The Quantum Dynamics of a Dilute Gas in a 3D BCC Optical Lattice
NASA Astrophysics Data System (ADS)
Reichl, Linda; Boretz, Yingyue
2015-03-01
The classical and quantum dynamics of a dilute gas of rubidium atoms, in a 3D body-centered cubic optical lattice, is studied for a range of polarizations of the laser beams forming the lattice. The relative polarization of the lasers determines the the structure of the potential energy seen by the rubidium atoms. If three pairs of in-phase mutually perpendicular laser beams, with the same wavelength, form the lattice, only a limited range of possible couplings can be realized in the lab. We have determined the band structure of the BCC optical lattice for all theoretically possible couplings, and find that the band structure for lattices realizable in the lab, differs significantly from that expected for a BCC crystal. As coupling is increased, the lattice becomes increasingly chaotic and it becomes possible to produce band structure that has qualitative similarity to a BCC. Welch Foundation
NASA Astrophysics Data System (ADS)
Tamura, Ryuji
2015-03-01
We investigated and clarified the superstructures formed by tetrahedra in the bcc lattice within the framework of second-order transitions. Compliance with both the Landau and Lifshitz conditions was investigated for all possible superstructures and, based on this, we demonstrate that bcc crystals that contain tetrahedra at an inversion center can exhibit a variety of second-order transitions, which are regarded as a new type of diffusionless order-disorder transition with antiferroic orientational orders. Finally, we show that the transition gives rise to a new glassy state. Breaking of the local inversion symmetry may lead to a new orientational glass, which is reminiscent of spin glasses in magnetism.
The sign-factor of the 3D Ising model on dual BCC lattice
NASA Astrophysics Data System (ADS)
Khachatryan, Sh.; Sedrakyan, A.
2002-01-01
We modify the two-dimensional model for the sign-factor of the regular 3D Ising model (3DIM) presented by Kavalov and Sedrakyan (Phys. Lett. 173B (1986) 449 and Nucl. Phys. 285B (1987) 264) for the case of dual to body centered cubic (DBCC) three-dimensional lattice. The advantage of this lattice is in an absence of self-intersections of the two-dimensional surfaces embedded there. We investigate simpler case of the model with scalar fermions (instead of SU(2) needed for 3DIM) and have found it's spectrum, which appeared to be massless. We reformulate the model by use of R-matrix formalism and a new interesting structure appears in a necessity to introduce three-particle R(3)ijk-matrices. We formulate the integrability property of the model for more general case.
Helium-vacancy cluster in a single bcc iron crystal lattice
NASA Astrophysics Data System (ADS)
Gao, N.; Victoria, M.; Chen, J.; Van Swygenhoven, H.
2011-06-01
The properties of the cluster HenV, an iron vacancy with an increasing number of He atoms, is studied with molecular statics and molecular dynamics simulations. A study of the binding energy of the self-interstitial atom (SIA) and the He, shows that from n = 6 the HenV cluster is stable and cannot shrink anymore, and from n = 16 the HenV2 cluster is stabilized by the emission of SIA in the form of a lang110rang dumbbell. Calculation of the pressure exercised by the HenV cluster shows local peak normal stress and shear stress values up to 9 GPa and 4 GPa, respectively. The local configurations of HenV suggest that with increasing helium content, a high symmetry configuration close to a face centered cubic lattice is formed.
NASA Astrophysics Data System (ADS)
Jacobs, Michel H. G.; Schmid-Fetzer, Rainer
2010-12-01
We use a lattice vibrational technique to derive thermophysical and thermochemical properties of the pure elements aluminum and iron in pressure-temperature space. This semi-empirical technique is based on either the Mie-Grüneisen-Debye (MGD) approach or an extension of Kieffer's model to incorporate details of the phonon spectrum. It includes treatment of intrinsic anharmonicity, electronic effects based on the free electron gas model, and magnetic effects based on the Calphad approach. We show that Keane's equation of state for the static lattice is better suitable to represent thermodynamic data for aluminum from 1 bar to pressures in the multi-megabar region relative to Vinet's universal and the Birch-Murnaghan equation of state. It appears that the MGD and Mie-Grüneisen-Kieffer approach produce similar results, but that the last one better represents heat capacity below room temperature. For iron we show that the high temperature behavior of thermal expansivity can be explained within the Calphad approach by a pressure-dependent Curie temperature with a slope between -1 and 0 K/GPa.
Metastable bcc phase formation in the Nb-Cr system
Thoma, D.J.; Schwarz, R.B.; Perepezko, J.H.; Plantz, D.H.
1993-08-01
Extended metastable bcc solid solutions of Nb-Xat.%Cr (X = 35, 50, 57, 77, 82, and 94) were synthesized by two-anvil splat-quenching. In addition, bcc (Nb-67at.%Cr) was prepared by mechanically alloying mixtures of niobium and chromium powders. The lattice parameters were measured by X-ray diffraction and the Young`s moduli were measured by low-load microindentation. The composition dependence of the lattice parameters and elastic moduli show a positive deviation with respect to a rule of mixtures. During continuous heating at 15C/min., the metastable precursor bcc phases decomposed at temperatures above 750C to uniformly refined microstructures.
NASA Astrophysics Data System (ADS)
Gai, Xiao; Smith, Roger; Kenny, S. D.
2016-03-01
The properties of inert gas bubbles in bcc Fe is examined using a combination of static energy minimisation, molecular dynamics and barrier searching methods with empirical potentials. Static energy minimisation techniques indicate that for small Ar and Xe bubbles, the preferred gas to vacancy ratio at 0 K is about 1:1 for Ar and varies between 0.5:1 and 0.9:1 for Xe. In contrast to interstitial He atoms and small He interstitial clusters, which are highly mobile in the lattice, Ar and Xe atoms prefer to occupy substitutional sites and any interstitials present in the lattice soon displace Fe atoms and become substitutional. If a pre-existing bubble is present then there is a capture radius around a bubble which extends up to the 6th neighbour position. Collision cascades can also enlarge an existing bubble by the capture of vacancies. Ar and Xe can diffuse through the lattice through vacancy driven mechanisms but with relatively high energy barriers of 1.8 and 2.0 eV respectively. This indicates that Ar and Xe bubbles are much harder to form than bubbles of He and that such gases produced in a nuclear reaction would more likely be dispersed at substitutional sites without the help of increased temperature or radiation-driven mechanisms.
Free energy contributions to the hcp-bcc transformation in transition metals
Moroni, E.G.; Grimvall, G.; Jarlborg, T.
1996-04-01
The electronic and vibrational free energies of some hcp and bcc transition metals are computed {ital ab} {ital initio}. The vibrational part is obtained from a total-energy calculation over lattices with atoms randomly displaced according to a Gaussian distribution. The relative importance of electronic and vibrational excitations in the stabilization of the high-temperature bcc structure is clarified. {copyright} {ital 1996 The American Physical Society.}
Some phonon effects in S(q) for bcc metals
NASA Astrophysics Data System (ADS)
Rosenfeld, A. M.; Stott, M. J.
1990-10-01
The static structure factor S(q) of solid bcc metals is investigated within the harmonic lattice approximation. Fine structure found in the diffuse background of S(q) is studied for the alkali metals and for Nb and Mo using realistic force constants that are consistent with observed phonon spectra. Ancillary peaks in S(q) between Bragg peaks previously found for Li and Na along high-symmetry directions in q space are shown to result from extended structure in S(q). These well-defined features that extend from Bragg peaks, and in some cases span them, are readily accounted for in terms of the frequencies and polarizations of the phonons. These features are in turn related to particular lattice deformations that occur in common phase transformations of the bcc lattice. A relationship is noted between the calculated structure and similar features observed in neutron- and x-ray-scattering studies of the martensitic transformation in the alkali metals, charge-density waves in K, and the ω phase in Zr-Nb alloys.
NASA Astrophysics Data System (ADS)
Burghardt, Wesley; McCready, Erica
We report in situ small-angle x-ray scattering (SAXS) investigations of a spherically-ordered block copolymer melt with a low styrene content (13%) resulting in spherical polystyrene microdomains ordered in BCC lattice. Melt annealing after clearing above the ODT produces ordered samples that have a macroscopically random orientation distribution of BCC 'grains'. Melt samples are subjected to uniaxial extensional flow in a counter-rotating drum extensional flow fixture housed in an oven with synchrotron x-ray access. During flow, initially isotropic diffraction rings in SAXS patterns become deformed, reflecting distortion of the BCC lattice. Diffracted intensity also concentrates azimuthally, indicating macroscopic alignment of the BCC lattice. There is evidence that extensional flow leads to progressive disordering of the BCC structure, with loss of higher order peaks and the emergence of a diffuse 'halo' of scattering. While the primary diffraction peak is visible in directions parallel and perpendicular to the stretching direction, the deformation of the lattice d-spacing follows affine deformation. Indications of ordering persist to higher strains in samples stretched at higher extension rates, and evidence of affine lattice deformation persists to very high strains (Hencky
First-principles study of interactions between substitutional solutes in bcc iron
NASA Astrophysics Data System (ADS)
Gorbatov, O. I.; Delandar, A. Hosseinzadeh; Gornostyrev, Yu N.; Ruban, A. V.; Korzhavyi, P. A.
2016-07-01
Using density functional theory based calculations, employing the locally self-consistent Green's function method and the projected augmented wave method, we develop a database of solute-solute interactions in dilute alloys of bcc Fe. Interactions within the first three coordination shells are computed for the ferromagnetic state as well as for the paramagnetic (disordered local moment) state of the iron matrix. The contribution of lattice relaxations to the defect interaction energy is investigated in the ferromagnetic state. Implications of the obtained results for modeling the phenomena of point defect clustering and phase precipitation in bcc Fe-based alloys and steel are discussed.
Combined molecular dynamics-spin dynamics simulations of bcc iron
Perera, Meewanage Dilina N; Yin, Junqi; Landau, David P; Nicholson, Don M; Stocks, George Malcolm; Eisenbach, Markus; Brown, Greg
2014-01-01
Using a classical model that treats translational and spin degrees of freedom on an equal footing, we study phonon-magnon interactions in BCC iron with combined molecular and spin dynamics methods. The atomic interactions are modeled via an empirical many-body potential while spin dependent interactions are established through a Hamiltonian of the Heisenberg form with a distance dependent magnetic exchange interaction obtained from first principles electronic structure calculations. The temporal evolution of translational and spin degrees of freedom was determined by numerically solving the coupled equations of motion, using an algorithm based on the second order Suzuki-Trotter decomposition of the exponential operators. By calculating Fourier transforms of space- and time-displaced correlation functions, we demonstrate that the the presence of lattice vibrations leads to noticeable softening and damping of spin wave modes. As a result of the interplay between lattice and spin subsystems, we also observe additional longitudinal spin wave excitations, with frequencies which coincide with that of the longitudinal lattice vibrations.
Helium bubbles in bcc Fe and their interactions with irradiation
NASA Astrophysics Data System (ADS)
Gai, Xiao; Lazauskas, Tomas; Smith, Roger; Kenny, Steven D.
2015-07-01
The properties of helium bubbles in a body-centred cubic (bcc) Fe lattice have been examined. The atomic configurations and formation energies of different He-vacancy complexes were determined. The 0 K results show that the most energetically favourable He to Fe vacancy ratio increases from about 1:1 for approximately 5 vacancies up to about 4:1 for 36 vacancies. The formation mechanisms for small He clusters have also been considered. Isolated interstitials and small clusters can diffuse quickly through the lattice. MD simulations of randomly placed interstitial He atoms at 500 K showed clustering over the time scale of nanoseconds with He clusters containing up to 4 atoms being mobile over this time scale. He clusters containing 4 or 5 atoms were shown to eject an Fe dumbbell interstitial which could then detach from the He cluster and diffuse with the remaining He-vacancy complex being effectively immobile. Collision cascades initiated near larger bubbles showed that Fe vacancies produced by the cascades readily become part of the He-vacancy complexes. Energy barriers for He to join an existing bubble as a function of the He-vacancy ratio are also calculated. These can be larger than the diffusion barrier in the pristine lattice, but are lower when the bubbles contain excess vacancies, thus indicating that bubble growth may be kinetically constrained.
Multiscale Simulation of Plasticity in bcc Metals
NASA Astrophysics Data System (ADS)
Weygand, Daniel; Mrovec, Matous; Hochrainer, Thomas; Gumbsch, Peter
2015-07-01
Significant progress in our understanding of plasticity in body-centered cubic (bcc) metals during the last decade has enabled rigorous multiscale modeling based on quantitative physical principles. Significant advances have been made at the atomistic level in the understanding of dislocation core structures and energetics associated with dislocation glide by using high-fidelity models originating from quantum mechanical principles. These simulations revealed important details about the influence of non-Schmid (nonglide) stresses on the mobility of screw dislocations in bcc metals that could be implemented to mesoscopic discrete dislocation simulations with atomistically informed dislocation mobility laws. First applications of dislocation dynamics simulations to studies of plasticity in small-scale bcc single crystals have been performed. Dislocation dynamics simulations inspired the development of continuum models based on advanced 3D dislocation density measures with evolution equations that naturally track dislocation motion. These advances open new opportunities and perspectives for future quantitative and materials-specific multiscale simulation methods to describe plastic deformation in bcc metals and their alloys.
Heats of formation of bcc binary alloys
NASA Technical Reports Server (NTRS)
Bozzolo, Guillermo; Ferrante, John; Smith, John R.
1991-01-01
The method of Bozzolo, Ferrante and Smith is applied for the calculation of alloy energies for bcc elements. The heat of formation of several alloys is computed with the help of the Connolly-Williams method within the tetrahedron approximation. The dependence of the results on the choice of different sets of ordered structures is discussed.
Cascade morphology transition in bcc metals
Setyawan, Wahyu; Selby, A.; Juslin, Niklas; Stoller, Roger E.; Wirth, Brian D.; Kurtz, Richard J.
2015-06-10
Energetic atom collisions in solids induce shockwaves with complex morphologies. In this paper, we establish the existence of a morphological transition in such cascades. The order parameter of the morphology is defined as the exponent, $b$, in the defect production curve as a function of cascade energy ($N_F$$ \\sim$$E_{MD}^b$). Response of different bcc metals can be compared in a consistent energy domain when the energy is normalized by the transition energy, $\\mu$, between the high- and the low-energy regime. Using Cr, Fe, Mo and W data, an empirical formula of $\\mu$ as a function of displacement threshold energy, $E_d$, is presented for bcc metals.
Cascade morphology transition in bcc metals.
Setyawan, Wahyu; Selby, Aaron P; Juslin, Niklas; Stoller, Roger E; Wirth, Brian D; Kurtz, Richard J
2015-06-10
Energetic atom collisions in solids induce shockwaves with complex morphologies. In this paper, we establish the existence of a morphological transition in such cascades. The order parameter of the morphology is defined as the exponent, b, in the defect production curve as a function of cascade energy (N(F) ~ E(MD)(b)). Response of different bcc metals can be compared in a consistent energy domain when the energy is normalized by the transition energy, μ, between the high- and the low-energy regime. Using Cr, Fe, Mo and W data, an empirical formula of μ as a function of displacement threshold energy, E(d), is presented for bcc metals. PMID:25985256
Sound velocities of bcc-Fe and Fe0.85Si0.15 alloy at high pressure and temperature
NASA Astrophysics Data System (ADS)
Liu, Jin; Lin, Jung-Fu; Alatas, Ahmet; Bi, Wenli
2014-08-01
Studying the velocity-density profiles of iron and iron-silicon alloy at high pressures and temperatures is critical for understanding the Earth’s core as well as the interiors of other planetary bodies. Here we have investigated the compressional wave velocity (VP) and density (ρ) profiles of polycrystalline bcc-Fe and Fe0.85Si0.15 alloy (8 wt.% Si) using in situ high-energy resolution inelastic X-ray scattering (HERIX) and synchrotron X-ray diffraction spectroscopies in an externally-heated diamond anvil cell (EHDAC) up to 15 GPa and 700 K. Based on the measured velocity-density (VP-ρ) and velocity-pressure (VP-P) relations of bcc-Fe at simultaneous high pressure and temperature (P-T) conditions, our results show a strong VP reduction at elevated temperatures at a constant density. Comparison of the VP-ρ profiles between the bcc-Fe and bcc-Fe0.85Si0.15 alloy indicates that the alloying effect of additional 8 wt.% Si on the VP-ρ relationship of bcc-Fe is predominant via a constant density decrease of approximately 0.6 g/cm3 (7%). Compared with the literature velocity results for bcc and hcp Fe-Si alloys, the bcc-Fe and Fe-Si alloys exhibit higher VP than their hcp phase counterparts at the given bcc-hcp transition pressures. Our results here strongly support the notion that high temperature has a strong effect on the VP of Fe and that the VP-ρ profile of Fe can be affected by structural and magnetic transitions. Analyses on literature elastic constants of the bcc Fe-Si alloys, as a function of P-T and Si content, show that the bcc phase displays extremely high VP anisotropy of 16-30% and VS splitting anisotropy of 40-90% at high temperatures, while the addition of Si further enhances the anisotropy. Due to the extremely high elastic anisotropy of the bcc Fe-Si alloy, a certain portion of the bcc Fe-Si alloy with the lattice-preferred orientation may produce VP and VS anisotropies to potentially account for the observed seismic anisotropy in the inner core.
Tuning ideal tensile strengths and intrinsic ductility of bcc refractory alloys.
Qi, Liang; Chrzan, D C
2014-03-21
An important theoretical ductility criterion for group V and VI metal-based refractory alloys in body-centered cubic (bcc) lattices is the mechanical failure mode of their perfect crystals under tension along the weakest direction [100]. Pure Mo and W fail by cleavage and are deemed intrinsically brittle. However, first-principles calculations show that alloying with group IV or V transition metals can transform these materials into ones that display intrinsically ductile behavior, failing in shear under [100] tension. Remarkably, this transition can be understood as an electron filling effect with the intrinsically ductile response the manifestation of a Jahn-Teller distortion. PMID:24702389
Weinberger, Christopher R.; Tucker, Garritt J.; Foiles, Stephen M.
2013-02-01
It is well known that screw dislocation motion dominates the plastic deformation in body-centered-cubic metals at low temperatures. The nature of the nonplanar structure of screw dislocations gives rise to high lattice friction, which results in strong temperature and strain rate dependence of plastic flow. Thus the nature of the Peierls potential, which is responsible for the high lattice resistance, is an important physical property of the material. However, current empirical potentials give a complicated picture of the Peierls potential. Here, we investigate the nature of the Peierls potential using density functional theory in the bcc transition metals. The results show that the shape of the Peierls potential is sinusoidal for every material investigated. Furthermore, we show that the magnitude of the potential scales strongly with the energy per unit length of the screw dislocation in the material.
Lattice Green's functions in all dimensions
NASA Astrophysics Data System (ADS)
Guttmann, Anthony J.
2010-07-01
We give a systematic treatment of lattice Green's functions (LGF) on the d-dimensional diamond, simple cubic, body-centred cubic and face-centred cubic lattices for arbitrary dimensionality d >= 2 for the first three lattices, and for 2 <= d <= 5 for the hyper-fcc lattice. We show that there is a close connection between the LGF of the d-dimensional hyper-cubic lattice and that of the (d - 1)-dimensional diamond lattice. We give constant-term formulations of LGFs for each of these lattices in all dimensions. Through a still under-developed connection with Mahler measures, we point out an unexpected connection between the coefficients of the sc, bcc and diamond LGFs and some Ramanujan-type formulae for 1/π.
BCC skin cancer diagnosis based on texture analysis techniques
NASA Astrophysics Data System (ADS)
Chuang, Shao-Hui; Sun, Xiaoyan; Chang, Wen-Yu; Chen, Gwo-Shing; Huang, Adam; Li, Jiang; McKenzie, Frederic D.
2011-03-01
In this paper, we present a texture analysis based method for diagnosing the Basal Cell Carcinoma (BCC) skin cancer using optical images taken from the suspicious skin regions. We first extracted the Run Length Matrix and Haralick texture features from the images and used a feature selection algorithm to identify the most effective feature set for the diagnosis. We then utilized a Multi-Layer Perceptron (MLP) classifier to classify the images to BCC or normal cases. Experiments showed that detecting BCC cancer based on optical images is feasible. The best sensitivity and specificity we achieved on our data set were 94% and 95%, respectively.
The nanostructure and hydrogenation reaction of Mg50Co50 BCC alloy prepared by ball-milling.
Matsuda, J; Shao, H; Nakamura, Y; Akiba, E
2009-05-20
Mg50Co50 alloy before and after hydrogenation was investigated by means of transmission electron microscopy (TEM). Mg50Co50 alloy before hydrogenation was found to contain crystals not larger than 5 nm in size. Selected-area electron diffraction patterns (SAEDPs) revealed that these nanocrystals have a body-centered cubic (BCC) structure with a lattice parameter of about 0.3 nm. Distribution of Mg and Co elements in the Mg50Co50 alloy was uniform, indicated by energy dispersive x-ray spectroscopy (EDS) analysis. Crystallization and decomposition occurred in the Mg50Co50 alloy during hydrogenation. A large number of crystals larger than 10 nm were observed in the hydrogenated sample. The SAEDPs showed polycrystalline rings corresponding to the BCC phase and the Co metal phase. The existence of Mg-rich Mg-Co crystals and Co particles was also confirmed by TEM-EDS analysis. PMID:19420663
The nanostructure and hydrogenation reaction of Mg50Co50 BCC alloy prepared by ball-milling
NASA Astrophysics Data System (ADS)
Matsuda, J.; Shao, H.; Nakamura, Y.; Akiba, E.
2009-05-01
Mg50Co50 alloy before and after hydrogenation was investigated by means of transmission electron microscopy (TEM). Mg50Co50 alloy before hydrogenation was found to contain crystals not larger than 5 nm in size. Selected-area electron diffraction patterns (SAEDPs) revealed that these nanocrystals have a body-centered cubic (BCC) structure with a lattice parameter of about 0.3 nm. Distribution of Mg and Co elements in the Mg50Co50 alloy was uniform, indicated by energy dispersive x-ray spectroscopy (EDS) analysis. Crystallization and decomposition occurred in the Mg50Co50 alloy during hydrogenation. A large number of crystals larger than 10 nm were observed in the hydrogenated sample. The SAEDPs showed polycrystalline rings corresponding to the BCC phase and the Co metal phase. The existence of Mg-rich Mg-Co crystals and Co particles was also confirmed by TEM-EDS analysis.
Thermodynamics of plastic flow of BCC metals from atomistic studies of isolated screw dislocations
NASA Astrophysics Data System (ADS)
Gröger, Roman; Vitek, Vaclav
2015-03-01
The thermodynamic description of dislocation glide in BCC metals depends crucially on the shape of the Peierls barrier that 1 / 2 < 111 > screw dislocations have to overcome when moving in the lattice. While the height of this barrier can be obtained unequivocally using saddle-point search algorithms such as the Nudged Elastic Band (NEB) method, its exact shape depends on the chosen approximation of the transition pathway of the system. We formulate a procedure that allows to identify the position of the dislocation directly from the displacements of atoms in its core. We investigate the performance of this model by calculating curved paths of a 1 / 2 < 111 > screw dislocation in tungsten from a series of images obtained recently using the NEB method at zero applied stress and for positive/negative shear stresses perpendicular to the slip direction. The Peierls barriers plotted along these curved paths are shown to be quite different from those obtained previously by assuming a straight dislocation path. We demonstrate how these results can be utilized to develop a new thermodynamic model of plasticity of BCC metals that is systematically linked to the atomic-level properties of isolated 1 / 2 < 111 > screw dislocations.
Dahmen, U.
1980-11-01
In a recent communication, Bhadeshia gives an explanation of the tent-shaped surface relief effects observed in association with Widmanstatten ferrite and lower bainite. Based on his proposed explanation he concludes that the diffusional fcc{yields}bcc transformation tn steel takes place by a displacive mechanism. This conclusion is disputed by Aaronson in a letter following that of Bhadeshia by showing that the experimentally observed orientation relationships are different from the one ( Nishyama-Wassermann ( N-W) , necessary for Bhadeshia' s mechanism. He therefore holds that the transformation is purely diffusional and that "any attempts to understand diffusional phase transformations in terms of a shear mechanism are counterproductive." The purpose of the present note is (l) to show that Bhadeshia's mechanism seems to be based on a misinterpretation of the lattice symmetries resulting from the fcc {yields} bcc transformation and cannot explain a tent-shaped surface relief even if the N-W orientation relationship is followed, and (2) to suggest that attempts to formally understand diffusional phase transformations in terms of shear may not always be counterproductive.
Ginzburg-Landau theory for the solid-liquid interface of bcc elements
NASA Technical Reports Server (NTRS)
Shih, W. H.; Wang, Z. Q.; Zeng, X. C.; Stroud, D.
1987-01-01
Consideration is given to a simple order-parameter theory for the interfacial tension of body-centered-cubic solids in which the principal order parameter is the amplitude of the density wave at the smallest nonzero reciprocal-lattice vector of the solid. The parameters included in the theory are fitted to the measured heat of fusion, melting temperature, and solid-liquid density difference, and to the liquid structure factor and its temperature derivative at freezing. Good agreement is found with experiment for Na and Fe and the calculated anisotropy of the surface tension among different crystal faces is of the order of 2 percent. On the basis of various assumptions about the universal behavior of bcc crystals at melting, the formalism predicts that the surface tension is proportional to the heat of fusion per surface atom.
The HCP To BCC Phase Transformation in Ti Characterized by Nanosecond Electron Microscopy
Campbell, G; LaGrange, T; King, W; Colvin, J; Ziegler, A; Browning, N; Kleinschmidt, H; Bostanjoglo, O
2005-06-21
The general class of martensitic phase transformations occurs by a rapid lattice-distortive mechanism, where kinetics and morphology of the transformation are dominated by the strain energy. Since transformation is diffusionless, phase fronts propagate through a crystal with great speed that can approach the speed of sound. We have observed a particular example of this class of phase transformation, the hexagonal close packed (HCP) to body centered cubic (BCC) transformation in titanium that is driven by a rapid increase in temperature. We have used a novel nanosecond electron microscope (the dynamic transmission electron microscope, DTEM) to acquire diffraction and imaging information on the transformation, which is driven in-situ by nanosecond laser irradiation. Using nanosecond exposure times that are possible in the DTEM, data can be collected about the transient events in these fast transformations. We have identified the phase transformation with diffraction patterns and correlated the time of the phase transformation with calculated conditions in the sample.
Dislocations With Edge Components in Nanocrystalline bcc Mo
G. M. Cheng; W. Z. Xu; W. W. Jian; H. Yuan; M. H. Tsai; Y. T. Zhu; Y. F. Zhang; Paul C. Millett
2013-07-01
We report high-resolution transmission electron microscopy (HRTEM) observation of a high density of dislocations with edge components (approximately 1016 m-2) in nanocrystalline (NC) body-centered cubic (bcc) Mo prepared by high-pressure torsion. We also observed for the first time of the 1/2 <111> and <001> pure edge dislocations in NC Mo. Crystallographic analysis and image simulations reveal that the best way using HRTEM to study dislocations with edge components in bcc systems is to take images along <110> zone axis, from which it is possible to identify 1/2 <111> pure edge dislocations, and edge components of 1/2 <111> and <001> mixed dislocations. The <001> pure edge dislocations can only be identified from <100> zone axis. The high density of dislocations with edge components is believed to play a major role in the reduction of strain rate sensitivity in NC bcc metals and alloys.
Dislocation nucleation in bcc Ta single crystals studied by nanoindentation
Biener, M M; Biener, J; Hodge, A M; Hamza, A V
2007-08-08
The study of dislocation nucleation in closed-packed metals by nanoindentation has recently attracted much interest. Here, we address the peculiarities of the incipient plasticity in body centered cubic (bcc) metals using low index Ta single-crystals as a model system. The combination of nanoindentation with high-resolution atomic force microscopy provides us with experimental atomic-scale information on the process of dislocation nucleation and multiplication. Our results reveal a unique deformation behavior of bcc Ta at the onset of plasticity which is distinctly different from that of closed-packed metals. Most noticeable, we observe only one rather than a sequence of discontinuities in the load-displacement curves. This and other differences are discussed in context of the characteristic plastic deformation behavior of bcc metals.
Theoretical elastic moduli of ferromagnetic bcc Fe alloys.
Zhang, Hualei; Punkkinen, Marko P J; Johansson, Börje; Vitos, Levente
2010-07-14
The polycrystalline elastic parameters of ferromagnetic Fe(1-x)M(x) (M = Al, Si, V, Cr, Mn, Co, Ni, Rh; 0 ≤ x ≤ 0.1) random alloys in the body centered cubic (bcc) crystallographic phase have been calculated using first-principles alloy theory in combination with statistical averaging methods. With a few exceptions, the agreement between the calculated and the available experimental data for the polycrystalline aggregates is satisfactory. All additions considered here decrease the bulk modulus (B) and Poisson's ratio (ν) of bcc Fe. The complex composition dependence of the C(44) single-crystal elastic constant is reflected in the polycrystalline shear modulus (G), Young's modulus (E), and Debye temperature (Θ). The polycrystalline anisotropy of bcc Fe is increased by all additions, and Al, Si, Ni, and Rh yield the largest alloying effects. PMID:21399255
Hot electron spin attenuation lengths of bcc Fe34Co66—Room temperature Magnetocurrent of 1200%
NASA Astrophysics Data System (ADS)
Heindl, E.; Kefes, C.; Soda, M.; Vancea, J.; Back, C. H.
2009-11-01
We investigate spin-dependent hot electron transport through metallic epitaxial spin valves by ballistic electron magnetic microscopy (BEMM). By variation of the thickness of one of the ferromagnetic layers we determine the spin dependent attenuation lengths which reflect hot electron transport along the vicinity of the [1 0 0]-axis of the bcc Fe34Co66 lattice. The majority spin attenuation length is more than 6 times larger than that of the minority spins within the measured energy interval of 1.3 up to 2 eV above the Fermi level. Consequently a Magnetocurrent effect exceeding 1200% accompanied by a monotonic bias voltage behavior is observed at room temperature.
Crystallographic Lattice Boltzmann Method.
