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‣ Fast and spectrally accurate summation of 2-periodic Stokes potentials

Lindbo, Dag; Tornberg, Anna-Karin
Tipo: Artigo de Revista Científica
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We derive a Ewald decomposition for the Stokeslet in planar periodicity and a novel PME-type O(N log N) method for the fast evaluation of the resulting sums. The decomposition is the natural 2P counterpart to the classical 3P decomposition by Hasimoto, and is given in an explicit form not found in the literature. Truncation error estimates are provided to aid in selecting parameters. The fast, PME-type, method appears to be the first fast method for computing Stokeslet Ewald sums in planar periodicity, and has three attractive properties: it is spectrally accurate; it uses the minimal amount of memory that a gridded Ewald method can use; and provides clarity regarding numerical errors and how to choose parameters. Analytical and numerical results are give to support this. We explore the practicalities of the proposed method, and survey the computational issues involved in applying it to 2-periodic boundary integral Stokes problems.

‣ Novel Effects in High-Tc Granular Superconductors Predicted via Model of 3D Josephson Junction Arrays

Sergeenkov, Sergei
Tipo: Artigo de Revista Científica
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Despite the fact that Josephson Junction Arrays (JAA) have been actively studied for decades, they continue to contribute to the variety of intriguing and peculiar phenomena (both fundamental and important for potential applications) providing at the same time a useful tool for testing new theoretical ideas. To give just a few recent examples, it is sufficient to mention paramagnetic Meissner effect (PME) as well as the recently introduced thermophase and piezophase effects suggesting, respectively, a direct influence of a thermal gradient and an applied stress on phase difference between the adjacent grains. At the same time, an artificially prepared islands of superconducting grains, well-described by the various models of JJAs, proved useful in studying the charging effects in these systems, ranging from Coulomb blockade of Cooper pair tunneling and Bloch oscillations to propagation of quantum ballistic vortices. The present paper reviews some of the recently suggested novel effects which should manifest themselves either in weak-links-bearing superconductors or in artificially prepared JJAs. In Section 2 we consider the appearance of an electric-field induced magnetization (analog of the so-called magnetoelectric effect) in a model granular superconductor. The dual effect...

‣ Non-uniform continuity of the semiflow map associated to the porous medium equation

Matioc, Bogdan--Vasile
Tipo: Artigo de Revista Científica
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We prove that the semiflow map associated to the evolution problem for the porous medium equation (PME) is real-analytic as a function of the initial data in $H^s(\mathbb{S})$, $s>7/2,$ at any fixed positive time, but it is not uniformly continuous. More precisely, we construct two sequences of exact positive solutions of the PME which at initial time converge to zero in $H^s(\mathbb{S})$, but such that the limit inferior of the difference of the two sequences is bounded away from zero in $H^s(\mathbb{S})$ at any later time.

‣ On the superconductivity of the Li$_{x}$RhB$_{y}$ compositions

Takeya, H.; ElMassalami, M.; Amorim, H. S.; Fujii, H.; Mochiku, T.; Takano, Y.
Tipo: Artigo de Revista Científica
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We observed superconductivity ($T_{c}$ $\simeq$2-3 K) in Li$_{x}$RhB$_{y}$ intermetallics wherein $x$ and $y$ vary over a wide compositional range. The crystal structure consists of cubic unit-cell ($a$ $\simeq$ 12.1 \AA ) with centro-symmetric space group $Pn\bar{3}n$. A weak but positive pressure-induced increase of $T_{c}$ was observed. The correlations between the composition and each of the followings were followed over a wide range of $x$ and $y$: the unit-cell dimensions, $T_{c}$ , Sommerfeld coefficient $\gamma$, Debye temperature $\theta_{D}$, and critical fields $H$$_{c1} and H$$_{c2}$. The thermal evolution of the electronic specific heat within the superconducting phase was observed to follow a quadratic-in-$T$ behavior. In addition, a paramagnetic Meissner Effect (PME) is manifested during a low-field-cooled magnetization cycle. This manifestation of quadratic-in-$T$ behavior and PME feature will be discussed.; Comment: 6 pages, 8 figures, to appear in Materials Research Express (2014)

‣ How to mesh up Ewald sums (I): A theoretical and numerical comparison of various particle mesh routines

