefficients, effective atomic and electron numbers, mean free paths and kermas for PbO, barite and some boron ores  

NASA Astrophysics Data System (ADS)

The total mass attenuation coefficients, ?m, for PbO, barite, colemanite, tincal and ulexite were determined at 80.1, 302.9, 356.0, 661.7 and 1250.0 keV photon energies by using NaI (Tl) scintillation detector. Effective atomic number, Zeff, effective electron number, Neff, total atomic cross-section, ?t, total electronic cross-section, ?e, mean free path, mfp, and kerma relative to air were determined experimentally and theoretically. The theoretical mass attenuation coefficients were estimated using mixture rule. The calculated values were compared with the experimental values for all samples.

Un, A.; Sahin, Y.

2011-07-01

16

A new calculation on the stopping power and mean free path for low energy electrons in toluene over energy range of 20-10000 eV.  

PubMed

A new calculation of the stopping powers (SP) and inelastic mean free paths (IMFP) for electrons in toluene at energies below 10 keV has been presented. The calculation is based on the dielectric model and on an empirical evaluation approach of optical energy loss function (OELF). The reliability for the evaluated OELFs of several hydrocarbons with available experimental optical data has been systematically checked. For toluene, using the empirical OELF, the evaluated mean ionization potential, is compared with that given by Bragg's rule, and the calculated SP at 10 keV is also compared with the Bethe-Bloch prediction. The present results for SP and IMFP provide an alternative basic data for the study on the energy deposition of low-energy electrons transport through toluene, and also show that the method used in this work may be a good one for evaluating the SP and IMFP for hydrocarbons. PMID:19138526

Tan, Zhenyu; Xia, Yueyuan; Liu, Xiangdong; Zhao, Mingwen; Zhang, Liming

2008-12-06

17

Stopping power and mean free path for low-energy electrons in ten scintillators over energy range of 20-20,000 eV.  

PubMed

Systematic calculations of the stopping powers (SP) and inelastic mean free paths (IMFP) for 20-20,000eV electrons in a group of 10 important scintillators have been carried out. The calculations are based on the dielectric model including the Born-Ochkur exchange correction and the optical energy loss functions (OELFs) are empirically evaluated because of the lack of available experimental optical data for the scintillators under consideration. The evaluated OELFs are examined by both the f-sum rule and the calculation of mean ionization potential. The SP and IMFP data presented here are the first results for the 10 scintillators over the energy range of 20-20,000eV, and are of key importance for the investigation of liquid scintillation counting. PMID:21880497

Tan, Zhenyu; Xia, Yueyuan

2011-08-22

18

Electron stopping power and inelastic mean free path in amino acids and protein over the energy range of 20-20,000 eV.  

PubMed

Systematic calculations of stopping power (SPs) and inelastic mean free path (IMFP) values for 20-20,000 eV electrons in a group of 15 amino acids and a simple protein have been performed. The calculations are based on the dielectric response model and take into account the exchange effect between the incident electron and target electrons. The optical energy-loss functions for the 15 investigated amino acids and the protein are evaluated by using an empirical approach, because of the lack of experimental optical data. For all the considered materials, the calculated mean ionization potentials are in good agreement with those given by Bragg's rule, and the evaluated SP values at 20 keV converge well to the Bethe-Bloch predictions. The data shown represent the first results of SP and IMFP, for these 15 amino acids and the protein in the energy range below 20 keV, and might be useful for studies of various radiation effects in these materials. In addition, the average energy deposited by inelastic scattering of the electrons on this group of 15 amino acids, on the protein, on Formvar and on DNA, respectively, has been estimated for energies below 20 keV. The dependences of the average energy deposition on the electron energy are given. These results are important for any detailed studies of radiation-induced inactivation of proteins and the DNA. PMID:16733724

Tan, Zhenyu; Xia, Yueyuan; Zhao, Mingwen; Liu, Xiangdong

2006-05-30

19

Positron Mean Free Paths Between 50eV and 40KeV  

SciTech Connect

We have developed a formalism for calculating the inelastic mean free paths (IMFP's) of positrons by extending an existing semiempirical method developed by Tanuma, Powell, and Penn that is currently used for electrons. As examples, we have calculated the positron IMFP's in the 50 eV to 40 keV energy range for four different elements. In the intermediate-energy region, roughly 200 eV to 2 keV, the inelastic mean free paths for both positrons and electrons are similar. However, at lower energies, differences in correlation and exchange between the leptons during inelastic scattering events result in the inelastic mean free path being smaller for the positrons than for the electrons. At the highest energies, above about 2 keV, the relativistic correction suggested by Inokuti to the scattering cross section has been made. The resulting relativistic inelastic mean free paths are reasonable upper limits for both leptons below the bremsstrahlung regime.

Farrell, Helen Honora; Denison, A. B.

2004-03-01

20

Interplanetary scattering mean free path: Collisionless wave-damping effects  

SciTech Connect

Waves with small wavelengths are responsible for the scattering of particles with small values of ..mu.. (where ..mu.. is the cosine of the particle pitch angle). However, these waves are strongly damped in a thermal plasma by the process of collisionless cyclotron damping. This damping severely reduces theamplitude of the waves at small wavelengths and, in turn, results in a severely reduced scattering rate near ..mu.. = 0. This effect allows the mean free path for some particles to be much longer than the value estimated from standard quasi-linear theory. To demonstrate this point, a specific model for the observed interplanetary magnetic turbulence spectrum is chosen which is consistent with current observational constraints and also reflects the effects of collisionless wave damping. The consequences of these assumptions for the calculation of the interplanetary mean free path are explored. Specific predictions of the theory regarding the propagation of protons are best described by considering three separate kinetic energy regimes.

Davila, J.M.; Scott, J.S.

1984-10-01

21

Inverse mean free path, stopping power CSDA range, and straggling in Ni, Cu, and Au for electrons of energy S or equal 10 keV calculated from a statistical model  

Microsoft Academic Search

A statistical model is described and employed to calculate inverse mean free path, stopping power, and mean square energy loss for electrons of energy from 0.5 eV to 10 keV above the Fermi level in several solids. From these calculations, electron range in the continuous-slowing-down approximation and straggling are evaluated for electron energies from 10 eV to 10 keV. Table

J. C. Ashley; C. J. Tung; R. H. Ritchie; V. E. Anderson

1976-01-01

22

Neutrino mean free paths in cold symmetric nuclear matter  

SciTech Connect

The neutrino mean free paths (NMFP) for scattering and absorption in cold symmetric nuclear matter (SNM) are calculated using two-body effective interactions and one-body effective weak operators obtained from realistic models of nuclear forces using correlated basis theory. The infinite system is modeled in a box with periodic boundary conditions and the one particle-hole (p-h) response functions are calculated using the Tamm-Dancoff approximation (TDA). For the densities {rho}=(1/2), 1 (3/2){rho}{sub 0}, where {rho}{sub 0} is the equilibrium density of SNM, the strength of the response is shifted to higher energy transfers when compared to a noninteracting Fermi gas (FG). This and the weakness of effective operators compared to the bare operators, significantly reduces the cross sections, enhancing the NMFP by factors of {approx}2.5-3.5 at the densities considered. The NMFP at the equilibrium density {rho}{sub 0} are also calculated using the TDA and random phase approximation (RPA) using zero range Skyrme-like effective interactions with parameters chosen to reproduce the equation of state and spin-isospin susceptibilities of matter. Their results indicate that RPA corrections to correlated TDA may further increase the NMFP by {approx}25% to 3-4 times those in a noninteracting FG. Finally, the sums and the energy weighted sums of the Fermi and Gamow-Teller responses obtained from the correlated ground state are compared with those of the 1 p-h response functions to extract the sum and mean energies of multi p-h contributions to the weak response. The relatively large mean energy of the multi p-h excitations suggests that they may not contribute significantly to low energy NMFP.

Cowell, S.; Pandharipande, V.R. [Department of Physics, University of Illinois at Urbana-Champaign, 1110 W. Green St., Urbana, Illinois 61801 (United States)

2004-09-01

23

Broadband mean free path of diffuse light in polydisperse ensembles of scatterers for white light-emitting diode lighting.  

PubMed

We study the diffuse transport of light through polymer slabs containing TiO(2) scattering particles. The slabs are diffuser plates typical of a commercial white light-emitting diode (LED) module (Fortimo). We have measured the diffuse transmission and reflection properties over a broad wavelength range (470-840 nm) from which we derive the transport mean free path using the theory of light diffusion. With increasing scatterer density, the mean free path becomes shorter. The mean free path increases with wavelength; hence, blue light is scattered more strongly than red light. To interpret the results, we propose an ab initio model without adjustable parameters for the mean free path by using Mie theory. We include inhomogeneous broadening as a result of the size distribution of the scattering particles as measured by dynamic light scattering. Surprisingly, the calculated mean free path decreases with wavelength, at variance with our experiments, which is caused by particles with radii R in excess of 0.25 ?m. Close inspection of the scatterers by electron microscopy reveals that large particles (R>0.4 ?m) consist of clusters of small particles (R<0.13 ?m). Therefore, we have improved our model by only taking into account the individual scatterers within the clusters. This model predicts mean free paths in good agreement with our experimental results. We discuss consequences of our results to white LED lighting modules. PMID:23669667

Vos, Willem L; Tukker, Teus W; Mosk, Allard P; Lagendijk, Ad; IJzerman, Wilbert L

2013-04-20

24

Universal phonon mean free path spectra in crystalline semiconductors at high temperature  

NASA Astrophysics Data System (ADS)

Thermal conductivity in non-metallic crystalline materials results from cumulative contributions of phonons that have a broad range of mean free paths. Here we use high frequency surface temperature modulation that generates non-diffusive phonon transport to probe the phonon mean free path spectra of GaAs, GaN, AlN, and 4H-SiC at temperatures near 80 K, 150 K, 300 K, and 400 K. We find that phonons with MFPs greater than 230 +/- 120 nm, 1000 +/- 200 nm, 2500 +/- 800 nm, and 4200 +/- 850 nm contribute 50% of the bulk thermal conductivity of GaAs, GaN, AlN, and 4H-SiC near room temperature. By non-dimensionalizing the data based on Umklapp scattering rates of phonons, we identified a universal phonon mean free path spectrum in small unit cell crystalline semiconductors at high temperature.

Freedman, Justin P.; Leach, Jacob H.; Preble, Edward A.; Sitar, Zlatko; Davis, Robert F.; Malen, Jonathan A.

2013-10-01

25

Universal phonon mean free path spectra in crystalline semiconductors at high temperature.  

PubMed

Thermal conductivity in non-metallic crystalline materials results from cumulative contributions of phonons that have a broad range of mean free paths. Here we use high frequency surface temperature modulation that generates non-diffusive phonon transport to probe the phonon mean free path spectra of GaAs, GaN, AlN, and 4H-SiC at temperatures near 80?K, 150?K, 300?K, and 400?K. We find that phonons with MFPs greater than 230 120?nm, 1000 200?nm, 2500 800?nm, and 4200 850?nm contribute 50% of the bulk thermal conductivity of GaAs, GaN, AlN, and 4H-SiC near room temperature. By non-dimensionalizing the data based on Umklapp scattering rates of phonons, we identified a universal phonon mean free path spectrum in small unit cell crystalline semiconductors at high temperature. PMID:24129328

Freedman, Justin P; Leach, Jacob H; Preble, Edward A; Sitar, Zlatko; Davis, Robert F; Malen, Jonathan A

2013-10-16

26

Universal phonon mean free path spectra in crystalline semiconductors at high temperature  

PubMed Central

Thermal conductivity in non-metallic crystalline materials results from cumulative contributions of phonons that have a broad range of mean free paths. Here we use high frequency surface temperature modulation that generates non-diffusive phonon transport to probe the phonon mean free path spectra of GaAs, GaN, AlN, and 4H-SiC at temperatures near 80?K, 150?K, 300?K, and 400?K. We find that phonons with MFPs greater than 230 120?nm, 1000 200?nm, 2500 800?nm, and 4200 850?nm contribute 50% of the bulk thermal conductivity of GaAs, GaN, AlN, and 4H-SiC near room temperature. By non-dimensionalizing the data based on Umklapp scattering rates of phonons, we identified a universal phonon mean free path spectrum in small unit cell crystalline semiconductors at high temperature.

Freedman, Justin P.; Leach, Jacob H.; Preble, Edward A.; Sitar, Zlatko; Davis, Robert F.; Malen, Jonathan A.

2013-01-01

27

Wave-particle interaction in parallel transport of long mean-free-path plasmas along open field magnetic field lines  

NASA Astrophysics Data System (ADS)

A tokamak fusion reactor dumps a large amount of heat and particle flux to the divertor through the scrape-off plasma (SOL). Situation exists either by necessity or through deliberate design that the SOL plasma attains long mean-free-path along large segments of the open field lines. The rapid parallel streaming of electrons requires a large parallel electric field to maintain ambipolarity. The confining effect of the parallel electric field on electrons leads to a trap/passing boundary in the velocity space for electrons. In the normal situation where the upstream electron source populates both the trapped and passing region, a mechanism must exist to produce a flux across the electron trap/passing boundary. In a short mean-free-path plasma, this is provided by collisions. For long mean-free-path plasmas, wave-particle interaction is the primary candidate for detrapping the electrons. Here we present simulation results and a theoretical analysis using a model distribution function of trapped electrons. The dominating electromagnetic plasma instability and the associated collisionless scattering, that produces both particle and energy fluxes across the electron trap/passing boundary in velocity space, are discussed.

Guo, Zehua; Tang, Xianzhu

2012-03-01

28

Light exotic particles in strange matter: Energy flux and mean free path  

NASA Astrophysics Data System (ADS)

We calculate the axion and photino energy flux from degenerate strange quark and compare it with the corresponding flux from nucleon matter. We also calculate the mean free path of axions in strange matter and examine the constraints on axion decay constant.

Jha, R. N.; Goyal, Ashok; Anand, J. D.

1991-12-01

29

Light exotic particles in strange matter: energy flux and mean free path.  

NASA Astrophysics Data System (ADS)

The authors calculate the axion and photino energy flux from degenerate strange quark matter and compare it with the corresponding flux from nucleon matter. They also calculate the mean free path of axions in strange matter and examine the constraints on axion decay constant.

Iha, R. N.; Goyal, A.; Anand, J. D.

1991-12-01

30

An experimental method for calibration of the plasmon mean free path.  

PubMed

Transmission electron microscopy specimens in the form of elongated, conical needles were made using a dual-beam focused ion beam system, allowing the specimen thickness to be geometrically determined for a range of thickness values. From the same samples electron energy loss maps were acquired and the plasmon mean free path (lambda) for inelastic scattering was determined experimentally from the measured values of specimen thickness. To test the method lambda was determined for Ni (174 +/- 17 nm), alpha-Al(2)O(3) (143 +/- 14 nm), Si (199 +/- 20 nm) and amorphous SiO(2) (238 +/- 12 nm), and compared both to experimental values of lambda taken from the literature and to calculated values. The calculated values of lambda significantly underestimate the true sample thickness for high accelerating voltages (300 kV) and large collection angles. A linear dependence of lambda on thickness was confirmed for t/lambda < 0.5-0.6, but this method also provides an approach for calibrating lambda at sample thicknesses for which multiple scattering occurs, thus expanding the thickness range over which electron energy loss spectroscopy can be used to determine the absolute sample thickness (t/lambda > 0.6). The experimental method proposed in this contribution offers a means to calibrate lambda for any type of material or phase that can be milled using a focused ion beam system. PMID:19941556

Meltzman, H; Kauffmann, Y; Thangadurai, P; Drozdov, M; Baram, M; Brandon, D; Kaplan, W D

2009-12-01

31

Effects of mean free path on the preferentially orientated growth of AlN thin films  

NASA Astrophysics Data System (ADS)

AlN thin films grown respectively with (1 0 0) and (0 0 2) orientations were prepared by radio frequency (RF) magnetic sputtering method, under different working pressure or substrate target distance. The thin films were characterized by X-ray diffraction (XRD). Effects of the mean free path of Al atoms, correlated with the working pressure and target substrate distance, on the preferentially orientated growth of AlN thin films were investigated in detail. It was observed that when the ratio of the mean free path of Al atoms to the substrate target distance (R=?/d) was less than 0.14, the thin films were (1 0 0) preferentially orientated.

Chen, Yiping; Wang, Ruzhi; Wang, Bo; Xing, Tao; Song, Xuemei; Zhu, Mankang; Yan, Hui

2005-10-01

32

Theoretical explanation of the large observed cosmic ray parallel mean free paths in the solar system  

NASA Astrophysics Data System (ADS)

In recent observations large parallel mean free paths of charged cosmic rays which interact with the solar wind plasma have been discovered. The traditional approach to reproduce these measurements theoretically is the assumption that a large part of the magnetic fluctuations is in so-called 2D modes which are expected to be ineffective in scattering. By considering very recent results of particle scattering studies is is argued in this article that the assumption that 2D modes are ineffective in scattering is ruther questionable. By replacing the standard form of the wavespectrum of the magnetic fluctuations by a more realistic model an alternative explanation of observed cosmic ray parallel mean free paths in the solar system is provided by this article.

Shalchi, A.

2007-07-01

33

The Energy Dependence of the Fragmentation Parameters and Interaction Mean Free Paths in Nuclear Emulsion for Heavy Cosmic Ray Nuclei.  

National Technical Information Service (NTIS)

It is generally assumed that the fragmentation parameters and absorption mean free paths used in extrapolating the charge spectrum of cosmic ray nuclei to the top of the atmosphere are independent of energy. However recent results suggest that these quant...

T. E. Cleghorn

1967-01-01

34

Two-Dimensional Graphene with Structural Defects: Elastic Mean Free Path, Minimum Conductivity, and Anderson Transition  

NASA Astrophysics Data System (ADS)

Quantum transport properties of disordered graphene with structural defects (Stone-Wales and divacancies) are investigated using a realistic ?-?* tight-binding model elaborated from ab initio calculations. Mean free paths and semiclassical conductivities are then computed as a function of the nature and density of defects (using an order-N real-space Kubo-Greenwood method). By increasing the defect density, the decay of the semiclassical conductivities is predicted to saturate to a minimum value of 4e2/?h over a large range (plateau) of carrier density (>0.51014cm-2). Additionally, strong contributions of quantum interferences suggest that the Anderson localization regime could be experimentally measurable for a defect density as low as 1%.

Lherbier, Aurlien; Dubois, Simon M.-M.; Declerck, Xavier; Roche, Stephan; Niquet, Yann-Michel; Charlier, Jean-Christophe

2011-01-01

35

Magnetic field fluctuation geometry as a possible solution to the proton mean free path discrepancy problem  

NASA Astrophysics Data System (ADS)

Phenomenological values of the mean free path of solar energetic protons in the solar wind are compared to the values derived from measurements of the interplanetary magnetic fields by a standard quasilinear theory (QLT) for a slab geometry of the fields. For strongly scattering events the quality of QLT decreases systematically if the angle between the directions of the mean magnetic field and the solar wind velocity increases. We show that this surprising result can be interpreted as evidence for magnetic fluctuations with oblique wave number vectors, i.e. for field geometries differing from the slab model. We compare results from the standard slab model with those from three models with more complicated fluctuation geometries. Best results are obtained for a model with a 2D component of the fluctuations and for one with radially oriented wave vectors.

Jaekel, U.; Wanner, W.; Schlickeiser, R.; Wibberenz, G.

1994-11-01

36

Mean free path spectra as a tool to understand thermal conductivity in bulk and nanostructures  

NASA Astrophysics Data System (ADS)

A rigorous derivation is given for the thermal conductivity accumulation function and mean free path (MFP) spectrum of an isotropic bulk material. The key physical insight is to express the kinetic theory integral in terms of MFP rather than frequency. Extending this framework to incorporate boundary scattering in nanostructures leads to an integral equation that transforms a material's bulk MFP spectrum into the size-dependent thermal conductivity of the nanostructure. The kernel of this transform represents the boundary scattering rule for the particular type of nanostructure. The principal benefit of this transform is that it requires only a single function, the material's bulk MFP spectrum, or equivalently its accumulation function. Explicit knowledge of the material's dispersion relation and frequency-dependent bulk MFPs is not needed, nor is a summation over polarizations, because the bulk MFP spectrum already contains this information in exactly the form required to evaluate boundary scattering. The utility of this framework is demonstrated through a case study of six models for the phonon thermal conductivity of silicon: three analytical, one gray, and two numerical.

Yang, Fan; Dames, Chris

2013-01-01

37

NUCLEAR PHYSICS: Probing the Dissipation Mechanism in Ternary Reactions of 197Au+197Au by Mean Free Path of Nucleons  

NASA Astrophysics Data System (ADS)

the collision of very heavy nuclei 197Au+197Au at 15 A MeV has been studied within the improved quantum molecular dynamics model. A class of ternary events satisfying nearly complete balance of mass numbers is selected. The experimental mass distributions for the system 197Au+197Au ternary fission fragments, the heaviest (A1), the intermediate (A2) and the lightest (A3), are reproduced well. The mean free path of nucleons in the reaction system is studied and the shorter mean free path is responsible for the ternary fission with three mass comparable fragments, in which the two-body dissipation mechanism plays a dominant role.

Tian, Jun-Long; Li, Xian; Yan, Shi-Wei; Wu, Xi-Zhen; Li, Zhu-Xia

2009-08-01

38

Non-Fermi liquid correction to the neutrino mean free path and emissivity in neutron star beyond the leading order  

NASA Astrophysics Data System (ADS)

In this work we have derived the expressions of the mean free path (MFP) and emissivity of the neutrinos by incorporating non-Fermi liquid (NFL) corrections upto next to leading order (NLO). We have shown how such corrections affect the cooling of the neutron star composed of quark matter core.

Adhya, Souvik P.; Roy, P. K.; Dutt-Mazumder, Abhee K.

2013-04-01

39

Extraction of mean free path of charge carriers in CdZnTe crystals from measured full-energy peaks  

NASA Astrophysics Data System (ADS)

The mean free path of electrons (?e) and holes (?h) in a CdZnTe crystal were extracted from measured /?-ray spectra. First, distortion due to Gaussian broadening was unfolded from full-energy peaks in the /?-ray spectra. As a result, only tailing was left in the unfolded peaks. Second, ?e and ?h were determined by fitting tailing of full-energy peaks in calculated response functions to that in the unfolded /?-ray spectra. The response functions were calculated using EGS4 with the Hecht equation coded. 2.5102cm for ?e and 5.010-2cm for ?h were obtained in an electric field of 2500V/cm: the /?? products were found to be 1.010-1cm2/V for electrons and 2.010-5cm2/V for holes. This method is reasonable to obtain ?e and ?h, which are necessary to correct the distortion caused by tailing in X-ray spectra.

Miyajima, Satoshi; Sakuragi, Hideaki; Matsumoto, Masao

2002-06-01

40

The mean free path of mobile dislocations in doped NaCl single crystals measured by N.M.R. between room temperature and 300C  

Microsoft Academic Search

The mean free path of mobile dislocations has been measured by determining the spinlattice relaxation rate of deforming NaCl single crystals as a function of temperature and of the concentration of Ca impurities. The latter may influence the magnitude of the mean free path, but this depends to a large extent on the point-defect configuration. The degree of association and

W. H. M. Alsem; J. Th; M. de Hosson; R. Muenter; H. Tamler; O. Kanert

1982-01-01

41

Temperature-dependent ballistic transport in a channel with length below the scattering-limited mean free path  

NASA Astrophysics Data System (ADS)

The temperature-dependent ballistic transport, using nonequilibrium Arora distribution function (NEADF), is shown to result in mobility degradation with reduction in channel length, in direct contrast to expectation of a collision-free transport. The ballistic mean free path (mfp) is much higher than the scattering-limited long-channel mfp, yet the mobility is amazingly lower. High-field effects, converting stochastic velocity vectors to streamlined ones, are found to be negligible when the applied voltage is less than the critical voltage appropriate for a ballistic mfp, especially at cryogenic temperatures. Excellent agreement with the experimental data on a metal-oxide-semiconductor field-effect transistor is obtained. The applications of NEADF are shown to cover a wide spectrum, covering regimes from the scattering-limited to ballistic, from nondegenerate to degenerate, from nanowire to bulk, from low- to high-temperature, and from a low electric field to an extremely high electric field.

Arora, Vijay K.; Zainal Abidin, Mastura Shafinaz; Tan, Michael L. P.; Riyadi, Munawar A.

2012-03-01

42

The dependence of the parallel and perpendicular mean free paths on the rigidity of the solar energetic particles: Theoretical model versus observations  

NASA Astrophysics Data System (ADS)

Context. The dependence of the parallel and perpendicular mean free paths on the particle rigidity is an important topic in the studies of the diffusion and propagation of charged energetic particles in a large-scale turbulent magnetic field. Aims: In this work, we investigate the dependence of the parallel and perpendicular mean free paths on the rigidity of solar energetic particles (SEPs) by means of both the theoretical model and spacecraft observations with regard to several typical SEP events. Methods: A direct method developed by previous studies and derived from the focused transport equation and Taylor-Green-Kubo (TGK) formulation is applied to explicitly determine the parallel and perpendicular mean free paths of SEPs in a turbulent and spatially varying magnetic field. Results: We find that the parallel and perpendicular mean free paths, ?? and ??, of energetic protons monotonically decrease with increasing particle rigidity, and the ratio ??/?? monotonically increases with particle rigidity, when the magnetic turbulence is weak. Taking a series of typical SEP events together, it can also be seen that the ratio ??/?? of the perpendicular to the parallel mean free paths remains in the range of 0.001-0.2.

He, H.-Q.; Wan, W.

2013-09-01

43

A Direct Approach to Determine the Perpendicular Mean Free Path of Solar Energetic Particles in a Turbulent and Spatially Varying Magnetic Field  

NASA Astrophysics Data System (ADS)

A direct approach to explicitly determine the perpendicular mean free path of solar energetic particles (SEPs) with the influences of parallel diffusion and composite dynamical turbulence in a spatially varying magnetic field is presented. As theoretical applications of the direct approach, we investigate the inherent relations between the perpendicular mean free path and various parameters concerning physical properties of SEPs as well as those of interplanetary conditions such as solar wind and the turbulent magnetic field. Comparisons of the perpendicular mean free paths with and without adiabatic focusing are also presented. The direct method shows encouraging agreement with spacecraft observations, suggesting it is a reliable and useful tool in theoretical investigations and space weather forecasting. In the typical parameter regimes, ??(r) ? r0.29 and ??(r)-??(r) ? r-0.58 in the inner heliosphere; ??(r) ? r0.11 and ??(r)-??(r) ? r-0.22 in the outer heliosphere. Generally, the ratio ??-?? is in the range 0.01 - 0.20. However, when the turbulence strength ?B-B is sufficiently large, the ratio ??-?? would approach or exceed unity. The critical value of the turbulence strength ?B-B for 100 MeV protons is about 2.34, beyond which the perpendicular mean free path ?? becomes increasingly larger than the parallel mean free path ??.

He, Hongqing; Wan, Weixing

2013-04-01

44

Proton Inelastic Mean Free Path in a Group of Organic Materials in 0.05-10 MeV Range  

NASA Astrophysics Data System (ADS)

Inelastic mean free paths (MFPs) of 0.05-10 MeV protons in a group of 10 organic compounds are systematically calculated. The calculations are based on the method newly derived from the Ashley optical-data model and from the higher-order correction terms in stopping power calculations. Especially, in this method the new and empirical Bloch correction for the inelastic MFP is given. An evaluation for the optical energy loss function is incorporated into the present calculations because of the lack of available experimental optical data for the considered organic compounds expect for kapton. The proton inelastic MFPs for these 10 organic compounds in the energy range from 0.05 to 10 MeV are presented here for the first time, and the combination of these inelastic MFP data and our previous data of stopping power calculation for these bioorganic compounds may form a useful database for Monte Carlo track-structure studies of various radiation effects on these materials.

Tan, Zhen-Yu; Xia, Yue-Yuan; Zhao, Ming-Wen; Liu, Xiang-Dong

2010-11-01

45

A Direct Approach for Determining the Perpendicular Mean Free Path of Solar Energetic Particles in a Turbulent and Spatially Varying Magnetic Field  

NASA Astrophysics Data System (ADS)

A direct approach for explicitly determining the perpendicular mean free path of solar energetic particles (SEPs) influenced by parallel diffusion and composite dynamical turbulence in a spatially varying magnetic field is presented. As theoretical applications of the direct approach, we investigate the inherent relations between the perpendicular mean free path and various parameters concerning physical properties of SEPs as well as those of interplanetary conditions such as the solar wind and the turbulent magnetic field. Comparisons of the perpendicular mean free paths with and without adiabatic focusing are also presented. The direct method shows encouraging agreement with spacecraft observations, suggesting it is a reliable and useful tool for use in theoretical investigations and space weather forecasting.

He, H.-Q.; Wan, W.

2012-12-01

46

Parallel transport of long mean-free-path plasmas along open magnetic field lines: Plasma profile variation  

SciTech Connect

Parallel transport of long mean-free-path plasma along an open magnetic field line is characterized by strong temperature anisotropy, which is driven by two effects. The first is magnetic moment conservation in a non-uniform magnetic field, which can transfer energy between parallel and perpendicular degrees of freedom. The second is decompressional cooling of the parallel temperature due to parallel flow acceleration by conventional presheath electric field which is associated with the sheath condition near the wall surface where the open magnetic field line intercepts the discharge chamber. To the leading order in gyroradius to system gradient length scale expansion, the parallel transport can be understood via the Chew-Goldbeger-Low (CGL) model which retains two components of the parallel heat flux, i.e., q{sub n} associated with the parallel thermal energy and q{sub s} related to perpendicular thermal energy. It is shown that in addition to the effect of magnetic field strength (B) modulation, the two components (q{sub n} and q{sub s}) of the parallel heat flux play decisive roles in the parallel variation of the plasma profile, which includes the plasma density (n), parallel flow (u), parallel and perpendicular temperatures (T{sub Parallel-To} and T{sub Up-Tack }), and the ambipolar potential ({phi}). Both their profile (q{sub n}/B and q{sub s}/B{sup 2}) and the upstream values of the ratio of the conductive and convective thermal flux (q{sub n}/nuT{sub Parallel-To} and q{sub s}/nuT{sub Up-Tack }) provide the controlling physics, in addition to B modulation. The physics described by the CGL model are contrasted with those of the double-adiabatic laws and further elucidated by comparison with the first-principles kinetic simulation for a specific but representative flux expander case.

Guo Zehua; Tang Xianzhu [Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States)

2012-08-15

47

Parallel transport of long mean-free-path plasmas along open magnetic field lines: Plasma profile variation  

NASA Astrophysics Data System (ADS)

Parallel transport of long mean-free-path plasma along an open magnetic field line is characterized by strong temperature anisotropy, which is driven by two effects. The first is magnetic moment conservation in a non-uniform magnetic field, which can transfer energy between parallel and perpendicular degrees of freedom. The second is decompressional cooling of the parallel temperature due to parallel flow acceleration by conventional presheath electric field which is associated with the sheath condition near the wall surface where the open magnetic field line intercepts the discharge chamber. To the leading order in gyroradius to system gradient length scale expansion, the parallel transport can be understood via the Chew-Goldbeger-Low (CGL) model which retains two components of the parallel heat flux, i.e., qn associated with the parallel thermal energy and qs related to perpendicular thermal energy. It is shown that in addition to the effect of magnetic field strength (B) modulation, the two components (qn and qs) of the parallel heat flux play decisive roles in the parallel variation of the plasma profile, which includes the plasma density (n), parallel flow (u), parallel and perpendicular temperatures (T? and T?), and the ambipolar potential (?). Both their profile (qn/B and qs/B2) and the upstream values of the ratio of the conductive and convective thermal flux (qn/nuT? and qs/nuT?) provide the controlling physics, in addition to B modulation. The physics described by the CGL model are contrasted with those of the double-adiabatic laws and further elucidated by comparison with the first-principles kinetic simulation for a specific but representative flux expander case.

Guo, Zehua; Tang, Xian-Zhu

2012-08-01

48

The Energy Dependence of the Fragmentation Parameters and Mean Free Paths of Cosmic Ray Nuclei with Z Greater Than or Equal to 10(+).  

National Technical Information Service (NTIS)

The fragmentation parameters and interaction mean free paths for VH and H-type cosmic ray nuclei have been measured as functions of energy in nuclear emulsions. It is found that they are essentially independent of energy over the range from 100 MeV/n to 3...

T. F. Cleghorn P. S. Freier C. J. Waddington

1967-01-01

49

Flux pinning mechanism in BaFe1.9Ni0.1As2 single crystals: Evidence for fluctuation in mean free path induced pinning  

NASA Astrophysics Data System (ADS)

The flux pinning mechanism of BaFe1.9Ni0.1As2 superconducting crystals have been investigated systematically by magnetic measurements up to 13 T at various temperatures. The field dependence of the critical current density, Jc, was analysed within the collective pinning model. A remarkably good agreement between the experimental results and theoretical ?l pinning curve is obtained, which indicates that pinning in BaFe1.9Ni0.1As2 crystal originates from spatial variation of the mean free path. Moreover, the normalized pinning force density, Fp, curves versus h = B/Birr (Birr is the irreversibility field) were scaled using the Dew-Hughes model. Analysis suggests that point pinning alone cannot explain the observed field variation of Fp.

Shahbazi, M.; Wang, X. L.; Choi, K. Y.; Dou, S. X.

2013-07-01

50

Evaluations of proton inelastic mean free paths for 12 elemental solids over the energy range from 0.05 to 10 MeV  

NASA Astrophysics Data System (ADS)

The systematical calculations of the inelastic mean free paths (MFPs) of 0.05-10 MeV protons in 12 elemental solids (Al, Si, Ni, Cu, Mo, Rh, Ag, W, Os, Ir, Pt, Au) have been performed. The calculations are based on the algorithm derived from Ashleys optical-data model including the higher-order corrections to stopping power (SP) for protons. The prominence and necessity of the higher-order corrections are demonstrated by calculating the proton SPs for the 12 solids using Ashleys optical-data model and by comparing the calculated SPs with the experimental results, the tabulated values and other corresponding theoretical evaluations. The algorithm of evaluating the proton inelastic MFP is described. In this algorithm, the Barkas-effect correction and the Bloch correction are taken into account, the minimum impact parameter from Lindhard is used in the Barkas-effect correction, and an empirical estimation of a free parameter involved in the Bloch correction to the inelastic MFP is proposed. The evaluated inelastic MFPs of 0.05-10 MeV protons for the 12 solids under two different cases, i.e. the higher-order corrections not being considered and the Barkas-effect correction and the Bloch correction being included, are presented in the tabulated form and are first results for these solids. These numerical results provide an alternative basic data for the Monte Carlo studies on low-energy proton transport in these 12 solids.

Tan, Zhenyu; Xia, Yueyuan

2011-02-01

51

Proton inelastic mean free path in amino acids and protein over the energy range of 0.05-10 MeV  

NASA Astrophysics Data System (ADS)

The inelastic mean free paths (MFP) of 0.05-10 MeV protons in a group of 15 amino acids and a protein have been systematically calculated. The calculations are based on the method newly derived from the Ashleys optical-data model and from the higher-order correction terms in stopping power calculations. Especially, in this method the new and empirical Bloch correction for the inelastic MFP was given. An evaluation for the optical energy loss function is incorporated into the present calculations because of the lack of available experimental optical data for the bioorganic compounds under consideration. The proton inelastic MFPs for these 15 amino acids and the protein in the energy range from 0.05 to 10 MeV are presented here for the first time, and the combination of these inelastic MFP data and our previous data of the SP calculations for these bioorganic compounds may form a useful database for Monte Carlo track-structure studies of various radiation effects in these materials.

Tan, Zhenyu; Xia, Yueyuan; Liu, Xiangdong; Zhao, Mingwen

2010-09-01

52

Direct measurement of product of the electron mobility and mean free drift time of CdZnTe semiconductors using position sensitive single polarity charge sensing detectors  

NASA Astrophysics Data System (ADS)

This article describes novel techniques to directly measure the electron mobility and mean free drift time product ?e?e in semiconductor detectors. These methods are based on newly developed single polarity charge sensing and depth sensing techniques. Compared with conventional methods based on the Hecht relation, the new methods do not involve curve fitting, are less sensitive to the variation of pulse rise times, and allow the use of higher energy ? rays typical of many applications.

He, Z.; Knoll, G. F.; Wehe, D. K.

1998-11-01

53

Direct measurement of product of the electron mobility and mean free drift time of CdZnTe semiconductors using position sensitive single polarity charge sensing detectors  

SciTech Connect

This article describes novel techniques to directly measure the electron mobility and mean free drift time product {mu}{sub e}{tau}{sub e} in semiconductor detectors. These methods are based on newly developed single polarity charge sensing and depth sensing techniques. Compared with conventional methods based on the Hecht relation, the new methods do not involve curve fitting, are less sensitive to the variation of pulse rise times, and allow the use of higher energy {gamma} rays typical of many applications. {copyright} {ital 1998 American Institute of Physics.}

He, Z.; Knoll, G.F.; Wehe, D.K. [Department of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, Michigan 48109 (United States)

1998-11-01

54

The Relation between Relaxation Time, Mean Free Path, Collision Time and Drift Velocity--Pitfalls and a Proposal for an Approach Illustrating the Essentials  

ERIC Educational Resources Information Center

|The collision model is frequently introduced to describe electronic conductivity in solids. Depending on the chosen approach, the introduction of the collision time can lead to erroneous results for the average velocity of the electrons, which enters the expression for the electrical conductivity. In other textbooks, correct results are obtained

Jakoby, Bernhard

2009-01-01

55

Mean free path in soccer and gases  

NASA Astrophysics Data System (ADS)

The trajectories of the molecules in an ideal gas and of the ball in a soccer game are compared. The great difference between these motions and some similarities are discussed. This example could be suitable for discussing many concepts in kinetic theory in a way that can be pictured by students for getting a more intuitive understanding. It could be suitable for an introductory course in vacuum techniques or undergraduate courses in kinetic theory of gases. Without going into the slightly harder quantitative results, the analysis presented might be used for introducing some ideas of kinetic theory qualitatively to high school students.

Luzuriaga, J.

2010-09-01

56

Mean Free Path in Soccer and Gases  

ERIC Educational Resources Information Center

|The trajectories of the molecules in an ideal gas and of the ball in a soccer game are compared. The great difference between these motions and some similarities are discussed. This example could be suitable for discussing many concepts in kinetic theory in a way that can be pictured by students for getting a more intuitive understanding. It

Luzuriaga, J.

2010-01-01

57

A DRIFT ORDERED SHORT MEAN-FREE DESCRIPTION FOR PARTIALLY IONIZED MAGNETIZED PLASMA  

SciTech Connect

Effects of neutral particles, most prominently the associated heat flux and viscosity, can be very important or even dominant at the edge of a tokamak and so must be self-consistently accounted for in a description of magnetized tokamak edge plasma. To the best of our knowledge, this has only been done so far for short mean-free path plasma under MHD-like Braginskii's orderings i.e. assuming that species velocities are on the order of the ion thermal speed. Since plasma flows in modern tokamaks are usually slow compared with the ion thermal speed (at least in the absence of strong external momentum sources) it is more appropriate to use drift orderings in which the plasma flow velocity is instead comparable with the diamagnetic heat flow divided by pressure. Employing drift orderings and evaluating species distribution functions through second order in the small gyroradius and mean-free path expansion parameters allows accounting for the important effects of heat fluxes on species momentum transport (viscosities), which are missing from the large flow ordered treatments. In this work we consider short mean-free path plasma consisting of electrons and single species of singly-charged ions and neutrals. We neglect neutral-neutral and elastic electron-neutral collisions and approximate the neutral-ion charge-exchange cross-section with a constant. We employ drift orderings to evaluate ion, neutral, and electron heat fluxes, viscosity tensors, and momentum and energy exchange terms and formulate a self-consistent system of electron, ion, and neutral fluid equations, thereby generalizing the drift-ordered treatment of fully ionized plasma.

SIMAKOV, ANDERI N. [Los Alamos National Laboratory

2007-02-08

58

Phonon-mediated path-interference in electronic energy transfer  

NASA Astrophysics Data System (ADS)

Motivated by the recent observations of quantum coherence in light-harvesting antennae, we present a formalism to quantify the contribution of path-interference in phonon-mediated electronic energy transfer. The transfer rate between two molecules is computed by considering the quantum mechanical amplitudes associated with pathways connecting the initial and final sites. This includes contributions from classical pathways, but also terms arising from their interference. By treating the vibrational modes of the molecules as a non-Markovian harmonic oscillator bath, we compute the first-order path-interference correction to the classical transfer rate. We show that the correction due to path-interference may have either a vibrational or an electronic character, and may exceed the contribution of the indirect classical pathways.

Hossein-Nejad, Hoda; Olaya-Castro, Alexandra; Scholes, Gregory D.

2011-03-01

59

Data path development for multiple electron beam maskless lithography  

NASA Astrophysics Data System (ADS)

Electron beam lithography has been used in the production of integrated circuits for decades. However, due to the limitation of throughput it was not a viable solution for high volume manufacturing and its biggest application is the production of semiconductor masks. For many considerations it has particularly now become desirable to eliminate the semiconductor mask and introduce maskless lithography for semiconductor fabrication. Multiple Electron Beam Maskless Lithography (MEBML2) has been proposed as a solution to overcome the traditional source current limitation of an electron beam system by using many thousands of parallel electron beamlets to write a pattern directly on the wafer. In developing the MEBML2 tool the challenges have shifted and, in absence of the mask, the system data path has emerged as one of the central challenges. The main theme in the data path development is bandwidth. The required raw bandwidth at the patterning beams is determined by throughput and resolution, i.e. pixel size and number of intensity modulation levels. To achieve a production worthy throughput at 10 wafers per hour in a Gaussian-beam-based maskless lithography system, by writing 3.5-nm pixels at 2 levels (on/off) which is required for the 22-nm lithography node, the required aggregate bandwidth at the beam blanker array is up to 45 Tbit/s. Such a large bandwidth requirement means that the data path architecture is mainly characterized by the bandwidth of the data streams in the system. Compression techniques can be used to reduce the intermediate data stream bandwidth requirements and consequently lead to simplifying the system design, reducing power consumption and footprint, but come at the cost of increased data processing complexity and possible limitations on throughput. In this paper we will show results from the development of a prototype data path for the Gaussian-beam-based maskless lithography system. A new concept for data processing and storage is proposed. The vertex-based processing and storage technique is adopted to reduce memory usage considerably, with only modest requirements on the hardware resources. It reveals that a realistically implementable data path system for the maskless lithography tool in high volume manufacturing is feasible.

Krecinic, Faruk; Lin, Shy-Jay; Chen, Jack J. H.

2011-03-01

60

PATH  

NSDL National Science Digital Library

Started in the 1970s as an agency to assist men and women in gaining access to a variety of birth control methods, PATH has since expanded its focus to provide "sustainable, culturally relevant [health] solutions, enabling communities worldwide to break longstanding cycles of poor health." The PATH website has more than a dozen videos and slideshows available to visitors at the "Our Multimedia" link near the bottom right hand corner of the homepage. A three-minute video entitled "Better Nutrition For Life" educates visitors about an innovative rice product that could bring greater nutrition to millions of malnourished people where rice is a staple food. The product is Ultra Rice, and is actually fortified pasta that looks, cooks, and tastes like rice, but is fortified with nutrients. The "rice" can be fortified with the needed nutrients the particular population being served is lacking. A slideshow about TB in the Ukraine, explains to visitors why there has been a resurgence of TB in Eastern Europe, and how PATH and its partners set out to help control it throughout the region.

61

Comparison between Path Lengths Traveled by Solar Electrons and Ions in Ground-Level Enhancement Events  

NASA Astrophysics Data System (ADS)

We have examined the Wind/3DP/SST electron and Wind/EPACT/LEMT ion data to investigate the path length difference between solar electrons and ions in the ground-level enhancement (GLE) events in solar cycle 23. Assuming that the onset time of metric type II or decameter-hectometric (DH) type III radio bursts is the solar release time of non-relativistic electrons, we have found that within an error range of 10% the deduced path length of low-energy (~27 keV) electrons from their release site near the Sun to the 1 AU observer is consistent with the ion path length deduced by Reames from the onset time analysis. In addition, the solar longitude distribution and IMF topology of the GLE events examined are in favor of the coronal mass ejection-driven shock acceleration origin of observed non-relativistic electrons. We have also found an increase of electron path lengths with increasing electron energies. The increasing rate of path lengths is correlated with the pitch angle distribution (PAD) of peak electron intensities locally measured, with a higher rate corresponding to a broader PAD. The correlation indicates that the path length enhancement is due to the interplanetary scattering experienced by first arriving electrons. The observed path length consistency implies that the maximum stable time of magnetic flux tubes, along which particles transport, could reach 4.8 hr.