Namburi, Manjusha; Krithivasan, Siddharth; Ansumali, Santosh
2016-01-01
Current approaches to Direct Numerical Simulation (DNS) are computationally quite expensive for most realistic scientific and engineering applications of Fluid Dynamics such as automobiles or atmospheric flows. The Lattice Boltzmann Method (LBM), with its simplified kinetic descriptions, has emerged as an important tool for simulating hydrodynamics. In a heterogeneous computing environment, it is often preferred due to its flexibility and better parallel scaling. However, direct simulation of realistic applications, without the use of turbulence models, remains a distant dream even with highly efficient methods such as LBM. In LBM, a fictitious lattice with suitable isotropy in the velocity space is considered to recover Navier-Stokes hydrodynamics in macroscopic limit. The same lattice is mapped onto a cartesian grid for spatial discretization of the kinetic equation. In this paper, we present an inverted argument of the LBM, by making spatial discretization as the central theme. We argue that the optimal spatial discretization for LBM is a Body Centered Cubic (BCC) arrangement of grid points. We illustrate an order-of-magnitude gain in efficiency for LBM and thus a significant progress towards feasibility of DNS for realistic flows. PMID:27251098
Crystallographic Lattice Boltzmann Method
Namburi, Manjusha; Krithivasan, Siddharth; Ansumali, Santosh
2016-01-01
Current approaches to Direct Numerical Simulation (DNS) are computationally quite expensive for most realistic scientific and engineering applications of Fluid Dynamics such as automobiles or atmospheric flows. The Lattice Boltzmann Method (LBM), with its simplified kinetic descriptions, has emerged as an important tool for simulating hydrodynamics. In a heterogeneous computing environment, it is often preferred due to its flexibility and better parallel scaling. However, direct simulation of realistic applications, without the use of turbulence models, remains a distant dream even with highly efficient methods such as LBM. In LBM, a fictitious lattice with suitable isotropy in the velocity space is considered to recover Navier-Stokes hydrodynamics in macroscopic limit. The same lattice is mapped onto a cartesian grid for spatial discretization of the kinetic equation. In this paper, we present an inverted argument of the LBM, by making spatial discretization as the central theme. We argue that the optimal spatial discretization for LBM is a Body Centered Cubic (BCC) arrangement of grid points. We illustrate an order-of-magnitude gain in efficiency for LBM and thus a significant progress towards feasibility of DNS for realistic flows. PMID:27251098
Crystallographic Lattice Boltzmann Method
NASA Astrophysics Data System (ADS)
Namburi, Manjusha; Krithivasan, Siddharth; Ansumali, Santosh
2016-06-01
Current approaches to Direct Numerical Simulation (DNS) are computationally quite expensive for most realistic scientific and engineering applications of Fluid Dynamics such as automobiles or atmospheric flows. The Lattice Boltzmann Method (LBM), with its simplified kinetic descriptions, has emerged as an important tool for simulating hydrodynamics. In a heterogeneous computing environment, it is often preferred due to its flexibility and better parallel scaling. However, direct simulation of realistic applications, without the use of turbulence models, remains a distant dream even with highly efficient methods such as LBM. In LBM, a fictitious lattice with suitable isotropy in the velocity space is considered to recover Navier-Stokes hydrodynamics in macroscopic limit. The same lattice is mapped onto a cartesian grid for spatial discretization of the kinetic equation. In this paper, we present an inverted argument of the LBM, by making spatial discretization as the central theme. We argue that the optimal spatial discretization for LBM is a Body Centered Cubic (BCC) arrangement of grid points. We illustrate an order-of-magnitude gain in efficiency for LBM and thus a significant progress towards feasibility of DNS for realistic flows.
Lattice vibrations and instabilities in tungsten phases from electronic structure calculations
NASA Astrophysics Data System (ADS)
Grimvall, G.; Einarsdotter, K.; Sadigh, B.; Köpe, B.; Ozolinš, V.
1998-03-01
Phonon dispersion curves are calculated for bcc and fcc W, as a function of atomic volume. The range of phonon stability in the fcc phase is mapped out in the Brillouin zone. Incipient instabilities in the bcc phase are studied, and compared with related instabilities in, e.g., bcc Ti and Zr. A molecular-dynamics type analysis is also performed. Implications are discussed for binary phase diagrams AB where elements A and B have different lattice structures, one of them being dynamically unstable.
Madden-Julian Oscillation simulated in BCC climate models
NASA Astrophysics Data System (ADS)
Zhao, Chongbo; Ren, Hong-Li; Song, Lianchun; Wu, Jie
2015-12-01
This study evaluates the ability of four versions BCC (Beijing Climate Center or National Climate Center) models (BCC_AGCM2.1, BCC_AGCM2.2, BCC_CSM1.1 and BCC_CSM1.1m) in simulating the MJO phenomenon using the outputs of the AMIP (Atmospheric Model Intercomparison Project) and historical runs. In general, the models can simulate some major characteristics of the MJO, such as the intensity, the periodicity, the propagation, and the temporal/spatial evolution of the MJO signals in the tropics. There are still some biases between the models and the observation/reanalysis data, such as the overestimated total intraseasonal variability, but underestimated MJO intensity, shorter significant periodicity, and excessive westward propagation. The differences in the ability of simulating the MJO between AMIP and historical experiments are also significant. Compared to the AMIP runs, the total intraseasonal variability is reduced and more realistic, however the ratio between the MJO and its westward counterpart decreases in the historical runs. This unrealistic simulation of the zonal propagation might have been associated with the greater mean precipitation over the Pacific and corresponded to the exaggeration of the South Pacific Convergence Zone structure in precipitation mean state. In contrast to the T42 versions, the improvement of model resolution demonstrate more elaborate topography, but the enhanced westward propagation signals over the Arabia Sea followed. The underestimated (overestimated) MJO variability over eastern Indian Ocean (Pacific) was assumed to be associated with the mean state. Three sets of sensitive experiments using BCC_CSM1.1m turn out to support this argument.
Nonadiabaticity in the iron bcc to hcp phase transformation
NASA Astrophysics Data System (ADS)
Johnson, Donald F.; Carter, Emily A.
2008-03-01
Iron is known to undergo a pressure-induced phase transition from the ferromagnetic (FM) body-centered-cubic (bcc) α-phase to the nonmagnetic (NM) hexagonal-close-packed (hcp) ɛ-phase, with a large observed pressure hysteresis whose origin is still a matter of debate. Long ago, Burgers [Physica (Amsterdam) 1, 561 (1934)] proposed an adiabatic pathway for bcc to hcp transitions involving crystal shear followed by atom shuffles. However, a quantum mechanics search in six-dimensional stress-strain space reveals a much lower energy path, where the crystal smoothly shears along the entire path while the atoms shuffle only near the transition state (TS). The energy profile for this phase transition path exhibits a cusp at the TS and closely follows bcc and hcp diabatic energy wells. Both the cusp and the overlap with diabatic energy surfaces are hallmarks of nonadiabaticity, analogous to, e.g., electron transfer (ET) reactions in liquids. Fluctuations in the positions of FM bcc iron atoms near the TS induce magnetic quenching (akin to solvent fluctuations inducing ET), which then promotes NM hcp iron formation (akin to solvent reorganization after ET). We propose that the nonadiabatic nature of this transition at the atomic scale may contribute to the observed pressure hysteresis.
The invariant line and precipitate morphology in fcc-bcc systems
Weatherly, G.C.; Zhang, W.Z. . Dept. of Materials Science and Engineering)
1994-09-01
Second-phase precipitates in many face-centered cubic-body-centered cubic (fcc-bcc) systems (e.g., Ni-Cr, Cu-Cr, Fe-Cu, and [alpha]-[gamma] stainless steels) have a lath-shaped morphology, the long axis of the lath being an invariant line of the transformation. The invariant line direction and major (habit) facet plane of the product phase can be predicted by O-lattice (O-line) models. For N-W-, and K-S-oriented precipitates, the habit plane is shown to be an unrotated plane of the transformation. This contains a single set of dislocations lying parallel to the invariant line, with their Burgers vector in the habit plane. Structural ledge models for the habit-plane interface also are considered. For the range of lattice parameter ratios of interest in this study, structural ledge and O-line models can make almost identical predictions as to the optimum habit plane. A variety of elasticity calculations for the energy of fully constrained or fully relaxed precipitates is presented. These models are shown to have limited predictive capabilities. It is suggested that better atomic matching along or near to the invariant line direction might explain the preference for K-S-related precipitates in many systems.
Dislocation dynamics: simulation of plastic flow of bcc metals
Lassila, D H
2001-02-20
This is the final report for the LDRD strategic initiative entitled ''Dislocation Dynamic: Simulation of Plastic Flow of bcc Metals'' (tracking code: 00-SI-011). This report is comprised of 6 individual sections. The first is an executive summary of the project and describes the overall project goal, which is to establish an experimentally validated 3D dislocation dynamics simulation. This first section also gives some information of LLNL's multi-scale modeling efforts associated with the plasticity of bcc metals, and the role of this LDRD project in the multiscale modeling program. The last five sections of this report are journal articles that were produced during the course of the FY-2000 efforts.
Shear Modulus and Dislocations in bcc Solid ^3He
NASA Astrophysics Data System (ADS)
Cheng, Zhi Gang; Souris, Fabien; Beamish, John
2016-05-01
The shear modulus of hcp ^4He decreases significantly above ˜ 200 mK, as ^3He impurities unbind from dislocations, unpinning them, and softening the crystal. Here we report shear modulus measurements on a fermi quantum solid: bcc ^3He. In contrast to previous low-frequency measurements, which did not show dislocation softening in this system, we have observed a drop in shear modulus, accompanied by a dissipation peak, which we attribute to the unpinning of dislocations as ^4He impurities unbind. For large stresses, impurities cannot pin the dislocations and the low temperature stiffening is suppressed. At high frequencies, the modulus changes and dissipation peaks shift to higher temperature, indicating that the unbinding is thermally activated. For a 58 bar bcc ^3He crystal, we find an activation energy of 0.27 K, smaller than the 0.7 K binding energy for ^3He impurities in hcp ^4He.
Ab initio calculations of grain boundaries in bcc metals
NASA Astrophysics Data System (ADS)
Scheiber, Daniel; Pippan, Reinhard; Puschnig, Peter; Romaner, Lorenz
2016-03-01
In this study, we compute grain boundary (GB) properties for a large set of GBs in bcc transition metals with a special focus on W, Mo and Fe using ab initio density functional theory (DFT) and semi-empirical second nearest neighbour modified embedded atom method (2NN-MEAM) potentials. The GB properties include GB energies, surface energies, GB excess volume and work of separation, which we analyse and then compare to experimental data. We find that the used 2NN-MEAM potentials can predict general trends of GB properties, but do not always reproduce the GB ground state structure and energy found with DFT. In particular, our results explain the experimental finding that W and Mo prefer intergranular fracture, while other bcc metals prefer transgranular cleavage.
Formation of dislocation loops during He clustering in bcc Fe
NASA Astrophysics Data System (ADS)
Gao, N.; Van Swygenhoven, H.; Victoria, M.; Chen, J.
2011-11-01
The clustering of helium in bcc (body centered cubic) iron and the growth of a helium bubble are simulated at the atomistic level for the helium-rich vacancy-poor condition. It is shown that a \\frac{1}{2}\\langle 111\\rangle dislocation loop is formed as a sequential collection of <111> crowdions, the latter being the most stable self-interstitial atom configuration in the presence of a He cluster.
Formation of bcc and fcc during the coalescence of free and supported Fe and Ni clusters.
Li, Guojian; Wang, Qiang; Sui, Xudong; Wang, Kai; Wu, Chun; He, Jicheng
2015-09-01
The formation of bcc and fcc during the coalescence of free and supported Fe and Ni clusters has been studied by molecular dynamics simulation using an embedded atom method. Structural evolution of the clusters, coalesced under varying temperature, Ni content and substrate conditions, was explored by interatomic energy, snapshots, pair distribution functions and bond order parameters. The results show that the formation of bcc and fcc is strongly related to Ni content, substrate and coalescence temperature. Free clusters coalesced at 1200 K form bcc at lower Ni contents with fcc forming at higher Ni concentrations and no observable coexistence of bcc and fcc. Differences in coalescence at 1000 K result from the coexistence of bcc and fcc within the Ni range of 50-70%. Free clusters supported on disordered Ni substrates were shown to transform from spherical morphology to islands of supported clusters with preferred epitaxial orientation. The Ni content required to form bcc and fcc coexistence on supported clusters at 1000 K decreased to 30-50% Ni. Free clusters possessing bcc and fcc generally stacked along the bcc (110) and fcc (111) facets, whereas supported clusters stacked along the (111) bcc and (100) fcc planes. Structural transformation was induced by clusters containing greater numbers of atoms. Spread over the substrate enhanced interatomic energy, order substrates affect the epitaxial growth direction and increase the melting points of the supported clusters. This study can be used to predict the nature of fcc and bcc formation in Fe-Ni films. PMID:26234423
Electronic selection rules controlling dislocation glide in bcc metals.
Jones, Travis E; Eberhart, Mark E; Clougherty, Dennis P; Woodward, Chris
2008-08-22
The validity of the structure-property relationships governing the low-temperature deformation behavior of many bcc metals was brought into question with recent ab initio density functional studies of isolated screw dislocations in Mo and Ta. These relationships were semiclassical in nature, having grown from atomistic investigations of the deformation properties of the group V and VI transition metals. We find that the correct form for these structure-property relationships is fully quantum mechanical, involving the coupling of electronic states with the strain field at the core of long a/<2111> screw dislocations. PMID:18764636
Electronic Selection Rules Controlling Dislocation Glide in bcc Metals
NASA Astrophysics Data System (ADS)
Jones, Travis E.; Eberhart, Mark E.; Clougherty, Dennis P.; Woodward, Chris
2008-08-01
The validity of the structure-property relationships governing the low-temperature deformation behavior of many bcc metals was brought into question with recent ab initio density functional studies of isolated screw dislocations in Mo and Ta. These relationships were semiclassical in nature, having grown from atomistic investigations of the deformation properties of the group V and VI transition metals. We find that the correct form for these structure-property relationships is fully quantum mechanical, involving the coupling of electronic states with the strain field at the core of long a/2⟨111⟩ screw dislocations.
Hierarchical Characterization of Deformation Heterogeneities inBCC crystals
Magid, Karen R.; Lilleodden, Erica T.; Tamura, Nobumichi; Florando, Jeff; Lassila, Dave; Barabash, Rozaliya I.; Morris, J.W.
2005-01-01
Deformation behavior of body-centered cubic (BCC) metals is being investigated by white beam x-ray microdiffraction to characterize the dislocation structure that results from uniaxial compression experiments. The measurements were performed on molybdenum single crystals and a tantalum bicrystal as part of a hierarchical characterization effort. Results show heterogeneities in the deformed structure and misorientation maps consistent with results obtained from Orientation Imaging Microscopy (OIM). Additionally, the technique allows for the determination of the active glide systems as well as of the dislocation densities in function of the position in the sample.
Atomistic Simulation of Defect Properties in BCC Tantalum
Yang, L H; Soderlind, P; Moriarty, J A
2002-04-19
The fundamental atomic-level properties of point and line defects in bcc Ta have been simulated by means of quantum-based multi-ion interatomic potentials derived from the model generalized pseudopotential theory (MGPT). The potentials have been applied to the calculations of point defect formation and migration energies. The results are then compared with the ab-initio electronic-structure results and experimental data, which in turn provide rigorous validation tests of the MGPT potentials. Robust and accurate two- and three-dimensional Green's function (GF) techniques have been developed for static and dynamic simulations of single a/2<111> screw dislocation properties in bcc Ta. The transformation of the dislocation core under the influence of external stress was studied in detail using static GF method. Finite-temperature GF simulation reveals multiple-kink (thermal-kink) formation under an applied stress and the corresponding thermal-kink configuration entropy is estimated to be around 5.23k{sub B}.
NASA Astrophysics Data System (ADS)
Alling, B.; Körmann, F.; Grabowski, B.; Glensk, A.; Abrikosov, I. A.; Neugebauer, J.
2016-06-01
We study the impact of lattice vibrations on magnetic and electronic properties of paramagnetic bcc and fcc iron at finite temperature, employing the disordered local moments molecular dynamics (DLM-MD) method. Vibrations strongly affect the distribution of local magnetic moments at finite temperature, which in turn correlates with the local atomic volumes. Without the explicit consideration of atomic vibrations, the mean local magnetic moment and mean field derived magnetic entropy of paramagnetic bcc Fe are larger compared to paramagnetic fcc Fe, which would indicate that the magnetic contribution stabilizes the bcc phase at high temperatures. In the present study we show that this assumption is not valid when the coupling between vibrations and magnetism is taken into account. At the γ -δ transition temperature (1662 K), the lattice distortions cause very similar magnetic moments of both bcc and fcc structures and hence magnetic entropy contributions. This finding can be traced back to the electronic densities of states, which also become increasingly similar between bcc and fcc Fe with increasing temperature. Given the sensitive interplay of the different physical excitation mechanisms, our results illustrate the need for an explicit consideration of vibrational disorder and its impact on electronic and magnetic properties to understand paramagnetic Fe. Furthermore, they suggest that at the γ -δ transition temperature electronic and magnetic contributions to the Gibbs free energy are extremely similar in bcc and fcc Fe.
Calculation of body-centered-cubic lattice sums with an application to ferromagnetism.
NASA Technical Reports Server (NTRS)
Wintucky, E. G.
1972-01-01
The lattice sums for the bcc lattice are recalculated using the method of Flax and Raich to obtain more general expressions, valid for all temperatures, in terms of a Langevin function and its derivatives. Formulas are presented which enable easy numerical evaluation. A comparison with well-known low-temperature expansions and with the results of direct numerical integration demonstrates the validity at low temperatures of the more general expressions calculated here.
NASA Astrophysics Data System (ADS)
Li, Xiaojie; Schönecker, Stephan; Li, Ruihuan; Li, Xiaoqing; Wang, Yuanyuan; Zhao, Jijun; Johansson, Börje; Vitos, Levente
2016-07-01
To examine the effect of neutron transmutation on tungsten as the first wall material of fusion reactors, the elastic properties of W1‑x‑y Re x Os y (0 ⩽ x, y ⩽ 6%) random alloys in body centered cubic (bcc) structure are investigated systematically using the all-electron exact muffin-tin orbitals (EMTO) method in combination with the coherent-potential approximation (CPA). The calculated lattice constant and elastic properties of pure W are consistent with available experiments. Both Os and Re additions reduce the lattice constant and increase the bulk modulus of W, with Os having the stronger effect. The polycrystalline shear modulus, Young’s modulus and the Debye temperature increase (decrease) with the addition of Re (Os). Except for C 11, the other elastic parameters including C 12, C 44, Cauchy pressure, Poisson ratio, B/G, increase as a function of Re and Os concentration. The variations of the latter three parameters and the trend in the ratio of cleavage energy to shear modulus for the most dominant slip system indicate that the ductility of the alloy enhances with increasing Re and Os content. The calculated elastic anisotropy of bcc W slightly increases with the concentration of both alloying elements. The estimated melting temperatures of the W–Re–Os alloy suggest that Re or Os addition will reduce the melting temperature of pure W solid. The classical Labusch–Nabarro model for solid-solution hardening predicts larger strengthening effects in W1‑y Os y than in W1‑x Re x . A strong correlation between C‧ and the fcc–bcc structural energy difference for W1‑x‑y Re x Os y is revealed demonstrating that canonical band structure dictates the alloying effect on C‧. The structural energy difference is exploited to estimate the alloying effect on the ideal tensile strength in the [0 0 1] direction.
Bornyakov, V.G.
2005-06-01
Possibilities that are provided by a lattice regularization of QCD for studying nonperturbative properties of QCD are discussed. A review of some recent results obtained from computer calculations in lattice QCD is given. In particular, the results for the QCD vacuum structure, the hadron mass spectrum, and the strong coupling constant are considered.
A new method for development of bond-order potentials for transition bcc metals
NASA Astrophysics Data System (ADS)
Lin, Yi-Shen; Mrovec, M.; Vitek, V.
2014-04-01
A new development of numerical bond-order potentials (BOPs) for the non-magnetic transition metals V, Nb, Ta, Cr, Mo and W is presented. The principles on which the BOPs have been set up are the same as in earlier developments (Aoki et al 2007 Prog. Mater. Sci. 52 154). However, the bond integrals are based on the recently advanced method of parametrization of tight-binding from DFT calculations (Madsen et al 2011 Phys. Rev. B 83 4119, Urban et al 2011 Phys. Rev. B 84 155119) and do not require any screening. At the same time, the functional form of the environmentally dependent repulsion is identified with the functional form of the repulsion arising from the overlap of s and p electrons in argon as proposed in Aoki and Kurokawa (2007 J. Phys.: Condens. Matter 19 136228). This is justified by the same physical origin of the environment dependent repulsion, which in transition metals arises from the overlap of s electrons that are being squeezed into the ion core regions under the influence of the strong covalent d bonds. The testing of the developed BOPs involves investigation of alternative higher energy structures, transformation paths connecting the bcc structure with other structures via continuously distorted configurations, evaluation of the vacancy formation energy and calculation of phonon spectra. In all cases, the BOP calculations are in more than satisfactory agreement with either DFT calculations and/or available experimental data. The calculated γ-surfaces for {1 0 1} planes all suggest that the core of 1/2<1 1 1> screw dislocations is non-degenerate in the transition metals. This is also in full agreement with available calculations that account fully for the quantum-mechanical nature of the d electrons that provide the bulk of the bonding in transition metals. The testing of developed BOPs clearly demonstrates that they are transferable to structures well outside the regime of the ideal bcc lattice and are suitable for investigating the
The physical and mechanical metallurgy of advanced O+BCC titanium alloys
NASA Astrophysics Data System (ADS)
Cowen, Christopher John
This thesis comprises a systematic study of the microstructural evolution, phase transformation behavior, elevated-temperature creep behavior, room-temperature and elevated-temperature tensile behavior, and room-temperature fatigue behavior of advanced titanium-aluminum-niobium (Ti-Al-Nb) alloys with and without boron additions. The specific alloys studied were: Ti-5A1-45Nb (at%), Ti-15Al-33Nb (at%), Ti-15Al-33Nb-0.5B (at%), Ti-15Al-33Nb-5B (at%), Ti-21Al-29Nb (at%), Ti-22Al-26Nb (at%), and Ti-22Al-26Nb-5B (at%). The only alloy composition that had been previously studied before this thesis work began was Ti-22Al-26Nb (at%). Publication in peer-reviewed material science journals of the work performed in this thesis has made data available in the scientific literature that was previously non-existent. The knowledge gap for Ti-Al-Nb phase equilibria over the compositional range of Ti-23Al-27Nb (at%) to Ti-12Al-38Nb (at%) that existed before this work began was successfully filled. The addition of 5 at% boron to the Ti-15Al-33Nb alloy produced 5-9 volume percent boride phase needles within the microstructure. The chemical composition of the boride phase measured by electron microprobe was determined to be approximately B 2TiNb. The lattice parameters of the boride phase were simulated through density functional theory calculations by collaborators at the Air Force Research Laboratory based on the measured composition. Using the simulated lattice parameters, electron backscatter diffraction kikuchi patterns and selected area electron diffraction patterns obtained from the boride phase were successfully indexed according to the space group and site occupancies of the B27 orthorhombic crystal structure. This suggests that half the Ti (c) Wyckoff positions are occupied by Ti atoms and the other half are occupied by Nb atoms in the boride phase lattice. Creep deformation behavior is the main focus of this thesis and in particular understanding the dominant creep
Cubic to tetragonal crystal lattice reconstruction during ordering or decomposition
Cheong, Byung-kl
1992-09-01
This thesis studied thermodynamic stability and morphology of product phases in diffusional phase transformations involving cubic-to-tetragonal crystal lattice reconstructions. Two different kinds of diffusional transformations were examined: L1{sub 0} ordering (fcc to fct lattice change) and decomposition of off-stoichiometric B2 ordering alloys accompanying bcc to fcc Bain transformation. In the first case, Fe-45 at.% Pd alloys were studied by TEM; in the second, the Bain strain relaxation during decomposition of hyper-eutectoid Cu-9.04 wt% Be alloy was studied. CuAu and InMg were also studied.
Plastic anisotropy and dislocation trajectory in BCC metals
NASA Astrophysics Data System (ADS)
Dezerald, Lucile; Rodney, David; Clouet, Emmanuel; Ventelon, Lisa; Willaime, François
2016-05-01
Plasticity in body-centred cubic (BCC) metals at low temperatures is atypical, marked in particular by an anisotropic elastic limit in clear violation of the famous Schmid law applicable to most other metals. This effect is known to originate from the behaviour of the screw dislocations; however, the underlying physics has so far remained insufficiently understood to predict plastic anisotropy without adjustable parameters. Here we show that deviations from the Schmid law can be quantified from the deviations of the screw dislocation trajectory away from a straight path between equilibrium configurations, a consequence of the asymmetrical and metal-dependent potential energy landscape of the dislocation. We propose a modified parameter-free Schmid law, based on a projection of the applied stress on the curved trajectory, which compares well with experimental variations and first-principles calculations of the dislocation Peierls stress as a function of crystal orientation.
Plastic anisotropy and dislocation trajectory in BCC metals
Dezerald, Lucile; Rodney, David; Clouet, Emmanuel; Ventelon, Lisa; Willaime, François
2016-01-01
Plasticity in body-centred cubic (BCC) metals at low temperatures is atypical, marked in particular by an anisotropic elastic limit in clear violation of the famous Schmid law applicable to most other metals. This effect is known to originate from the behaviour of the screw dislocations; however, the underlying physics has so far remained insufficiently understood to predict plastic anisotropy without adjustable parameters. Here we show that deviations from the Schmid law can be quantified from the deviations of the screw dislocation trajectory away from a straight path between equilibrium configurations, a consequence of the asymmetrical and metal-dependent potential energy landscape of the dislocation. We propose a modified parameter-free Schmid law, based on a projection of the applied stress on the curved trajectory, which compares well with experimental variations and first-principles calculations of the dislocation Peierls stress as a function of crystal orientation. PMID:27221965
Plastic anisotropy and dislocation trajectory in BCC metals.
Dezerald, Lucile; Rodney, David; Clouet, Emmanuel; Ventelon, Lisa; Willaime, François
2016-01-01
Plasticity in body-centred cubic (BCC) metals at low temperatures is atypical, marked in particular by an anisotropic elastic limit in clear violation of the famous Schmid law applicable to most other metals. This effect is known to originate from the behaviour of the screw dislocations; however, the underlying physics has so far remained insufficiently understood to predict plastic anisotropy without adjustable parameters. Here we show that deviations from the Schmid law can be quantified from the deviations of the screw dislocation trajectory away from a straight path between equilibrium configurations, a consequence of the asymmetrical and metal-dependent potential energy landscape of the dislocation. We propose a modified parameter-free Schmid law, based on a projection of the applied stress on the curved trajectory, which compares well with experimental variations and first-principles calculations of the dislocation Peierls stress as a function of crystal orientation. PMID:27221965
2011-01-01
Background A close association between maize roots and Burkholderia cepacia complex (BCC) bacteria has been observed in different locations globally. In this study we investigated by MultiLocus Restriction Typing (MLRT) the genetic diversity and relationships among Burkholderia cenocepacia IIIB and BCC6 populations associated with roots of maize plants cultivated in geographically distant countries (Italy and Mexico), in order to provide new insights into their population structure, evolution and ecology. Results The 31 B. cenocepacia IIIB and 65 BCC6 isolates gave rise to 29 and 39 different restriction types (RTs), respectively. Two pairs of isolates of B. cenocepacia IIIB and BCC6, recovered from both Italian and Mexican maize rhizospheres, were found to share the same RT. The eBURST (Based Upon Related Sequence Types) analysis of MLRT data grouped all the B. cenocepacia IIIB isolates into four clonal complexes, with the RT-4-complex including the 42% of them, while the majority of the BCC6 isolates (94%) were grouped into the RT-104-complex. These two main clonal complexes included RTs shared by both Italian and Mexican maize rhizospheres and a clear relationship between grouping and maize variety was also found. Grouping established by eBURST correlated well with the assessment using unweighted-pair group method with arithmetic mean (UPGMA). The standardized index of association values obtained in both B. cenocepacia IIIB and BCC6 suggests an epidemic population structure in which occasional clones emerge and spread. Conclusions Taken together our data demonstrate a wide dispersal of certain B. cenocepacia IIIB and BCC6 isolates in Mexican and Italian maize rhizospheres. Despite the clear relationship found between the geographic origin of isolates and grouping, identical RTs and closely related isolates were observed in geographically distant regions. Ecological factors and selective pressure may preferably promote some genotypes within each local microbial
ERIC Educational Resources Information Center
Parris, Richard
2011-01-01
Given a segment that joins two lattice points in R[superscript 3], when is it possible to form a lattice cube that uses this segment as one of its twelve edges? A necessary and sufficient condition is that the length of the segment be an integer. This paper presents an algorithm for finding such a cube when the prime factors of the length are…
Csébfalvi, Balázs
2013-09-01
In this paper, Cosine-Weighted B-spline (CWB) filters are proposed for interpolation on the optimal Body-Centered Cubic (BCC) lattice. We demonstrate that our CWB filters can well exploit the fast trilinear texture-fetching capability of modern GPUs, and outperform the state-of-the-art box-spline filters not just in terms of efficiency, but in terms of visual quality and numerical accuracy as well. Furthermore, we rigorously show that the CWB filters are better tailored to the BCC lattice than the previously proposed quasi-interpolating BCC B-spline filters, because they form a Riesz basis; exactly reproduce the original signal at the lattice points; but still provide the same approximation order. PMID:23846091
NASA Astrophysics Data System (ADS)
Ou, X.; Sietsma, J.; Santofimia, M. J.
2016-06-01
Molecular dynamics simulations have been used to study the effects of different orientation relationships between fcc and bcc phases on the bcc/fcc interfacial propagation in pure iron systems at 300 K. Three semi-coherent bcc/fcc interfaces have been investigated. In all the cases, results show that growth of the bcc phase starts in the areas of low potential energy and progresses into the areas of high potential energy at the original bcc/fcc interfaces. The phase transformation in areas of low potential energy is of a martensitic nature while that in the high potential energy areas involves occasional diffusional jumps of atoms.
