5,162 research outputs found
Nonlinear Realization of the Local Conform-Affine Symmetry Group for Gravity in the Composite Fiber Bundle Formalism
A gauge theory of gravity based on a nonlinear realization (NLR) of the local
Conform-Affine (CA) group of symmetry transformations is presented. The coframe
fields and gauge connections of the theory are obtained. The tetrads and
Lorentz group metric are used to induce a spacetime metric. The inhomogenously
transforming (under the Lorentz group) connection coefficients serve as
gravitational gauge potentials used to define covariant derivatives
accommodating minimal coupling of matter and gauge fields. On the other hand,
the tensor valued connection forms serve as auxillary dynamical fields
associated with the dilation, special conformal and deformational (shear)
degrees of freedom inherent in the bundle manifold. The bundle curvature of the
theory is determined. Boundary topological invariants are constructed. They
serve as a prototype (source free) gravitational Lagrangian. The Bianchi
identities, covariant field equations and gauge currents are obtained.Comment: 24 pages. to appear in IJGMM
Spectral and localization properties of the Dirichlet wave guide with two concentric Neumann discs
Bound states of the Hamiltonian describing a quantum particle living on three
dimensional straight strip of width are investigated. We impose the Neumann
boundary condition on the two concentric windows of the radii and
located on the opposite walls and the Dirichlet boundary condition on the
remaining part of the boundary of the strip. We prove that such a system
exhibits discrete eigenvalues below the essential spectrum for any .
When and tend to the infinity, the asymptotic of the eigenvalue is
derived. A comparative analysis with the one-window case reveals that due to
the additional possibility of the regulating energy spectrum the anticrossing
structure builds up as a function of the inner radius with its sharpness
increasing for the larger outer radius. Mathematical and physical
interpretation of the obtained results is presented; namely, it is derived that
the anticrossings are accompanied by the drastic changes of the wave function
localization. Parallels are drawn to the other structures exhibiting similar
phenomena; in particular, it is proved that, contrary to the two-dimensional
geometry, at the critical Neumann radii true bound states exist.Comment: 25 pages, 7 figure
Propagation of axions in a strongly magnetized medium
The polarization operator of an axion in a degenerate gas of electrons
occupying the ground-state Landau level in a superstrong magnetic field G is investigated in a model with a
tree-level axion-electron coupling. It is shown that a dynamic axion mass,
which can fall within the allowed range of values , is generated under the conditions of strongly
magnetized neutron stars. As a result, the dispersion relation for axions is
appreciably different from that in a vacuum.Comment: RevTex, no figures, 13 pages, Revised version of the paper published
in J. Exp. Theor. Phys. {\bf 88}, 1 (1999
On the motion of a heavy rigid body in an ideal fluid with circulation
Chaplygin's equations describing the planar motion of a rigid body in an
unbounded volume of an ideal fluid involved in a circular flow around the body
are considered. Hamiltonian structures, new integrable cases, and partial
solutions are revealed, and their stability is examined. The problems of
non-integrability of the equations of motion because of a chaotic behavior of
the system are discussed.Comment: 25 pages, 4 figure
In Escherichia coli ammonia inhibits cytochrome bo3 but activates cytochrome bd-I
Interaction of two redox enzymes of Escherichia coli, cytochrome bo3 and cytochrome bd-I, with ammonium sulfate/ammonia at pH 7.0 and 8.3 was studied using high-resolution respirometry and absorption spectroscopy. At pH 7.0, the oxygen reductase activity of none of the enzymes is affected by the ligand. At pH 8.3, cytochrome bo3 is inhibited by the ligand, with 40% maximum inhibition at 100 mM (NH4 )2SO4 . In contrast, the activity of cytochrome bd-I at pH 8.3 increases with increasing the ligand concentration, the largest increase (140%) is observed at 100 mM (NH4 )2SO4 . In both cases, the effector molecule is apparently not NH4+ but NH3 . The ligand induces changes in absorption spectra of both oxidized cytochromes at pH 8.3. The magnitude of these changes increases as ammonia concentration is increased, yielding apparent dissociation constants Kdapp of 24.3 ± 2.7 mM (NH4 )2SO4 (4.9 ± 0.5 mM NH3 ) for the Soret region in cytochrome bo3, and 35.9 ± 7.1 and 24.6 ± 12.4 mM (NH4 )2SO4 (7.2 ± 1.4 and 4.9 ± 2.5 mM NH3 ) for the Soret and visible regions, respectively, in cytochrome bd-I. Consistently, addition of (NH4)2SO4 to cells of the E. coli mutant containing cytochrome bd-I as the only terminal oxidase at pH 8.3 accelerates the O2 consumption rate, the highest one (140%) being at 27 mM (NH4 )2SO4 . We discuss possible molecular mechanisms and physiological significance of modulation of the enzymatic activities by ammonia present at high concentration in the intestines, a niche occupied by E. coli
