Spatial Charge Density Research Articles

Cage-like structure and charge hollow in the immiscible Cu–Ta system

Ground states of the immiscible Cu–Ta system are investigated by a hybrid approach of first-principles calculation and cluster expansion method. The obtained pairs, triplets, and tetrads cluster interactions show that the closer-packed cluster may not always contribute to stabilize structures. A CuTa 7 phase with a cage-like structure is predicted to be stable at ground state. The obtained spatial valence charge density shows that there exists a charge hollow within this cage-like structure, contributing to stabilize CuTa 7 phase.

Singular charge density at the center of the pion?

We relate the three-dimensional infinite momentum frame spatial charge density of the pion to its electromagnetic form factor ${F}_{\ensuremath{\pi}}({Q}^{2})$. Diverse treatments of the measured form factor data including phenomenological fits, nonrelativistic quark models, the application of perturbative quantum chromodynamics (QCD), QCD sum rules, holographic QCD, and the Nambu--Jona-Lasinio (NJL) model all lead to the result that the charge density at the center of the pion has a logarithmic divergence. Relativistic constituent quark models do not display this singularity. Future measurements planned for larger values of ${Q}^{2}$ may determine whether or not a singularity actually occurs.

Superstructures in arrays of rotated graphene layers: Electronic structure calculations

We have applied first-principles total-energy electronic structure calculations in the local-density approximation to obtain the spatial charge density for a three-layer graphene array with the top surface rotated by $21.8\ifmmode^\circ\else\textdegree\fi{}$. It is possible to identify superstructures over the rotated layer, known as moir\'e patterns, in this system. This kind of superstructure has been reported experimentally during scanning tunneling microscopy (STM) measurements. Within this framework, we show that the simulated STM images are strongly influenced by the relative rotation of the top layer with respect to the underlying graphite. In this particular case we relate the stacking sequence of atoms to the intensity maxima at different tip-surface distances.

Identification of the electroelastic coupling from full multi-physical fields measured at the micrometre scale

Metal coated microcantilevers are used as transducers of their electrochemical environment. Using the metallic layer of these cantilevers as a working electrode allows one to modify the electrochemical state of the cantilever surface. Since the mechanical behaviour of micrometre scale objects is significantly surface-driven, this environment modification induces bending of the cantilever. Using a full-field interferometric measurement set-up to monitor the objects then provides an optical phase map, which is found to originate from both electrochemical and mechanical effects. The scaling of the electrochemically-induced phase with respect to the surface charge density is modelled according to Gouy–Chapman–Stern theory, whereas the relationship between the mechanical effect and the surface charge density is analysed. An identification technique is described to determine a modelling of the electroelastic coupling and to identify the spatial charge density distribution from full-field phase measurements. Minimizing the least-squares gap between the measured phase and a statically admissible phase field, the mechanical effect is found to be charge-driven. The charge density field is also found to be singular on the cantilever edge, and the shear stress versus charge density is found to be non-linear.

Effect of the surface characteristics of Methanosarcina barkeri on immobilization to support materials

To realize a highly efficient anaerobic treatment, it is necessary to immobilize high concentrations of methanogens within a fermenter. In this study, we experimentally examine the effect of the surface characteristics of the acetate-utilizing methanogen Methanosarcina barkeri (JCM 10043) on immobilization to support materials. To this aim, we measured the electrostatic and hydrophobic properties of M. barkeri. The electrophoretic mobility of M. barkeri decreased with increasing ionic strength of the cell suspension and was fitted by the Ohshima equation using two parameters: spatial charge density in the polyelectrolyte region ( ZeN = –1.15 × 10 6 C/m 3) and the softness parameter (1/λ = 3.35 × 10 −9 m). M. barkeri showed an affinity to n-hexadecane, with adhesion of more than 60%. M. barkeri showed hydrophobicity relative to Escherichia coli. We also carried out a microbial adhesion test to support materials. M. barkeri showed better adhesion to the anion-exchange resin than to the hydrophobic resin. The microbial cells adhered uniformly to the anion-exchange resin, while coagulated cells adhered non-uniformly to the hydrophobic resin. M. barkeri showed poor adhesion to the cation-exchange resin. The anion-exchange resin is most effective in immobilizing M. barkeri within the fermenter.

