548 research outputs found
Asymmetric double barrier resonant tunneling structures with improved characteristics
We present a self-consistent calculation, based on the global coherent
tunnelling model, and show that structural asymmetry of double barrier resonant
tunnelling structures significantly modifies the current-voltage
characteristics compared to the symmetric structures. In particular, a suitably
designed asymmetric structure can produce much larger peak current and absolute
value of the negative differential conductivity than its commonly used
symmetric counterpart.Comment: 1 paper, 3 figure
General 2 charge geometries
Two charge BPS horizon free supergravity geometries are important in
proposals for understanding black hole microstates. In this paper we construct
a new class of geometries in the NS1-P system, corresponding to solitonic
strings carrying fermionic as well as bosonic condensates. Such geometries are
required to account for the full microscopic entropy of the NS1-P system. We
then briefly discuss the properties of the corresponding geometries in the dual
D1-D5 system.Comment: 44 page
Ricci-flat deformation of orbifolds and localized tachyonic modes
We study Ricci-flat deformations of orbifolds in type II theory. We obtain a
simple formula for mass corrections to the twisted modes due to the
deformations, and apply it to originally tachyonic and massless states in
several examples. In the case of supersymmetric orbifolds, we find that
tachyonic states appear when the deformation breaks all the supersymmetries. We
also study nonsupersymmetric orbifolds C^2/Z_{2N(2N+1)}, which is T-dual to N
type 0 NS5-branes. For N>=2, we compute mass corrections for states, which have
string scale tachyonic masses. We find that the corrected masses coincide to
ones obtained by solving the wave equation for the tachyon field in the smeared
type 0 NS5-brane background geometry. For N=1, we show that the unstable mode
representing the bubble creation is the unique tachyonic mode.Comment: 20 pages, minor collection
Dynamics of Logamediate and Intermediate Scenarios in the Dark Energy Filled Universe
We have considered a model of two component mixture i.e., mixture of
Chaplygin gas and barotropic fluid with tachyonic field. In the case, when they
have no interaction then both of them retain their own properties. Let us
consider an energy flow between barotropic and tachyonic fluids. In both the
cases we find the exact solutions for the tachyonic field and the tachyonic
potential and show that the tachyonic potential follows the asymptotic
behavior. We have considered an interaction between these two fluids by
introducing a coupling term. Finally, we have considered a model of three
component mixture i.e., mixture of tachyonic field, Chaplygin gas and
barotropic fluid with or without interaction. The coupling functions decays
with time indicating a strong energy flow at the initial period and weak stable
interaction at later stage. To keep the observational support of recent
acceleration we have considered two particular forms (i) Logamediate Scenario
and (ii) Intermediate Scenario, of evolution of the Universe. We have examined
the natures of the recent developed statefinder parameters and slow-roll
parameters in both scenarios with and without interactions in whole evolution
of the universe.Comment: 28 pages, 20 figure
D-brane anti-D-brane effective action and brane interaction in open string channel
We construct the effective action of a -brane-anti--brane system by
making use of the non-abelian extension of tachyonic DBI action. We succeed the
construction by restricting the Chan-Paton factors of two non-BPS -branes
in the action to the Chan-Paton factors of a system. For the
special case that both branes are coincident, the action reduces to the one
proposed by A. Sen. \\The effective potential indicates that
when branes separation is larger than the string length scale, there are two
minima in the tachyon direction. As branes move toward each other under the
gravitational force, the tachyon tunneling from false to true vacuum may make a
bubble formation followed by a classical evolution of the bubble. On the other
hand, when branes separation is smaller than the string length scale, the
potential shows one maximum and one minimum. In this case, a homogeneous
tachyon rolling in real time makes an attractive potential for the branes
distance. This classical force is speculated to be the effective force between
the two branes.Comment: Latex, 14 pages, 1 figure, the version appears in JHE
Non-BPS D-Branes in Light-Cone Green-Schwarz Formalism
Non-BPS D-branes are difficult to describe covariantly in a manifestly
supersymmetric formalism. For definiteness we concentrate on type IIB string