Tan, Lun C.; Malandraki, Olga E.; Reames, Donald V.; Ng, Chee K.; Wang, Linghua; Patsou, Ioanna; Papaioannou, Athanasios

2013-05-01

62

Elastic backscattering of electrons: determination of physical parameters of electron transport processes by elastic peak electron spectroscopy  

Microsoft Academic Search

This review paper is concerned with elastic peak electron spectroscopy (EPES) and information it gives on electronic transport phenomena. Experimental methods are described for determining the physical parameters such as the inelastic mean free path (IMFP), the life time of hot electrons, the elastic- and inelastic-scattering cross sections, the surface excitation parameter (SEP), etc. used by Auger electron spectroscopy, X-ray

G. Gergely

2002-01-01

63

Inelastic Interactions of Swift Electrons in Solids.  

National Technical Information Service (NTIS)

Theoretical calculations of electron mean free paths and electron slowing-down spectra in solids are described. These calculations involve (a) the use of an electron gas model to approximate the response of conduction band electrons in metals, (b) the app...

C. J. Tung R. H. Ritchie J. C. Ashley V. E. Anderson

1976-01-01

64

Variation of the electronic dipole polarizability on the reaction path.  

PubMed

The reaction force and the electronic flux, first proposed by Toro-Labb et al. (J Phys Chem A 103:4398, 1999) have been expressed by the existing conceptual DFT apparatus. The critical points (extremes) of the chemical potential, global hardness and softness have been identified by means of the existing and computable energy derivatives: the Hellman-Feynman force, nuclear reactivity and nuclear stiffness. Specific role of atoms at the reaction center has been unveiled by indicating an alternative method of calculation of the reaction force and the reaction electronic flux. The electron dipole polarizability on the IRC has been analyzed for the model reaction HF + CO?HCOF. The electron polarizability determined on the IRC ? e (?) was found to be reasonably parallel to the global softness curve S(?). The softest state on the IRC (not TS) coincides with zero electronic flux. PMID:23525962

J?drzejewski, Mateusz; Ordon, Piotr; Komorowski, Ludwik

2013-03-23

65

Effects of elastic and inelastic scattering in giving electrons tortuous paths in matter  

Microsoft Academic Search

Heavy charged particles travel in essentially straight lines in matter, while electrons travel in tortuous paths. Frequent multiple elastic Coulomb scattering by atomic nuclei is often cited as the reason for this electron behavior. Heavy charged particles also undergo multiple Coulomb scattering. However, because they are massive, significant deflections occur only in rare, close encounters with nuclei. In contrast to

J. E. Turner; R. N. Hamm

1995-01-01

66

Parasitic Effects of Grounding Paths on Common-Mode EMI Filter's Performance in Power Electronics Systems  

SciTech Connect

High-frequency common-mode (CM) electromagnetic-interference (EMI) noise is difficult to suppress in electronics systems. EMI filters are used to suppress CM noise, but their performance is greatly affected by the parasitic effects of the grounding paths. In this paper, the parasitic effects of the grounding paths on an EMI filter's performance are investigated in a motor-drive system. The effects of the mutual inductance between two grounding paths are explored. Guidelines for the grounding of CM EMI filters are derived. Simulations and experiments are finally carried out to verify the theoretical analysis.

Wang, Shuo [ORNL; Maillet, Yoann [Virginia Polytechnic Institute and State University (Virginia Tech); Wang, Fei [ORNL; Lai, Rixin [General Electric; Luo, Fang [Virginia Polytechnic Institute and State University (Virginia Tech); Boroyevich, Dushan [Virginia Polytechnic Institute and State University (Virginia Tech)

2010-01-01

67

Electronic Transport in Mesoscopic Systems  

Microsoft Academic Search

Recent advances in semiconductor technology have made possible the fabrication of structures whose dimensions are much smaller than the mean free path of an electron. This book gives the first thorough account of the theory of electronic transport in such mesoscopic systems. Beginning with coverage of fundamental concepts, the book presents a detailed account of transmission function formalism which is

Supriyo Datta

1997-01-01

68

Vibrational Analysis of a Shipboard Free Electron Laser Beam Path.  

National Technical Information Service (NTIS)

This thesis explores the deployment of a free electron laser (FEL) weapon system in a shipboard vibration environment. A concept solid model of a shipboard FEL is developed and used as a basis for a finite element model which is subjected to vibration sim...

B. M. Gallant

2011-01-01

69

Nonequilibrium heterogeneous catalysis in the long mean-free-path regime  

NASA Astrophysics Data System (ADS)

It is shown that a standard principle of traditional catalysisthat a catalyst does not alter the final thermodynamic equilibrium of a reactioncan fail in low-pressure, heterogeneous gas-surface reactions. Kinetic theory for this epicatalysis is presented, and two well-documented experimental examples are detailed: surface ionized plasmas and hydrogen dissociation on refractory metals. This phenomenon should be observable over a wide range of temperatures and pressures, and for a broad spectrum of heterogeneous reactions. By transcending some constraints of equilibrium thermodynamics, epicatalysis might provide additional control parameters and synthetic routes for reactions, and enable product streams boosted in thermochemical energy or desirable species.

Sheehan, D. P.

2013-09-01

70

Isoscalar giant-resonance energies and long-mean-free path nuclear fluid dynamics  

SciTech Connect

The collisionless Vlasov equation is solved by a moment expansion and truncated after the fourth moments. This scheme yields fluid dynamical equations for the eigenvibrations of a nucleus which are beyond the usual elastic equations. These Euler equations are solved exactly and also approximately for the normal parity (electric, surface) modes 2/sup +/, first excited 2/sup +/, 3/sup -/, 4/sup +/, the abnormal parity (magnetic, twist) modes 1/sup +/, 2/sup -/, and the compression (breathing and squeezing) modes 0/sup +/, 1/sup -/. Without adjustable parameters, agreement of the resulting giant resonance energies with experimental data, where available, is reasonably good. Since the corrections due to the inclusion of the third and fourth moments enhance the energies by up to 35%, the moment expansion converges only slowly.

Hasse, R.W.; Ghosh, G.; Winter, J.; Lumbroso, A.

1982-05-01

71

Mean free-path length theory of predatorprey interactions: Application to juvenile salmon migration  

Microsoft Academic Search

Ecological theory traditionally describes predatorprey interactions in terms of a law of mass action in which the prey mortality rate depends on the density of predators and prey. This simplifying assumption makes population-based models more tractable but ignores potentially important behaviors that characterize predatorprey dynamics. Here, we expand traditional predatorprey models by incorporating directed and random movements of both predators

James J. Anderson; Eliezer Gurarie; Richard W. Zabel

2005-01-01

72

Path Integrals for Electronic Densities, Reactivity Indices, and Localization Functions in Quantum Systems  

PubMed Central

The density matrix theory, the ancestor of density functional theory, provides the immediate framework for Path Integral (PI) development, allowing the canonical density be extended for the many-electronic systems through the density functional closure relationship. Yet, the use of path integral formalism for electronic density prescription presents several advantages: assures the inner quantum mechanical description of the system by parameterized paths; averages the quantum fluctuations; behaves as the propagator for time-space evolution of quantum information; resembles Schrdinger equation; allows quantum statistical description of the system through partition function computing. In this framework, four levels of path integral formalism were presented: the Feynman quantum mechanical, the semiclassical, the Feynman-Kleinert effective classical, and the Fokker-Planck non-equilibrium ones. In each case the density matrix or/and the canonical density were rigorously defined and presented. The practical specializations for quantum free and harmonic motions, for statistical high and low temperature limits, the smearing justification for the Bohrs quantum stability postulate with the paradigmatic Hydrogen atomic excursion, along the quantum chemical calculation of semiclassical electronegativity and hardness, of chemical action and Mulliken electronegativity, as well as by the Markovian generalizations of Becke-Edgecombe electronic focalization functions all advocate for the reliability of assuming PI formalism of quantum mechanics as a versatile one, suited for analytically and/or computationally modeling of a variety of fundamental physical and chemical reactivity concepts characterizing the (density driving) many-electronic systems.

Putz, Mihai V.

2009-01-01

73

Electronic band structure and Li diffusion paths in (LaLi)TiO 3  

Microsoft Academic Search

The electronic structure and the Li diffusion paths in the lithium doped lanthanum titanate have been studied. The band dispersion and the density of states (DOS) are calculated using the linear-muffin-tin-orbital (LMTO) method. The model structure used contains La-rich and La deficient layers, with the 2ap2ap2ap unit cell and base centered C symmetry. The primitive cell contains 20 atoms represented

Shinji Ono; Yusuke Seki; Shoji Kashida; Michisuke Kobayashi

2006-01-01

74

A Born-Oppenheimer approximation for path integrals with an application to electron solvation in polarizable fluids  

Microsoft Academic Search

Path integral techniques are very useful for exploring electron and proton solvation and electron transfer in po- larizable molecular fluids. * The solvated electron or proton is found to sustain energy excitations which are small com- pared to the energy spacing in the molecular electronic energy levels. This means that the dipolar fluctuations in the molecules, the source of molecular

J. Cao; B. J. Berne

1993-01-01

75

On the path length of an excess electron interacted with optical phonons in a molecular chain  

NASA Astrophysics Data System (ADS)

We show that in a molecular chain with dispersionless phonons at zero temperature, a quasistationary moving soliton state of an excess electron is possible. As the soliton velocity vanishes, the path length of the excess electron exponentially tends to infinity. It is demonstrated that in the presence of dispersion, when the soliton initial velocity exceeds the maximum group velocity of the chain, the soliton slows down until it reaches the maximum group velocity and then moves stationarily at this maximum group velocity. A conclusion is made of the fallacy of some works were the existence of moving polarons in a dispersionless medium is considered infeasible.

Lakhno, V. D.

2008-08-01

76

Communication: Predictive partial linearized path integral simulation of condensed phase electron transfer dynamics  

NASA Astrophysics Data System (ADS)

A partial linearized path integral approach is used to calculate the condensed phase electron transfer (ET) rate by directly evaluating the flux-flux/flux-side quantum time correlation functions. We demonstrate for a simple ET model that this approach can reliably capture the transition between non-adiabatic and adiabatic regimes as the electronic coupling is varied, while other commonly used semi-classical methods are less accurate over the broad range of electronic couplings considered. Further, we show that the approach reliably recovers the Marcus turnover as a function of thermodynamic driving force, giving highly accurate rates over four orders of magnitude from the normal to the inverted regimes. We also demonstrate that the approach yields accurate rate estimates over five orders of magnitude of inverse temperature. Finally, the approach outlined here accurately captures the electronic coherence in the flux-flux correlation function that is responsible for the decreased rate in the inverted regime.

Huo, Pengfei; Miller, Thomas F.; Coker, David F.

2013-10-01

77

All-electron path integral Monte Carlo simulations of warm dense matter: application to water and carbon plasmas.  

PubMed

We develop an all-electron path integral Monte Carlo method with free-particle nodes for warm dense matter and apply it to water and carbon plasmas. We thereby extend path integral Monte Carlo studies beyond hydrogen and helium to elements with core electrons. Path integral Monte Carlo results for pressures, internal energies, and pair-correlation functions compare well with density functional theory molecular dynamics calculations at temperatures of (2.5-7.5)10(5) K, and both methods together form a coherent equation of state over a density-temperature range of 3-12 g/cm(3) and 10(4)-10(9) K. PMID:22540485

Driver, K P; Militzer, B

2012-03-16

78

Quantitative Analysis by Auger Electron Spectroscopy  

Microsoft Academic Search

A short review is presented of the theoretical background of a physical model for the quantification of Auger electron spectroscopy (AES) for surface analysis. The recent studies on the data-base for the inelastic mean free paths (IMFP) by Seah and Dench and systematic calculations of the backscattering factors (R) by Shimizu and Ichimura have now enabled standard quantitative corrections comparable

Ryuichi Shimizu

1983-01-01

79

Particle-in-cell simulations of electron dynamics in low pressure discharges with magnetic fields  

Microsoft Academic Search

In modern low pressure plasma discharges, the electron mean free path often exceeds the device dimensions. Under such conditions the electron velocity distribution function may significantly deviate from Maxwellian, which strongly affects the discharge properties. The description of such plasmas has to be kinetic and often requires the use of numerical methods. This thesis presents the study of kinetic effects

Dmytro Sydorenko

2006-01-01

80

Impact of electron-phonon scattering on the performance of carbon nanotube interconnects for GSI  

Microsoft Academic Search

While electron mean-free path in carbon nanotubes can be as large as several micrometers for small bias voltages, for large biases electrons get backscattered by optical and zone-boundary phonons and nanotube resistance can increase by more than 100 times. This letter reveals this kind of backscattering has a small impact (error <25%) in most interconnect applications of carbon nanotubes in

Azad Naeemi; James D. Meindl

2005-01-01

81

Path-integral molecular-dynamics calculation of the conduction-band energy of excess electrons in fluid argon  

Microsoft Academic Search

Path-integral molecular-dynamics simulations are used to calculate the energy V0 of excess electrons in liquid argon at various densities in the gas-phase region. Using the same electron-argon interaction potential, V0 is also calculated within the framework of the Wigner-Seitz model. Both calculations agree with experiment. The excess electron is found to be delocalized over about a thousand atoms, which could

J.-M. Lopez-Castillo; Y. Frongillo; B. Plenkiewicz; J.-P. Jay-Gerin

1990-01-01

82

Self-similar variables and the problem of nonlocal electron heat conductivity  

Microsoft Academic Search

Self-similar solutions of the collisional electron kinetic equation are obtained for the plasmas with one and three dimensional plasma parameter inhomogeneities and arbitrary Z(sub eff). For the plasma parameter profiles characterized by the ratio of the mean free path of thermal electrons with respect to electron-electron collisions, lambda(sub T), to the scale length of electron temperature variation, L, one obtains

S. I. Krasheninnikov; O. G. Bakunin

1993-01-01

83

Gravimetric and astro-geodetic geoids and mean free-air anomalies in India  

NASA Astrophysics Data System (ADS)

A preliminary gravimetric geoid with respect to the International Spheroid and the latest astro-geodetic geoid computed on the Everest and International Spheroids are given in the form of undulation maps over the Indian Subcontinent. 10x10 mean free-air anomalies (modified) on the Geodetic Reference System, 1967 (GRS-67) are also given for the whole country in the form of a chart. For the purpose of computing the gravimetric geoid, 50x50 mean free-air anomalies were used outside the area bounded by latitudes 00 to 400 N and longitudes 600 to 1000 E and 10x10 mean free-air anomalies within these limits. The anomalies partly computed by Survey of India and mostly collected from other sources (such as B.G.I.) were utilised for this purpose. The astro-geodetic geoid is based on the astronomical data observed in India up to 1978.

Khosla, K. L.; Arur, M. G.; Bains, P. S.

1982-09-01

84

Theory of probing attosecond electron wave packets via two-path interference of angle-resolved photoelectrons  

SciTech Connect

We study theoretically the electron wave packet generated by an attosecond pulse train (APT) which is probed with a time-delayed infrared (IR) laser pulse. The APT creates an excited state and a continuum electron wave packet. By ionizing the excited state with an IR, a delayed new continuum electron wave packet is created. The interference of the wave packets from the two paths, as reflected in angle-resolved photoelectron spectra, is analyzed analytically. Using the analytical expressions, we examine the possibility of retrieving information on the electron wave packet generated by the APT.

Choi, N. N. [School of Natural Science, Kumoh National Institute of Technology, Gumi, Gyeongbuk 730-701 (Korea, Republic of); J. R. Macdonald Laboratory, Physics Department, Kansas State University, Manhattan, Kansas 66506-2604 (United States); Jiang, T. F. [J. R. Macdonald Laboratory, Physics Department, Kansas State University, Manhattan, Kansas 66506-2604 (United States); Institute of Physics, National Chiao Tung University, Hsinchu 30010, Taiwan (China); Morishita, T. [Department of Applied Physics and Chemistry, University of Electro-Communications, 1-5-1 Chofu-ga-oka, Chofu-shi, Tokyo, 182-8585 (Japan); PRESTO, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012 (Japan); Lee, M.-H. [School of Natural Science, Kumoh National Institute of Technology, Gumi, Gyeongbuk 730-701 (Korea, Republic of); Lin, C. D. [J. R. Macdonald Laboratory, Physics Department, Kansas State University, Manhattan, Kansas 66506-2604 (United States)

2010-07-15

85

Simplified Monte Carlo simulation of elastic electron scattering in limited media  

Microsoft Academic Search

A practical method to simulate elastic electron scattering is described which represents an effective improvement upon currently available approximate simulation methods. Single scattering events are simulated by means of a differential cross section model given by a simple analytical expression, whose parameters are determined so as to exactly reproduce the values of the mean free path and the first and

J. Bar; J. Sempau; J. M. Fernndez-Varea; F. Salvat

1994-01-01

86

Electronphonon coupling and phonon-drag thermopower of a very low mobility 2DEG  

Microsoft Academic Search

It has been predicted that the phonon drag thermopower will be enhanced in very low mobility samples. The effect occurs when ql<1 where q is the phonon wave number and l the electron mean free path. We present experimental data and detailed numerical calculations which confirm this prediction.

R. Fletcher; M. Tsaousidou; P. T. Coleridge; Y. Feng; Z. R. Wasilewski

2002-01-01

87

Electron-Phonon Scattering in Metallic Single-Walled Carbon Nanotubes  

Microsoft Academic Search

Electron scattering rates in metallic single-walled carbon nanotubes are studied using an atomic force microscope as an electrical probe. From scaling of the resistance of the same nanotube with length in the low and high bias regimes, the mean free paths for both regimes are inferred. The observed scattering rates are consistent with calculations for acoustic phonon scattering at low

Sami Rosenblatt; Yuval Yaish; Vera Sazonova; Hande Ustunel; Stephan Braig; Tomas A. Arias; Piet W. Brouwer; Paul L. McEuen

2004-01-01

88

P3-34: Design of a gridded electron gun using the particle path and the FEM approach  

Microsoft Academic Search

The XMGUN was used to design a 30kV, 4.6A and 0.88?Perv axis-symmetric high power electron gun, with grid and shadow grid, working under the space charge limited flow to be used in a TWT. The XMGUN uses the particle path, instead of the motion equation, to perform the macroparticles ray tracing and the FEM approach. It was observed a current

Csar C. Xavier; Cludio C. Motta

2010-01-01

89

Path integral approach to the photoionization of one active electron atom by a pulsed Fock state: Fluctuations contribution  

SciTech Connect

We model in a fully quantum mechanical way the dynamics of an atom of one optically active electron interacting with a pulsed Fock state which is linearly polarized. We use path integral methods. We derive the system's sign solved propagator which gives full information on its dynamics. We apply our method to the ionization of atomic hydrogen by a one-photon pulsed Fock state and study the contribution of the electromagnetic fluctuations.

Thrapsaniotis, E. G. [52 Vianou street, 13671 Athens, Aharnes (Greece)

2012-08-15

90

Hooke's Atom in an Arbitrary External Electric Field: Analytical Solutions of Two-Electron Problem by Path Integral Approach  

NASA Astrophysics Data System (ADS)

By using the path integral approach, we investigate the problem of Hooke's atom (two electrons interacting with Coulomb potential in an external harmonic-oscillator potential) in an arbitrary time-dependent electric field. For a certain infinite set of discrete oscillator frequencies, we obtain the analytical solutions. The ground state polarization of the atom is then calculated. The same result is also obtained through linear response theory.

Cai, Liang; Zhang, Ping; Yang, Tao; Pan, Xiao-Yin

2011-04-01

91

Effects of elastic-electron scattering on measurements of silicon dioxide film thicknesses by X-ray photoelectron spectroscopy  

Microsoft Academic Search

It is now customary for the effects of elastic-electron scattering to be ignored in measurements of the thicknesses of overlayer films by X-ray photoelectron spectroscopy (XPS). It is known, however, that elastic scattering can cause the effective attenuation length (EAL), needed for the thickness measurement, to be different from the corresponding inelastic mean free path (IMFP). We have investigated the

C. J. Powell; A. Jablonski

2001-01-01

92

Coherent control of the electron quantum paths for the generation of single ultrashort atto second laser pulse  

SciTech Connect

We report a mechanism and a realizable approach for the coherent control of the generation of an isolated and ultrashort atto second (as) laser pulse from atoms by optimizing the two-color laser fields with a proper time delay. Optimizing the laser pulse shape allows the control of the electron quantum paths and enables high-harmonic generation from the long- and short-trajectory electrons to be enhanced and split near the cutoff region. In addition, it delays the long-trajectory electron emission time and allows the production of extremely short atto second pulses in a relatively narrow time duration. As a case study, we show that an isolated 30 as pulse with a bandwidth of 127 eV can be generated directly from the contribution of long-trajectory electrons alone.

Liu, I-Lin [Center for Quantum Science and Engineering, Department of Physics, National Taiwan University, Taipei 10617, Taiwan (China); Li, Peng-Cheng [Center for Quantum Science and Engineering, Department of Physics, National Taiwan University, Taipei 10617, Taiwan (China); College of Physics and Electronic Engineering, Northwest Normal University, Lanzhou 730070 (China); Chu, Shih-I [Center for Quantum Science and Engineering, Department of Physics, National Taiwan University, Taipei 10617, Taiwan (China); Department of Chemistry, University of Kansas, Lawrence, Kansas 66045 (United States)

2011-09-15

93

Quantized conductance in extended electron waveguides fabricated on shallow etched modulation-doped GaAs\\/AlGaAs heterostructures  

Microsoft Academic Search

By high-resolution electron beam lithography and wet chemical etching extended electron waveguides with lengths up to 20?m were fabricated on modulation-doped GaAs\\/AlGaAs heterostructures showing conductance quantization. For short electron waveguides 15 well-resolved steps in the conductance trace are observable. Conductance quantization is observed up to wire lengths comparable to the transport mean free path of electrons in the unconstrained two-dimensional

F. Beuscher; L. Worschech; B. Weidner; A. Forchel

2000-01-01

94

NIST data resources for surface analysis by X-ray photoelectron spectroscopy and Auger electron spectroscopy  

Microsoft Academic Search

A description is given of data resources that are available from the National Institute of Standards and Technology (NIST) for X-ray photoelectron spectroscopy (XPS) and Auger-electron spectroscopy. NIST currently has three databases available: an XPS Database, an Electron Elastic-Scattering Cross-Section Database, and an Electron Inelastic-Mean-Free-Path Database. NIST also offers Standard Test Data (STD) for XPS, a set of simulated XPS

C. J. Powell; A. Jablonski; A. Naumkin; A. Kraut-Vass; J. M. Conny; J. R. Rumble

2001-01-01

95

The path to ubiquitous and low-cost organic electronic appliances on plastic  

Microsoft Academic Search

Organic electronics are beginning to make significant inroads into the commercial world, and if the field continues to progress at its current, rapid pace, electronics based on organic thin-film materials will soon become a mainstay of our technological existence. Already products based on active thin-film organic devices are in the market place, most notably the displays of several mobile electronic

Stephen R. Forrest

2004-01-01

96

Path-integral Monte Carlo simulations for interacting few-electron quantum dots with spin-orbit coupling  

NASA Astrophysics Data System (ADS)

We develop path-integral Monte Carlo simulations for a parabolic two-dimensional (2D) quantum dot containing N interacting electrons in the presence of Dresselhaus and/or Rashba spin-orbit couplings. Our method solves in a natural way the spin contamination problem and allows for numerically exact finite-temperature results at weak spin-orbit coupling. For N<10 electrons, we present data for the addition energy, the particle density, and the total spin S in the Wigner molecule regime of strong Coulomb interactions. We identify magic numbers at N=3 and N=7 via a peak in the addition energy. These magic numbers differ both from weak interaction and classical predictions, and are stable with respect to (weak) spin-orbit couplings.

Weiss, Stephan; Egger, R.

2005-12-01

97

Temperature-dependent solid-state electron transport through bacteriorhodopsin: experimental evidence for multiple transport paths through proteins.  

PubMed

Electron transport (ETp) across bacteriorhodopsin (bR), a natural proton pump protein, in the solid state (dry) monolayer configuration, was studied as a function of temperature. Transport changes from thermally activated at T > 200 K to temperature independent at <130 K, similar to what we have observed earlier for BSA and apo-azurin. The relatively large activation energy and high temperature stability leads to conditions where bR transports remarkably high current densities above room temperature. Severing the chemical bond between the protein and the retinal polyene only slightly affected the main electron transport via bR. Another thermally activated transport path opens upon retinal oxime production, instead of or in addition to the natural retinal. Transport through either or both of these paths occurs on a background of a general temperature-independent transport. These results lead us to propose a generalized mechanism for ETp across proteins, in which tunneling and hopping coexist and dominate in different temperature regimes. PMID:22296717

Sepunaru, Lior; Friedman, Noga; Pecht, Israel; Sheves, Mordechai; Cahen, David

2012-02-27

98

Electron transport measurement in locally strained graphene  

NASA Astrophysics Data System (ADS)

Strain engineering is a promising method for controlling electron transport in graphene; Spatial variation of gauge fields produced by non-uniform strain in graphene causes electron scattering, leading to modulation of the electronic state such as band gap formation. We have succeeded in introducing local strain to graphene, by inserting designed dielectric nanostructures between the graphene sheet and its substrate. [1] The transport measurement of strained graphene has revealed that improvement of the mean free path is crucial for clear demonstration of effect of lattice strain on electron transport.[4pt] [1] H. Tomori et al., Appl. Phys. Express 4, 075102 (2011).

Tomori, Hikari; Kanda, Akinobu; Ootuka, Youiti; Karube, Hiromasa; Kanda, Akinobu

2013-03-01

99

Nuclear quantum effects in the electronic structure of C 2H 4: a combined Feynman path integralab initio approach  

Microsoft Academic Search

A tight-binding equipped Feynman path integral Monte Carlo formalism has been linked to a HartreeFock Hamiltonian to derive the electronic properties of C2H4 considering the quantum character of the nuclei. Configurationally averaged electronic quantities are compared with single-configuration results. The potential energy of the vibrational problem is caused by an energetic up-shift of the electron-nuclear interaction of the electronic Hamiltonian

Rafael Ram??rez; Eduardo Hernndez; Joachim Schulte; Michael C Bhm

1998-01-01

100

Feynman Path Integral Approach to Electron Diffraction for One and Two Slits: Analytical Results  

ERIC Educational Resources Information Center

|In this paper we present an analytic solution of the famous problem of diffraction and interference of electrons through one and two slits (for simplicity, only the one-dimensional case is considered). In addition to exact formulae, various approximations of the electron distribution are shown which facilitate the interpretation of the results.

Beau, Mathieu

2012-01-01

101

Determination of the Path Loss from Passenger Electronic Devices to Radio Altimeter with Additional EMI Test  

NASA Astrophysics Data System (ADS)

Emitters of current and future wireless ultra wideband technology (UWB) inside the cabin should not interfere with any aircraft system. Especially the radio altimeter (RA) system using antennas mounted outside the fuselage is potentially sensitive to UWB devices in the frequency range between 4.1 and 4.8 GHz. The measurement of the interference path loss (IPL) to the RA is therefore of interest and is presented for different aircraft. The need of a high dynamic setup with low parasitic coupling in the IPL measurement is stressed. In addition, electromagnetic interference (EMI) tests with different transmitted signals are made, showing that the susceptibility of the RA system actually increases with UWB modulation.

Schr, J.; Nunes, R. R.

2012-05-01

102

Different paths to successthe growth of the electronics sector in Ireland and Israel  

Microsoft Academic Search

Both Ireland and Israel have, over the last three decades, established internationally competitive electronics industries. Israeli electronics has its origins in locally initiated R&D, is dominated by indigenously owned firms, and has its main export-market strengths in the research-intensive leading-edge markets for telecommunications and medical diagnostic equipment. In contrast, large-scale, US-owned plants producing computer equipment and components dominate the Irish

Stephen Roper; Amnon Frenkel

2000-01-01

103

GHz surface acoustic waves as a probe of composite fermions in two-dimensional electron systems  

Microsoft Academic Search

By providing the low-disorder two-dimensional electron system at submicron dimensions, important new length scales are revealed. Using surface acoustic waves of wavelength ??1 ?m to measure the 2D electron conductivity, enhanced conductivity is observed at even-denominator filling factors v =, : this represents measurement of composite fermions' conductivity at a dimension smaller than the quasiparticles' mean free path. At smaller

R. L. Willett; L. N. Pfeiffer

1996-01-01

104

Silicon electronics on silk as a path to bioresorbable, implantable devices  

NASA Astrophysics Data System (ADS)

Many existing and envisioned classes of implantable biomedical devices require high performance electronics/sensors. An approach that avoids some of the longer term challenges in biocompatibility involves a construction in which some parts or all of the system resorbs in the body over time. This paper describes strategies for integrating single crystalline silicon electronics, where the silicon is in the form of nanomembranes, onto water soluble and biocompatible silk substrates. Electrical, bending, water dissolution, and animal toxicity studies suggest that this approach might provide many opportunities for future biomedical devices and clinical applications.

Kim, Dae-Hyeong; Kim, Yun-Soung; Amsden, Jason; Panilaitis, Bruce; Kaplan, David L.; Omenetto, Fiorenzo G.; Zakin, Mitchell R.; Rogers, John A.

2009-09-01

105

Electron transport in single-walled carbon nanotubes  

NASA Astrophysics Data System (ADS)

We present electron transport experiments on single walled carbon nanotubes (SWNTs). By measuring the linear scaling of resistance with length, we determine an unusually long mean free path of Lm 1 mum at room temperature. From the temperature dependence of the mean free path for over 10 samples we show that inelastic scattering with acoustic phonons are the main source of scattering at room temperature and experimentally determine the electron-acoustic phonon strength. Disorder ultimately limits the low temperature mean free path (Lm 10 mum), which we show by employing scanning gate microscopy. We analyze the non-linear scaling of resistance with length and temperature to further elucidate the nature of this disorder. In general, we find that transport in 1-dimension is dominated by the strongest defect along the channel. For larger source-drain voltage (VSD > 0.2 V), we show that L m is significantly reduced in both metallic and semiconducting SWNTs, due to electrons scattering with the higher energy phonons. In semiconducting samples, when the Fermi energy is close to the energy band gap, we observe an anomalous conductance dip. Finally, we utilize locally controlled gate structures to fabricate a series of tunable barriers to form a superlattice and observe its mini-band structure superimposed on that of the nanotubes.

Purewal, Meninder S.

106

First milestone on the path toward a table-top free-electron laser (FEL)  

Microsoft Academic Search

Latest developments in the field of laser-wakefield accelerators (LWFAs) have led to relatively stable electron beams in terms of peak energy, charge, pointing and divergence from mm-sized accelerators. Simulations and LWFA theory indicate that these beams have low transverse emittances and ultrashort bunch durations on the order of ~10 fs. These features make LWFAs perfectly suitable for driving high-brightness X-ray

M. Fuchs; R. Weingartner; A. Popp; Zs. Major; S. Becker; J. Osterhoff; T. Seggebrock; R. Hrlein; G. D. Tsakiris; U. Schramm; T. P. Rowlands-Rees; S. M. Hooker; D. Habs; F. Krausz; S. Karsch; F. Grner

2010-01-01

107

Classical transport calculations of multilayer conductance in Co/Cu using the path-integral method  

SciTech Connect

Classical transport calculations are carried out for a multilayer cobalt-copper system, using the path-integral method. Both metals are approximated with free-electron descriptions, but with different mean free paths fit to the observed bulk conductivities. These alone produce discontinuities in the slope, but not in the values, of conductance-versus-total-thickness plots. If specularity coefficients are defined for transmission at interfaces and reflection at surfaces, values different from one produce a discontinuous drop in such curves. Comparison of the calculated conductance of copper layers with the data of Bailey et al. [J. Appl. Phys. 85, 7345 (1999)] suggests a specularity coefficient for reflection of some 22% and transmission of approximately 14%. This simple model can readily be extended to giant magnetoresistance by assuming different specularity coefficients for spin-up and -down electrons. (c) 2000 The American Physical Society.

Harrison, Walter A. [Los Alamos National Laboratory, T1, B221 Los Alamos, New Mexico 87545 (United States)

2000-03-15

108

Characterization of thin films on the nanometer scale by Auger electron spectroscopy and X-ray photoelectron spectroscopy  

Microsoft Academic Search

We describe two NIST databases that can be used to characterize thin films from Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) measurements. First, the NIST Electron Effective-Attenuation-Length Database provides values of effective attenuation lengths (EALs) for user-specified materials and measurement conditions. The EALs differ from the corresponding inelastic mean free paths on account of elastic-scattering of the signal

C. J. Powell; A. Jablonski; W. S. M. Werner; W. Smekal

2005-01-01

109

Magnetic turbulent electron transport in a reversed field pinch  

SciTech Connect

A model of magnetic turbulent electron transport is presented. The model, based on the thermal conduction theory of Rechester and Rosenbluth, entails a Boltzmann description of electron dynamics in the long mean-free-path limit and quantitatively describes the salient features of superthermal electron measurements in the RFP edge plasma. Included are predictions of the mean superthermal electron energy, current density, and power flux asymmetry. A discussion of the transport model, the assumptions implicit in the model, and the relevance of this work to more general issue of magnetic turbulent transport in toroidal systems is presented. 32 refs., 3 figs.

Schoenberg, K.; Moses, R.

1990-01-01

110

Electrons and nuclei of C 6H 6 and C 6D 6; a combined Feynman path integral ab initio approach  

Microsoft Academic Search

We have linked an ab initio approach of the HartreeFock (HF) type to the Feynman path integral quantum Monte Carlo (PIMC) formalism in order to study C6H6 and C6D6 under consideration of the quantum character of the nuclei and electrons. The combination of the statistical Monte Carlo approach with an electronic Hamiltonian offers the possibility to study the influence of

Michael C. Bhm; Rafael Ram??rez; Joachim Schulte

1998-01-01

111

Electron spin resonance and exchange paths in the orthorhombic dimer system Sr2VO4  

NASA Astrophysics Data System (ADS)

We report on susceptibility and electron spin resonance (ESR) measurements at X- and Q-band frequencies of Sr2VO4 with orthorhombic symmetry. In this dimer system, the V4+ ions are in tetrahedral environment and are coupled by an antiferromagnetic intradimer exchange constant J/kB? 100 K to form a singlet ground state without any phase transitions between room temperature and 2 K. Based on an extended Hckel tight-binding analysis, we identify the strongest exchange interaction to occur between two inequivalent vanadium sites via two intermediate oxygen ions. The ESR absorption spectra can be well fitted by a single Lorentzian line and the temperature dependence of the ESR intensity, and the dc susceptibility can be modeled by using the Bleaney-Bowers approach for independent dimers. The temperature dependence of the ESR linewidth at X-band frequency can be modeled by a superposition of a linear increase with temperature with a slope ? =1.35 Oe/K and a thermally activated behavior with an activation energy ?/kB =1418 K, both of which point to spin-phonon coupling as the dominant relaxation mechanism in this compound.

Deisenhofer, J.; Schaile, S.; Teyssier, J.; Wang, Zhe; Hemmida, M.; von Nidda, H.-A. Krug; Eremina, R. M.; Eremin, M. V.; Viennois, R.; Giannini, E.; van der Marel, D.; Loidl, A.

2012-12-01

112

Ambipolar transport via trapped-electron whistler instability along open magnetic field lines.  

PubMed

An open field line plasma is bounded by a chamber wall which intercepts the magnetic field. Steady state requires an upstream plasma source balancing the particle loss to the boundary. In cases where the electrons have a long mean free path, ambipolarity in parallel transport critically depends on collisionless detrapping of the electrons via wave-particle interaction. The trapped-electron whistler instability, whose nonlinear saturation produces a spectrum of whistler waves that is responsible for the electron detrapping flux, is shown to be an unusually robust kinetic instability, which is essential to the universality of the ambipolar constraint in plasma transport. PMID:23030098

Guo, Zehua; Tang, Xian-Zhu

2012-09-28

113

Substrate surface corrugation effects on the electronic transport in graphene nanoribbons  

NASA Astrophysics Data System (ADS)

In this work, the electronic transport in armchair graphene nanoribbons in the presence of surface corrugation is studied. The non-equilibrium Green's function along with tight-binding model for describing the electronic band structure is employed to investigate the electronic properties of graphene nanoribbons. The effects of surface corrugation parameters, such as corrugation amplitude and correlation length, on the electronic properties of the graphene nanoribbons are studied. The mean free path of carriers is extracted and its dependency on the corrugation amplitude and correlation length is investigated.

Babaee Touski, Shoeib; Pourfath, Mahdi

2013-09-01

114

Tornado Paths  

NSDL National Science Digital Library

This website catalogs all the tornado paths in the United States since 1950. The tornado path data is overlaid onto a Google Maps base for easy browsing and manipulation of the map view. Clicking on individual tornados provides the user with information such as its Fujita rating, the amount of damage caused by the tornado, the size of the path that the tornado made, and the length of time the tornado was on the ground.

Samson, Perry; Michigan, University O.

115

Thickness Difference: A New Filtering Tool for Quantitative Electron Diffraction  

SciTech Connect

A new way of filtering electron diffraction patterns has been discovered. Patterns from slightly different specimen thicknesses beyond the mean free path for inelastic scattering are subtracted. Only thickness sensitive information (dominantly elastic) remains. Thermal diffuse scattering and Borrmann effects are removed in addition to the inelastic signal eliminated by conventional energy filtering. One application is quantitative convergent beam electron diffraction without an energy filter. Structure factors for {alpha}-Al{sub 2}O{sub 3} have been measured with an average uncertainty of 0.25%.

Nakashima, Philip N. H. [Department of Materials Engineering, Monash University, Clayton Vic 3800 (Australia); Monash Centre for Electron Microscopy, Monash University, Clayton Vic 3800 (Australia)

2007-09-21

116

MetaPath: an electronic knowledge base for collating, exchanging and analyzing case studies of xenobiotic metabolism.  

PubMed

The MetaPath knowledge base was developed for the purpose of archiving, sharing and analyzing experimental data on metabolism, metabolic pathways and crucial supporting metadata. The MetaPath system grew out of the need to compile and organize the results of metabolism studies into a systematic database to facilitate data comparisons and evaluations. Specialized MetaPath data evaluation tools facilitate the review of pesticide metabolism data submitted for regulatory risk assessments as well as exchange of results of complex analyses used in regulation and research. Customized screen editors called Composers were developed to automate data entry into MetaPath while also streamlining the production of agency specific study summaries such as the Data Evaluation Records (DER) used by the US EPA Office of Pesticide Programs. Efforts are underway through an Organization for Economic Co-operation and Development (OECD) work group to extend the use of DER Composers as harmonized templates for rat metabolism, livestock residue, plant residue and environmental degradation studies. PMID:22414578

Kolanczyk, Richard C; Schmieder, Patricia; Jones, William J; Mekenyan, Ovanes G; Chapkanov, Atanas; Temelkov, Stanislav; Kotov, Stefan; Velikova, Maria; Kamenska, Verginia; Vasilev, Krasimir; Veith, Gilman D

2012-03-07

117

Influence of emitted electrons transiting between surfaces on plasma-surface interaction  

NASA Astrophysics Data System (ADS)

Emitted electrons are accelerated back into the plasma by the sheath. If their mean free path is large, they can propagate directly to another surface without suffering collisions. We analyze the effects of ``transit'' on plasma-surface interaction. When transit occurs, surfaces exchanging electrons are intricately coupled. All surfaces float more negatively than they would if the emission collisionally remixed with the bulk plasma. Asymmetries of the system drive a net ``transit current'' between the surfaces, which influences their potential difference. The larger the initial energy spread of the emitted electrons, the larger the potential difference.

Campanell, Michael; Wang, Hongyue

2013-09-01

118

Energy spectrum estimates for the ion-electron emission in the radioisotope battery  

NASA Astrophysics Data System (ADS)

It is described the simple practical model of express-estimates of the ion-electron emission (IEE) induced by the fast ion fluxes in the emitter of radioisotope source of current. Under calculations performed it is taken into account the analytical approximations for medium stopping power and electrons inelastic mean free path, the source of suprathermal electrons (SE) generation and the probabilities of SE arriving at the film surface and their passage through surface potential barrier. The model developed may be used for computer simulations of processes in solid-state plasma of emitter films in the radioisotope battery.

Balebanov, V. M.; Erokhin, N. S.; Mikhailovskaya, L. A.

2004-03-01

119

SMART scattering matrices for single collision electron monte carlo calculations  

SciTech Connect

An angular redistribution function for electron scattering based on Goudsmit-Saunderson theory has been implemented in a Monte Carlo electron transport code in the form of a scattering matrix that the authors term SMART (simulating many accumulative Rutherford trajectories). These matrices were originally developed for use with discrete ordinates electron transport codes. An essential characteristic of this scattering theory is a large effective mean-free-path for electrons, much larger in fact than the true single collision mean-free-path. When this theory is applied to single collision analog Monte Carlo calculations, excellent results are obtained for the principal quantities of interest, transmission and reflection spectra, and energy deposition. A derivation of the SMART scattering matrix is presented, using the method of weighted residuals to obtain the discretized form of the Spencer-Lewis equation for electron transport. Results of Monte Carlo calculations for electron transport in aluminum slabs for both beam source and isotropic source configurations are given. These results are compared with similar benchmark calculations made with the TIGER code series.

Filippone, W.L.; Woolf, S.

1988-11-01

120

Path Sensitization in Critical Path Problem  

Microsoft Academic Search

Since the delay of a circuit is determined by the delay of its longest sensitizable paths (such paths are called critical paths), the problem of estimating the delay of a circuit is called critical path problem. One important aspect of the critical path problem is to decide whether a path is sensitizable. A framework which allows various previously proposed path

Hsi-chuan Chen; David Hung-chang Du

1991-01-01

121

Investigation of the relative stabilities of various allotropic phases of elemental tellurium under pressure and their interconversion paths by electronic structure calculations and crystal structure analyses  

SciTech Connect

Elemental tellurium adopts a number of different structures under pressure. The relative stabilities of these allotropes and the interconversion between them were examined on the basis of first principles electronic band structure calculations with and without external pressure. The relative stabilities of the allotropes were also analyzed by estimating the overall strength of covalent bonding on the basis of the Te-Te overlap populations determined from tight-binding electronic structure calculations. The crystal structures of the allotropes were analyzed to determine how one form can transform into another form under pressure. Our study leads to the energy profile of tellurium as a function of pressure consistent with experiment, shows that the relative stabilities of the allotropic phases of Te are mainly governed by the overall strength of covalent bonding, and that all the allotropes are intimately related in structure, and one form can be readily converted to another form under pressure via a simple interconversion path.

Soulard, C. [Institut des Materiaux Jean Rouxel (UMR 6502), Laboratoire de Chimie des Solides, 2 rue de la Houssiniere, BP 32229, 44322 Nantes Cedex 3 (France); Rocquefelte, X. [Institut des Materiaux Jean Rouxel (UMR 6502), Laboratoire de Chimie des Solides, 2 rue de la Houssiniere, BP 32229, 44322 Nantes Cedex 3 (France); Evain, M. [Institut des Materiaux Jean Rouxel (UMR 6502), Laboratoire de Chimie des Solides, 2 rue de la Houssiniere, BP 32229, 44322 Nantes Cedex 3 (France); Jobic, S. [Institut des Materiaux Jean Rouxel (UMR 6502), Laboratoire de Chimie des Solides, 2 rue de la Houssiniere, BP 32229, 44322 Nantes Cedex 3 (France)]. E-mail: stephane.jobic@cnrs-imm.fr; Koo, H.-J. [Department of Chemistry, Kyung Hee University, Seoul 130-701 (Korea, Republic of); Whangbo, M.-H. [Department of Chemistry, North Carolina State University, Raleigh, NC 27695-8204 (United States)

2004-12-01

122

Convoy electron production by heavy ions in solids  

SciTech Connect

The term convoy electron refers to those electrons ejected in fast ion-atom and ion-solid collisions closely matched in vector velocity to that of the incident heavy particles responsible for their ejection. Similarities and differences among electrons ejected into such states through binary electron capture to continuum and electron loss to continuum processes in single ion-atom encounters are compared and contrasted to more complex ejection processes occurring in solid targets. Puzzles posed by the apparent strong projectile Z dependence but weak emergent ion charge dependence of the yield in the case of solid targets are reviewed. Very recent progress in resolving these puzzles has been made by recent observations that the apparent mean free path for electron scattering out of the forward direction within the target is observed to be an order of magnitude greater than that for free electrons of equal velocity provided the projectile charge is high. 13 references, 2 figures, 1 table.

Sellin, I.A.

1984-01-01

123

Elastic scattering of 200keV electrons in elemental solids: experimental observation of atomic-number-dependent oscillatory behavior.  