STABILITY IN BCC TRANSITION METALS: MADELUNG AND BAND-ENERGY EFFECTS DUE TO ALLOYING
Landa, A; Soderlind, P; Ruban, A; Peil, O; Vitos, L
2009-08-28
The phase stability of the bcc Group VB (V, Nb, and Ta) transition metals is explored by first-principles electronic-structure calculations. Alloying with a small amount of a neighboring metal can either stabilize or destabilize the bcc phase. This counterintuitive behavior is explained by competing mechanisms that dominate depending on particular dopand. We show that band-structure effects dictate stability when a particular Group VB metal is alloyed with its nearest neighbors within the same d-transition series. In this case, the neighbor with less (to the left) and more (to the right) d electrons, destabilize and stabilize bcc, respectively. When alloying with neighbors of different d-transition series, electrostatic Madelung energy dominates over the band energy and always stabilizes the bcc phase.
Properties of Helium Defects in BCC and FCC Metals Investigated with Density Functional Theory
Zu, Xiaotao T.; Yang, Li; Gao, Fei; Peng, SM; Heinisch, Howard L.; Long, XG; Kurtz, Richard J.
2009-08-03
The relative stability of single He defects in bcc and fcc metals is investigated using ab initio calculations based on density functional theory (DFT). The results indicate that the tetrahedral position is energetically more favorable for a He interstitial than the octahedral site in bcc metals, but the relative stability of He defects in fcc metals varies, depending on local environments. The He formation energies in bcc Fe and fcc Ni at the tetrahedral and octahedral positions with and without spin polarization are investigated. It is of interest to find that the magnetism of host atoms does not directly affect the relative stabilities of He in interstitial sites in bcc Fe and fcc Ni.
Mechanical properties of irradiated multi-phase polycrystalline BCC materials
NASA Astrophysics Data System (ADS)
Song, Dingkun; Xiao, Xiazi; Xue, Jianming; Chu, Haijian; Duan, Huiling
2015-04-01
Structure materials under severe irradiations in nuclear environments are known to degrade because of irradiation hardening and loss of ductility, resulting from irradiation-induced defects such as vacancies, interstitials and dislocation loops, etc. In this paper, we develop an elastic-viscoplastic model for irradiated multi-phase polycrystalline BCC materials in which the mechanical behaviors of individual grains and polycrystalline aggregates are both explored. At the microscopic grain scale, we use the internal variable model and propose a new tensorial damage descriptor to represent the geometry character of the defect loop, which facilitates the analysis of the defect loop evolutions and dislocation-defect interactions. At the macroscopic polycrystal scale, the self-consistent scheme is extended to consider the multiphase problem and used to bridge the individual grain behavior to polycrystal properties. Based on the proposed model, we found that the work-hardening coefficient decreases with the increase of irradiation-induced defect loops, and the orientation/loading dependence of mechanical properties is mainly attributed to the different Schmid factors. At the polycrystalline scale, numerical results for pure Fe match well with the irradiation experiment data. The model is further extended to predict the hardening effect of dispersoids in oxide-dispersed strengthened steels by the considering the Orowan bowing. The influences of grain size and irradiation are found to compete to dominate the strengthening behaviors of materials.
Dislocations and Plasticity in bcc Transition Metals at High Pressure
Yang, L H; Tang, M; Moriarty, J A
2009-01-23
Using first-principles electronic structure calculations, quantum-based atomistic simulations and atomistically informed dislocation dynamics (DD) simulations, we have studied individual dislocation behavior and the multiscale modeling of single-crystal plasticity in the prototype bcc transition metals Ta, Mo and V under both ambient and high pressure conditions. The primary focus in this work is on the pressure-dependent structure, mobility and interaction of a/2<111> screw dislocations, which dominate the plastic deformation properties of these materials. At the electronic scale, first-principles calculations of elasticity, ideal strength and generalized stacking fault energy surfaces have been used to validate quantum-based multi-ion interatomic potentials. At the atomistic scale, these potentials have been used in flexible Green's function boundary condition simulations to study the core structure, Peierls stress {tau}{sub P}, thermally activated kink-pair formation and mobility below {tau}{sub P}, and phonon-drag mobility above {tau}{sub P}. These results have then been distilled into analytic velocity laws and used directly in predictive microscale DD simulations of flow stress and resolved yield stress over wide ranges of pressure, temperature and strain rate.
An Atomistic study of Helium Resolution in bcc Iron
Stoller, Roger E; Stewart, David M
2011-01-01
The evolution of gas-stabilized bubbles in irradiated materials can be a significant factor in the microstructural processes that lead to mechanical property and dimensional changes in structural materials exposed to high-energy neutrons. Helium generation and accumulation is particularly important under DT fusion irradiation conditions. Although the process of ballistic resolutioning of gas from bubbles has been long-discussed in the literature, there have been few computational studies of this mechanism. Resolutioning could limit bubble growth by ejecting gas atoms back into the metal matrix. A detailed atomistic study of ballistic He resolutioning from bubbles in bcc iron has been carried out using molecular dynamics. A newly-developed Fe-He interatomic potential was employed, with the iron matrix described by the potential of Ackland and co-workers from 1997. The primary variables examined were: irradiation temperature (100 and 600K), iron knock-on atom energy (5 and 20 keV), bubble radius (~0.5 and 1.0 nm), and He-to-vacancy ratio in the bubble (0.25, 0.5 and 1.0) in order to obtain an assessment of this dynamic resolutioning mechanism. The results presented here focus on the 5 keV cascades which indicate a modest, but potentially significant level of He removal by this process.
Cesium under pressure: First-principles calculation of the bcc-to-fcc phase transition
NASA Astrophysics Data System (ADS)
Carlesi, S.; Franchini, A.; Bortolani, V.; Martinelli, S.
1999-05-01
In this paper we present the ab initio calculation of the structural properties of cesium under pressure. The calculation of the total energy is done in the local-density approximation of density-functional theory, using a nonlocal pseudopotential including the nonlinear core corrections proposed by Louie et al. The calculation of the pressure-volume diagram for both bcc and fcc structures allows us to prove that the transition from bcc to fcc structure is a first-order transition.
Solid-liquid interface free energies of pure bcc metals and B2 phases
Wilson, S. R.; Gunawardana, K. G. S. H.; Mendelev, M. I.
2015-04-07
The solid-liquid interface (SLI) free energy was determined from molecular dynamics (MD) simulation for several body centered cubic (bcc) metals and B2 metallic compounds (space group: Pm3¯m ; prototype: CsCl). In order to include a bcc metal with a low melting temperature in our study, a semi-empirical potential was developed for Na. Two additional synthetic “Na” potentials were also developed to explore the effect of liquid structure and latent heat on the SLI free energy. The obtained MD data were compared with the empirical Turnbull, Laird, and Ewing relations. All three relations are found to predict the general trend observedmore » in the MD data for bcc metals obtained within the present study. However, only the Laird and Ewing relations are able to predict the trend obtained within the sequence of “Na” potentials. The Laird relation provides the best prediction for our MD data and other MD data for bcc metals taken from the literature. Overall, the Laird relation also agrees well with our B2 data but requires a proportionality constant that is substantially different from the bcc case. It also fails to explain a considerable difference between the SLI free energies of some B2 phases which have nearly the same melting temperature. In contrast, this difference is satisfactorily described by the Ewing relation. Thus, the Ewing relation obtained from the bcc dataset also provides a reasonable description of the B2 data.« less
Solid-liquid interface free energies of pure bcc metals and B2 phases
Wilson, S. R.; Gunawardana, K. G. S. H.; Mendelev, M. I.
2015-04-07
The solid-liquid interface (SLI) free energy was determined from molecular dynamics (MD) simulation for several body centered cubic (bcc) metals and B2 metallic compounds (space group: Pm3¯m ; prototype: CsCl). In order to include a bcc metal with a low melting temperature in our study, a semi-empirical potential was developed for Na. Two additional synthetic “Na” potentials were also developed to explore the effect of liquid structure and latent heat on the SLI free energy. The obtained MD data were compared with the empirical Turnbull, Laird, and Ewing relations. All three relations are found to predict the general trend observed in the MD data for bcc metals obtained within the present study. However, only the Laird and Ewing relations are able to predict the trend obtained within the sequence of “Na” potentials. The Laird relation provides the best prediction for our MD data and other MD data for bcc metals taken from the literature. Overall, the Laird relation also agrees well with our B2 data but requires a proportionality constant that is substantially different from the bcc case. It also fails to explain a considerable difference between the SLI free energies of some B2 phases which have nearly the same melting temperature. In contrast, this difference is satisfactorily described by the Ewing relation. Thus, the Ewing relation obtained from the bcc dataset also provides a reasonable description of the B2 data.
Plasticity and Failure in Nanocrystalline BCC Metals via MD Simulation
Rudd, R E
2010-02-12
Advances in the ability to generate extremely high pressures in dynamic experiments such as at the National Ignition Facility has motivated the need for special materials optimized for those conditions as well as ways to probe the response of these materials as they are deformed. We need to develop a much deeper understanding of the behavior of materials subjected to high pressure, especially the effect of rate at the extremely high rates encountered in those experiments. Here we use large-scale molecular dynamics (MD) simulations of the high-rate deformation of nanocrystalline tantalum at pressures less than 100 GPa to investigate the processes associated with plastic deformation for strains up to 100%. We focus on 3D polycrystalline systems with typical grain sizes of 10-20 nm. We also study a rapidly quenched liquid (amorphous solid) tantalum. We apply a constant volume (isochoric), constant temperature (isothermal) shear deformation over a range of strain rates, and compute the resulting stress-strain curves to large strains for both uniaxial and biaxial compression. We study the rate dependence and identify plastic deformation mechanisms. The identification of the mechanisms is facilitated through a novel technique that computes the local grain orientation, returning it as a quaternion for each atom. This analysis technique is robust and fast, and has been used to compute the orientations on the fly during our parallel MD simulations on supercomputers. We find both dislocation and twinning processes are important, and they interact in the weak strain hardening in these extremely fine-grained microstructures. We also present some results on void growth in nanocrystalline BCC metals under tension.
NASA Astrophysics Data System (ADS)
Wang, Fang; Xin, Xiaoge; Wang, Zaizhi; Cheng, Yanjie; Zhang, Jie; Yang, Song
2014-05-01
The abilities of BCC_AGCM2.1 and BCC_AGCM2.2 to simulate the annual-mean cloud vertical structure (CVS) were evaluated through comparison with GCM-Oriented CALIPSO Cloud Product (CALIPSO-GOCCP) data. BCC_AGCM2.2 has a dynamical core and physical processes that are consistent with BCC_AGCM2.1, but has a higher horizontal resolution. Results showed that both BCC_AGCM versions underestimated the global-mean total cloud cover (TCC), middle cloud cover (MCC) and low cloud cover (LCC), and that BCC_AGCM2.2 underestimated the global-mean high cloud cover (HCC). The global-mean cloud cover shows a systematic decrease from BCC_AGCM2.1 to BCC_AGCM2.2, especially for HCC. Geographically, HCC is significantly overestimated in the tropics, particularly by BCC_AGCM2.1, while LCC is generally overestimated over extra-tropical lands, but significantly underestimated over most of the oceans, especially for subtropical marine stratocumulus clouds. The leading EOF modes of CVS were extracted. The BCC_AGCMs perform well in reproducing EOF1, but with a larger variance explained. The two models also capture the basic features of EOF3, except an obvious deficiency in eigenvector peaks. EOF2 has the largest simulation biases in both position and strength of eigenvector peaks. Furthermore, we investigated the effects of CVS on relative shortwave and longwave cloud radiative forcing (RSCRF and RLCRF). Both BCC_AGCM versions successfully reproduce the sign of regression coefficients, except for RLCRF in PC1. However, the RSCRF relative contributions from PC1 and PC2 are overestimated, while the relative contribution from PC3 is underestimated in both BCC_AGCM versions. The RLCRF relative contribution is underestimated for PC2 and overestimated for PC3.
Csébfalvi, Balázs
2010-01-01
In this paper, we demonstrate that quasi-interpolation of orders two and four can be efficiently implemented on the Body-Centered Cubic (BCC) lattice by using tensor-product B-splines combined with appropriate discrete prefilters. Unlike the nonseparable box-spline reconstruction previously proposed for the BCC lattice, the prefiltered B-spline reconstruction can utilize the fast trilinear texture-fetching capability of the recent graphics cards. Therefore, it can be applied for rendering BCC-sampled volumetric data interactively. Furthermore, we show that a separable B-spline filter can suppress the postaliasing effect much more isotropically than a nonseparable box-spline filter of the same approximation power. Although prefilters that make the B-splines interpolating on the BCC lattice do not exist, we demonstrate that quasi-interpolating prefiltered linear and cubic B-spline reconstructions can still provide similar or higher image quality than the interpolating linear box-spline and prefiltered quintic box-spline reconstructions, respectively. PMID:20224143
BCc1, the novel antineoplastic nanocomplex, showed potent anticancer effects in vitro and in vivo
Kalanaky, Somayeh; Hafizi, Maryam; Fakharzadeh, Saideh; Vasei, Mohammad; Langroudi, Ladan; Janzamin, Ehsan; Hashemi, Seyed Mahmoud; Khayamzadeh, Maryam; Soleimani, Masoud; Akbari, Mohammad Esmaeil; Nazaran, Mohammad Hassan
2016-01-01
Purpose In spite of all the efforts and researches on anticancer therapeutics, an absolute treatment is still a myth. Therefore, it is necessary to utilize novel technologies in order to synthesize smart multifunctional structures. In this study, for the first time, we have evaluated the anticancer effects of BCc1 nanocomplex by vitro and in vivo studies, which is designed based on the novel nanochelating technology. Methods Human breast adenocarcinoma cell line (MCF-7) and mouse embryonic fibroblasts were used for the in vitro study. Antioxidant potential, cell toxicity, apoptosis induction, and CD44 and CD24 protein expression were evaluated after treatment of cells with different concentrations of BCc1 nanocomplex. For the in vivo study, mammary tumor-bearing female Balb/c mice were treated with different doses of BCc1 and their effects on tumor growth rate and survival were evaluated. Results BCc1 decreased CD44 protein expression and increased CD24 protein expression. It induced MCF-7 cell apoptosis but at the same concentrations did not have negative effects on mouse embryonic fibroblasts viability and protected them against oxidative stress. Treatment with nanocomplex increased survival and reduced the tumor size growth in breast cancer-bearing balb/c mice. Conclusion These results demonstrate that BCc1 has the capacity to be assessed as a new anticancer agent in complementary studies. PMID:26766901
NASA Astrophysics Data System (ADS)
Vairavel, Mathayan; Sundaravel, Balakrishnan; Panigrahi, Binaykumar
2016-09-01
There are contradictory theoretical predictions of lattice location of oxygen interstitial atom at tetrahedral and octahedral interstices in bcc Fe. For validating these predictions, 300 keV O18 ions with fluence of 5 × 1015 ions/cm2 are implanted into bcc Fe single crystals at room temperature and annealed at 400 °C. The Rutherford backscattering spectrometry (RBS) and nuclear reaction analysis (NRA)/channeling measurements are carried out with 850 keV protons. The lattice location of implanted O18 is analysed using the α-particles yield from O18(p,α)N15 nuclear reaction. The tilt angular scans of α-particle yield along <110> and <100> axial directions are performed at room temperature. Lattice location of O18 is found to be at tetrahedral interstitial site by comparing the experimental scan with simulated scans using FLUX7 software.
Daulton, T. L.
1998-10-23
The kinetics which drive cascade formation and subsequent collapse into point-defect clusters is investigated by analyzing the microstructure produced in situ by low fluence 100 keV Kr ion irradiations of fcc-Cu over a wide temperature range (18-873 K). The yield of collapsed point-defect clusters is demonstrated unequivocally to be temperature dependent, remaining approximately constant up to lattice temperatures of 573 K and then abruptly decreasing with increasing temperature. This drop in yield is not caused by defect loss during or following ion irradiation. This temperature dependence can be explained by a thermal spike effect. These in-situ yield measurements are compared to previous ex-situ yield measurements in fcc-Ni and bcc-Mo.
Modeling of the magnetic free energy of self-diffusion in bcc Fe
NASA Astrophysics Data System (ADS)
Sandberg, N.; Chang, Z.; Messina, L.; Olsson, P.; Korzhavyi, P.
2015-11-01
A first-principles based approach to calculating self-diffusion rates in bcc Fe is discussed with particular focus on the magnetic free energy associated with diffusion activation. First, the enthalpies and entropies of vacancy formation and migration in ferromagnetic bcc Fe are calculated from standard density functional theory methods in combination with transition state theory. Next, the shift in diffusion activation energy when going from the ferromagnetic to the paramagnetic state is estimated by averaging over random spin states. Classical and quantum mechanical Monte Carlo simulations within the Heisenberg model are used to study the effect of spin disordering on the vacancy formation and migration free energy. Finally, a quasiempirical model of the magnetic contribution to the diffusion activation free energy is applied in order to connect the current first-principles results to experimental data. The importance of the zero-point magnon energy in modeling of diffusion in bcc Fe is stressed.
Application of STEM characterization for investigating radiation effects in BCC Fe-based alloys
Parish, Chad M.; Field, Kevin G.; Certain, Alicia G.; Wharry, Janelle P.
2015-04-20
This paper provides a general overview of advanced scanning transmission electron microscopy (STEM) techniques used for characterization of irradiated BCC Fe-based alloys. Advanced STEM methods provide the high-resolution imaging and chemical analysis necessary to understand the irradiation response of BCC Fe-based alloys. The use of STEM with energy dispersive x-ray spectroscopy (EDX) for measurement of radiation-induced segregation (RIS) is described, with an illustrated example of RIS in proton- and self-ion irradiated T91. Aberration-corrected STEM-EDX for nanocluster/nanoparticle imaging and chemical analysis is also discussed, and examples are provided from ion-irradiated oxide dispersion strengthened (ODS) alloys. In conclusion, STEM techniques for void, cavity, and dislocation loop imaging are described, with examples from various BCC Fe-based alloys.
Kinetics of disorder-to-fcc phase transition via an intermediate bcc state
Liu Yongsheng; Nie Huifen; Bansil, Rama; Steinhart, Milos; Bang, Joona; Lodge, Timothy P.
2006-06-15
Time-resolved small-angle x-ray scattering measurements reveal that a long-lived intermediate bcc state forms when a poly(styrene-b-isoprene) diblock copolymer solution in an isoprene selective solvent is rapidly cooled from the disordered micellar fluid at high temperature to an equilibrium fcc state. The kinetics of the epitaxial growth of the [111] fcc peak from the [110] bcc peak was obtained by fitting the scattering data to a simple model of the transformation. The growth of the [111] fcc peak agrees with the Avrami model of nucleation and growth kinetics with an exponent n=1.4, as does the initial decay of the [110] bcc peak, with an exponent n=1.3. The data were also found to be in good agreement with the Cahn model of grain boundary nucleation and growth.
Application of STEM characterization for investigating radiation effects in BCC Fe-based alloys
Parish, Chad M.; Field, Kevin G.; Certain, Alicia G.; Wharry, Janelle P.
2015-04-20
This paper provides a general overview of advanced scanning transmission electron microscopy (STEM) techniques used for characterization of irradiated BCC Fe-based alloys. Advanced STEM methods provide the high-resolution imaging and chemical analysis necessary to understand the irradiation response of BCC Fe-based alloys. The use of STEM with energy dispersive x-ray spectroscopy (EDX) for measurement of radiation-induced segregation (RIS) is described, with an illustrated example of RIS in proton- and self-ion irradiated T91. Aberration-corrected STEM-EDX for nanocluster/nanoparticle imaging and chemical analysis is also discussed, and examples are provided from ion-irradiated oxide dispersion strengthened (ODS) alloys. In conclusion, STEM techniques for void,more » cavity, and dislocation loop imaging are described, with examples from various BCC Fe-based alloys.« less
Random-field Ising model on isometric lattices: Ground states and non-Porod scattering
NASA Astrophysics Data System (ADS)
Bupathy, Arunkumar; Banerjee, Varsha; Puri, Sanjay
2016-01-01
We use a computationally efficient graph cut method to obtain ground state morphologies of the random-field Ising model (RFIM) on (i) simple cubic (SC), (ii) body-centered cubic (BCC), and (iii) face-centered cubic (FCC) lattices. We determine the critical disorder strength Δc at zero temperature with high accuracy. For the SC lattice, our estimate (Δc=2.278 ±0.002 ) is consistent with earlier reports. For the BCC and FCC lattices, Δc=3.316 ±0.002 and 5.160 ±0.002 , respectively, which are the most accurate estimates in the literature to date. The small-r behavior of the correlation function exhibits a cusp regime characterized by a cusp exponent α signifying fractal interfaces. In the paramagnetic phase, α =0.5 ±0.01 for all three lattices. In the ferromagnetic phase, the cusp exponent shows small variations due to the lattice structure. Consequently, the interfacial energy Ei(L ) for an interface of size L is significantly different for the three lattices. This has important implications for nonequilibrium properties.
Ab initio study of the anharmonic lattice dynamics of iron at the γ -δ phase transition
NASA Astrophysics Data System (ADS)
Lian, Chao-Sheng; Wang, Jian-Tao; Chen, Changfeng
2015-11-01
We report calculations of phonon dispersions of iron (Fe) at its γ -δ phase transition using a self-consistent ab initio lattice dynamical method in conjunction with an effective magnetic force approach via the antiferromagnetic approximation. Our results show that anharmonic phonon-phonon interactions play a crucial role in stabilizing the δ -Fe phase in the open bcc lattice. In contrast, the lattice dynamics of the close-packed fcc γ -Fe phase are dominated by magnetic interactions. Simultaneous considerations of the lattice anharmonic and magnetic interactions produced temperature-dependent phonon dispersions for δ -Fe and γ -Fe phases in excellent agreement with recent experimental measurements. The present results highlight the key role of lattice anharmonicity in determining the structural stability of iron at high temperatures, which has significant implications for other high-temperature paramagnetic metals like Ce and Pu.
Lattice gas and lattice Boltzmann computational physics
Chen, S.
1993-05-01
Recent developments of the lattice gas automata method and its extension to the lattice Boltzmann method have provided new computational schemes for solving a variety of partial differential equations and modeling different physics systems. The lattice gas method, regarded as the simplest microscopic and kinetic approach which generates meaningful macroscopic dynamics, is fully parallel and can be easily programmed on parallel machines. In this talk, the author will review basic principles of the lattice gas and lattice Boltzmann method, its mathematical foundation and its numerical implementation. A detailed comparison of the lattice Boltzmann method with the lattice gas technique and other traditional numerical schemes, including the finite-difference scheme and the pseudo-spectral method, for solving the Navier-Stokes hydrodynamic fluid flows, will be discussed. Recent achievements of the lattice gas and the the lattice Boltzmann method and their applications in surface phenomena, spinodal decomposition and pattern formation in chemical reaction-diffusion systems will be presented.
Fermi Surface of Nanocrystalline Embedded Particles in Materials: bcc Cu in Fe
NASA Astrophysics Data System (ADS)
Nagai, Y.; Chiba, T.; Tang, Z.; Akahane, T.; Kanai, T.; Hasegawa, M.; Takenaka, M.; Kuramoto, E.
2001-10-01
We report that a positron can act as a probe to directly reveal electronic structures of nanocrystalline embedded particles in materials. The Fermi surface (FS) of ``bcc'' Cu nanoparticles in an Fe matrix is observed as the first example. A two-dimensional angular correlation of the positron annihilation radiation (2D-ACAR) method is used to measure the momentum distribution which reflects the FS topology. The obtained 2D-ACAR spectra show strong and characteristic anisotropy associated with the necks of the FS around the \\{110\\} Brillouin zone boundaries of the bcc Cu, which are well reproduced by full-potential linearized argumented plane-wave calculations.
Superconductivity of BCC Zr and Zr-Nb alloys at high pressures
NASA Astrophysics Data System (ADS)
Ponyatovsky, E. G.; Bashkin, I. O.; Tissen, V. G.; Nefedova, M. V.
2009-09-01
The superconducting transition temperature T c of bcc Zr is measured at pressures to 64 GPa. The T c value gradually decreases as pressure is increased. For the Zr-Nb alloys, there are found anomalies in the T c ( P) dependences at rather low pressures. The anomalies are discussed within the available theoretical models. We assume on the basis of the T c ( P) experimental data for the Zr-Nb alloys that the T c ( P) curve for bcc zirconium has a maximum in the metastability region.
ERIC Educational Resources Information Center
Scott, Paul
2006-01-01
A lattice is a (rectangular) grid of points, usually pictured as occurring at the intersections of two orthogonal sets of parallel, equally spaced lines. Polygons that have lattice points as vertices are called lattice polygons. It is clear that lattice polygons come in various shapes and sizes. A very small lattice triangle may cover just 3…
Antitubercular Lanostane Triterpenes from Cultures of the Basidiomycete Ganoderma sp. BCC 16642.
Isaka, Masahiko; Chinthanom, Panida; Sappan, Malipan; Danwisetkanjana, Kannawat; Boonpratuang, Thitiya; Choeyklin, Rattaket
2016-01-22
Sixteen new lanostane triterpenoids (1-16), together with 26 known compounds (17-42), were isolated from cultures of the basidiomycete Ganoderma sp. BCC 16642. Antitubercular activities of these Ganoderma lanostanoids against Mycobacterium tuberculosis H37Ra were evaluated, and structure-activity relationships are proposed. PMID:26716912
Boron diffusion in bcc-Fe studied by first-principles calculations
NASA Astrophysics Data System (ADS)
Xianglong, Li; Ping, Wu; Ruijie, Yang; Dan, Yan; Sen, Chen; Shiping, Zhang; Ning, Chen
2016-03-01
The diffusion mechanism of boron in bcc-Fe has been studied by first-principles calculations. The diffusion coefficients of the interstitial mechanism, the B-monovacancy complex mechanism, and the B-divacancy complex mechanism have been calculated. The calculated diffusion coefficient of the interstitial mechanism is D0 = 1.05 × 10-7 exp (-0.75 eV/kT) m2 · s-1, while the diffusion coefficients of the B-monovacancy and the B-divacancy complex mechanisms are D1 = 1.22 × 10-6 f1 exp (-2.27 eV/kT) m2 · s-1 and D2 ≈ 8.36 × 10-6 exp (-4.81 eV/kT) m2 · s-1, respectively. The results indicate that the dominant diffusion mechanism in bcc-Fe is the interstitial mechanism through an octahedral interstitial site instead of the complex mechanism. The calculated diffusion coefficient is in accordance with the reported experiment results measured in Fe-3%Si-B alloy (bcc structure). Since the non-equilibrium segregation of boron is based on the diffusion of the complexes as suggested by the theory, our calculation reasonably explains why the non-equilibrium segregation of boron is not observed in bcc-Fe in experiments. Project supported by the National Natural Science Foundation of China (Grant No. 51276016) and the National Basic Research Program of China (Grant No. 2012CB720406).
Solubility of hydrogen and deuterium in bcc-uranium-titanium alloys
Powell, G.L.; Kirkpatrick, J.R.
1996-08-05
For the bcc-U-Ti alloy system, H and D solubility measurements have been made on 12 alloy specimens ranging in composition from pure U to pure Ti and temperature range bounded by 900 K to 1,500 K. The results are described by a model within a standard error of 3%.
Crystallization pathways of liquid-bcc transition for a model iron by fast quenching
Pan, Shao-Peng; Feng, Shi-Dong; Qiao, Jun-Wei; Wang, Wei-Min; Qin, Jing-Yu
2015-01-01
We report simulations on the local structural evolution in the liquid-bcc transition of a model iron. Fourteen main Voronoi polyhedra are chosen as the representatives of short-range orders (SROs) and their transformations during crystallization are also investigated. Thus, the crystallization pathways for the main SROs are drawn. Our results also show that the transformations between two SROs in the crystallization pathways can be classified into two categories, first the enlargement of coordination number, second the transformation of local symmetry from five-fold to four-fold. The former reduces the potential energy while the latter increases it. It is found that the potential energy cannot decease monotonously whatever crystallization pathway is chosen to transform the icosahedral SRO to bcc SRO. Therefore, the latter transformation might provide the energy barrier of crystallization. We propose two transformation styles among SROs. All the transformations in the crystallization pathways can be achieved according to the styles. Moreover, the two transformation styles indicates that the bcc structure is more similar to liquid than other crystals. That might be the reason why the first phase nucleated during a rapid cooling process should be bcc crystal. PMID:26581525
Multilayer Relaxation and Surface Energies of FCC and BCC Metals Using Equivalent Crystal Theory
NASA Technical Reports Server (NTRS)
Rodriguez, Agustin M.; Bozzolo, Guillermo; Ferrante, John
1993-01-01
The multilayer relaxation of fcc and bcc metal surfaces is calculated using equivalent crystal theory. The results for changes in interplanar spacings of planes close to the surface and the ensuing surface energies are discussed in reference to other theoretical results and compared to available experimental data. The calculation includes high-index surfaces for which no other theoretical results are known.
NASA Astrophysics Data System (ADS)
Tang, Zhi; Gao, Michael C.; Diao, Haoyan; Yang, Tengfei; Liu, Junpeng; Zuo, Tingting; Zhang, Yong; Lu, Zhaoping; Cheng, Yongqiang; Zhang, Yanwen; Dahmen, Karin A.; Liaw, Peter K.; Egami, Takeshi
2013-12-01
The crystal lattice type is one of the dominant factors for controlling the mechanical behavior of high-entropy alloys (HEAs). For example, the yield strength at room temperature varies from 300 MPa for the face-centered-cubic (fcc) structured alloys, such as the CoCrCuFeNiTi x system, to about 3,000 MPa for the body-centered-cubic (bcc) structured alloys, such as the AlCoCrFeNiTi x system. The values of Vickers hardness range from 100 to 900, depending on lattice types and microstructures. As in conventional alloys with one or two principal elements, the addition of minor alloying elements to HEAs can further alter their mechanical properties, such as strength, plasticity, hardness, etc. Excessive alloying may even result in the change of lattice types of HEAs. In this report, we first review alloying effects on lattice types and properties of HEAs in five Al-containing HEA systems: Al x CoCrCuFeNi, Al x CoCrFeNi, Al x CrFe1.5MnNi0.5, Al x CoCrFeNiTi, and Al x CrCuFeNi2. It is found that Al acts as a strong bcc stabilizer, and its addition enhances the strength of the alloy at the cost of reduced ductility. The origins of such effects are then qualitatively discussed from the viewpoints of lattice-strain energies and electronic bonds. Quantification of the interaction between Al and 3 d transition metals in fcc, bcc, and intermetallic compounds is illustrated in the thermodynamic modeling using the CALculation of PHAse Diagram method.
