3,803 research outputs found
D-branes in the WZW model
It is stated in the literature that D-branes in the WZW-model associated with
the gluing condition J = - \bar{J} along the boundary correspond to branes
filling out the whole group volume. We show instead that the end-points of open
strings are rather bound to stay on `integer' conjugacy classes. In the case of
SU(2) level k WZW model we obtain k-1 two dimensional Euclidean D-branes and
two D particles sitting at the points e and -e.Comment: 2 pages, LaTe
Using supernova neutrinos to monitor the collapse, to search for gravity waves and to probe neutrino masses
We discuss the importance of observing supernova neutrinos. By analyzing the
SN1987A observations of Kamiokande-II, IMB and Baksan, we show that they
provide a 2.5{\sigma} support to the standard scenario for the explosion. We
discuss in this context the use of neutrinos as trigger for the search of the
gravity wave impulsive emission. We derive a bound on the neutrino mass using
the SN1987A data and argue, using simulated data, that a future galactic
supernova could probe the sub-eV region.Comment: 8 pages, 1 figure. Proceeding for the Galileo-Xu Guangqi meeting: The
Sun, the Stars, the Universe and General Relativity; October 26-30, 2009,
Shanghai (China). Accepted for publication at International Journal of Modern
Physics
Physical Principles of the Amplification of Electromagnetic Radiation Due to Negative Electron Masses in a Semiconductor Superlattice
In a superlattice placed in crossed electric and magnetic fields, under
certain conditions, the inversion of electron population can appear at which
the average energy of electrons is above the middle of the miniband and the
effective mass of the electron is negative. This is the implementation of the
negative effective mass amplifier and generator (NEMAG) in the superlattice. It
can result in the amplification and generation of terahertz radiation even in
the absence of negative differential conductivity.Comment: 5 pages, 3 figure
Resummations in the Bloch-Nordsieck model
We studied different levels of resummations of the exactly solvable
Bloch-Nordsieck model in order to be able to compare the approximations with an
exact result. We studied one-loop perturbation theory, 2PI resummation and
Schwinger-Dyson equations truncated in a way to maintain Ward-identities. At
all levels we carefully performed renormalization. We found that although the
2PI resummation does not exhibit infrared sensitivity at the mass shell (the
one-loop perturbation theory does), but it is still far from the exact
solution. The method of truncated Schwinger-Dyson equations, however, is exact
in this model, so it provides a new way of solving the Bloch-Nordsieck model.
This method can also be generalized to other, more complicated theories.Comment: 12 pages, 3 figures, revtex
On interrelations between Sibgatullin's and Alekseev's approaches to the construction of exact solutions of the Einstein-Maxwell equations
The integral equations involved in Alekseev's "monodromy transform" technique
are shown to be simple combinations of Sibgatullin's integral equations and
normalizing conditions. An additional complex conjugation introduced by
Alekseev in the integrands makes his scheme mathematically inconsistent;
besides, in the electrovac case all Alekseev's principal value integrals
contain an intrinsic error which has never been identified before. We also
explain how operates a non-trivial double-step algorithm devised by Alekseev
for rewriting, by purely algebraic manipulations and in a different (more
complicated) parameter set, any particular specialization of the known
analytically extended N-soliton electrovac solution obtained in 1995 with the
aid of Sibgatullin's method.Comment: 7 pages, no figures, section II extende
Nonlinear dynamics and band transport in a superlattice driven by a plane wave
A quantum particle transport induced in a spatially-periodic potential by a
propagating plane wave has a number important implications in a range of
topical physical systems. Examples include acoustically driven semiconductor
superlattices and cold atoms in optical crystal. Here we apply kinetic
description of the directed transport in a superlattice beyond standard linear
approximation, and utilize exact path-integral solutions of the semiclassical
transport equation. We show that the particle drift and average velocities have
non-monotonic dependence on the wave amplitude with several prominent extrema.
Such nontrivial kinetic behaviour is related to global bifurcations developing
with an increase of the wave amplitude. They cause dramatic transformations of
the system phase space and lead to changes of the transport regime. We describe
different types of phase trajectories contributing to the directed transport
and analyse their spectral content
One-dimensional Chern-Simons theory
We study a one-dimensional toy version of the Chern-Simons theory. We
construct its simplicial version which comprises features of a low-energy
effective gauge theory and of a topological quantum field theory in the sense
of Atiyah.Comment: 37 page
Magnetoresistance of compensated semimetals in confined geometries
Two-component conductors -- e.g., semi-metals and narrow band semiconductors
-- often exhibit unusually strong magnetoresistance in a wide temperature
range. Suppression of the Hall voltage near charge neutrality in such systems
gives rise to a strong quasiparticle drift in the direction perpendicular to
the electric current and magnetic field. This drift is responsible for a strong
geometrical increase of resistance even in weak magnetic fields. Combining the
Boltzmann kinetic equation with sample electrostatics, we develop a microscopic
theory of magnetotransport in two and three spatial dimensions. The compensated
Hall effect in confined geometry is always accompanied by electron-hole
recombination near the sample edges and at large-scale inhomogeneities. As the
result, classical edge currents may dominate the resistance in the vicinity of
charge compensation. The effect leads to linear magnetoresistance in two
dimensions in a broad range of parameters. In three dimensions, the
magnetoresistance is normally quadratic in the field, with the linear regime
restricted to rectangular samples with magnetic field directed perpendicular to
the sample surface. Finally, we discuss the effects of heat flow and
temperature inhomogeneities on the magnetoresistance.Comment: 22 pages, 7 figures, published versio
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