PubMed

Mean free path of elastic electron scattering ?(el) has been measured with a 200keV transmission electron microscope for a wide range of stable elemental solids. An oscillating behavior versus atomic number Z has been revealed, such that, within one row of the periodic table, ?(el) exhibits minimum (maximum) for elements with completed (empty) outer d shells. These ?(el)(Z) oscillations are attributed to Z dependence of the atomic density, and their importance for the interpretation of electron microscope images is demonstrated. PMID:21825363

Iakoubovskii, Konstantin; Mitsuishi, Kazutaka

2009-03-20

124

Electronic thermal conductivity measurements in intrinsic graphene  

NASA Astrophysics Data System (ADS)

The electronic thermal conductivity of graphene and two-dimensional Dirac materials is of fundamental interest and can play an important role in the performance of nanoscale devices. We report the electronic thermal conductivity Ke in suspended graphene in the nearly intrinsic regime over a temperature range of 20-300 K. We present a method to extract Ke using two-point dc electron transport at low bias voltages, where the electron and lattice temperatures are decoupled. We find Ke ranging from 0.5 to 11 W/m K over the studied temperature range. The data are consistent with a model in which heat is carried by quasiparticles with the same mean free path and velocity as graphene's charge carriers.

Yi?en, S.; Tayari, V.; Island, J. O.; Porter, J. M.; Champagne, A. R.

2013-06-01

125

High-coherence electron bunches produced by femtosecond photoionization  

NASA Astrophysics Data System (ADS)

With the development of ultrafast electron and X-ray sources it is becoming possible to study structural dynamics with atomic-level spatial and temporal resolution. Because of their short mean free path, electrons are particularly well suited for investigating surfaces and thin films, such as the challenging and important class of membrane proteins. To perform single-shot diffraction experiments on protein crystals, an ultracold electron source was proposed, based on near-threshold photoionization of laser-cooled atoms, which is capable of producing electron pulses of both high intensity and high coherence. Here we show that high coherence electron pulses can be produced by femtosecond photoionization, opening up a new regime of ultrafast structural dynamics experiments. The transverse coherence turns out to be much better than expected on the basis of the large bandwidth of the femtosecond ionization laser pulses. This surprising result can be explained by analysis of classical electron trajectories.

Engelen, W. J.; van der Heijden, M. A.; Bakker, D. J.; Vredenbregt, E. J. D.; Luiten, O. J.

2013-04-01

126

On spin-flip radiation from an electron moving with a helical path in a magnetized plasma  

Microsoft Academic Search

We study certain features of radiation arising from spin flipping of an electron or positron gyrating with a general pitch in a magneto-active plasma. Numerical analysis of the power spectrum is carried out, and a comparison is made between this radiation and the normal synchrotron radiation. The essential idea of application to astrophysical radiation in the electromagnetic ?-wave range is

P. C. W. fung; W. Y. P. Fung

1981-01-01

127

Term Paths  

NSDL National Science Digital Library

Students follow several pathways using anatomical directions on a simulated "body" produced from a copy of a school building's fire evacuation plan. The main hallways are designated as major blood vessels and the various areas of the school, the head, chest, abdomen, etc. Students complete several pathways using anatomical terms as directions. For example, one of my paths begins, "Ex- ot-, ad- superior, ecto- derm-, peri-frontal, circum- rhino-, " which loosely means, exit the ear, go to the superior region, outside the skin, around the frontal region, around the nose. At the end of each path I leave a clue that lets me know the students actually made it. The combined clues form a sentence.

Cynthia Ann Radle (McCullough High School REV)

1995-06-30

128

A transmission electron microscope for lecture demonstrations  

NASA Astrophysics Data System (ADS)

A simple transmission electron microscope (TEM) suitable for lecture demonstrations is described. In this TEM electrons are created in a glow discharge between two parallel electrodes in air at a reduced pressure. The electrons are collimated by a small hole in the anode, focused by a solenoid that acts as an electromagnetic lens, and imaged on a thin layer of phosphor deposited inside an Erlenmeyer flask. An image of a biological sample placed between the source and the lens can be magnified about 20 times. The microscope uses inexpensive components that can be quickly assembled during a demonstration. The TEM provides a visual and memorable display that highlights phenomena such as mean-free-path, charged particle optics, electrical discharges, and cathodoluminescence.

Panitz, J. A.; Rempfer, Gertrude

2006-11-01

129

Carbon nanotubes: Electrons in one dimension  

NASA Astrophysics Data System (ADS)

The work presented in this thesis will discuss transport measurements on individual single-walled nanotubes (SWNTs) and SWNT bundles. SWNTs, which are essentially rolled-up sheets of graphite, are either one-dimensional (1D) metals or 1D semiconductors depending on how they are rolled-up. Measurements on both metallic and semiconducting SWNTs will be presented. Chapter 1 will present an introductory overview to the thesis, discussing prior related experimental work and introducing basic concepts that are used in subsequent chapters. Chapter 2 discusses the experimental methods we have used to study transport in SWNTs. Chapters 3 and 4 discuss low temperature measurements of metallic SWNTs. Chapter 3 will discuss the low temperature behavior of the conductance of a SWNT bundle, or rope, that shows quantum mechanical effects resulting from the finite size of the sample. Chapter 4 will discuss how these finite size effects can be used to experimentally study the quantum level structure in metallic nanotubes and the effects of an applied magnetic field. In chapters 5 and 6, we discuss transport measurements of semiconducting SWNTs. In chapter 5, we show that semiconducting SWNT's can be doped with potassium. Chapter 6 presents experiment and theory that indicate that the elastic mean free path in metallic tubes is far longer than in semiconducting tubes. Chapters 7 and 8 address the effects of electron-electron (e-e) interactions on the transport properties of metallic SWNTs. Chapter 7 discusses some theoretical aspects of 1D wires when e-e interactions are taken account, giving a simplified picture of the Luttinger-liquid state expected for a 1D system of interacting electrons. Finally, chapter 8 will discuss measurements on metallic samples with extremely long mean free paths. These experiments show evidence of this Luttinger-liquid behavior, in which the electron-electron interactions lead to a qualitatively different ground state than what would be expected with Fermi-liquid theory.

Bockrath, Marc William

130

Electron dynamics in inhomogeneous magnetic fields.  

PubMed

This review explores the dynamics of two-dimensional electrons in magnetic potentials that vary on scales smaller than the mean free path. The physics of microscopically inhomogeneous magnetic fields relates to important fundamental problems in the fractional quantum Hall effect, superconductivity, spintronics and graphene physics and spins out promising applications which will be described here. After introducing the initial work done on electron localization in random magnetic fields, the experimental methods for fabricating magnetic potentials are presented. Drift-diffusion phenomena are then described, which include commensurability oscillations, magnetic channelling, resistance resonance effects and magnetic dots. We then review quantum phenomena in magnetic potentials including magnetic quantum wires, magnetic minibands in superlattices, rectification by snake states, quantum tunnelling and Klein tunnelling. The third part is devoted to spintronics in inhomogeneous magnetic fields. This covers spin filtering by magnetic field gradients and circular magnetic fields, electrically induced spin resonance, spin resonance fluorescence and coherent spin manipulation. PMID:21393794

Nogaret, Alain

2010-06-04

131

Electron Heat Conduction in the Solar Wind: Transition from Spitzer-Hrm to the Collisionless Limit  

NASA Astrophysics Data System (ADS)

We use a statistically significant set of measurements to show that the field-aligned electron heat flux q ? in the solar wind at 1 AU is consistent with the Spitzer-Hrm collisional heat flux q sh for temperature gradient scales larger than a few mean free paths LT >~ 3.5?fp. This represents about 65% of the measured data and corresponds primarily to high ?, weakly collisional plasma ("slow solar wind"). In the more collisionless regime ?fp/LT >~ 0.28, the electron heat flux is limited to q ?/q 0 ~ 0.3, independent of mean free path, where q 0 is the "free-streaming" value; the measured q ? does not achieve the full q 0. This constraint q ?/q 0 ~ 0.3 might be attributed to wave-particle interactions, effects of an interplanetary electric potential, or inherent flux limitation. We also show a ? e dependence to these results that is consistent with a local radial electron temperature profile Te ~ r -? that is a function of the thermal electron beta ? = ?(? e ) and that the ? dependence of the collisionless regulation constraint is not obviously consistent with a whistler heat flux instability. It may be that the observed saturation of the measured heat flux is a simply a feature of collisional transport. We discuss the results in a broader astrophysical context.

Bale, S. D.; Pulupa, M.; Salem, C.; Chen, C. H. K.; Quataert, E.

2013-06-01

132

Low Energy Electron Spin Filter  

NASA Astrophysics Data System (ADS)

A new type of low energy electron spin filter has been investigated. Ultrathin (nanometer scale) ferromagnetic free standing micro-film [50micron by 50 micron mesh] arrays have been fabricated. These films exhibit high electron transmission rate (on the order of 1%) at low electron kinetic energies (energies from 5 to 30 eV have been measured). The composition of the films is a 2.0nm Au/1.5nm Co/3.0nm C sandwitch, in which the 3nm-thick C "substrate" supports the 1.5nm-thick ferromagnetic (Co) and 2nm-thick Au cap layers. The deleterious effects of sources of stray electrons (e.g. pinholes) are dealt with by measuring the electron transmission as a function of the cap layer thickness. This measurement also determines directly the low energy electron mean free path in the cap layer. Magneto-optical Kerr Effect (MOKE) measurements of these films have also been performed, which indicate that these films exhibit reasonably high remanence. These results imply that a high figure of merit spin detector can be designed based upon this electron spin filter.

Wu, X.; Hulbert, S. L.; Sinkovic, B.

1997-03-01

133

Efficient path profiling  

Microsoft Academic Search

A path profile determines how many times each acyclic path in a routine executes. This type of profiling subsumes the more common basic block and edge profiling, which only approximate path frequencies. Path profiles have many potential uses in program performance tuning, profile-directed compilation, and software test coverage. This paper describes a new algorithm for path profiling. This simple, fast

Thomas Ball; James R. Larus

1996-01-01

134

A novel method for patient exit and entrance dose prediction based on water equivalent path length measured with an amorphous silicon electronic portal imaging device.  

PubMed

In vivo dosimetry is one of the quality assurance tools used in radiotherapy to monitor the dose delivered to the patient. Electronic portal imaging device (EPID) images for a set of solid water phantoms of varying thicknesses were acquired and the data fitted onto a quadratic equation, which relates the reduction in photon beam intensity to the attenuation coefficient and material thickness at a reference condition. The quadratic model is used to convert the measured grey scale value into water equivalent path length (EPL) at each pixel for any material imaged by the detector. For any other non-reference conditions, scatter, field size and MU variation effects on the image were corrected by relative measurements using an ionization chamber and an EPID. The 2D EPL is linked to the percentage exit dose table, for different thicknesses and field sizes, thereby converting the plane pixel values at each point into a 2D dose map. The off-axis ratio is corrected using envelope and boundary profiles generated from the treatment planning system (TPS). The method requires field size, monitor unit and source-to-surface distance (SSD) as clinical input parameters to predict the exit dose, which is then used to determine the entrance dose. The measured pixel dose maps were compared with calculated doses from TPS for both entrance and exit depth of phantom. The gamma index at 3% dose difference (DD) and 3 mm distance to agreement (DTA) resulted in an average of 97% passing for the square fields of 5, 10, 15 and 20 cm. The exit dose EPID dose distributions predicted by the algorithm were in better agreement with TPS-calculated doses than phantom entrance dose distributions. PMID:20019398

Kavuma, Awusi; Glegg, Martin; Metwaly, Mohamed; Currie, Garry; Elliott, Alex

2009-12-17

135

A novel method for patient exit and entrance dose prediction based on water equivalent path length measured with an amorphous silicon electronic portal imaging device  

NASA Astrophysics Data System (ADS)

In vivo dosimetry is one of the quality assurance tools used in radiotherapy to monitor the dose delivered to the patient. Electronic portal imaging device (EPID) images for a set of solid water phantoms of varying thicknesses were acquired and the data fitted onto a quadratic equation, which relates the reduction in photon beam intensity to the attenuation coefficient and material thickness at a reference condition. The quadratic model is used to convert the measured grey scale value into water equivalent path length (EPL) at each pixel for any material imaged by the detector. For any other non-reference conditions, scatter, field size and MU variation effects on the image were corrected by relative measurements using an ionization chamber and an EPID. The 2D EPL is linked to the percentage exit dose table, for different thicknesses and field sizes, thereby converting the plane pixel values at each point into a 2D dose map. The off-axis ratio is corrected using envelope and boundary profiles generated from the treatment planning system (TPS). The method requires field size, monitor unit and source-to-surface distance (SSD) as clinical input parameters to predict the exit dose, which is then used to determine the entrance dose. The measured pixel dose maps were compared with calculated doses from TPS for both entrance and exit depth of phantom. The gamma index at 3% dose difference (DD) and 3 mm distance to agreement (DTA) resulted in an average of 97% passing for the square fields of 5, 10, 15 and 20 cm. The exit dose EPID dose distributions predicted by the algorithm were in better agreement with TPS-calculated doses than phantom entrance dose distributions.

Kavuma, Awusi; Glegg, Martin; Metwaly, Mohamed; Currie, Garry; Elliott, Alex

2010-01-01

136

Flight Path Displays.  

National Technical Information Service (NTIS)

Aircraft display technology has advanced to the state where the flight path display, an integrated format on which both the vertical and horizontal path are graphically represented, is feasible. This report researches efforts made to design flight paths f...

D. A. Warner

1979-01-01

137

Decoupling initial electron beam parameters for Monte Carlo photon beam modelling by removing beam-modifying filters from the beam path.  

PubMed

A new method is presented to decouple the parameters of the incident e(-) beam hitting the target of the linear accelerator, which consists essentially in optimizing the agreement between measurements and calculations when the difference filter, which is an additional filter inserted in the linac head to obtain uniform lateral dose-profile curves for the high energy photon beam, and flattening filter are removed from the beam path. This leads to lateral dose-profile curves, which depend only on the mean energy of the incident electron beam, since the effect of the radial intensity distribution of the incident e- beam is negligible when both filters are absent. The location of the primary collimator and the thickness and density of the target are not considered as adjustable parameters, since a satisfactory working Monte Carlo model is obtained for the low energy photon beam (6 MV) of the linac using the same target and primary collimator. This method was applied to conclude that the mean energy of the incident e- beam for the high energy photon beam (18 MV) of our Elekta SLi Plus linac is equal to 14.9 MeV. After optimizing the mean energy, the modelling of the filters, in accordance with the information provided by the manufacturer, can be verified by positioning only one filter in the linac head while the other is removed. It is also demonstrated that the parameter setting for Bremsstrahlung angular sampling in BEAMnrc ('Simple' using the leading term of the Koch and Motz equation or 'KM' using the full equation) leads to different dose-profile curves for the same incident electron energy for the studied 18 MV beam. It is therefore important to perform the calculations in 'KM' mode. Note that both filters are not physically removed from the linac head. All filters remain present in the linac head and are only rotated out of the beam. This makes the described method applicable for practical usage since no recommissioning process is required. PMID:16333165

De Smedt, B; Reynaert, N; Flachet, F; Coghe, M; Thompson, M G; Paelinck, L; Pittomvils, G; De Wagter, C; De Neve, W; Thierens, H

2005-12-06

138

Variance reduction unique to coupled electron/photon Monte Carlo transport  

SciTech Connect

The disparity between the mean free paths of electrons and photons leads to unique variance-reduction procedures for the coupled transport at intermediate energies. It is electrons that ultimately deposit charge and energy, yet electron transport is much more expensive than photon transport. The objective, then, is to control the variance of the energy, angular, and spatially dependent populations of the two species within the generation and transport of the cascade in such a way that all contributions to electron flux in the vicinity of the detector are represented to a degree commensurate with their importance. However, in [beta]/[gamma] transport, biasing techniques are often unique to one or the other of the species.

Halbleib, J.A.; Kensek, R.P.; Valdez, G.D.

1993-01-01

139

Total cross sections for positron and electron scattering from pyrimidine  

NASA Astrophysics Data System (ADS)

In this paper we report original measurements of total cross sections for positron scattering from the important biomolecule pyrimidine. The energy range of these measurements was 0.3-45 eV, while the energy resolution was ~260 meV. In addition, we report theoretical results, calculated within the independent atom-screened additivity rule (IAM-SCAR) formalism, for the corresponding electron impact total cross sections. In that case the energy range is 1-10 000 eV. Total cross sections are very important input data for codes that seek to simulate charged-particle tracks in matter, as they define the mean-free path between collisions. As the present data and computations are to the best of our knowledge the first total cross sections to be reported for either positron or electron scattering from pyrimidine, they fill an important void in our available knowledge in the literature.

Zecca, A.; Chiari, L.; Garca, G.; Blanco, F.; Trainotti, E.; Brunger, M. J.

2010-11-01

140

Electron and phonon transport in Co-doped FeV0.6Nb0.4Sb half-Heusler thermoelectric materials  

NASA Astrophysics Data System (ADS)

The electron and phonon transport characteristics of n-type Fe1-xCoxV0.6Nb0.4Sb half-Heusler thermoelectric compounds is analyzed. The acoustic phonon scattering is dominant in the carrier transport. The deformation potential of Edef = 14.1 eV and the density of state effective mass m* ~ 2.0 me are derived under a single parabolic band assumption. The band gap is calculated to be ~0.3 eV. Electron and phonon mean free paths are estimated based on the low and high temperature measurements. The electron mean free path is higher than the phonon one above room temperature, which is consistent with the experimental result that the electron mobility decreases more than the lattice thermal conductivity by grain refinement to enhance boundary scattering. A maximum ZT value of ~0.33 is obtained at 650 K for x = 0.015, an increase by ~60% compared with FeVSb. The optimal doping level is found to be ~3.0 1020 cm-3 at 600 K.

Fu, Chenguang; Liu, Yintu; Xie, Hanhui; Liu, Xiaohua; Zhao, Xinbing; Jeffrey Snyder, G.; Xie, Jian; Zhu, Tiejun

2013-10-01

141

DISCUSS: Critical Path Analysis  

NSDL National Science Digital Library

This module, by Barrie Baker and Neville Hunt of Coventry University, introduces critical path analysis and addresses the following topics: Networks, Critical paths, Floats, Activity-on-node (AON) networks. Excel spreadsheets are used to provide examples and exercises.

Hunt, Neville; Baker, Barrie

2009-04-23

142

Path planning for UAVs  

Microsoft Academic Search

In this paper, a two step path-planning algorithm for UAVs is proposed. The algorithm generates a stealthy path through a set of enemy radar sites of known location, and provides an intuitive way to trade-off stealth versus path length. In the first step, a suboptimal rough-cut path is generated through the radar sites by constructing and searching a graph based

S. A. Bortoff; E. Hartford

2000-01-01

143

Internal tests and improvements of the Krook model for nonlocal electron energy transport in laser produced plasmas  

SciTech Connect

Electron thermal transport in a laser produced plasma cannot be described with a local approximation in many regions of a laser produced plasma because the electron mean free path is longer than the temperature gradient scale length. Since a Krook model for the electron Vlasov equation is analytically solvable in the nonlocal limit, one can find simple expressions for the electron thermal flux in the nonlocal limit, and these can be economically incorporated into fluid simulations. The Krook model provides reasonable descriptions of both preheat and flux limitation. We have analyzed the Krook model in a series of publications. Here we streamline the presentation of the model, show qualitatively just what the model predicts in certain situations, give internal tests to check the validity of the model, and provide more accurate analytic approximations to the integral formulas which the model gives rise to.

Colombant, Denis G.; Manheimer, Wallace M. [Naval Research Laboratory, Code 6730, Plasma Physics Division, Washington, DC 20375 (United States)

2010-11-15

144

Electron Transport and Band Structure in Phosphorus-Doped Polycrystalline Silicon Films  

SciTech Connect

We study transport mechanisms, effective mass, and band structure by measuring the resistivity, Hall, and Seebeck and Nernst coefficients in heavily phosphorus-doped polycrystalline silicon films made by thermal crystallization of amorphous silicon. We observe a change in transport mechanism which results in an increase in electron mobility from 10% to 80% of the single-crystal silicon mobility as the carrier concentration increases from 10{sup 19} to 10{sup 20} cm{sup -3}. Our measurements of effective mass at the Fermi level indicate that as the carrier concentration increases, there is a shift from impurity-band transport to conduction-band transport, and that the electron effective mass is lower in the impurity band than in the conduction band of Si. The shift to conduction-band transport improves electron mobility with carrier density by improving intragrain carrier mean free path lengths and relaxation times.

Young, D. L.; Branz, H. M.; Liu, F.; Reedy, R.; To, B.; Wang, Q.

2009-01-01

145

Walden's Paths - Ensemble Edition  

NSDL National Science Digital Library

Waldens Paths enables users of digital document collections (e.g. the Web) to exploit these documents by reusing them for previously unintended audiences in an academic setting. Authors of paths (usually educators) overlay a linear, directed meta-structure over the Web documents and recontextualize these by adding explanatory text to achieve their curricular goals. Paths do not modifythe structure or content of the Web resources that they include. The creation of a path over pre-organized content (e.g. books, Web pages) to reorganize and associate related information serves to facilitate easy retrieval and communication. Waldens Paths displays the information that the path points to in conjunction with the textual annotations added by the author of the path.

2011-01-04

146

The control of the electron quantum paths for the generation of isolated attosecond pulse using a three-color laser scheme  

NASA Astrophysics Data System (ADS)

We theoretically propose a three-color laser scheme to enhance the high-order harmonic intensity and generate an isolated attosecond pulse. By adding a 3 fs, 1600 nm laser pulse to a synthesized two-color laser field (5 fs, 800 nm and 10 fs, 1200 nm), the harmonic intensity is effectively enhanced and an isolated attosecond pulse with duration 41 as is generated. In this scheme, the short trajectory is suppressed, the selection of the long quantum path can be achieved. We also investigate emission time of harmonics in terms of the time-frequency analysis and the semi-classical three-step model to illustrate the physical mechanism of high-order harmonic generation.

Ge, Xin-Lei; Xia, Chang-Long; Liu, Xue-Shen

2012-11-01

147

Unified description of coherent and dissipative electron transport  

NASA Astrophysics Data System (ADS)

We apply the Landauer-Bttiker theory of dc electron transport to a chain of scatterers, which are coupled to an external heat bath. In this approach, coherent and dissipative transport are treated in a unified manner, and the suppression of quantum coherence effects for increasing coupling with the heat bath can be described. In particular, we find that the resonant behavior of the conductance with respect to the Fermi wave vector disappears gradually with increasing coupling. For intermediate coupling, resonance effects are only found for sufficiently small samples. They disappear asymptotically with increasing sample size. The characteristic length for this dephasing effect corresponds to the mean free path, which is derived from the exponential decay of the transmission coefficient in the presence of inelastic scattering.

Maschke, K.; Schreiber, M.

1991-08-01

148

Simulations of photon-assisted tunneling using the Fokker-Planck equation to model the scattering of electrons within the emitting metal tip.  

PubMed

A method to simulate photon-assisted tunneling is developed, and applied to model laser-assisted field emission from metals. Our simulations show that most of the exchange of quanta between the electrons and the radiation occurs within the emitting metal tip. In typical experiments (lambda = 670 nm with tungsten metal) the depth of penetration for the radiation is four times the mean free path for electrons at the Fermi level, so it is necessary to allow for scattering. We use a Floquet expansion with the time-dependent Schrdinger equation to allow for the exchange of quanta between the electrons and the radiation field. Multiparticle effects are modeled with the density functional theory within the local density approximation for the Kohn-Sham exchange and correlation, and the Fokker-Planck formulation is used to determine the effects of scattering on the energy distribution of the electrons. PMID:11770749

Hagmann, M J; Mousa, M S; Brugat, M

2001-10-01

149

Transverse electron transport in layered metallic systems: Giant magnetoresistance and injection of spins  

SciTech Connect

The distribution functions for electrons differing in the sign of the spin projection and belonging to different layers (ferromagnetic and nonmagnetic) are determined from the system of Boltzmann kinetic equations. These functions make it possible to derive integral equations for electrochemical potentials for arbitrary ratios of characteristic lengths (layer thicknesses and momentum-and diffusion mean free paths) and to describe transverse electron transport both in the bulk and in the surface regions of the multilayer structure. The expressions for the effective contact resistance are derived and the value of the transverse ohmic resistance of the structure is found, as well as its spin-dependent part determined by the values of injection factors {gamma}. The values of nonequilibrium spin polarizations, which are also connected with coefficients {gamma}, are determined. The values of {gamma} are calculated for various relations between the characteristic parameters of the given system and for various types of magnetic order.

Kravchenko, V. Ya. [Russian Academy of Sciences, Institute of Solid State Physics (Russian Federation)], E-mail: krav@issp.ac.ru

2006-05-15

150

Resistivity of thin gold films on mica induced by electron-surface scattering: Application of quantitative scanning tunneling microscopy  

NASA Astrophysics Data System (ADS)

We report a comparison between the resistivity measured on thin gold films deposited on mica, with predictions based upon classical theories of size effects (Drude's, Sondheimer's and Calecki's), as well as predictions based upon quantum theories of electron-surface scattering (the modified theory of Sheng, Xing and Wang, the theory of Tesanovic, Jaric and Maekawa, and that of Trivedi and Aschroft). From topographic images of the surface recorded with a Scanning Tunneling Microscope, we determined the rms roughness amplitude, ? and the lateral correlation length, ? corresponding to a Gaussian representation of the average height-height autocorrelation function, describing the roughness of each sample in the scale of length set by the Fermi wave length. Using (?, ?) as input data, we present a rigorous comparison between resistivity data and predictions based upon the theory of Calecki as well as quantum theoretical predictions without adjustable parameters. The resistivity was measured on gold films of different thickness evaporated onto mica substrates, between 4 K and 300 K. The resistivity data covers the range 0.1 < x(T) < 6.8, for 4 K < T < 300 K, where x(T) is the ratio between film thickness and electron mean free path in the bulk at temperature T. We experimentally identify electron-surface and electron-phonon scattering as the microscopic electron scattering mechanisms giving rise to the macroscopic resistivity. The different theories are all capable of estimating the thin film resistivity to an accuracy better than 10%; however the mean free path and the resistivity characterizing the bulk turn out to depend on film thickness. Surprisingly, only the Sondheimer theory and its quantum version, the modified theory of Sheng, Xing and Wang, predict and increase in resistivity induced by size effects that seems consistent with published galvanomagnetic phenomena also arising from electron-surface scattering measured at low temperatures.

Robles, Marcelo E.; Gonzalez-Fuentes, Claudio A.; Henriquez, Ricardo; Kremer, German; Moraga, Luis; Oyarzun, Simn; Suarez, Marco Antonio; Flores, Marcos; Munoz, Raul C.

2012-02-01

151

The SunPath  

NSDL National Science Digital Library

This site features an interactive applet that models the Sun's path from a geocentric view. It calculates and visualizes the position of the Sun based on latitude and time, and allows students to simulate the Sun's position and path for an hour, a day, a month or a year.

University, Australian N.

152

Shortest path algorithms  

Microsoft Academic Search

Theshortest path problem is considered from a computational point of view. Eight algorithms which solve theshortest path tree problem on directed graphs are presented, together with the results of wide-ranging experimentation designed to compare their relative performances on different graph topologies. The focus of this paper is on the implementation of the different data structures used in the algorithms. A

Giorgio Gallo; Stefano Pallottino

1988-01-01

153

Visualizing Paths in Context  

Microsoft Academic Search

Data about movement through a space is increasingly becoming available for capture and analysis. In many applications, this data is captured or modeled as transitions between a small number of areas of interests, or a finite set of states, and these transitions constitute paths in the space. Similarities and differences between paths are of great importance to such analyses, but

Fabio Pellacini; Lori Lorigo; Geri Gay

2006-01-01

154

Agricultural robot path tracking based on predictable path  

Microsoft Academic Search

In order to realize the path tracking of agricultural robot in complex farmland environment and improve its speed and accuracy, a kind of agricultural robot path tracking method based predictable path was put forward in this paper. The definition of predictable path and fitting were presented, the positioning principles and methods were described in detail. The implementation method of path

Chi Gao; Yougang Su; Hui Ma

2010-01-01

155

Path-summation waveforms  

NASA Astrophysics Data System (ADS)

In this paper, we examine an efficient, practical method to calculate approximate, finite-frequency waveforms for the early signals from a point source in 3-D acoustic media with smoothly varying velocity and constant density. In analogy to the use of Feynman path integrals in quantum physics, we obtain an approximate waveform solution for the scalar wave equation by a Monte Carlo summation of elementary signals over a representative sample of all possible paths between a source and observation point. The elementary signal is formed from the convolution of the source time function with a time derivative of the Green's function for the homogeneous problem. For each path, this elementary signal is summed into a time series at a traveltime obtained from an integral of slowness along the path. The constructive and destructive interference of these signals produces the approximate waveform response for the range of traveltimes covered by the sampled paths. We justify the path-summation technique for a smooth medium using a heuristic construction involving the Helmholtz-Kirchhoff integral theorem. The technique can be applied to smooth, but strongly varying and complicated velocity structures. The approximate waveform includes geometrical spreading, focusing, defocusing and phase changes, but does not fully account for multiple scattering. We compare path-summation waveforms with the exact solution for a 3-D geometry involving a low-velocity spherical inclusion, and with finite-difference waveforms for a 2-D structure with realistic, complicated velocity variations. In contrast to geometrical-ray methods, the path-summation approach reproduces finite-frequency wave phenomena such as diffraction and does not exhibit singular behaviour. Relative to the finite-difference numerical method, the path-summation approach requires insignificant computer memory and, depending on the number of waveforms required, up to one to two orders of magnitude less computing time. The sampled paths and associated traveltimes produced by the path summation give a relation between the medium and the signal on the waveform that is not available with finite-difference and finite-element methods. Furthermore, the speed and accuracy of the path-summation method may be sufficient to allow 3-D waveform inversion using stochastic, non-linear, global search methods.

Lomax, Anthony

1999-09-01

156

Tortuous path chemical preconcentrator  

SciTech Connect

A non-planar, tortuous path chemical preconcentrator has a high internal surface area having a heatable sorptive coating that can be used to selectively collect and concentrate one or more chemical species of interest from a fluid stream that can be rapidly released as a concentrated plug into an analytical or microanalytical chain for separation and detection. The non-planar chemical preconcentrator comprises a sorptive support structure having a tortuous flow path. The tortuosity provides repeated twists, turns, and bends to the flow, thereby increasing the interfacial contact between sample fluid stream and the sorptive material. The tortuous path also provides more opportunities for desorption and readsorption of volatile species. Further, the thermal efficiency of the tortuous path chemical preconcentrator is comparable or superior to the prior non-planar chemical preconcentrator. Finally, the tortuosity can be varied in different directions to optimize flow rates during the adsorption and desorption phases of operation of the preconcentrator.

Manginell, Ronald P. (Albuquerque, NM); Lewis, Patrick R. (Albuquerque, NM); Adkins, Douglas R. (Albuquerque, NM); Wheeler, David R. (Albuquerque, NM); Simonson, Robert J. (Cedar Crest, NM)

2010-09-21

157

Follow the Paths  

NSDL National Science Digital Library

In this lesson, younger students will be introduced to the various orbital paths that are used for satellites. Using a globe and a satellite model or a large picture of Earth, the teacher will introduce three types of orbital paths (polar, elliptical, and geosynchronous). The students should be able to define 'satellite', define the three types of orbits, describe how satellites orbit the Earth, and understand how they are slowed down by drag from the atmosphere.

158

Path integrals and parastatistics  

NASA Astrophysics Data System (ADS)

The propagator and corresponding path integral for a system of identical particles obeying parastatistics are derived. It is found that the statistical weights of topological sectors of the path integral for parafermions and parabosons are simply related through multiplication by the parity of the permutation of the final positions of the particles. Appropriate generalizations of statistics are proposed obeying unitarity and factorizability (strong cluster decomposition). The realization of simple maximal occupancy (Gentile) statistics is shown to require ghost states.

Polychronakos, Alexios P.

1996-02-01

159

PathFinder Science  

NSDL National Science Digital Library

PathFinder Science contains research projects about water conservation, tardigrades, a winter bird survey, ozone, ultraviolet light and DNA, global warming, spot removal, lichens, stream monitoring, amphibian biomonitoring, and particulate monitoring. Free registration to the PathFinder Science Network offers the opportunity to be a part of the listserv, upload collaborative project data or publish research work. There are tools and tips to help students publish their research on the web.

160

Electron scattering at surfaces and grain boundaries in thin Au films  

NASA Astrophysics Data System (ADS)

The electron scattering at surfaces and grain boundaries is investigated using polycrystalline Au films deposited onto mica substrates. We vary the three length scales associated with: (i) electron scattering in the bulk, that at temperature T is characterized by the electronic mean free path in the bulk ?0(T); (ii) electron-surface scattering, that is characterized by the film thickness t; (iii) electron-grain boundary scattering, that is characterized by the mean grain diameter D. We varied independently the film thickness from approximately 50 nm to about 100 nm, and the typical grain size making up the samples from 12 nm to 160 nm. We also varied the scale of length associated with electron scattering in the bulk by measuring the resistivity of each specimen at temperatures T, 4 K < T < 300 K. Cooling the samples to 4 K increases ?0(T) by approximately 2 orders of magnitude. Detailed measurements of the grain size distribution as well as surface roughness of each sample were performed with a Scanning Tunnelling Microscope (STM). We compare, for the first time, theoretical predictions with resistivity data employing the two theories available that incorporate the effect of both electron-surface as well as electron-grain boundary scattering acting simultaneously: the theory of A.F. Mayadas and M. Shatzkes, Phys. Rev. 1 1382 (1970) (MS), and that of G. Palasantzas, Phys. Rev. B 58 9685 (1998). We eliminate adjustable parameters from the resistivity data analysis, by using as input the grain size distribution as well as the surface roughness measured with the STM on each sample. The outcome is that both theories provide a fair representation of both the temperature as well as the thickness dependence of the resistivity data, but yet there are marked differences between the resistivity predicted by these theories. In the case of the MS theory, when the average grain diameter D is significantly smaller than ?0(300) = 37 nm, the electron mean free path in the bulk at 300 K, the effect of electron-grain boundary scattering dominates the increase in resistivity of the film over the bulk, and the electronic mean free path, ?D(4), computed from Drude's model at 4 K, is similar to the grain diameter D. The increase in resistivity attributable to electron-grain boundary scattering can be as large as 220 at low temperatures, for samples made out of 12 nm grains. On the contrary, when D is significantly larger than ?0(300), then electron-surface scattering dominates the increase in resistivity. When D is comparable to ?0(300), there is a cross over where both electron-surface and electron-grain boundary scattering do contribute to increasing the resistivity of the film over that of the bulk. These predictions are in sharp contrast with those based upon the theory of Palasantzas, that predicts an increase in resistivityattributable to electron-grain boundary/surface scatteringthat turns out to be essentially unity regardless of the size of the grains making up the sample.

Henriquez, Ricardo; Flores, Marcos; Moraga, Luis; Kremer, German; Gonzlez-Fuentes, Claudio; Munoz, Raul C.

2013-05-01

161

Thermal conductivity of the pyrochlore superconductor KOs2O6: strong electron correlations and fully gapped superconductivity.  

PubMed

To elucidate the nature of the superconducting ground state of the geometrically frustrated pyrochlore KOs2O6 (Tc=9.6 K), the thermal conductivity was measured down to low temperatures (approximately Tc/100). We found that the quasiparticle mean free path is strikingly enhanced below a transition at Tp=7.8 K, indicating enormous electron inelastic scattering in the normal state. In magnetic fields, the conduction at T-->0 K is nearly constant up to approximately 0.4Hc2, in contrast with the rapid growth expected for superconductors with an anisotropic gap. This unambiguously indicates a fully gapped superconductivity, in contrast with previous studies. These results highlight that KOs2O6 is unique among superconductors with strong electron correlations. PMID:16907273

Kasahara, Y; Shimono, Y; Shibauchi, T; Matsuda, Y; Yonezawa, S; Muraoka, Y; Hiroi, Z

2006-06-23

162

Generation and accretion of electrons in complex plasmas with cylindrical particles  

NASA Astrophysics Data System (ADS)

This paper presents an analytical model for the physical understanding of the charging of cylindrical dust particles in an open complex plasma system. Two different mechanisms, viz., thermionic emission and photoelectric emission have been considered for the electron generation from the charged cylindrical dust particles; the corresponding expressions for the rate of emission of electrons and their mean energy have been derived. A simple approach has been adopted to derive the expression for the rate of electron accretion to the dust particle. Further a new expression for the mean energy associated with the accreted electrons due to cylindrical dust particle has been derived and presented. An interesting comparison of results obtained in the case of spherical and cylindrical dust particles has also been made. Using these expressions, a formalism has been developed for the electronic processes in an illuminated dust cloud with cylindrical particles, on the basis of charge neutrality condition and number and energy balance of electrons; the charge carried by the cylindrical dust particles, electron temperature, and electron density corresponding to a given situation have been determined. The limitation of the applicability of the theory, viz., that the mean free path of an electron for accretion by dust particles be less than the dimension of the dust cloud has been pointed out.

Sodha, Mahendra Singh; Mishra, S. K.; Misra, Shikha

2009-12-01

163

A high-energy electron scattering study of the electronic structure and elemental composition of O-implanted Ta films used for the fabrication of memristor devices  

NASA Astrophysics Data System (ADS)

High-energy electron scattering is used to investigate Ta films implanted with 10 keV O ions. These films are of interest as they have been used for the fabrication of memristors. High-energy electron scattering is used with incoming electron energies ranging from 5 to 40 keV. The inelastic mean free path, and hence the probing depth, is at these energies of the same order as the range of the implanted ions. At the same time, we can distinguish the mass of the atom that scattered the electron elastically, due to the dependence of the recoil energy on the mass of the scatterer. This allows us to determine quantitatively the atomic composition near the surface from the signal of electrons that have scattered elastically but not inelastically. Electrons that have scattered inelastically as well as elastically provide us with information on the possible electronic excitations. Their signal is used to monitor the presence of the Ta2O5 phase near the surface (characterised by a significant band gap of ~=4.5 eV), and estimate at what depth below the surface pure Ta metal is present. In this way, we obtain a fairly detailed picture of the elemental composition and electronic properties of these films.

Vos, M.; Grande, P. L.; Nandi, S. K.; Venkatachalam, D. K.; Elliman, R. G.

2013-08-01

164

Sampling diffusive transition paths  

SciTech Connect

We address the problem of sampling double-ended diffusive paths. The ensemble of paths is expressed using a symmetric version of the Onsager-Machlup formula, which only requires evaluation of the force field and which, upon direct time discretization, gives rise to a symmetric integrator that is accurate to second order. Efficiently sampling this ensemble requires avoiding the well-known stiffness problem associated with sampling infinitesimal Brownian increments of the path, as well as a different type of stiffness associated with sampling the coarse features of long paths. The fine-features sampling stiffness is eliminated with the use of the fast sampling algorithm (FSA), and the coarse-feature sampling stiffness is avoided by introducing the sliding and sampling (S&S) algorithm. A key feature of the S&S algorithm is that it enables massively parallel computers to sample diffusive trajectories that are long in time. We use the algorithm to sample the transition path ensemble for the structural interconversion of the 38-atom Lennard-Jones cluster at low temperature.

F. Miller III, Thomas; Predescu, Cristian

2006-10-12

165

Image simulation for atomic resolution secondary electron image.  

PubMed

It has been demonstrated recently that an atomic resolution secondary electron (SE) image can be achieved with a scanning transmission electron microscope (STEM) equipped with a probe-aberration corrector. Its high sensitivity to the surface structure provides a powerful tool to simultaneously study both surface and bulk structure in the STEM, in the combination with the annular dark field (ADF) image. To quantitatively explain the atomic resolution SE image and retrieve surface-structure information, an image simulation is required. Here, we develop a method to simultaneously calculate, for the first time, the atomic resolution SE and ADF-STEM images, based on the multislice method with a frozen-phonon approximation. An object function for secondary electrons, derived from the inelastic scattering, is used to calculate the intensity distribution of the secondary electrons emitted from each slice. The simulations show that the SE image contrast is sensitive to the surface structure and the electron inelastic mean free path, but insensitive to specimen thickness when the thickness is more than 5 nm. The simulated SE images for SrTiO(3) crystal show good agreement with the experimental observations. PMID:22940532

Wu, Lijun; Egerton, R F; Zhu, Yimei

2012-07-04

166

Electron-phonon mediated heat flow in disordered graphene  

NASA Astrophysics Data System (ADS)

We calculate the heat flux and electron-phonon thermal conductance in a disordered graphene sheet, going beyond a Fermis golden rule approach to fully account for the modification of the electron-phonon interaction by disorder. Using the Keldysh technique combined with standard impurity averaging methods in the regime kFl?1 (where kF is the Fermi wave vector and l is the mean free path), we consider both scalar potential (i.e., deformation potential) and vector-potential couplings between electrons and phonons. We also consider the effects of electronic screening at the Thomas-Fermi level. We find that the temperature dependence of the heat flux and thermal conductance is sensitive to the presence of disorder and screening, and reflects the underlying chiral nature of electrons in graphene and the corresponding modification of their diffusive behavior. In the case of weak screening, disorder enhances the low-temperature heat flux over the clean system (changing the associated power law from T4 to T3), and the deformation potential dominates. For strong screening, both the deformation potential and vector-potential couplings make comparable contributions, and the low-temperature heat flux obeys a T5 power law.

Chen, Wei; Clerk, Aashish A.

2012-09-01

167

Modified predictive formula for the electron stopping power  

SciTech Connect

We report an improved predictive formula for the electron stopping power (SP) based on an analysis and fit of SPs and electron inelastic mean free paths (IMFPs) calculated from optical data for 37 elemental solids and energies between 200 eV and 30 keV. The formula is a function of energy, density, and IMFP, and is recommended for solids with atomic numbers larger than 6. While the mean deviation between predicted and calculated SPs was 7.25%, larger deviations were found for four additional materials, Li (22.2%), Be (17.9%), graphite (15.3%), and diamond (15.7%). The predictive SP formula can be applied to multicomponent materials. Test comparisons for two compounds, guanine and InSb, showed average deviations of 16.0% and 19.1%, respectively. The improved SP formula is expected to be useful in simulations of electron trajectories in solids with the continuous slowing-down approximation (e.g., in Auger-electron spectroscopy and electron microprobe analysis)

Jablonski, A. [Institute of Physical Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44/52, 01-224 Warsaw (Poland); Tanuma, S. [Materials Analysis Station, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047 (Japan); Powell, C. J. [Surface and Microanalysis Science Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8370 (United States)

2008-03-15

168

Bond Paths Are Not Chemical Bonds  

NASA Astrophysics Data System (ADS)

This account takes to task papers that criticize the definition of a bond path as a criterion for the bonding between the atoms it links by mistakenly identifying it with a chemical bond. It is argued that the notion of a chemical bond is too restrictive to account for the physics underlying the broad spectrum of interactions between atoms and molecules that determine the properties of matter. A bond path on the other hand, as well as being accessible to experimental verification and subject to the theorems of quantum mechanics, is applicable to any and all of the interactions that account for the properties of matter. It is shown that one may define a bond path operator as a Dirac observable, making the bond path the measurable expectation value of a quantum mechanical operator. Particular attention is given to van der Waals interactions that traditionally are assumed to represent attractive interactions that are distinct from chemical bonding. They are assumed by some to act in concert with Pauli repulsions to account for the existence of condensed states of molecules. It is such dichotomies of interpretation that are resolved by the experimental detection of bond paths and the delineation of their properties in molecular crystals. Specific criticisms of the stabilization afforded by the presence of bond paths derived from spectroscopic measurements performed on dideuteriophenanthrene are shown to be physically unsound. The concept of a bond path as a "bridge of density" linking bonded atoms was introduced by London in 1928 following the definition of the electron density by Schrdinger in 1926. These papers marked the beginning of the theory of atoms in molecules linked by bond paths.

Bader, Richard F. W.

2009-09-01

169

Surface sensitivity of low energy electron Mssbauer spectroscopy (LEEMS) using57Fe  

NASA Astrophysics Data System (ADS)

Very low energy electrons (LEE) (E e ?15 eV) are produced with high intensity directly by Mssbauerabsorption and conversion in the case of57Fe [1, 4, 5]. These electrons should be very surface sensitive due to their very low attenuation length compared to the 7.3 keV K-Conversion electrons of57Fe [5, 11]. We have examined the surface sensitivity of these resonant LEE, using nonresonant56Fe metal and56Fe stainless steel foils coated with about 20 and 50 57Fe, respectively. They were exposed to air after evaporation: The 20 samples are found to be fully oxidized [5]. Depth Selective Conversion Electron Mssbauer Spectroscopy (DCEMS), performed with a high transmission orange type magnetic spectrometer [5, 6, 13] reveals a two layer structure of the 50 samples. Low Energy Electron Mssbauer Spectroscopy (LEEMS) [5] is found to be significantly more surface sensitive than conventional DCEMS, but not as surface sensitive as Auger Electron Mssbauer Spectroscopy (AEMS) using LMM-Auger electrons of 500 600 eV, as expected due to the different mean free path. But because of the very low intensity of these Auger electrons this mode appears to be not very useful for practical application.

Klingelhfer, G.; Imkeller, U.; Kankeleit, E.; Stahl, B.