NASA Astrophysics Data System (ADS)
Ashokan, V.; Abou Ghantous, M.; Khater, A.
2015-12-01
Ferromagnetic nanojunctions … Fe[Fe1-cCoc ] ℓ Fe …, with ℓ is the number of layers which constitute the nanojunction, based on Fe/Co alloy are considered for the first time in this work. We model the salient magnetic properties of the layered ferromagnetic nanostructures between magnetically ordered iron leads. The effective field theory (EFT) Ising spin method is used to compute reliable Jav exchange values for the VCA Fe/Co alloy materials in comparison with experimental data and compared to existing DFT calculated exchange interactions. The new set of exchange interaction values between pairs of nearest neighbors atom in the alloy are deduced and agree with previous known measurement of lattice constant for this alloy. Using the combined EFT and mean field theory (MFT) spin methods, the sublattice magnetizations of the Fe and Co sites on the individual bcc basal planes of the layered nanostructures, are calculated and analyzed. The sublattice magnetizations, effective magnetic moments per site, and the possible ferromagnetic order of the layers [Fe1-cCoc ] ℓ on the individual bcc atomic planes of the embedded nanostructures for all temperatures and in particular for TcFe ≤ T ≤Tα→γ are presented as a function of temperature and thicknesses of the layered ferromagnetic nanostructures, for different stable concentrations c=0.25, 0.5 and 0.75. In the absence of first principles calculations for these basic physical variables for the layered nanostructures between iron leads, the combined EFT and MFT approach yields the only available information for them at present in the absence of a possible Curie temperature for these alloys. These variables are necessary for certain spin dynamic computations, as for the ballistic magnon transport across embedded nanojunctions in magnonics. The model is general, and may applied directly to other composite magnetic elements and embedded nanostructures.
Twin nucleation in Fe-based bcc alloys—modeling and experiments
NASA Astrophysics Data System (ADS)
Ojha, A.; Sehitoglu, H.; Patriarca, L.; Maier, H. J.
2014-10-01
We develop an analytical expression for twin nucleation stress in bcc metal and alloys considering generalized planar fault energy and the dislocations bounding the twin nucleus. We minimize the total energy to predict the twinning stress relying only on parameters that are obtained through atomistic calculations, thus excluding the need for any empirical constants. We validate the present approach by means of precise measurements of the onset of twinning in bcc Fe-50at% Cr single crystals showing excellent agreement. The experimental observations of the three activated slip systems of symmetric configuration in relation to the twinning mechanism are demonstrated via transmission electron microscopy techniques along with digital image correlation. We then confirm the validity of the model for Fe, Fe-25at% Ni and Fe-3at% V alloys compared with experiments from the literature to show general applicability.
NASA Astrophysics Data System (ADS)
Singh, D.; Clougherty, D. P.; MacLaren, J. M.; Albers, R. C.; Wang, C. S.
1991-10-01
The influence of local-spin-dependent correlation effects on the predicted stable ground-state phase of iron is reexamined with use of general-potential linearized augmented-plane-wave calculations. Differences in the form of the Vosko-Wilk-Nusair (VWN) local-spin-density functional used in previous studies are noted, since in previous studies significant additional approximations were made with respect to those of Vosko, Wilk, and Nusan [Can. J. Phys. 58, 1200 (1980)] and of MacLaren, Clougherty, and Albers [Phys. Rev. B 42, 3205 (1990)]. While the results of previous linear muffin-tin orbital calculations using the VWN functional predict a bcc ferromagnetic ground state, the present calculations show that the VWN spin-correlation effects fail to stabilize a bcc ground state. Considerable sensitivity to the form of the spin interpolation is found.
Topology of the spin-polarized charge density in bcc and fcc iron.
Jones, Travis E; Eberhart, Mark E; Clougherty, Dennis P
2008-01-11
We report the first investigation of the topology of spin-polarized charge density, specifically in bcc and fcc iron. While the total spin-density is found to possess the topology of the non-magnetic prototypical structures, the spin-polarized charge densities of bcc and high-spin fcc iron are atypical. In these cases, the two spin densities are correlated: the spin-minority electrons have directional bond paths and deep minima, while the spin-majority electrons fill these holes, reducing bond directionality. The presence of distinct spin topologies allows us to show that the two phase changes seen in fcc iron (paramagnetic to low-spin and low-spin to high-spin) are different. The former follows the Landau symmetry-breaking paradigm and proceeds without a topological transformation, while the latter involves a topological catastrophe. PMID:18232817
Topology of the Spin-Polarized Charge Density in bcc and fcc Iron
NASA Astrophysics Data System (ADS)
Jones, Travis E.; Eberhart, Mark E.; Clougherty, Dennis P.
2008-01-01
We report the first investigation of the topology of spin-polarized charge density, specifically in bcc and fcc iron. While the total spin-density is found to possess the topology of the non-magnetic prototypical structures, the spin-polarized charge densities of bcc and high-spin fcc iron are atypical. In these cases, the two spin densities are correlated: the spin-minority electrons have directional bond paths and deep minima, while the spin-majority electrons fill these holes, reducing bond directionality. The presence of distinct spin topologies allows us to show that the two phase changes seen in fcc iron (paramagnetic to low-spin and low-spin to high-spin) are different. The former follows the Landau symmetry-breaking paradigm and proceeds without a topological transformation, while the latter involves a topological catastrophe.
Giant in-plane magnetic anisotropy in epitaxial bcc Co/Fe(110) bilayers
NASA Astrophysics Data System (ADS)
Ślezak, M.; Ślezak, T.; Matlak, K.; Matlak, B.; DróŻdŻ, P.; Giela, T.; Wilgocka-Ślezak, D.; Pilet, N.; Raabe, J.; Kozioł-Rachwał, A.; Korecki, J.
2016-07-01
We report on in-plane magnetic anisotropy in epitaxial bcc Co/Fe(110) bilayers on W(110). The magnetic surface anisotropy in the Co/Fe(110) bilayers exhibited a strong nonmonotonic dependence on Co coverage. Magneto-optical studies revealed a sharp maximum of the magnetic surface anisotropy, 2.44 mJ /m2 , at dCo=5 Å . This giant interfacial magnetic anisotropy allowed a small fraction of a Co monolayer to reorient the magnetization of the bulk-like Fe film. We conclude that the mono- and double-layer bcc Co(110) exhibited in-plane magnetic anisotropy with a [1 1 ¯0 ] easy axis.
Multi-scale modeling of the iron bcc arrow hcp martensitic phase transformation
NASA Astrophysics Data System (ADS)
Caspersen, Kyle; Carter, Emily; Lew, Adrian; Ortiz, Michael
2004-03-01
Pressures exceeding 10 GPa induce a martensitic phase transformation in iron, where ferro-magnetic bcc transforms into non-magnetic hcp. The transition pressure is not known precisely, but is thought to depend strongly on shear. To investigate the properties of this transformation and the role of shear, we have developed a multi-scale iron model. This model contains a free energy derived from an ab-initio based non-linear elastic expansion, a kinematically compatible spinodal decomposition of phases, ab-initio based interfacial energies, and a dependence on the bcc rightarrow hcp transformation path(s). The model shows spinodal decomposition behavior (with a slight expected deviation) as well as predicting 10 GPa to be the transformation pressure. Additionally, the model predicted that the inclusion of shear facilitates the transformation, causing transformation pressure to decrease.
Importance of Shear in the bcc-to-hcp Transformation in Iron
NASA Astrophysics Data System (ADS)
Caspersen, Kyle J.; Lew, Adrian; Ortiz, Michael; Carter, Emily A.
2004-09-01
Iron shows a pressure-induced martensitic phase transformation from the ground state ferromagnetic bcc phase to a nonmagnetic hcp phase at ≈13 GPa. The exact transformation pressure (TP) and pathway are not known. Here we present a multiscale model containing a quantum-mechanics-based multiwell energy function accounting for the bcc and hcp phases of Fe and a construction of kinematically compatible and equilibrated mixed phases. This model suggests that shear stresses have a significant influence on the bcc↔hcp transformation. In particular, the presence of modest shear accounts for the scatter in measured TPs. The formation of mixed phases also provides an explanation for the observed hysteresis in TP.
Pressure Induced Metal-Nonmetal and FCC-BCC Transitions in Calcium*
NASA Astrophysics Data System (ADS)
Wang, G. M.; Blaisten-Barojas, E.; Papaconstantopoulos, D. A.
2001-04-01
The band structure of fcc and bcc calcium at different densities is obtained with the Augmented Plane Wave (APW) method using a soft-core approximation and Gaspar-Kohn-Sham potential. A tight-binding(TB) model is then built successfully to reproduce the first principles band structure and density of states. Properties examined within TB include bulk modulus, elastic constants, metal-nonmetal transition and fcc to bcc structural transition under pressure. Results are in an excellent agreement with experimental observations. Several dynamical properties of calcium under pressure are further explored with TB molecular dynamics at finite temperature. *Work supported in part by the Office of Naval Research grant N00014-98-1-0832
Yield Functions and Plastic Potentials for BCC Metals and Possibly Other Materials
Christensen, R M
2005-09-29
Yield functions and plastic potentials are expressed in terms of the invariants of the stress tensor for polycrystalline metals and other isotropic materials. The plastic volume change data of Richmond is used to evaluate the embedded materials properties for some bcc metals and one polymer. A general form for the plastic potential is found that is intended to represent and cover a wide range of materials types.
Gerberich, W.W.
1992-12-31
Objective was to study fatigue where a combination of low temperature and cyclic loading produced cyclic cleavage in bcc Fe-base systems. Both dislocation dynamics and quasi-statics of crack growth were probed. This document reviews progress over the past 6 years: hydrogen embrittlement and cleavage, computations (stress near crack tip), dislocation emission from grain boundaries, fracture process zones, and understanding brittle fracture at the atomistic/dislocation scales and at the microscopic/macroscopic scale.
[A microstructural approach to fatigue crack processes in poly crystalline BCC materials
Gerberich, W.W.
1992-01-01
Objective was to study fatigue where a combination of low temperature and cyclic loading produced cyclic cleavage in bcc Fe-base systems. Both dislocation dynamics and quasi-statics of crack growth were probed. This document reviews progress over the past 6 years: hydrogen embrittlement and cleavage, computations (stress near crack tip), dislocation emission from grain boundaries, fracture process zones, and understanding brittle fracture at the atomistic/dislocation scales and at the microscopic/macroscopic scale.
A low-surface energy carbon allotrope: the case for bcc-C6.
Yin, Wen-Jin; Chen, Yuan-Ping; Xie, Yue-E; Liu, Li-Min; Zhang, S B
2015-06-01
Graphite may be viewed as a low-surface-energy carbon allotrope with little layer-layer interaction. Other low-surface-energy allotropes but with much stronger layer-layer interaction may also exist. Here, we report a first-principles prediction for one of the known carbon allotropes, bcc-C6 (a body centered carbon allotrope with six atoms per primitive unit), that should have exceptionally low-surface energy and little size dependence down to only a couple layer thickness. This unique property may explain the existence of the relatively-high-energy bcc-C6 during growth. The electronic properties of the bcc-C6 thin layers can also be intriguing: the (111), (110), and (001) thin layers have direct band gap, indirect band gap, and metallic character, respectively. The refrained chemical reactivity of the thin layers does not disappear after cleaving, as lithium-doped (Li-doped) 3-layers (111) has a noticeably increased binding energy of H2 molecules with a maximum storage capacity of 10.8 wt%. PMID:25959535
Palmer, R.B.
1987-05-01
This paper looks at, and compares three types of damping ring lattices: conventional, wiggler lattice with finite ..cap alpha.., wiggler lattice with ..cap alpha.. = 0, and observes the attainable equilibrium emittances for the three cases assuming a constraint on the attainable longitudinal impedance of 0.2 ohms. The emittance obtained are roughly in the ratio 4:2:1 for these cases.
Mechanism of Void Nucleation and Growth in bcc Fe: Atomistic Simulations at Experimental Time Scales
Fan Yue; Kushima, Akihiro; Yip, Sidney; Yildiz, Bilge
2011-03-25
Evolution of small-vacancy clusters in bcc Fe is simulated using a multiscale approach coupling an atomistic activation-relaxation method for sampling transition-state pathways with environment-dependent reaction coordinate calculations and a kinetic Monte Carlo simulation to reach time scales on the order of {approx}10{sup 4} s. Under vacancy-supersaturated condition, di- and trivacancy clusters form and grow by coalescence (Ostwald ripening). For cluster size greater than four we find a transition temperature of 150 deg. C for accelerated cluster growth, as observed in positron annihilation spectroscopy experiments. Implications for the mechanism of stage-IV radiation-damage-recovery kinetics are discussed.
Heats of Segregation of BCC Binaries from Ab Initio and Quantum Approximate Calculations
NASA Technical Reports Server (NTRS)
Good, Brian S.
2003-01-01
We compare dilute-limit segregation energies for selected BCC transition metal binaries computed using ab initio and quantum approximate energy methods. Ab initio calculations are carried out using the CASTEP plane-wave pseudopotential computer code, while quantum approximate results are computed using the Bozzolo-Ferrante-Smith (BFS) method with the most recent parameters. Quantum approximate segregation energies are computed with and without atomistic relaxation. Results are discussed within the context of segregation models driven by strain and bond-breaking effects. We compare our results with full-potential quantum calculations and with available experimental results.
Heats of Segregation of BCC Binaries from ab Initio and Quantum Approximate Calculations
NASA Technical Reports Server (NTRS)
Good, Brian S.
2004-01-01
We compare dilute-limit heats of segregation for selected BCC transition metal binaries computed using ab initio and quantum approximate energy methods. Ab initio calculations are carried out using the CASTEP plane-wave pseudopotential computer code, while quantum approximate results are computed using the Bozzolo-Ferrante-Smith (BFS) method with the most recent LMTO-based parameters. Quantum approximate segregation energies are computed with and without atomistic relaxation, while the ab initio calculations are performed without relaxation. Results are discussed within the context of a segregation model driven by strain and bond-breaking effects. We compare our results with full-potential quantum calculations and with available experimental results.
A numerical study of crack initiation in a bcc iron system based on dynamic bifurcation theory
NASA Astrophysics Data System (ADS)
Li, Xiantao
2014-10-01
Crack initiation under dynamic loading conditions is studied under the framework of dynamic bifurcation theory. An atomistic model for BCC iron is considered to explicitly take into account the detailed molecular interactions. To understand the strain-rate dependence of the crack initiation process, we first obtain the bifurcation diagram from a computational procedure using continuation methods. The stability transition associated with a crack initiation, as well as the connection to the bifurcation diagram, is studied by comparing direct numerical results to the dynamic bifurcation theory [R. Haberman, SIAM J. Appl. Math. 37, 69-106 (1979)].
A numerical study of crack initiation in a bcc iron system based on dynamic bifurcation theory
Li, Xiantao
2014-10-28
Crack initiation under dynamic loading conditions is studied under the framework of dynamic bifurcation theory. An atomistic model for BCC iron is considered to explicitly take into account the detailed molecular interactions. To understand the strain-rate dependence of the crack initiation process, we first obtain the bifurcation diagram from a computational procedure using continuation methods. The stability transition associated with a crack initiation, as well as the connection to the bifurcation diagram, is studied by comparing direct numerical results to the dynamic bifurcation theory [R. Haberman, SIAM J. Appl. Math. 37, 69–106 (1979)].
Haritakun, Rachada; Sappan, Malipan; Suvannakad, Rapheephat; Tasanathai, Kanoksri; Isaka, Masahiko
2010-01-01
A novel cyclodepsipeptide, cordycommunin (1), and two dihydroisocoumarins (2 and 3) were isolated from the insect pathogenic fungus Ophiocordyceps communis BCC 16475. The absolute configurations of the amino acid residues of 1 were addressed by application of Marfey's method. Cordycommunin (1) showed growth inhibition of Mycobacterium tuberculosis H37Ra with an MIC value of 15 microM. This compound also exhibited weak cytotoxicity to KB cells with an IC50 of 45 microM, while it was inactive against BC, NCI-H187, and Vero cell lines at a concentration of 88 microM (50 microg/mL). PMID:20028029
Dynamics of the BCC-HCP transition of solid 4He
NASA Astrophysics Data System (ADS)
Fujii, Hirosuke; Okumura, Yuji; Matsumoto, Koichi; Okuda, Yuichi
2000-07-01
We are optically studying the phase transition from BCC to HCP of solid 4He (1.43 K) at melting pressure to investigate how the crystal changes its structure when it is in equilibrium with superfluid. When the temperature was changed slowly through the transition, new phase seeds were nucleated somewhere and those seeds grew in the superfluid phase. The interesting point is that growth of the new seeds is more favorable than the direct solid-solid Martensitic transition. Whereas at the transition of 1.78 K where the solid is in equilibrium with the normal fluid, the transition occurred as an ordinary Martensitic transition.
Modelling irradiation-induced softening in BCC iron by crystal plasticity approach
NASA Astrophysics Data System (ADS)
Xiao, Xiazi; Terentyev, Dmitry; Yu, Long; Song, Dingkun; Bakaev, A.; Duan, Huiling
2015-11-01
Crystal plasticity model (CPM) for BCC iron to account for radiation-induced strain softening is proposed. CPM is based on the plastically-driven and thermally-activated removal of dislocation loops. Atomistic simulations are applied to parameterize dislocation-defect interactions. Combining experimental microstructures, defect-hardening/absorption rules from atomistic simulations, and CPM fitted to properties of non-irradiated iron, the model achieves a good agreement with experimental data regarding radiation-induced strain softening and flow stress increase under neutron irradiation.
Simulation of He embrittlement at grain boundaries in bcc transition metals
NASA Astrophysics Data System (ADS)
Suzudo, Tomoaki; Yamaguchi, Masatake
2015-10-01
To investigate what atomic properties largely determine vulnerability to He embrittlement at grain boundaries (GB) of bcc metals, we introduce a computational model composed of first principles density functional theory and a He segregation rate theory model. Predictive calculations of He embrittlement at the first wall of the future DEMO fusion concept reactor indicate that variation in the He embrittlement originated not only from He production rate related to neutron irradiation, but also from the He segregation energy at the GB that has a systematic trend in the periodic table.
NASA Astrophysics Data System (ADS)
Zuo, Jinqing; Ren, Hong-Li; Wu, Jie; Nie, Yu; Li, Qiaoping
2016-09-01
The subseasonal variability and predictability of the Arctic Oscillation/North Atlantic Oscillation (AO/NAO) is evaluated using a full set of hindcasts generated from the Beijing Climate Center Atmospheric General Circulation Model version 2.2 (BCC_AGCM2.2). It is shown that the predictability of the monthly mean AO/NAO index varies seasonally, with the highest predictability during winter (December-March) and the lowest during autumn (August-November), with respect to both observations and BCC_AGCM2.2 results. As compared with the persistence prediction skill of observations, the model skillfully predicts the monthly mean AO/NAO index with a one-pentad lead time during all winter months, and with a lead time of up to two pentads in December and January. During winter, BCC_AGCM2.2 exhibits an acceptable skill in predicting the daily AO/NAO index of ∼9 days, which is higher than the persistence prediction skill of observations of ∼4 days. Further analysis suggests that improvements in the simulation of storm track activity, synoptic eddy feedback, and troposphere-stratosphere coupling in the Northern Hemisphere could help to improve the prediction skill of subseasonal AO/NAO variability by BCC_AGCM2.2 during winter. In particular, BCC_AGCM2.2 underestimates storm track activity intensity but overestimates troposphere-stratosphere coupling, as compared with observations, thus providing a clue to further improvements in model performance.
Kim, Mi-Sun; Jang, Jichan; AB Rahman, Nurlilah Binte; Pethe, Kevin; Berry, Edward A.; Huang, Li-Shar
2015-01-01
Recently, energy production pathways have been shown to be viable antitubercular drug targets to combat multidrug-resistant tuberculosis and eliminate pathogen in the dormant state. One family of drugs currently under development, the imidazo[1,2-a]pyridine derivatives, is believed to target the pathogen's homolog of the mitochondrial bc1 complex. This complex, denoted cytochrome bcc, is highly divergent from mitochondrial Complex III both in subunit structure and inhibitor sensitivity, making it a good target for drug development. There is no soluble cytochrome c in mycobacteria to transport electrons from the bcc complex to cytochrome oxidase. Instead, the bcc complex exists in a “supercomplex” with a cytochrome aa3-type cytochrome oxidase, presumably allowing direct electron transfer. We describe here purification and initial characterization of the mycobacterial cytochrome bcc-aa3 supercomplex using a strain of M. smegmatis that has been engineered to express the M. tuberculosis cytochrome bcc. The resulting hybrid supercomplex is stable during extraction and purification in the presence of dodecyl maltoside detergent. It is hoped that this purification procedure will potentiate functional studies of the complex as well as crystallographic studies of drug binding and provide structural insight into a third class of the bc complex superfamily. PMID:25861988
Janse Van Rensburg, E.J.
1996-12-31
The geometry of polygonal knots in the cubic lattice may be used to define some knot invariants. One such invariant is the minimal edge number, which is the minimum number of edges necessary (and sufficient) to construct a lattice knot of given type. In addition, one may also define the minimal (unfolded) surface number, and the minimal (unfolded) boundary number; these are the minimum number of 2-cells necessary to construct an unfolded lattice Seifert surface of a given knot type in the lattice, and the minimum number of edges necessary in a lattice knot to guarantee the existence of an unfolded lattice Seifert surface. In addition, I derive some relations amongst these invariants. 8 refs., 5 figs., 2 tabs.
Lattice Dynamical Interpretation of the Structure of \\zeta-Phase AgZn
NASA Astrophysics Data System (ADS)
Yamada, Yasusada; Noda, Yukio
1988-04-01
The crystal structure of \\zeta-phase AgZn has been reconsidered. It is shown that the static structure of \\zeta-AgZn is expressible in terms of (i) two types of lattice waves (phonon modes) and (ii) one type of probability density wave of Zn atom site-occupation. It is noted that the relevant phonon modes are the soft modes existing commonly in bcc Hume-Rothery alloys. It is pointed out that \\zeta-AgZn shares the common origin of stabilization with 9R martensites and ω-phase.
Zarkevich, N. A.; Johnson, D. D.
2015-05-12
We revisit results from decades of pressure experiments on the bcc ↔ hcp transformations in iron, which are sensitive to non-hydrostatic conditions and sample size. We emphasize the role of martensitic stress in the observed pressure hysteresis and address the large spread in values for onset pressures of the nucleating phase. From electronic-structure calculations, we find a bcc ↔ hcp equilibrium coexistence pressure of 8.4 GPa. Accounting for non-hydrostatic martensitic stress and a stress-dependent transition barrier, we suggest a pressure inequality for better comparison to experiment and observed hysteresis. We construct the equation of state for bcc and hcp phases under hydrostatic pressure, and compare to experiments and previous calculations.
Zarkevich, N. A.; Johnson, D. D.
2015-05-12
We revisit results from decades of pressure experiments on the bcc ↔ hcp transformations in iron, which are sensitive to non-hydrostatic conditions and sample size. We emphasize the role of martensitic stress in the observed pressure hysteresis and address the large spread in values for onset pressures of the nucleating phase. From electronic-structure calculations, we find a bcc ↔ hcp equilibrium coexistence pressure of 8.4 GPa. Accounting for non-hydrostatic martensitic stress and a stress-dependent transition barrier, we suggest a pressure inequality for better comparison to experiment and observed hysteresis. We construct the equation of state for bcc and hcp phasesmore » under hydrostatic pressure, and compare to experiments and previous calculations.« less
NASA Astrophysics Data System (ADS)
Liu, J. B.; Johnson, D. D.
2009-04-01
Using density-functional theory, we calculate the potential-energy surface (PES), minimum-energy pathway (MEP), and transition state (TS) versus hydrostatic pressure σhyd for the reconstructive transformation in Fe from body-centered cubic (bcc) to hexagonal closed-packed (hcp). At fixed σhyd , the PES is described by coupled shear (γ) and shuffle (η) modes and is determined from structurally minimized hcp-bcc energy differences at a set of (η,γ) . We fit the PES using symmetry-adapted polynomials, permitting the MEP to be found analytically. The MEP is continuous and fully explains the transformation and its associated magnetization and volume discontinuity at TS. We show that σhyd (while not able to induce shear) dramatically alters the MEP to drive reconstruction by a shuffle-only mode at ≤30GPa , as observed. Finally, we relate our polynomial-based results to Landau and nudge-elastic-band approaches and show they yield incorrect MEP in general.
Ab initio guided design of bcc Mg-Li alloys for ultra light-weight applications
NASA Astrophysics Data System (ADS)
Friák, Martin; Counts, William Art; Raabe, Dierk; Neugebauer, Jörg
2009-03-01
Ab initio calculations are becoming increasingly useful to engineers interested in designing new alloys because these calculations are able to accurately predict basic material properties only knowing the atomic composition of the material. In this paper, fundamental physical properties (like formation energies and elastic constants) of 11 bcc Mg-Li compounds are calculated using density-functional theory (DFT) and compared with available experimental data. These DFT-determined properties are in turn used to calculate engineering parameters like (i) specific Young's modulus (Y/ρ) or (ii) bulk over shear modulus ratio (B/G) differentiating between brittle and ductile behavior. The engineering parameters are then used to identify alloys that have optimal mechanical properties needed for a light weight structural material. It was found that the stiffest bcc magnesium-lithium alloys contain about 70 at.% Mg while the most ductile alloys have 0-20 at.% Mg. The specific modulus for alloys with 70 at.% Mg is equal to that of Al-Mg alloys. An Ashby map containing Y/ρ vs. B/G shows that it is not possible to increase both Y/ρ and B/G by changing only the composition or local order of a binary alloy (W. A. Counts, M. Fri'ak, D. Raabe and J. Neugebauer, Acta Mater 57 (2009) 69-76).
Anisotropy analysis of the thermal stresses and strain energies in BCC metal films
NASA Astrophysics Data System (ADS)
Zhang, Jian-Min; Zhang, Yan; Xu, Ke-Wei
2005-11-01
A thin polycrystalline film tightly bonded to a thick substrate of different thermal expansion coefficients will experience thermal stresses when the temperature is changed in applications. Calculations of these stresses and the corresponding strain energies for grains having various crystallographic orientations (hkl) relative to the film surface were made for a polycrystalline film composed of the BCC metal Cr, Fe, K, Li, Mo, Na, Nb, Ta, V and W, respectively. Neglecting W, which is isotropic, the stresses and strain energy densities are not grain orientation (hkl) dependence. The remaining BCC metals may be grouped into two classes. In the first group (Cr, Mo, Nb and V), the (1 0 0)-oriented grains have the highest stresses and strain energy densities and (1 1 1)-oriented grains the lowest, the stresses σ1 and σ2 in the film surface and strain energy densities in other (hkl)-oriented grains decrease linearly with increasing angle between (hkl) and (1 0 0), and have σ1<σ2 excepting in (1 0 0)- and (1 1 1)-oriented grains. The contrary conclusions are obtained for the second group (Fe, K, Li, Na and Ta).
Microscopic Origin of Heisenberg and Non-Heisenberg Exchange Interactions in Ferromagnetic bcc Fe
NASA Astrophysics Data System (ADS)
Kvashnin, Y. O.; Cardias, R.; Szilva, A.; Di Marco, I.; Katsnelson, M. I.; Lichtenstein, A. I.; Nordström, L.; Klautau, A. B.; Eriksson, O.
2016-05-01
By means of first principles calculations, we investigate the nature of exchange coupling in ferromagnetic bcc Fe on a microscopic level. Analyzing the basic electronic structure reveals a drastic difference between the 3 d orbitals of Eg and T2 g symmetries. The latter ones define the shape of the Fermi surface, while the former ones form weakly interacting impurity levels. We demonstrate that, as a result of this, in Fe the T2 g orbitals participate in exchange interactions, which are only weakly dependent on the configuration of the spin moments and thus can be classified as Heisenberg-like. These couplings are shown to be driven by Fermi surface nesting. In contrast, for the Eg states, the Heisenberg picture breaks down since the corresponding contribution to the exchange interactions is shown to strongly depend on the reference state they are extracted from. Our analysis of the nearest-neighbor coupling indicates that the interactions among Eg states are mainly proportional to the corresponding hopping integral and thus can be attributed to be of double-exchange origin. By making a comparison to other magnetic transition metals, we put the results of bcc Fe into context and argue that iron has a unique behavior when it comes to magnetic exchange interactions.
Change in the thermophysical properties of BCC iron during isothermal compression
NASA Astrophysics Data System (ADS)
Magomedov, M. N.
2015-11-01
Equation of state P( V/ V 0, T) and baric dependences of the thermodynamic properties of bcc iron are obtained using the Mie-Lennard-Jones interatomic pair potential and the Einstein model of a crystal without any adjustable parameters. The calculations performed along two isotherms at 300 and 1500 K from P = 0 to 8000 kbar = 800 GPa (i.e., to V/ V 0 = 0.5) show good agreement with the experimental data. Baric graphical dependences are obtained for the following properties: isothermal bulk modulus B T and B'( P), isochoric specific heat C v and C v ' ( P), isobaric specific heat C p , thermal expansion coefficient α p and α p ' ( P), and specific surface energy of (100) face σ and σ'( P). Analytical approximations are obtained for baric dependences B'( P), α p ( P), and σ'( P). It is shown that, at P → ∞, functions B T ( P) and σ( P) for bcc iron change linearly and function α p ' ( P) tends toward zero.