Deserno, Markus; Holm, Christian
Tipo: Artigo de Revista Científica
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Standard Ewald sums, which calculate e.g. the electrostatic energy or the force in periodically closed systems of charged particles, can be efficiently speeded up by the use of the Fast Fourier Transformation (FFT). In this article we investigate three algorithms for the FFT-accelerated Ewald sum, which attracted a widespread attention, namely, the so-called particle-particle-particle-mesh (P3M), particle mesh Ewald (PME) and smooth PME method. We present a unified view of the underlying techniques and the various ingredients which comprise those routines. Additionally, we offer detailed accuracy measurements, which shed some light on the influence of several tuning parameters and also show that the existing methods -- although similar in spirit -- exhibit remarkable differences in accuracy. We propose combinations of the individual components, mostly relying on the P3M approach, which we regard as most flexible.; Comment: 18 pages, 8 figures included, revtex style

‣ A porous medium equation involving the infinity-Laplacian. Viscosity solutions and asymptotic behaviour

Portilheiro, Manuel; Vazquez, Juan Luis
Tipo: Artigo de Revista Científica
Português
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We study a nonlinear porous medium type equation involving the infinity Laplacian operator. We first consider the problem posed on a bounded domain and prove existence of maximal nonnegative viscosity solutions. Uniqueness is obtained for strictly positive solutions with Lipschitz in time data. We also describe the asymptotic behaviour for the Dirichlet problem in the class of maximal solutions. We then discuss the Cauchy problem posed in the whole space. As in the standard porous medium equation (PME), solutions which start with compact support exhibit a free boundary propagating with finite speed, but such propagation takes place only in the direction of the spatial gradient. The description of the asymptotic behaviour of the Cauchy Problem shows that the asymptotic profile and the rates of convergence and propagation exactly agree for large times with a one-dimensional PME.; Comment: 33 pages

‣ Paramagnetic Meissner effect in mesoscopic samples

Schweigert, V. A.; Peeters, F. M.
Tipo: Artigo de Revista Científica
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Using the non-linear Ginzburg-Landau (GL) theory, we study the magnetic response of different shaped samples in the field-cooled regime (FC). For high external magnetic fluxes, the conventional diamagnetic response under cooling down can be followed by the paramagnetic Meissner effect (PME). A second-order transition from a giant vortex state to a multi-vortex state, with the same vorticity, occurs at the second critical field which leads to the suppression of PME.; Comment: 4 pages, 6 figures

‣ Estimating Masses in Clusters of Galaxies

Aceves, Hector; Perea, Jaime
Tipo: Artigo de Revista Científica
Português
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The projected (PME) and virial mass estimator (VME) are revisited and tested using an N-body equilibrium system. It is found that the PME can overestimate the mass by approx 40% if a cluster is sampled only about its effective radius, Re. The exact value of this error depends on the mass profile of the cluster. The VME can yield errors of approx 20% if we are also able to sample about one Re. Both estimators yield the correct total mass for an equilibrium system. Theoretically, if we know the total extent of the system, the VME provides an accurate mass at different radii provided its gravitational potential term is correctly taken into consideration. The VME can provide acceptable masses for equilibrium systems with anisotropic velocity dispersions.; Comment: 7 pages, 6 figures, to appear in `Observational Cosmology: The development of galaxy systems', eds. G. Giuricin, et al., ASP Conference Series. Minor change to match the Conference version

‣ Maximum entropy as a consequence of Bayes' theorem in differentiable manifolds

Davis, Sergio
Tipo: Artigo de Revista Científica
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Bayesian inference and the principle of maximum entropy (PME) are usually presented as separate but complementary branches of inference, the latter playing a central role in the foundations of Statistical Mechanics. In this work it is shown that the PME can be derived from Bayes' theorem and the divergence theorem for systems whose states can be mapped to points in a differentiable manifold. In this view, entropy must be interpreted as the invariant measure (non-informative prior) on the space of probability densities.

‣ Paramagnetic Meissner effect in single crystal Yb(3)Rh(4)Sn(13)

Kulkarni, P.
Tipo: Artigo de Revista Científica
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Paramagnetic Meissner effect was observed in single crystal Yb(3)Rh(4)Sn(13). While field cooling the sample, at the onset of the superconducting transition at 7.92 K, the DC magnetization first goes through a minimum at 7.85 K, followed by the peak of the PME signal at 7.6 K and then crosses over to the diamagnetic state. The magnetization vs field curves are reversible above the minimum and imbibe a pinning characteristic below 7.85 K. The minimum can be attributed to the opposing contribution to the dc magnetization signal from surface superconductivity. The subsequent flux compression while cooling the sample below 7.85 K shows the rapid increase in the dc magnetization signal. PME signal is actually the diamagnetism opposing the flux compression.; Comment: 3 pages, 4 figures