1992-04-01

170

Production of electron-positron pairs by a photon propagating in a strongly magnetized thermal bath  

SciTech Connect

The production of electron-positron pairs by a photon propagating in a thermal bath in both zero and strong (B Much-Greater-Than 4.41 Multiplication-Sign 10{sup 13} G) magnetic fields has been considered. The mean free path has been calculated for the high-energy photon propagating through a thermodynamically equilibrium photon gas along the magnetic field lines so that the {gamma} {yields} e{sup -}e{sup +} decay is kinematically forbidden. It has been shown that the strong magnetic field suppresses the probability of the {gamma}{gamma} Prime {yields} e{sup -}e{sup +} process. The analyzed process can be useful for analysis of possible mechanisms of the generation of the e{sup -}e{sup +} plasma in the regions of the polar caps of magnetars.

Dunaev, M. A., E-mail: yarosla@rambler.ru; Mikheev, N. V., E-mail: mikheev@uniyar.ac.ru [Yaroslavl State University (Russian Federation)

2012-03-15

171

Monte Carlo simulations of electron thermalization in alkali iodide and alkaline-earth fluoride scintillators  

SciTech Connect

A Monte Carlo model of electron thermalization in inorganic scintillators, which was developed and applied to CsI in a previous publication [Wang et al., J. Appl. Phys. 110, 064903 (2011)], is extended to another material of the alkali halide class, NaI, and to two materials from the alkaline-earth halide class, CaF2 and BaF2. This model includes electron scattering with both longitudinal optical (LO) and acoustic phonons as well as the effects of internal electric fields. For the four pure materials, a significant fraction of the electrons recombine with self-trapped holes and the thermalization distance distributions of the electrons that do not recombine peak between approximately 25 and 50 {per_thousand}nm and extend up to a few hundreds of nanometers. The thermalization time distributions of CaF2, BaF2, NaI, and CsI extend to approximately 0.5, 1, 2, and 7 ps, respectively. The simulations show that the LO phonon energy is a key factor that affects the electron thermalization process. Indeed, the higher the LO phonon energy is, the shorter the thermalization time and distance are. The thermalization time and distance distributions show no dependence on the incident {gamma}-ray energy. The four materials also show different extents of electron-hole pair recombination due mostly to differences in their electron mean free paths (MFPs), LO phonon energies, initial densities of electron-hole pairs, and static dielectric constants. The effect of thallium doping is also investigated for CsI and NaI as these materials are often doped with activators. Comparison between CsI and NaI shows that both the larger size of Cs+ relative to Na+, i.e., the greater atomic density of NaI, and the longer electron mean free path in NaI compared to CsI contribute to an increased probability for electron trapping at Tl sites in NaI versus CsI.

Wang, Zhiguo; Xie, YuLong; Campbell, Luke W.; Gao, Fei; Kerisit, Sebastien N.

2012-07-01

172

Monte Carlo simulations of electron thermalization in alkali iodide and alkaline-earth fluoride scintillators  

NASA Astrophysics Data System (ADS)

A Monte Carlo model of electron thermalization in inorganic scintillators, which was developed and applied to CsI in a previous publication [Wang et al., J. Appl. Phys. 110, 064903 (2011)], is extended to another material of the alkali halide class, NaI, and to two materials from the alkaline-earth halide class, CaF2 and BaF2. This model includes electron scattering with both longitudinal optical (LO) and acoustic phonons as well as the effects of internal electric fields. For the four pure materials, a significant fraction of the electrons recombine with self-trapped holes and the thermalization distance distributions of the electrons that do not recombine peak between approximately 25 and 50 nm and extend up to a few hundreds of nanometers. The thermalization time distributions of CaF2, BaF2, NaI, and CsI extend to approximately 0.5, 1, 2, and 7 ps, respectively. The simulations show that the LO phonon energy is a key factor that affects the electron thermalization process. Indeed, the higher the LO phonon energy is, the shorter the thermalization time and distance are. The thermalization time and distance distributions show no dependence on the incident ?-ray energy. The four materials also show different extents of electron-hole pair recombination due mostly to differences in their electron mean free paths (MFPs), LO phonon energies, initial densities of electron-hole pairs, and static dielectric constants. The effect of thallium doping is also investigated for CsI and NaI as these materials are often doped with activators. Comparison between CsI and NaI shows that both the larger size of Cs+ relative to Na+, i.e., the greater atomic density of NaI, and the longer electron mean free path in NaI compared to CsI contribute to an increased probability for electron trapping at Tl sites in NaI versus CsI.

Wang, Zhiguo; Xie, YuLong; Campbell, Luke W.; Gao, Fei; Kerisit, Sebastien

2012-07-01

173

Monte Carlo simulations of electron thermalization in alkali iodide and alkaline-earth fluoride scintillators  

SciTech Connect

A Monte Carlo model of electron thermalization in inorganic scintillators, which was developed and applied to CsI in a previous publication [Wang et al., J. Appl. Phys. 110, 064903 (2011)], is extended to another material of the alkali halide class, NaI, and to two materials from the alkaline-earth halide class, CaF{sub 2} and BaF{sub 2}. This model includes electron scattering with both longitudinal optical (LO) and acoustic phonons as well as the effects of internal electric fields. For the four pure materials, a significant fraction of the electrons recombine with self-trapped holes and the thermalization distance distributions of the electrons that do not recombine peak between approximately 25 and 50 nm and extend up to a few hundreds of nanometers. The thermalization time distributions of CaF{sub 2}, BaF{sub 2}, NaI, and CsI extend to approximately 0.5, 1, 2, and 7 ps, respectively. The simulations show that the LO phonon energy is a key factor that affects the electron thermalization process. Indeed, the higher the LO phonon energy is, the shorter the thermalization time and distance are. The thermalization time and distance distributions show no dependence on the incident {gamma}-ray energy. The four materials also show different extents of electron-hole pair recombination due mostly to differences in their electron mean free paths (MFPs), LO phonon energies, initial densities of electron-hole pairs, and static dielectric constants. The effect of thallium doping is also investigated for CsI and NaI as these materials are often doped with activators. Comparison between CsI and NaI shows that both the larger size of Cs{sup +} relative to Na{sup +}, i.e., the greater atomic density of NaI, and the longer electron mean free path in NaI compared to CsI contribute to an increased probability for electron trapping at Tl sites in NaI versus CsI.

Wang Zhiguo; Gao Fei; Kerisit, Sebastien [Fundamental and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352 (United States); Xie Yulong [Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352 (United States); Campbell, Luke W. [National Security Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352 (United States)

2012-07-01

174

Following the Path  

ERIC Educational Resources Information Center

This article profiles Diane Stanley, an author and illustrator of children's books. Although she was studying to be a medical illustrator in graduate school, Stanley's path changed when she got married and had children. As she was raising her children, she became increasingly enamored of the colorful children's books she would check out of the

Rodia, Becky

2004-01-01

175

Paths of Innovation  

Microsoft Academic Search

In 1903 the Wright brothers' airplane travelled a couple of hundred yards. Today fleets of streamlined jets transport millions of people each day to cities worldwide. Between discovery and application, between invention and widespread use, there is a world of innovation, of tinkering, improvement and adaptation. This is the world David Mowery and Nathan Rosenberg map out in Paths of

David C. Mowery; Nathan Rosenberg

176

Intrusion Path Analysis.  

National Technical Information Service (NTIS)

The design and implementation of an Intrusion Path Analysis (IPA) function came about as a result of the upgrades to the security systems at the Savannah River Site (SRS), near Aiken, South Carolina. The stated requirements for IPA were broad, leaving opp...

R. D. Hardwick

1989-01-01

177

(Intrusion Path Analysis)  

Microsoft Academic Search

The design and implementation of an Intrusion Path Analysis (IPA) function came about as a result of the upgrades to the security systems at the Savannah River Site (SRS), near Aiken, South Carolina. The stated requirements for IPA were broad, leaving opportunity for creative freedom during design and development. The essential elements were that it: be based on alarm and

Hardwick

1989-01-01

178

Paths to Remarriage  

Microsoft Academic Search

High divorce rates and the traditionally discrepant ages at death for husbands and wives indicate a need for a more complete understanding of the paths to remarriage in contemporary America. This study uses data from the U.S. Bureau of the Census' Current Population Survey to examine the extent and timing of remarriage, social factors associated with remarriage, and the impact

Graham B. Spanier; Paul C. Glick

1980-01-01

179

Path to the Profession  

ERIC Educational Resources Information Center

A growing number of institutions are being more deliberate about bringing in fundraisers who fit the culture of the development department and about assessing skills and providing training that fill specific needs. Development shops are paying more attention to cultivating their staffs, staying attuned to employees' needs and creating career paths

Coleman, Toni

2012-01-01

180

Scattering of 7.3 keV conversion electrons from a 57Co source covered gradually by gold absorbers of various thicknesses  

NASA Astrophysics Data System (ADS)

The conversion electrons emitted with initial energy of 7.3 keV from the K-shell of 57Fe atoms were measured with an electrostatic spectrometer set to the instrumental resolution of 7 eV. The source /- 57Co on an Al backing /- was covered in successive steps by six evaporated Au layers and changes of the line shapes were recorded. The thicknesses of the Au absorbers determined by neutron activation analysis varied from 2.7 to 21 nm, i.e. up to about three times the inelastic mean free path of the 7.3 keV electrons. The individual elastic and inelastic scattering events in the Al backing, Au absorbers and carbonaceous contamination overlayer were simulated by the Monte Carlo method. When the Au energy loss function derived by Yoshikawa et al. was applied in MC simulations, a reasonable agreement with measured line shapes was reached even for the thickest Au absorbers.

Dragoun, O.; palek, A.; Kovalk, A.; Yakushev, E. A.; Ryav, M.; Frna, J.; Brabec, V.; Novgorodov, A. F.; Liljequist, D.

2002-08-01

181

Exploring highly correlated materials via electron pair emission: the case of NiO/Ag(100).  

PubMed

Metal oxides like NiO are usually termed 'highly correlated', because the material properties are decisively determined by the electron-electron interaction. This makes them interesting candidates for electron pair spectroscopy which is particularly sensitive to the electron correlation. We have prepared ultrathin NiO/Ag(100) films and studied the electron pair emission upon electron impact. Compared to the metal substrate we observe an increase of the coincidence intensity by a factor of 8 for NiO. Thickness dependent measurements prove that this enhancement is an intrinsic effect rather than due to a mean free path increase of the oxide. The Nel temperature T(N) of NiO films displays a thickness dependence which allows us to tune T(N). We performed temperature dependent measurements and observed no temperature dependence of the coincidence spectra. This proves that the electron pair emission probes the local correlation rather than long range order. An enhanced coincidence intensity was also found for other oxide phases compared to their corresponding metal phases. PMID:23399964

Schumann, F O; Behnke, L; Li, C H; Kirschner, J

2013-02-12

182

Convoy electrons from atomic and molecular heavy ion collisions with solids  

NASA Astrophysics Data System (ADS)

Energy distributions of convoy electrons have been measured for different projectiles ( 0203 MeV/u < Ep/Mp < 2.7 MeV/u) traversing thin carbon foils ( 2 ?g/cm2 < px < 20 ?g/cm2 ). The motivation of our study is to elucidate the mechanisms of convoy electron production in ion-solid collisions. The analysis of the convoy peak's shape and yield dependence on dwell time td, velocity vp, incident charge state qi and atomic number zp of the projectile shows significant deviations from theories of ECC and ELC in single collision systems. In non-equilibrium of the projectile charge ( td < 2 fs ) the convoy electron yield Yc of atomic heavy ions depends strongly on qi and exhibits an exponential dependence on the dwell time. Here the ELC process prevails. In the higher dwell time regime ( td > 2 fs ) the convoy electron yield Yc is described by Yc zp 3.2Ep -1.3. In comparison to free isotachic electrons the convoy electrons have an effective mean free path about two up to ten times higher. The value depends on the velocity vp and the nuclear charge zp and suggests a strong correlation between the convoy electron and the traversing ion. We discuss the observed convoy production and their strong dwell time dependence ( td < 30 fs ).

Koschar, P.; Latz, R.; Kemmler, J.; Burkhard, M.; Frischkorn, H. J.; Hofmann, D.; Schader, J.; Schramm, R.; Groeneveld, K. O.; Brenig, M.; Elston, S.; Sellin, I. A.; Meckbach, W.

183

Calculation of electron-molecule reactions of NO decomposition in pulsed corona discharges  

NASA Astrophysics Data System (ADS)

In recent years there has been increasing research interest in the removal of nitrogen-oxides from exhaust gases using a pulsed corona discharge reactor. The pulsed streamer corona produces energetic electrons that excite, ionize and dissociate gas molecules, and by forming radicals that enhance the gas-phase chemical reactions which reduce the pollutant's concentration. The decomposition ratio of these substances is dependant on the gas composition, concentration, energy distribution of fast electrons, and other parameters. For a detailed analysis of the phenomena, knowledge of chemical reaction mechanisms is essential. The reaction rates of possible molecule-molecule reactions can in most cases be found in the literature, but the rates of the electron-molecule collision reactions depend on the energy of free electrons. With the knowledge of the field distribution, the energy of the free electrons and the reaction rates can be found. In this paper the authors present a method, in which the reaction rates of the electron- molecule collision are determined. The model is based on a calculation of: the energy of free electrons in the time and space varying field, considering the mean free path and the energy-dependent reaction cross sections of molecules. Knowing the rates, it is possible to solve the reaction kinetic equations, and to get the time-evolution of byproducts, and the decomposition ratio of the pollutant gases.

Horvth, M.; Kiss, E.

2008-12-01

184

Physical Mechanisms of the Electron Energy Distribution Function Control in Inhomogeneous Non-stationary Plasma  

NASA Astrophysics Data System (ADS)

To predict the main scenarios of electron distribution function (EDF) control, first of all it is necessary to develop a kind of roadmap of formation a different modes of the EDF in the inhomogeneous unsteady plasma. The analysis shows that the time scales are determined by the ratio between the transient time tL (the characteristic time of electron transport through the plasma volume) and the relaxation times te of the EDF momentum tm (on velocity direction) and energy. Accordingly, for the spatial variable it is the ratio between the characteristic size of plasma L and an electron mean free path l (momentum relaxation) and a length of energy relaxation of electron energy le. A significant difference between the scale of momentum relaxation and energy te >> tm, le >> l (reaching two or more orders of magnitude), allows to predict the possible modes of the EDF formation, with various degrees of selectivity effects on different groups of electrons (from a local EDF when L >> le and it is possible to affect only the entire ensemble of electrons) and the nonlocal EDF, when L << le and different groups of electrons behave independently of each other and it is possible to influence only on the interest profiles.

Kudryavtsev, Anatoly; Tsendin, Lev

2011-11-01

185

Electron kinetics and non-Joule heating in near-collisionless inductively coupled plasmas  

SciTech Connect

Electron kinetics in an inductively coupled plasma sustained by a coaxial solenoidal coil is studied for the near-collisionless regime when the electron mean free path is large compared to the tube radius. Emphasis is placed on the influence of the oscillatory magnetic field induced by the coil current and the finite dimension of the plasma on electron heating and formation of the electron distribution function (EDF). A nonlocal approach to the solution of the Boltzmann equation is developed for the near-collisionless regime when the traditional two-term Legendre expansion for the EDF is not applicable. Dynamic Monte Carlo (DMC) simulations are performed to calculate the EDF and electron heating rate in argon in the pressure range 0.3{endash}10 mTorr and driving frequency range 2{endash}40 MHz, for given distributions of electromagnetic fields. The wall potential {phi}{sub w} in DMC simulations is found self-consistently with the EDF. Simulation results indicate that the EDF of trapped electrons with total energy {var_epsilon}{lt}e{phi}{sub w} is almost isotropic and is a function solely of {var_epsilon}, while the EDF of untrapped electrons with {var_epsilon}{gt}e{phi}{sub w} is notably anisotropic and depends on the radial position. These results are in agreement with theoretical analysis. {copyright} {ital 1997} {ital The American Physical Society}

Kolobov, V.I. [Plasma Processing Laboratory, Department of Chemical Engineering, University of Houston, Houston, Texas 77204-4792 (United States); Lymberopoulos, D.P. [Applied Materials, 3100 Bowers Avenue, Santa Clara, California 95054 (United States); Economou, D.J. [Plasma Processing Laboratory, Department of Chemical Engineering, University of Houston, Houston, Texas 77204-4792 (United States)

1997-03-01

186

Electron-phonon interaction in impure polycrystalline metals  

NASA Astrophysics Data System (ADS)

We calculate the electron-phonon scattering rate in polycrystalline metals, e.g., Ti1-xAlx, in the limit of dilute impurity concentration. We consider the additional contribution due to the Umklapp process of impurity scattering, which has been neglected in all previous nearly free-electron calculations but is important for the present problem. We find that, as a result of including the Umklapp process, the scattering rate in the dirty limit qTl<<1 is enhanced by the disorder due to substitutional impurities and random lattice shift of crystallites, where qT=thermal phonon wave vector and l=electron mean free path. Specifically, we obtain the scattering rate 1/?ep~T2l-1, where T=temperature, in agreement with previous experiments both in order of magnitude and in functional dependence. This work satisfactorily explains the long-standing discrepancy between theories and experiments regarding the effect of disorder on electron-phonon scattering, for the case of polycrystalline metals with dilute impurity concentration.

Jan, W.; Wu, G. Y.; Wei, H.-S.

2001-10-01

187

Exact train pathing  

Microsoft Academic Search

Suppose we are given a schedule of train movements over a rail network into which a new train is to be included. The origin\\u000a and the destination are specified for the new train; it is required that a schedule (including the path) be determined for\\u000a it so as to minimize the time taken without affecting the schedules for the old

Viswanath Nagarajan; Abhiram G. Ranade

2008-01-01

188

Adaptive Fast Path Architecture  

Microsoft Academic Search

Adaptive Fast Path Architecture (AFPA) is a software architecture that dramatically improves the efficiency, and therefore the capacity, of Web and other network servers. The architecture includes a RAM-based cache that serves static content and a reverse proxy that can distribute requests for dynamic content to multiple servers. These two mechanisms are combined using a ?exible layer-7 (content-based) routing facility.

Elbert C. Hu; Philippe Joubert; Robert B. King; Jason D. Lavoie; John M. Tracey

2001-01-01

189

PATHS groundwater hydrologic model  

SciTech Connect

A preliminary evaluation capability for two-dimensional groundwater pollution problems was developed as part of the Transport Modeling Task for the Waste Isolation Safety Assessment Program (WISAP). Our approach was to use the data limitations as a guide in setting the level of modeling detail. PATHS Groundwater Hydrologic Model is the first level (simplest) idealized hybrid analytical/numerical model for two-dimensional, saturated groundwater flow and single component transport; homogeneous geology. This document consists of the description of the PATHS groundwater hydrologic model. The preliminary evaluation capability prepared for WISAP, including the enhancements that were made because of the authors' experience using the earlier capability is described. Appendixes A through D supplement the report as follows: complete derivations of the background equations are provided in Appendix A. Appendix B is a comprehensive set of instructions for users of PATHS. It is written for users who have little or no experience with computers. Appendix C is for the programmer. It contains information on how input parameters are passed between programs in the system. It also contains program listings and test case listing. Appendix D is a definition of terms.

Nelson, R.W.; Schur, J.A.

1980-04-01

190

Particle-in-Cell Simulation for the Control of Electron Energy Distribution of Dielectric Barrier Discharges at Atmospheric Pressure  

NASA Astrophysics Data System (ADS)

Recently, atmospheric pressure plasmas attract lots of interests for the useful applications such as surface modification and bio-medical treatment. In this study, a particle-in-cell Monte Carlo collision (PIC-MCC) simulation was adopted to investigate the discharge characteristics of a planar micro dielectric barrier discharge (DBD) with a driving frequency from 1 MHz to 50 MHz and with a gap distance from 60 to 500 micrometers. The variation of control parameters such as the gap distance, the driving wave form, and the applied voltage results in the change in the electron energy distribution function (EEDF). Through the relation between the ionization mean free path and the gap size, a significant change of EEDFs is achievable with the decrease of gap distance. Therefore, it is possible to categorize the operation range of DBDs for its applications by controlling the interactions between plasmas and neutral gas for the generation of preferable radicals.

Bae, Hyo Won; Yel Lee, Jung; Lee, Ho-Jun; Lee, Hae June

2011-11-01

191

Electron Transport Behavior on Gate Length Scaling in Sub-50 nm GaAs Metal Semiconductor Field Effect Transistors  

SciTech Connect

Short channel GaAs Metal Semiconductor Field Effect Transistors (MESFETs) have been fabricated with gate length to 20 nm, in order to examine the characteristics of sub-50 nm MESFET scaling. Here the rise in the measured transconductance is mainly attributed to electron velocity overshoot. For gate lengths below 40 nm, however, the transconductance drops suddenly. The behavior of velocity overshoot and its degradation is investigated and simulated by using a transport model based on the retarded Langevin equation (RLE). This indicates the existence of a minimum acceleration length needed for the carriers to reach the overshoot velocity. The argument shows that the source resistance must be included as an internal element, or appropriate boundary condition, of relative importance in any model where the gate length is comparable to the inelastic mean free path of the carriers.

Han, Jaeheon [Department of Electronic Engineering, Kangnam University, 111 Gugal-dong, Giheung-gu, Yongin-city, Gyeonggi-do, Korea 446-702 (Korea, Republic of)

2011-12-23

192

Stationary electron velocity distribution function in crossed electric and magnetic fields with collisions  

SciTech Connect

Analytical studies and numerical simulations show that the electron velocity distribution function in a Hall thruster discharge with crossed electric and magnetic fields is not Maxwellian. This is due to the fact that the mean free path between collisions is greater than both the Larmor radius and the characteristic dimensions of the discharge channel. However in numerical models of Hall thrusters, a hydrodynamic approach is often used to describe the electron dynamics, because discharge simulation in a fully kinetic approach requires large computing resources and is time consuming. A more accurate modeling of the electron flow in the hydrodynamic approximation requires taking into account the non-Maxwellian character of the distribution function and finding its moments, an approach that reflects the properties of electrons drifting in crossed electric and magnetic fields better than the commonly used Euler or Navier-Stokes approximations. In the present paper, an expression for the electron velocity distribution function in rarefied spatially homogeneous stationary plasma with crossed electric and magnetic fields and predominance of collisions with heavy particles is derived in the relaxation approximation. The main moments of the distribution function including longitudinal and transversal temperatures, the components of the viscous stress tensor, and of the heat flux vector are calculated. Distinctive features of the hydrodynamic description of electrons with a strongly non-equilibrium distribution function and the prospects for further development of the proposed approach for calculating the distribution function in spatially inhomogeneous plasma are discussed.

Shagayda, Andrey [Department of Electrophysics, Keldysh Research Centre, Moscow 125438 (Russian Federation)

2012-08-15

193

Low-temperature electron dephasing times in polycrystalline disordered metals  

NASA Astrophysics Data System (ADS)

We have systematically investigated the electron dephasing times ?_? in various three-dimensional polycrystalline impure metals with distinct material characteristics. (1) We have studied the electron-phonon scattering time ?_ep in the limit of ql << 1 (where q being the wave number of thermal phonons, and l being the electron elastic mean free path). We observe ?_ep-1 ~ T^2 in very many alloys. However, we do not find any universal dependence of ?_ep-1 on l. (2) At very low temperatures, we observe a saturation of ?_? in many samples. The value of ?_? (T arrow 0) falls basically in the range from 0.005 to 0.5 ns and, particularly, we find ?_? (T arrow 0) ~ D^- ?, with ? close to or slightly larger than 1, for the diffusion constant D varies from about 0.1 to 10 cm^2/s. Our observation seems to suggest a universal saturation length of ?D ?_? ~ 1000 at T arrow 0 in three-dimensional polycrystalline impure metals.

Lin, J. J.

2001-03-01

194

Fast Advection of Magnetic Fields by Hot Electrons  

SciTech Connect

Experiments where a laser-generated proton beam is used to probe the megagauss strength self-generated magnetic fields from a nanosecond laser interaction with an aluminum target are presented. At intensities of 10{sup 15} W cm{sup -2} and under conditions of significant fast electron production and strong heat fluxes, the electron mean-free-path is long compared with the temperature gradient scale length and hence nonlocal transport is important for the dynamics of the magnetic field in the plasma. The hot electron flux transports self-generated magnetic fields away from the focal region through the Nernst effect [A. Nishiguchi et al., Phys. Rev. Lett. 53, 262 (1984)] at significantly higher velocities than the fluid velocity. Two-dimensional implicit Vlasov-Fokker-Planck modeling shows that the Nernst effect allows advection and self-generation transports magnetic fields at significantly faster than the ion fluid velocity, v{sub N}/c{sub s{approx_equal}}10.

Willingale, L.; Thomas, A. G. R.; Krushelnick, K. [Blackett Laboratory, Imperial College London, London, SW7 2BZ (United Kingdom); Center for Ultrafast Optical Science, Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, Michigan 48109-2099 (United States); Nilson, P. M.; Kaluza, M. C.; Dangor, A. E.; Evans, R. G.; Fernandes, P.; Haines, M. G.; Kamperidis, C.; Kingham, R. J.; Ridgers, C. P.; Sherlock, M.; Wei, M. S.; Najmudin, Z. [Blackett Laboratory, Imperial College London, London, SW7 2BZ (United Kingdom); Bandyopadhyay, S.; Notley, M. [Central Laser Facility, Rutherford Appleton Laboratory, Chilton, Oxon, OX11 0QX (United Kingdom); Minardi, S.; Tatarakis, M. [Centre for Plasma Physics and Lasers, Department of Electronics, Technological Educational Institute of Crete, Chania, Crete (Greece); Rozmus, W. [Department of Physics, University of Alberta, Edmonton, Alberta (Canada)

2010-08-27

195

Electron pressure is less than nkT in the solar wind  

NASA Astrophysics Data System (ADS)

We claim that in the solar wind beyond 5 R odot electron pressure P e is less than n e kT e The reason is that though particles with random velocities beyond thermal velocity cannot collide hence collisionless particles below the thermal velocity can collide and exert pressure We find alpha equiv P e n e kT e density scale height mean free path 1 2 and the adiabatic relation of T e propto n e 2 alpha 3 where alpha approx 0 4 near 1AU and alpha approx 1 below 5 R odot Similarly thermal conductivity for alpha 1 is shown to scale sim T e 0 instead of the classical T e 5 2 Then thermal energy equation gives T e propto heliocentric distance -0 4 in rough accord with the observations We find that observed large line widths of OVI and Ly alpha in the polar coronal hole arise from large projected velocities of the solar wind hence no need of very high perpendicular temperature Usual Boltzmann equation always leads to P e n e kT e by taking moment of m e v In every textbook the elementary length scale of e g d x in the collision term to be multiplied to the distribution function is adopted as much larger than the mean free paths ensuring that mutual collisions of identical particles produce no momentum gain All the other terms are however supposed not to include collisions within the same d x which is a contradiction By solving this we show P e le n e kT e if collisions are not entirely

Hirayama, T.

196

Portage and Path Dependence*  

PubMed Central

We examine portage sites in the U.S. South, Mid-Atlantic, and Midwest, including those on the fall line, a geomorphological feature in the southeastern U.S. marking the final rapids on rivers before the ocean. Historically, waterborne transport of goods required portage around the falls at these points, while some falls provided water power during early industrialization. These factors attracted commerce and manufacturing. Although these original advantages have long since been made obsolete, we document the continuing importance of these portage sites over time. We interpret these results as path dependence and contrast explanations based on sunk costs interacting with decreasing versus increasing returns to scale.

Bleakley, Hoyt; Lin, Jeffrey

2012-01-01

197

Byrds Flight Path  

NSDL National Science Digital Library

The Ohio State Universitys Library web site notes As a navigational aviator, Byrd pioneered in the technology that would be the foundation for modern polar exploration and investigation. As a decorated and much celebrated hero, Byrd drew popular attention to areas of the world that became focal points of scientific investigation in numerous disciplines. More information about Admiral Richard E. Byrd can be found at (http:--www.lib.ohio-state.edu-arvweb-polar-byrd-byrd.htm). The next animation, #1001, shows Byrds plane as it follows the flight path presented in this animation.

Biddlecome, Tom; Snodgrass, Stuart; Newcombe, Marte; Jezek, Ken

1999-11-08

198

Semiclassical Ehrenfest paths  

NASA Astrophysics Data System (ADS)

Trajectories are a central concept in our understanding of classical phenomena and also in rationalizing quantum mechanical effects. In this work we provide a way to determine semiclassical paths, approximations to quantum averages in phase space, directly from classical trajectories. We avoid the need of intermediate steps, like particular solutions to the Schroedinger equation or numerical integration in phase space by considering the system to be initially in a coherent state and by assuming that its early dynamics is governed by the Heller semiclassical approximation. Our result is valid for short propagation times only, but gives non-trivial information on the quantum-classical transition.

Liberalquino, Rafael; Parisio, Fernando

2013-08-01

199

The Path of Pollution  

NSDL National Science Digital Library

This lesson plan has students follow the path of pollution from the Chernobyl nuclear accident. They will name and locate countries to which radiation traveled and describe how air pollution moves from one area to another. In addition they will compare the chronological sequence of radiation travel with geographic distribution. Provided are: essential elements, objective, focus, a list of materials, background information, procedure, and ideas for extension and evaluation. Links provide the history of the accident and the data required to complete the investigation.

200

The California PATH Database  

NSDL National Science Digital Library

The well known Berkeley Digital Library SunSite, discussed in the February 9, 1996 Scout Report, has recently added a new resource to its collection. The PATH database, maintained by the Harmer E. Davis Transportation Library at the University of California, is "the world's largest bibliographical database pertaining to Intelligent Transportation Systems (ITS)." It is searchable and browsable (Browse by ITS Thesaurus Term), and contains over 9,000 records and abstracts "including monographs, journal articles, conference papers, technical reports, theses and selected media coverage," dating back to the 1940s.

1997-01-01

201

Photodeactivation paths in norbornadiene.  

PubMed

The first high level ab initio quantum-chemical calculations of potential energy surfaces (PESs) for low-lying singlet excited states of norbornadiene in the gas phase are presented. The optimization of the stationary points (minima and conical intersections) and the recalculation of the energies were performed using the multireference configuration interaction with singles (MR-CIS) and the multiconfigurational second-order perturbation (CASPT2) methods, respectively. It was shown that the crossing between valence V2 and Rydberg R1 states close to the Franck-Condon (FC) point permits an easy population switch between these states. Also, a new deactivation path in which the doubly excited state with (?3)(2) configuration (DE) has a prominent role in photodeactivation from the R1 state due to the R1/DE and the DE/V1 conical intersections very close to the R1 and DE minima, respectively, was proposed. Subsequent deactivation from the V1 to the ground state goes through an Olivucci-Robb-type conical intersection that adopts a rhombic distorted geometry. The deactivation path has negligible barriers, thereby making ultrafast radiationless decay to the ground state possible. PMID:23553256

Antol, Ivana

2013-04-03

202

pathChirp: Efficient Available Bandwidth Estimation for Network Paths  

SciTech Connect

This paper presents pathChirp, a new active probing tool for estimating the available bandwidth on a communication network path. Based on the concept of ''self-induced congestion,'' pathChirp features an exponential flight pattern of probes we call a chirp. Packet chips offer several significant advantages over current probing schemes based on packet pairs or packet trains. By rapidly increasing the probing rate within each chirp, pathChirp obtains a rich set of information from which to dynamically estimate the available bandwidth. Since it uses only packet interarrival times for estimation, pathChirp does not require synchronous nor highly stable clocks at the sender and receiver. We test pathChirp with simulations and Internet experiments and find that it provides good estimates of the available bandwidth while using only a fraction of the number of probe bytes that current state-of-the-art techniques use.

Cottrell, Les

2003-04-30

203

Free Path Lengths in Quasi Crystals  

NASA Astrophysics Data System (ADS)

The Lorentz gas is a model for a cloud of point particles (electrons) in a distribution of scatterers in space. The scatterers are often assumed to be spherical with a fixed diameter d, and the point particles move with constant velocity between the scatterers, and are specularly reflected when hitting a scatterer. There is no interaction between point particles. An interesting question concerns the distribution of free path lengths, i.e. the distance a point particle moves between the scattering events, and how this distribution scales with scatterer diameter, scatterer density and the distribution of the scatterers. It is by now well known that in the so-called Boltzmann-Grad limit, a Poisson distribution of scatterers leads to an exponential distribution of free path lengths, whereas if the scatterer distribution is periodic, the free path length distribution asymptotically behaves as a power law. This paper considers the case when the scatters are distributed on a quasi crystal, i.e. non periodically, but with a long range order. Simulations of a one-dimensional model are presented, showing that the quasi crystal behaves very much like a periodic crystal, and in particular, the distribution of free path lengths is not exponential.

Wennberg, Bernt

2012-06-01

204

pathChirp: Efficient Available Bandwidth Estimation for Network Paths  

Microsoft Academic Search

This paper presents pathChirp, a new active probing tool for estimating the available bandwidth on a communication network path. Based on the concept of ''self-induced congestion,'' pathChirp features an exponential flight pattern of probes we call a chirp. Packet chips offer several significant advantages over current probing schemes based on packet pairs or packet trains. By rapidly increasing the probing

Les

2003-01-01

205

Inelastic scattering of electron and light ion beams in organic polymers  

NASA Astrophysics Data System (ADS)

We have calculated the inelastic mean free path, stopping power, and energy-loss straggling of swift electron, proton, and ?-particle beams in a broad incident energy range in four organic polymers: poly(methyl methacrylate) (PMMA), Kapton, polyacetylene (PA), and poly(2-vinylpyridine) (P2VP). These calculations have been done through a suitable description of their optical properties and its extension into the whole momentum and energy transfer excitation spectrum. For electrons, we take into account the exchange effect between the projectile and the target electrons, while the charge-state fractions have been considered for ions. Our results are compared with other models and with the available experimental data. An excellent agreement with experimental data is obtained in the case of proton and ?-particle beams in Kapton and a reasonably good agreement has been achieved for electron beams in PMMA, Kapton, and PA. We have parameterized by means of simple analytical expressions our results for electron beams interacting with these four polymers, which can be easily implemented in Monte Carlo calculations.

de Vera, Pablo; Abril, Isabel; Garcia-Molina, Rafael

2011-05-01

206

The role of geomagnetic field configuration on the GPS signal paths through the magnetosphere and ionosphere.  

NASA Astrophysics Data System (ADS)

For several positions on GPS satellite orbit in respect to the geomagnetic field lines, the propagation of signals transmitted towards the same receiver at the Earth surface, is examined. The changes in electron density height distribution are estimated taking into account the plasmapause for appropriate propagation paths. The total electron content (TEC) is calculated in the ionosphere up to 650 km. The group path delay is calculated for the quasi-longitudinal and quasi-transversal propagation paths.

Grubor, D. P.; uli?, D. M.; Jeli?, . V.

1995-04-01

207

Steered transition path sampling.  

PubMed

We introduce a path sampling method for obtaining statistical properties of an arbitrary stochastic dynamics. The method works by decomposing a trajectory in time, estimating the probability of satisfying a progress constraint, modifying the dynamics based on that probability, and then reweighting to calculate averages. Because the progress constraint can be formulated in terms of occurrences of events within time intervals, the method is particularly well suited for controlling the sampling of currents of dynamic events. We demonstrate the method for calculating transition probabilities in barrier crossing problems and survival probabilities in strongly diffusive systems with absorbing states, which are difficult to treat by shooting. We discuss the relation of the algorithm to other methods. PMID:22779577

Guttenberg, Nicholas; Dinner, Aaron R; Weare, Jonathan

2012-06-21

208

Steered transition path sampling  

NASA Astrophysics Data System (ADS)

We introduce a path sampling method for obtaining statistical properties of an arbitrary stochastic dynamics. The method works by decomposing a trajectory in time, estimating the probability of satisfying a progress constraint, modifying the dynamics based on that probability, and then reweighting to calculate averages. Because the progress constraint can be formulated in terms of occurrences of events within time intervals, the method is particularly well suited for controlling the sampling of currents of dynamic events. We demonstrate the method for calculating transition probabilities in barrier crossing problems and survival probabilities in strongly diffusive systems with absorbing states, which are difficult to treat by shooting. We discuss the relation of the algorithm to other methods.

Guttenberg, Nicholas; Dinner, Aaron R.; Weare, Jonathan

2012-06-01

209

Path-integral approach to the scattering theory of quantum transport  

NASA Astrophysics Data System (ADS)

The scattering theory of quantum transport relates transport properties of disordered mesoscopic conductors to their transfer matrix T. We introduce an approach to the statistics of transport quantities which expresses the probability distribution of T as a path integral. The path integal is derived for a model of conductors with broken time-reversal invariance in arbitrary dimensions. It is applied to the Dorokhov-Mello-Pereyra-Kumar (DMPK) equation which describes quasi-one-dimensional wires. We use the equivalent channel model whose probability distribution for the eigenvalues of TT is equivalent to the DMPK equation independent of the values of the forward scattering mean free paths. We find that infinitely strong forward scattering corresponds to diffusion on the coset space of the transfer-matrix group. It is shown that the saddle-point of the path integral corresponds to ballistic conductors with large conductances. We solve the saddle-point equation and recover random-matrix theory from the saddle-point approximation to the path integral.

Endesfelder, D.

1998-05-01

210

Probability and shape of the spectral line of a single bulk characteristic energy loss of a fast electron in a medium with electron absorption and strong spatial dispersion  

SciTech Connect

The probability of single characteristic energy loss of a fast electron in a reflection experiment has been calculated. Unlike many works concerning this subject, the bremsstrahlung of bulk plasmons in the non- Cherenkov ranges of frequencies and wavevectors of a plasmon has been taken into account. The contributions to the probability of single loss and to the shape of the spectral line from a quantum correction that is due to the interference of elastic and inelastic electron scattering events have been determined. The probability has been calculated in the kinetic approximation for the relative permittivity, where the short-wavelength range of the plasmon spectrum is correctly taken into account. In view of these circumstances, the expression for the mean free path of the electron with respect to the emission of a bulk plasmon that was obtained by Pines [D. Pines, Elementary Excitations in Solids (Benjamin, New York, 1963)] has been refined. The coherence length of the fast electron in the medium-energy range under consideration has been estimated. The shape of the spectral line of energy losses in the non-Cherenkov frequency range has been determined. It has been shown that the probability of the single emission of the bulk plasmon incompletely corresponds to the Poisson statistics.

Libenson, B. N., E-mail: libenson-b@yandex.ru [St. Petersburg State Polytechnic University (Russian Federation)

2011-10-15

211

A distributed shortest path protocol  

NASA Astrophysics Data System (ADS)

We present a distributed protocol for obtaining the shortest paths between all pairs of nodes in a network with weighted links. The protocol is based on an extension of the Dijkstra (centralized) shortest path algorithm and uses collaboration between neighboring nodes to transfer the information needed at the nodes for the successive construction of the shortest paths. A formal description of the protocol is given by indicating the exact algorithm performed by each node. The validation proofs are greatly simplified by separating the communication mechanism from the computation at the nodes, the latter being the transposition of the Dijkstra shortest path algorithm to the decentralized protocol.

Zerbib, F. B. M.; Segall, A.

1981-06-01

212

Insights into the mechanisms on chemical reactions: reaction paths for chemical reactions. [Li + HF; OH + H  

Microsoft Academic Search

We report reaction paths for two prototypical chemical reactions: Li + HF, an electron transfer reaction, and OH + H, an abstraction reaction. In the first reaction we consider the connection between the energetic terms in the reaction path Hamiltonian and the electronic changes which occur upon reaction. In the second reaction we consider the treatment of vibrational effects in

T. H. Jr. Dunning; E. Rosen; R. A. Eades

1987-01-01

213

Theory and simulations of current drive via injection of an electron beam in the ACT-1 device  

SciTech Connect

One- and two-dimensional particle simulations of beam-plasma interaction have been carried out in order to understand current drive experiments that use an electron beam injected into the ACT-1 device. Typically, the beam velocity along the magnetic field is V = 10/sup 9/ cm/sec while the thermal velocity of the background electrons is v/sub t/ = 10/sup 8//cm. The ratio of the beam density to the background density is about 10% so that a strong beam-plasma instability develops causing rapid diffusion of beam particles. For both one- and two- dimensional simulations, it is found that a significant amount of beam and background electrons is accelerated considerably beyond the initial beam velocity when the beam density is more than a few percent of the background plasma density. In addition, electron distribution along the magnetic field has a smooth negative slope, f' (v/sub parallel/) < 0, for v/ sub parallel/ > 0 extending v/sub parallel/ = 1.5 V approx. 2 V, which is in sharp contrast to the predictions from quasilinear theory. An estimate of the mean-free path for beam electrons due to Coulomb collisions reveals that the beam electrons can propagate a much longer distance than is predicted from a quasilinear theory, due to the presence of a high energy tail. These simulation results agree well with the experimental observations from the ACT-1 device.

Okuda, H.; Horton, R.; Ono, M.; Wong, K.L.

1985-02-01

214

MeV to GeV electron propagation and modulation: results of the KET-telescope onboard ULYSSES  

NASA Astrophysics Data System (ADS)

The KET instrument onboard Ulysses is continuously monitoring the flux of MeV to GeV electrons. I summarize here the main results obtained with data from the ~ 7 MeV and > 300 MeV electron KET channels, collected from launch up to the end of 1995 after completion of the Ulysses prime mission. Regarding low energy electrons, the stress is put on the study of Jovian electron events, which were used to derive the values of the three diagonal diffusion coefficients in interplanetary space. The ~ 7 MeV parallel mean free path is found to be large (~ 1 AU). The radial diffusion coefficient perpendicular to the average magnetic field is ~ 0.5 % of that along the field, while the ratio of the polar diffusion coefficient to the radial one is less than unity. Regarding the high energy electrons, the KET data show that the flux at ~ 300 MeV has been constant during the full period, whereas the maximum recovery, at ~ 700 MeV, was by a factor of ~ 5. Regarding the comparison with proton modulation, the e/p ratio strongly decreased in 1991, which is tentatively correlated with the reversal of the sun's magnetic field. During the fast latitude scan of Ulysses, under solar minimum conditions, no significant latitude dependent feature is found for Galactic electrons, implying a much smaller, if any, latitudinal gradient compared to that of protons, which is already small.

Ferrando, P.

1997-05-01

215

SPECIAL ISSUE DEVOTED TO MULTIPLE RADIATION SCATTERING IN RANDOM MEDIA: Stability of the HenyeyGreenstein phase function and fast path integration under conditions of multiple light scattering  

NASA Astrophysics Data System (ADS)

It is shown that the stability of the HenyeyGreenstein phase function allows the calculation rate of light propagation through strongly scattering objects to be drastically increased by using the same a priori information on interaction processes as in the initial formulation of the problem. The increase in the calculation rate is accompanied by a gradual impairment of simulation accuracy from the accuracy of the Monte-Carlo method to that of the diffusion approximation. By using a standard assumption about the statistical independence of the photon mean free path and photon scattering angle, an exact analytic expression relating the effective number of scattering events with the optical path is obtained.

Petnikova, V. M.; Tret'yakov, Evgeniy V.; Shuvalov, Vladimir V.

2006-11-01

216

Fuzzy shortest path problems incorporating interactivity among paths  

Microsoft Academic Search

This paper deals with a shortest path problem on a network in which a fuzzy number, instead of a real number, is assigned to each arc length. Such a problem is ill-posed because each arc cannot be identified as being either on the shortest path or not. Therefore, based on the possibility theory, we introduce the concept of degree of

Shinkoh Okada

2004-01-01

217

Stabilizing Intelligent Route Control: Randomized Path Monitoring, Randomized Path Switching or History-Aware Path Switching?  

Microsoft Academic Search

Multihoming Intelligent Route Control (IRC) plays a significant role in improving the performance of Internet accesses. However,\\u000a in a competitive environment, IRC systems may introduce persistent route oscillations, causing significant performance degradation.\\u000a In this study, three design alternatives to cope with this issue are investigated: Randomized Path Monitoring, Randomized\\u000a Path Switching and History-aware Path Switching. The simulation results show that

Alexandre Fonte; Jos Martins; Marilia Curado; Edmundo Monteiro

2008-01-01

218

Electron Tunneling Studies in Tantalum Overlayers on Niobium and in Tantalum and Niobium Nitride Films  

Microsoft Academic Search

Tunneling measurements have been performed on tantalum surface layers on niobium. The thickness of the tantalum layer ranges from 10 to 100 A. The critical current, bound-state energy, phonon structure, and oxide barrier shape are investigated. The results are compared with an extended version of the Gallagher theory which accounts for both the finite mean free path in the Ta

Elie Khalil Track

1988-01-01

219

Measurements of direct path and folded path optical scintillation path weightings  

NASA Astrophysics Data System (ADS)

A theoretical prediction by Dr. Avihu Ze'evi of the relative contribution to the optical scintillation by different points along the path was described by a weight function for direct and exact folded path spherical wave sources. In an effort to verify this prediction a turbulence chamber was built to allow a controlled turbulence source to be moved and measured at different path positions in conjunction with scintillation measurements. The experimental results follow Dr. Ze'evi's general pattern but both sources are less weighted at the detector end than predicted and the folded path is more heavily weighted at the target end than predicted.

