9,679 research outputs found
The Rolling Tachyon Boundary Conformal Field Theory on an Orbifold
We consider the non-trivial boundary conformal field theory with exactly
marginal boundary deformation. In recent years this deformation has been
studied in the context of rolling tachyons and S-branes in string theory. Here
we study the problem directly from an open string point of view, at one loop.
We formulate the theory of the Z_2 reflection orbifold. To do so, we extend
fermionization techniques originally introduced by Polchinski and Thorlacius.
We also explain how to perform the open string computations at arbitrary
(rational) radius, by consistently constructing the corresponding shift
orbifold, and show in what sense these are related to known boundary states. In
a companion paper, we use these results in a cosmological context involving
decaying branes.Comment: 23 page
Fractional S-branes on a Spacetime Orbifold
Unstable D-branes are central objects in string theory, and exist also in
time-dependent backgrounds. In this paper we take first steps to studying brane
decay in spacetime orbifolds. As a concrete model we focus on the R^{1,d}/Z_2
orbifold. We point out that on a spacetime orbifold there exist two kinds of
S-branes, fractional S-branes in addition to the usual ones. We investigate
their construction in the open string and closed string boundary state
approach. As an application of these constructions, we consider a scenario
where an unstable brane nucleates at the origin of time of a spacetime, its
initial energy then converting into energy flux in the form of closed strings.
The dual open string description allows for a well-defined description of this
process even if it originates at a singular origin of the spacetime.Comment: 22 pages, 6 eps figure
Mechanically-Induced Transport Switching Effect in Graphene-based Nanojunctions
We report a theoretical study suggesting a novel type of electronic switching
effect, driven by the geometrical reconstruction of nanoscale graphene-based
junctions. We considered junction struc- tures which have alternative
metastable configurations transformed by rotations of local carbon dimers. The
use of external mechanical strain allows a control of the energy barrier
heights of the potential profiles and also changes the reaction character from
endothermic to exothermic or vice-versa. The reshaping of the atomic details of
the junction encode binary electronic ON or OFF states, with ON/OFF
transmission ratio that can reach up to 10^4-10^5. Our results suggest the
possibility to design modern logical switching devices or mechanophore sensors,
monitored by mechanical strain and structural rearrangements.Comment: 10 pages, 4 figure
Brane Decay from the Origin of Time
We present a novel scenario where matter in a spacetime originates from a
decaying brane at the origin of time. The decay could be considered as a ``Big
Bang''-like event at X^0=0. The closed string interpretation is a
time-dependent spacetime with a semi-infinite time direction, with the initial
energy of the brane converted into energy flux from the origin. The open string
interpretation can be viewed as a string theoretic non-singular initial
condition.Comment: 5 pages, 2 eps figure
Asymmetric Non-Abelian Orbifolds and Model Building
The rules for the free fermionic string model construction are extended to
include general non-abelian orbifold constructions that go beyond the real
fermionic approach. This generalization is also applied to the asymmetric
orbifold rules recently introduced. These non-abelian orbifold rules are quite
easy to use. Examples are given to illustrate their applications.Comment: 30 pages, Revtex 3.
Poincar\'{e} gauge theory of gravity
A Poincar\'{e} gauge theory of (2+1)-dimensional gravity is developed.
Fundamental gravitational field variables are dreibein fields and Lorentz gauge
potentials, and the theory is underlain with the Riemann-Cartan space-time. The
most general gravitational Lagrangian density, which is at most quadratic in
curvature and torsion tensors and invariant under local Lorentz transformations
and under general coordinate transformations, is given. Gravitational field
equations are studied in detail, and solutions of the equations for weak
gravitational fields are examined for the case with a static, \lq \lq spin"less
point like source. We find, among other things, the following: (1)Solutions of
the vacuum Einstein equation satisfy gravitational field equations in the
vacuum in this theory. (2)For a class of the parameters in the gravitational
Lagrangian density, the torsion is \lq \lq frozen" at the place where \lq \lq
spin" density of the source field is not vanishing. In this case, the field
equation actually agrees with the Einstein equation, when the source field is
\lq \lq spin"less. (3)A teleparallel theory developed in a previous paper is
\lq \lq included as a solution" in a limiting case. (4)A Newtonian limit is
obtainable, if the parameters in the Lagrangian density satisfy certain
conditions.Comment: 27pages, RevTeX, OCU-PHYS-15
Multifrequency observations of BL Lacertae in 1988
Simultaneous multiwavelength observations of BL Lacertae were performed on two occasions separated by month in 1988 June and July, covering the radio, submillimeter, infrared, optical, ultraviolet, and X-ray wave bands. In the wide-band photon spectra, the X-ray flux lies clearly above the extension of radio ultraviolet continuum as expected. The slope of the X-ray spectra is significantly flatter than that at optical ultraviolet regimes, and its spectral index 0.7-1.0 corresponds to the slope at submillimeter band. Comparison with earlier observations, in fact, indicates that the X-ray flux is correlated with the submillimeter band, and not with the others, and supports the SSC model
Large N reduction on group manifolds
We show that the large N reduction holds on group manifolds. Large N field
theories defined on group manifolds are equivalent to some corresponding matrix
models. For instance, gauge theories on S^3 can be regularized in a gauge
invariant and SO(4) invariant manner.Comment: 21 pages, 4 figures, typos corrected, a reference adde
Scaling and the Fractal Geometry of Two-Dimensional Quantum Gravity
We examine the scaling of geodesic correlation functions in two-dimensional
gravity and in spin systems coupled to gravity. The numerical data support the
scaling hypothesis and indicate that the quantum geometry develops a
non-perturbative length scale. The existence of this length scale allows us to
extract a Hausdorff dimension. In the case of pure gravity we find d_H approx.
3.8, in support of recent theoretical calculations that d_H = 4. We also
discuss the back-reaction of matter on the geometry.Comment: 16 pages, LaTeX format, 8 eps figure
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