Ebonex-Supported PtM Anode Catalysts for PEM Water Electrolysis
The work presents a research on the preparation of Pt-based bimetallic catalysts dispersed on commercial Magnelli phase titania (Ebonex@) by sol gel method and investigation of their activity toward the oxygen evolution reaction (OER) in polymer electrolyte membrane water electrolysis (PEMWE). The catalytic support is also used for preparation of a carbon-free gas diffusion layer (ET30) integrated in the oxygen electrode of the membrane electrode assembly (MEA). The performance characteristics of MEA with PtM/Ebonex on ET30 are investigated in a laboratory PEMWE and compared to those of MEA with commercial carbon-based GDL with the same anode catalyst. It is proven that the chemical nature and electron density of the second metal have an essential effect on the catalyst surface structure and properties, including the lattice parameter, particle size, and electronic surface state state which in turn, reflect on the electrochemical behavior and catalytic activity. The catalysts PtCr/Ebonex and PtMn/Ebonex having deficiency of electrons in the valent d-orbital do not form an alloy with Pt and have lower catalytic activity. In contrast, the metallic components in PtFe/Ebonex and PtCo/Ebonex form a solid solution which results in changes in the catalyst structure and surface electron state, leading to enhanced OER efficieny compared to pure Pt/Ebonex
Microsecond Time-Resolved Absorption Spectroscopy Used to Study CO Compounds of Cytochrome bd from Escherichia coli
Cytochrome bd is a tri-heme (b558, b595, d) respiratory oxygen reductase that is found in many bacteria including pathogenic
species. It couples the electron transfer from quinol to O2 with generation of an electrochemical proton gradient. We
examined photolysis and subsequent recombination of CO with isolated cytochrome bd from Escherichia coli in oneelectron
reduced (MV) and fully reduced (R) states by microsecond time-resolved absorption spectroscopy at 532-nm
excitation. Both Soret and visible band regions were examined. CO photodissociation from MV enzyme possibly causes fast
(t,1.5 ms) electron transfer from heme d to heme b595 in a small fraction of the protein, not reported earlier. Then the
electron migrates to heme b558 (t,16 ms). It returns from the b-hemes to heme d with t,180 ms. Unlike cytochrome bd in
the R state, in MV enzyme the apparent contribution of absorbance changes associated with CO dissociation from heme d is
small, if any. Photodissociation of CO from heme d in MV enzyme is suggested to be accompanied by the binding of an
internal ligand (L) at the opposite side of the heme. CO recombines with heme d (t,16 ms) yielding a transient
hexacoordinate state (CO-Fe2+
-L). Then the ligand slowly (t,30 ms) dissociates from heme d. Recombination of CO with a
reduced heme b in a fraction of the MV sample may also contribute to the 30-ms phase. In R enzyme, CO recombines to
heme d (t,20 ms), some heme b558 (t,0.2–3 ms), and finally migrates from heme d to heme b595 (t,24 ms) in ,5% of the
enzyme population. Data are consistent with the recent nanosecond study of Rappaport et al. conducted on the
membranes at 640-nm excitation but limited to the Soret band. The additional phases were revealed due to differences in
excitation and other experimental conditions
The Dynamics of a Rigid Body in Potential Flow with Circulation
We consider the motion of a two-dimensional body of arbitrary shape in a
planar irrotational, incompressible fluid with a given amount of circulation
around the body. We derive the equations of motion for this system by
performing symplectic reduction with respect to the group of volume-preserving
diffeomorphisms and obtain the relevant Poisson structures after a further
Poisson reduction with respect to the group of translations and rotations. In
this way, we recover the equations of motion given for this system by Chaplygin
and Lamb, and we give a geometric interpretation for the Kutta-Zhukowski force
as a curvature-related effect. In addition, we show that the motion of a rigid
body with circulation can be understood as a geodesic flow on a central
extension of the special Euclidian group SE(2), and we relate the cocycle in
the description of this central extension to a certain curvature tensor.Comment: 28 pages, 2 figures; v2: typos correcte
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