Costantine, A. G.

1983-06-01

220

Path Diversity Aware Interdomain Routing  

Microsoft Academic Search

AbstractAs the Internet becomes the critical information infrastructure for both personal and business applications, fast and reliable routing protocols need to be designed to maintain the performance of those applications in the presence of failures. Todays interdomain routing protocol, BGP, is known to be slow in reacting and recovering from network failures. Increasing path diversity by advertising multiple paths is

Feng Wang; Lixin Gao

2009-01-01

221

Fracture paths and ultrananocrystalline diamond  

Microsoft Academic Search

We use the simulated fracture of ultrananocrystalline diamond (UNCD) to illustrate how different fracture paths can result in different predictions of system properties. At zero temperature, the system is unable to explore the potential energy surface far from the fracture path being investigated. This can result in misleading predictions for the mechanical properties of UNCD. In non-zero temperature simulations, the

Jeffrey T. Paci; Lipeng Sun; Ted Belytschko; George C. Schatz

2005-01-01

222

Path Analysis: A Brief Introduction.  

ERIC Educational Resources Information Center

Path analysis is presented as a technique that can be used to test on a priori model based on a theoretical conceptualization involving a network of selected variables. This being an introductory source, no previous knowledge of path analysis is assumed, although some understanding of the fundamentals of multiple regression analysis might be

Carducci, Bernardo J.

223

Obstacle avoidance and path planning  

Microsoft Academic Search

Outlines the state-of-the-art in obstacle avoidance and path planning for industrial robots that is practical on the current generation of computer hardware. Describes practical vehicle planners and planning for manipulators. Summarizes that obstacle avoidance and path planning are techniques with differing goals. Sonar is the standard method of obstacle avoidance systems which is largely limited by the reliability of the

Stephen Cameron

1994-01-01

224

Electron affinity of pentacene thin film studied by radiation-damage free inverse photoemission spectroscopy  

NASA Astrophysics Data System (ADS)

The electron affinity of pentacene thin films has been evaluated during the last decades, but it is still under controversial due to varieties of film quality and radiation damages of the films introduced during inverse photoemission spectroscopy (IPES) experiment together with insufficient energy resolution of the instruments. We employed the near-ultraviolet IPES with a better energy resolution 0.27 ~ 0.32 eV and using lower energy electron beams (0 eV <= Ei <= 4.9 eV) to study the unoccupied states of pentacene thin film. Due to a large mean-free-path of the electron in this energy region, the threshold electron affinity of the bulk of pentacene film was precisely determined to be 2.70 +/- 0.03 eV. Using the threshold ionization energy of 4.90 +/- 0.05 eV determined by ultraviolet photoemission spectroscopy, the band-gap energy of the pentacene film is obtained to be 2.20 +/- 0.06 eV.

Han, Weining; Yoshida, Hiroyuki; Ueno, Nobuo; Kera, Satoshi

2013-09-01

225

Path Integral Simulation of Positronium in Materials  

NASA Astrophysics Data System (ADS)

A path integral Monte Carlo technique is used to simulate positronium (Ps) in an insulator. The primitive propagator is used, with a pair of interacting chains representing the positron and electron. We calculate the energy and radial distribution function for Ps enclosed in a hard, spherical cavity, and the polarizability of the model Ps in the presence of an electrostatic field. The model can be extended to calculate the "pick-off" annihilation rate of the positron within the open volume of an insulating solid.

Bug, Amy L. R.; Sterne, P. A.; Larrimore, Lisa; McFarland, Robert

2000-03-01

226

Simple model of bulk and surface excitation effects to inelastic scattering in low-energy electron beam irradiation of multi-walled carbon nanotubes  

SciTech Connect

The effect of bulk and surface excitations to inelastic scattering in low-energy electron beam irradiation of multi-walled carbon nanotubes (MWNTs) is studied using the dielectric formalism. Calculations are based on a semiempirical dielectric response function for MWCNTs determined by means of a many-pole plasmon model with parameters adjusted to available experimental spectroscopic data under theoretical sum-rule constrains. Finite-size effects are considered in the context of electron gas theory via a boundary correction term in the plasmon dispersion relations, thus, allowing a more realistic extrapolation of the electronic excitation spectrum over the whole energy-momentum plane. Energy-loss differential and total inelastic scattering cross sections as a function of electron energy and distance from the surface, valid over the energy range {approx}50-30,000 eV, are calculated with the individual contribution of bulk and surface excitations separated and analyzed for the case of normally incident and escaping electrons. The sensitivity of the results to the various approximations for the spatial dispersion of the electronic excitations is quantified. Surface excitations are shown to have a strong influence upon the shape and intensity of the energy-loss differential cross section in the near surface region whereas the general notion of a spatially invariant inelastic mean free path inside the material is found to be of good approximation.

Kyriakou, Ioanna; Emfietzoglou, Dimitris [Medical Physics Lab, University of Ioannina Medical School, 451 10 Ioannina (Greece); Garcia-Molina, Rafael [Departamento de Fisica - CIOyN, Universidad de Murcia, E-30100 Murcia (Spain); Abril, Isabel [Departament de Fisica Aplicada, Universitat d'Alacant, Apartat 99, E-03080 Alacant (Spain); Kostarelos, Kostas [Nanomedicine Lab, Centre for Drug Delivery Research, The School of Pharmacy, University of London, London WC1N 1AX (United Kingdom)

2011-09-01

227

An introduction to critical paths.  

PubMed

A critical path defines the optimal sequencing and timing of interventions by physicians, nurses, and other staff for a particular diagnosis or procedure. Critical paths are developed through collaborative efforts of physicians, nurses, pharmacists, and others to improve the quality and value of patient care. They are designed to minimize delays and resource utilization and to maximize quality of care. Critical paths have been shown to reduce variation in the care provided, facilitate expected outcomes, reduce delays, reduce length of stay, and improve cost-effectiveness. The approach and goals of critical paths are consistent with those of total quality management (TQM) and can be an important part of an organization's TQM process. PMID:15739581

Coffey, Richard J; Richards, Janet S; Remmert, Carl S; LeRoy, Sarah S; Schoville, Rhonda R; Baldwin, Phyllis J

228

Squeezed States and Path Integrals.  

National Technical Information Service (NTIS)

The continuous-time regularization scheme for defining phase-space path integrals is briefly reviewed as a method to define a quantization procedure that is completely covariant under all smooth canonical coordinate transformations. As an illustration of ...

I. Daubechies J. R. Klauder

1992-01-01

229

Formal language constrained path problems  

SciTech Connect

In many path finding problems arising in practice, certain patterns of edge/vertex labels in the labeled graph being traversed are allowed/preferred, while others are disallowed. Motivated by such applications as intermodal transportation planning, the authors investigate the complexity of finding feasible paths in a labeled network, where the mode choice for each traveler is specified by a formal language. The main contributions of this paper include the following: (1) the authors show that the problem of finding a shortest path between a source and destination for a traveler whose mode choice is specified as a context free language is solvable efficiently in polynomial time, when the mode choice is specified as a regular language they provide algorithms with improved space and time bounds; (2) in contrast, they show that the problem of finding simple paths between a source and a given destination is NP-hard, even when restricted to very simple regular expressions and/or very simple graphs; (3) for the class of treewidth bounded graphs, they show that (i) the problem of finding a regular language constrained simple path between source and a destination is solvable in polynomial time and (ii) the extension to finding context free language constrained simple paths is NP-complete. Several extensions of these results are presented in the context of finding shortest paths with additional constraints. These results significantly extend the results in [MW95]. As a corollary of the results, they obtain a polynomial time algorithm for the BEST k-SIMILAR PATH problem studied in [SJB97]. The previous best algorithm was given by [SJB97] and takes exponential time in the worst case.

Barrett, C.; Jacob, R.; Marathe, M.

1997-07-08

230

Real Time Path Integral Methods  

Microsoft Academic Search

An efficient methodology has been developed for simulating the long-time dynamics of quantum dissipative processes modeled in terms of a system coupled to a multidimensional harmonic bath. The starting point is expression of the path integral in terms of a quasi-adiabatic propagator which allows large time increments. The resulting quasi-adiabatic propagator path integral is evaluated on a system-specific discrete variable

Nancy Makri

1998-01-01

231

Formal Language Constrained Path Problems  

Microsoft Academic Search

Given an alphabet ?, a (directed) graph G whose edges are weighted and ?-labeled, and a formal language L ? ??, the formal-language-constrained shortest\\/simple path problem con- sists of finding a shortest (simple) path p in G complying with the additional constraint that l(p) ? L. Here l(p) denotes the unique word obtained by concatenating the ?-labels of the edges

Christopher L. Barrett; Riko Jacob; Madhav V. Marathe

1998-01-01

232

A Database of >20 keV Electron Green's Functions of Interplanetary Transport at 1 AU  

NASA Astrophysics Data System (ADS)

We use interplanetary transport simulations to compute a database of electron Green's functions, i.e., differential intensities resulting at the spacecraft position from an impulsive injection of energetic (>20 keV) electrons close to the Sun, for a large number of values of two standard interplanetary transport parameters: the scattering mean free path and the solar wind speed. The nominal energy channels of the ACE, STEREO, and Wind spacecraft have been used in the interplanetary transport simulations to conceive a unique tool for the study of near-relativistic electron events observed at 1 AU. In this paper, we quantify the characteristic times of the Green's functions (onset and peak time, rise and decay phase duration) as a function of the interplanetary transport conditions. We use the database to calculate the FWHM of the pitch-angle distributions at different times of the event and under different scattering conditions. This allows us to provide a first quantitative result that can be compared with observations, and to assess the validity of the frequently used term beam-like pitch-angle distribution.

Agueda, N.; Vainio, R.; Sanahuja, B.

2012-10-01

233

Energy loss of proton, alpha particle, and electron beams in hafnium dioxide films  

SciTech Connect

The electronic stopping power, S, of HfO{sub 2} films for proton and alpha particle beams has been measured and calculated. The experimental data have been obtained by the Rutherford backscattering technique and cover the range of 120-900 and 120-3000 keV for proton and alpha particle beams, respectively. Theoretical calculations of the energy loss for the same projectiles have been done by means of the dielectric formalism using the Mermin energy loss function--generalized oscillator strength (MELF-GOS) model for a proper description of the HfO{sub 2} target on the whole momentum-energy excitation spectrum. At low projectile energies, a nonlinear theory based on the extended Friedel sum rule has been employed. The calculations and experimental measurements show good agreement for protons and a quite good one for alpha particles. In particular, the experimental maximums of both stopping curves (around 120 and 800 keV, respectively) are well reproduced. On the basis of this good agreement, we have also calculated the inelastic mean-free path (IMFP) and the stopping power for electrons in HfO{sub 2} films. Our results predict a minimum value of the IMFP and a maximum value of the S for electrons with energies around 120 and 190 eV, respectively.

Behar, Moni; Fadanelli, Raul C.; Nagamine, Luiz C. C. M. [Instituto de Fisica, Universidade Federal do Rio Grande do Sul, Av. Bento Goncalves 9500, 91501-970 Porto Alegre, RS (Brazil); Abril, Isabel; Denton, Cristian D. [Departament de Fisica Aplicada, Universitat dAlacant, Apartat 99, E-03080 Alacant (Spain); Garcia-Molina, Rafael [Departamento de Fisica-CIOyN, Universidad de Murcia, Apartado 4021, E-30080 Murcia (Spain); Arista, Nestor R. [Division Colisiones Atomicas, Centro Atomico Bariloche, RA-8400 San Carlos de Bariloche (Argentina)

2009-12-15

234

A DATABASE OF >20 keV ELECTRON GREEN'S FUNCTIONS OF INTERPLANETARY TRANSPORT AT 1 AU  

SciTech Connect

We use interplanetary transport simulations to compute a database of electron Green's functions, i.e., differential intensities resulting at the spacecraft position from an impulsive injection of energetic (>20 keV) electrons close to the Sun, for a large number of values of two standard interplanetary transport parameters: the scattering mean free path and the solar wind speed. The nominal energy channels of the ACE, STEREO, and Wind spacecraft have been used in the interplanetary transport simulations to conceive a unique tool for the study of near-relativistic electron events observed at 1 AU. In this paper, we quantify the characteristic times of the Green's functions (onset and peak time, rise and decay phase duration) as a function of the interplanetary transport conditions. We use the database to calculate the FWHM of the pitch-angle distributions at different times of the event and under different scattering conditions. This allows us to provide a first quantitative result that can be compared with observations, and to assess the validity of the frequently used term beam-like pitch-angle distribution.

Agueda, N.; Sanahuja, B. [Departament d'Astronomia i Meteorologia, Institut de Ciencies del Cosmos, Universitat de Barcelona, Barcelona (Spain); Vainio, R. [Department of Physics, University of Helsinki, Helsinki (Finland)

2012-10-15

235

Energy loss of proton, ? particle, and electron beams in hafnium dioxide films  

NASA Astrophysics Data System (ADS)

The electronic stopping power, S , of HfO2 films for proton and alpha particle beams has been measured and calculated. The experimental data have been obtained by the Rutherford backscattering technique and cover the range of 120-900 and 120-3000 keV for proton and alpha particle beams, respectively. Theoretical calculations of the energy loss for the same projectiles have been done by means of the dielectric formalism using the Mermin energy loss functiongeneralized oscillator strength (MELF-GOS) model for a proper description of the HfO2 target on the whole momentum-energy excitation spectrum. At low projectile energies, a nonlinear theory based on the extended Friedel sum rule has been employed. The calculations and experimental measurements show good agreement for protons and a quite good one for alpha particles. In particular, the experimental maximums of both stopping curves (around 120 and 800 keV, respectively) are well reproduced. On the basis of this good agreement, we have also calculated the inelastic mean-free path (IMFP) and the stopping power for electrons in HfO2 films. Our results predict a minimum value of the IMFP and a maximum value of the S for electrons with energies around 120 and 190 eV, respectively.

Behar, Moni; Fadanelli, Raul C.; Abril, Isabel; Garcia-Molina, Rafael; Denton, Cristian D.; Nagamine, Luiz C. C. M.; Arista, Nstor R.

2009-12-01

236

Validation of Non-Local Electron Transport Approaches, Application to Shock Ignition  

NASA Astrophysics Data System (ADS)

For laser-plasma interactions at moderate intensities the conduction of heat cannot be captured by the classical Spitzer-H"arm expression and an accurate treatment for non-local electron transport is necessary. A suitable method needs to discriminate between local electrons, that behave in accordance to the classical thermal conduction, and non-local electrons, that have very long mean free paths and diffuse energy all over the physical domain. Two widely known and promising schemes are examined in detail: SNB [Schurtz et al. PoP (2000)] and CMG [Manheimer et al. PoP (2008)]. Both models have been implemented in the hydrodynamic code DUED and benchmarked against the fully kinetic Vlasov-Fokker-Plank codes OSHUN and KETS. Both schemes calculate the right amount of flux in the limit of steep temperature gradients, and for the test problem of hot-spot relaxation they are both generally well-behaved at hydrodynamic time-scales (30 ?ei). However, at kinetic time-scales (up to 30 ?ei) the SNB model better approximates the kinetic solution. 1D and 2D shock ignition simulations will be presented and the role of non-local effects in the implosion and ignition stages will be discussed.

Marocchino, Alberto; Atzeni, S.; Schiavi, A.; Tzoufras, M.; Mallet, J.; Nicola, Ph. D.; Feugeas, J. L.; Tikhonchuk, V.; Bell, A. R.

2012-10-01

237

E-beam ionized channel guiding of an intense relativistic electron beam  

DOEpatents

An Intense Relativistic Electron Beam (IREB) is guided through a curved path by ionizing a channel in a gas with electrons from a filament, and confining the electrons to the center of the path with a magnetic field extending along the path. The magnetic field is preferably generated by a solenoid extending along the path.

Frost, C.A.; Godfrey, B.B.; Kiekel, P.D.; Shope, S.L.

1986-04-25

238

Probing Hot Electron Flow Generated on Pt Nanoparticles with Au/TiO2 Schottky Diodes during Catalytic CO Oxidation  

SciTech Connect

Hot electron flow generated on colloid platinum nanoparticles during exothermic catalytic carbon monoxide oxidation was directly detected with Au/TiO{sub 2} diodes. Although Au/TiO{sub 2} diodes are not catalytically active, platinum nanoparticles on Au/TiO{sub 2} exhibit both chemicurrent and catalytic turnover rate. Hot electrons are generated on the surface of the metal nanoparticles and go over the Schottky energy barrier between Au and TiO{sub 2}. The continuous Au layer ensures that the metal nanoparticles are electrically connected to the device. The overall thickness of the metal assembly (nanoparticles and Au thin film) is comparable to the mean free path of hot electrons, resulting in ballistic transport through the metal. The chemicurrent and chemical reactivity of nanoparticles with citrate, hexadecylamine, hexadecylthiol, and TTAB (Tetradecyltrimethylammonium Bromide) capping agents were measured during catalytic CO oxidation at pressures of 100 Torr O{sub 2} and 40 Torr CO at 373-513 K. We found that chemicurrent yield varies with each capping agent, but always decreases with increasing temperature. We suggest that this inverse temperature dependence is associated with the influence of charging effects due to the organic capping layer during hot electron transport through the metal-oxide interface.

Park, Jeong Y.; Lee, Hyunjoo; Renzas, J. Russell; Zhang, Yawen; Somorjai, G.A.

2008-05-01

239

Experimental Study on Fast Electrons Transport in Ultra-intense Laser Irradiated Solid Targets by Transition Radiation  

NASA Astrophysics Data System (ADS)

The experiment was performed with SILEX laser facility(Ti-saphhire) at LFRC in China. The SILEX parameter: wavelength 0.8?m, duration 35fs, output power 280TW, contrast 5*105, The focal spot ?10?m(F/1.7), intensity on target surface 1*10^19W/cm^2(F/3). The main diagnostic equipments are the electron spectrometer, OMA spectrometer, optical streak camera. Some experimental results are given: The spectrum of optical emission from rear surface is rather narrow around some particular frequencies(1?, 2?, 3?), We ascribe and confirm that the spike-like spectral line that is coherent transition radiation; The coherent light is also seen on time-integrated image with ring-patter due to Weibel instability of the fast electron transport; Obtained experimental cure of target thickness vs OTR image intensity is relative to mean free path of fast electron; The measuring optical transition radiation(OTR) duration of 171ps much longer than 1ps duration of fast electron transport target, the possible explanation is that the OTR duration to be determined magnetic diffusion time.

Zhijian, Zheng; Guangcan, Wang; Yuqiu, Gu

2008-11-01

240

Variables Related to Sexual Coercion: A Path Model.  

ERIC Educational Resources Information Center

For a study, a path model was developed and tested representing variables that relate to sexual coercion. Specifically, sex, who asked, who paid, relational stage, past/current relationship, attitudes, drug or alcohol use, coercion strategies, and victim resistance were proposed and their effects examined. Data were collected by using electronic

Emmers-Sommer, Tara M.; Allen, Mike

241

A Unified Approach to Path Problems  

Microsoft Academic Search

A general method is described for solving path problems on directed graphs. Such path problems include finding shortest paths, solving sparse systems of hnear equaUons, and carrying out global flow analysis of computer programs The method consists of two steps First, a collecUon of regular expressions representmg sets of paths m the graph Is constructed This can be done by

Robert Endre Tarjan; TAR JAN

1981-01-01

242

Underwater path planing using fast marching algorithms  

Microsoft Academic Search

In this paper, new tools for obstacle avoidance and path planning for underwater vehicles are presented. The authors' technique, based on a level set formulation of the path planning problem, extracts optimal paths from complex and continuous environments in a complete and consistent manner. Fast marching algorithm is known to be efficient for finding cost optimal path in mobile robotics

Clment Ptrs; Yan Pailhast; Yvan Petilloti; Dave Lanes

2005-01-01

243

Paths, Trees and Cycles in Tournaments  

Microsoft Academic Search

We survey results on paths, trees and cycles in tournaments. The main subjects are hamiltonian paths and cycles, vertex and arc disjoint paths with prescribed endvertices, arc-pancyclicity, oriented paths, trees and cycles in tour- naments. Several unsolved problems are included.

Jrgen Bang-Jensen; Gregory Gutin

244

Triple quantum dots: Two path transport and electrostatic stability diagram  

NASA Astrophysics Data System (ADS)

We present analysis of measurements on a triple quantum dot with a two path setup. The electron transport along both paths is recorded as well as charging data measured with a quantum point contact. The results are compared with an electrostatic model. This model allows for the full visualization of the three dimensional stability diagram for triple quantum dots. It is found that this diagram contains a central diamond shaped plane at which one specific quantum cellular automata (QCA) process occurs. Moreover quadruple points are found with four degenerate charge configurations. The QCA process defines the quadruple points and limits their number to six.

Rogge, M. C.; Haug, R. J.

2010-02-01

245

On the general false path problem in timing analysis  

Microsoft Academic Search

The false path problem is often referred to as the problem of detecting the longest sensitizable path (A path which is not a false path is a sensitizable path). The term false path is not clearly defined. In this paper, we first give a clear and precise definition of a false path. Then the general false path problem is formulated.

David Hung-Chang Du; S. H. Yen; Subbarao Ghanta

1989-01-01

246

Hierarchical Encoded Path Views for Path Query Processing: An Optimal Model and Its Performance Evaluation  

Microsoft Academic Search

Efficient path computation is essential for applications such as intelligent transportation systems (ITS) and network routing. In ITS navigation systems, many path requests can be submitted over the same, typically huge, transportation network within a small time window. While path precomputation (path view) would provide an efficient path query response, it raises three problems which must be addressed: 1) precomputed

Ning Jing; Yun-wu Huang; Elke A. Rundensteiner

1998-01-01

247

Career Paths of Academic Deans.  

ERIC Educational Resources Information Center

|This paper examines various career paths leading to deanship and considers the implications of the findings for women and minorities who aspire to this position. The paper is part of a larger study of academic deanship conducted by the Center for Academic Leadership at Washington State University between October 1996 and January 1997. Data for

Wolverton, Mimi; Gonzales, Mary Jo

248

Employer Resource Manual. Project Path.  

ERIC Educational Resources Information Center

|Project Path at Illinois' College of DuPage was established to provide pre-employment training and career counseling for disabled students. To encourage the integration of qualified individuals with disabilities into the workplace, the project compiled this resource manual for area businesses, providing tips for interacting with disabled people

Kane, Karen R.; Del George, Eve

249

Complexity of Path Forming Games.  

National Technical Information Service (NTIS)

For a number of two player games where players alternately choose the next vertex of a simple or elementary path in a graph, the authors considers the problem to determine whether for a given game instance there is a winning strategy for the first player....

H. L. Bodlaender

1989-01-01

250

Alternative Nuclear Paths To 2050  

Microsoft Academic Search

he circumstances surrounding nuclear power worldwide and the importance that may be given to issues affecting its future development point toward very different alternative paths over the next 50 years. Economic deregulation, lack of competitiveness in some countries, negative public perception and concerns about waste issues suggest that nuclear power might decrease progressively with a potential phase-out of the technology

Ivan Vera; Evelyne Bertel; Geoffrey Stevens

251

Thinking on the Write Path  

ERIC Educational Resources Information Center

The present paper underscores the importance of the cognitive orientation of English as a Foreign Language (EFL) students in their success in writing courses. A few suggestions are made as to how EFL teachers can put their students on the right cognitive path in their writings.

Salmani-Nodoushan, Mohammad Ali

2007-01-01

252

Beam wander experiments: terrestrial path  

Microsoft Academic Search

We report on a set of measurements made in December 2005 by researchers from the University of Central Florida, SPAWAR's Innovative Science and Technology Experiment Facility (ISTEF), Harris Corporation, NASA Kennedy Space Center, and Northrop Grumman. The experiments were conducted on the Shuttle Landing Facility (SLF) at Kennedy Space Center (KSC) over terrestrial paths of 1, 2, and 5 km.

R. L. Phillips; L. C. Andrews; J. Stryjewski; B. Griffis; M. Borbath; D. Galus; G. Burdge; K. Green; C. Kim; D. Stack; C. Harkrider; D. Wayne; D. Hand; J. Kiriazes

2006-01-01

253

Perceived Shrinkage of Motion Paths  

ERIC Educational Resources Information Center

|We show that human observers strongly underestimate a linear or circular trajectory that a luminous spot follows in the dark. At slow speeds, observers are relatively accurate, but, as the speed increases, the size of the path is progressively underestimated, by up to 35%. The underestimation imposes little memory load and does not require

Sinico, Michele; Parovel, Giulia; Casco, Clara; Anstis, Stuart

2009-01-01

254

The path to adaptive microsystems  

Microsoft Academic Search

Scaling trends in microsystems are discussed frequently in the technical community, providing a short-term perspective on the future of integrated microsystems. This paper looks beyond the leading edge of technological development, focusing on new microsystem design paradigms that move far beyond today's systems based on static components. We introduce the concept of Adaptive Microsystems and outline a path to realizing

John C. Zolper; Michael J. Biercuk

2006-01-01

255

Size-dependent giant-magnetoresistance in millimeter scale GaAs/AlGaAs 2D electron devices.  

PubMed

Large changes in the electrical resistance induced by the application of a small magnetic field are potentially useful for device-applications. Such Giant Magneto-Resistance (GMR) effects also provide new insights into the physical phenomena involved in the associated electronic transport. This study examines a "bell-shape" negative GMR that grows in magnitude with decreasing temperatures in mm-wide devices fabricated from the high-mobility GaAs/AlGaAs 2-Dimensional Electron System (2DES). Experiments show that the span of this magnetoresistance on the magnetic-field-axis increases with decreasing device width, W, while there is no concurrent Hall resistance, Rxy, correction. A multi-conduction model, including negative diagonal-conductivity, and non-vanishing off-diagonal conductivity, reproduces experimental observations. The results suggest that a size effect in the mm-wide 2DES with mm-scale electron mean-free-paths is responsible for the observed "non-ohmic" size-dependent negative GMR. PMID:24067264

Mani, R G; Kriisa, A; Wegscheider, W

2013-09-25

256

Total cross-sections for positron and electron scattering from ?-tetrahydrofurfuryl alcohol  

NASA Astrophysics Data System (ADS)

In this paper, we report original measurements of total cross-sections (TCSs) for positron scattering from an important biomolecule, ?-tetrahydrofurfuryl alcohol (THFA). The energy range of these measurements was 0.15-50.15 eV, whereas the energy resolution was ~260 meV. In addition, we report theoretical results, calculated within the independent-screened additivity rule (IAM-SCAR) formalism, on the corresponding electron impact total cross-sections. In this case, the energy range is 1-10 000 eV. With the advent of new particle track simulation codes, which incorporate accurate atomic and molecular data in order to provide interaction details at the nanoscale, interest in positron and electron TCSs has enjoyed something of a recent renaissance as they specify the mean free path between collisions in such codes. Because the present data are, to the best of our knowledge, the first TCSs to be reported for positron scattering from THFA, they fill an important void in the knowledge available to us from the literature.

Zecca, A.; Chiari, L.; Garca, G.; Blanco, F.; Trainotti, E.; Brunger, M. J.

2011-06-01

257

TOPICAL REVIEW: Electron transport in indium arsenide nanowires  

NASA Astrophysics Data System (ADS)

The vapor--liquid--solid growth of semiconductor nanowires led to the implementation of engineered electronic and optoelectronic one-dimensional nanostructures with outstanding promise for device applications. To realize this promise, detailed understanding and control over their growth, crystal structure, and transport properties and their combined impact on device performance is vital. Here, we review our work on electron transport in InAs nanowires in a variety of device schemes. First, we provide a brief introduction and historical perspective on growth and transport studies in InAs NWs. Second, we discuss and present experimental measurements of ballistic transport in InAs nanowires over ~200 nm length scale, which indicates a large electron mean free path and correlates with the high electron mobility measured on similar nanowires. Third, we devise a device model that enables accurate estimation of transport coefficients from field-effect transistor measurements by taking into account patristic device components. We utilize this model to reveal the impact of surface states, diameter, lateral and vertical fields, as well as crystal structure, on electron transport and transport coefficient calculation. We show in these studies that electron transport in InAs nanowires is dominated by surface state effects that introduce measurement artifacts in parameter extraction, reduce electron mobility for smaller diameters, and degrade the subthreshold characteristics of transistors made of Zinc Blende InAs nanowires. This device model is also used for isolating vertical and lateral field effects on electron transport in nanowire transistor channels and explaining observed negative differential conductance and mobility degradation at high injection fields, which is supported by electro-thermal simulations and microstructure failure analysis. We adopt the concept of lack of inversion symmetry in polar III-V materials and the resultant spontaneous polarization charges perpendicular to the electron transport trajectory in twinned Wurtzite nanowires to explain compensation of surface charges for this type of nanowires and their enhanced subthreshold characteristics over transistors made of Zinc Blende ones. Fourth, we discuss the combined effects of surface states and field variations in InAs nanowire transistor channels to shed light on the local electrostatic behavior in 1D channels studied by scanning probe measurements. Fifth, we survey and benchmark results on nanowire transistor performance and demonstrate the superiority of InAs nanowires for high-on currents and high-speed applications. Finally, we implement a novel integration scheme for InAs nanowires on Si substrates that enables vertical alignment and electrical isolation between nanowires which is necessary for achieving multifunctional devices per single chip.

Dayeh, Shadi A.

2010-02-01

258

Physarum can compute shortest paths.  

PubMed

Physarum polycephalum is a slime mold that is apparently able to solve shortest path problems. A mathematical model has been proposed by Tero et al. (Journal of Theoretical Biology, 244, 2007, pp. 553-564) to describe the feedback mechanism used by the slime mold to adapt its tubular channels while foraging two food sources s(0) and s(1). We prove that, under this model, the mass of the mold will eventually converge to the shortest s(0)-s(1) path of the network that the mold lies on, independently of the structure of the network or of the initial mass distribution. This matches the experimental observations by Tero et al. and can be seen as an example of a "natural algorithm", that is, an algorithm developed by evolution over millions of years. PMID:22732274

Bonifaci, Vincenzo; Mehlhorn, Kurt; Varma, Girish

2012-06-23

259

E-beam ionized channel guiding of an intense relativistic electron beam  

DOEpatents

An IREB is guided through a curved path by ionizing a channel in a gas with electrons from a filament, and confining the electrons to the center of the path with a magnetic field extending along the path. The magnetic field is preferably generated by a solenoid extending along the path. 2 figs.

Frost, C.A.; Godfrey, B.B.; Kiekel, P.D.; Shope, S.L.

1988-05-10

260

Time optimal paths for high speed maneuvering  

SciTech Connect

Recent theoretical results have completely solved the problem of determining the minimum length path for a vehicle with a minimum turning radius moving from an initial configuration to a final configuration. Time optimal paths for a constant speed vehicle are a subset of the minimum length paths. This paper uses the Pontryagin maximum principle to find time optimal paths for a constant speed vehicle. The time optimal paths consist of sequences of axes of circles and straight lines. The maximum principle introduces concepts (dual variables, bang-bang solutions, singular solutions, and transversality conditions) that provide important insight into the nature of the time optimal paths. We explore the properties of the optimal paths and present some experimental results for a mobile robot following an optimal path.

Reister, D.B.; Lenhart, S.M.

1993-01-01

261

NASA: a path dependent organization  

Microsoft Academic Search

Mission agencies like NASA are complex systems, connecting people with science and technology to accomplish the desired tasks. Path dependence can help explain why NASA and other mission agencies often sacrifice long-term capabilities for short-term survival. The National Aeronautics and Space Act of 1958, followed by President Kennedys challenge, catapulted NASA to the moon, encouraged human exploration, and sped up

David Bruggeman

2002-01-01

262

Accelerating cleanup: Paths to closure  

SciTech Connect

This document was previously referred to as the Draft 2006 Plan. As part of the DOE`s national strategy, the Richland Operations Office`s Paths to Closure summarizes an integrated path forward for environmental cleanup at the Hanford Site. The Hanford Site underwent a concerted effort between 1994 and 1996 to accelerate the cleanup of the Site. These efforts are reflected in the current Site Baseline. This document describes the current Site Baseline and suggests strategies for further improvements in scope, schedule and cost. The Environmental Management program decided to change the name of the draft strategy and the document describing it in response to a series of stakeholder concerns, including the practicality of achieving widespread cleanup by 2006. Also, EM was concerned that calling the document a plan could be misconstrued to be a proposal by DOE or a decision-making document. The change in name, however, does not diminish the 2006 vision. To that end, Paths to Closure retains a focus on 2006, which serves as a point in time around which objectives and goals are established.

Edwards, C.

1998-06-30

263

Path integral on star graph  

NASA Astrophysics Data System (ADS)

In this paper we study path integral for a single spinless particle on a star graph with N edges, whose vertex is known to be described by U(N) family of boundary conditions. After carefully studying the free particle case, both at the critical and off-critical levels, we propose a new path integral formulation that correctly captures all the scale-invariant subfamily of boundary conditions realized at fixed points of boundary renormalization group flow. Our proposal is based on the folding trick, which maps a scalar-valued wave function on star graph to an N-component vector-valued wave function on half-line. All the parameters of scale-invariant subfamily of boundary conditions are encoded into the momentum independent weight factors, which appear to be associated with the two distinct path classes on half-line that form the cyclic group Z2. We show that, when bulk interactions are edge-independent, these weight factors are generally given by an N-dimensional unitary representation of Z2. Generalization to momentum dependent weight factors and applications to worldline formalism are briefly discussed.

Ohya, Satoshi

2012-06-01

264

Multiple Paths to Encephalization and Technical Civilizations  

NASA Astrophysics Data System (ADS)

We propose consideration of at least two possible evolutionary paths for the emergence of intelligent life with the potential for technical civilization. The first is the path via encephalization of homeothermic animals; the second is the path to swarm intelligence of so-called superorganisms, in particular the social insects. The path to each appears to be facilitated by environmental change: homeothermic animals by decreased climatic temperature and for swarm intelligence by increased oxygen levels.

Schwartzman, David; Middendorf, George

2011-12-01

265

Theory of bright-field scanning transmission electron microscopy for tomography  

NASA Astrophysics Data System (ADS)

Radiation transport theory is applied to electron microscopy of samples composed of one or more materials. The theory, originally due to Goudsmit and Saunderson, assumes only elastic scattering and an amorphous medium dominated by atomic interactions. For samples composed of a single material, the theory yields reasonable parameter-free agreement with experimental data taken from the literature for the multiple scattering of 300-keV electrons through aluminum foils up to 25 ?m thick. For thin films, the theory gives a validity condition for Beer's law. For thick films, a variant of Molire's theory [V. G. Molire, Z. Naturforschg. 3a, 78 (1948)] of multiple scattering leads to a form for the bright-field signal for foils in the multiple-scattering regime. The signal varies as [t ln(e1-2?t/?)]-1 where t is the path length of the beam, ? is the mean free path for elastic scattering, and ? is Euler's constant. The Goudsmit-Saunderson solution interpolates numerically between these two limits. For samples with multiple materials, elemental sensitivity is developed through the angular dependence of the scattering. From the elastic scattering cross sections of the first 92 elements, a singular-value decomposition of a vector space spanned by the elastic scattering cross sections minus a delta function shows that there is a dominant common mode, with composition-dependent corrections of about 2%. A mathematically correct reconstruction procedure beyond 2% accuracy requires the acquisition of the bright-field signal as a function of the scattering angle. Tomographic reconstructions are carried out for three singular vectors of a sample problem with four elements Cr, Cu, Zr, and Te. The three reconstructions are presented jointly as a color image; all four elements are clearly identifiable throughout the image.

Levine, Zachary H.

2005-02-01

266

Theory of bright-field scanning transmission electron microscopy for tomography  

SciTech Connect

Radiation transport theory is applied to electron microscopy of samples composed of one or more materials. The theory, originally due to Goudsmit and Saunderson, assumes only elastic scattering and an amorphous medium dominated by atomic interactions. For samples composed of a single material, the theory yields reasonable parameter-free agreement with experimental data taken from the literature for the multiple scattering of 300-keV electrons through aluminum foils up to 25 {mu}m thick. For thin films, the theory gives a validity condition for Beer's law. For thick films, a variant of Moliere's theory [V. G. Moliere, Z. Naturforschg. 3a, 78 (1948)] of multiple scattering leads to a form for the bright-field signal for foils in the multiple-scattering regime. The signal varies as [t ln(e{sup 1-2{gamma}}t/{tau})]{sup -1} where t is the path length of the beam, {tau} is the mean free path for elastic scattering, and {gamma} is Euler's constant. The Goudsmit-Saunderson solution interpolates numerically between these two limits. For samples with multiple materials, elemental sensitivity is developed through the angular dependence of the scattering. From the elastic scattering cross sections of the first 92 elements, a singular-value decomposition of a vector space spanned by the elastic scattering cross sections minus a delta function shows that there is a dominant common mode, with composition-dependent corrections of about 2%. A mathematically correct reconstruction procedure beyond 2% accuracy requires the acquisition of the bright-field signal as a function of the scattering angle. Tomographic reconstructions are carried out for three singular vectors of a sample problem with four elements Cr, Cu, Zr, and Te. The three reconstructions are presented jointly as a color image; all four elements are clearly identifiable throughout the image.

Levine, Zachary H. [National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8410 (United States)

2005-02-01

267

Lateral Path Controller Design for Autonomous Airship  

Microsoft Academic Search

According to the path control problem for unmanned autonomous airship, the lateral path mathematic model was introduced based on the scheme of control system and a kind of lateral path design method of autonomous airship is proposed based on fuzzy logic and adaptive sliding mode control (ASMC). The movement model and dynamic model of autonomous airship is derived from considering

Guo Jian-guo; Zhou Jun

2010-01-01

268

Quantifying the Causes of Path Inflation  

Microsoft Academic Search

Researchers have shown that the Internet exhibits path inflation - end-to-end paths can be significantly longer than necessary. We present a trace-driven study of 65 ISPs that characterizes the root causes of path inflation, namely topology and routing policy choices within an ISP, between pairs of ISPs, and across the global Inter- net. To do so, we develop and validate

Neil Spring; Ratul Mahajan; Thomas Anderson

2003-01-01

269

Chip layout optimization using critical path weighting  

Microsoft Academic Search

A chip layout procedure for optimizing the performance of critical timing paths in a synchronous digital circuit is presented. The procedure uses the path analysis data produced by a static timing analysis program to generate weights for critical nets on clock and data paths. These weights are then used to bias automatic placement and routing in the layout program. This

A. E. Dunlop; V. D. Agrawal; D. N. Deutsch; M. F. Jukl; P. Kozak; M. Wiesel

1984-01-01

270

Chip layout optimization using critical path weighting  

Microsoft Academic Search

A chip layout procedure for optimizing the performance of critical timing paths in a synchronous digital circuit is presented. The procedure uses the path analysis data produced by a static timing analysis program to generate weights for critical nets on clock and data paths. These weights are then used to bias automatic placement and routing in the layout program. This

A. E. Dunlop; V. D. Agrawal; D. N. Deutsch; M. F. Jukl; P. Kazak

1988-01-01

271

Multilinear decomposition of human walking paths  

Microsoft Academic Search

In a previous work, the authors have shown how the Principal Components Analysis (PCA) of a set of human walking paths provides sufficient information to derive a linear human-like path generator based on examples. The present work aims to provide an analysis of human walking paths from the perspective of multilinear algebra, using the n-mode Singular Value Decomposition (SVD). This

Christian A. Ramirez; M. Castela?n; G. Arechavaleta

2010-01-01

272

Randomized path coloring on binary trees  

Microsoft Academic Search

Motivated by the problem of WDM routing in all-optical networks, we study the following NP-hard problem. We are given a directed binary tree T and a set R of directed paths on T. We wish to assign colors to paths of R, in such way that no two paths that share a directed arc of T are assigned the same

Vincenzo Auletta; Ioannis Caragiannis; Christos Kaklamanis; Pino Persiano

2002-01-01

273

Removing False Paths from Combinational Modules 1  

Microsoft Academic Search

The existence of false paths complicates the task of accurate tim- ing analysis significantly. A technique to remove false paths from a combinational circuit without degrading its performance h as a prac- tical value since topological timing analysis is then good e nough to estimate the performance of false-path-free circuits accu rately. One can think of the KMS algorithm (1)

Yuji Kukimoto; Robert K. Brayton

274

Hypertext Paths and the World-Wide Web: Experiences with Walden's Paths  

Microsoft Academic Search

Walden's Paths applies the concept of hypertextual paths to the World-Wide Web. Walden's Paths is being developed for use in the K-12 school environment. The heterogene- ity of the Web coupled with the desirability of supporting the teacher-student relationship make this an interesting and challenging project. We describe the Walden's Paths imple- mentation, discuss ...

Richard Furuta; Frank M. Shipman III; Catherine C. Marshall; Donald Brenner; Hao-wei Hsieh

1997-01-01

275

Preserving Topology Confidentiality in Inter-Domain Path Computation Using a Path-Key-Based Mechanism  

Microsoft Academic Search

Multiprotocol Label Switching (MPLS) and Generalized MPLS (GMPLS) Traffic Engineering (TE) Label Switched Paths (LSPs) may be computed by Path Computation Elements (PCEs). Where the TE LSP crosses multiple domains, such as Autonomous Systems (ASes), the path may be computed by multiple PCEs that cooperate, with each responsible for computing a segment of the path. However, in some cases (e.g.,

A. Farrel

276

Evolutionary path of UHF secondary imagery dissemination systems (SIDS)  

Microsoft Academic Search

Secondary imagery dissemination (SID) is the process of post-collection and electronic dissemination of digital imagery and associated data. SID systems (SIDS) provide exploited, non-original, annotated, anonymous-source imagery from intelligence, fusion, and dissemination centers. Ultra-high frequency (UHF) operation penetrates double-canopy jungle or heavy rain to provide imagery from an INTEL center to highly-mobile tactical users. The evolutionary path of UHF SIDS

Ernie Franke

2000-01-01

277

Equilibrium and Dynamical Path Integral Methods in Bacterial Photosynthesis  

Microsoft Academic Search

Classical molecular dynamics simulations have provided a wealth of information on processes in biological systems. In spite\\u000a of the spectacular success and the insights gained from such simulations, processes involving electron or exciton transfer\\u000a are inherently quantum mechanical and thus not amenable to a classical description. This chapter focuses on the use of path\\u000a integral methods for simulating the equilibrium

Nancy Makri

278

Diffusion Path Theorems for Ternary Diffusion Couples  

NASA Astrophysics Data System (ADS)

A review is given of 17 theorems concerning diffusion paths in ternary diffusion couples published by Kirkaldy and Brown in 1963. An additional 11 theorems are given herein that were taken from work published on diffusion paths after that time. The new theorems are concerned primarily with diffusion paths that result from crossing multiple-phase regions in an interdiffusion zone. The theorems describe a method for classifying microstructural boundaries between the regions and a catalog of diffusion path features that are unique to each type of boundary. In addition, a proposal is given for how to plot diffusion paths in quaternary and higher order systems.

Morral, John E.

2012-10-01

279

Longest Path Problems on Ptolemaic Graphs  

NASA Astrophysics Data System (ADS)

Longest path problem is a problem for finding a longest path in a given graph. While the graph classes in which the Hamiltonian path problem can be solved efficiently are widely investigated, there are few known graph classes such that the longest path problem can be solved efficiently. Polynomial time algorithms for finding a longest cycle and a longest path in a Ptolemaic graph are proposed. Ptolemaic graphs are the graphs that satisfy the Ptolemy inequality, and they are the intersection of chordal graphs and distance-hereditary graphs. The algorithms use the dynamic programming technique on a laminar structure of cliques, which is a recent characterization of Ptolemaic graphs.

Takahara, Yoshihiro; Teramoto, Sachio; Uehara, Ryuhei

280

Colloidal quantum dot photovoltaics: a path forward.  

PubMed

Colloidal quantum dots (CQDs) offer a path toward high-efficiency photovoltaics based on low-cost materials and processes. Spectral tunability via the quantum size effect facilitates absorption of specific wavelengths from across the sun's broad spectrum. CQD materials' ease of processing derives from their synthesis, storage, and processing in solution. Rapid advances have brought colloidal quantum dot photovoltaic solar power conversion efficiencies of 6% in the latest reports. These achievements represent important first steps toward commercially compelling performance. Here we review advances in device architecture and materials science. We diagnose the principal phenomenon-electronic states within the CQD film band gap that limit both current and voltage in devices-that must be cured for CQD PV devices to fulfill their promise. We close with a prescription, expressed as bounds on the density and energy of electronic states within the CQD film band gap, that should allow device efficiencies to rise to those required for the future of the solar energy field. PMID:21967723

Kramer, Illan J; Sargent, Edward H

2011-10-12

281

Attention trees and semantic paths  

NASA Astrophysics Data System (ADS)

In the last few decades several techniques for image content extraction, often based on segmentation, have been proposed. It has been suggested that under the assumption of very general image content, segmentation becomes unstable and classification becomes unreliable. According to recent psychological theories, certain image regions attract the attention of human observers more than others and, generally, the image main meaning appears concentrated in those regions. Initially, regions attracting our attention are perceived as a whole and hypotheses on their content are formulated; successively the components of those regions are carefully analyzed and a more precise interpretation is reached. It is interesting to observe that an image decomposition process performed according to these psychological visual attention theories might present advantages with respect to a traditional segmentation approach. In this paper we propose an automatic procedure generating image decomposition based on the detection of visual attention regions. A new clustering algorithm taking advantage of the Delaunay- Voronoi diagrams for achieving the decomposition target is proposed. By applying that algorithm recursively, starting from the whole image, a transformation of the image into a tree of related meaningful regions is obtained (Attention Tree). Successively, a semantic interpretation of the leaf nodes is carried out by using a structure of Neural Networks (Neural Tree) assisted by a knowledge base (Ontology Net). Starting from leaf nodes, paths toward the root node across the Attention Tree are attempted. The task of the path consists in relating the semantics of each child-parent node pair and, consequently, in merging the corresponding image regions. The relationship detected in this way between two tree nodes generates, as a result, the extension of the interpreted image area through each step of the path. The construction of several Attention Trees has been performed and partial results will be shown.

