988 research outputs found
Tachyon Dynamics and the Effective Action Approximation
Recently effective actions have been extensively used to describe tachyon
condensation in string theory. While the various effective actions which have
appeared in the literature have very similar properties for static
configurations, they differ for time-dependent tachyons. In this paper we
discuss general properties of non-linear effective Lagrangians which are first
order in derivatives. In particular we show that some observed properties, such
as asymptotically vanishing pressure, are rather generic features, although the
quantative features differ. On the other hand we argue that certain features of
marginal tachyon profiles are beyond the reach of any first order Lagrangian
description. We also point out that an effective action, proposed earlier,
captures the dynamics of tachyons well.Comment: References added and confusing reference clarifie
On open superstring partition function in inhomogeneous rolling tachyon background
We consider open superstring partition function Z on the disc in
time-dependent tachyon background T= f(x_i) e^{m x_0} where the profile
function f depends on spatial coordinates. We compute Z to second order in
derivatives of f and compare the result with some previously suggested
effective actions depending only on the first derivatives of the tachyon field.
We also compute the target-space stress-energy tensor in this background and
demonstrate its conservation in the ``on-shell'' case of the linear profile f=
f_0 + q_i x_i corresponding to a marginal perturbation. We comment on the role
of the rolling tachyon with linear spatial profile in the decay of an unstable
D-brane.Comment: 32p, latex, v2: introduction revised to expand and correct comments
on the paper by Kutasov and Niarchos, v3: corrected typos and formulas in
section 4, final version to appear in JHE
Impact of Electrodes on Recombination in Bulk Heterojunction Organic Solar Cells
In recent years, the efficiency of organic solar cells (OSCs) has increased to more than 13%, although different barriers are on the way for reaching higher efficiencies. One crucial barrier is the recombination of charge carriers, which can either occur as the bulk recombination of photogenerated charges or the recombination of photogenerated charges and electrodic induced charges (EICs). This work studies the impact of EICs on the recombination lifetime in OSCs. To this end, the net recombination lifetime of photogenerated charge carriers in the presence of EICs is measured by means of conventional and newly developed transient photovoltage techniques. Moreover, a new approach has been introduced to exclusively measure the bulk recombination lifetime, i.e., in the absence of EICs; this approach was conducted by depositing transparent insulating layers on both sides of the OSC active layer. An examination of these approaches on OSCs with different active layer materials, thicknesses, and varying light intensities determined that the EICs can only reduce the recombination lifetime of the photogenerated charges in OSCs with very weak recombination strength. This work supports that for OSCs with highly reduced recombination strength, eliminating the recombination of photogenerated charges and EICs is critical for achieving better performance. Therefore, the use of a proper blocking layer suppresses EIC recombination in systems with very weak recombination.</p
The Guppy Effect as Interference
People use conjunctions and disjunctions of concepts in ways that violate the
rules of classical logic, such as the law of compositionality. Specifically,
they overextend conjunctions of concepts, a phenomenon referred to as the Guppy
Effect. We build on previous efforts to develop a quantum model that explains
the Guppy Effect in terms of interference. Using a well-studied data set with
16 exemplars that exhibit the Guppy Effect, we developed a 17-dimensional
complex Hilbert space H that models the data and demonstrates the relationship
between overextension and interference. We view the interference effect as, not
a logical fallacy on the conjunction, but a signal that out of the two
constituent concepts, a new concept has emerged.Comment: 10 page
D-Brane Effective Actions and Particle Production near the Beginning of the Tachyon Condensation
In this paper we will study the quantum field theory of fluctuation modes
around the classical solution that describes tachyon condensation on unstable
D-brane.We will calculate the number of particle produced near the beginning of
the rolling tachyon process. We will perform this calculation for different
tachyon effective actions and we will find that the rate of the particle
production strongly depends on the form of the effective action used for the
description of the early stage of the tachyon condensation.Comment: 21 page
Singular tachyon kinks from regular profiles
We demonstrate how Sen's singular kink solution of the Born-Infeld tachyon
action can be constructed by taking the appropriate limit of initially regular
profiles. It is shown that the order in which different limits are taken plays
an important role in determining whether or not such a solution is obtained for
a wide class of potentials. Indeed, by introducing a small parameter into the
action, we are able circumvent the results of a recent paper which derived two
conditions on the asymptotic tachyon potential such that the singular kink
could be recovered in the large amplitude limit of periodic solutions. We show
that this is explained by the non-commuting nature of two limits, and that
Sen's solution is recovered if the order of the limits is chosen appropriately.Comment: 7 pages, 3 figures. References adde
Tachyon condensation in open-closed p-adic string theory
We study a simple model of p-adic closed and open strings. It sheds some
light on the dynamics of tachyon condensation for both types of strings. We
calculate the effect of static and decaying D-brane configurations on the
closed string background. For closed string tachyons we find lumps analogous to
D-branes. By studying their fluctuation spectrum and the D-branes they admit,
we argue that closed string lumps should be interpreted as spacetimes of lower
dimensionality described by some noncritical p-adic string theory.Comment: 21 pages, 3 figures; v2: discussion of the fluctuations of the double
lump substantially improve
Dissolving D0-brane into D2-brane with background B-field
D0-branes on a D2-brane with a constant background B-field are unstable due
to the presence of a tachyonic mode and expected to dissolve into the D2-brane
to formulate a constant D0-charge density. In this paper we study such a
dissolution process in terms of a noncommutative gauge theory. Our results show
that the localized D0-brane spreads out over all of space on the D2-brane as
the tachyon rolls down into a stable vacuum. D0-branes on a D2-brane can be
described as unstable solitons in a noncommutative gauge theory in 2+1
dimensions in the Seiberg-Witten limit. In contrast to the case of annihilation
of a non-BPS D-brane, we are free from difficulty of disappearance of DOF,
since there exist open strings after the tachyon condensation. We solve an
equation of motion of the gauge field numerically, and our results show that
the localized soliton smears over all of noncommutative space. In addition, we
evaluate distributions of D-brane charge, F-string charge, and energy density
via formulas derived in Matrix theory. Our results show that the initial
singularities of D0-charge and energy density are resolved by turning on the
tachyon, and they disperse over the whole space on the D2-brane during the
tachyon condensation process.Comment: 42 pages, 20 figures, JHEP style; references added, clarifications
added in section 3.1; references adde
Optimal behavior of viscoelastic flow at resonant frequencies
The global entropy generation rate in the zero-mean oscillatory flow of a
Maxwell fluid in a pipe is analyzed with the aim at determining its behavior at
resonant flow conditions. This quantity is calculated explicitly using the
analytic expression for the velocity field and assuming isothermal conditions.
The global entropy generation rate shows well-defined peaks at the resonant
frequencies where the flow displays maximum velocities. It was found that
resonant frequencies can be considered optimal in the sense that they maximize
the power transmitted to the pulsating flow at the expense of maximum
dissipation.Comment: Paper accepted to be published in Phys. Rev.
Tachyon Kinks in Boundary String Field Theory
We study tachyon kinks with and without electromagnetic fields in the context
of boundary string field theory. For the case of pure tachyon only an array of
kink-antikink is obtained. In the presence of electromagnetic coupling, all
possible static codimension-one soliton solutions such as array of
kink-antikink, single topological BPS kink, bounce, half kink, as well as
nonBPS topological kink are found, and their properties including the
interpretation as branes are analyzed in detail. Spectrum of the obtained kinks
coincides with that of Dirac-Born-Infeld type effective theory.Comment: LaTex, 29 pages, 17 Figure
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