Magnetic Evolution of the <100> Interstitial Loop Formation Process in bcc Iron
NASA Astrophysics Data System (ADS)
Xu, Haixuan; Stoller, Roger; Stocks, G. Malcolm
2013-03-01
Interstitial loops are a signature of radiation damage in materials and are only observed in systems far from equilibrium state due to their high formation energies (approximately 4eV). Unlike other bcc metals, in which the interstitial loops are almost exclusively 1/2 <111> type, two types of loops, <100>and 1/2 <111> are identified in bcc iron. Although 1/2 <111> loops can be formed directly by atomic displacment cascades, the mechanism of <100> loop formation had remained undetermined since they were observed fifty years ago. Recently, the formation mechanism has been discovered using self-evolving atomistic kinetic Monte Carlo (SEAKMC) simulations. Here we describe the influence of magnetism in the corresponding loop formation process using the ab initio locally self-consistent multiple-scattering (LSMS) method. Significant magnetic moment changes during the loop formation process are observed and their effect on the loop stability are evaluated. In addition, the effects of <100> loop formation on the microstructural evolution and material properties will be discussed. Work supported by the Center for Defect Physics, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.
Solving the Puzzle of ⟨100⟩ Interstitial Loop Formation in bcc Iron
NASA Astrophysics Data System (ADS)
Xu, Haixuan; Stoller, Roger E.; Osetsky, Yury N.; Terentyev, Dmitry
2013-06-01
The interstitial loop is a unique signature of radiation damage in structural materials for nuclear and other advanced energy systems. Unlike other bcc metals, two types of interstitial loops, 1/2⟨111⟩ and ⟨100⟩, are formed in bcc iron and its alloys. However, the mechanism by which ⟨100⟩ interstitial dislocation loops are formed has remained undetermined since they were first observed more than fifty years ago. We describe our atomistic simulations that have provided the first direct observation of ⟨100⟩ loop formation. The process was initially observed using our self-evolving atomistic kinetic Monte Carlo method, and subsequently confirmed using molecular dynamics simulations. Formation of ⟨100⟩ loops involves a distinctly atomistic interaction between two 1/2⟨111⟩ loops, and does not follow the conventional assumption of dislocation theory, which is Burgers vector conservation between the reactants and the product. The process observed is different from all previously proposed mechanisms. Thus, our observations might provide a direct link between experiments and simulations and new insights into defect formation that may provide a basis to increase the radiation resistance of these strategic materials.
Nanoscale Twinning and Martensitic Transformation in Shock-Deformed BCC Metals
Hsiung, L L
2005-03-22
Shock-induced twinning and martensitic transformation in BCC-based polycrystalline metals (Ta and U-6wt%Nb) have been observed and studied using transmission electron microscopy (TEM). The length-scale of domain thickness for both twin lamella and martensite phase is found to be smaller than 100 nm. While deformation twinning of {l_brace}112{r_brace}<111>-type is found in Ta when shock-deformed at 15 GPa, both twinning and martensitic transformation are found in Ta when shock-deformed at 45 GPa. Similar phenomena of nanoscale twinning and martensitic transformation are also found in U6Nb shock-deformed at 30 GPa. Since both deformation twinning and martensitic transformation occurred along the {l_brace}211{r_brace}{sub b} planes associated with high resolved shear stresses, it is suggested that both can be regarded as alternative paths for shear transformations to occur in shock-deformed BCC metals. Heterogeneous nucleation mechanisms for shock-induced twinning and martensitic transformation are proposed and discussed.
Equation of state of bcc-Mo by static volume compression to 410 GPa
Akahama, Yuichi; Hirao, Naohisa; Ohishi, Yasuo; Singh, Anil K.
2014-12-14
Unit cell volumes of Mo and Pt have been measured simultaneously to ≈400 GPa by x-ray powder diffraction using a diamond anvil cell and synchrotron radiation source. The body-centered cubic (bcc) phase of Mo was found to be stable up to 410 GPa. The equation of state (EOS) of bcc-Mo was determined on the basis of Pt pressure scale. A fit of Vinet EOS to the volume compression data gave K{sub 0} = 262.3(4.6) GPa, K{sub 0}′ = 4.55(16) with one atmosphere atomic volume V{sub 0} = 31.155(24) A{sup 3}. The EOS was in good agreement with the previous ultrasonic data within pressure difference of 2.5%–3.3% in the multimegabar range, though the EOS of Mo proposed from a shock compression experiment gave lower pressure by 7.2%–11.3% than the present EOS. The agreement would suggest that the Pt pressure scale provides an accurate pressure value in an ultra-high pressure range.
Trap effect of vacancy on hydrogen diffusivity in bcc-Fe
NASA Astrophysics Data System (ADS)
Zhu, Deqiong; Oda, Takuji
2016-02-01
Evaluation of tritium effective diffusivity in structural materials with defects is essential to appropriately estimate the accumulation and leakage of tritium in fusion reactors. In the present work, we focus on the trap effect of vacancy on the diffusivity of hydrogen in bcc-Fe. The hydrogen effective diffusivities in different bcc-Fe systems with varied hydrogen/vacancy ratios (H/V ratios) are evaluated by using molecular statics (MS) and molecular dynamics (MD) simulations, and are compared with that in the system without vacancy. The hydrogen effective diffusivity is obviously decreased due to the vacancy trap effect. The trap effect is more profound in small H/V-ratio systems due to the dependence of V-H binding energy on the number of trapped H atoms. The trapped H atoms hardly contribute to hydrogen diffusivity because of the difficulty in detrapping and small mobility of V-H complex. This fact results in a good correlation between the effective diffusivity of hydrogen and the fraction of H atoms solute in the bulk without being trapped by vacancy.
Diffusion of hydrogen within idealized grains of bcc Fe: A kinetic Monte Carlo study
NASA Astrophysics Data System (ADS)
Du, Yaojun A.; Rogal, Jutta; Drautz, Ralf
2012-11-01
Structural defects in materials such as vacancies, grain boundaries, and dislocations may trap hydrogen and a local accumulation of hydrogen at these defects can lead to the degradation of the materials properties. An important aspect in obtaining insight into hydrogen-induced embrittlement on the atomistic level is to understand the diffusion of hydrogen in these materials. In our study we employ kinetic Monte Carlo (kMC) simulations to investigate hydrogen diffusion in bcc iron within different microstructures. All input data to the kMC model, such as available sites, solution energies, and diffusion barriers, are obtained from first-principles calculations. We find that hydrogen mainly diffuses within the interface region with an overall diffusivity that is lower than in pure bcc Fe bulk. The concentration dependence of the diffusion coefficient is strongly nonlinear and the diffusion coefficient may even decrease with an increasing hydrogen concentration. To describe the macroscopic diffusion coefficient we derive an analytic expression as a function of hydrogen concentrations and temperatures which is in excellent agreement with our numerical results for idealized microstructures.
Neutron scattering studies of Ti-Cr-V bcc alloy with the residual hydrogen and deuterium
NASA Astrophysics Data System (ADS)
Mori, K.; Iwase, K.; Sugiyama, M.; Kofu, M.; Yamamuro, O.; Onodera, Y.; Otomo, T.; Fukunaga, T.
2012-02-01
Hydrogen and deuterium pressure-composition (P-C) isotherm measurements were carried out for Ti0.31Cr0.49V0.20 bcc alloy. As a result, the residual hydrogen-to-metal ratio (H/M)res was almost the same as the residual deuterium-to-metal one (D/M)res; the (H/M)res and (D/M)res were approximately 0.5. Furthermore, the neutron scattering experiments were conducted with Ti0.31Cr0.49V0.20 alloys including the residual hydrogen and/or deuterium. It was found that H atoms absorbed in the first absorption cycle up to H/M = 0.5 are hardly bound in the desorption cycle as the residual hydrogen, whereas D atoms are mainly bound in Ti0.31Cr0.49V0.20 bcc alloy as the residual deuterium when D2 gas is partially used in the H absorption reaction.
Microscopic Origin of Heisenberg and Non-Heisenberg Exchange Interactions in Ferromagnetic bcc Fe.
Kvashnin, Y O; Cardias, R; Szilva, A; Di Marco, I; Katsnelson, M I; Lichtenstein, A I; Nordström, L; Klautau, A B; Eriksson, O
2016-05-27
By means of first principles calculations, we investigate the nature of exchange coupling in ferromagnetic bcc Fe on a microscopic level. Analyzing the basic electronic structure reveals a drastic difference between the 3d orbitals of E_{g} and T_{2g} symmetries. The latter ones define the shape of the Fermi surface, while the former ones form weakly interacting impurity levels. We demonstrate that, as a result of this, in Fe the T_{2g} orbitals participate in exchange interactions, which are only weakly dependent on the configuration of the spin moments and thus can be classified as Heisenberg-like. These couplings are shown to be driven by Fermi surface nesting. In contrast, for the E_{g} states, the Heisenberg picture breaks down since the corresponding contribution to the exchange interactions is shown to strongly depend on the reference state they are extracted from. Our analysis of the nearest-neighbor coupling indicates that the interactions among E_{g} states are mainly proportional to the corresponding hopping integral and thus can be attributed to be of double-exchange origin. By making a comparison to other magnetic transition metals, we put the results of bcc Fe into context and argue that iron has a unique behavior when it comes to magnetic exchange interactions. PMID:27284671
Boone, Marc; Suppa, Mariano; Miyamoto, Makiko; Marneffe, Alice; Jemec, Gregor; Del Marmol, Veronique
2016-06-01
High-definition optical coherence tomography (HD-OCT) features of basal cell carcinoma (BCC) have recently been defined. We assessed in vivo optical properties (IV-OP) of BCC, by HD-OCT. Moreover their critical values for BCC subtype differentiation were determined. The technique of semi-log plot whereby an exponential function becomes linear has been implemented on HD-OCT signals. The relative attenuation factor (µraf ) at different skin layers could be assessed.. IV-OP of superficial BCC with high diagnostic accuracy (DA) and high negative predictive values (NPV) were (i) decreased µraf in lower part of epidermis and (ii) increased epidermal thickness (E-T). IV-OP of nodular BCC with good to high DA and NPV were (i) less negative µraf in papillary dermis compared to normal adjacent skin and (ii) significantly decreased E-T and papillary dermal thickness (PD-T). In infiltrative BCC (i) high µraf in reticular dermis compared to normal adjacent skin and (ii) presence of peaks and falls in reticular dermis had good DA and high NPV. HD-OCT seems to enable the combination of in vivo morphological analysis of cellular and 3-D micro-architectural structures with IV-OP analysis of BCC. This permits BCC sub-differentiation with higher accuracy than in vivo HD-OCT analysis of morphology alone. PMID:27375943
Boone, Marc; Suppa, Mariano; Miyamoto, Makiko; Marneffe, Alice; Jemec, Gregor; Del Marmol, Veronique
2016-01-01
High-definition optical coherence tomography (HD-OCT) features of basal cell carcinoma (BCC) have recently been defined. We assessed in vivo optical properties (IV-OP) of BCC, by HD-OCT. Moreover their critical values for BCC subtype differentiation were determined. The technique of semi-log plot whereby an exponential function becomes linear has been implemented on HD-OCT signals. The relative attenuation factor (µraf) at different skin layers could be assessed.. IV-OP of superficial BCC with high diagnostic accuracy (DA) and high negative predictive values (NPV) were (i) decreased µraf in lower part of epidermis and (ii) increased epidermal thickness (E-T). IV-OP of nodular BCC with good to high DA and NPV were (i) less negative µraf in papillary dermis compared to normal adjacent skin and (ii) significantly decreased E-T and papillary dermal thickness (PD-T). In infiltrative BCC (i) high µraf in reticular dermis compared to normal adjacent skin and (ii) presence of peaks and falls in reticular dermis had good DA and high NPV. HD-OCT seems to enable the combination of in vivo morphological analysis of cellular and 3-D micro-architectural structures with IV-OP analysis of BCC. This permits BCC sub-differentiation with higher accuracy than in vivo HD-OCT analysis of morphology alone. PMID:27375943
Flat Band Quastiperiodic Lattices
NASA Astrophysics Data System (ADS)
Bodyfelt, Joshua; Flach, Sergej; Danieli, Carlo
2014-03-01
Translationally invariant lattices with flat bands (FB) in their band structure possess irreducible compact localized flat band states, which can be understood through local rotation to a Fano structure. We present extension of these quasi-1D FB structures under incommensurate lattices, reporting on the FB effects to the Metal-Insulator Transition.
Laterally closed lattice homomorphisms
NASA Astrophysics Data System (ADS)
Toumi, Mohamed Ali; Toumi, Nedra
2006-12-01
Let A and B be two Archimedean vector lattices and let be a lattice homomorphism. We call that T is laterally closed if T(D) is a maximal orthogonal system in the band generated by T(A) in B, for each maximal orthogonal system D of A. In this paper we prove that any laterally closed lattice homomorphism T of an Archimedean vector lattice A with universal completion Au into a universally complete vector lattice B can be extended to a lattice homomorphism of Au into B, which is an improvement of a result of M. Duhoux and M. Meyer [M. Duhoux and M. Meyer, Extended orthomorphisms and lateral completion of Archimedean Riesz spaces, Ann. Soc. Sci. Bruxelles 98 (1984) 3-18], who established it for the order continuous lattice homomorphism case. Moreover, if in addition Au and B are with point separating order duals (Au)' and B' respectively, then the laterally closedness property becomes a necessary and sufficient condition for any lattice homomorphism to have a similar extension to the whole Au. As an application, we give a new representation theorem for laterally closed d-algebras from which we infer the existence of d-algebra multiplications on the universal completions of d-algebras.
NASA Astrophysics Data System (ADS)
Bergner, Georg; Catterall, Simon
2016-08-01
We discuss the motivations, difficulties and progress in the study of supersymmetric lattice gauge theories focusing in particular on 𝒩 = 1 and 𝒩 = 4 super-Yang-Mills in four dimensions. Brief reviews of the corresponding lattice formalisms are given and current results are presented and discussed. We conclude with a summary of the main aspects of current work and prospects for the future.
Björner, Anders
1987-01-01
A continuous analogue to the partition lattices is presented. This is the metric completion of the direct limit of a system of embeddings of the finite partition lattices. The construction is analogous to von Neumann's construction of a continuous geometry over a field F from the finite-dimensional projective geometries over F. PMID:16593874
Honeycomb lattices with defects
NASA Astrophysics Data System (ADS)
Spencer, Meryl A.; Ziff, Robert M.
2016-04-01
In this paper, we introduce a variant of the honeycomb lattice in which we create defects by randomly exchanging adjacent bonds, producing a random tiling with a distribution of polygon edges. We study the percolation properties on these lattices as a function of the number of exchanged bonds using an alternative computational method. We find the site and bond percolation thresholds are consistent with other three-coordinated lattices with the same standard deviation in the degree distribution of the dual; here we can produce a continuum of lattices with a range of standard deviations in the distribution. These lattices should be useful for modeling other properties of random systems as well as percolation.
NASA Astrophysics Data System (ADS)
Dremov, V. V.; Ionov, G. V.; Sapozhnikov, F. A.; Smirnov, N. A.; Karavaev, A. V.; Vorobyova, M. A.; Ryzhkov, M. V.
2015-09-01
The present work is devoted to classical molecular dynamics investigation into microscopic mechanisms of the bcc-hcp transition in iron. The interatomic potential of EAM type used in the calculations was tested for the capability to reproduce ab initio data on energy evolution along the bcc-hcp transformation path (Burgers deformation + shuffe) and then used in the large-scale MD simulations. The large-scale simulations included constant volume deformation along the Burgers path to study the origin and nature of the plasticity, hydrostatic volume compression of defect free samples above the bcc to hcp transition threshold to observe the formation of new phase embryos, and the volume compression of samples containing screw dislocations to study the effect of the dislocations on the probability of the new phase critical embryo formation. The volume compression demonstrated high level of metastability. The transition starts at pressure much higher than the equilibrium one. Dislocations strongly affect the probability of the critical embryo formation and significantly reduce the onset pressure of transition. The dislocations affect also the resulting structure of the samples upon the transition. The formation of layered structure is typical for the samples containing the dislocations. The results of the simulations were compared with the in-situ experimental data on the mechanism of the bcc-hcp transition in iron.
Courant, E.D.; Garren, A.A.
1985-10-01
A realistic, distributed interaction region (IR) lattice has been designed that includes new components discussed in the June 1985 lattice workshop. Unlike the test lattices, the lattice presented here includes utility straights and the mechanism for crossing the beams in the experimental straights. Moreover, both the phase trombones and the dispersion suppressors contain the same bending as the normal cells. Vertically separated beams and 6 Tesla, 1-in-1 magnets are assumed. Since the cells are 200 meters long, and have 60 degree phase advance, this lattice has been named RLD1, in analogy with the corresponding test lattice, TLD1. The quadrupole gradient is 136 tesla/meter in the cells, and has similar values in other quadrupoles except in those in the IR`s, where the maximum gradient is 245 tesla/meter. RLD1 has distributed IR`s; however, clustered realistic lattices can easily be assembled from the same components, as was recently done in a version that utilizes the same type of experimental and utility straights as those of RLD1.
NASA Astrophysics Data System (ADS)
You, Y.; Yan, M. F.
2013-05-01
C and N atoms are the most frequent foreign interstitial atoms (FIAs), and often incorporated into the surface layers of steels to enhance their properties by thermochemical treatments. Al, Si, Ti, V, Cr, Mn, Co, Ni, Cu, Nb and Mo are the most common alloying elements in steels, also can be called foreign substitutional atoms (FSAs). The FIA and FSA interactions play an important role in the diffusion of C and N atoms, and the microstructures and mechanical properties of surface modified layers. Ab initio calculations based on the density functional theory are carried out to investigate FIA interactions with FSA in ferromagnetic bcc iron. The FIA-FSA interactions are analyzed systematically from five aspects, including interaction energies, density of states (DOS), bond populations, electron density difference maps and local magnetic moments.
Surface Segregation Energies of BCC Binaries from Ab Initio and Quantum Approximate Calculations
NASA Technical Reports Server (NTRS)
Good, Brian S.
2003-01-01
We compare dilute-limit segregation energies for selected BCC transition metal binaries computed using ab initio and quantum approximate energy method. Ab initio calculations are carried out using the CASTEP plane-wave pseudopotential computer code, while quantum approximate results are computed using the Bozzolo-Ferrante-Smith (BFS) method with the most recent parameterization. Quantum approximate segregation energies are computed with and without atomistic relaxation. The ab initio calculations are performed without relaxation for the most part, but predicted relaxations from quantum approximate calculations are used in selected cases to compute approximate relaxed ab initio segregation energies. Results are discussed within the context of segregation models driven by strain and bond-breaking effects. We compare our results with other quantum approximate and ab initio theoretical work, and available experimental results.
Synergetic effects of Mn and Si in the interaction with point defects in bcc Fe
NASA Astrophysics Data System (ADS)
Bakaev, A.; Terentyev, D.; He, X.; Van Neck, D.
2014-12-01
The interaction of Mn, Si and Cr with a vacancy and self-interstitial defects in BCC Fe has been analyzed using ab initio calculations. While the interaction of the considered solute clusters with a single vacancy is linearly additive, there is a considerable synergetic effect in the case of self-interstitial atoms, found to bind strongly with Mn-Si pairs. The latter therefore act as deep trapping configurations for self-interstitials. At the same time, the presence of the point defects nearby weakly attractive Mn-Si pairs significantly enhances the solute-solute binding. The revealed effects are rationalized on the basis of charge density and local magnetic moment distributions.
Magnetic properties and atomic ordering of BCC Heusler alloy Fe2MnGa ribbons
NASA Astrophysics Data System (ADS)
Xin, Yuepeng; Ma, Yuexing; Luo, Hongzhi; Meng, Fanbin; Liu, Heyan
2016-05-01
The electronic structure, atomic disorder and magnetic properties of the Heusler alloy Fe2MnGa have been investigated experimentally and theoretically. BCC Fe2MnGa ribbon samples were prepared. Experimentally, a saturation magnetic moment (3.68 μB at 5 K) much larger than the theoretical value (2.04 μB) has been reported. First-principles calculations indicate that the difference is related to the Fe-Mn disorder between A, B sites, as can also be deduced from the XRD pattern. L21 type Fe2MnGa is a ferrimagnet with antiparallel Fe and Mn spin moments. However, when Fe-Mn disorder occurs, part of Mn moments will be parallel to Fe moments, and the Fe moments also clearly increase simultaneously. All this results in a total moment of 3.74 μB, close to the experimental value.
FAST TRACK COMMUNICATION: Finite-temperature magnetism in bcc Fe under compression
NASA Astrophysics Data System (ADS)
Sha, Xianwei; Cohen, R. E.
2010-09-01
We investigate the contributions of finite-temperature magnetic fluctuations to the thermodynamic properties of bcc Fe as functions of pressure. First, we apply a tight-binding total-energy model parameterized to first-principles linearized augmented plane-wave computations to examine various ferromagnetic, anti-ferromagnetic, and noncollinear spin spiral states at zero temperature. The tight-binding data are fit to a generalized Heisenberg Hamiltonian to describe the magnetic energy functional based on local moments. We then use Monte Carlo simulations to compute the magnetic susceptibility, the Curie temperature, heat capacity, and magnetic free energy. Including the finite-temperature magnetism improves the agreement with experiment for the calculated thermal expansion coefficients.
Vacancy-mediated fcc/bcc phase separation in Fe1 -xNix ultrathin films
NASA Astrophysics Data System (ADS)
Menteş, T. O.; Stojić, N.; Vescovo, E.; Ablett, J. M.; Niño, M. A.; Locatelli, A.
2016-08-01
The phase separation occurring in Fe-Ni thin films near the Invar composition is studied by using high-resolution spectromicroscopy techniques and density functional theory calculations. Annealed at temperatures around 300 ∘C ,Fe0.70Ni0.30 films on W(110) break into micron-sized bcc and fcc domains with compositions in agreement with the bulk Fe-Ni phase diagram. Ni is found to be the diffusing species in forming the chemical heterogeneity. The experimentally determined energy barrier of 1.59 ±0.09 eV is identified as the vacancy formation energy via density functional theory calculations. Thus, the principal role of the surface in the phase separation process is attributed to vacancy creation without interstitials.
Patra, Anirban; McDowell, David L.
2016-03-25
We use a continuum crystal plasticity framework to study the effect of microstructure and mesoscopic factors on dislocation channeling and flow localization in an irradiated model bcc alloy. For simulated dislocation channeling characteristics we correlate the dislocation and defect densities in the substructure, local Schmid factor, and stress triaxiality, in terms of their temporal and spatial evolution. A metric is introduced to assess the propensity for localization and is correlated to the grain-level Schmid factor. We also found that localization generally takes place in grains with a local Schmid factor in the range 0.42 or higher. Surface slip step heightsmore » are computed at free surfaces and compared to relevant experiments.« less
Solute/impurity diffusivities in bcc Fe: A first-principles study
NASA Astrophysics Data System (ADS)
Zhang, Chong; Fu, Jie; Li, Ruihuan; Zhang, Pengbo; Zhao, Jijun; Dong, Chuang
2014-12-01
Chinese low activation martensitic steel (CLAM) has been designed with decreased W content and increased Ta content to improve performance. We performed first-principles calculations to investigate the diffusion properties of solute element (Cr, W, Mn, V, Ta) and C diffusion with a nearby solute element inside bcc Fe. The self-diffusion coefficients and solute diffusion coefficients in Fe host were derived using the nine-frequency model. A relatively lower diffusivity was observed for W in paramagnetic state, implying enriched W concentration inside Fe host. The solute atom interacts strongly with C impurity, depending on the interatomic distance. According to our calculations, formation of Ta carbide precipitates is energetically preferred by trapping C impurity around Ta atom. Our theoretical results are helpful for investigating the evolution of microstructure of steels for engineering applications.
Intaraudom, Chakapong; Nitthithanasilp, Sutichai; Rachtawee, Pranee; Boonruangprapa, Tanapong; Prabpai, Samran; Kongsaeree, Palangpon; Pittayakhajonwut, Pattama
2015-12-01
Ten compounds including nine phenalenone derivatives (five bipolarides and four bipolarols) and a sesterterpene acid (bipolarenic acid), were isolated from a marine isolated of the fungus Lophiostoma bipolare (BCC25910), along with the known compounds, (-)-scleroderolide, (-)-sclerodin, and oxasetin. Chemical structures were elucidated based on NMR spectroscopic data and HRESIMS analysis. In addition, the absolute configurations of the phenalenones were resolved using specific rotations and chemical means, while the relative configuration of bipolarenic acid was confirmed by X-ray crystallographic analysis. The compounds were evaluated for biological activity against the Plasmodium falciparum K-1 strain, Candida albicans, and Bacillus cereus, and for cytotoxicity against both cancerous and non-cancerous cells. PMID:26582262
Computer Simulations of Interstitial Loop Growth Kinetics in irradiated bcc Fe
Li, Yulan; Hu, Shenyang Y.; Henager, Charles H.; Deng, Huiqiu; Gao, Fei; Sun, Xin; Khaleel, Mohammad A.
2012-08-01
The growth kinetics of (001) [001] interstitial loops in bcc Fe is studied by phase-field modeling. The effect of defect (vacancy/interstitial) concentration, generation, recombination, sink strength, and elastic interaction on the growth kinetics of interstitial loops is systematically simulated. Results show that the elastic interaction between the defects and interstitial loops speeds up the growth kinetics and affects the morphology of the interstitial loops. Linear growth rate, i.e., the loop average radius is linear to time, under both aging and irradiation are predicted, which is in agreement with experimental observation. The results also show that the interstitial loop growth rate, which is directly related to the sink strength of the interstitial loop for interstitials, increases linearly with the initial interstitial concentration during aging while changing logarithmically with the interstitial generation rate under irradiation.
Moment Mapping of bcc Fe1-xMnx Alloy Films on MgO(001)
NASA Astrophysics Data System (ADS)
Idzerda, Yves; Bhatkar, Harsh; Arenholz, Elke
2015-03-01
The magnetic moments of ~ 20 nm single crystal films of compositionally graded Fe1-xMnx films (0.1 <= x <= 0.2) grown on MgO(001) are determined by spatially resolved moment mapping using X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (MCD). RHEED measurements confirmed that the growth of Fe1-xMnx films remained epitaxial and in the bcc phase up to x =0.35 but, like Fe growth, is rotated 45 degree with respect to the MgO(001) surface net. This is beyond the bulk bcc stability limit of x =0.12. Both magnetometry and XMCD measurements show that the net magnetic moment of these alloy films behave similarly to the bulk behavior, with a gradual moment reduction at low Mn concentrations followed by an abrupt departure from the Slater-Pauling curve and disappearance of the moment at x =0.15. By generating a compositional variation around this critical concentration and subsequently using spatially resolved mapping of the X-ray absorption at the Fe and Mn L3-edge using linear and circular polarized soft X-rays, the local composition and elemental moments can be simultaneously mapped across the surface of the sample. The Fe moment is found to gradually reduce with increasing Mn content with a very abrupt decline at x =0.15. Surprisingly, the Mn moment shows a very small net moment (<0.1 muB) at all compositions, suggesting a complicated Mn spin structure.
Superalloy Lattice Block Structures
NASA Technical Reports Server (NTRS)
Nathal, M. V.; Whittenberger, J. D.; Hebsur, M. G.; Kantzos, P. T.; Krause, D. L.
2004-01-01
Initial investigations of investment cast superalloy lattice block suggest that this technology will yield a low cost approach to utilize the high temperature strength and environmental resistance of superalloys in lightweight, damage tolerant structural configurations. Work to date has demonstrated that relatively large superalloy lattice block panels can be successfully investment cast from both IN-718 and Mar-M247. These castings exhibited mechanical properties consistent with the strength of the same superalloys measured from more conventional castings. The lattice block structure also accommodates significant deformation without failure, and is defect tolerant in fatigue. The potential of lattice block structures opens new opportunities for the use of superalloys in future generations of aircraft applications that demand strength and environmental resistance at elevated temperatures along with low weight.
ORGINOS,K.
2003-01-07
I review the current status of hadronic structure computations on the lattice. I describe the basic lattice techniques and difficulties and present some of the latest lattice results; in particular recent results of the RBC group using domain wall fermions are also discussed. In conclusion, lattice computations can play an important role in understanding the hadronic structure and the fundamental properties of Quantum Chromodynamics (QCD). Although some difficulties still exist, several significant steps have been made. Advances in computer technology are expected to play a significant role in pushing these computations closer to the chiral limit and in including dynamical fermions. RBC has already begun preliminary dynamical domain wall fermion computations [49] which we expect to be pushed forward with the arrival of QCD0C. In the near future, we also expect to complete the non-perturbative renormalization of the relevant derivative operators in quenched QCD.
NASA Astrophysics Data System (ADS)
Weidner, Carrie; Yu, Hoon; Anderson, Dana
2016-05-01
In this work, we report on progress towards performing interferometry using atoms trapped in an optical lattice. That is, we start with atoms in the ground state of an optical lattice potential V(x) =V0cos [ 2 kx + ϕ(t) ] , and by a prescribed phase function ϕ(t) , transform from one atomic wavefunction to another. In this way, we implement the standard interferometric sequence of beam splitting, propagation, reflection, reverse propagation, and recombination. Through the use of optimal control techniques, we have computationally demonstrated a scalable accelerometer that provides information on the sign of the applied acceleration. Extension of this idea to a two-dimensional shaken-lattice-based gyroscope is discussed. In addition, we report on the experimental implementation of the shaken lattice system.
Model calculations of edge dislocation defects and vacancies in α-Iron lattice
NASA Astrophysics Data System (ADS)
Petrov, L.; Troev, T.; Nankov, N.; Popov, E.
2010-01-01
Two models of defects in perfect α-iron lattice were discussed. In the perfect bcc iron lattice 42×42×42 ao (ao = 2,87 Å) an edge dislocation was created, moving the second half of the bulk on one ao distance. This action generates a little volume in the middle of the bulk witch increases of the positron lifetime (PLT) calculated using the superimposed-atom method of Puska and Nieminen [1]. The result of 118 ps PLT in simple edge dislocation's model is in a good concurrence with earlier publications and experimental data [2]. Through the dislocation line one, two and three vacancies were localized. These models give the results for PLT of 146, 157 and 167 ps respectively. The computer simulations were performed using Finnis-Sinclair (FS) N-body potential.
Automated Lattice Perturbation Theory
Monahan, Christopher
2014-11-01
I review recent developments in automated lattice perturbation theory. Starting with an overview of lattice perturbation theory, I focus on the three automation packages currently "on the market": HiPPy/HPsrc, Pastor and PhySyCAl. I highlight some recent applications of these methods, particularly in B physics. In the final section I briefly discuss the related, but distinct, approach of numerical stochastic perturbation theory.