Giusti, Christian; Pieroni, Goffredo G.; Pieroni, Laura

2007-03-01

282

Evaluation of guidewire path reproducibility  

PubMed Central

The number of minimally invasive vascular interventions is increasing. In these interventions, a variety of devices are directed to and placed at the site of intervention. The device used in almost all of these interventions is the guidewire, acting as a monorail for all devices which are delivered to the intervention site. However, even with the guidewire in place, clinicians still experience difficulties during the interventions. As a first step toward understanding these difficulties and facilitating guidewire and device guidance, we have investigated the reproducibility of the final paths of the guidewire in vessel phantom models on different factors: user, materials and geometry. Three vessel phantoms (vessel diameters ?4 mm) were constructed having tortuousity similar to the internal carotid artery from silicon tubing and encased in Sylgard elastomer. Several trained users repeatedly passed two guidewires of different flexibility through the phantoms under pulsatile flow conditions. After the guidewire had been placed, rotational c-arm image sequences were acquired (9 in. II mode, 0.185 mm pixel size), and the phantom and guidewire were reconstructed (5123, 0.288 mm voxel size). The reconstructed volumes were aligned. The centerlines of the guidewire and the phantom vessel were then determined using region-growing techniques. Guidewire paths appear similar across users but not across materials. The average root mean square difference of the repeated placement was 0.170.02 mm (plastic-coated guidewire), 0.730.55 mm (steel guidewire) and 1.150.65 mm (steel versus plastic-coated). For a given guidewire, these results indicate that the guidewire path is relatively reproducible in shape and position.

Schafer, Sebastian; Hoffmann, Kenneth R.; Noel, Peter B.; Ionita, Ciprian N.; Dmochowski, Jacek

2008-01-01

283

Evaluation of guidewire path reproducibility.  

PubMed

The number of minimally invasive vascular interventions is increasing. In these interventions, a variety of devices are directed to and placed at the site of intervention. The device used in almost all of these interventions is the guidewire, acting as a monorail for all devices which are delivered to the intervention site. However, even with the guidewire in place, clinicians still experience difficulties during the interventions. As a first step toward understanding these difficulties and facilitating guidewire and device guidance, we have investigated the reproducibility of the final paths of the guidewire in vessel phantom models on different factors: user, materials and geometry. Three vessel phantoms (vessel diameters approximately 4 mm) were constructed having tortuousity similar to the internal carotid artery from silicon tubing and encased in Sylgard elastomer. Several trained users repeatedly passed two guidewires of different flexibility through the phantoms under pulsatile flow conditions. After the guidewire had been placed, rotational c-arm image sequences were acquired (9 in. II mode, 0.185 mm pixel size), and the phantom and guidewire were reconstructed (512(3), 0.288 mm voxel size). The reconstructed volumes were aligned. The centerlines of the guidewire and the phantom vessel were then determined using region-growing techniques. Guidewire paths appear similar across users but not across materials. The average root mean square difference of the repeated placement was 0.17 +/- 0.02 mm (plastic-coated guidewire), 0.73 +/- 0.55 mm (steel guidewire) and 1.15 +/- 0.65 mm (steel versus plastic-coated). For a given guidewire, these results indicate that the guidewire path is relatively reproducible in shape and position. PMID:18561663

Schafer, Sebastian; Hoffmann, Kenneth R; Nol, Peter B; Ionita, Ciprian N; Dmochowski, Jacek

2008-05-01

284

Three-Dimensional Electron Realm in VSe2 by Soft-X-Ray Photoelectron Spectroscopy: Origin of Charge-Density Waves  

NASA Astrophysics Data System (ADS)

The resolution of angle-resolved photoelectron spectroscopy (ARPES) in three-dimensional (3D) momentum k is fundamentally limited by ill defined surface-perpendicular wave vector k? associated with the finite photoelectron mean free path. Pushing ARPES into the soft-x-ray energy region sharpens the k? definition, allowing accurate electronic structure investigations in 3D materials. We apply soft-x-ray ARPES to explore the 3D electron realm in a paradigm transition metal dichalcogenide VSe2. Essential to break through the dramatic loss of the valence band photoexcitation cross section at soft-x-ray energies is the advanced photon flux performance of our synchrotron instrumentation. By virtue of the sharp 3D momentum definition, the soft-x-ray ARPES experimental band structure and Fermi surface of VSe2 show a textbook clarity. We identify pronounced 3D warping of the Fermi surface and show that its concomitant nesting acts as the precursor for the exotic 3D charge-density waves in VSe2. Our results demonstrate the immense potential of soft-x-ray ARPES to explore details of 3D electronic structure.

Strocov, Vladimir N.; Shi, Ming; Kobayashi, Masaki; Monney, Claude; Wang, Xiaoqiang; Krempasky, Juraj; Schmitt, Thorsten; Patthey, Luc; Berger, Helmuth; Blaha, Peter

2012-08-01

285

Three-dimensional electron realm in VSe2 by soft-x-ray photoelectron spectroscopy: origin of charge-density waves.  

PubMed

The resolution of angle-resolved photoelectron spectroscopy (ARPES) in three-dimensional (3D) momentum k is fundamentally limited by ill defined surface-perpendicular wave vector k(perpendicular) associated with the finite photoelectron mean free path. Pushing ARPES into the soft-x-ray energy region sharpens the k(perpendicular) definition, allowing accurate electronic structure investigations in 3D materials. We apply soft-x-ray ARPES to explore the 3D electron realm in a paradigm transition metal dichalcogenide VSe2. Essential to break through the dramatic loss of the valence band photoexcitation cross section at soft-x-ray energies is the advanced photon flux performance of our synchrotron instrumentation. By virtue of the sharp 3D momentum definition, the soft-x-ray ARPES experimental band structure and Fermi surface of VSe2 show a textbook clarity. We identify pronounced 3D warping of the Fermi surface and show that its concomitant nesting acts as the precursor for the exotic 3D charge-density waves in VSe2. Our results demonstrate the immense potential of soft-x-ray ARPES to explore details of 3D electronic structure. PMID:23002761

Strocov, Vladimir N; Shi, Ming; Kobayashi, Masaki; Monney, Claude; Wang, Xiaoqiang; Krempasky, Juraj; Schmitt, Thorsten; Patthey, Luc; Berger, Helmuth; Blaha, Peter

2012-08-20

286

Ideal two-dimensional electron systems with a giant Rashba-type spin splitting in real materials: surfaces of bismuth tellurohalides.  

PubMed

Spintronics is aimed at actively controlling and manipulating the spin degrees of freedom in semiconductor devices. A promising way to achieve this goal is to make use of the tunable Rashba effect that relies on the spin-orbit interaction in a two-dimensional electron system immersed in an inversion-asymmetric environment. The spin-orbit-induced spin splitting of the two-dimensional electron state provides a basis for many theoretically proposed spintronic devices. However, the lack of semiconductors with large Rashba effect hinders realization of these devices in actual practice. Here we report on a giant Rashba-type spin splitting in two-dimensional electron systems that reside at tellurium-terminated surfaces of bismuth tellurohalides. Among these semiconductors, BiTeCl stands out for its isotropic metallic surface-state band with the ?-point energy lying deep inside the bulk band gap. The giant spin splitting of this band ensures a substantial spin asymmetry of the inelastic mean free path of quasiparticles with different spin orientations. PMID:23004307

Eremeev, S V; Nechaev, I A; Koroteev, Yu M; Echenique, P M; Chulkov, E V

2012-06-13

287

Electron localization effects on the low-temperature high-field magnetoresistivity of three-dimensional amorphous superconductors  

SciTech Connect

The electrical resistivity {rho} of three-dimensional amorphous superconducting films a-Mo{sub 3}Si and a-Nb{sub 3}Ge is measured in magnetic fields {mu}{sub 0}H up to 30 T. At low temperatures and at magnetic fields above the upper critical field H{sub c2}, {rho} is temperature independent and decreases as a function of magnetic field. This field dependence is consistent with localization theory in the high-field limit [{mu}{sub 0}H{gt}{h_bar}/(4eL{sub {phi}}{sup 2}), where L{sub {phi}} is the phase-coherence length]. Above the superconducting transition temperature T{sub c}, the temperature dependence of the conductivity is consistent with inelastic scattering processes which are destructive to the phase coherence for electron localization, thereby allowing estimates for L{sub {phi}}(T). The Hall effect data on a-Mo{sub 3}Si, in conjunction with the resistivity data, allow the determination of the carrier concentration and mean free path. The upper critical field is comparable to (in a-Mo{sub 3}Si) and significantly larger than (in a-Nb{sub 3}Ge) the Clogston-Chandrasekhar paramagnetic limit. This phenomenon is discussed in the context of electron localization. {copyright} {ital 1998} {ital The American Physical Society}

Samoilov, A.V.; Yeh, N. [Department of Physics 114-36, California Institute of Technology, Pasadena, California 91125 (United States); Tsuei, C.C. [IBM, Thomas Watson Research Center, Yorktown Heights, New York 10598 (United States)

1998-01-01

288

[Botulinum toxin: the misguided path].  

PubMed

Botulinum toxin is widely used and has become a popular mass phenomenon in aesthetic medicine. Considerable scientific data concerning the biopsychosocial impact of botulinum toxin use have become available. The bidirectional interaction of mimic and emotion, described as the facial feedback hypothesis, is particularly influenced, as is mimicry. Furthermore, botulinum toxin can cause dysfunction of face harmony including false laughing or the "frozen face". As a result, complex psychosocial disturbances can occur and may affect social interaction and cause flattening of affect. Thus one must ask whether in the future botulinum toxin will continue to be employed in aesthetic dermatology or perhaps be regarded as a misguided path. PMID:23636411

Harth, W

2013-06-01

289

Electron Spectroscopic Methods for the Investigation of Liquid Surface Structure  

NASA Astrophysics Data System (ADS)

The goal of our work is the elucidation of the surface structure of liquids on the molecular level. We use several electron spectroscopies and related techniques. In photoelectron spectroscopy the observation depth is governed by the mean free path of the photoelectrons which in turn depends on the kinetic energy. Using synchrotron radiation as source of photons the energy and thus the observation depth can be controlled. Such measurements allow to reconstruct concentration depth profiles on a scale length of ca. 0.2 nm. Perfect surface sensitivity is achieved by MIES (= Metastable Induced Electron Spectroscopy) because the metastable helium atoms do not penetrate into the liquid. Recently we have applied ISS (= Ion Scattering Spectroscopy) to liquid surfaces, again a technique of perfect surface sensitivity which yields the elemental composition. Among the systems studied are pure polar solvents footnote[2]H. Morgner and W.A. Mller Mol. Phys. 57 (1986) 623, H. Morgner, J. Oberbrodhage and M.Wulf J. Electr. Spectr. Rel. Phen. 61 (1993) 183-92, liquid mixturesfootnote[3]H. Morgner, J. Oberbrodhage, K. Richter and K. Roth Mol. Phys. 73 (1991) 1295 - 1306, H. Morgner and M. Wulf J.Electr.Spectr.Rel.Phen. (1995) in print, solutions of surface active salts footnote[4] H. Morgner and J. Oberbrodhage J. Phys.: Condens. Matter 7 (1995) 7427-7441, H. Morgner, J. Oberbrodhage, K. Richter and K. Roth J. Phys.: Condens. Matt. 3 (1991) 5639 - 55 and the spontaneous formation of tenside films on liquid surfaces footnote[5]H. Morgner, J. Oberbrodhage and K. Richter Mol. Phys. 76 (1992) 813-22, H. Morgner, K. Richter and M.Wulf Mol. Phys. 79 (1993) 169-177.

Morgner, Harald

1996-03-01

290

Path integration: effect of curved path complexity and sensory system on blindfolded walking.  

PubMed

Path integration refers to the ability to integrate continuous information of the direction and distance traveled by the system relative to the origin. Previous studies have investigated path integration through blindfolded walking along simple paths such as straight line and triangles. However, limited knowledge exists regarding the role of path complexity in path integration. Moreover, little is known about how information from different sensory input systems (like vision and proprioception) contributes to accurate path integration. The purpose of the current study was to investigate how sensory information and curved path complexity affect path integration. Forty blindfolded participants had to accurately reproduce a curved path and return to the origin. They were divided into four groups that differed in the curved path, circle (simple) or figure-eight (complex), and received either visual (previously seen) or proprioceptive (previously guided) information about the path before they reproduced it. The dependent variables used were average trajectory error, walking speed, and distance traveled. The results indicated that (a) both groups that walked on a circular path and both groups that received visual information produced greater accuracy in reproducing the path. Moreover, the performance of the group that received proprioceptive information and later walked on a figure-eight path was less accurate than their corresponding circular group. The groups that had the visual information also walked faster compared to the group that had proprioceptive information. Results of the current study highlight the roles of different sensory inputs while performing blindfolded walking for path integration. PMID:22840893

Koutakis, Panagiotis; Mukherjee, Mukul; Vallabhajosula, Srikant; Blanke, Daniel J; Stergiou, Nicholas

2012-07-27

291

Effect of strain path change on the evolution of texture and microstructure during rolling of copper and nickel  

Microsoft Academic Search

The effect of strain path change during rolling has been investigated for copper and nickel using X-ray diffraction and electron back scatter diffraction as well as crystal plasticity simulations. Four different strain paths namely: (i) unidirectional rolling; (ii) reverse rolling; (iii) two-step cross rolling and (iv) multi-step cross rolling were employed to decipher the effect of strain path change on

N. P. Gurao; S. Sethuraman; Satyam Suwas

2011-01-01

292

Analysis of the electron and ion fluxes to the wall of a hot-filament discharge device  

NASA Astrophysics Data System (ADS)

The current densities of ions and electrons to the wall of a hot-filament discharge device are examined both experimentally and theoretically. The ion current to the wall as a function of neutral gas pressure is found theoretically from a model of the sheath and presheath that includes charge-exchange collisions of the ions with neutrals. The electron current is found from a model based upon the energy distributions of secondary electrons from ionization of the neutral gas and of secondary electrons from the wall. In a hot-filament discharge device with argon plasma (density (0.2-4.5)109 cm-3, electron temperature 0.14-0.21 eV, and pressure 0.3-12 mTorr), a gridded energy analyzer is placed behind a slit in the wall and the current collected is recorded as a function of the retarding potential. The dependence of the collector current on the grid bias potential identifies the electrons in the 10-65 eV range as being mostly secondaries from ionization and those in the 0-10 eV range as being mostly secondaries from the wall. Ions are collected at the most negative grid bias voltages. The measured ion currents are within about 40% of values calculated from a model with charge exchange collisions of ions, and at the highest pressure differ by about a factor of 3 from the collisionless value, indicating that the Bohm ion current should be corrected for collisions when the charge exchange mean free path is less than about 0.2 of the plasma radius.

Wang, Xu; Knappmiller, Scott; Robertson, Scott; Sternovsky, Zoltan

2007-04-01

293

Calculations of stopping powers of 100 eV-30 keV electrons in 31 elemental solids  

SciTech Connect

We present calculated electron stopping powers (SPs) for 31 elemental solids (Li, Be, glassy C, graphite, diamond, Na, Mg, K, Sc, Ti, V, Fe, Y, Zr, Nb, Mo, Ru, Rh, In, Sn, Cs, Gd, Tb, Dy, Hf, Ta, W, Re, Os, Ir, and Bi). These SPs were determined with an algorithm previously used for the calculation of electron inelastic mean free paths and from energy-loss functions (ELFs) derived from experimental optical data. The SP calculations were made for electron energies between 100 eV and 30 keV and supplement our earlier SP calculations for ten additional solids (Al, Si, Cr, Ni, Cu, Ge, Pd, Ag, Pt, and Au). Plots of SP versus atomic number for the group of 41 solids show clear trends. Multiple peaks and shoulders are seen that result from the contributions of valence-electron and various inner-shell excitations. Satisfactory agreement was found between the calculated SPs and values from the relativistic Bethe SP equation with recommended values of the mean excitation energy (MEE) for energies above 10 keV. We determined effective MEEs versus maximum excitation energy from the ELFs for each solid. Plots of effective MEE versus atomic number showed the relative contributions of valence-electron and different core-electron excitations to the MEE. For a maximum excitation energy of 30 keV, our effective MEEs agreed well for Be, graphite, Na, Al, and Si with recommended MEEs; a difference for Li was attributed to sample oxidation in the SP measurements for the recommended MEE. Substantially different effective MEEs were found for the three carbon allotropes (graphite, diamond, and glassy C)

Tanuma, S. [Materials Analysis Station, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047 (Japan); Powell, C. J.; Penn, D. R. [National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8370 (United States)

2008-03-15

294

Analysis of the electron and ion fluxes to the wall of a hot-filament discharge device  

SciTech Connect

The current densities of ions and electrons to the wall of a hot-filament discharge device are examined both experimentally and theoretically. The ion current to the wall as a function of neutral gas pressure is found theoretically from a model of the sheath and presheath that includes charge-exchange collisions of the ions with neutrals. The electron current is found from a model based upon the energy distributions of secondary electrons from ionization of the neutral gas and of secondary electrons from the wall. In a hot-filament discharge device with argon plasma (density (0.2-4.5)x10{sup 9} cm{sup -3}, electron temperature 0.14-0.21 eV, and pressure 0.3-12 mTorr), a gridded energy analyzer is placed behind a slit in the wall and the current collected is recorded as a function of the retarding potential. The dependence of the collector current on the grid bias potential identifies the electrons in the 10-65 eV range as being mostly secondaries from ionization and those in the 0-10 eV range as being mostly secondaries from the wall. Ions are collected at the most negative grid bias voltages. The measured ion currents are within about 40% of values calculated from a model with charge exchange collisions of ions, and at the highest pressure differ by about a factor of 3 from the collisionless value, indicating that the Bohm ion current should be corrected for collisions when the charge exchange mean free path is less than about 0.2 of the plasma radius.

Wang Xu; Knappmiller, Scott; Robertson, Scott; Sternovsky, Zoltan [Department of Physics, University of Colorado, Boulder, Colorado 80309-0390 (United States); Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder Colorado 80309-0392 (United States)

2007-04-15

295

Energetic electrons in the magnetosheath and upstream of the bow shock  

SciTech Connect

The characteristics and transport of >200 keV electrons upstream of Earth's bow shock and in the outer magnetosheath are studied using nearly 4 years of data from the California Institute of Technology Electron/Isotope Spectrometer abroad IMP 8. In the upstream region, elevated electron intensities ranging from near the background level approx.0.1 (cm/sup 2/ s sr)/sup -1/ up to approx.100 (cm/sup 2/ s sr)/sup -1/ are observed about 2% of the time and typically persist for less than 5 min. These burst electrons stream intensely (xiapprox.0.5 to 1.5) sunward along the interplanetary magnetic field line. With the aid of a new coordinate system, it is demonstrated that the upstream bursts are most frequently seen on interplanetary field lines that trace to the inner magnetosheath, and that the total energy transported sunward by >200 keV upstream electrons averages approx.1.6 x 10/sup 14/ ergs/s, a figure comparable to the tailward energy flow observed in the permanent layer of energetic electrons adjacent to the magnetopause. Bursts of energetic electrons in the outer magnetosheath have time scales similar to those observed upstream, but they are typically approx.10 times more intense and much less anisotropic (xi< or approx. =0.4). The net flow of these electrons is toward the shock and very nearly equals the net flow away from the shock in the adjacent upstream region. The magnetosheath data suggest that energetic electron transport in this region may be described by a leaky box model with a scattering mean free path approx.0.6 R/sub E/. It is concluded that the energetic (>200 keV) component of upstream electrons originates downstream of the bow shock. Energetic electrons in the upstream region, in the outer magnetosheath, and in the magnetopause electron layer appear to be closely related and probably have a common origin in the inner magnetosheath or possibly the magnetosphere.

Bieber, J.W.; Stone, E.C.

1982-01-01

296

Optimum gradient of mountain paths.  

PubMed

By combining the experiment results of R. Margaria (Atti Accad. Naz. Lincei Memorie 7: 299-368, 1938), regarding the metabolic cost of gradient locomotion, together with recent insights on gait biomechanics, a prediction about the most economical gradient of mountain paths (approximately 25%) is obtained and interpreted. The pendulum-like mechanism of walking produces a waste of mechanical work against gravity within the gradient range of up to 15% (the overall efficiency is dominated by the low transmission efficiency), whereas for steeper values only the muscular efficiency is responsible for the (slight) metabolic change (per meter of vertical displacement) with respect to gradient. The speeds at the optimum gradient turned out to be approximately 0.65 m/s (+0.16 m/s vertical) and 1.50 m/s (-0.36 m/s vertical), for uphill and downhill walking, respectively, and the ascensional energy expenditure was 0.4 and 2.0 ml O2.kg body mass-1.vertical m-1 climbed or descended. When the metabolic power becomes a burden, as in high-altitude mountaineering, the optimum gradient should be reduced. A sample of real mountain path gradients, experimentally measured, mimics the obtained predictions. PMID:8594031

Minetti, A E

1995-11-01

297

Electron localization in water clusters. I. Electron--water pseudopotential  

SciTech Connect

A local pseudopotential for the interaction of an electron with a water molecule in the electronic ground state is developed. The potential contains Coulomb, adiabatic polarization, exclusion, and exchange contributions. The potential is suitable for a description of excess electron states in water clusers, and for studies of electron solvation in water. Quantum path integral molecular dynamics simulations of electron localization in water clusters employing this potential yield results in agreement with available experimental data and all-electron quantum chemical calculations.

Barnett, R.N.; Landman, U.; Cleveland, C.L.; Jortner, J.

1988-04-01

298

A numerical model and scaling relationship for energetic electron beams propagating in air  

SciTech Connect

Scaling relationships for energy loss and scattering are combined with a particle code to construct a purely algebraic expression for the energy deposited by an energetic electron beam injected into field-free homogeneous air. An algebraic formulation is possible because the mean free paths for the major collisional processes depend similarly on density and energy above 1 keV. Accordingly, the spatial behavior of an initially cold pencil beam is approximately self-similar when expressed in terms of the nominal beam range, provided the beam energy at injection exceeds several keV. Since a warm and broad beam can always be decomposed into a series of cold pencil beams, the total energy deposited can be obtained through a simple sum. With such a model, the ionization and excitation generated by a beam can be computed quickly and easily at every point in space. Similar formulations can be developed for other media using particle codes or experimental data. In liquids or solids, the energy deposited is quickly converted to heat and ultimately to melting and/or vaporization of the material.

Fernsler, R. F.; Slinker, S. P. [Plasma Physics Division, Naval Research Laboratory, Washington, DC 20375 (United States); Lambrakos, S. G. [Materials Science and Technology Division, Naval Research Laboratory, Washington, DC 20375 (United States)

2008-09-15

299

Electron pressure is less than nekTe in the solar wind  

NASA Astrophysics Data System (ADS)

In the solar wind beyond distance r? 10RSUN, mean free paths Lmfp become larger than the density scale height Ln, namely `collisionless'. However particles with random velocities V smaller than thermal speeds VT can still collide, and exert a pressure P. Below shows P? nkT. The collision distance L is proportional to V 4/n in general. In a rarefied solar wind it is limited by Ln (=r/2 for density n ? r-2 ). For electrons we thus expect Pe/nekTe= (V/VTe)2 =(Ln/Lmfp)1/2? ? _? 1. This shows Pe? 0.5nekTe at 1AU. Then the adiabatic relation is dlogTe /dlog ne= 2? /3 ? 1/3, but not 2/3 near 1AU. Since the diffusion coefficient of thermal conduction is LV (? LL1/4), the thermal conductivity becomes ? = ? (Spitzer) (Ln/Lmfp) 5/ 4, which scales Te0 instead of Te5/2. The thermal energy equation including thermal conduction then leads Te? quad r-0.4 in accord with Helios (r-0. 3) and Ulysses (r-0. 6). Observed large line-widths in the polar coronal hole are readily reproduced by projected Doppler widths of almost constant wind speeds of 800km/s already at 3-5RSUN because of the observed low density (Hirayama, T., COSPAR Colloq.13,13,2002), hence no need of very high temperatures in polar holes.

Hirayama, T.

300

Perturbation-theoretic studies of the anomalous electronic structure and transport properties of quasicrystals  

SciTech Connect

The question of whether scattering by the almost periodic lattice potentials of recently discovered metallic quasicrystals is able to explain the observed short mean free paths (about 1 A) in these materials is discussed. It is shown that scattering by a three-dimensional almost periodic potential cannot account for such resistivities, but a model which includes structural defects in the three-dimensional Penrose tiling and large scattering of electrons due to s-d resonant scattering can account for the experimental observations. It is also shown in perturbation theory that if a dc current is set up in the crystal (e.g., by an electric field which is switched on and then quickly switched off), the current will decay slowly to zero with an anomalous logarithmic dependence on time for one- and three-dimensional quasicrystals. This is most likely due to the extremely long-wavelength Fourier components possessed by the quasiperiodic potential. The same phenomena are not found for two-dimensional quasicrystals.

Sokoloff, J.B.

1987-10-15

301

Perturbation-theoretic studies of the anomalous electronic structure and transport properties of quasicrystals  

NASA Astrophysics Data System (ADS)

The question of whether scattering by the almost periodic lattice potentials of recently discovered metallic quasicrystals is able to explain the observed short mean free paths (about 1 Å) in these materials is discussed. It is shown that scattering by a three-dimensional almost periodic potential cannot account for such resistivities, but a model which includes structural defects in the three-dimensional Penrose tiling and large scattering of electrons due to s-d resonant scattering can account for the experimental observations. It is also shown in perturbation theory that if a dc current is set up in the crystal (e.g., by an electric field which is switched on and then quickly switched off), the current will decay slowly to zero with an anomalous logarithmic dependence on time for one- and three-dimensional quasicrystals. This is most likely due to the extremely long-wavelength Fourier components possessed by the quasiperiodic potential. The same phenomena are not found for two-dimensional quasicrystals.

Sokoloff, J. B.

1987-10-01

302

Path tracking control of a manipulator with passive joints  

Microsoft Academic Search

A method is proposed of path tracking control of a manipulator with passive joints, i.e. having no actuators. A desired path is geometrically specified in operational space. The position of the manipulator is controlled to follow the desired path. In this method, a path coordinate system based on the desired path is defined in operational space. The path coordinates consist

Hirohiko ARAI; Kazuo TANIE; Susumu TACHI

1991-01-01

303

OSLoptimal single-loop guide paths for AGVS  

Microsoft Academic Search

This study deals with the Automated Guided Vehicle System guide path design problem. We suggest a single closed loop guide path layout configuration as an alternative to conventional but more complex guide path designs. The benefits of using a simple guide path versus more complicated guide paths are discussed. A procedure for designing an optimal single loop guide path for

J. M. A. TANCHOCOf; DAVID SINRIECH

1992-01-01

304

Recombination of radical pairs in high magnetic fields: A path integral-Monte Carlo treatment  

Microsoft Academic Search

The magnetic field dependence of photoinduced geminate hydrogen (electron) transfer processes originates from a Brownian motion of radical pairs, a hyperfine coupling, and Zeeman-induced precession of the unpaired electron spins and a spin-selective recombination. In this paper we introduce a Wiener path integral description for these processes. This approach entails an integration over all possible diffusion trajectories of two functionals,

Klaus Schulten; Irving R. Epstein

1979-01-01

305

Spatial and frequency coherence of oblique, one-hop, high-frequency paths  

SciTech Connect

We consider the effect of random index of refraction fluctuations upon long-distance, ionospherically-reflected, hf paths. Along with deterministic effects such as multipath and dispersion, such fluctuations have a deleterious impact on hf communication including nonabsorptive fading, time-of-arrival spread, angle-of-arrival spread, and Doppler spread. We develop a formalism to calculate the mutual coherence functions for spatial and frequency separations based upon a path integral solution of the parabolic wave equation for a single refracted path through an ionosphere which contains random electron density fluctuations. The statistics of the hf path depend directly on the strength and statistics of the electron density fluctuations; we model the spatial power spectrum of the density fluctuation as a power law behavior versus frequency and with outer and inner scales.

Fitzgerald, T.J.

1995-10-01

306

Integrated assignment and path planning  

NASA Astrophysics Data System (ADS)

A surge of interest in unmanned systems has exposed many new and challenging research problems across many fields of engineering and mathematics. These systems have the potential of transforming our society by replacing dangerous and dirty jobs with networks of moving machines. This vision is fundamentally separate from the modern view of robotics in that sophisticated behavior is realizable not by increasing individual vehicle complexity, but instead through collaborative teaming that relies on collective perception, abstraction, decision making, and manipulation. Obvious examples where collective robotics will make an impact include planetary exploration, space structure assembly, remote and undersea mining, hazardous material handling and clean-up, and search and rescue. Nonetheless, the phenomenon driving this technology trend is the increasing reliance of the US military on unmanned vehicles, specifically, aircraft. Only a few years ago, following years of resistance to the use of unmanned systems, the military and civilian leadership in the United States reversed itself and have recently demonstrated surprisingly broad acceptance of increasingly pervasive use of unmanned platforms in defense surveillance, and even attack. However, as rapidly as unmanned systems have gained acceptance, the defense research community has discovered the technical pitfalls that lie ahead, especially for operating collective groups of unmanned platforms. A great deal of talent and energy has been devoted to solving these technical problems, which tend to fall into two categories: resource allocation of vehicles to objectives, and path planning of vehicle trajectories. An extensive amount of research has been conducted in each direction, yet, surprisingly, very little work has considered the integrated problem of assignment and path planning. This dissertation presents a framework for studying integrated assignment and path planning and then moves on to suggest an exact mathematical model and solution techniques. The approach adopted is based upon the very flexible New Product Development model but also blends many features from other approaches. Solution methods using branch and bound and construction heuristics are developed and tested on several example problems, including a military scenario featuring unmanned air vehicles.

Murphey, Robert A.

307

Sequential path entanglement for quantum metrology.  

PubMed

Path entanglement is a key resource for quantum metrology. Using path-entangled states, the standard quantum limit can be beaten, and the Heisenberg limit can be achieved. However, the preparation and detection of such states scales unfavourably with the number of photons. Here we introduce sequential path entanglement, in which photons are distributed across distinct time bins with arbitrary separation, as a resource for quantum metrology. We demonstrate a scheme for converting polarization Greenberger-Horne-Zeilinger entanglement into sequential path entanglement. We observe the same enhanced phase resolution expected for conventional path entanglement, independent of the delay between consecutive photons. Sequential path entanglement can be prepared comparably easily from polarization entanglement, can be detected without using photon-number-resolving detectors, and enables novel applications. PMID:23648653

Jin, Xian-Min; Peng, Cheng-Zhi; Deng, Youjin; Barbieri, Marco; Nunn, Joshua; Walmsley, Ian A

2013-01-01

308

Sequential Path Entanglement for Quantum Metrology  

NASA Astrophysics Data System (ADS)

Path entanglement is a key resource for quantum metrology. Using path-entangled states, the standard quantum limit can be beaten, and the Heisenberg limit can be achieved. However, the preparation and detection of such states scales unfavourably with the number of photons. Here we introduce sequential path entanglement, in which photons are distributed across distinct time bins with arbitrary separation, as a resource for quantum metrology. We demonstrate a scheme for converting polarization Greenberger-Horne-Zeilinger entanglement into sequential path entanglement. We observe the same enhanced phase resolution expected for conventional path entanglement, independent of the delay between consecutive photons. Sequential path entanglement can be prepared comparably easily from polarization entanglement, can be detected without using photon-number-resolving detectors, and enables novel applications.

Jin, Xian-Min; Peng, Cheng-Zhi; Deng, Youjin; Barbieri, Marco; Nunn, Joshua; Walmsley, Ian A.

2013-05-01

309

Quantifying Kinetic Paths of Protein Folding  

PubMed Central

We propose a new approach to activated protein folding dynamics via a diffusive path integral framework. The important issues of kinetic paths in this situation can be directly addressed. This leads to the identification of the kinetic paths of the activated folding process, and provides a direct tool and language for the theoretical and experimental community to understand the problem better. The kinetic paths giving the dominant contributions to the long-time folding activation dynamics can be quantitatively determined. These are shown to be the instanton paths. The contributions of these instanton paths to the kinetics lead to the bell-like shape folding rate dependence on temperature, which is in good agreement with folding kinetic experiments and simulations. The connections to other approaches as well as the experiments of the protein folding kinetics are discussed.

Wang, Jin; Zhang, Kun; Lu, Hongyang; Wang, Erkang

2005-01-01

310

Enhancement of the stability of resistive switching characteristics by conduction path reconstruction  

NASA Astrophysics Data System (ADS)

In this study, a Pt/BiFeO3/TiN device was fabricated and the resistance switching characteristics were investigated. After the first forming process, the conduction path was formed and exhibited unstable bipolar switching characteristics. Subsequently, the original conduction path was destroyed thoroughly by high negative bias. By reconstructing the conduction path after a second forming process (re-forming process), the device exhibits stable bipolar switching characteristics. Transmission electron microscopy analysis indicates that the stability of switching behavior was enhanced because of the joule heating effect, and is an easy way to improve the resistance switching characteristics.

Huang, Jheng-Jie; Chang, Ting-Chang; Yu, Chih-Cheng; Huang, Hui-Chun; Chen, Yu-Ting; Tseng, Hsueh-Chih; Yang, Jyun-Bao; Sze, Simon M.; Gan, Der-Shin; Chu, Ann-Kuo; Lin, Jian-Yang; Tsai, Ming-Jinn

2013-07-01

311

Semiclassical series from path integrals  

SciTech Connect

We derive the semiclassical series for the partition function in Quantum Statistical Mechanics (QSM) from its path integral representation. Each term of the series is obtained explicitly from the (real) minima of the classical action. The method yields a simple derivation of the exact result for the harmonic oscillator, and an accurate estimate of ground-state energy and specific heat for a single-well quartic anharmonic oscillator. As QSM can be regarded as finite temperature field theory at a point, we make use of the field-theoretic language of Feynman diagrams to illustrate the non-perturbative character of the series: it contains all powers of ({Dirac_h}/2{pi}) and graphs with any number of loops; the usual perturbative series corresponds to a subset of the diagrams of the semiclassical series. We comment on the application of our results to other potentials, to correlation functions and to field theories in higher dimensions.

Carvalho, C. A. A. de; Cavalcanti, R. M. [Instituto de Fisica, Universidade Federal do Rio de Janeiro, Cx. Postal 68528, CEP 21945-970, Rio de Janeiro, RJ (Brazil); Institute for Theoretical Physics, University of California, Santa Barbara, California 93106-4030 (United States)

1999-07-13

312

The path to adaptive microsystems  

NASA Astrophysics Data System (ADS)

Scaling trends in microsystems are discussed frequently in the technical community, providing a short-term perspective on the future of integrated microsystems. This paper looks beyond the leading edge of technological development, focusing on new microsystem design paradigms that move far beyond today's systems based on static components. We introduce the concept of Adaptive Microsystems and outline a path to realizing these systems-on-a-chip. The role of DARPA in advancing future components and systems research is discussed, and specific DARPA efforts enabling and producing adaptive microsystems are presented. In particular, we discuss efforts underway in the DARPA Microsystems Technology Office (MTO) including programs in novel circuit architectures (3DIC), adaptive imaging and sensing (AFPA, VISA, MONTAGE, A-to-I) and reconfigurable RF/Microwave devices (SMART, TFAST, IRFFE).

Zolper, John C.; Biercuk, Michael J.

2006-06-01

313

Slant path atmospheric extinction measurements  

NASA Astrophysics Data System (ADS)

A commercial Fourier transform spectrometer is being used to collect 0.06/cm 2-14-micron slant path atmospheric spectra using the sun as the source of radiation. Using a Lambert plot approach, 10-15 spectra are collected at different air masses. The logarithm of the intensity versus air mass is used to calculate the extinctance for the atmosphere at arbitrary frequencies within the bandwidth. Over the past three years, spectra have been collected on over 120 days at White Sands Missile Range, NM. Up to a factor of ten variation in the atmospheric extinction is observed at many CO2 laser frequencies which correlate with atmospheric water vapor measurements. Variations in the atmospheric extinction for DF frequencies are smaller and do not correlate with the water vapor measurements.

Gutman, W. M.; Hanley, S. T.; Walters, D. L.

1981-01-01

314

ANALYSIS OF CROSSING PATH CRASH COUNTERMEASURE SYSTEMS  

Microsoft Academic Search

This paper summarizes the results of an analysis of promising countermeasure systems for crossing path crashes, and thus provides a foundation for setting research priorities under the United States (U.S.) Department of Transportation's Intelligent Vehicle Initiative. Crossing path crashes involve one moving vehicle cutting across the path of another, which amounted to 1.72 million police-reported crashes in the U.S. based

Wassim G. Najm; Jonathan A. Koopmann; David L. Smith

315

Optimizing Regular Path Expressions Using Graph Schemas  

Microsoft Academic Search

Several languages, such as LOREL and UnQL, support querying of semi-structured data. Others, such as WebSQL and WebLog, query Web sites. All these languages model data as labeled graphs and use regular path expressions to express queries that traverse arbitrary paths in graphs. Naive execution of path expressions is inefficient, however, because it often requires exhaustive graph search. We describe

Mary F. Fernandez; Dan Suciu

1998-01-01

316

Efficient computation of geodesic shortest paths  

Microsoft Academic Search

This paper describes an ecient algorithm for the geodesic shortest path problem, i.e. the problemof nding shortest paths between pairs of points on the surface of a 3-dimensional polyhedron suchthat the path is constrained to lie on the surface of the polyhedron. We use the wavefront methodand show an O(nlog2n) time bound for this problem, when there are O(n) vertices

Sanjiv Kapoor; Hauz Khan

1999-01-01

317

Path planning for a mobile robot  

Microsoft Academic Search

Two problems for path planning of a mobile robot are considered. The first problem is to find a shortest-time, collision-free path for the robot in the presence of stationary obstacles in two dimensions. The second problem is to determine a collision-free path (greedy in time) for a mobile robot in an environment of moving obstacles. The environment is modeled in

Christos Alexopoulos; Paul M. Griffin

1992-01-01

318

Maximum Flux Transition Paths of Conformational Change.  

PubMed

Given two metastable states A and B of a biomolecular system, the problem is to calculate the likely paths of the transition from A to B. Such a calculation is more informative and more manageable if done for a reduced set of collective variables chosen so that paths cluster in collective variable space. The computational task becomes that of computing the "center" of such a cluster. A good way to define the center employs the concept of a committor, whose value at a point in collective variable space is the probability that a trajectory at that point will reach B before A. The committor "foliates" the transition region into a set of isocommittors. The maximum flux transition path is defined as a path that crosses each isocommittor at a point which (locally) has the highest crossing rate of distinct reactive trajectories. This path is based on the same principle as the minimum resistance path of Berkowitz et al (1983), but it has two advantages: (i) the path is invariant with respect to a change of coordinates in collective variable space and (ii) the differential equations that define the path are simpler. It is argued that such a path is nearer to an ideal path than others that have been proposed with the possible exception of the finite-temperature string method path. To make the calculation tractable, three approximations are introduced, yielding a path that is the solution of a nonsingular two-point boundary-value problem. For such a problem, one can construct a simple and robust algorithm. One such algorithm and its performance is discussed. PMID:20890401

Zhao, Ruijun; Shen, Juanfang; Skeel, Robert D

2010-08-10

319

Applications of Visible Light Path Laser Projector  

Microsoft Academic Search

We proposed and developed a method to visualize the light path of a laser with jetting mists along light axis of a laser pointer.\\u000a The estimation accuracy for the position of a laser spot occluded behind an object was improved when the light path of the\\u000a laser was visualized. In this paper, we propose visible light path laser projector (VLLP)

Nobuchika Sakata; Shu Okamoto; Shogo Nishida

2009-01-01

320

Randomized path coloring on binary trees  

Microsoft Academic Search

Motivated by the problem of WDM routing in all-optical networks, we study the following NP-hard problem. We are given a di- rected binary tree T and a set R of directed paths on T.W e wish to assign colors to paths in R, in such a way that no two paths that share ad irected arc ofT are assigned the

Vincenzo Auletta; Ioannis Caragiannis; Christos Kaklamanis; Pino Persiano

2000-01-01

321

Path instability of a rising bubble.  

PubMed

We model the problem of path instability of a rising bubble by considering the bubble as a spheroidal body of fixed shape, and we solve numerically the coupled fluid-body problem. Numerical results show that this model exhibits path instability for large enough values of the control parameters. The corresponding characteristics of the zigzag and spiral paths are in good agreement with experimental observations. Analysis of the vorticity field behind the bubble reveals that a wake instability leading to a double threaded wake is the primary cause of the path instability. PMID:11800955

Mougin, Guillaume; Magnaudet, Jacques

2001-12-19

322

Free-Electron lasers: Twisted light from an electron beam  

NASA Astrophysics Data System (ADS)

A relativistic electron beam travelling on an undulating path interacts with a laser and emits light carrying orbital angular momentum. The wavelengths of these bright twisted-light beams can go down to those of hard X-rays.

Couprie, Marie-Emmanuelle

2013-09-01

323

Path planning and path tracking control of unmanned ground vehicles (UGVs)  

Microsoft Academic Search

Unmanned ground vehicles (UGVs) will be playing increasingly important role in the future battlefields. How to automatically guide and control UGVs under varying environment conditions represents a challenging issue. This paper presents a novel approach to achieving path planning and path tracking of UGVs under dynamic environments. We apply the topology theory to find the optimal path given any starting

Liguo Weng; D. Y. Song

2005-01-01

324

Path-Goal Theory of Leadership.  

National Technical Information Service (NTIS)

The paper reviews the path-goal theory of leadership. This theory states that a leader's behavior is important for good performance as a function of its impact on subordinates' perceptions of paths to goals and the attractiveness of the goals. When leader...

R. J. House T. R. Mitchell

1975-01-01

325

Cooperative organic mine avoidance path planning  

NASA Astrophysics Data System (ADS)

The JHU/APL Path Planning team has developed path planning techniques to look for paths that balance the utility and risk associated with different routes through a minefield. Extending on previous years' efforts, we investigated real-world Naval mine avoidance requirements and developed a tactical decision aid (TDA) that satisfies those requirements. APL has developed new mine path planning techniques using graph based and genetic algorithms which quickly produce near-minimum risk paths for complicated fitness functions incorporating risk, path length, ship kinematics, and naval doctrine. The TDA user interface, a Java Swing application that obtains data via Corba interfaces to path planning databases, allows the operator to explore a fusion of historic and in situ mine field data, control the path planner, and display the planning results. To provide a context for the minefield data, the user interface also renders data from the Digital Nautical Chart database, a database created by the National Geospatial-Intelligence Agency containing charts of the world's ports and coastal regions. This TDA has been developed in conjunction with the COMID (Cooperative Organic Mine Defense) system. This paper presents a description of the algorithms, architecture, and application produced.

McCubbin, Christopher B.; Piatko, Christine D.; Peterson, Adam V.; Donnald, Creighton R.; Cohen, David

2005-06-01

326

Ray path deviation in a nonhemispherical dome  

Microsoft Academic Search

A calculation is presented for meridional rays of the ray path deviation in a conformal dome in which both the shape of the inner and outer surfaces are specified in detail so as to examine under what conditions the ray path deviation might be minimized by independently adjusting the shape of the inner surface. Specific results are given for the

Richard I. Joseph; Michael E. Thomas

2001-01-01

327

The stable paths problem and interdomain routing  

Microsoft Academic Search

Dynamic routing protocols such as RIP and OSPF essentially implement distributed algorithms for solving the shortest paths problem. The border gateway protocol (BGP) is currently the only interdomain routing protocol deployed in the Internet. BGP does not solve a shortest paths problem since any interdomain protocol is required to allow policy-based metrics to override distance-based metrics and enable autonomous systems

Timothy G. Griffin; F. Bruce Shepherd; Gordon T. Wilfong

2002-01-01

328

Adaptively Ubiquitous Learning in Campus Math Path  

ERIC Educational Resources Information Center

|The purposes of this study are to develop and evaluate the instructional model and learning system which integrate ubiquitous learning, computerized adaptive diagnostic testing system and campus math path learning. The researcher first creates a ubiquitous learning environment which is called "adaptive U-learning math path system". This system

Shih, Shu-Chuan; Kuo, Bor-Chen; Liu, Yu-Lung

2012-01-01

329

Modeling Robot Path Planning with CD  

Microsoft Academic Search

Robotic systems are usually built as independent a gents that collaborate to accomplish a specific task. Analysis of robot path planning consists of route plan- ning and path generation. We will show how to apply the Cell-DEVS formalism and the CD++ toolkit for these tasks. We present a Cell -DEVS model for route planning, which, based on the obstacles, finds

Gabriel A. Wainer

2006-01-01

330

Hausdorff Dimension of Operator Stable Sample Paths  

Microsoft Academic Search

The Hausdorff dimension of the sample paths of a stochastic process with stationary independent operator stable increments is computed. With probability one, every sample path has the same dimension, depending on the real parts of the eigenvalues of the operator stable exponent.