theory in flat background in light-cone Green-Schwarz formalism. We study both
the boundary state and the boundary conformal field theory descriptions of
these D-branes with manifest SO(8) covariance and go through various
consistency checks. We analyze Sen's original construction of non-BPS D-branes
given in terms of an orbifold boundary conformal field theory. We also directly
study the relevant world-sheet theory by deriving the open string boundary
condition from the covariant boundary state. Both these methods give the same
open string spectrum which is consistent with the boundary state, as required
by the world-sheet duality. The boundary condition found in the second method
is given in terms of bi-local fields that are quadratic in Green-Schwarz
fermions. We design a special ``doubling trick'' suitable to handle such
boundary conditions and prescribe rules for computing all possible correlation
functions without boundary insertions. This prescription has been tested by
computing disk one-point functions of several classes of closed string states
and comparing the results with the boundary state computation.Comment: 47 pages, 1 figure. Typos corrected, references added and slight
modification of certain explanation made. Version accepted for publication in
JHE
Matrix Cosmology
Exact time-dependent solutions of c=1 string theory are described using the
free fermion formulation. One such class of solutions describes draining of the
Fermi sea and has a spacetime interpretation as closed string tachyon
condensation. A second class of solutions, corresponding to droplets of Fermi
liquid orbiting in phase space, describes closed cosmologies which bounce
through singularities.Comment: 21 pages, 6 figures, v2: added references, minor additions and
correction
Tachyonic Inflation in a Warped String Background
We analyze observational constraints on the parameter space of tachyonic
inflation with a Gaussian potential and discuss some predictions of this
scenario. As was shown by Kofman and Linde, it is extremely problematic to
achieve the required range of parameters in conventional string
compactifications. We investigate if the situation can be improved in more
general compactifications with a warped metric and varying dilaton. The
simplest examples are the warped throat geometries that arise in the vicinity
of of a large number of space-filling D-branes. We find that the parameter
range for inflation can be accommodated in the background of D6-branes wrapping
a three-cycle in type IIA. We comment on the requirements that have to be met
in order to realize this scenario in an explicit string compactification.Comment: Latex, JHEP class, 20 pages, 4 figures. v2: references added, small
error in section 7 corrected, published versio
Non-minimally Coupled Tachyonic Inflation in Warped String Background
We show that the non-minimal coupling of tachyon field to the scalar
curvature, as proposed by Piao et al, with the chosen coupling parameter does
not produce the effective potential where the tachyon field can roll down from
T=0 to large along the slope of the potential. We find a correct choice of
the parameters which ensures this requirement and support slow-roll inflation.
However, we find that the cosmological parameter found from the analysis of the
theory are not in the range obtained from observations. We then invoke warped
compactification and varying dilaton field over the compact manifold, as
proposed by Raeymaekers, to show that in such a setup the observed parameter
space can be ensured.Comment: minor typos corrected and references adde
Off-Shell Interactions for Closed-String Tachyons
Off-shell interactions for localized closed-string tachyons in C/Z_N
superstring backgrounds are analyzed and a conjecture for the effective height
of the tachyon potential is elaborated. At large N, some of the relevant
tachyons are nearly massless and their interactions can be deduced from the
S-matrix. The cubic interactions between these tachyons and the massless fields
are computed in a closed form using orbifold CFT techniques. The cubic
interaction between nearly-massless tachyons with different charges is shown to
vanish and thus condensation of one tachyon does not source the others. It is
shown that to leading order in N, the quartic contact interaction vanishes and
the massless exchanges completely account for the four point scattering
amplitude. This indicates that it is necessary to go beyond quartic
interactions or to include other fields to test the conjecture for the height
of the tachyon potential.Comment: 37 pages, 3 figures, LaTeX, JHEP class. Typos corrected, references
added, published versio
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