Legless locomotion in lattices
NASA Astrophysics Data System (ADS)
Schiebel, Perrin; Dai, Jin; Gong, Chaohui; Serrano, Miguel M.; Mendelson, Joseph R., III; Choset, Howie; Goldman, Daniel I.
2015-03-01
By propagating waves from head to tail, limbless organisms like snakes can traverse terrain composed of rocks, foliage, soil and sand. Previous research elucidated how rigid obstacles influence snake locomotion by studying a model terrain-symmetric lattices of pegs placed in hard ground. We want to understand how different substrate-body interaction modes affect performance in desert-adapted snakes during transit of substrates composed of both rigid obstacles and granular media (GM). We tested Chionactis occipitalis, the Mojave shovel-nosed snake, in two laboratory treatments: lattices of 0 . 64 cm diameter obstacles arrayed on both a hard, slick substrate and in a GM of ~ 0 . 3 mm diameter glass particles. For all lattice spacings, d, speed through the hard ground lattices was less than that in GM lattices. However, maximal undulation efficiencies ηu (number of body lengths advanced per undulation cycle) in both treatments were comparable when d was intermediate. For other d, ηu was lower than this maximum in hard ground lattices, while on GM, ηu was insensitive to d. To systematically explore such locomotion, we tested a physical robot model of the snake; performance depended sensitively on base substrate, d and body wave parameters.
NASA Astrophysics Data System (ADS)
Knuth, Kevin H.
2009-12-01
Previous derivations of the sum and product rules of probability theory relied on the algebraic properties of Boolean logic. Here they are derived within a more general framework based on lattice theory. The result is a new foundation of probability theory that encompasses and generalizes both the Cox and Kolmogorov formulations. In this picture probability is a bi-valuation defined on a lattice of statements that quantifies the degree to which one statement implies another. The sum rule is a constraint equation that ensures that valuations are assigned so as to not violate associativity of the lattice join and meet. The product rule is much more interesting in that there are actually two product rules: one is a constraint equation arises from associativity of the direct products of lattices, and the other a constraint equation derived from associativity of changes of context. The generality of this formalism enables one to derive the traditionally assumed condition of additivity in measure theory, as well introduce a general notion of product. To illustrate the generic utility of this novel lattice-theoretic foundation of measure, the sum and product rules are applied to number theory. Further application of these concepts to understand the foundation of quantum mechanics is described in a joint paper in this proceedings.
NASA Astrophysics Data System (ADS)
Dézerald, Lucile; Ventelon, Lisa; Willaime, François; Clouet, Emmanuel; Rodney, David
2014-06-01
Ab initio methods, based on the Density Functional Theory (DFT), have been extensively used to study point defects and defect clusters in materials. Present HPC resources and DFT codes now allow similar investigations to be performed on dislocations. The study of these extended defects requires not only larger simulation cells but also a higher accuracy because the energy differences, which are involved, are rather small, typically 50-to-100 meV for supercells containing 50-to-500 atoms. The topology of the Peierls potential of screw dislocations with 1/2 <111>Burgers vector, i.e. the 2D energy landscape seen by these dislocations, is being completely revisited by DFT calculations. From results obtained in all body-centered cubic (bcc) transition metals, except Cr (V, Nb, Ta, Mo, W and Fe), using the PWSCF code, which is part of the Quantum-Espresso package, we concluded that the 2D Peierls potentials have two common features: the single-hump shape of the barrier between two minima of the potential, and the presence of a maximum - and not a minimum as predicted by most empirical potentials - around the split core. In iron, the topology of the Peierls potential is reversed compared to the classical sinusoidal picture: the location of the saddle point and the maximum are indeed inverted with unexpected flat regions. The first results obtained within the framework of the PRACE project, DIMAIM (DIslocations in Metals using Ab Initio Methods), started at the beginning of 2013, will also be presented. In particular, in order to address the twinning-antitwinning asymmetry often observed in bcc metals and regarded as the major contribution to the breakdown of Schmid's law, we have determined the crystal orientation dependence of the Peierls stress, i.e. the critical stress required for dislocation motion. These computationally most expensive simulations were performed on the PRACE Tier-0 system at Barcelona Supercomputing Center (Marenostrum III). The scalability results
ERIC Educational Resources Information Center
Bronx Community Coll., NY.
As part of an on-going study, all 1,818 graduates and 6,847 non-graduates who left Bronx Community College (BCC) in 1975-76 were surveyed by mail, with a return of 1,681 responses representing 52.4% of graduate and 20% of the non-graduate sample. Findings included the following: (1) 63% of graduates and 48% of non-graduates responding continued…
Latticed pentamode acoustic cloak
Chen, Yi; Liu, Xiaoning; Hu, Gengkai
2015-01-01
We report in this work a practical design of pentamode acoustic cloak with microstructure. The proposed cloak is assembled by pentamode lattice made of a single-phase solid material. The function of rerouting acoustic wave round an obstacle has been demonstrated numerically. It is also revealed that shear related resonance due to weak shear resistance in practical pentamode lattices punctures broadband feature predicted based on ideal pentamode cloak. As a consequence, the latticed pentamode cloak can only conceal the obstacle in segmented frequency ranges. We have also shown that the shear resonance can be largely reduced by introducing material damping, and an improved broadband performance can be achieved. These works pave the way for experimental demonstration of pentamode acoustic cloak. PMID:26503821
Lattice Boltzmann Stokesian dynamics.
Ding, E J
2015-11-01
Lattice Boltzmann Stokesian dynamics (LBSD) is presented for simulation of particle suspension in Stokes flows. This method is developed from Stokesian dynamics (SD) with resistance and mobility matrices calculated using the time-independent lattice Boltzmann algorithm (TILBA). TILBA is distinguished from the traditional lattice Boltzmann method (LBM) in that a background matrix is generated prior to the calculation. The background matrix, once generated, can be reused for calculations for different scenarios, thus the computational cost for each such subsequent calculation is significantly reduced. The LBSD inherits the merits of the SD where both near- and far-field interactions are considered. It also inherits the merits of the LBM that the computational cost is almost independent of the particle shape. PMID:26651812
Lattice Boltzmann Stokesian dynamics
NASA Astrophysics Data System (ADS)
Ding, E. J.
2015-11-01
Lattice Boltzmann Stokesian dynamics (LBSD) is presented for simulation of particle suspension in Stokes flows. This method is developed from Stokesian dynamics (SD) with resistance and mobility matrices calculated using the time-independent lattice Boltzmann algorithm (TILBA). TILBA is distinguished from the traditional lattice Boltzmann method (LBM) in that a background matrix is generated prior to the calculation. The background matrix, once generated, can be reused for calculations for different scenarios, thus the computational cost for each such subsequent calculation is significantly reduced. The LBSD inherits the merits of the SD where both near- and far-field interactions are considered. It also inherits the merits of the LBM that the computational cost is almost independent of the particle shape.
Catterall, Simon; Kaplan, David B.; Unsal, Mithat
2009-03-31
We provide an introduction to recent lattice formulations of supersymmetric theories which are invariant under one or more real supersymmetries at nonzero lattice spacing. These include the especially interesting case of N = 4 SYM in four dimensions. We discuss approaches based both on twisted supersymmetry and orbifold-deconstruction techniques and show their equivalence in the case of gauge theories. The presence of an exact supersymmetry reduces and in some cases eliminates the need for fine tuning to achieve a continuum limit invariant under the full supersymmetry of the target theory. We discuss open problems.
Suwannarangsee, Surisa; Arnthong, Jantima; Eurwilaichitr, Lily; Champreda, Verawat
2014-10-01
Enzymatic hydrolysis of lignocellulosic biomass into fermentable sugars is a key step in the conversion of agricultural by-products to biofuels and value-added chemicals. Utilization of a robust microorganism for on-site production of biomass-degrading enzymes has gained increasing interest as an economical approach for supplying enzymes to biorefinery processes. In this study, production of multi-polysaccharide-degrading enzymes from Aspergillus aculeatus BCC199 by solid-state fermentation was improved through the statistical design approach. Among the operational parameters, yeast extract and soybean meal as well as the nonionic surfactant Tween 20 and initial pH were found as key parameters for maximizing production of cellulolytic and hemicellulolytic enzymes. Under the optimized condition, the production of FPase, endoglucanase, β-glucosidase, xylanase, and β-xylosidase was achieved at 23, 663, 88, 1,633, and 90 units/g of dry substrate, respectively. The multi-enzyme extract was highly efficient in the saccharification of alkaline-pretreated rice straw, corn cob, and corn stover. In comparison with commercial cellulase preparations, the BCC199 enzyme mixture was able to produce remarkable yields of glucose and xylose, as it contained higher relative activities of β-glucosidase and core hemicellulases (xylanase and β-xylosidase). These results suggested that the crude enzyme extract from A. aculeatus BCC199 possesses balanced cellulolytic and xylanolytic activities required for the efficient saccharification of lignocellulosic biomass feedstocks, and supplementation of external β-glucosidase or xylanase was dispensable. The work thus demonstrates the high potential of A. aculeatus BCC199 as a promising producer of lignocellulose-degrading enzymes for the biomass conversion industry. PMID:25001556
Computational study of atomic mobility for the bcc phase of the U-Pu-Zr ternary system
NASA Astrophysics Data System (ADS)
Li, Weibang; Hu, Rui; Cui, Y.-W.; Zhong, Hong; Chang, Hui; Li, Jinshan; Zhou, Lian
2010-12-01
Experimental diffusion data in literature has been evaluated to assess the atomic mobility for the bcc phase in the U-Pu-Zr system by means of the DICTRA-type (Diffusion Controlled TRAnsformation) phenomenological treatment. The developed mobility database has been validated by comprehensive comparisons made between the experimental and calculated diffusion coefficients, as well as other interesting details resulting from interdiffusion, e.g. the concentration profile and the diffusion path of diffusion couples.
Zhou, Dongwen; Visessanguan, Wonnop; Chaikaew, Siriporn; Benjakul, Soottawat; Oda, Kohei; Wlodawer, Alexander
2014-01-01
Histamine dehydrogenase (HADH) catalyzes the oxidative deamination of histamine, resulting in the production of imidazole acetaldehyde and an ammonium ion. The enzyme isolated from the newly identified halophilic archaeon Natrinema gari BCC 24369 is significantly different from the previously described protein from Nocardioides simplex. This newly identified HADH comprises three subunits with molecular weights of 49.0, 24.7 and 23.9 kDa, respectively, and is optimally active under high-salt conditions (3.5–5 M NaCl). As a step in the exploration of the unique properties of the protein, the HADH heterotrimer was purified and crystallized. Crystals were obtained using the sitting-drop vapor-diffusion method from a solution composed of 0.2 M calcium chloride dihydrate, 0.1 M HEPES pH 7.5, 28% PEG 400. Diffraction data were collected at −173°C to a resolution limit of 2.4 Å on the Southeast Regional Collaborative Access Team (SER-CAT) beamline 22-ID at the Advanced Photon Source, Argonne National Laboratory. The crystals belonged to the monoclinic space group C2, with unit-cell parameters a = 211.9, b = 58.6, c = 135.4 Å, β = 103.0°. The estimated Matthews coefficient is 3.21 Å3 Da−1, corresponding to 61.7% solvent content. PMID:25005094
3D dislocation dynamics: stress-strain behavior and hardening mechanisms in FCC and BCC metals
Hirth, J P; Rhee, M; Zhib, H M; de la Rubia, T D
1999-02-19
A dislocation dynamics (DD) model for plastic deformation, connecting the macroscopic mechanical properties to basic physical laws governing dislocation mobility and related interaction mechanisms, has been under development. In this model there is a set of critical reactions that determine the overall results of the simulations, such as the stress-strain curve. These reactions are, annihilation, formation of jogs, junctions, and dipoles, and cross-slip. In this paper we discuss these reactions and the manner in which they influence the simulated stress- strain behavior in fcc and bcc metals. In particular, we examine the formation (zipping) and strength of dipoles and junctions, and effect of jogs, using the dislocation dynamics model. We show that the strengths (unzipping) of these reactions for various configurations can be determined by direct evaluation of the elastic interactions. Next, we investigate the phenomenon of hardening in metals subjected to cascade damage dislocations. The microstructure investigated consists of small dislocation loops decorating the mobile dislocations. Preliminary results reveal that these loops act as hardening agents, trapping the dislocations and resulting in increased hardening.
A new mechanism of loop formation and transformation in bcc iron without dislocation reaction
NASA Astrophysics Data System (ADS)
Chen, J.; Gao, N.; Jung, P.; Sauvage, T.
2013-10-01
Structure and kinetics of dislocation loops in α-Fe is an active field in material science, due to their implications on fundamental understanding as well as application of structural materials in irradiation environments. Recent computer simulations provoke new conceptions, which call for experimental verification. The present investigation reports transmission electron microscopy of small interstitial dislocation loops (2.5-10 nm diameters) in bcc iron, irradiated with 25 MeV α-particles at 573 K up to 0.13 dpa. The observed <1 0 0> and ½<1 1 1> loops have habit planes of (1 0 0), and (1 1 0), (1 1 1) and (2 1 1), respectively. Furthermore it is observed that loops also contain ½<1 1 1>{2 1 1} and <1 0 0>{1 0 0} components which are considered as intermediate stages of transformation of ½<1 1 1> loops to <1 0 0>. Based on these observations, a new mechanism of loop formation and transformation by self-interstitial atoms aggregation is proposed, with concurrent molecular dynamic simulations supporting the kinetic feasibility of the proposed process.
Atomistic studies of nucleation of He clusters and bubbles in bcc iron
NASA Astrophysics Data System (ADS)
Yang, L.; Deng, H. Q.; Gao, F.; Heinisch, H. L.; Kurtz, R. J.; Hu, S. Y.; Li, Y. L.; Zu, X. T.
2013-05-01
Atomistic simulations of the nucleation of He clusters and bubbles in bcc iron at 800 K have been carried out using the newly developed Fe-Fe interatomic potential, along with Ackland potential for the Fe-Fe interactions. Microstructure changes were analyzed in detail. We found that a He cluster with four He atoms is able to push out an iron interstitial from the cluster, creating a Frenkel pair. Small He clusters and self-interstitial atom (SIA) can migrate in the matrix, but He-vacancy (He-V) clusters are immobile. Most SIAs form <111> clusters, and only the dislocation loops with a Burgers vector of b = 1/2 <111> appear in the simulations. SIA clusters (or loops) are attached to He-V clusters for He implantation up to 1372 appm, while the He-V cluster-loop complexes with more than one He-V cluster are formed at the He concentration of 2057 appm and larger.
Grain Size Dependence of Uniform Elongation in Single-Phase FCC/BCC Metals
NASA Astrophysics Data System (ADS)
Liu, Haiting; Shen, Yao; Ma, Jiawei; Zheng, Pengfei; Zhang, Lei
2016-07-01
We studied the dependence of uniform elongation on grain size in the range of submicron to millimeter for single-phase FCC/BCC metals by reviewing recent experimental results and applying crystal plasticity finite element method simulation. In the order of increasing grain size, uniform elongation can be divided into three stages, namely low elongation stage, nearly constant elongation stage, and decreased elongation with large scatters stage. Low elongation stage features a dramatic increase near the critical grain size at the end of the stage, which is primarily attributed to the emergence of dislocation cell size transition from ultrafine to mid-size grain. Other factors can be neglected due to their negligible influence on overall variation trend. In nearly constant elongation stage, uniform elongation remains unchanged at a high level in general. As grain size keeps growing, uniform elongation starts decreasing and becomes scattered upon a certain grain size, indicating the initiation of decreased elongation with large scatters stage. It is shown that the increase is not linear or smooth but rather sharp at the end of low elongation stage, leading to a wider range in nearly constant elongation stage. The grain size dependence of uniform elongation can serve as a guiding principle for designing small uniaxial tensile specimens for mechanical testing, where size effect matters in most cases.
An experimental statistical analysis of stress projection factors in BCC tantalum
Carroll, J. D.; Clark, B. G.; Buchheit, T. E.; Boyce, B. L.; Weinberger, C. R.
2013-10-01
Crystallographic slip planes in body centered cubic (BCC) metals are not fully understood. In polycrystals, there are additional confounding effects from grain interactions. This paper describes an experimental investigation into the effects of grain orientation and neighbors on elastic–plastic strain accumulation. In situ strain fields were obtained by performing digital image correlation (DIC) on images from a scanning electron microscope (SEM) and from optical microscopy. These strain fields were statistically compared to the grain structure measured by electron backscatter diffraction (EBSD). Spearman rank correlations were performed between effective strain and six microstructural factors including four Schmid factors associated with the <111> slip direction, grain size, and Taylor factor. Modest correlations (~10%) were found for a polycrystal tension specimen. The influence of grain neighbors was first investigated by re-correlating the polycrystal data using clusters of similarly-oriented grains identified by low grain boundary misorientation angles. Second, the experiment was repeated on a tantalum oligocrystal, with through-thickness grains. Much larger correlation coefficients were found in this multicrystal due to the dearth of grain neighbors and subsurface microstructure. Finally, a slip trace analysis indicated (in agreement with statistical correlations) that macroscopic slip often occurs on {110}<111> slip systems and sometimes by pencil glide on maximum resolved shear stress planes (MRSSP). These results suggest that Schmid factors are suitable for room temperature, quasistatic, tensile deformation in tantalum as long as grain neighbor effects are accounted for.
Heats of Segregation of BCC Metals Using Ab Initio and Quantum Approximate Methods
NASA Technical Reports Server (NTRS)
Good, Brian; Chaka, Anne; Bozzolo, Guillermo
2003-01-01
Many multicomponent alloys exhibit surface segregation, in which the composition at or near a surface may be substantially different from that of the bulk. A number of phenomenological explanations for this tendency have been suggested, involving, among other things, differences among the components' surface energies, molar volumes, and heats of solution. From a theoretical standpoint, the complexity of the problem has precluded a simple, unified explanation, thus preventing the development of computational tools that would enable the identification of the driving mechanisms for segregation. In that context, we investigate the problem of surface segregation in a variety of bcc metal alloys by computing dilute-limit heats of segregation using both the quantum-approximate energy method of Bozzolo, Ferrante and Smith (BFS), and all-electron density functional theory. In addition, the composition dependence of the heats of segregation is investigated using a BFS-based Monte Carlo procedure, and, for selected cases of interest, density functional calculations. Results are discussed in the context of a simple picture that describes segregation behavior as the result of a competition between size mismatch and alloying effects
Influence of point defects on grain boundary mobility in bcc tungsten.
Borovikov, Valery; Tang, Xian-Zhu; Perez, Danny; Bai, Xian-Ming; Uberuaga, Blas P; Voter, Arthur F
2013-01-23
Atomistic computer simulations were performed to study the influence of radiation-induced damage on grain boundary (GB) sliding processes in bcc tungsten (W), the divertor material in the ITER tokamak and the leading candidate for the first wall material in future fusion reactors. In particular, we calculated the average sliding-friction force as a function of the number of point defects introduced into the GB for a number of symmetric tilt GBs. In all cases the average sliding-friction force at fixed shear strain rate depends on the number of point defects introduced into the GB, and in many cases introduction of these defects reduces the average sliding-friction force by roughly an order of magnitude. We have also observed that as the number of interstitials in the GB is varied, the direction of the coupled GB motion sometimes reverses, causing the GB to migrate in the opposite direction under the same applied shear stress. This could be important in the microstructural evolution of polycrystalline W under the harsh radiation environment in a fusion reactor, in which high internal stresses are present and frequent collision cascades generate interstitials and vacancies. PMID:23238084
Elastic constants of bcc shape-memory binary alloys: Effect of the configurational ordering
NASA Astrophysics Data System (ADS)
Castán, T.; Planes, A.
1988-10-01
The relationship between the elastic shear modulus C'=1/2(C11-C12) and the atomic order state in a shape-memory binary alloy AxB1-x above its martensitic transition temperature is analyzed. We first present a simple method to evaluate the elastic constants in binary alloys, assuming the atoms interact via a two-body Morse potential. For CuZn and AgZn alloys, the potential parameters corresponding to the different A-A, B-B, and A-B pairs are determined from experimental data of the elastic constant C' for different alloy compositions. We next calculate C' at 0 K as a function of the ordering state. To do this, we use atomic configurations obtained with a Monte Carlo simulation of the Ising model for a bcc binary alloy, at each temperature Ti. We obtain a linear relationship between C' and the short-range-order parameter η. We also show that the deviations from the linear behavior observed when C' is represented against the square of the long-rang-order parameter come mainly from the critical behavior of the system near the order-disorder temperature Tc.
On the interaction between perfect interstitial clusters and a vacancy in BCC, FCC and HCP metals
Puigvi, Mary Angels; Serra, Anna; de Diego, Nieves; Osetskiy, Yury N; Bacon, David J
2004-01-01
Point defects and defect clusters have been observed in metals irradiated by high-energy particles. Interactions of these defects between themselves and with existing microstructure features cause microstructure evolution and lead to changes in mechanical and physical properties of the irradiated materials. Models for prediction of radiation-induced changes should include details of reactions involving defects, and so in this paper we present the results of atomic-scale computer modelling of interactions between a cluster of self-interstitial atoms (SIAs) and a single vacancy in models of bcc, fcc and hcp metals. The vacancy is taken to lie on or within the glide prism of the cluster. This type of reaction is considered to be one of the most frequent because formation of SIA clusters, particularly glissile clusters, is commonly observed in high-energy displacement cascades in all metals. The interaction depends strongly on the dislocation nature of the cluster and therefore these interactions are different in the three crystal structures. Vacancy-SIA recombination, in particular, is inhibited by dissociation of the SIA loop on its glide prism.
Vibrational contributions to the stability of point defects in bcc iron: A first-principles study
NASA Astrophysics Data System (ADS)
Lucas, G.; Schäublin, R.
2009-09-01
The purpose of this study is to investigate the modes of vibration of the self-interstitial atoms and the vacancy in bcc iron and to estimate how the vibrational properties can affect the stability of these defects. The phonon density of states of the vacancy and the self-interstitials have been calculated within the quasi harmonic approximation using density functional theory calculations. It was observed that self-interstitial atoms have several localized high frequency modes of vibration related to the stretching of the dumbbell bond, but also soft modes favoring their migration. From the phonon density of states, the vibrational contributions to the free energy have been estimated for finite temperatures. Results are compared to previous work performed by others using empirical potentials. We found a rather large formation entropy for the vacancy, SVf = 4.08 kB. Our results show that the vibrational entropy can have a significant influence on the formation of the point defects even at moderate temperature. Possible consequences on the mobility of these defects are also discussed.
Atomic-scale simulations of material behaviors and tribology properties for BCC metal film
NASA Astrophysics Data System (ADS)
H, D. Aristizabal; P, A. Parra; P, López; E, Restrepo-Parra
2016-01-01
This work has two main purposes: (i) introducing the basic concepts of molecular dynamics analysis to material scientists and engineers, and (ii) providing a better understanding of instrumented indentation measurements, presenting an example of nanoindentation and scratch test simulations. To reach these purposes, three-dimensional molecular dynamics (MD) simulations of nanoindentation and scratch test technique were carried out for generic thin films that present BCC crystalline structures. Structures were oriented in the plane (100) and placed on FCC diamond substrates. A pair wise potential was employed to simulate the interaction between atoms of each layer and a repulsive radial potential was used to represent a spherical tip indenting the sample. Mechanical properties of this generic material were obtained by varying the indentation depth and dissociation energy. The load-unload curves and coefficient of friction were found for each test; on the other hand, dissociation energy was varied showing a better mechanical response for films that present grater dissociation energy. Structural change evolution was observed presenting vacancies and slips as the depth was varied. Project supported by la DirecciónNacional de Investigación of the Universidad Nacional de Colombia, “the Theoretical Study of Physical Properties of Hard Materials for Technological Applications” (Grant No. 20101007903).
Atomistic studies of nucleation of He clusters and bubbles in BCC iron
Yang, Li; Deng, Huiqiu; Gao, Fei; Heinisch, Howard L.; Kurtz, Richard J.; Hu, Shenyang Y.; Li, Yulan; Zu, Xiaotao
2013-05-15
Atomistic simulations of the nucleation of He clusters and bubbles in bcc iron at 800 K have been carried out using the newly developed Fe-He interatomic potential, along with Ackland potential for the Fe-Fe interactions. Microstructure changes were analyzed in detail. We found that a He cluster with four He atoms is able to push out an iron interstitial from the cluster, creating a Frenkel pair. Small He clusters and self-interstitial atom (SIA) can migrate in the matrix, but He-vacancy (He-V) clusters are immobile. Most SIAs form <111> clusters, and only the dislocation loops with a Burgers vector of b =1/2<111> appear in the simulations. SIA clusters (or loops) are attached to He-V clusters for He implantation up to 1372 appm, while the He-V cluster-loop complexes with more than one He-V cluster are formed at the He concentration of 2057 appm and larger.
Supong, Khomsan; Thawai, Chitti; Choowong, Wilunda; Kittiwongwattana, Chokchai; Thanaboripat, Dusanee; Laosinwattana, Chamroon; Koohakan, Prommart; Parinthawong, Nonglak; Pittayakhajonwut, Pattama
2016-05-01
An endophytic actinomycete strain BCC72023 was isolated from rice (Oryza sativa L.) and identified as the genus Streptomyces, based on phenotypic, chemotaxonomic and 16S rRNA gene sequence analyses. The strain showed 99.80% similarity compared with Streptomyces samsunensis M1463(T). Chemical investigation led to the isolation of three macrolides, efomycins M (1), G (2) and oxohygrolidin (3), along with two polyethers, abierixin (4) and 29-O-methylabierixin (5). To our knowledge, this is the first report of efomycin M being isolated from a natural source. The compounds were identified using spectroscopic techniques and comparison with previously published data. All compounds exhibited antimalarial activity against the Plasmodium falciparum, K-1 strain, a multidrug-resistant strain, with IC50 values in a range of 1.40-5.23 μg/ml. In addition, these compounds were evaluated for biological activity against Mycobacterium tuberculosis, Bacillus cereus, Colletotrichum gloeosporioides and Colletotrichum capsici, as well as cytotoxicity against both cancerous (MCF-7, KB, NCI-H187) and non-cancerous (Vero) cells. PMID:26809052
Herchenroeder, J.W.
1988-01-01
The high temperature bcc allotrope of a rare earth metal has the potential for substantially different magnetic properties than the room temperature hexagonal (hcp or dcp) counterpart because of its more symmetrical crystal field. The stabilization by alloying and quenching of this bcc phase was studied for La-M alloys where M is a non-rare earth metal from Group II or III. The factors influencing the stabilization, such as size of M and quench rate, are discussed. {gamma}La (bcc) could be retained over a composition range around the eutectoid composition by Mg or Cd alloying. A comparison of T{sub o} curves of the various alloy systems suggest that the eutectoid temperature of the La-M system must be approximately equal to or less than a critical T{sub o} temperature of 515{degree}C if the bcc phase is to be retained by quenching. The thermal stability of {beta}Gd (bcc) was investigated by DTA and isothermal annealing. It was found to transform to an intermediate phase before reverting to the equilibrium phases in contrast to {gamma}La alloys which decompose directly on heating to the equilibrium phases. Bcc {beta}Gd and {beta}Dy stabilized by Mg additions exhibit spin glass-like behavior. Both systems show field cooling effects in the magnetic susceptibility which is indicative of spin freezing reactions.
NASA Astrophysics Data System (ADS)
Weidner, Carrie; Yu, Hoon; Anderson, Dana
2015-05-01
This work introduces a method to perform interferometry using atoms trapped in an optical lattice. Starting at t = 0 with atoms in the ground state of a lattice potential V(x) =V0cos [ 2 kx + ϕ(t) ] , we show that it is possible to transform from one atomic wavefunction to another by a prescribed shaking of the lattice, i.e., by an appropriately tailored time-dependent phase shift ϕ(t) . In particular, the standard interferometer sequence of beam splitting, propagation, reflection, reverse propagation, and recombination can be achieved via a set of phase modulation operations {ϕj(t) } . Each ϕj(t) is determined using a learning algorithm, and the split-step method calculates the wavefunction dynamics. We have numerically demonstrated an interferometer in which the shaken wavefunctions match the target states to better than 1 % . We carried out learning using a genetic algorithm and optimal control techniques. The atoms remain trapped in the lattice throughout the full interferometer sequence. Thus, the approach may be suitable for use in an dynamic environment. In addition to the general principles, we discuss aspects of the experimental implementation. Supported by the Office of Naval Research (ONR) and Northrop Grumman.
Andreas S. Kronfeld
2002-09-30
After reviewing some of the mathematical foundations and numerical difficulties facing lattice QCD, I review the status of several calculations relevant to experimental high-energy physics. The topics considered are moments of structure functions, which may prove relevant to search for new phenomena at the LHC, and several aspects of flavor physics, which are relevant to understanding CP and flavor violation.
Feng Haidong; Siegel, Warren
2006-08-15
We propose some new simplifying ingredients for Feynman diagrams that seem necessary for random lattice formulations of superstrings. In particular, half the fermionic variables appear only in particle loops (similarly to loop momenta), reducing the supersymmetry of the constituents of the type IIB superstring to N=1, as expected from their interpretation in the 1/N expansion as super Yang-Mills.
NASA Astrophysics Data System (ADS)
Singh, Kevin; Geiger, Zachary; Senaratne, Ruwan; Rajagopal, Shankari; Fujiwara, Kurt; Weld, David; Weld Group Team
2015-05-01
Quasiperiodicity is intimately involved in quantum phenomena from localization to the quantum Hall effect. Recent experimental investigation of quasiperiodic quantum effects in photonic and electronic systems have revealed intriguing connections to topological phenomena. However, such experiments have been limited by the absence of techniques for creating tunable quasiperiodic structures. We propose a new type of quasiperiodic optical lattice, constructed by intersecting a Gaussian beam with a 2D square lattice at an angle with an irrational tangent. The resulting potential, a generalization of the Fibonacci lattice, is a physical realization of the mathematical ``cut-and-project'' construction which underlies all quasiperiodic structures. Calculation of the energies and wavefunctions of atoms loaded into the proposed quasiperiodic lattice demonstrate a fractal energy spectrum and the existence of edge states. We acknowledge support from the ONR (award N00014-14-1-0805), the ARO and the PECASE program (award W911NF-14-1-0154), the AFOSR (award FA9550-12-1-0305), and the Alfred P. Sloan foundation (grant BR2013-110).