‣ Reentrant AC magnetic susceptibility in Josephson-junction arrays: An alternative explanation for the paramagnetic Meissner effect

Barbara, P.; Araujo-Moreira, F. M.; Cawthorne, A. B.; Lobb, C. J.
Tipo: Artigo de Revista Científica
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The paramagnetic Meissner effect (PME) measured in high $T_{C}$ granular superconductors has been attributed to the presence of $\pi$-junctions between the grains. Here we present measurements of complex AC magnetic susceptibility from two-dimensional arrays of conventional (non $\pi$) Nb/Al/AlOx/Nb Josephson junctions. We measured the susceptibility as a function of the temperature $T$, the AC amplitude of the excitation field, $h_{AC}$, and the external magnetic field, $H_{DC}$. The experiments show a strong paramagnetic contribution from the multi-junction loops, which manifests itself as a reentrant screening at low temperature, for values of $h_{AC}$ higher than 50 mOe. A highly simplified model, based on a single loop containing four junction, accounts for this paramagnetic contribution and the range of parameters in which it appears. This model offers an alternative explanation of PME which does not involve $\pi$-junctions.; Comment: PDF file, 6 two-columns pages, 9 figures

‣ Anamalous positive magnetization signal at the onset of normal to superconductor transition in single crystal Nb sphere

Kulkarni, P. D.; Banerjee, S. S.; Suderow, H.; Takeya, H.; Ramakrishnan, S.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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We report the observation of positive magnetization signal at the onset of superconducting transition in isothemral and isomagnetic measurements in single crystal Niobium sphere. The PME displays two regimes, viz., low applied magnetic fields H < 4 Oe) and higher fields H > 120 Oe, and the PME signal is absent at the intermediate magnetic fields. Subsequently, the isothermal magnetization data shows the presence of positive signal at the superconductor to normal transition. Using this data we are able to plot the H(c3)-T(c3) line, at which the positive magnetization signal nucleates, in the phase diagram of single crystal Nb.; Comment: 4 pages, 6 figures

‣ Aronson-B\'enilan estimates for the porous medium equation under the Ricci flow

Cao, Huai-Dong; Zhu, Meng
Tipo: Artigo de Revista Científica
Português
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In this paper we study the porous medium equation (PME) coupled with the Ricci flow on complete manifolds with bounded nonnegative curvature operator. In particular, we derive Aronson-B\'enilan and Li-Yau-Hamilton type differential Harnack estimates for positive solutions to the PME, with a linear forcing term, under the Ricci flow.; Comment: Minor changes to the abstract and remark 1.5

‣ Evaluation of the efficacy of Ultra-High Pressure Homogenization technology to improve the safety and quality of liquid foods and especially of orange juice

Fonte: [Barcelona] : Universitat Autònoma de Barcelona. Departament de Ciència Animal i dels Aliments, Publicador: [Barcelona] : Universitat Autònoma de Barcelona. Departament de Ciència Animal i dels Aliments,
Tipo: Tesis i dissertacions electròniques; info:eu-repo/semantics/doctoralThesis Formato: application/pdf
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Descripció del recurs: 01 setembre 2012; El principal objetivo de esta tesis ha sido evaluar la capacidad de la tecnología de ultra alta presión de homogeneización (UHPH) como alternativa a la pasteurización convencional para garantizar la seguridad y calidad de los alimentos líquidos. Para probar la eficacia de la UHPH y garantizar la seguridad microbiana de los alimentos se inoculó Listeria monocytogenes y/o S. enterica serovar Senftenberg 775W en diferentes alimentos líquidos como son huevo entero líquido, leche y zumo de frutas (naranja y uva). y se trataron a 150, 200 y 250 MPa en el caso del huevo liquido y a 200, 300 y 400 MPa en el de leche y zumos de frutas, realizándose la evaluación de su supervivencia durante el almacenamiento en refrigeración a 4ºC durante 20 días en las muestras de huevo y 12 días en las muestras de leche y zumo de frutas. Los recuentos de células viables y dañadas se realizaron utilizando agar triptona soya enriquecida con extracto de levadura y el mismo medio suplementado con sal. Para evaluar los factores que pudieran afectar a la inactivación bacteriana en la eficacia del tratamiento UHPH, se estudió la influencia de la concentración bacteriana baja (3 log CFU/ml) y alta (7 log CFU/ml) en huevo y leche...