First-principles calculations of the structural stability and magnetic property of themetastable phases in the equilibrium immiscible Co–Au system

To reveal the energetic sequence of the alloy phases in the Co–Au system, the latticeconstants, cohesive energies, and bulk modulus of the fcc Au, hcp Co, the B1, B2, andL10 structured CoAuphases, and the D03, L12,and D019 structured Co3Au and CoAu3 phases, respectively, are acquired by first-principles calculations within the generalized-gradientapproximation (GGA) as well as within the local density approximation (LDA). Inaddition, the magnetic moment of the Co atom in the studied phases are also calculated.To further examine the structural stability, the elastic constants of the studied phases arecalculated and the results suggest that the fcc-type structures could be elastically stable atCo/Au = 1:3, 1:1, and 3:1, whereas the hcp-type structures could be stable atCo/Au = 1:3 and 3:1. Moreover, the spatial valence charge density (SVCD) and spin density of thestudied phases are also calculated to clarify the physical origin of the structuralstability. It turns out that, in the relatively stable phases, the high SVCDs mostlydistribute between the similar atoms, thus forming the attractive covalent bonding tostabilize the respective structures, and that the spin density may also play animportant role in influencing the stability of the ferromagnetic metastable phases.

Role of spatial valence charge density on the metastability of an immiscible binary metal system at equilibrium

In the equilibrium immiscible Ag--Ru system, the elastic constants and phonon spectra of the possible ${\mathrm{Ag}}_{3}\mathrm{Ru}$ and $\mathrm{Ag}{\mathrm{Ru}}_{3}$ in $\mathrm{D}{0}_{3},\mathrm{L}{1}_{2}$, and $\mathrm{D}{0}_{19}$ structures, respectively, are first obtained by the lattice dynamics calculation, which shows that only $\mathrm{L}{1}_{2}$ and $\mathrm{D}{0}_{19}\phantom{\rule{0.3em}{0ex}}\mathrm{Ag}{\mathrm{Ru}}_{3}$ phases can relatively be stable. The spatial valence charge density (SVCD) is then acquired by first principles calculation for all the studied structures. It turns out that the high SVCDs distribute mostly between the similar atoms in the relatively stable phases, while located mainly between the dissimilar atoms in all the unstable structures, indicating that the SVCD plays a decisive role in determining the metastability, i.e., the structural stability of the nonequilibrium phase, in an equilibrium immiscible binary metal system.

Density-functional calculations of defect formation energies using supercell methods: Defects in diamond

Density-functional theory combined with periodic boundary conditions is used to systematically study the dependence of defect formation energy on supercell size for diamond containing vacancy and self-interstitial defects. We investigate the effect of the electrostatic energy due to the neutralization of charged supercells and the effect of the alignment of the valence band maximum (VBM) on the formation energy. For negatively charged vacancies and positively charged interstitials, the formation energies show a clear dependence on supercell size, and the electrostatic corrections agree with the trend given by the Makov-Payne scheme (Ref. 28). For positively charged vacancies and negatively charged interstitials, the size dependence and the electrostatic corrections are quite weak. An analysis of the spatial charge density distributions reveals that these large variations in electrostatic terms with defect type originate from differences in the screening of the defect-localized charge, as explained by using a simple electron-gas model. Several VBM alignment schemes are also tested. The best agreement between the calculated and asymptotically exact ionization levels is obtained when the levels are based on the formation energies referenced to the VBM of the defect-containing supercell.

Influence of growth phase on bacterial cell electrokinetic characteristics examined by soft particle electrophoresis theory

The influence of incubation time on the electrokinetic properties of Escherichia coli, Pseudomonas putida, Alcaligenes faecalis, and Alcaligenes sp., was examined by electrophoretic mobility measurements and the results were discussed based on Ohshima's soft particle theory. The electrophoretic mobility of E. coli plotted against incubation time revealed that the mobility gradually increased from the outset of incubation to 7 h, which corresponded to the exponential growth and early stationary phase, and then decreased. For all strains, electrophoretic mobility leveled off to nonzero values in accordance with the increase in ionic concentration, which was a characteristic feature of soft particles. Soft particle analysis was carried out at the outset of incubation and the time when mobility reached a maximum in order to obtain the spatial charge density ( ZN) and cell surface softness (1/ λ). ZN at the maximum electrophoretic mobility was more negative than that at the outset of incubation for E. coli, P. putida, and A. faecalis, while the ZN of Alcaligenes sp. almost remained unchanged. 1/ λ decreased for E. coli, but increased for Alcaligenes sp. and A. faecalis. These findings indicated that the cell growth phase affects both ZN and 1/ λ, although the dependencies of these parameters are unique to each bacterial strain.

Curvature and Charge Modulations in Lamellar DNA−Lipid Complexes

To model the possible formation of coupled spatial corrugations and charge density modulations in lamellar DNA−lipid complexes, we use a free energy functional which includes the electrostatic, lipid mixing, and elastic degrees of freedom in a self-consistent manner. We find that the balance of forces favors membrane corrugations that are expected to be stable with respect to thermal membrane undulations for a certain range of lipid (charged and uncharged) composition. This may lead to locking between DNA strands in adjacent galleries of the complex. Furthermore, the possibility of membrane corrugations renders the lamellar complex more stable with respect to another, hexagonal, DNA−lipid phase.

