7,506 research outputs found
Nitrogen front evolution in purged polymer electrolyte membrane fuel cell with dead-ended anode
In this paper, we model and experimentally verify the evolution of liquid water and nitrogen fronts along the length of the anode channel in a proton exchange membrane fuel cell operating with a dead-ended anode that is fed by dry hydrogen. The accumulation of inert nitrogen and liquid water in the anode causes a voltage drop, which is recoverable by purging the anode. Experiments were designed to clarify the effect of N-2 blanketing, water plugging of the channels, and flooding of the gas diffusion layer. The observation of each phenomenon is facilitated by simultaneous gas chromatography measurements on samples extracted from the anode channel to measure the nitrogen content and neutron imaging to measure the liquid water distribution. A model of the accumulation is presented, which describes the dynamic evolution of a N-2 blanketing front in the anode channel leading to the development of a hydrogen starved region. The prediction of the voltage drop between purge cycles during nonwater plugging channel conditions is shown. The model is capable of describing both the two-sloped behavior of the voltage decay and the time at which the steeper slope begins by capturing the effect of H-2 concentration loss and the area of the H-2 starved region along the anode channel
Four-loop verification of algorithm for Feynman diagrams summation in N=1 supersymmetric electrodynamics
A method of Feynman diagrams summation, based on using Schwinger-Dyson
equations and Ward identities, is verified by calculating some four-loop
diagrams in N=1 supersymmetric electrodynamics, regularized by higher
derivatives. In particular, for the considered diagrams correctness of an
additional identity for Green functions, which is not reduced to the gauge Ward
identity, is proved.Comment: 14 pages, 9 figure
Spectroscopic evidence for strong correlations between local superconducting gap and local Altshuler-Aronov density-of-states suppression in ultrathin NbN films
Disorder has different profound effects on superconducting thin films. For a
large variety of materials, increasing disorder reduces electronic screening
which enhances electron-electron repulsion. These fermionic effects lead to a
mechanism described by Finkelstein: when disorder combined to electron-electron
interactions increases, there is a global decrease of the superconducting
energy gap and of the critical temperature , the ratio
/ remaining roughly constant. In addition, in most films an
emergent granularity develops with increasing disorder and results in the
formation of inhomogeneous superconducting puddles. These gap inhomogeneities
are usually accompanied by the development of bosonic features: a pseudogap
develops above the critical temperature and the energy gap
starts decoupling from . Thus the mechanism(s) driving the appearance of
these gap inhomogeneities could result from a complicated interplay between
fermionic and bosonic effects. By studying the local electronic properties of a
NbN film with scanning tunneling spectroscopy (STS) we show that the
inhomogeneous spatial distribution of is locally strongly correlated
to a large depletion in the local density of states (LDOS) around the Fermi
level, associated to the Altshuler-Aronov effect induced by strong electronic
interactions. By modelling quantitatively the measured LDOS suppression, we
show that the latter can be interpreted as local variations of the film
resistivity. This local change in resistivity leads to a local variation of
through a local Finkelstein mechanism. Our analysis furnishes a purely
fermionic scenario explaining quantitatively the emergent superconducting
inhomogeneities, while the precise origin of the latter remained unclear up to
now.Comment: 11 pages, 4 figure
Towards proof of new identity for Green functions in N=1 supersymmetric electrodynamics
For the N=1 supersymmetric massless electrodynamics, regularized by higher
derivatives, we describe a method, by which one can try to prove the new
identity for the Green functions, which was proposed earlier. Using this method
we show that some contribution to the new identity are really 0.Comment: 16 pages, 1 figure, an error corrected, significant change
Twisting the N=2 String
The most general homogeneous monodromy conditions in string theory
are classified in terms of the conjugacy classes of the global symmetry group
. For classes which generate a discrete subgroup \G,
the corresponding target space backgrounds {\bf C}^{1,1}/\G include half
spaces, complex orbifolds and tori. We propose a generalization of the
intercept formula to matrix-valued twists, but find massless physical states
only for (untwisted) and (\`a la Mathur
and Mukhi), as well as for being a parabolic element of . In
particular, the sixteen -twisted sectors of the string are
investigated, and the corresponding ground states are identified via
bosonization and BRST cohomology. We find enough room for an extended multiplet
of `spacetime' supersymmetry, with the number of supersymmetries being
dependent on global `spacetime' topology. However, world-sheet locality for the
chiral vertex operators does not permit interactions among all massless
`spacetime' fermions.Comment: 42 pages, LaTeX, no figures, 120 kb, ITP-UH-24/93, DESY 93-191
(abstract and introduction clarified, minor corrections added
The property of maximal transcendentality in the N=4 SYM
We show results for the universal anomalous dimension gamma_{uni}(j) of
Wilson twist-2 operators in the N=4 Supersymmetric Yang-Mills theory in the
first three orders of perturbation theory. These expressions are obtained by
extracting the most complicated contributions from the corresponding anomalous
dimensions in QCD.Comment: 6 pages, published in the Proceedings of International Bogolyubov
Conference "Problems of Theoretical and Mathematical Physics" (dedicated to
the 100th anniversary of the birth of N.N. Bogolyubov (1909-1992)), Dubna,
Russia, August 21 - 27, 2009 (Phys.Part.Nucl. in press
Space-Time Supersymmetry of Extended Fermionic Strings in Dimensions
The fermionic string theory is revisited in light of its recently
proposed equivalence to the non-compact fermionic string model. The
issues of space-time Lorentz covariance and supersymmetry for the BRST
quantized strings living in uncompactified dimensions are
discussed. The equivalent local quantum supersymmetric field theory appears to
be the most transparent way to represent the space-time symmetries of the
extended fermionic strings and their interactions. Our considerations support
the Siegel's ideas about the presence of Lorentz symmetry as well as
at least one self-dual space-time supersymmetry in the theory of the
fermionic strings, though we do not have a compelling reason to argue about the
necessity of the {\it maximal} space-time supersymmetry. The world-sheet
arguments about the absence of all string massive modes in the physical
spectrum, and the vanishing of all string-loop amplitudes in the Polyakov
approach, are given on the basis of general consistency of the theory.Comment: 29 pages, LaTeX, ITP-UH-1/9
Electron-Phonon Coupling in Highly-Screened Graphene
Photoemission studies of graphene have resulted in a long-standing
controversy concerning the strength of the experimental electron-phonon
interaction in comparison with theoretical calculations. Using high-resolution
angle-resolved photoemission spectroscopy we study graphene grown on a copper
substrate, where the metallic screening of the substrate substantially reduces
the electron-electron interaction, simplifying the comparison of the
electron-phonon interaction between theory and experiment. By taking the
nonlinear bare bandstructure into account, we are able to show that the
strength of the electron-phonon interaction does indeed agree with theoretical
calculations. In addition, we observe a significant bandgap at the Dirac point
of graphene.Comment: Submitted to Phys. Rev. Lett. on July 20, 201
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