‣ Oxygen Atom Transfer and Oxidative Water Incorporation in Cuboidal Mn_(3)MO_n Complexes Based on Synthetic, Isotopic Labeling, and Computational Studies

Kanady, Jacob S.; Mendoza-Cortés, José L.; Tsui, Emily Y.; Nielsen, Robert J.; Goddard, William A., III; Agapie, Theodor
Fonte: American Chemical Society Publicador: American Chemical Society
Tipo: Article; PeerReviewed Formato: application/pdf; chemical/x-cif; chemical/x-cif
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The oxygen-evolving complex (OEC) of photosystem II contains a Mn_(4)CaO_n catalytic site, in which reactivity of bridging oxidos is fundamental to OEC function. We synthesized structurally relevant cuboidal Mn_(3)MO_n complexes (M = Mn, Ca, Sc; n = 3,4) to enable mechanistic studies of reactivity and incorporation of μ_(3)-oxido moieties. We found that Mn^(IV)_(3)CaO_4 and Mn^(IV)_(3)ScO_4 were unreactive toward trimethylphosphine (PMe_3). In contrast, our Mn^(III)_(2)Mn^(IV)_(2)O_4 cubane reacts with this phosphine within minutes to generate a novel Mn^(III)_(4)O_3 partial cubane plus Me_(3)PO. We used quantum mechanics to investigate the reaction paths for oxygen atom transfer to phosphine from Mn^(III)_(2)Mn^(IV)_(2)O_4 and Mn^(IV)_(3)CaO_4. We found that the most favorable reaction path leads to partial detachment of the CH_(3)COO^– ligand, which is energetically feasible only when Mn(III) is present. Experimentally, the lability of metal-bound acetates is greatest for Mn^(III)_(2)Mn^(IV)_(2)O_4. These results indicate that even with a strong oxygen atom acceptor, such as PMe_3, the oxygen atom transfer chemistry from Mn_(3)MO_4 cubanes is controlled by ligand lability, with the Mn^(IV)_(3)CaO_4 OEC model being unreactive. The oxidative oxide incorporation into the partial cubane...

‣ The Coordination Chemistry of “[BP_3]NiX” Platforms: Targeting Low-Valent Nickel Sources as Promising Candidates to L_3Ni=E and L_3Ni≡E Linkages

MacBeth, Cora E.; Thomas, J. Christopher; Betley, Theodore A.; Peters, Jonas C.
Fonte: American Chemical Society Publicador: American Chemical Society
Tipo: Article; PeerReviewed Formato: application/pdf; chemical/x-cif
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A series of divalent, monovalent, and zerovalent nickel complexes supported by the electron-releasing, monoanionic tris(phosphino)borate ligands [PhBP_3] and [PhBP^(iPr)_3] ([PhBP_3] = [PhB(CH_2PPh_2)_3]-, [PhBP^(iPr)_3] = [PhB(CH_2PiPr_2)_3]-) have been synthesized to explore fundamental aspects of their coordination chemistry. The pseudotetrahedral, divalent halide complexes [PhBP_3]NiCl (1), [PhBP_3]NiI (2), and [PhBP^(iPr)_3]NiCl (3) were prepared by the metalation of [PhBP_3]Tl or [PhBP^(iPr)_3]Tl with (Ph_3P)_2NiCl_2, NiI_2, and (DME)NiCl_2 (DME = 1,2-dimethoxyethane), respectively. Complex 1 is a versatile precursor to a series of complexes accessible via substitution reactions including [PhBP_3]Ni(N_3) (4), [PhBP_3]Ni(OSiPh_3) (5), [PhBP_3]Ni(O-p-tBu-Ph) (6), and [PhBP_3]Ni(S-p-tBu-Ph) (7). Complexes 2−5 and 7 have been characterized by X-ray diffraction (XRD) and are pseudotetrahedral monomers in the solid state. Complex 1 reacts readily with oxygen to form the four-electron-oxidation product, {[PhB(CH_2P(O)Ph_2)_2(CH_2PPh_2)]NiCl} (8A or 8B), which features a solid-state structure that is dependent on its method of crystallization. Chemical reduction of 1 using Na/Hg or other potential 1-electron reductants generates a product that arises from partial ligand degradation...