Neural mesh iteration for the FE computation of charged-particle trajectories in vacuum electron devices

In this paper, a neural mesh-adaptation iterative procedure is presented for the finite-element solution of coupled steady-state electromechanical problems describing the motion of charged particles inside vacuum electron devices. In the procedure at each step a neural algorithm performs a mesh refinement in which the mesh "density" is related to the current estimate of the three-dimensional (3-D) spatial charge density distribution. Convergence to an accurate solution is obtained with a reduced computational effort in comparison with conventional fixed mesh iterative techniques. An example of application to the 3-D calculation of electron trajectories inside the collector of a travelling wave tube is shown in order to illustrate the advantages of the iterative scheme.

Three-dimensional FE modeling of multistage depressed TWT collectors

In this paper a fully three-dimensional (3-D) finite element (FE) approach for the electromagnetic analysis of multistage depressed travelling wave tube (TWT) collectors is presented. In the model a coupled stationary electromechanical problem is iteratively solved; convergence to solution is obtained by using a congruent representation of the unknown 3-D spatial charge density. A FE computer code, based on the above iterative scheme, has been developed and used to carry out several 3-D analyses of TWT collectors.

Nonlocal dielectric functions of molecular liquids from computer simulations

The longitudinal dielectric function, χ( k)=1−1/ ε L( k), of both polar and nonpolar liquid environments is a key ingredient to modern theories of solvent effects upon charge transfer reactions in solution and solvation dynamics processes in general. In this work, we present a molecular dynamics simulation study of the dielectric function of several molecular liquids ranging from associating (H 2O, CH 3OH) and nonassociating ((CH 3) 2SO, CH 3CN) polar liquids to nondipolar ones (C 6H 6, CCl 4 and CO 2) aiming to better understand the relationships between the structure of the χ( k) functions to the charge–charge spatial correlations in each liquid. We show that the characteristic main peak of χ( k), located at some wave vector k ∗ , whose value depends on the liquid, arises from spatial charge density fluctuations of wavelength 2 π/k ∗ for all liquids considered, with the exception of CO 2. Further details about the shape of the χ( k) functions for the simulated liquids are discussed in terms of intra and intermolecular charge–charge correlations.

Semi-classical orbits in a caterpillar like Sinaı̈ billiard

The transport properties of a caterpillar-like Sinai billiard are investigated in the ballistic regime. The experimental kinetic coefficients exhibit pronounced features, characteristic of this kind of billiard. A Monte-Carlo style simulation of the semi-classical electron motion is performed in order to model the transmission probabilities Tij of an ideal caterpillar structure. By visualising the spatial charge density distributions, the main features in the transmission probabilities are related to special electron orbits which dominate the electron dynamics at a given B. Finally, Poincaré sections are calculated close to the commensurability condition. The appearance of stability islands surrounded by the stochastic sea characterises the presence of regular electron skipping orbits involved in the ballistic transport phenomena.

Experimental evaluation of the measurement of isotope ratios by time of flight mass spectrometry with laser ablation

Time of flight mass spectrometry (TOF-MS) in combination with laser ablation (LA) applied for low isotope composition measurements is limited by energy and spatial dispersion of ions, spatial charge density, and some other factors. In order to reduce these factors that make the extraction of the plasma ions difficult, we used a second laser to create ions by non-isotope-selective photoionization of neutral particles. The photoionization was carried out in uranium vapor beam by the third harmonic of a pulsed Nd:YAG laser. This work presents the results of tentative experiments to estimate the characteristics of the LA-TOF-MS method with non-selective photoionization.

Interfacial electronic density of states in phthalocyanine derivative Langmuir–Blodgett films determined by surface potential measurement

Surface potentials of phthalocyanine derivative Langmuir–Blodgett (LB) films on metal electrodes were studied. Two kinds of phthalocyanine derivatives (C6PcCu and CuttbPc) were examined as a function of the number of deposited layers. The distributions of spatial excess charge density and electronic density of states at the metal/film interface were determined. The difference in the electrostatic phenomena observed in these two phthalocyanine LB films was attributed to the difference in the density of surface states of these two films.

Some Comments on London Theory for Superconductors

The basic formulae of London theory for superconductors are reviewed. Moreover, an expression for the spatial charge density in a type-II superconductor is obtained; this equation is associated with sinusoidal oscillations. Considerations on both penetration depth and coherence length are exposed.