Peter Becker-Kern; Mark M. Meerschaert; Hans-Peter Scheffler

2003-01-01

331

Bounding the error of path loss models  

Microsoft Academic Search

In this paper we analyze the efficacy of basic path loss models at predicting median path loss in urban environments. We attempt to bound the practical error of these models and look at how they may hinder practical wireless applications, and in particular dynamic spectrum access networks. This analysis is made using a large set of measurements from production networks

Caleb Phillips; Douglas Sicker; Dirk Grunwald

2011-01-01

332

Paths in m-ary interval trees  

Microsoft Academic Search

We introduce the m-ary interval tree, a random structure that underlies interval division and simultaneous parking problems. Certain significant paths in the m-ary interval trees are considered. When appropriately normed, the length of these paths are shown to converge in distribution to a normal random variable. The work extends the study of incomplete binary interval trees in Itoh and Mahmoud

Mehri Javanian; Hosam M. Mahmoud; Mohammad Vahidi-asl

2004-01-01

333

Diversion path analysis handbook. Volume I. Methodology  

Microsoft Academic Search

Diversion Path Analysis (DPA) is a procedure for analyzing internal controls of a facility in order to identify vulnerabilities to successful diversion of material by an adversary. The internal covert threat is addressed but the results are also applicable to the external overt threat. The diversion paths are identified. Complexity parameters include records alteration or falsification, multiple removals of sub-threshold

M. D. K. Maltese; K. E. Goodwin; J. C. Schleter

1976-01-01

334

Judgments of path, not heading, guide locomotion.  

PubMed

To steer a course through the world, people are almost entirely dependent on visual information, of which a key component is optic flow. In many models of locomotion, heading is described as the fundamental control variable; however, it has also been shown that fixating points along or near one's future path could be the basis of an efficient control solution. Here, the authors aim to establish how well observers can pinpoint instantaneous heading and path, by measuring their accuracy when looking at these features while traveling along straight and curved paths. The results showed that observers could identify both heading and path accurately (approximately 3 degrees ) when traveling along straight paths, but on curved paths they were more accurate at identifying a point on their future path (approximately 5 degrees ) than indicating their instantaneous heading (approximately 13 degrees ). Furthermore, whereas participants could track changes in the tightness of their path, they were unable to accurately track the rate of change of heading. In light of these results, the authors suggest it is unlikely that heading is primarily used by the visual system to support active steering. PMID:16478328

Wilkie, Richard M; Wann, John P

2006-02-01

335

Reducing power with dynamic critical path information  

Microsoft Academic Search

Recent research has shown that dynamic information regarding instruction criticality can be used to increase microprocessor performance. Critical path information can also be used in processors to achieve a better balance of power and performance. This paper uses the output of a dynamic critical path predictor to decrease the power consumption of key portions of the processor without incurring a

John S. Seng; Eric S. Tune; Dean M. Tullsen

2001-01-01

336

Analysis of probabilistic roadmaps for path planning  

Microsoft Academic Search

We provide an analysis of a path planning method which uses probabilistic roadmaps. This method has proven very successful in practice, but the theoretical understanding of its performance is still limited. Assuming that a path ? exists between two configurations a and b of the robot, we study the dependence of the failure probability to connect a and b, on:

Lydia E. Kavraki; Mihail N. Kolountzakis; Jean-claude Latombe

1998-01-01

337

Path Planning Algorithms for Agricultural Machines  

Microsoft Academic Search

If the field plot shape is not rectangular and if it contains obstacles, the coverage path planning problem is hard to solve for a non-omnidirectional machine. Scientists have developed several algorithms to solve this coverage path planning problem, but all of them have pros and cons. If the machines were omnidirectional and turning times were decreased to insignificant, the problem

T. Oksanen; A. Visala

2007-01-01

338

Path Integrals in Lattice Quantum Chromodynamics  

NASA Astrophysics Data System (ADS)

I discuss the use of path integrals to study strong-interaction physics from first principles. The underlying theory is cast into path integrals which are evaluated numerically using Monte Carlo methods on a space-time lattice. Examples are given in progress made in nuclear physics.

Lee, F. X.

2008-11-01

339

Fatigue failure paths for offshore platform inspection  

Microsoft Academic Search

A closed- form, reliability-based procedure is developed to identify fatigue failure paths of offshore structures and assess the notional probability of system failure through these paths. The procedure utilizes the Miners rule node fatigue failure reliability model developed by Wirsching. Effects of load redistribution following the fatigue failure of a node on the time to failure of remaining unfailed nodes

Demir I. Karsan; Ashok Kumar

1990-01-01

340

The path dependence of deformation texture development  

SciTech Connect

It is demonstrated for the case of three different strain paths, all of which end up with the same, elongated specimen shape, that the texture developed during straining is path dependent. This is true both for experiments on aluminum polycrystals and for simulations using the LApp code.

Takeshita, T.; Kocks, U.F.; Wenk, H.R.

1987-01-01

341

Evaluation of Calcine Disposition Path Forward  

SciTech Connect

This document describes an evaluation of the baseline and two alternative disposition paths for the final disposition of the calcine wastes stored at the Idaho Nuclear Technology and Engineering Center at the Idaho National Engineering and Environmental Laboratory. The pathways are evaluated against a prescribed set of criteria and a recommendation is made for the path forward.

Birrer, S.A.; Heiser, M.B.

2003-02-26

342

Evaluation of Calcine Disposition - Path Forward  

SciTech Connect

This document describes an evaluation of the baseline and two alternative disposition paths for the final disposition of the calcine wastes stored at the Idaho Nuclear Technology and Engineering Center at the Idaho National Engineering and Environmental Laboratory. The pathways are evaluated against a prescribed set of criteria and a recommendation is made for the path forward.

Steve Birrer

2003-02-01

343

White Noise Path Integrals in Stochastic Neurodynamics  

NASA Astrophysics Data System (ADS)

The white noise path integral approach is used in stochastic modeling of neural activity, where the primary dynamical variables are the relative membrane potentials, while information on transmembrane ionic currents is contained in the drift coefficient. The white noise path integral allows a natural framework and can be evaluated explicitly to yield a closed form for the conditional probability density.

Carpio-Bernido, M. Victoria; Bernido, Christopher C.

2008-06-01

344

Bergman Kernel from Path Integral  

NASA Astrophysics Data System (ADS)

We rederive the expansion of the Bergman kernel on Khler manifolds developed by Tian, Yau, Zelditch, Lu and Catlin, using path integral and perturbation theory, and generalize it to supersymmetric quantum mechanics. One physics interpretation of this result is as an expansion of the projector of wave functions on the lowest Landau level, in the special case that the magnetic field is proportional to the Khler form. This is relevant for the quantum Hall effect in curved space, and for its higher dimensional generalizations. Other applications include the theory of coherent states, the study of balanced metrics, noncommutative field theory, and a conjecture on metrics in black hole backgrounds discussed in [24]. We give a short overview of these various topics. From a conceptual point of view, this expansion is noteworthy as it is a geometric expansion, somewhat similar to the DeWitt-Seeley-Gilkey et al short time expansion for the heat kernel, but in this case describing the long time limit, without depending on supersymmetry.

Douglas, Michael R.; Klevtsov, Semyon

2010-01-01

345

Dynamics of dielectric breakdown paths.  

PubMed

We investigate the dynamics and geometry of dielectric breakdown paths of needle defects of arbitrary residual resistivity in an otherwise homogeneous medium using a time-dependent electrical-circuit model. The circuit model consists of a semi-infinite lattice of capacitors in parallel with resistors that break down to a lower (residual) resistance. The breakdown occurs if the local field across a resistor exceeds a critical value for a breakdown delay time. We consider cases where the initial resistance is infinite or finite and where the residual resistance is finite or zero. We consider the model for the case where the applied field reaches the critical value adiabatically. We find that, as in the quasistatic case, the breakdown grows either one dimensionally or spreads with a fractal dimension (bifurcates) depending on the values of residual resistance and breakdown delay time. Also, we find that the propagation velocity of the needle oscillates spontaneously. We give the phase diagram for bifurcation and oscillations. We derive a simplified recursive map approximation to explain this behavior. PMID:16241371

Boksiner, Jeffrey; Leath, P L

2003-06-27

346

Path integral for inflationary perturbations  

NASA Astrophysics Data System (ADS)

The quantum theory of cosmological perturbations in single-field inflation is formulated in terms of a path integral. Starting from a canonical formulation, we show how the free propagators can be obtained from the well-known gauge-invariant quadratic action for scalar and tensor perturbations, and determine the interactions to arbitrary order. This approach does not require the explicit solution of the energy and momentum constraints, a novel feature which simplifies the determination of the interaction vertices. The constraints and the necessary imposition of gauge conditions is reflected in the appearance of various commuting and anticommuting auxiliary fields in the action. These auxiliary fields are not propagating physical degrees of freedom but need to be included in internal lines and loops in a diagrammatic expansion. To illustrate the formalism we discuss the tree-level three-point and four-point functions of the inflaton perturbations, reproducing the results already obtained by the methods used in the current literature. Loop calculations are left for future work.

Prokopec, Tomislav; Rigopoulos, Gerasimos

2010-07-01

347

Electron Tunneling Studies in Tantalum Overlayers on Niobium and in Tantalum and Niobium Nitride Films  

NASA Astrophysics Data System (ADS)

Tunneling measurements have been performed on tantalum surface layers on niobium. The thickness of the tantalum layer ranges from 10 to 100 A. The critical current, bound-state energy, phonon structure, and oxide barrier shape are investigated. The results are compared with an extended version of the Gallagher theory which accounts for both the finite mean free path in the Ta overlayers and suppression of the I_{rm c}R product due to strong electron-phonon coupling effects. Excellent fits to the data yield a value of the intrinsic scattering probability for electrons at the Ta/Nb interface of r^2 = 0.01. In addition, a new fabrication technique--dual ion-beam sputtering --is used to deposit thin films of NbN. The properties of these films and of tunnel junctions formed with NbN as base electrode and native-oxide as well as artificial barriers are reported. A universal empirical correlation is found between the average barrier height phi and the effective barrier width d for measured junctions. This correlation, which holds both for our data and for available data in the literature for oxide -barrier junctions, is discussed in the general context of oxide growth and compared with results for artificial tunnel barriers. Finally, high quality Ta/PbBi tunnel junction of area <=q1 mu m^2 and current density 10 ^3-10^5 A/cm ^2 are produced using a window geometry. The electrical noise properties of these junctions are investigated. Discrete voltage switching events allow the identification of the effect of single localized states in the oxide barrier. The voltage and temperature dependence of the switching rates are consistent with a microscopic model based on the emission and capture of individual electrons at the localized sites within the barrier.

Track, Elie Khalil

348

Using computerized tomography to determine ionospheric structures. Part 2, A method using curved paths to increase vertical resolution  

SciTech Connect

A method is presented to unfold the two-dimensional vertical structure in electron density by using data on the total electron content for a series of paths through the ionosphere. The method uses a set of orthonormal basis functions to represent the vertical structure and takes advantage of curved paths and the eikonical equation to reduce the number of iterations required for a solution. Curved paths allow a more thorough probing of the ionosphere with a given set of transmitter and receiver positions. The approach can be directly extended to more complex geometries.

Vittitoe, C.N.

1993-08-01

349

The Path of Human Evolution  

NASA Astrophysics Data System (ADS)

A complex series of evolutionary steps, contingent upon a dynamic environmental context and a long biological heritage, have led to the ascent of Homo sapiens as a dominant component of the modern biosphere. In a field where missing links still abound and new discoveries regularly overturn theoretical paradigms, our understanding of the path of human evolution has made tremendous advances in recent years. Two major trends characterize the development of the hominin clade subsequent to its origins with the advent of upright bipedalism in the Late Miocene of Africa. One is a diversification into two prominent morphological branches, each with a series of 'twigs' representing evolutionary experimentation at the species or subspecies level. The second important trend, which in its earliest manifestations cannot clearly be ascribed to one or the other branch, is the behavioral complexity of an increasing reliance on technology to expand upon limited inherent morphological specializations and to buffer the organism from its environment. This technological dependence is directly associated with the expansion of hominin range outside Africa by the genus Homo, and is accelerated in the sole extant form Homo sapiens through the last 100 Ka. There are interesting correlates between the evolutionary and behavioral patterns seen in the hominin clade and environmental dynamics of the Neogene. In particular, the tempo of morphological and behavioral innovation may be tracking major events in Neogene climatic development as well as reflecting intervals of variability or stability. Major improvements in analytical techniques, coupled with important new collections and a growing body of contextual data are now making possible the integration of global, regional and local environmental archives with an improved biological understanding of the hominin clade to address questions of coincidence and causality.

Feibel, C. S.

2004-12-01

350

Tunneling investigation of the electron scattering effect on the momentum-dependent energy gap distribution in MgB2  

NASA Astrophysics Data System (ADS)

We have studied the effect of electron scattering on the momentum-dependent energy gap distributions in MgB2 by measuring planar tunnel junctions made from epitaxial MgB2 thin films on different substrates, of different thicknesses, with different junction interface properties, and different counter-electrode materials. The phonon features in the tunneling spectra indicated that the native oxide barrier is mainly MgO with estimated barrier thickness ranging from 1.1 to 1.5 nm and the average barrier height from 1.7 to 2.6 eV. When tunneling into the ab-plane occurred in very clean films, both the ? (~1.8 meV) and ? (~7.2 meV) gaps were observed with fine structures in the conductance peaks, indicating a distribution of gap values due to anisotropic electron-phonon interaction. The ? gap was enhanced (~7.9 meV) in MgB2 thin films on SiC substrates which had Tc values over 40 K due to epitaxial tensile strain. As the MgB2 film thickness decreased from 100 nm to 33 nm, the ? gap distribution range was narrowed from ~1.7 meV to ~1.4 meV, and the fine peak structures disappeared due to the reduced electron mean free path limited by the film thickness. The fine peak structures were also gradually smeared out when the junction resistance increased. The results show that the electron scattering, either from the MgB2 film or the junction interface or barrier, can smear out the gap distribution structures in the tunneling spectra. Deterioration of the MgB2 film surface was also found to cause an increase in the ? gap value, likely due to an enhancement of interband scattering.

Dai, Wenqing; Chen, Ke; Xi, X. X.; Li, Qi

2013-02-01

351

Iterative summation of path integrals for nonequilibrium molecular quantum transport  

NASA Astrophysics Data System (ADS)

We formulate and apply a nonperturbative numerical approach to the nonequilibrium current I(V) through a voltage-biased molecular conductor. We focus on a single electronic level coupled to an unequilibrated vibration mode (Anderson-Holstein model), which can be mapped to an effective three-state problem. Performing an iterative summation of real-time path integral (ISPI) expressions, we accurately reproduce known analytical results in three different limits. We then study the crossover regime between those limits and show that the Franck-Condon blockade persists in the quantum-coherent low-temperature limit, with a nonequilibrium smearing of step features in the IV curve.

Htzen, R.; Weiss, S.; Thorwart, M.; Egger, R.

2012-03-01

352

Mach flow angularity probes for scramjet engine flow path diagnostics  

NASA Astrophysics Data System (ADS)

Mach-flow angularity (MFA) probes were developed for use in scramjet flow path probe rakes. Prototype probes were fabricated to demonstrate the assembly processes (numerical control machining, furnace brazing, and electron beam welding). Tests of prototype probes confirmed the thermal durability margins and life cycle. Selected probes were calibrated in air at Mach numbers from 1.75 to 6.0. Acceptance criteria for the production probes stressed thermal durability and pressure (and, consequently, Mach number) measurement quality. This new water-cooled MFA probe has 0.397-cm shaft diameter and is capable of withstanding heat fluxes of 2.724 kW/sq cm.

Jalbert, Paul A.; Hiers, Robert S., Jr.

353

Steering Chiral Swimmers along Noisy Helical Paths  

NASA Astrophysics Data System (ADS)

Chemotaxis along helical paths towards a target releasing a chemoattractant is found in sperm cells and many microorganisms. We discuss the stochastic differential geometry of the noisy helical swimming path of a chiral swimmer. A chiral swimmer equipped with a simple feedback system can navigate in a concentration gradient of chemoattractant. We derive an effective equation for the alignment of helical paths with a concentration gradient which is related to the alignment of a dipole in an external field and discuss the chemotaxis index.

Friedrich, Benjamin M.; Jlicher, Frank

2009-08-01

354

Quickest Paths for Different Network Router Mechanisms  

SciTech Connect

The quickest path problem deals with the transmission of a message of size {sigma} from a source to a destination with the minimum end-to-end delay over a network with bandwidth and delay constraints on the links. We consider four basic modes and two variations for the message delivery at the nodes reacting the mechanisms such as circuit switching, Internet protocol, and their combinations. For each of first three modes, we present O(m{sup 2} + mn log n) time algorithm to compute the quickest path for a given message size {sigma}. For the last mode, the quickest path can be computed in O(m + n log n) time.

Rao, NSV

2000-10-16

355

Middle path for electricity options and sustainable development.  

National Technical Information Service (NTIS)

In a landmark article in Foreign Affairs in October 1976, Amory Lovins presented his vision of two vastly different and seemingly irreconcilable paths that energy provision might take into the future. One path was a ''hard'' path, characterized by extensi...

J. I. Mills J. S. Herring

1994-01-01

356

Individual carbon nanotubes for quantum electronic and quantum photonic devices  

NASA Astrophysics Data System (ADS)

Carbon nanotubes (CNTs) are promising materials since their unique one dimensional geometry leads to remarkable physical properties such as ballistic transport, long mean free path, large direct band gaps, high mechanical tensile strength and strong exciton binding energies, which make them attractive candidates for applications in high-performance nanoelectronics and nanophotonics. CNT-based field-effect transistors (CNT-FETs) are considered to be ideally suited for future nanoelectronics. Single CNT-FETs made by depositing metal electrodes on top of individual CNTs with E-beam lithography have achieved great performance but are limited for massive large area integrated circuit fabrication. Therefore, this thesis demonstrates characteristics of CNT-FETs made by registered in-plane growth utilizing tailored nanoscale catalyst patterns and chemical vapor deposition (CVD), resulting in CNT arrays directly bridging source and drain. The demonstrated access to individual CNTs with pronounced semiconducting behavior opens also the possibility to form more advanced nanoelectronic structures such as CNT quantum dots. CNT-based single electron transistors (CNT-SETS) are promising for quantum electronic devices operating with ultra-low power consumption and allow fundamental studies of electron transport. In addition to existing CNT-SETS based on individual CNTs, we have fabricated the first CNT-SETS based on in-plane grown CNTs using the CVD technique. The demonstrated utilization of registered in-plane growth opens possibilities to create novel SET device geometries which are more complex, i.e. laterally ordered and scalable, as required for advanced quantum electronic devices. Blinking and spectral diffusion are hallmarks of nanoscale light emitters and a challenge for creating stable fluorescent biomarkers or efficient nonclassical light sources. The studies of blinking of CNTs are still in the explorative stage. In this thesis, I show the first experimental demonstration of the suppression of blinking and spectral diffusion of individual CNTs by manipulation of their dielectric environment, resulting in five fold enhanced light emission. Such results open many new device applications in CNT nanophotonics, such as efficient CNT-based single photon sources. CNT-based FETs, SETS and light emitters studied in this thesis demonstrate the great potential for CNTs as optoelectronic material in future nanoelectronic and nanophotonic device applications.

Ai, Nan

357

A chemist building paths to cell biology.  

PubMed

Galileo is reported to have stated, "Measure what is measurable and make measurable what is not so." My group's trajectory in cell biology has closely followed this philosophy, although it took some searching to find this path. PMID:24174456

Weibel, Douglas B

2013-11-01

358

UV Laser Long-Path Absorption Spectroscopy.  

National Technical Information Service (NTIS)

Long path Differential Optical Absorption Spectroscopy (DOAS) using a picosecond UV laser as a light source was developed in our institute. Tropospheric OH radicals are measured by their rotational absorption lines around 308 nm. The spectra are obtained ...

H. Dorn T. Brauers R. Neuroth

1994-01-01

359

Diversion Path Analysis Handbook. Volume I. Methodology.  

National Technical Information Service (NTIS)

Diversion Path Analysis (DPA) is a procedure for analyzing internal controls of a facility in order to identify vulnerabilities to successful diversion of material by an adversary. The internal covert threat is addressed but the results are also applicabl...

M. D. K. Maltese K. E. Goodwin J. C. Schleter

1976-01-01

360

Path integration while ignoring irrelevant movement.  

PubMed

Participants attempted to return to the origin of travel after following an outbound path by locomotion on foot (Experiments 1-3) or in a virtual visual environment (Experiment 4). Critical conditions interrupted the outbound path with verbal distraction or irrelevant, to-be-ignored movements. Irrelevant movement, real or virtual, had greater effects than verbal or cognitive distraction, indicating inability to ignore displacement during path integration. Effects of the irrelevant movement's direction (backward vs. rightward) and location (1st vs. 2nd leg of path) indicated that participants encoded a configural representation of the pathway and then cognitively compensated for the movement, producing errors directly related to the demands of compensation. An encoding-error model fit to the data indicated that backward movement produced downward rescaling, whereas movement that led to implied rotation (rightward on 2nd leg) produced distortions of shape and scale. PMID:10682296

May, M; Klatzky, R L

2000-01-01

361

Expedite Departure Path (EDP) Operational Concept.  

National Technical Information Service (NTIS)

This document describes the operational concept for the Expedite Departure Path (EDP) tool. EDP is part of the Center-TRACON Automation System (CTAS). CTAS provides computer intelligence for planning and controlling arrival and departure traffic within se...

C. W. Johnson D. R. Isaacson K. K. Lee

1999-01-01

362

The prediction of radio-path characteristics  

NASA Astrophysics Data System (ADS)

The paper examines algorithms for the long-term prediction of radio-path characteristics in the ionosphere, the main characteristic being the MUF at a given distance. The proposed approach is based on long-term memories called DATA BANKS. Attention is given to the characteritics of the various banks, including the BANK OF CITIES, the BANK OF RADIO PATHS, the REFERENCE DATA BANK, and the OUTPUT DATA BANK.

Gitina, G. M.; Kalinin, Iu. K.

363

A Path Algorithm for Constrained Estimation  

PubMed Central

Many least-square problems involve affine equality and inequality constraints. Although there are a variety of methods for solving such problems, most statisticians find constrained estimation challenging. The current article proposes a new path-following algorithm for quadratic programming that replaces hard constraints by what are called exact penalties. Similar penalties arise in l1 regularization in model selection. In the regularization setting, penalties encapsulate prior knowledge, and penalized parameter estimates represent a trade-off between the observed data and the prior knowledge. Classical penalty methods of optimization, such as the quadratic penalty method, solve a sequence of unconstrained problems that put greater and greater stress on meeting the constraints. In the limit as the penalty constant tends to ?, one recovers the constrained solution. In the exact penalty method, squared penalties!are replaced by absolute value penalties, and the solution is recovered for a finite value of the penalty constant. The exact path-following method starts at the unconstrained solution and follows the solution path as the penalty constant increases. In the process, the solution path hits, slides along, and exits from the various constraints. Path following in Lasso penalized regression, in contrast, starts with a large value of the penalty constant and works its way downward. In both settings, inspection of the entire solution path is revealing. Just as with the Lasso and generalized Lasso, it is possible to plot the effective degrees of freedom along the solution path. For a strictly convex quadratic program, the exact penalty algorithm can be framed entirely in terms of the sweep operator of regression analysis. A few well-chosen examples illustrate the mechanics and potential of path following. This article has supplementary materials available online.

Zhou, Hua; Lange, Kenneth

2013-01-01

364

Source selectable path diversity via routing deflections  

Microsoft Academic Search

We present the design of a routing system in which end-systems set tags to select non-shortest path routes as an alternative to explicit source routes. Routers collectively generate these routes by using tags as hints to independently deflect packets to neighbors t hat lie off the shortest-path. We show how this can be done simply, by local extensions of the

Xiaowei Yang; David Wetherall

2006-01-01

365

Robust real-time NURBS path interpolators  

Microsoft Academic Search

In a review of the real-time non-uniform rational B-splines (NURBS) path interpolation method in CNC controllers, it was found that none of the NURBS interpolators described in the literature has the necessary robustness against an extreme knot distribution. The problems begin with the calculation of the total length of the NURBS path: most interpolators handle knots as a global curve

W. T. Lei; S. B. Wang

2009-01-01

366

47 CFR 22.515 - Permissible communications paths.  

Code of Federal Regulations, 2012 CFR

...false Permissible communications paths. 22.515...Telecommunication FEDERAL COMMUNICATIONS COMMISSION (CONTINUED...515 Permissible communications paths. Mobile stations may communicate only with and through base stations....

2012-10-01

367

47 CFR 22.515 - Permissible communications paths.  

Code of Federal Regulations, 2011 CFR

...false Permissible communications paths. 22.515...Telecommunication FEDERAL COMMUNICATIONS COMMISSION (CONTINUED...515 Permissible communications paths. Mobile stations may communicate only with and through base stations....

2011-10-01

368

Quantum cosmology based on discrete Feynman paths  

SciTech Connect

Although the rules for interpreting local quantum theory imply discretization of process, Lorentz covariance is usually regarded as precluding time quantization. Nevertheless a time-discretized quantum representation of redshifting spatially-homogeneous universe may be based on discrete-step Feynman paths carrying causal Lorentz-invariant action--paths that not only propagate the wave function but provide a phenomenologically-promising elementary-particle Hilbert-space basis. In a model under development, local path steps are at Planck scale while, at a much larger ''wave-function scale'', global steps separate successive wave-functions. Wave-function spacetime is but a tiny fraction of path spacetime. Electromagnetic and gravitational actions are ''at a distance'' in Wheeler-Feynman sense while strong (color) and weak (isospin) actions, as well as action of particle motion, are ''local'' in a sense paralleling the action of local field theory. ''Nonmaterial'' path segments and ''trivial events'' collaborate to define energy and gravity. Photons coupled to conserved electric charge enjoy privileged model status among elementary fermions and vector bosons. Although real path parameters provide no immediate meaning for ''measurement'', the phase of the complex wave function allows significance for ''information'' accumulated through ''gentle'' electromagnetic events involving charged matter and ''soft'' photons. Through its soft-photon content the wave function is an ''information reservoir''.

Chew, Geoffrey F.

2002-10-10

369

Equivalence of trans paths in ion channels  

NASA Astrophysics Data System (ADS)

We explore stochastic models for the study of ion transport in biological cells. Analysis of these models explains and explores an interesting feature of ion transport observed by biophysicists. Namely, the average time it takes ions to cross certain ion channels is the same in either direction, even if there is an electric potential difference across the channels. It is shown for simple single ion models that the distribution of a path (i.e., the history of location versus time) of an ion crossing the channel in one direction has the same distribution as the time-reversed path of an ion crossing the channel in the reverse direction. Therefore, not only is the mean duration of these paths equal, but other measures, such as the variance of passage time or the mean time a path spends within a specified section of the channel, are also the same for both directions of traversal. The feature is also explored for channels with interacting ions. If a system of interacting ions is in reversible equilibrium (net flux is zero), then the equivalence of the left-to-right trans paths with the time-reversed right-to-left trans paths still holds. However, if the system is in equilibrium, but not reversible equilibrium, then such equivalence need not hold.

Alvarez, Juan; Hajek, Bruce

2006-04-01

370

Electronics 2010: A New Systems Approach to Teaching Electronics  

NSDL National Science Digital Library

These are the slides for the MATEC NetWorks Webinar that was held on May 9, 2008 for Electronics 2010: A New Systems Approach to Teaching Electronics. Presented by Tom McGlew, a thirty year veteran to the electronics field, the presentation discusses new paths for electronics and the changing role of the technician, and the implications for curriculum. The goals of the eSyst project are also discussed, the needed curriculum changes and updates, and ideas for assessment.

2008-11-26

371

A Direct Path Interference Cancellation Approach to Passive Radar Based on FM Radio transmitter  

Microsoft Academic Search

Target detection by non-cooperative illuminator is a research hotspot in electronic warfare field, with four countering' potential advantages. One of the major problems in bistatic radars with continuous waves is the direct path interference (DPI). The conventional solution to this problem is to use an adaptive antenna by steering null towards the interference. Unfortunately the null depth obtained by this

Zhu Jiabing; Tao Liang; Hong Yi

2006-01-01

372

Simple signal path technique for matching circuit in packaged semiconductor diode laser  

NASA Astrophysics Data System (ADS)

Improvement in frequency bandwidth of a packaged diode laser is investigated by the circuit impedance matching. To overcome the structural limitation of the electronic package for high frequencies, employing conventional lumped element impedance matching technique in conjunction with the manipulation of signal path of the package connections showed significant enhancement for 3-dB bandwidth to more than 6 GHz.

Han, J. H.

2008-06-01

373

Minimal entropy probability paths between genome families.  

PubMed

We develop a metric for probability distributions with applications to biological sequence analysis. Our distance metric is obtained by minimizing a functional defined on the class of paths over probability measures on N categories. The underlying mathematical theory is connected to a constrained problem in the calculus of variations. The solution presented is a numerical solution, which approximates the true solution in a set of cases called rich paths where none of the components of the path is zero. The functional to be minimized is motivated by entropy considerations, reflecting the idea that nature might efficiently carry out mutations of genome sequences in such a way that the increase in entropy involved in transformation is as small as possible. We characterize sequences by frequency profiles or probability vectors, in the case of DNA where N is 4 and the components of the probability vector are the frequency of occurrence of each of the bases A, C, G and T. Given two probability vectors a and b, we define a distance function based as the infimum of path integrals of the entropy function H( p) over all admissible paths p(t), 0 < or = t< or =1, with p(t) a probability vector such that p(0)=a and p(1)=b. If the probability paths p(t) are parameterized as y(s) in terms of arc length s and the optimal path is smooth with arc length L, then smooth and "rich" optimal probability paths may be numerically estimated by a hybrid method of iterating Newton's method on solutions of a two point boundary value problem, with unknown distance L between the abscissas, for the Euler-Lagrange equations resulting from a multiplier rule for the constrained optimization problem together with linear regression to improve the arc length estimate L. Matlab code for these numerical methods is provided which works only for "rich" optimal probability vectors. These methods motivate a definition of an elementary distance function which is easier and faster to calculate, works on non-rich vectors, does not involve variational theory and does not involve differential equations, but is a better approximation of the minimal entropy path distance than the distance //b-a//(2). We compute minimal entropy distance matrices for examples of DNA myostatin genes and amino-acid sequences across several species. Output tree dendograms for our minimal entropy metric are compared with dendograms based on BLAST and BLAST identity scores. PMID:15133624

Ahlbrandt, Calvin; Benson, Gary; Casey, William

2003-12-02

374

Perfect discretization of reparametrization invariant path integrals  

NASA Astrophysics Data System (ADS)

To obtain a well-defined path integral one often employs discretizations. In the case of gravity and reparametrization-invariant systems, the latter of which we consider here as a toy example, discretizations generically break diffeomorphism and reparametrization symmetry, respectively. This has severe implications, as these symmetries determine the dynamics of the corresponding system. Indeed we will show that a discretized path integral with reparametrization-invariance is necessarily also discretization independent and therefore uniquely determined by the corresponding continuum quantum mechanical propagator. We use this insight to develop an iterative method for constructing such a discretized path integral, akin to a Wilsonian RG flow. This allows us to address the problem of discretization ambiguities and of an anomaly-free path integral measure for such systems. The latter is needed to obtain a path integral, that can act as a projector onto the physical states, satisfying the quantum constraints. We will comment on implications for discrete quantum gravity models, such as spin foams.

Bahr, Benjamin; Dittrich, Bianca; Steinhaus, Sebastian

2011-05-01

375

Oscillatory fracture paths in thin elastic sheets  

NASA Astrophysics Data System (ADS)

We report results on a novel mode of quasi-static oscillatory crack propagation in thin elastic sheets [1]. A cutting tip of width w is perpendicularly driven through a thin brittle polymer film, held along its lateral boundaries, that progressively cuts the material as it advances [2]. For thin enough cutting tips (w < wc with w_c=0.2mm) the fracture path is straight. However, for w > w_c, the crack follows a well defined and highly reproducible oscillatory path. The amplitude and wavelength of the oscillatory crack paths scale linearly with the width of the cutting tip over a wide range of length scales but are independent of both of the cutting speed and the width of the sheet. We develop a model for this phenomena, based on the coupling between crack propagation and out-of-plane deformations of the film, which exhibits behaviour in remarkably good agreement with the experimental crack paths. [1] B. Roman, P.M. Reis, B. Audoly , S. DeVilliers, V. Vigui, D. Vallet, Oscillatory fracture paths in thin elastic sheets C.R. de Mecanique, vol 331, no 12, p. 811--816 (2003). [2] http://www.lmm.jussieu.fr/platefracture/

Reis, Pedro Miguel; Audoly, Basile; Roman, Benoit

2004-03-01

376

Unbiased sampling of lattice Hamilton path ensembles.  

PubMed

Hamilton paths, or Hamiltonian paths, are walks on a lattice which visit each site exactly once. They have been proposed as models of globular proteins and of compact polymers. A previously published algorithm [Mansfield, Macromolecules 27, 5924 (1994)] for sampling Hamilton paths on simple square and simple cubic lattices is tested for bias and for efficiency. Because the algorithm is a Metropolis Monte Carlo technique obviously satisfying detailed balance, we need only demonstrate ergodicity to ensure unbiased sampling. Two different tests for ergodicity (exact enumeration on small lattices, nonexhaustive enumeration on larger lattices) demonstrate ergodicity unequivocally for small lattices and provide strong support for ergodicity on larger lattices. Two other sampling algorithms [Ramakrishnan et al., J. Chem. Phys. 103, 7592 (1995); Lua et al., Polymer 45, 717 (2004)] are both known to produce biases on both 2x2x2 and 3x3x3 lattices, but it is shown here that the current algorithm gives unbiased sampling on these same lattices. Successive Hamilton paths are strongly correlated, so that many iterations are required between statistically independent samples. Rules for estimating the number of iterations needed to dissipate these correlations are given. However, the iteration time is so fast that the efficiency is still very good except on extremely large lattices. For example, even on lattices of total size 10x10x10 we are able to generate tens of thousands of uncorrelated Hamilton paths per hour of CPU time. PMID:17059235

Mansfield, Marc L

2006-10-21

377

Unbiased sampling of lattice Hamilton path ensembles  

NASA Astrophysics Data System (ADS)

Hamilton paths, or Hamiltonian paths, are walks on a lattice which visit each site exactly once. They have been proposed as models of globular proteins and of compact polymers. A previously published algorithm [Mansfield, Macromolecules 27, 5924 (1994)] for sampling Hamilton paths on simple square and simple cubic lattices is tested for bias and for efficiency. Because the algorithm is a Metropolis Monte Carlo technique obviously satisfying detailed balance, we need only demonstrate ergodicity to ensure unbiased sampling. Two different tests for ergodicity (exact enumeration on small lattices, nonexhaustive enumeration on larger lattices) demonstrate ergodicity unequivocally for small lattices and provide strong support for ergodicity on larger lattices. Two other sampling algorithms [Ramakrishnan et al., J. Chem. Phys. 103, 7592 (1995); Lua et al., Polymer 45, 717 (2004)] are both known to produce biases on both 222 and 333 lattices, but it is shown here that the current algorithm gives unbiased sampling on these same lattices. Successive Hamilton paths are strongly correlated, so that many iterations are required between statistically independent samples. Rules for estimating the number of iterations needed to dissipate these correlations are given. However, the iteration time is so fast that the efficiency is still very good except on extremely large lattices. For example, even on lattices of total size 101010 we are able to generate tens of thousands of uncorrelated Hamilton paths per hour of CPU time.

Mansfield, Marc L.

2006-10-01

378

Path analysis in genetic epidemiology: a critique.  

PubMed Central

Path analysis, a form of general linear structural equation models, is used in studies of human genetics data to discern genetic, environmental, and cultural factors contributing to familial resemblance. It postulates a set of linear and additive parametric relationships between phenotypes and genetic and cultural variables and then essentially uses the assumption of multivariate normality to estimate and perform tests of hypothesis on parameters. Such an approach has been advocated for the analysis of genetic epidemiological data by D. C. Rao, N. Morton, C. R. Cloninger, L. J. Eaves, and W. E. Nance, among others. This paper reviews and evaluates the formulations, assumptions, methodological procedures, interpretations, and applications of path analysis. To give perspective, we begin with a discussion of path analysis as it occurs in the form of general linear causal models in several disciplines of the social sciences. Several specific path analysis models applied to lipoprotein concentrations, IQ, and twin data are then reviewed to keep the presentation self-contained. The bulk of the critical discussion that follows is directed toward the following four facets of path analysis: (1) coherence of model specification and applicability to data; (2) plausibility of modeling assumptions; (3) interpretability and utility of the model; and (4) validity of statistical and computational procedures. In the concluding section, a brief discussion of the problem of appropriate model selection is presented, followed by a number of suggestions of essentially model-free alternative methods of use in the treatment of complex structured data such as occurs in genetic epidemiology.

Karlin, S; Cameron, E C; Chakraborty, R

1983-01-01

379

Orientation Specificity in Spatial Memory: What Makes a Path Different From a Map of the Path?  

Microsoft Academic Search

Three studies investigated the factors that lead spatial information to be stored in an orientation-specific versus orientation-free manner. In Experiment 1, we replicated the findings of Presson and Hazelrigg (1984) that learning paths from a small map versus learning the paths directly from viewing a world leads to different functional characteristics of spatial memory. Whether the route display was presented

Clark C. Presson; Nina DeLange; Mark D. Hazelrigg

1989-01-01

380

Adaptive path planning algorithm for cooperating unmanned air vehicles  

SciTech Connect

An adaptive path planning algorithm is presented for cooperating Unmanned Air Vehicles (UAVs) that are used to deploy and operate land-based sensor networks. The algorithm employs a global cost function to generate paths for the UAVs, and adapts the paths to exceptions that might occur. Examples are provided of the paths and adaptation.

Cunningham, C T; Roberts, R S

2001-02-08

381

k-Link Shortest Paths in Weighted Subdivisions  

Microsoft Academic Search

We study the shortest path problem in weighted polygonal subdivisions of the plane, with the additional constraint of an upper bound, k, on the number of links (segments) in the path. We prove structural properties of optimal paths and utilize these results to ob- tain approximation algorithms that yield a path having O(k) links and weighted length at most (1

Ovidiu Daescu; Joseph S. B. Mitchell; Simeon C. Ntafos; James D. Palmer; Chee-keng Yap

2005-01-01

382

A Genetic Robot Path Planner with Fuzzy Logic Adaptation  

Microsoft Academic Search

The paper develops a combined genetic algorithm and fuzzy logic approach to path planning for a mobile robot operating in rough environments. Path planning consists of a description of the environment using a fuzzy logic framework, and a two-stage planner. A global planner determines the path that optimizes a combination of terrain roughness and path curvature. A local planner uses

Mahmoud Tarokh

2007-01-01

383

Off-axis paths in spherical mirror interferometers  

Microsoft Academic Search

vO = 24r, and g being integers, the rays retrace their paths. These ray paths give rise to additional resonances which were observed. Pictures of the points of reflection are reproduced. The theory is in good agreement with the experi- mental observations. In laser amplifiers these ray paths enable one to obtain long effective path lengths in the active medium

D. Herriott; H. Kogelnik; R. Kompfner

1964-01-01

384

Fermionic path integrals and local anomalies  

NASA Astrophysics Data System (ADS)

No doubt, the subject of path integrals proved to be an immensely fruitful human, i.e. Feynman's idea. No wonder it is more timely than ever. Some even claim that it is the most daring, innovative and revolutionary idea since the days of Heisenberg and Bohr. It is thus likely to generate enthusiasm, if not addiction among physicists who seek simplicity together with perfection. Professor Devreese's long-lasting interest in, if not passion on the subject stems from his firm conviction that, beyond being the tool of choice, path integration provides the key to all quantum phenomena, be it in solid state, atomic, molecular or particle physics as evidenced by the impressive list of publications at the address http://lib.ua.ac.be/AB/a867.html. In this note, I review a pitfall of fermionic path integrals and a way to get around it in situations relevant to the Standard Model of particle physics.

Roepstorff, G.

2003-05-01

385

Molecular path control in zeolite membranes  

PubMed Central

We report molecular simulations of diffusion in confinement showing a phenomenon that we denote as molecular path control (MPC); depending on loading, molecules follow a preferred pathway. MPC raises the important question to which extent the loading may affect the molecular trajectories in nanoporous materials. Through MPC one is able to manually adjust the ratio of the diffusivities through different types of pores, and as an application one can direct the flow of diffusing particles in membranes forward or sideward by simply adjusting the pressure, without the need for mechanical parts like valves. We show that the key ingredient of MPC is the anisotropic nature of the nanoporous material that results in a complex interplay between different diffusion paths as a function of loading. These paths may be controlled by changing the loading, either through a change in pressure or temperature.

Dubbeldam, D.; Beerdsen, E.; Calero, S.; Smit, B.

2005-01-01

386

Quickest Paths for Different Network Router Mechanisms  

SciTech Connect

The quickest path problem deals with the transmission of a message of size {sigma} from a source to a destination with the minimum end-to-end delay over a network with bandwidth and delay constraints on the links. The authors consider four basic modes and two variations for the message delivery at the nodes reflecting the mechanisms such as circuit switching, Internet protocol, and their combinations. For each of the first three modes, they present O(m{sup 2} + mn log n) time algorithm to compute the quickest path for a given message size {sigma}. For the last mode, the quickest path can be computed in O(m + n log n) time.

Rao, N.S.V.; Grimmell, W.C.; Radhakrishnan, S.; Bang, Y.C.

2000-06-01

387

Quantum state of wormholes and path integral  

SciTech Connect

The quantum state of a wormhole can be represented by a path integral over all asymptotically Euclidean four-geometries and all matter fields which have prescribed values, the arguments of the wave function, on a three-surface {ital S} which divides the spacetime manifold into two disconnected parts. The ground-state wave function is picked out by requiring that there be no matter excitations in the asymptotic region. Once the path integrals over the lapse and shift functions are evaluated, the requirement that the spacetime be asymptotically Euclidean can be accomplished by fixing the asymptotic gravitational momentum in the remaining path integral. It is claimed that no wave function exists which corresponds to asymptotic field configurations such that the effective gravitational constant is negative in the asymptotic region. The wormhole wave functions are worked out in minisuperspace models with massless minimal and conformal scalar fields.

Garay, L.J. (Instituto de Optica, Consejo Superior de Investigaciones Cientificas, Serrano 121, E-28006 Madrid (Spain))

1991-08-15

388

Transition-path sampling of -hairpin folding  

NASA Astrophysics Data System (ADS)

We examine the dynamical folding pathways of the C-terminal -hairpin of protein G-B1 in explicit solvent at room temperature by means of a transition-path sampling algorithm. In agreement with previous free-energy calculations, the resulting path ensembles reveal a folding mechanism in which the hydrophobic residues collapse first followed by backbone hydrogen-bond formation, starting with the hydrogen bonds inside the hydrophobic core. In addition, the path ensembles contain information on the folding kinetics, including solvent motion. Using the recently developed transition interface sampling technique, we calculate the rate constant for unfolding of the protein fragment and find it to be in reasonable agreement with experiments. The results support the validation of using all-atom force fields to study protein folding.

Bolhuis, Peter G.

2003-10-01

389

Information Flow Analysis via Path Condition Refinement  

NASA Astrophysics Data System (ADS)

We present a new approach to information flow control (IFC), which exploits counterexample-guided abstraction refinement (CEGAR) technology. The CEGAR process is built on top of our existing IFC analysis in which illegal flows are characterized using program dependence graphs (PDG) and path conditions (as described in [12]). Although path conditions provide an already precise abstraction that can be used to generate witnesses to the illegal flow, they may still cause false alarms. Our CEGAR process recognizes false witnesses by executing them and monitoring their executions, and eliminates them by automatically refining path conditions in an iterative way as needed. The paper sketches the foundations of CEGAR and PDG-based IFC, and describes the approach in detail. An example shows how the approach finds illegal flow, and demonstrates how CEGAR eliminates false alarms.