NASA Astrophysics Data System (ADS)
Schaich, David
2016-03-01
Lattice field theory provides a non-perturbative regularization of strongly interacting systems, which has proven crucial to the study of quantum chromodynamics among many other theories. Supersymmetry plays prominent roles in the study of physics beyond the standard model, both as an ingredient in model building and as a tool to improve our understanding of quantum field theory. Attempts to apply lattice techniques to supersymmetric field theories have a long history, but until recently these efforts have generally encountered insurmountable difficulties related to the interplay of supersymmetry with the lattice discretization of spacetime. In recent years these difficulties have been overcome for a class of theories that includes the particularly interesting case of maximally supersymmetric Yang-Mills (N = 4 SYM) in four dimensions, which is a cornerstone of AdS/CFT duality. In combination with computational advances this progress enables practical numerical investigations of N = 4 SYM on the lattice, which can address questions that are difficult or impossible to handle through perturbation theory, AdS/CFT duality, or the conformal bootstrap program. I will briefly review some of the new ideas underlying this recent progress, and present some results from ongoing large-scale numerical calculations, including comparisons with analytic predictions.
ERIC Educational Resources Information Center
Scott, Paul
2006-01-01
A "convex" polygon is one with no re-entrant angles. Alternatively one can use the standard convexity definition, asserting that for any two points of the convex polygon, the line segment joining them is contained completely within the polygon. In this article, the author provides a solution to a problem involving convex lattice polygons.
Fabricius, Eva-Maria; Kruse-Boitschenko, Ute; Khoury, Reem; Wildner, Gustav-Paul; Raguse, Jan-Dirk; Klein, Martin; Hoffmeister, Bodo
2009-12-01
In previous studies we demonstrated telomerase activity in frozen tissues from BCC and their tumor-free margins by the PCR ELISA. In this study we examined in the same frozen sections immunohistochemical presence of hTERT in the nucleus. After fixation in acetone and methanol followed by steaming we used for visualization the antigen-antibody reactions by APAAP. This was the best method of preparation of the frozen sections in our preliminary hTERT-study with squamous cell carcinomas. This study was supplemented with antibodies against Ki-67, nucleolin, common leucocyte antigen CD45 and mutated p53. The immunoreactive scores were determined and included the comparison with telomerase activity. The investigation of hTERT expression was performed in the tissues of 41 patients with BCC and control tissues of 14 patients without tumor. Eleven commercial antibodies were used for a nuclear staining of hTERT expression. With the anti-hTERT antibodies we looked for both satisfactory distribution and intensity of immunohistochemical labeling in the carcinomas and in the squamous epithelia of the tumor centers, of the tumor-free margins and of the control tissues. The hTERT expression in the BCC was distributed heterogeneously. The score values established by the anti-hTERT antibodies used were variably or significantly increased. In the stroma they tended to be negative, so we disregarded stroma hTERT. Proof of hTERT did not differ uniformly from telomerase activity. We compared the high with the lower median hTERT values in the Kaplan-Meier curve. Patients with lower hTERT scores in the center or tumor margin as shown by some of the antibodies suffered relapse earlier. Finally, we compared the hTERT expression in BCC tissues with the hTERT scores in HNSCC tissues from our previous study. Only one anti-hTERT antibody (our Ab 7) yielded significantly higher scores in BCC than in HNSCC. PMID:19885561
NASA Astrophysics Data System (ADS)
Kapoor, Monica
A series of high-strength low-carbon bcc-Cu- & B2-NiAl-precipitation-strengthened ferritic steels with Mn, Cu, Ni and Al were studied. The yield strength of these alloys increases with the amount of alloying elements. A maximum strength of 1600 MPa, with 12.40 at. % elements, is achieved which is about 30 % higher than the strength of previously reports NUCu (Northwestern Copper) alloys. All the alloys studied attain a maximum hardness within 1--2 h of aging at 500°C--550°C. Aging at a lower temperature and solution treating at a higher temperature can increase the hardness of all the alloys. The lower aging temperature is limited to 500°C by the slow precipitation kinetics observed at 400°C. The higher solution treatment temperature is limited to 1050°C by the adverse impact on toughness in dilute alloys. The primary strengthening contribution is due to combined precipitation of bcc Cu and NiAl-type intermetallic precipitates. The composition, structure and morphology evolution of the precipitates from the 1600 MPa alloy was studied using atom probe tomography and transmission electron microscopy, as a function of aging time at 550°C. Near the peak hardness, the equiaxed bcc Cu-alloyed precipitates have substantial amounts of Fe and are coherent with the Fe matrix. On subsequent aging, the Cu-alloyed precipitates are progressively enriched with Cu and elongate to transform to the 9R phase. The number density of the Cu-alloyed and NiAl-type precipitate is similar near peak hardness indicating that NiAl-type precipitates nucleate on Cu-alloyed precipitates. Almost all Cu-alloyed precipitates are enveloped on one side by ordered NiAl-type precipitates after aging from 2 h to 100 h. Cu-alloyed precipitates coarsen slower than NiAl-type precipitates because of three possible reasons: interfacial energy differences between the two types of precipitates, slower diffusion kinetics of Cu through the ordered B2 NiAl envelope around the bcc Cu-alloyed precipitate
Bohemen, S. M. C. van; Sietsma, J.; Zwaag, S. van der
2006-10-01
The formation mechanisms of two hcp {alpha} phase morphologies in Ti-4.5Fe-6.8Mo-1.5Al have been investigated by optical microscopy (OM), atomic force microscopy (AFM), electron probe microanalysis (EPMA) and dilatometry. At relatively high temperatures primary {alpha} forms predominantly on prior bcc {beta} grain boundaries, whereas at lower temperatures so-called bainitic {alpha} plates nucleate both at grain boundaries and intragranularly. This morphological transition with decreasing temperature is associated with a change in transformation mechanism. The combined results of EPMA, OM and dilatometry show that the growth of these bainitic {alpha} plates is partitionless, and not accompanied by a volume change. Subsequently, a post-transformation redistribution of Fe takes place, which causes a dilatation that can be modelled based on the diffusion of Fe and the interface-area density. This mechanism as well as the formed microstructure are similar to bainite in steel, and therefore we chose to denote this transformation product as bainitic {alpha}. In addition, the AFM observations on bainitic {alpha} plates show an invariant plane strain surface relief with tilt angles that are consistent with the Burgers' transformation model based on shear. In contrast, the AFM results show that the formation of primary {alpha} is accompanied by an irregular dip on a free surface, which is in agreement with the volume decrease measured using dilatometry. Furthermore, the EPMA results show that primary {alpha} is formed by a partitioning transformation. The change in transformation mechanism with decreasing temperature is supported by the observed trend in both the dilatation and the volume fraction {alpha} as a function of temperature.
Plasticity and Failure in Nanocrystalline BCC Metals via Molecular Dynamics Simulation
Rudd, R E
2010-09-29
Advances in the ability to generate extremely high pressures in dynamic experiments such as at the National Ignition Facility has motivated the need for special materials optimized for those conditions as well as ways to probe the response of these materials as they are deformed. We need to develop a much deeper understanding of the behavior of materials subjected to high pressure, especially the effect of rate at the extremely high rates encountered in those experiments. Here we use large-scale molecular dynamics (MD) simulations of the high-rate deformation of nanocrystalline tantalum at pressures less than 100 GPa to investigate the processes associated with plastic deformation for strains up to 100%. We focus on 3D polycrystalline systems with typical grain sizes of 10-20 nm. We also study a rapidly quenched liquid (amorphous solid) tantalum. We apply a constant volume (isochoric), constant temperature (isothermal) shear deformation over a range of strain rates, and compute the resulting stress-strain curves to large strains for both uniaxial and biaxial compression. We study the rate dependence and identify plastic deformation mechanisms. The identification of the mechanisms is facilitated through a novel technique that computes the local grain orientation, returning it as a quaternion for each atom. This analysis technique is robust and fast, and has been used to compute the orientations on the fly during our parallel MD simulations on supercomputers. We find both dislocation and twinning processes are important, and they interact in the weak strain hardening in these extremely fine-grained microstructures. We also present some results on void growth in nanocrystalline BCC metals under tension.
NASA Astrophysics Data System (ADS)
Hsu, Hsiao-Ping; Nadler, Walder; Grassberger, Peter
2005-07-01
The scaling behavior of randomly branched polymers in a good solvent is studied in two to nine dimensions, modeled by lattice animals on simple hypercubic lattices. For the simulations, we use a biased sequential sampling algorithm with re-sampling, similar to the pruned-enriched Rosenbluth method (PERM) used extensively for linear polymers. We obtain high statistics of animals with up to several thousand sites in all dimension 2⩽d⩽9. The partition sum (number of different animals) and gyration radii are estimated. In all dimensions we verify the Parisi-Sourlas prediction, and we verify all exactly known critical exponents in dimensions 2, 3, 4, and ⩾8. In addition, we present the hitherto most precise estimates for growth constants in d⩾3. For clusters with one site attached to an attractive surface, we verify the superuniversality of the cross-over exponent at the adsorption transition predicted by Janssen and Lyssy.
Gupta, R.
1998-12-31
The goal of the lectures on lattice QCD (LQCD) is to provide an overview of both the technical issues and the progress made so far in obtaining phenomenologically useful numbers. The lectures consist of three parts. The author`s charter is to provide an introduction to LQCD and outline the scope of LQCD calculations. In the second set of lectures, Guido Martinelli will discuss the progress they have made so far in obtaining results, and their impact on Standard Model phenomenology. Finally, Martin Luescher will discuss the topical subjects of chiral symmetry, improved formulation of lattice QCD, and the impact these improvements will have on the quality of results expected from the next generation of simulations.
Lattice Boltzmann morphodynamic model
NASA Astrophysics Data System (ADS)
Zhou, Jian Guo
2014-08-01
Morphological change due to sediment transport is a common natural phenomenon in real flows. It involves complex processes of erosion and deposition such as those along beaches and in river beds, imposing a strong strain on human beings. Studying and understanding morphodynamic evolution are essential to protect living environment. Although there are conventional numerical methods like finite difference method and finite volume method for forecast of morphological change by solving flow and morphodynamic equations, the methods are too complex/inefficient to be applied to a real large scale problem. To overcome this, a lattice Boltzmann method is developed to simulate morphological evolution under flows. It provides an alternative way of studying morphodynamics at the full advantages of the lattice Boltzmann methodology. The model is verified by applications to the evolution of one and two dimensional sand dunes under shallow water flows.
Kronfeld, A.S.; Allison, I.F.; Aubin, C.; Bernard, C.; Davies, C.T.H.; DeTar, C.; Di Pierro, M.; Freeland, E.D.; Gottlieb, Steven; Gray, A.; Gregor, E.; Heller, U.M.; Hetrick, J.E.; El-Khadra, Aida X.; Levkova, L.; Mackenzie, P.B.; Maresca, F.; Menscher, D.; Nobes, M.; Okamoto, M.; Oktay, M.B.; /Fermilab /Glasgow U. /Columbia U. /Washington U., St. Louis /Utah U. /DePaul U. /Art Inst. of Chicago /Indiana U. /Ohio State U. /Arizona U. /APS, New York /U. Pacific, Stockton /Illinois U., Urbana /Cornell U., LEPP /Simon Fraser U. /UC, Santa Barbara
2005-09-01
In the past year, we calculated with lattice QCD three quantities that were unknown or poorly known. They are the q{sup 2} dependence of the form factor in semileptonic D {yields} K/{nu} decay, the decay constant of the D meson, and the mass of the B{sub c} meson. In this talk, we summarize these calculations, with emphasis on their (subsequent) confirmation by experiments.
Multipole plasmonic lattice solitons
Kou Yao; Ye Fangwei; Chen Xianfeng
2011-09-15
We theoretically demonstrate a variety of multipole plasmonic lattice solitons, including dipoles, quadrupoles, and necklaces, in two-dimensional metallic nanowire arrays with Kerr-type nonlinearities. Such solitons feature complex internal structures with an ultracompact mode size approaching or smaller than one wavelength. Their mode sizes and the stability characteristics are studied in detail within the framework of coupled mode theory. The conditions to form and stabilize these highly confined solitons are within the experimentally achievable range.
Nucleon Structure from Lattice QCD
David Richards
2007-09-05
Recent advances in lattice field theory, in computer technology and in chiral perturbation theory have enabled lattice QCD to emerge as a powerful quantitative tool in understanding hadron structure. I describe recent progress in the computation of the nucleon form factors and moments of parton distribution functions, before proceeding to describe lattice studies of the Generalized Parton Distributions (GPDs). In particular, I show how lattice studies of GPDs contribute to building a three-dimensional picture of the proton, I conclude by describing the prospects for studying the structure of resonances from lattice QCD.
Lattice QCD for parallel computers
NASA Astrophysics Data System (ADS)
Quadling, Henley Sean
Lattice QCD is an important tool in the investigation of Quantum Chromodynamics (QCD). This is particularly true at lower energies where traditional perturbative techniques fail, and where other non-perturbative theoretical efforts are not entirely satisfactory. Important features of QCD such as confinement and the masses of the low lying hadronic states have been demonstrated and calculated in lattice QCD simulations. In calculations such as these, non-lattice techniques in QCD have failed. However, despite the incredible advances in computer technology, a full solution of lattice QCD may still be in the too-distant future. Much effort is being expended in the search for ways to reduce the computational burden so that an adequate solution of lattice QCD is possible in the near future. There has been considerable progress in recent years, especially in the research of improved lattice actions. In this thesis, a new approach to lattice QCD algorithms is introduced, which results in very significant efficiency improvements. The new approach is explained in detail, evaluated and verified by comparing physics results with current lattice QCD simulations. The new sub-lattice layout methodology has been specifically designed for current and future hardware. Together with concurrent research into improved lattice actions and more efficient numerical algorithms, the very significant efficiency improvements demonstrated in this thesis can play an important role in allowing lattice QCD researchers access to much more realistic simulations. The techniques presented in this thesis also allow ambitious QCD simulations to be performed on cheap clusters of commodity computers.
Toward lattice fractional vector calculus
NASA Astrophysics Data System (ADS)
Tarasov, Vasily E.
2014-09-01
An analog of fractional vector calculus for physical lattice models is suggested. We use an approach based on the models of three-dimensional lattices with long-range inter-particle interactions. The lattice analogs of fractional partial derivatives are represented by kernels of lattice long-range interactions, where the Fourier series transformations of these kernels have a power-law form with respect to wave vector components. In the continuum limit, these lattice partial derivatives give derivatives of non-integer order with respect to coordinates. In the three-dimensional description of the non-local continuum, the fractional differential operators have the form of fractional partial derivatives of the Riesz type. As examples of the applications of the suggested lattice fractional vector calculus, we give lattice models with long-range interactions for the fractional Maxwell equations of non-local continuous media and for the fractional generalization of the Mindlin and Aifantis continuum models of gradient elasticity.
Chui, C. P.; Zhou, Yan
2014-08-15
The understanding of the magnetovolume effect lacks explicit consideration of spin-lattice coupling at the atomic level, despite abundant theoretical and experimental studies throughout the years. This research gap is filled by the recently developed spin-lattice dynamics technique implemented in this study, which investigates the magnetovolume effect of isotropic body-centered-cubic (BCC) iron, a topic that has previously been subject to macroscopic analysis only. This approach demonstrates the magnetic anomaly followed by the volumetric changes associated with the effect, each characterized by the corresponding field-induced inflection temperature. The temperature of the heat capacity peaks is useful in determining the temperature for retarding the atomic volume increase. Moreover, this work shows the correlation between the effects of temperature and field strength in determining the equilibrium atomic volume of a ferromagnetic material under a magnetic field.
A Mechanical Lattice Aid for Crystallography Teaching.
ERIC Educational Resources Information Center
Amezcua-Lopez, J.; Cordero-Borboa, A. E.
1988-01-01
Introduces a 3-dimensional mechanical lattice with adjustable telescoping mechanisms. Discusses the crystalline state, the 14 Bravais lattices, operational principles of the mechanical lattice, construction methods, and demonstrations in classroom. Provides lattice diagrams, schemes of the lattice, and various pictures of the lattice. (YP)
Influence of Ni on the lattice stability of Fe-Ni alloys at multimegabar pressures
NASA Astrophysics Data System (ADS)
Vekilova, O. Yu.; Simak, S. I.; Ponomareva, A. V.; Abrikosov, I. A.
2012-12-01
The lattice stability trends of the primary candidate for Earth's core material, the Fe-Ni alloy, were examined from first principles. We employed the exact muffin-tin orbital method (EMTO) combined with the coherent potential approximation (CPA) for the treatment of alloying effects. It was revealed that high pressure reverses the trend in the relative stabilities of the body-centered cubic (bcc), face-centered cubic (fcc), and hexagonal close-packed (hcp) phases observed at ambient conditions. In the low pressure region the increase of Ni concentration in the Fe-Ni alloy enhances the bcc phase destabilization relative to the more close-packed fcc and hcp phases. However, at 300 GPa (Earth's core pressure), the effect of Ni addition is opposite. The reverse of the trend is associated with the suppression of the ferromagnetism of Fe when going from ambient pressures to pressure conditions corresponding to those of Earth's core. The first-principles results are explained in the framework of the canonical band model.
Frequency of occurrence of LCS per cascade in bcc iron by PKA energy 30 keV
NASA Astrophysics Data System (ADS)
Elias, A.; Driss Khodja, M.
2009-11-01
The frequency of occurrence per cascade in bcc iron is studied by PKA energy 30keV using the fitting of potentials on linear collision sequences (LCS). Two embedded atom potentials that essentially differ by their repulsive branches are used in classical molecular dynamic (MD) with the code DYMOKA and its binary collision approximation (BCA) The Molière and Born Mayer potentials are used in the Binary Collision Approximation (BCA). The linear collision sequences (LCS) in iron can be generated with Marlowe code in BCA, using the potentials parameters. The BCA is used to accumulate cascade statistics and to build linear collision sequences distributions.
NASA Astrophysics Data System (ADS)
Tougou, Kouichi; Shikata, Akihito; Kawase, Uchu; Onitsuka, Takashi; Fukumoto, Ken-ichi
2015-10-01
To investigate the effect of irradiation hardening of structural materials due to cavity formation in BCC metals for nuclear applications, an in-situ transmission electron microscopy (TEM) observation in tensile test was performed for the helium ion-irradiated specimens of pure molybdenum and pure iron. The obstacle barrier strength, α was calculated from the bow-out dislocation based on line tension model, and the obstacle barrier strengths of cavity in pure molybdenum and pure iron were about 0.5-0.7. The fractions of cross-slip generation of dislocation of screw type due to interaction with the cavities were about 16-18 % for pure molybdenum.
Lattice-induced nonadiabatic frequency shifts in optical lattice clocks
Beloy, K.
2010-09-15
We consider the frequency shift in optical lattice clocks which arises from the coupling of the electronic motion to the atomic motion within the lattice. For the simplest of three-dimensional lattice geometries this coupling is shown to affect only clocks based on blue-detuned lattices. We have estimated the size of this shift for the prospective strontium lattice clock operating at the 390-nm blue-detuned magic wavelength. The resulting fractional frequency shift is found to be on the order of 10{sup -18} and is largely overshadowed by the electric quadrupole shift. For lattice clocks based on more complex geometries or other atomic systems, this shift could potentially be a limiting factor in clock accuracy.
Herchenroeder, J.W.
1989-02-01
The high temperature bcc allotrope of a rare earth metal has the potential for substantially different magnetic properties than the room temperature hexagonal (hcp or dhcp) counterpart because of its more symmetrical crystal field. The stabilization by alloying and quenching of this bcc phase was studied for La-M alloys where M is an non-rare earth metal from Group II or III. The factors influencing the stabilization, such as size of M and quench rate, are discussed. ..gamma..La (bcc) could be retained over a composition range around the eutectoid composition by Mg or Cd alloying. A comparison of T/sub o/ curves of the various alloy systems suggest that the eutectoid temperature of the La-M system must be approximately equal to or less than a critical T/sub o/ temperature of 515/degree/C if the bcc phase is to be retained by quenching. The thermal stability of ..beta..Gd (bcc) was investigated by DTA and isothermal annealing. It was found to transform to an intermediate phase before reverting to the equilibrium phases in contrast to ..gamma..La alloys which decompose directly on heating to the equilibrium phases. 71 refs., 52 figs., 7 tabs.
Murphy, Mark P; Caraher, Emma
2015-11-01
Bacteria of the Burkholderia cepacia complex (Bcc) persist in the airways of people with cystic fibrosis (CF) despite the continuous recruitment of neutrophils. Most members of Bcc are multidrug resistant and can form biofilms. As such, we sought to investigate whether biofilm formation plays a role in protecting Bcc bacteria from neutrophils. Using the neutrophil-like, differentiated cell line, dHL60, we have shown for the first time that Bcc biofilms are enhanced in the presence of these cells. Biofilm biomass was greater following culture in the presence of dHL60 cells than in their absence, likely the result of incorporating dHL60 cellular debris into the biofilm. Moreover, we have demonstrated that mature biofilms (cultured for up to 72 h) induced necrosis in the cells. Established biofilms also acted as a barrier to the migration of the cells and masked the bacteria from being recognized by the cells; dHL60 cells expressed less IL-8 mRNA and secreted significantly less IL-8 when cultured in the presence of biofilms, with respect to planktonic bacteria. Our findings provide evidence that biofilm formation can, at least partly, enable the persistence of Bcc bacteria in the CF airway and emphasize a requirement for anti-biofilm therapeutics. PMID:26371179
Optical properties of bcc transition metals in the range 0-40eV
NASA Astrophysics Data System (ADS)
Romaniello, P.; de Boeij, P. L.; Carbone, F.; van der Marel, D.
2006-02-01
We present a systematic analysis of the optical properties of bcc transition metals in the groups VB: V, Nb, and Ta, and VIB: paramagnetic Cr, Mo, and W. For this we use our formulation of time-dependent current-density-functional theory for the linear response of metals. The calculated dielectric and electron energy-loss functions are compared with our ellipsometry measurements and with data reported in literature, showing an overall good agreement. The experimental data of the dielectric functions presented by Nestell and Christy and by Weaver differ mostly in the low-frequency region. However, we found that their reflectivity data are in very good agreement up to about 3eV . We attribute this apparent discrepancy to the Drude-like extrapolation model used by Weaver in the Kramers-Kronig procedure to extract the optical constants from their reflectivity data. Our experiments are in good agreement with Nestell and Christy’s data. The calculated absorption spectra show some deviations from the experiments, in particular in the 3d metals. We assign the spectra in terms of transitions between pairs of bands and we analyze which parts of the Brillouin zone are mainly involved in the absorption. Our results suggest that the blueshift of some spectral features in our calculations can be attributed mainly to the incorrect description of the virtual d bands by the approximations used for the ground state exchange-correlation functional. These virtual bands are too weakly bound by the local density and generalized gradient approximations, in particular in the 3d metals. We calculate separately the inter- and intraband contributions to the absorption and we show using a k•p analysis that, within the scalar-relativistic approximation, interband transitions contribute to the absorption already at frequencies well below 0.5eV . This finding makes questionable the Drude-like behavior normally assumed in the experimental analysis of the linear response. We find that the
Single identities for lattice theory and for weakly associative lattices
McCune, W.; Padmanabhan, R.
1995-03-13
We present a single identity for the variety of all lattices that is much simpler than those previously known to us. We also show that the variety of weakly associative lattices is one-based, and we present a generalized one-based theorem for subvarieties of weakly associative lattices that can be defined with absorption laws. The automated theorem-proving program OTTER was used in substantial way to obtain the results.
Matsuoka, H.
1985-01-01
The thermodynamic consequences of QCD are explored in the framework of lattice gauge theory. Attention is focused upon the nature of the chiral symmetry restoration transition at finite temperature and at finite baryon density, and possible strategies for identifying relevant thermodynamic phases are discussed. Some numerical results are presented on the chiral symmetry restoration in the SU(2) gauge theory at high baryon density. The results suggest that with T approx. = 110 MeV there is a second order restoration transition at the critical baryon density n/sub B//sup c/ approx. = 0.62 fm/sup -3/.
Computation of the lattice Green function for a dislocation
NASA Astrophysics Data System (ADS)
Tan, Anne Marie Z.; Trinkle, Dallas R.
2016-08-01
Modeling isolated dislocations is challenging due to their long-ranged strain fields. Flexible boundary condition methods capture the correct long-range strain field of a defect by coupling the defect core to an infinite harmonic bulk through the lattice Green function (LGF). To improve the accuracy and efficiency of flexible boundary condition methods, we develop a numerical method to compute the LGF specifically for a dislocation geometry; in contrast to previous methods, where the LGF was computed for the perfect bulk as an approximation for the dislocation. Our approach directly accounts for the topology of a dislocation, and the errors in the LGF computation converge rapidly for edge dislocations in a simple cubic model system as well as in BCC Fe with an empirical potential. When used within the flexible boundary condition approach, the dislocation LGF relaxes dislocation core geometries in fewer iterations than when the perfect bulk LGF is used as an approximation for the dislocation, making a flexible boundary condition approach more efficient.
Molecular Dynamics Study of Void Growth and Dislocations in Dynamic Fracture of FCC and BCC Metals
Seppala, E T; Belak, J; Rudd, R E
2003-06-17
Void growth with concomitant dislocation formation has been studied in single crystal face-centered-cubic and body-centered-cubic metals using molecular dynamics method with Embedded-Atom and Finnis-Sinclair potentials for copper and tantalum, respectively. We have concentrated on the quantitative analysis of the void shape evolution, on the structure of dislocations, which emerge from the void, and on the continuum measures such as plastic strain. The effects of strain-rate, differences between lattice structures, and loading conditions as uniaxial, biaxial, and triaxial expansion on the shape of the void and on the dislocations have been investigated.
Rasmussen, S. |; Smith, J.R. |
1995-05-01
We present a new style of molecular dynamics and self-assembly simulation, the Lattice Polymer Automaton (LPA). In the LPA all interactions, including electromagnetic forces, are decomposed and communicated via propagating particles, {open_quotes}photons.{close_quotes} The monomer-monomer bondforces, the molecular excluded volume forces, the longer range intermolecular forces, and the polymer-solvent interactions may all be modeled with propagating particles. The LPA approach differs significantly from both of the standard approaches, Monte Carlo lattice methods and Molecular Dynamics simulations. On the one hand, the LPA provides more realism than Monte Carlo methods, because it produces a time series of configurations of a single molecule, rather than a set of causally unrelated samples from a distribution of configurations. The LPA can therefore be used directly to study dynamical properties; one can in fact watch polymers move in real time. On the other hand, the LPA is fully discrete, and therefore much simpler than traditional Molecular Dynamics models, which are continuous and operate on much shorter time scales. Due to this simplicity it is possible to simulate longer real time periods, which should enable the study of molecular self-organization on workstations supercomputers are not needed.
Quantum Gases in Optical Lattices
NASA Astrophysics Data System (ADS)
Barmettler, Peter; Kollath, Corinna
2015-09-01
The experimental realization of correlated quantum phases with ultracold gases in optical lattices and their theoretical understanding has witnessed remarkable progress during the last decade. In this review we introduce basic concepts and tools to describe the many-body physics of quantum gases in optical lattices. This includes the derivation of effective lattice Hamiltonians from first principles and an overview of the emerging quantum phases. Additionally, state-of-the-art numerical tools to quantitatively treat bosons or fermions on different lattices are introduced.
Nuclear Physics and Lattice QCD
Beane, Silas
2003-11-01
Impressive progress is currently being made in computing properties and interac- tions of the low-lying hadrons using lattice QCD. However, cost limitations will, for the foreseeable future, necessitate the use of quark masses, Mq, that are signif- icantly larger than those of nature, lattice spacings, a, that are not significantly smaller than the physical scale of interest, and lattice sizes, L, that are not sig- nificantly larger than the physical scale of interest. Extrapolations in the quark masses, lattice spacing and lattice volume are therefore required. The hierarchy of mass scales is: L 1 j Mq j â ºC j a 1 . The appropriate EFT for incorporating the light quark masses, the finite lattice spacing and the lattice size into hadronic observables is C-PT, which provides systematic expansions in the small parame- ters e m L, 1/ Lâ ºC, p/â ºC, Mq/â ºC and aâ ºC . The lattice introduces other unphysical scales as well. Lattice QCD quarks will increasingly be artificially separated
NASA Astrophysics Data System (ADS)
Nishijima, Masahiko; Matsuura, Makoto; Takenaka, Kana; Takeuchi, Akira; Ofuchi, Hironori; Makino, Akihiro
2014-05-01
A role of Cu on the nanocrystallization of an Fe85.2Si1B9P4Cu0.8 alloy was investigated by X-ray absorption fine structure (XAFS) and transmission electron microscopy (TEM). The Cu K-edge XAFS results show that local structure around Cu is disordered for the as-quenched sample whereas it changes to fcc-like structure at 613 K. The fcc Cu-clusters are, however, thermodynamically unstable and begin to transform into bcc structure at 638 K. An explicit bcc structure is observed for the sample annealed at 693 K for 600 s in which TEM observation shows that precipitated bcc-Fe crystallites with ˜12 nm are homogeneously distributed. The bcc structure of the Cu-clusters transforms into the fcc-type again at 973 K, which can be explained by the TEM observations; Cu segregates at grain boundaries between bcc-Fe crystallites and Fe3(B,P) compounds. Combining the XAFS results with the TEM observations, the structure transition of the Cu-clusters from fcc to bcc is highly correlated with the preliminary precipitation of the bcc-Fe which takes place prior to the onset of the first crystallization temperature, Tx1 = 707 K. Thermodynamic analysis suggests that an interfacial energy density γ between an fcc-Cu cluster and bcc-Fe matrix dominates at a certain case over the structural energy between fcc and bcc Cu, ΔGfcc - bcc, which causes phase transition of the Cu clusters from fcc to bcc structure.