Influence of site-directed modifications on the formation of iron cores in ferritin

The structure and crystal chemical properties of iron cores of reconstituted recombinant human ferritins and their site-directed variants have been studied by transmission electron microscopy and electron diffraction. The kinetics of Fe uptake have been compared spectrophotometrically. Recombinant L and H-chain ferritins, and recombinant H-chain variants incorporating modifications in the threefold (Asp131 → His or Glu134 → Ala) and fourfold (Leu169 → Arg) channels, at the partially buried ferroxidase sites (Glu62,His65 → Lys,Gly), a putative nucleation site on the inner surface (Glu61,Glu64,Glu67 → Ala), and both the ferroxidase and nucleation sites (Glu62,His65 → Lys,Gly and Glu61,Glu64,Glu67 → Ala), were investigated. An additional H-chain variant, incorporating substitution of the last ten C-terminal residues for those of the L-chain protein, was also studied. Most of the proteins assimilated iron to give discrete electron-dense cores of the Fe(III) hydrated oxide, ferrihydrite (Fe 2 O 3 . n H 2 O). No differences were observed for variants modified in the three- or fourfold channels compared with the unmodified H-chain ferritin. The recombinant L-chain ferritin and H-chain variant depleted of the ferroxidase site, however, showed markedly reduced uptake kinetics and comprised cores of increased diameter and regularity. Depletion of the inner surface Glu residues, whilst maintaining the ferroxidase site, resulted in a partially reduced rate of Fe uptake and iron cores of wider particle size distribution. Modification of both ferroxidase and inner surface Glu residues resulted in complete inhibition of iron uptake and deposition. No cores were observed by electron microscopy although negative staining showed that the protein shell was intact. The general requirement of an appropriate spatial charge density across the cavity surface rather than specific amino acid residues could explain how, in spite of an almost complete lack of identity between the amino acid sequences of bacterioferritin and mammalian ferritins, ferrihydrite is deposited within the cavity of both proteins under similar reconstitution conditions.

Influence of site-directed modifications on the formation of iron cores in ferritin

The structure and crystal chemical properties of iron cores of reconstituted recombinant human ferritins and their site-directed variants have been studied by transmission electron microscopy and electron diffraction. The kinetics of Fe uptake have been compared spectrophotometrically. Recombinant L and H-chain ferritins, and recombinant H-chain variants incorporating modifications in the threefold (Asp131 → His or Glu134 → Ala) and fourfold (Leu169 → Arg) channels, at the partially buried ferroxidase sites (Glu62,His65 → Lys,Gly), a putative nucleation site on the inner surface (Glu61,Glu64,Glu67 → Ala), and both the ferroxidase and nucleation sites (Glu62,His65 → Lys,Gly and Glu61,Glu64,Glu67 → Ala), were investigated. An additional H-chain variant, incorporating substitution of the last ten C-terminal residues for those of the L-chain protein, was also studied. Most of the proteins assimilated iron to give discrete electron-dense cores of the Fe(III) hydrated oxide, ferrihydrite (Fe 2O 3. nH 2O). No differences were observed for variants modified in the three- or fourfold channels compared with the unmodified H-chain ferritin. The recombinant L-chain ferritin and H-chain variant depleted of the ferroxidase site, however, showed markedly reduced uptake kinetics and comprised cores of increased diameter and regularity. Depletion of the inner surface Glu residues, whilst maintaining the ferroxidase site, resulted in a partially reduced rate of Fe uptake and iron cores of wider particle size distribution. Modification of both ferroxidase and inner surface Glu residues resulted in complete inhibition of iron uptake and deposition. No cores were observed by electron microscopy although negative staining showed that the protein shell was intact. The general requirement of an appropriate spatial charge density across the cavity surface rather than specific amino acid residues could explain how, in spite of an almost complete lack of identity between the amino acid sequences of bacterioferritin and mammalian ferritins, ferrihydrite is deposited within the cavity of both proteins under similar reconstitution conditions.

Determination of the trapped charge distribution in scaled silicon nitride MONOS nonvolatile memory devices by tunneling spectroscopy

In this paper an analytical model for extraction of the spatial trapped charge density in silicon nitride has been formulated with amphoteric trap statistics. The model includes the energy distribution of traps in nitride and the time-dispersive tunneling transitions from these nitride traps. We have examined charge loss from sub-100 Å nitride films with MONOS memory devices at 110 K at different gate bias conditions. We conclude that by maintaining depletion/weak-inversion at the Si surface, the primary charge loss mechanism is back-tunneling of charge from the nitride traps into the Si bands. The actual charge distribution in the nitride is a function of charge injection and trapping. We have demonstrated that modified Fowler-Nordheim tunneling of electrons into the nitride generates a non-uniform spatial distribution of trapped charge for a spatially uniform trap density in the nitride.