‣ Iridium(I) and Iridium(III) Complexes Supported by a Diphenolate Imidazolyl-Carbene Ligand

Weinberg, David R.; Hazari, Nilay; Labinger, Jay A.; Bercaw, John E.
Fonte: American Chemical Society Publicador: American Chemical Society
Tipo: Article; PeerReviewed Formato: application/pdf; chemical/x-cif
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Deprotonation of 1,3-di(2-hydroxy-5-tert-butylphenyl)imidazolium chloride (1a) followed by reaction with chloro-1,5-cyclooctadiene Ir(I) dimer affords the anionic Ir(I) complex [K][{OCO}Ir(cod)] (2: OCO = 1,3-di(2-hydroxy-5-tert-butylphenyl)imidazolyl; cod = 1,5-cyclooctadiene), the first Ir complex stabilized by a diphenolate imidazolyl-carbene ligand. In the solid state 2 exhibits square-planar geometry, with only one of the phenoxides bound to the metal center. Oxidation of 2 with 2 equiv of [FeCp_2][PF_6] generates the Ir(III) complex [{OCO}Ir(cod)(MeCN)][PF_6] (3). Reaction of 3 with H_2 results in the liberation of cyclooctane and a species capable of catalyzing the hydrogenation of cyclohexene to cyclohexane. Displacement of cyclooctadiene from 3 can be achieved by heating in acetonitrile to form [{OCO}Ir(MeCN)3][PF_6] (4) or by reaction with either PMe_3 or PCy_3 to generate [{OCO}Ir(PMe_3)_3][PF_6] (5) or [{OCO}Ir(PCy_3)_2(MeCN)][PF_6] (6), respectively. 6 reacts with CO in acetonitrile to give an equilibrium mixture of 6 and [{OCO}Ir(PCy_3)_2(CO)][PF_6] (7) and with chloride to generate [{OCO}Ir(PCy_3)_(2)Cl] (8). The solid-state structure of 8 shows that the diphenolate imidazolyl-carbene ligand is distorted from planarity; DFT calculations suggest this is due to an antibonding interaction between the phenolates and the metal center in the highest occupied molecular orbital (HOMO) of the complex. 8 undergoes two successive reversible one-electron oxidations in CH_(2)Cl_2 at −0.22 and at 0.58 V (vs ferrocene/ferrocenium); EPR spectra...

‣ Synthesis and reactivity of tantalum complexes supported by bidentate X2 and tridentate LX2 ligands with two phenolates linked to pyridine, thiophene, furan, and benzene connectors: mechanistic studies of the formation of a tantalum benzylidene and insertion chemistry for tantalum-carbon bonds

Agapie, Theodor; Day, Michael W.; Bercaw, John E.
Fonte: EDP Sciences Publicador: EDP Sciences
Tipo: Article; PeerReviewed Formato: application/pdf; application/pdf; application/pdf; application/pdf; application/pdf; application/pdf; application/pdf; application/pdf; application/pdf; application/pdf; application/pdf; application/pdf; application/pdf; other; other; other
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Using either alkane elimination or salt metathesis methods, tantalum complexes have been prepared with new ligand systems with tridentate bis(phenolate)donor (donor = pyridine, furan, and thiophene) or bidentate bis(phenolate)benzene arrangements. The ligand framework has two X-type phenolates connected to the flat heterocyclic L-type donor at the 2,6- or 2,5- positions or to the 2,6- positions of benzene via direct ring−ring (sp_2−sp_2) linkages. Solid-state structures of these complexes show that in all cases the ligands bind in a mer fashion, but with different geometries of the LX_2 frameworks. The pyridine-linked system binds in a Cs-fashion, the furan-linked system in a C2_v-fashion, and the thiophene-linked system in a C_1-fashion. A bis(phenolate)pyridine tantalum tribenzyl species (7), upon heating in the presence of dimethylphenylphosphine, generates a stable benzylidene complex by α-hydrogen abstraction with loss of toluene and PMe_2Ph trapping. This process was found to be independent of PMe_2Ph concentration with ΔH = 31.3 ± 0.6 kcal·mol−1 and ΔS = 3 ± 2 cal·mol−1·K−1, and the kinetic isotope effect kH/kD = 4.9 ± 0.4, consistent with a mechanism involving rate determining α-hydrogen abstraction with loss of toluene...

‣ Duraçao do desemprego e transições para o emprego formal, a inatividade e a informalidade

Reis, Maurício; Aguas, Marina
Tipo: info:eu-repo/semantics/article; info:eu-repo/semantics/publishedVersion; ; ; Formato: application/pdf