NASA Astrophysics Data System (ADS)
Hüter, Claas; Friák, Martin; Weikamp, Marc; Neugebauer, Jörg; Goldenfeld, Nigel; Svendsen, Bob; Spatschek, Robert
2016-06-01
We investigate nonlinear elastic deformations in the phase field crystal model and derived amplitude equation formulations. Two sources of nonlinearity are found, one of them is based on geometric nonlinearity expressed through a finite strain tensor. This strain tensor is based on the inverse right Cauchy-Green deformation tensor and correctly describes the strain dependence of the stiffness for anisotropic and isotropic behavior. In isotropic one- and two-dimensional situations, the elastic energy can be expressed equivalently through the left deformation tensor. The predicted isotropic low-temperature nonlinear elastic effects are directly related to the Birch-Murnaghan equation of state with bulk modulus derivative K'=4 for bcc. A two-dimensional generalization suggests K2D '=5 . These predictions are in agreement with ab initio results for large strain bulk deformations of various bcc elements and graphene. Physical nonlinearity arises if the strain dependence of the density wave amplitudes is taken into account and leads to elastic weakening. For anisotropic deformation, the magnitudes of the amplitudes depend on their relative orientation to the applied strain.
NASA Astrophysics Data System (ADS)
Palberg, Thomas; Wette, Patrick; Herlach, Dieter M.
2016-02-01
The interfacial free energy is a central quantity in crystallization from the metastable melt. In suspensions of charged colloidal spheres, nucleation and growth kinetics can be accurately measured from optical experiments. In previous work, from these data effective nonequilibrium values for the interfacial free energy between the emerging bcc nuclei and the adjacent melt in dependence on the chemical potential difference between melt phase and crystal phase were derived using classical nucleation theory (CNT). A strictly linear increase of the interfacial free energy was observed as a function of increased metastability. Here, we further analyze these data for five aqueous suspensions of charged spheres and one binary mixture. We utilize a simple extrapolation scheme and interpret our findings in view of Turnbull's empirical rule. This enables us to present the first systematic experimental estimates for a reduced interfacial free energy, σ0 ,b c c, between the bcc-crystal phase and the coexisting equilibrium fluid. Values obtained for σ0 ,b c c are on the order of a few kBT . Their values are not correlated to any of the electrostatic interaction parameters but rather show a systematic decrease with increasing size polydispersity and a lower value for the mixture as compared to the pure components. At the same time, σ0 also shows an approximately linear correlation to the entropy of freezing. The equilibrium interfacial free energy of strictly monodisperse charged spheres may therefore be still greater.
Detection of nonribosomal peptide synthetase genes in Xylaria sp. BCC1067 and cloning of XyNRPSA.
Paungmoung, Porntip; Punya, Jantira; Pongpattanakitshote, Somchai; Jeamton, Wattana; Vichisoonthonkul, Taweerat; Bhumiratana, Sakarindr; Tanticharoen, Morakot; Linne, Uwe; Marahiel, Mohamed A; Cheevadhanarak, Supapon
2007-09-01
Nonribosomal peptides, synthesized by nonribosomal peptide synthetases (NRPS), are an important group of diverse bioactive fungal metabolites. Xylaria sp. BCC1067, which is known to produce a variety of biologically active metabolites, was studied for gene encoding NRPS by two different PCR-based methods and seven different NRPS fragments were obtained. In addition, screening a genomic library with an amplified NRPS fragment as a probe identified a putative NRPS gene named XyNRPSA. The functionality of XyNRPSA for the production of a corresponding metabolite was probed by gene insertion inactivation. Comparing the disrupting metabolite profile with that of the wild type led to the identification of a speculated metabolite. The crude extract of Xylaria sp. BCC1067 also exhibits antifungal activity against the human pathogens Candida albicans and Trichophyton mentagrophytes. However, the evaluation of biological activity of the XyNRPSA product suggests that it is neither a compound with antifungal activity nor a siderophore. In the vicinity of XyNRPSA, a second gene (named XyPtB) was identified. Its localization and homology to orfB of the ergot alkaloid biosynthetic gene cluster suggests that XyPtB may be involved in XyNRPSA product biosynthesis. PMID:17623029
Point defect properties in hcp and bcc Zr with trace solute Nb revealed by ab initio calculations
NASA Astrophysics Data System (ADS)
Xin, X. K.; Lai, W. S.; Liu, B. X.
2009-08-01
The properties of simple point defect (i.e. vacancy, self and foreign interstitial atoms) in the hcp (alpha) and bcc (beta) Zr with trace solute Nb have been studied by ab initio calculations with VASP codes. The calculations indicate that the formation energies of vacancy and substitutional Nb atom are 1.94 eV and 0.68 eV in alpha Zr and 0.36 eV and 0.07 eV in beta Zr, respectively, while the binding energies of the nearest neighbor vacancy-substitutional Nb pair and the nearest neighbor substitutional Nb-Nb pair are 0.09 eV and 0.03 eV in alpha Zr and 2.78 eV and 0.72 eV in beta Zr, respectively. These results suggest that the Nb atoms are more likely to agglomerate and form precipitates in the beta Zr than in the alpha Zr. Thus, the α-Zr-β-Zr-β-Nb transition mechanism through in situ α to β transformation of Zr and the vacancy-assisted Nb diffusion for Nb conglomeration in beta Zr under irradiation is proposed to explain the existence of beta Nb and Zr precipitate mixtures observed in the experiments for the Zr-Nb alloy. In addition, the defect formation energies in bcc Nb are also presented.
Minakawa, Shigeyuki Ohtake, Mitsuru; Futamoto, Masaaki; Kirino, Fumiyoshi; Inaba, Nobuyuki
2015-05-07
Co{sub 100−x}Fe{sub x} and Ni{sub 100−y}Fe{sub y} (at. %, x = 0–30, y = 0–60) films of 10 nm thickness are prepared on GaAs(100) substrates at room temperature by using a radio-frequency magnetron sputtering system. The detailed growth behavior is investigated by in-situ reflection high-energy electron diffraction. (100)-oriented Co and Ni single-crystals with metastable bcc structure are formed in the early stage of film growth, where the metastable structure is stabilized through hetero-epitaxial growth. With increasing the thickness up to 2 nm, the Co and the Ni films start to transform into more stable hcp and fcc structures through atomic displacements parallel to bcc(110) slide planes, respectively. The stability of bcc phase is improved by adding a small volume of Fe atoms into a Co film. The critical thickness of bcc phase formation is thicker than 10 nm for Co{sub 100−x}Fe{sub x} films with x ≥ 10. On the contrary, the stability of bcc phase for Ni-Fe system is less than that for Co-Fe system. The critical thicknesses for Ni{sub 100−y}Fe{sub y} films with y = 20, 40, and 60 are 1, 3, and 5 nm, respectively. The Co{sub 100−x}Fe{sub x} single-crystal films with metastable bcc structure formed on GaAs(100) substrates show in-plane uniaxial magnetic anisotropies with the easy direction along GaAs[011], similar to the case of Fe film epitaxially grown on GaAs(100) substrate. A Co{sub 100−x}Fe{sub x} film with higher Fe content shows a higher saturation magnetization and a lower coercivity.
Thermal vacancies and phase separation in bcc mixtures of helium-3 and helium-4
Fraass, Benedick Andrew
1980-01-01
Thermal vacancy concentrations in crystals of /sup 3/He-/sup 4/He mixtures have been determined. A new x-ray diffractometer-position sensitive detector system is used to make measurements of the absolute lattice parameter of the helium crystals with an accuracy of 300 ppM, and measurements of changes in lattice parameters to better than 60 ppM. The phase separation of the concentrated /sup 3/He-/sup 4/He mixtures has been studied in detail with the x-ray measurements. Vacancy concentrations in crystals with 99%, 51%, 28%, 12%, and 0% /sup 3/He have been determined. Phase separation has been studied in mixed crystals with concentrations of 51%, 28%, and 12% /sup 3/He and melting pressures between 3.0 and 6.1 MPa. The phase separation temperatures determined in this work are in general agreement with previous work. The pressure dependence of T/sub c/, the phase separation temperature for a 50% mixture, is found to be linear: dT/sub c//dP = -34 mdeg/MPa. The x-ray measurements are used to make several comments on the low temperature phase diagram of the helium mixtures.
Elimination of spurious lattice fermion solutions and noncompact lattice QCD
Lee, T.D.
1997-09-22
It is well known that the Dirac equation on a discrete hyper-cubic lattice in D dimension has 2{sup D} degenerate solutions. The usual method of removing these spurious solutions encounters difficulties with chiral symmetry when the lattice spacing l {ne} 0, as exemplified by the persistent problem of the pion mass. On the other hand, we recall that in any crystal in nature, all the electrons do move in a lattice and satisfy the Dirac equation; yet there is not a single physical result that has ever been entangled with a spurious fermion solution. Therefore it should not be difficult to eliminate these unphysical elements. On a discrete lattice, particle hop from point to point, whereas in a real crystal the lattice structure in embedded in a continuum and electrons move continuously from lattice cell to lattice cell. In a discrete system, the lattice functions are defined only on individual points (or links as in the case of gauge fields). However, in a crystal the electron state vector is represented by the Bloch wave functions which are continuous functions in {rvec {gamma}}, and herein lies one of the essential differences.
Physical symmetry and lattice symmetry in the lattice Boltzmann method
Cao, N.; Chen, S.; Jin, S.; Martinez, D.
1997-01-01
The lattice Boltzmann method (LBM) is regarded as a specific finite difference discretization for the kinetic equation of the discrete velocity distribution function. We argue that for finite sets of discrete velocity models, such as LBM, the physical symmetry is necessary for obtaining the correct macroscopic Navier-Stokes equations. In contrast, the lattice symmetry and the Lagrangian nature of the scheme, which is often used in the lattice gas automaton method and the existing lattice Boltzmann methods and directly associated with the property of particle dynamics, is not necessary for recovering the correct macroscopic dynamics. By relaxing the lattice symmetry constraint and introducing other numerical discretization, one can also obtain correct hydrodynamics. In addition, numerical simulations for applications, such as nonuniform meshes and thermohydrodynamics can be easily carried out and numerical stability can be ensured by the Courant-Friedricks-Lewey condition and using the semi-implicit collision scheme. {copyright} {ital 1997} {ital The American Physical Society}
Optical Abelian lattice gauge theories
Tagliacozzo, L.; Celi, A.; Zamora, A.; Lewenstein, M.
2013-03-15
We discuss a general framework for the realization of a family of Abelian lattice gauge theories, i.e., link models or gauge magnets, in optical lattices. We analyze the properties of these models that make them suitable for quantum simulations. Within this class, we study in detail the phases of a U(1)-invariant lattice gauge theory in 2+1 dimensions, originally proposed by P. Orland. By using exact diagonalization, we extract the low-energy states for small lattices, up to 4 Multiplication-Sign 4. We confirm that the model has two phases, with the confined entangled one characterized by strings wrapping around the whole lattice. We explain how to study larger lattices by using either tensor network techniques or digital quantum simulations with Rydberg atoms loaded in optical lattices, where we discuss in detail a protocol for the preparation of the ground-state. We propose two key experimental tests that can be used as smoking gun of the proper implementation of a gauge theory in optical lattices. These tests consist in verifying the absence of spontaneous (gauge) symmetry breaking of the ground-state and the presence of charge confinement. We also comment on the relation between standard compact U(1) lattice gauge theory and the model considered in this paper. - Highlights: Black-Right-Pointing-Pointer We study the quantum simulation of dynamical gauge theories in optical lattices. Black-Right-Pointing-Pointer We focus on digital simulation of abelian lattice gauge theory. Black-Right-Pointing-Pointer We rediscover and discuss the puzzling phase diagram of gauge magnets. Black-Right-Pointing-Pointer We detail the protocol for time evolution and ground-state preparation in any phase. Black-Right-Pointing-Pointer We provide two experimental tests to validate gauge theory quantum simulators.
Characterization of projection lattices of Hilbert spaces
Szambien, H.H.
1986-09-01
The classical lattices of projections of Hilbert spaces over the real, the complex or the quaternion number field are characterized among the totality of irreducible, complete, orthomodular, atomic lattices satisfying the covering property. To this end, so-called paratopological lattices are introduced, i.e, lattices carrying a topology that renders the lattice operations restrictedly continuous.
Excitonic surface lattice resonances
NASA Astrophysics Data System (ADS)
Humphrey, A. D.; Gentile, M. J.; Barnes, W. L.
2016-08-01
Electromagnetic resonances are important in controlling light at the nanoscale. The most studied such resonance is the surface plasmon resonance that is associated with metallic nanostructures. Here we explore an alternative resonance, the surface exciton-polariton resonance, one based on excitonic molecular materials. Our study is based on analytical and numerical modelling. We show that periodic arrays of suitable molecular nanoparticles may support surface lattice resonances that arise as a result of coherent interactions between the particles. Our results demonstrate that excitonic molecular materials are an interesting alternative to metals for nanophotonics; they offer the prospect of both fabrication based on supramolecular chemistry and optical functionality arising from the way the properties of such materials may be controlled with light.
Advances in lattice gauge theory
Duke, D.W.; Owens, J.F.
1985-01-01
This book presents papers on advances in lattice gauge theory. Topics covered include fermion Monte Carlo algorithms, portrait of a proton, critical behavior in QCD, the standard Higgs-model on the lattice, analytic calculation of mass gaps, and simulation of discrete Euclidean quantum gravity.
Buckling modes in pantographic lattices
NASA Astrophysics Data System (ADS)
Giorgio, Ivan; Della Corte, Alessandro; dell'Isola, Francesco; Steigmann, David J.
2016-07-01
We study buckling patterns in pantographic sheets, regarded as two-dimensional continua consisting of lattices of continuously distributed fibers. The fibers are modeled as beams endowed with elastic resistance to stretching, shearing, bending and twist. Included in the theory is a non-standard elasticity due to geodesic bending of the fibers relative to the lattice surface. xml:lang="fr"
Berg,J.S.
2008-02-21
I give a brief introduction to the purpose and goals of the EMMA experiment and describe how they will impact the design of the main EMMA ring. I then describe the mathematical model that is used to describe the EMMA lattice. Finally, I show how the different lattice configurations were obtained and list their parameters.
Introduction to lattice gauge theory
NASA Astrophysics Data System (ADS)
Gupta, R.
The lattice formulation of Quantum Field Theory (QFT) can be exploited in many ways. We can derive the lattice Feynman rules and carry out weak coupling perturbation expansions. The lattice then serves as a manifestly gauge invariant regularization scheme, albeit one that is more complicated than standard continuum schemes. Strong coupling expansions: these give us useful qualitative information, but unfortunately no hard numbers. The lattice theory is amenable to numerical simulations by which one calculates the long distance properties of a strongly interacting theory from first principles. The observables are measured as a function of the bare coupling g and a gauge invariant cut-off approx. = 1/alpha, where alpha is the lattice spacing. The continuum (physical) behavior is recovered in the limit alpha yields 0, at which point the lattice artifacts go to zero. This is the more powerful use of lattice formulation, so in these lectures the author focuses on setting up the theory for the purpose of numerical simulations to get hard numbers. The numerical techniques used in Lattice Gauge Theories have their roots in statistical mechanics, so it is important to develop an intuition for the interconnection between quantum mechanics and statistical mechanics.
The structure of hcp-bcc interfaces in a Zr-Nb alloy
Zhang, W.Z.; Perovic, V.; Perovic, A.; Weatherly, G.C.; Purdy, G.R.
1998-06-12
Conventional and lattice resolution transmission electron microscopy have been used to study the interface structure of {beta}(b.c.c.) particles in the {alpha}(h.c.p.) matrix of a Zr-Nb alloy. The misfit at the habit plane is accommodated by a single set of [010]{sub {beta}} dislocations lying along the invariant line. At the side facet planes, a combination of two or three different sets of dislocations, [1{bar 1}1]{sub {beta}}, [111]{sub {beta}} and [001]{sub {beta}}, again lying along the invariant line, are required to relieve the misfit. The geometry of prominent facets, as well as the densities of the different dislocations (steps) required for strain relief at each facet, are found to be in excellent agreement with the predictions of a model based on an invariant line strain transformation.
NASA Astrophysics Data System (ADS)
Sinkler, Wharton
The present study concerns glass formation and the beta (bcc) phase in Ti-3d metal systems. beta phase stability is related to amorphization, because the formability and stability of metallic glasses depends on the relative thermodynamic instability of chemically disordered crystalline solid solution phases (Johnson 1986). Correlations are found in this series of alloys which support a connection between electronic characteristics of the bcc phase and the tendency for glass formation. Electron irradiation-induced amorphization in Ti-3d metal systems is investigated as a function of temperature and DeltaN, the group number difference between Ti and the solute. DeltaN is made continuous by using a series of pseudobinary Laves compounds Ti(M1_{x}M2 _{(1-x)}_2. For DeltaN <= 2.2 (between TiCr_2 and TiMn _2) low temperature irradiation damage induces oriented precipitation of the beta (bcc) solid solution phase from the damaged compound. For DeltaN > 2.2 amorphization occurs. beta-phase precipitation under irradiation suggests that beta phase stability is continuously enhanced as Delta N decreases. Diffuse omega scattering in the quenched Ti-Cr beta phase is investigated using electron diffraction and low temperature electron irradiation. A new model of the short range ordered atomic displacements causing the diffuse scattering is developed. Based on this model, it is proposed that the structure reflects chemical short range order. This is supported by irradiation results on the beta phase. A correlation is found between the diffuse scattering and the valence electron concentration. The explanation proposed for this correlation is that the chemical ordering in the beta phase is driven by Fermi surface nesting. Results of annealing of quenched beta Ti-Cr are presented, and are compared with reports of annealing-induced amorphization of this phase (Blatter et al. 1988; Yan et al. 1993). Amorphization is not reproduced. A metastable compound phase beta ^{''} precipitates
Nonlinear dust-lattice waves: a modified Toda lattice
Cramer, N. F.
2008-09-07
Charged dust grains in a plasma interact with a Coulomb potential, but also with an exponential component to the potential, due to Debye shielding in the background plasma. Here we investigate large-amplitude oscillations and waves in dust-lattices, employing techniques used in Toda lattice analysis. The lattice consists of a linear chain of particles, or a periodic ring as occurs in experimentally observed dust particle clusters. The particle motion has a triangular waveform, and chaotic motion for large amplitude motion of a grain.
Li, Guangqin; Kobayashi, Hirokazu; Kusada, Kohei; Taylor, Jared M; Kubota, Yoshiki; Kato, Kenichi; Takata, Masaki; Yamamoto, Tomokazu; Matsumura, Syo; Kitagawa, Hiroshi
2014-11-18
Presented here is the synthesis of an ordered bcc copper-palladium nanoalloy, via the decomposition of a Pd nanoparticle@metal-organic framework composite material. In situ XRD measurements were performed in order to understand the mechanism of the decomposition process. This result gives a further perspective into the synthesis of new nanomaterials via metal-organic framework decomposition. PMID:25251225
Kenneth Wilson and Lattice QCD
NASA Astrophysics Data System (ADS)
Ukawa, Akira
2015-09-01
We discuss the physics and computation of lattice QCD, a space-time lattice formulation of quantum chromodynamics, and Kenneth Wilson's seminal role in its development. We start with the fundamental issue of confinement of quarks in the theory of the strong interactions, and discuss how lattice QCD provides a framework for understanding this phenomenon. A conceptual issue with lattice QCD is a conflict of space-time lattice with chiral symmetry of quarks. We discuss how this problem is resolved. Since lattice QCD is a non-linear quantum dynamical system with infinite degrees of freedom, quantities which are analytically calculable are limited. On the other hand, it provides an ideal case of massively parallel numerical computations. We review the long and distinguished history of parallel-architecture supercomputers designed and built for lattice QCD. We discuss algorithmic developments, in particular the difficulties posed by the fermionic nature of quarks, and their resolution. The triad of efforts toward better understanding of physics, better algorithms, and more powerful supercomputers have produced major breakthroughs in our understanding of the strong interactions. We review the salient results of this effort in understanding the hadron spectrum, the Cabibbo-Kobayashi-Maskawa matrix elements and CP violation, and quark-gluon plasma at high temperatures. We conclude with a brief summary and a future perspective.
NASA Astrophysics Data System (ADS)
Bonny, G.; Terentyev, D.; Elena, J.; Zinovev, A.; Minov, B.; Zhurkin, E. E.
2016-05-01
Upon irradiation, iron based steels used for nuclear applications contain dislocation loops of both < 100 > and ½ < 111 > type. Both types of loops are known to contribute to the radiation hardening and embrittlement of steels. In the literature many molecular dynamics works studying the interaction of dislocations with dislocation loops are available. Recently, based on such studies, a thermo-mechanical model to threat the dislocation - dislocation loop (DL) interaction within a discrete dislocation dynamics framework was developed for ½ < 111 > loops. In this work, we make a literature review of the dislocation - DL interaction in bcc iron. We also perform molecular dynamics simulations to derive the stress-energy function for < 100 > loops. As a result we deliver the function of the activation energy versus activation stress for < 100 > loops that can be applied in a discrete dislocation dynamics framework.
Liu, Zhong-Li; Cai, Ling-Cang; Chen, Xiang-Rong; Wu, Qiang; Jing, Fu-Qian
2009-03-01
We report a detailed ab initio study for body-centered-cubic (bcc) Ta within the framework of the quasiharmonic approximation (QHA) to refine its thermal equation of state and thermodynamic properties. Based on the excellent agreement of our calculated phonon dispersion curve with experiment, the accurate thermal equations of state and thermodynamic properties are well reproduced. The thermal equation of state (EOS) and EOS parameters are considerably improved in our work compared with previous results by others. Furthermore, at high temperatures, the excellent agreement of our obtained thermal expansion and Hugoniot curves with experiments greatly verifies the validity of the quasiharmonic approximation at higher temperatures. It is known that pressure suppresses the vibrations of atoms from their equilibrium positions, i.e. the bondings among atoms are strengthened by pressure; for the same temperature, anharmonicity becomes less important at high pressure. Thus the highest valid temperature of the QHA can be reasonably extended to the larger range. PMID:21817394
NASA Astrophysics Data System (ADS)
Khlebnikova, Yu. V.; Rodionov, D. P.; Sazonova, V. A.; Egorova, L. Yu.; Kaletina, Yu. V.
2013-09-01
The structure of a titanium iodide single crystal obtained by zone melting has been studied by metallography, X-ray diffraction, and electron microscopy. It has been shown that the initial bcc titanium single crystal becomes a pseudosingle crystal upon cooling below the temperature of the β → α polymorphic transition. The pseudosingle crystal consists of macroscopic packets, i.e., crystals of lath morphology with a size of 0.1-0.5 cm2 in different crystal sections. Each packet consists of α-phase laths of the same orientation, which are separated by dislocation boundaries. A total of six different types of packets in the pseudosingle crystal volume is realized in accordance with the Burgers orientation relationships. The structural heredity in the titanium pseudosingle crystal after the cycle of the α → β → α transformations is confirmed.
The Role of Shear in the Onset of Iron's bcc to hcp Stress--Induced Phase Transition
NASA Astrophysics Data System (ADS)
Lew, A.; Caspersen, K.; Ortiz, M.; Carter, E.
2003-12-01
Iron presents a martensitic phase transition from bcc to hcp at an approximate pressure of 13 GPa. The exact onset pressure has been determined to have values ranging from 9 to 16 GPa by several different experimental results. We propose a multiscale model for Iron, with all necessary quantities computed exclusively from first--principles. In this model, we account for all shear components of the deformation, finding that they play a crucial role in its onset, even for very small amounts of shear. Briefly, the model consists of constructing the energy landscape in all six-components of strain. Then, for a given deformation of a representative sample, we minimize its energy by possibly accomodating martensitic laminates inside it, in the spirit of a spinodal decomposition. We will describe the model in detail and show how the onset pressure varies when shear is present, as well as the mixed states encoutered upon transformation.
Exact ab initio transport coefficients in bcc Fe-X (X=Cr, Cu, Mn, Ni, P, Si) dilute alloys
NASA Astrophysics Data System (ADS)
Messina, Luca; Nastar, Maylise; Garnier, Thomas; Domain, Christophe; Olsson, Pär
2014-09-01
Defect-driven diffusion of impurities is the major phenomenon leading to formation of embrittling nanoscopic precipitates in irradiated reactor pressure vessel (RPV) steels. Diffusion depends strongly on the kinetic correlations that may lead to flux coupling between solute atoms and point defects. In this work, flux coupling phenomena such as solute drag by vacancies and radiation-induced segregation at defect sinks are systematically investigated for six bcc iron-based dilute binary alloys, containing Cr, Cu, Mn, Ni, P, and Si impurities, respectively. First, solute-vacancy interactions and migration energies are obtained by means of ab initio calculations; subsequently, self-consistent mean field theory is employed in order to determine the exact Onsager matrix of the alloys. This innovative multiscale approach provides a more complete treatment of the solute-defect interaction than previous multifrequency models. Solute drag is found to be a widespread phenomenon that occurs systematically in ferritic alloys and is enhanced at low temperatures (as for instance RPV operational temperature), as long as an attractive solute-vacancy interaction is present, and that the kinetic modeling of bcc alloys requires the extension of the interaction shell to the second-nearest neighbors. Drag occurs in all alloys except Fe(Cr); the transition from dragging to nondragging regime takes place for the other alloys around (Cu, Mn, Ni) or above (P, Si) the Curie temperature. As far as only the vacancy-mediated solute migration is concerned, Cr depletion at sinks is foreseen by the model, as opposed to the other impurities which are expected to enrich up to no less than 1000 K. The results of this study confirm the current interpretation of the hardening processes in ferritic-martensitic steels under irradiation.
Legless locomotion in lattices
NASA Astrophysics Data System (ADS)
Schiebel, Perrin; Goldman, Daniel I.
2014-11-01
Little is known about interactions between an animal body and complex terrestrial terrain like sand and boulders during legless, undulatory travel (e.g. snake locomotion). We study the locomotor performance of Mojave shovel-nosed snakes (Chionactisoccipitalis , ~ 35 cm long) using a simplified model of heterogeneous terrain: symmetric lattices of obstacles. To quantify performance we measure mean forward speed and slip angle, βs, defined as the angle between the instantaneous velocity and tangent vectors at each point on the body. We find that below a critical peg density the presence of granular media results in high speed (~ 60 cm/s), low average slip (βs ~6°) snake performance as compared to movement in the same peg densities on hard ground (~ 25 cm/s and βs ~15°). Above this peg density, performance on granular and hard substrates converges. Speed on granular media decreases with increasing peg density to that of the speed on hard ground, while speed on hard ground remains constant. Conversely, βs on hard ground trends toward that on granular media as obstacle density increases.
Quantum vortices in optical lattices
Vignolo, P.; Fazio, R.; Tosi, M. P.
2007-08-15
A vortex in a superfluid gas inside an optical lattice can behave as a massive particle moving in a periodic potential and exhibiting quantum properties. In this paper we discuss these properties and show that the excitation of vortex dynamics in a two-dimensional lattice can lead to striking measurable changes in its dynamic response. It would be possible by means of Bragg spectroscopy to carry out the first direct measurement of the effective vortex mass. In addition, the experiments proposed here provide an alternative way to study the pinning to the underlying lattice and the dissipative damping.
Localized structures in Kagome lattices
Saxena, Avadh B; Bishop, Alan R; Law, K J H; Kevrekidis, P G
2009-01-01
We investigate the existence and stability of gap vortices and multi-pole gap solitons in a Kagome lattice with a defocusing nonlinearity both in a discrete case and in a continuum one with periodic external modulation. In particular, predictions are made based on expansion around a simple and analytically tractable anti-continuum (zero coupling) limit. These predictions are then confirmed for a continuum model of an optically-induced Kagome lattice in a photorefractive crystal obtained by a continuous transformation of a honeycomb lattice.
Lattice QCD clusters at Fermilab
Holmgren, D.; Mackenzie, Paul B.; Singh, Anitoj; Simone, Jim; /Fermilab
2004-12-01
As part of the DOE SciDAC ''National Infrastructure for Lattice Gauge Computing'' project, Fermilab builds and operates production clusters for lattice QCD simulations. This paper will describe these clusters. The design of lattice QCD clusters requires careful attention to balancing memory bandwidth, floating point throughput, and network performance. We will discuss our investigations of various commodity processors, including Pentium 4E, Xeon, Opteron, and PPC970. We will also discuss our early experiences with the emerging Infiniband and PCI Express architectures. Finally, we will present our predictions and plans for future clusters.
Reliability analysis of interdependent lattices
NASA Astrophysics Data System (ADS)
Limiao, Zhang; Daqing, Li; Pengju, Qin; Bowen, Fu; Yinan, Jiang; Zio, Enrico; Rui, Kang
2016-06-01
Network reliability analysis has drawn much attention recently due to the risks of catastrophic damage in networked infrastructures. These infrastructures are dependent on each other as a result of various interactions. However, most of the reliability analyses of these interdependent networks do not consider spatial constraints, which are found important for robustness of infrastructures including power grid and transport systems. Here we study the reliability properties of interdependent lattices with different ranges of spatial constraints. Our study shows that interdependent lattices with strong spatial constraints are more resilient than interdependent Erdös-Rényi networks. There exists an intermediate range of spatial constraints, at which the interdependent lattices have minimal resilience.
Preferential Cu precipitation at extended defects in bcc Fe: An atomistic study
Zhang, Yongfeng; Millett, Paul C.; Tonks, Michael R.; Bai, Xian-Ming; Biner, S. Bulent
2015-04-01
As a starting point to understand Cu precipitation in RPV alloys, molecular dynamics and Metropolis Monte-Carlo simulations are carried out to study the effect of lattice defects on Cu precipitation by taking Fe-Cu system as a model alloy. Molecular dynamics simulations show that owing to the high heat of mixing and positive size mismatch, Cu is attracted by vacancy type defects such as vacancies and voids, and tensile stress fields. In accordance, preferential precipitation of Cu is observed in Metropolis Monte-Carlo simulations at dislocations, prismatic loops and voids. The interaction of Cu with a stress field, e.g., that associated with a dislocation or a prismatic loop, is dominated by elastic effect and can be well described by the linear-elasticity theory. For prismatic loops, the attraction to Cu is found to be size-dependent with opposite trends displayed by vacancy and interstitial loops. The size-dependences can be explained by considering the stress fields produced by these loops. The current results will be useful for understanding the effect of neutron irradiation on Cu precipitation in reactor-pressure-vessel steels.