Taghdiri, Mana; Snelting, Gregor; Sinz, Carsten

390

On the spatial relationship between lightning discharges and propagation paths of perturbed subionospheric VLF/LF signals  

SciTech Connect

A study has been made of the spatial relationship between propagation paths of subionospheric VLF/LF signals exhibiting sudden amplitude perturbations (Trimpi events) and time correlated cloud-to-ground lightning flashes. On each of the 4 days examined the storm centers were located close to the signal path from the NAU transmitter (28.5-kHz) in Puerto Rico to Stanford (SU) and were at large distances from the propagation path of the 48.5-kHz transmitter signal from Nebraska to SU. Nevertheless, no Trimpi events were observed on the former path, while many were seen on the latter. Furthermore, the detected Trimpi perturbations of the 48.5-kHz signal received at Stanford were found to be associated with the lightning activity in the distant storm centers. Since the NAU-SU path lies entirely at L < 2 and the 48.5-SU path is located mostly at 2< L <3, the L dependent magnetospheric conditions which determine the level of lightning-induced electron precipitation are different along the two paths. Thus, the authors postulate that the observed difference in Trimpi occurence on the two paths was due to the different magnetospheric conditions. Hence the occurence of Trimpi events over the geographical region corresponding to L <3 may be more dominantly controlled by magnetospheric conditions than the source lightning distribution.

Wai-Yeung Yip; Inan, U.S. (Stanford Univ., CA (USA)); Orville, R.E. (State Univ. of New York, Albany (USA))

1991-01-01

391

Path planning for everday robotics with SANDROS  

SciTech Connect

We discuss the integration of the SANDROS path planner into a general robot simulation and control package with the inclusion of a fast geometry engine for distance calculations. This creates a single system that allows the path to be computed, simulated, and then executed on the physical robot. The architecture and usage procedures are presented. Also, we present examples of its usage in typical environments found in our organization. The resulting system is as easy to use as the general simulation system (which is in common use here) and is fast enough (example problems are solved in seconds) to be used interactively on an everyday basis.

Watterberg, P.; Xavier, P. [Sandia National Labs., Albuquerque, NM (United States); Hwang, Y. [Korea Inst. of Science and Technology, Seoul (Korea, Republic of)

1997-02-01

392

Practical path planning among movable obstacles  

SciTech Connect

Path planning among movable obstacles is a practical problem that is in need of a solution. In this paper an efficient heuristic algorithm that uses a generate-and-test paradigm: a good'' candidate path is hypothesized by a global planner and subsequently verified by a local planner. In the process of formalizing the problem, we also present a technique for modeling object interactions through contact. Our algorithm has been tested on a variety of examples, and was able to generate solutions within 10 seconds. 5 figs., 27 refs.

Chen, Pang C.; Hwang, Yong K.

1990-09-05

393

The (1+1) Dimensional Dirac Equation With Pseudoscalar Potentials: Path Integral Treatment  

NASA Astrophysics Data System (ADS)

The supersymetric path integrals in solving the problem of relativistic spinning particle interacting with pseudoscalar potentials is examined. The relative propagator is presented by means of path integral, where the spin degrees of freedom are described by odd Grassmannian variables and the gauge invariant part of the effective action has a form similar to the standard pseudoclassical action given by Berezin and Marinov. After integrating over fermionic variables (Grassmannian variables), the problem is reduced to a nonrelativistic one with an effective supersymetric potential. Some explicit examples are considered, where we have extracted the energy spectrum of the electron and the wave functions.

Haouat, S.; Chetouani, L.

2007-06-01

394

GHz surface acoustic waves as a probe of composite fermions in two-dimensional electron systems  

Microsoft Academic Search

By probing the low-disorder two-dimensional electron system at submicron dimensions, important new length scales are revealed. Using surface acoustic waves of wavelength lambda <~ 1 mum to measure the 2D electron conductivity, enhanced conductivity is observed at even-denominator filling factors v = 1\\/(2), 1\\/(4): this represents measurement of composite fermions' conductivity at a dimension smaller than the quasiparticles' mean free

R. L. Willett; L. N. Pfeiffer

1996-01-01

395

An exploratory look at supermarket shopping paths  

Microsoft Academic Search

We present analyses of an extraordinary new dataset that reveals the path taken by individual shoppers in an actual grocery store, as provided by RFID (radio frequency identification) tags located on their shopping carts. The analysis is performed using a multivariate clustering algorithm not yet seen in the marketing literature that is able to handle data sets with unique (and

Jeffrey S. Larson; Eric T. Bradlow; Peter S. Fader

2005-01-01

396

A Random Sampling Scheme for Path Planning  

Microsoft Academic Search

Several randomized path planners have been proposed dur ing the last few years. Their attractiveness stems from their applicability to virtually any type of robots, and their empir ically observed success. In this article, we attempt to present a unifying view of these planners and to theoretically explain their success. First, we introduce a general planning scheme that consists of

Jrme Barraquand; Lydia E. Kavraki; Jean-claude Latombe; Tsai-yen Li; Rajeev Motwani; Prabhakar Raghavan

1997-01-01

397

Graph Minors .XIII. The Disjoint Paths Problem  

Microsoft Academic Search

We describe an algorithm, which for fixed k ? 0 has running time O(|V(G)|3), to solve the following problem: given a graph G and k pairs of vertices of G, decide if there are k mutually vertex-disjoint paths of G joining the pairs.

Neil Robertson; Paul D. Seymour

1995-01-01

398

DPDAT: data path direct access testing  

Microsoft Academic Search

Data Path Direct Access Test, DPDAT, supports efficient structural test of targeted datapath blocks using existing non-datapath DFT features in conjunction with architectural transparency already present in surrounding datapath blocks. This new DFT technique allows ATPG patterns generated at logic block levels to be applied at the full chip without expensive DFT overhead. The results of investigating feasibility on Intel(R)

Kee Sup Kim; Rathish Jayabharathi; Craig Carstens; Praveen Vishakantaiah; Derek Feltham; Adrian Carbine

2001-01-01

399

Applications of Path Compression on Balanced Trees  

Microsoft Academic Search

Several fast algorithms are presented for computing functions defined on paths in trees under various assumpuons. The algorithms are based on tree mampulatton methods first used to efficiently represent equivalence relations. The algorithms have O((m + n)a(m + n, n)) running tunes, where m and n are measures of the problem size and a Is a functional reverse of Ackermann's

Robert Endre Tarjan

1979-01-01

400

SILK: Scout Paths in the Linux Kernel  

Microsoft Academic Search

SILK stands for Scout In the Linux Kernel, and is a port of the Scout operating system to run as a Linux kernel mod- ule. SILK forms a replacement networking subsystem for standard Linux 2.4 kernels. Linux applications create and use Scout paths via the Linux socket interface with virtually no modifications to the applications themselves. SILK pro- vides Linux

Andy Bavier; Thiemo Voigt; Mike Wawrzoniak; Larry Peterson; Per Gunningberg

2001-01-01

401

Path instability of a rising bubble  

NASA Astrophysics Data System (ADS)

Path instability of a rising bubble is studied by carrying out DNS around a freely moving ellipsoidal bubble with a prescribed aspect ratio. The flow field is obtained by solving the full Navier-Stokes equations and the velocity and rotation rate of the bubble are found by solving the Kirchhoff equations in a form suitable to vortical flows. The problem has two control parameters, ie the aspect ratio K and the Galileo number Ga. Beyond a certain critical value K(Ga) or Ga(K), the path of the bubble first bifurcates towards a plane zigzag whose geometrical characteristics are in good agreement with experimental observations. Detailed examination reveals that the transition to the zigzag path occurs through a supercritical Hopf bifurcation when K is set fixed, while it occurs through a more complex process when Ga is set fixed. Ultimately, the zigzag degenerates into a fully helical trajectory, as observed in experiments. Examining the vorticity field reveals a one-to-one correspondance between the topology of the wake and the nature of the path. The competition between creation of vorticity at the bubble surface and evacuation in the wake allows us to propose a consistent scenario for explaining physically the observed transitions.

Magnaudet, Jacques; Mougin, Guillaume

2001-11-01

402

Dynamic path-based software watermarking  

Microsoft Academic Search

Software watermarking is a tool used to combat software piracy by embedding identifying information into a program. Most existing proposals for software watermarking have the shortcoming that the mark can be destroyed via fairly straightforward semantics-preserving code transformations. This paper introduces path-based watermarking, a new approach to software watermarking based on the dynamic branching behavior of programs. The advantage of

C. Collberg; E. Carter; S. Debray; A. Huntwork; J. Kececioglu; C. Linn; M. Stepp

2004-01-01

403

Modeling DNA Dynamics by Path Integrals  

NASA Astrophysics Data System (ADS)

Complementary strands in DNA double helix show temporary fluctuational openings which are essential to biological functions such as transcription and replication of the genetic information. Such large amplitude fluctuations, known as the breathing of DNA, are generally localized and, microscopically, are due to the breaking of the hydrogen bonds linking the base pairs (bps). I apply imaginary time path integral techniques to a mesoscopic Hamiltonian which accounts for the helicoidal geometry of a short circular DNA molecule. The bps displacements with respect to the ground state are interpreted as time dependent paths whose amplitudes are consistent with the model potential for the hydrogen bonds. The portion of the paths configuration space contributing to the partition function is determined by selecting the ensemble of paths which fulfill the second law of thermodynamics. Computations of the thermodynamics in the denaturation range show the energetic advantage for the equilibrium helicoidal geometry peculiar of B-DNA. I discuss the interplay between twisting of the double helix and anharmonic stacking along the molecule backbone suggesting an interesting relation between intrinsic nonlinear character of the microscopic interactions and molecular topology.

Zoli, Marco

2013-02-01

404

Issues affecting specialty career paths in dermatology.  

PubMed

As the field of medicine changes, physicians deal with ever-increasing challenges and pressures. An overview of broad career paths within the specialty of dermatology is presented and important issues affecting these subspecialty tracks are discussed. These issues include increasing regulatory controls, the medical liability crisis, competitive forces, corporate and global outsourcing threats, managed care, and reimbursement. PMID:18189057

Leonard, Aimee L; Hanke, C William; Bechtel, Mark A; Brownell, Isaac; Buckel, Larry J; Hurwitz, Robert M; Hsiung, Sherry H; Paller, Amy S; Turchan, Kirsten

2007-12-01

405

Folded-path optical analysis gas cell  

DOEpatents

A folded-path gas cell employs an elliptical concave mirror in confronting relationship to two substantially spherical concave mirrors. At least one of the spherical mirrors, and usually both, are formed with an added cylindrical component to increase orthogonal focii coincidence and thereby to increase the radiation energy throughput characteristic of the cell.

Carangelo, Robert M. (Glastonbury, CT); Wright, David D. (Vershire, VT)

1995-01-01

406

Judgments of Path, Not Heading, Guide Locomotion  

Microsoft Academic Search

To steer a course through the world, people are almost entirely dependent on visual information, of which a key component is optic flow. In many models of locomotion, heading is described as the fundamental control variable; however, it has also been shown that fixating points along or near one's future path could be the basis of an efficient control solution.

Richard M. Wilkie; John P. Wann

2006-01-01

407

VIPER: an efficient vigorously sensitizable path extractor  

Microsoft Academic Search

Fast and correct timing verification is a critical issue in VLSI design. Several tim-ing verification algorithms have been proposed in the last few years. However, due to the huge computation time needed to eliminate false paths, existing algorithms have difficulty in performing timing verification for large circuits. This paper presents an efficient timing verification algorithm, with a new sensitization criterion,

Hoon Chang; Jacob A. Abraham

1993-01-01

408

LONG PATH LASER OZONE MONITOR EVALUATION  

EPA Science Inventory

The purpose of the study reported here was to evaluate a long path laser air pollution monitor developed for the U.S. Environmental Protection Agency (EPA) by the General Electric (GE) Company. The monitor was known as ILAMS (Infrared Laser Atmospheric Monitoring System) and desi...

409

A Modal Perspective on Path Constraints  

Microsoft Academic Search

We analyze several classes of path constraints for semistructured data and prove a umber of decidability and complexity results for such constraints. While some of these decidability results were known before, we believe that our improved complexity bounds are new. Our proofs are based on techniques from modal logic and automata theory. We believe that our modal logic perspective sheds

Natasha Alechina; Stphane Demri; Maarten De Rijke

2003-01-01

410

Faster algorithms for the shortest path problem  

Microsoft Academic Search

Efficient implementations of Dijkstra's shortest path algorithm are investigated. A new data structure, called the radix heap, is proposed for use in this algorithm. On a network with n vertices, m edges, and nonnegative integer arc costs bounded by C, a one-level form of radix heap gives a time bound for Dijkstra's algorithm of O(m + n log C). A

Ravindra K. Ahuja; Kurt Mehlhorn; James B. Orlin; Robert Endre Tarjan

1990-01-01

411

Animation: Path of 2010 Solar Eclipse  

NASA Video Gallery

On Sunday, 2010 July 11, a total eclipse of the Sun is visible from within a narrow corridor that traverses Earth's southern hemisphere. The path of the Moon's umbral shadow crosses the South Pacific Ocean where it makes no landfall except for Mangaia (Cook Islands) and Easter Island (Isla de Pascua).

Jim Wilson

2010-07-09

412

Parallel Path Planning with Multiple Evasion Strategies  

Microsoft Academic Search

Probabilistic path planning driven by a potential eld is a well established technique and has been successfully ex- ploited to solve complex problems arising in a variety of domains. However, planners implementing this approach are rather inecient in dealing with certain types of lo- cal minima occurring in the potential eld, especially those characterized by deep or large attraction basins.

Stefano Caselli; Monica Reggiani; Roberto Sbravati

2002-01-01

413

Scanning path optimization for ultrasound surgery  

Microsoft Academic Search

One of the problems in ultrasound surgery is the long treatment times when large tumour volumes are sonicated. Large tumours are usually treated by scanning the tumour volume using a sequence of individual focus points. During the scanning, it is possible that surrounding healthy tissue suffers from undesired temperature rise. The selection of the scanning path so that the tumour

Matti Malinenyx; Tomi Huttunen; Jari P. Kaipio; Kullervo Hynynen

2005-01-01

414

14 CFR 25.111 - Takeoff path.  

Code of Federal Regulations, 2010 CFR

...500 feet above the takeoff surface, or at which the transition from the takeoff to the en route configuration is completed and VFTO is reached, whichever point is higher. In addition (1) The takeoff path must be based on the procedures prescribed...

2010-01-01

415

14 CFR 25.111 - Takeoff path.  

Code of Federal Regulations, 2010 CFR

...500 feet above the takeoff surface, or at which the transition from the takeoff to the en route configuration is completed and VFTO is reached, whichever point is higher. In addition (1) The takeoff path must be based on the procedures prescribed...

2009-01-01

416

Judgments of Path, Not Heading, Guide Locomotion  

ERIC Educational Resources Information Center

To steer a course through the world, people are almost entirely dependent on visual information, of which a key component is optic flow. In many models of locomotion, heading is described as the fundamental control variable; however, it has also been shown that fixating points along or near one's future path could be the basis of an efficient

Wilkie, Richard M.; Wann, John P.

2006-01-01

417

The Critical Path Program in Taiwan  

Microsoft Academic Search

Taiwan's Critical Path Program facilitates the development process for niche products of new drug and medical devices. The program fosters a favorable regulatory environment by establishing guidance systems and identifying selected index cases for successful stories through achievement-based proactive consultation. A total of 27 index cases were selected from a total of 68 cases submitted. Two index cases have been

Bing Bing Lin; Chih-Hwa Lin; Herng-Der Chern

2009-01-01

418

First Order Paths in Ordered Trees  

Microsoft Academic Search

We give two sucient conditions on XPath like languages for having first order expressivity, meaning that every first order defin- able set of paths in an ordered node-labeled tree is definable in that XPath language. They are phrased in terms of expansions of naviga- tional (sometimes called \\

Maarten Marx

2005-01-01

419

Ambivalent Journey: Teacher Career Paths in Oman  

ERIC Educational Resources Information Center

|This study investigated the career paths of 625 university graduates who prepared to be secondary school teachers in Oman, their assessment of their current work situation, and the extent to which their initial commitment to teaching was related to their subsequent career satisfaction and intention to remain in teaching. While nearly all

Chapman, David W.; Al-Barwani, Thuwayba; Al Mawali, Fathiya; Green, Elizabeth

2012-01-01

420

Air Path Estimation on Diesel HCCI Engine  

Microsoft Academic Search

In this paper, we address the problem of air path variables estimation for an HCCI engine. Two observers are pro- posed. Both rely on physical assumptions on the com- bustion, but use different sensors. After proving conver- gence in the two cases, we carry out comparisons based on simulation results. We stress the impact of two particu- lar additional sensors

J. Chauvin; N. Petit; P. Rouchon; C. Vigild; Ford Forschungszentrum Aachen

2006-01-01

421

Shortest paths algorithms: theory and experimental evaluation  

Microsoft Academic Search

We conduct an extensive computational study of shortest paths algorithms, including some very recent algorithms. We also suggest new algorithms motivated by the experimental results and prove interesting theoretical results suggested by the experimental data. Our computational study is based on several natural problem classes which identify strengths and weaknesses of various algorithms. These problem classes and algorithm implementations form

Boris V. Cherkassky; Andrew V. Goldberg; Tomasz Radzikt

1994-01-01

422

The diagnosis and cure of garden paths  

Microsoft Academic Search

We propose that, for the human parser, recovery from garden paths consists in repairing the structure built so far, rather than reparsing the input. The difficulty of a repair is attributable not to the cost of effecting the structural alterations but to the cost of deducing which alterations are needed. The parser must diagnose its error in order to correct

Janet Dean Fodor; Atsu Inoue

1994-01-01

423

Fatigue failure paths for offshore platform inspection  

SciTech Connect

A closed- form, reliability-based procedure is developed to identify fatigue failure paths of offshore structures and assess the notional probability of system failure through these paths. The procedure utilizes the Miners rule node fatigue failure reliability model developed by Wirsching. Effects of load redistribution following the fatigue failure of a node on the time to failure of remaining unfailed nodes is taken into consideration. Primary fatigue failure paths are identified through use of a branch and bound procedure. The components that fall in these paths are relatively critical to platform integrity. A platform inspection, therefore, needs to be planned around these components, as the failure of these would ultimately lead to system failure. The use of the procedure is demonstrated by application to a plane frame representation of a fixed shallow-water offshore jacket. The procedure is general in nature and neither imposes any restrictions in terms of uniform strengths of members nor requires loads to a specific pattern or be shared equally among members.

Karsan, D.I. (Marine Tech. Section, Conoco Inc., Houston, TX (US)); Kumar, A. (Conoco Inc., Ponca City, OK (US))

1990-06-01

424

Administrator Career Paths and Decision Processes  

ERIC Educational Resources Information Center

|Purpose: The purpose of this paper is to present qualitative evidence on the processes and forces that shape school administrator career paths. Design/methodology/approach: An embedded case study approach is used to understand more than 100 administrator career transitions within the Delaware education system. Semi-structured interview data were

Farley-Ripple, Elizabeth N.; Raffel, Jeffrey A.; Welch, Jennie Christine

2012-01-01

425

Path Integral Approach to Noncommutative Quantum Mechanics  

Microsoft Academic Search

We consider Feynman's path integral approach to quantum mechanics with a noncommutativity in position and momentum sectors of the phase space. We show that a quantum-mechanical system with this kind of noncommutativity is equivalent to the another one with usual commutative coordinates and momenta. We found connection between quadratic classical Hamiltonians, as well as Lagrangians, in their commutative and noncommutative

Branko Dragovich; Zoran Rakic

2004-01-01

426

Robot Path Planning Using a Genetic Algorithm.  

National Technical Information Service (NTIS)

Robot path planning can refer either to a mobile vehicle such as a Mars Rover, or to an end effector on an arm moving through a cluttered workspace. In both instances there may exist many solutions, some of which are better than others, either in terms of...

T. F. Cleghorn P. T. Baffes L. Wang

1988-01-01

427

Intelligent path prediction for vehicular travel  

Microsoft Academic Search

This thesis presents a methodology for intelligent path prediction, predicting the motion of an observed vehicle by reasoning about the actions taken by the operator of the vehicle. The thesis considers only the problem of tracking a vehicle performing a transit mission, a mission that proceeds from a start location to a goal location guided by an intelligent planning criterion.

James Alan Krozel

1992-01-01

428

Fermionic Path Integral Simulations of Hot, Dense Hydrogen  

NASA Astrophysics Data System (ADS)

Fermionic path integral Monte Carlo simulations have been applied to study the equilibrium properties of the hydrogen and deuterium at high pressure and temperature. We use this technique to determine Hugoniot [1] and compare with recent Laser shock wave experiments by Da Silva et al. Further we investigate the occurence of various species in the hydrogen plasma such as H, H+, e, H2 and H2+ and compare with simulation based on a chemical model. This analysis is connected with the Mott transition, in which electrons move from bound to free states, and the plasma phase transition [2] involving the dissociation of H2. [0.2cm] [1] B. Militzer, W. Magro, and D. Ceperley, ``Fermionic Path-Integral Simulation of Dense Hydrogen'' in Proceedings of the International Conference on Strongly Coupled Coulomb Systems, Boston (1997). [0.2cm] [2] W. R. Magro, D. M. Ceperley , C. Pierleoni and B. Bernu, Phys. Rev. Letts. 76, 1240 (1996).

Militzer, Burkhard; Magro, William; Ceperley, David

1998-03-01

429

Flight Path Control Equipment for Producing Curved Flight Path Profiles with Microwave Landing Systems.  

National Technical Information Service (NTIS)

The characteristics of a flight path control instrument for producing curved approach profiles and guidance along these profiles are presented. For safety reasons, steep noise abatement approaches must be flown along curved profiles. The problems of flyab...

G. Schaenzer

1974-01-01

430

Relationship between electron density and effective densities of body tissues for stopping, scattering, and nuclear interactions of proton and ion beams  

SciTech Connect

Purpose: In treatment planning of charged-particle radiotherapy, patient heterogeneity is conventionally modeled as variable-density water converted from CT images to best reproduce the stopping power, which may lead to inaccuracies in the handling of multiple scattering and nuclear interactions. Although similar conversions can be defined for these individual interactions, they would be valid only for specific CT systems and would require additional tasks for clinical application. This study aims to improve the practicality of the interaction-specific heterogeneity correction. Methods: The authors calculated the electron densities and effective densities for stopping power, multiple scattering, and nuclear interactions of protons and ions, using the standard elemental-composition data for body tissues to construct the invariant conversion functions. The authors also simulated a proton beam in a lung-like geometry and a carbon-ion beam in a prostate-like geometry to demonstrate the procedure and the effects of the interaction-specific heterogeneity correction. Results: Strong correlations were observed between the electron density and the respective effective densities, with which the authors formulated polyline conversion functions. Their effects amounted to 10% differences in multiple-scattering angle and nuclear interaction mean free path for bones compared to those in the conventional heterogeneity correction. Although their realistic effect on patient dose distributions would be generally small, it could be at the level of a few percent when a carbon-ion beam traverses a large bone. Conclusions: The present conversion functions are invariant and may be incorporated in treatment planning systems with a common function relating CT number to electron density. This will enable improved beam dose calculation while minimizing initial setup and quality management of the user's specific system.

Kanematsu, Nobuyuki; Inaniwa, Taku; Koba, Yusuke [Department of Accelerator and Medical Physics, Research Center for Charged Particle Therapy, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage-ku, Chiba 263-8555 (Japan)

2012-02-15

431

Electronic transport in two-dimensional Si:P ?-doped layers  

NASA Astrophysics Data System (ADS)

We investigate theoretically two-dimensional (2D) electronic transport in Si:P ?-doped layers limited by charged-dopant scattering. Since the carrier density is approximately equal to the dopant impurity density, the density dependent transport shows qualitatively different behavior from that of the well studied 2D Si MOSFETs (metal-oxide-semiconductor field-effect transistors) where the carrier density is independent of the impurity density. We find that the density dependent mobility of the Si:P system shows nonmonotonic behavior which is exactly opposite of the nonmonotonicity observed in Si MOSFETsin the Si:P system the mobility first decreases with increasing density and then it increases slowly with increasing density above a typical density 1014 cm-2 (in contrast to Si MOSFETs where the mobility typically increases with density first and then slowly decreases at high density as surface roughness scattering dominates). In the low-density limit (or strong screening limit) mobility decreases inversely with increasing density, but in the high-density limit (or weak-screening limit) it slowly increases due to the finite width effects of the 2D layer. In the intermediate density regime (1/a<2kFmean free path increase monotonically with density. We also compare the transport scattering time relevant to the mobility and the single-particle relaxation time relevant to the quantum level broadening, finding that the transport scattering time could be much larger than the single-particle scattering time unlike in Si MOSFETs where they are approximately equal.

Hwang, E. H.; Das Sarma, S.

2013-03-01

432

An Improved Shortest Path Algorithm for Computing One-to-One Shortest Paths on Road Networks  

Microsoft Academic Search

Computing one-to-one shortest paths on road networks is a fundamental work in many practical applications, especially in network and transportation related analyses. Pallottino's graph growth algorithm implemented with two queues (TWO-Q) is recommended as one of the top candidates to this kind of problems in literature. However, as a label-correcting shortest path algorithm, original TWO-Q algorithm begins scan from the

JinFu Leng; Wen Zeng

2009-01-01

433

Common-path interference and oscillatory Zener tunneling in bilayer graphene p-n junctions  

PubMed Central

Interference and tunneling are two signature quantum effects that are often perceived as the yin and yang of quantum mechanics: a particle simultaneously propagating along several distinct classical paths versus a particle penetrating through a classically inaccessible region via a single least-action path. Here we demonstrate that the Dirac quasiparticles in graphene provide a dramatic departure from this paradigm. We show that Zener tunneling in gapped bilayer graphene, which governs transport through p-n heterojunctions, exhibits common-path interference that takes place under the tunnel barrier. Due to a symmetry peculiar to the gapped bilayer graphene bandstructure, interfering tunneling paths form conjugate pairs, giving rise to high-contrast oscillations in transmission as a function of the gate-tunable bandgap and other control parameters of the junction. The common-path interference is solely due to forward-propagating waves; in contrast to FabryProt-type interference in resonant-tunneling structures, it does not rely on multiple backscattering. The oscillations manifest themselves in the junction IV characteristic as N-shaped branches with negative differential conductivity. The negative dI/dV, which arises solely due to under-barrier interference, can enable new high-speed active-circuit devices with architectures that are not available in electronic semiconductor devices.

Nandkishore, Rahul; Levitov, Leonid

2011-01-01

434

Common-path interference and oscillatory Zener tunneling in bilayer graphene p-n junctions.  

PubMed

Interference and tunneling are two signature quantum effects that are often perceived as the yin and yang of quantum mechanics: a particle simultaneously propagating along several distinct classical paths versus a particle penetrating through a classically inaccessible region via a single least-action path. Here we demonstrate that the Dirac quasiparticles in graphene provide a dramatic departure from this paradigm. We show that Zener tunneling in gapped bilayer graphene, which governs transport through p-n heterojunctions, exhibits common-path interference that takes place under the tunnel barrier. Due to a symmetry peculiar to the gapped bilayer graphene bandstructure, interfering tunneling paths form conjugate pairs, giving rise to high-contrast oscillations in transmission as a function of the gate-tunable bandgap and other control parameters of the junction. The common-path interference is solely due to forward-propagating waves; in contrast to Fabry-Prot-type interference in resonant-tunneling structures, it does not rely on multiple backscattering. The oscillations manifest themselves in the junction I-V characteristic as N-shaped branches with negative differential conductivity. The negative dI/dV, which arises solely due to under-barrier interference, can enable new high-speed active-circuit devices with architectures that are not available in electronic semiconductor devices. PMID:21825159

Nandkishore, Rahul; Levitov, Leonid

2011-08-08

435

Paths in the minimally weighted path model are incompatible with Schramm-Loewner evolution  

NASA Astrophysics Data System (ADS)

We study numerically the geometrical properties of minimally weighted paths that appear in the minimally weighted path (MWP) model on two-dimensional lattices assuming a combination of periodic and free boundary conditions (BCs). Each realization of the disorder consists of a random fraction (1-?) of bonds with unit strength and a fraction ? of bond strengths drawn from a Gaussian distribution with zero mean and unit width. For each such sample, a path is forced to span the lattice along the direction with the free BCs. The path and a set of negatively weighted loops form a ground state. A ground state on such a lattice can be determined performing a nontrivial transformation of the original graph and applying sophisticated matching algorithms. Here we examine whether the geometrical properties of the paths are in accordance with the predictions of the Schramm-Loewner evolution (SLE). Measuring the fractal dimension, considering the winding angle statistics, and reviewing Schramm's left passage formula indicate that the paths cannot be described in terms of SLE.

Norrenbrock, C.; Melchert, O.; Hartmann, A. K.

2013-03-01

436

A path model of aircraft noise annoyance  

NASA Astrophysics Data System (ADS)

This paper describes the development and testing of a path model of aircraft noise annoyance by using noise and social survey data collected in the vicinity of Toronto International Airport. Path analysis is used to estimate the direct and indirect effects of seventeen independent variables on individual annoyance. The results show that the strongest direct effects are for speech interference, attitudes toward aircraft operations, sleep interruption and personal sensitivity to noise. The strongest indirect effects are for aircraft Leq(24) and sensitivity. Overall the model explains 41 percent of the variation in the annoyance reported by the 673 survey respondents. The findings both support and extend existing statements in the literature on the antecedents of annoyance.

Taylor, S. M.

1984-09-01

437

Volcanic eruption induced WWVB transmission path interruption  

NASA Astrophysics Data System (ADS)

It is reported that the 60 kHz transmission of WWVB from Fort Collins, Colorado, was not received in Calgary, Alberta, Canada, for about 11 h from 1109 UT to 2153 UT on July 23, 1980. It is suggested that this transmission path interruption is correlated with the 15 km height ash cloud due to the July 22, 1980 volcanic eruption of Mount St. Helens as it drifted eastward interrupting both the ground- and first hop sky-wave paths and that this ash cloud is the source of the conductivity and/or ionization necessary to produce this interruption. Small phase retardations are also reported which could be correlated with other Mount St. Helens volcanic events during May-July 1980.

Buckmaster, H. A.; Hansen, C. H.

1985-07-01

438

Adaptive path planning for flexible manufacturing  

SciTech Connect

Path planning needs to be fast to facilitate real-time robot programming. Unfortunately, current planning techniques are still too slow to be effective, as they often require several minutes, if not hours of computation. To overcome this difficulty, we present an adaptive algorithm that uses past experience to speed up future performance. It is a learning algorithm suitable for automating flexible manufacturing in incrementally-changing environments. The algorithm allows the robot to adapt to its environment by having two experience manipulation schemes: For minor environmental change, we use an object-attached experience abstraction scheme to increase the flexibility of the learned experience; for major environmental change, we use an on-demand experience repair scheme to retain those experiences that remain valid and useful. Using this algorithm, we can effectively reduce the overall robot planning time by re-using the computation result for one task to plan a path for another.

Chen, Pang C.

1994-08-01

439

Path integral learning of multidimensional movement trajectories  

NASA Astrophysics Data System (ADS)

This paper explores the use of Path Integral Methods, particularly several variants of the recent Path Integral Policy Improvement (PI2) algorithm in multidimensional movement parametrized policy learning. We rely on Dynamic Movement Primitives (DMPs) to codify discrete and rhythmic trajectories, and apply the PI2-CMA and PIBB methods in the learning of optimal policy parameters, according to different cost functions that inherently encode movement objectives. Additionally we merge both of these variants and propose the PIBB-CMA algorithm, comparing all of them with the vanilla version of PI2. From the obtained results we conclude that PIBB-CMA surpasses all other methods in terms of convergence speed and iterative final cost, which leads to an increased interest in its application to more complex robotic problems.

Andr, Joa~o.; Santos, Cristina; Costa, Lino

2013-10-01

440

14 CFR 171.267 - Glide path automatic monitor system.  

Code of Federal Regulations, 2013 CFR

...DEPARTMENT OF TRANSPORTATION (CONTINUED) NAVIGATIONAL FACILITIES NON-FEDERAL NAVIGATION FACILITIES Interim Standard Microwave Landing System (ISMLS) § 171.267 Glide path automatic monitor system. (a) The ISMLS glide path...

2013-01-01

441

Inter-Domain Path Authentication in Tactical MANETs.  

National Technical Information Service (NTIS)

This paper presents a lightweight probabilistic path authentication scheme for mobile ad hoc networks (MANETs) based upon a new cryptographic primitive composite MAC. The proposed path-authentication scheme allows us to reliably identify nodes on a route ...

M. Ivatsa S. Reidt

2008-01-01

442

Mission Geography: Paths (title provided or enhanced by cataloger)  

NSDL National Science Digital Library

Mission Geography uses existing NASA data and images where possible to engage students in active, hands-on inquiry, modeling the scientific method and developing students' understanding of environment-society relations and Earth science. This module looks at many different kinds of paths and considers why paths are where they are and how they look from space. People and animals make paths that take into account the terrain and other features of the landscape. Rivers, lava, smoke, and other natural phenomena follow paths. The module contains four investigations in which students interpret paths in their immediate environment, use literature to investigate paths, inspect unusual paths, and use satellite images to view paths of natural disasters. Each investigation is complete with overview, a list of materials and supplies, content preview, classroom procedures, worksheets, background, and evaluation.

443

Path integrals and constraints: Particle in a box  

Microsoft Academic Search

Equivalence between the lagrangian path integral and the hamiltonian path integral with constraints is shown for a particle in a one-dimensional box. Present address: Photo-Voltaics Division, Solar Energy Research Institute, Golden, CO 80401, USA.

Akira Inomata; Vijay A. Singh

1980-01-01

444

Approximate Euclidean shortest path in 3-space  

Microsoft Academic Search

Papadimitriou's approximation approach to the Euclidean shortest path (ESP) problem in 3-space is revisited. As this problem is NP-hard, his approach represents an important step towards practical algorithms. Unfortunately, there are non-trivial gaps in the original description. Besides giving a complete treatment, we also give an alternative to his subdivision method which has some nice properties. Among the tools needed

Joonsoo Choi; Jrgen Sellen; Chee-Keng Yap

1994-01-01

445

Business engagement on Twitter: a path analysis  

Microsoft Academic Search

Social media services, such as Twitter, enable commercial businesses to participate actively in online word-of-mouth communication.\\u000a In this project, we examined the potential influences of business engagement in online word-of-mouth communication on the\\u000a level of consumers engagement and investigated the trajectories of a business online word-of-mouth message diffusion in\\u000a the Twitter community. We used path analysis to examine 164,478 tweets

Mimi Zhang; Bernard J. Jansen; Abdur Chowdhury

446

MultiPath Reasoning in a Database  

Microsoft Academic Search

Rules have been used in a database context for several purposes:deductive database queries, active database triggers, and productionsystem programs. Exploring the search space for non-deterministicrule programs, however, has generally been available only in largemonolithic systems intended for artificial intelligence applications.The goal of this research is to provide multi-path reasoning fornon-deterministic rule programs in a database environment. Themotivation for this work

Dariush Navabi; Lois M. L. Delcambre

1997-01-01

447

Do-It-Yourself Critical Path Method.  

ERIC Educational Resources Information Center

This report describes the critical path method (CPM), a system for planning and scheduling work to get the best time-cost combination for any particular job. With the use of diagrams, the report describes how CPM works on a step-by-step basis. CPM uses a network to show which parts of a job must be done and how they would eventually fit together

Morris, Edward P., Jr.

448

The path forward: Monte Carlo Convergence discussion  

SciTech Connect

This is a summary of 'the path forward' discussion session of the NuInt09 workshop which focused on Monte Carlo event generators. The main questions raised as part of this discussion are: how to make Monte Carlo generators more reliable and how important it is to work on a universal Monte Carlo generator of events? In this contribution, several experts in the field summarize their views, as presented at the workshop.

Andreopoulos, Costas [Rutherford Appleton Laboratory, STFC Oxfordshire OX11 0QX (United Kingdom); Gallagher, Hugh [Tufts University, Medford, Massachusetts (United States); Hayato, Yoshinari [Kamioka Observatory, ICRR, University of Tokyo Higashi-Mozumi 456, Kamioka-cho, Hida-city Gifu 506-1205 (Japan); Sobczyk, Jan T. [Institute of Theoretical Physics, Wroclaw, University Poland (Poland); Walter, Chris [Department of Physics, Duke University, Durham, NC 27708 (United States); Zeller, Sam [Los Alamos National Laboratory, Los Alamos, NM (United States)

2009-11-25

449

The path forward: Final state effects discussion  

SciTech Connect

This is a summary of 'the path forward' discussion session of the NuInt09 workshop which focused on final state interactions effects. The main question raised as part of this discussion was how to validate various approaches to implement such effects in Monte Carlo generators of events. In this contribution, several experts in the field summarize their views, as presented at the workshop.

Dytman, Steven [Dept. of Physics and Astronomy, Univ. of Pittsburgh, Pittsburgh, PA 15260 (United States); Leitner, Tina [Institut fuer Theoretische Physik, Universitaet Giessen (Germany); Smirnov, George [CERN, Geneva, Switzerland, and JINR, Dubna (Russian Federation); Sobczyk, Jan T. [Institute of Theoretical Physics, Wroclaw, University Poland (Poland); Tacik, Roman [Department of Physics, University of Regina, Regina, SK, Canada, S4S 0A2 (Canada)

2009-11-25

450

Buzz Lightyear Connect It!: Flight Path Activity  

NSDL National Science Digital Library

In this activity, "space cadets" (learners) use writing and sequencing skills in addition to directional words/ordered pairs to guide Buzz Lightyear (from the movie "Toy Story") through a grid to reach his orbiter. Learners are encouraged to avoid all obstacles and use as many or as few steps as needed. Once completed, learners give their flight path to a friend and ask him/her to follow the directions to help Buzz reach his orbiter.

Administration, National A.

2011-12-07

451

Local path control for an autonomous vehicle  

Microsoft Academic Search

A control system for an autonomous robot cart designed to operate in well-structured environments such as offices and factories is described. The onboard navigation system comprises a reference-state generator, an error-feedback controller, and cart-location sensing using odometry. There is a convenient separation between the path guidance and control logic. Under normal operating conditions, the controller ensures that the errors between

Winston L. Nelson; Ingeniar J. Cox

1988-01-01

452

Power, Distributive Conflicts, and Multiple Growth Paths  

Microsoft Academic Search

This article shows that multiple growth paths may occurin a politico-economic model of endogenous growth. This multiplicityis characterized by the coexistence of the low-tax, low-capital-flightequilibrium and a high-tax, high-capital-flight equilibrium.The likelihood of multiplicity is crucially related to the structureof power in societynamely, it is necessary that the politicallydecisive agents have a greater access to international capitalmarkets than the average in

Gilles Saint Paul; Thierry Verdier

1997-01-01

453

A path integral approach to agent planning  

Microsoft Academic Search

Control theory is a mathematical description of how to act optimally to gain future rewards. In this paper We discuss a class of non-linear stochastic control problems that can be ecien tly solved using a path integral. In this control for- malism, the central concept of cost-to-go or value function becomes a free energy and methods and concepts from sta-

Hilbert J. Kappen; Wim Wiegerinck; B. van den Broek

454

On Yen's Path Logic for Petri Nets  

NASA Astrophysics Data System (ADS)

In [13], Yen defines a class of formulas for paths in Petri nets and claims that its satisfiability problem is expspace-complete. In this paper, we show that in fact the satisfiability problem for this class of formulas is as hard as the reachability problem for Petri nets. Moreover, we salvage almost all of Yens results by defining a fragment of this class of formulas for which the satisfiability problem is expspace-complete by adapting his proof.

Atig, Mohamed Faouzi; Habermehl, Peter

455

Paths to community empowerment: Organizing at home  

Microsoft Academic Search

This study examines how low-income minority communities build on their strengths to improve material conditions and how these\\u000a actions lead to further empowerment at the individual and group level, and increase engagement with civic life. Based on earlier\\u000a qualitative research, low-income limited equity housing co-ops were chosen as research. Using quantitative questionnaire data,\\u000a a path model was tested in which

Susan Saegert; Gary Winkel

1996-01-01

456

Flux Control in Networks of Diffusion Paths  

SciTech Connect

A class of optimization problems in networks of intersecting diffusion domains of a special form of thin paths has been considered. The system of equations describing stationary solutions is equivalent to an electrical circuit built of intersecting conductors. The solution of an optimization problem has been obtained and extended to the analogous electrical circuit. The interest in this network arises from, among other applications, an application to wave-particle diffusion through resonant interactions in plasma.

A. I. Zhmoginov and N. J. Fisch

2009-07-08

457

On Path Restoration for Censored Outcomes  

PubMed Central

Summary Dimension reduction, model and variable selection are ubiquitous concepts in modern statistical science and deriving new methods beyond the scope of current methodology is noteworthy. This note briefly reviews existing regularization methods for penalized least squares and likelihood for survival data and their extension to a certain class of penalized estimating function. We show that if ones goal is to estimate the entire regularized coefficient path using the observed survival data, then all current strategies fail for the Buckley-James estimating function. We propose a novel two-stage method to estimate and restore the entire Dantzig-regularized coefficient path for censored outcomes in a least-squares framework. We apply our methods to a microarray study of lung andenocarcinoma with sample size n = 200 and p = 1036 gene predictors and find 10 genes that are consistently selected across different criteria and an additional 14 genes that merit further investigation. In simulation studies, we found that the proposed path restoration and variable selection technique has the potential to perform as well as existing methods that begin with a proper convex loss function at the outset.

Johnson, Brent A.; Long, Qi; Chung, Matthias

2011-01-01

458

DICOM involving XML path-tag  

NASA Astrophysics Data System (ADS)

Digital Imaging and Communications in Medicine (DICOM) is a standard for handling, storing, printing, and transmitting information in medical imaging. XML (Extensible Markup Language) is a set of rules for encoding documents in machine-readable form which has become more and more popular. The combination of these two is very necessary and promising. Using XML tags instead of numeric labels in DICOM files will effectively increase the readability and enhance the clear hierarchical structure of DICOM files. However, due to the fact that the XML tags rely heavily on the orders of the tags, the strong data dependency has a lot of influence on the flexibility of inserting and exchanging data. In order to improve the extensibility and sharing of DICOM files, this paper introduces XML Path-Tag to DICOM. When a DICOM file is converted to XML format, adding simple Path-Tag into the DICOM file in place of complex tags will keep the flexibility of a DICOM file while inserting data elements and give full play to the advantages of the structure and readability of an XML file. Our method can solve the weak readability problem of DICOM files and the tedious work of inserting data into an XML file. In addition, we set up a conversion engine that can transform among traditional DICOM files, XML-DCM and XML-DCM files involving XML Path-Tag efficiently.

Zeng, Qiang; Yao, Zhihong; Liu, Lei

2011-03-01

459

Disorder-induced Limited Path Percolation  

NASA Astrophysics Data System (ADS)

We introduce a model of percolation induced by disorder, where an initially homogeneous network with links of equal weight is disordered by the introduction of heterogeneous weights for the links. We consider a pair of nodes i and j to be mutually reachable when the ratio ?ij of the optimal path length between them before and after the introduction of disorder does not increase beyond a tolerance ratio ?. These conditions reflect practical limitations of reachability better than the usual percolation model, which entirely disregards path length when defining connectivity and, therefore, communication. We find that this model leads to a first-order phase transition in both 2-dimensional lattices and in Erd?s-Rnyi networks, and in the case of the latter, the size of the discontinuity implies that the transition is effectively catastrophic, with almost all system pairs undergoing the change from reachable to unreachable. Using the theory of optimal path lengths under disorder, we are able to predict the percolation threshold. For real networks subject to changes while in operation, this model should perform better in predicting functional limits than current percolation models.

Lpez, E.; Braunstein, L. A.

2012-03-01

460

Transition path sampling of protein conformational changes  

NASA Astrophysics Data System (ADS)

Conformational changes in proteins often take place on long time scales compared to the molecular timescale. These long time scales, related to high free energy barriers, make such processes difficult to access with a straightforward molecular dynamics approach. The transition path sampling technique has been developed to overcome such timescale differences without assuming a predefined reaction coordinate. We review the transition path sampling methodology with the application of protein conformational change in mind. Using three case studies, based on previous work, we elucidate the strengths and pitfalls of the method. First, the extensive work on the folding of Trp-cage reveals how to sample parallel pathways, how to obtain rate constants, and how to extract reaction coordinates. The second case-study, on the folding of Trpzip4 ?-hairpin, illustrates how to treat long-lived intermediates in a conformational change. The final example showcases the light-triggered conformational transition of Photo-active Yellow Protein into its signaling state, highlighting the wealth of insight that can be gathered from a transition path sampling approach, including new hypotheses for reaction mechanisms. We end with an outlook discussing future developments and application of the methodology.

Juraszek, Jarek; Vreede, Jocelyne; Bolhuis, Peter G.

2012-03-01