225 research outputs found
Vacancy induced energy band gap changes of semiconducting zigzag single walled carbon nanotubes
In this work, we have examined how the multi-vacancy defects induced in the
horizontal direction change the energetics and the electronic structure of
semiconducting Single-Walled Carbon Nanotubes (SWCNTs). The electronic
structure of SWCNTs is computed for each deformed configuration by means of
real space, Order(N) Tight Binding Molecular Dynamic (O(N) TBMD) simulations.
Energy band gap is obtained in real space through the behavior of electronic
density of states (eDOS) near the Fermi level. Vacancies can effectively change
the energetics and hence the electronic structure of SWCNTs. In this study, we
choose three different kinds of semiconducting zigzag SWCNTs and determine the
band gap modifications. We have selected (12,0), (13,0) and (14,0) zigzag
SWCNTs according to n (mod 3) = 0, n (mod 3) = 1 and n (mod 3) = 2
classification. (12,0) SWCNT is metallic in its pristine state. The application
of vacancies opens the electronic band gap and it goes up to 0.13 eV for a di-
vacancy defected tube. On the other hand (13,0) and (14,0) SWCNTs are
semiconductors with energy band gap values of 0.44 eV and 0.55 eV in their
pristine state, respectively. Their energy band gap values decrease to 0.07 eV
and 0.09 eV when mono-vacancy defects are induced in their horizontal
directions. Then the di-vacancy defects open the band gap again. So in both
cases, the semiconducting-metallic - semiconducting transitions occur. It is
also shown that the band gap modification exhibits irreversible
characteristics, which means that band gap values of the nanotubes do not reach
their pristine values with increasing number of vacancies
Non-minimally Coupled Gravitational and Electromagnetic Fields: pp-Wave Solutions
We give the Lagrangian formulation of a generic non-minimally extended
Einstein-Maxwell theory with an action that is linear in the curvature and
quadratic in the electromagnetic field. We derive the coupled field equations
by a first order variational principle using the method of Lagrange
multipliers. We look for solutions describing plane-fronted Einstein-Maxwell
waves with parallel rays. We give a family of exact solutions associated with a
partially massless spin-2 photon and a partially massive spin-2 graviton
A Comparison of the LVDP and {\Lambda}CDM Cosmological Models
We compare the cosmological kinematics obtained via our law of linearly
varying deceleration parameter (LVDP) with the kinematics obtained in the
{\Lambda}CDM model. We show that the LVDP model is almost indistinguishable
from the {\Lambda}CDM model up to the near future of our universe as far as the
current observations are concerned, though their predictions differ
tremendously into the far future.Comment: 6 pages, 5 figures, 1 table, matches the version to be published in
International Journal of Theoretical Physic
Dimensionality, topology, energy, the cosmological constant, and signature change
Using the concept of real tunneling configurations (classical signature
change) and nucleation energy, we explore the consequences of an alternative
minimization procedure for the Euclidean action in multiple-dimensional quantum
cosmology. In both standard Hartle-Hawking type as well as Coleman type
wormhole-based approaches, it is suggested that the action should be minimized
among configurations of equal energy. In a simplified model, allowing for
arbitrary products of spheres as Euclidean solutions, the favoured space-time
dimension is 4, the global topology of spacelike slices being (hence predicting a universe of Kantowski-Sachs type). There is,
however, some freedom for a Kaluza-Klein scenario, in which case the observed
spacelike slices are . In this case, the internal space is a product
of two-spheres, and the total space-time dimension is 6, 8, 10 or 12.Comment: 34 pages, LaTeX, no figure
Propagating Torsion in 3D-Gravity and Dynamical Mass Generation
In this paper, fermions are minimally coupled to 3D-gravity where a dynamical
torsion is introduced. A Kalb-Ramond field is non-minimally coupled to these
fermions in a gauge-invariant way. We show that a 1-loop mass generation
mechanism takes place for both the 2-form gauge field and the torsion. As for
the fermions, no mass is dynamically generated: at 1-loop, there is only a mass
shift proportional to the Yukawa coupling whenever the fermions have a
non-vanishing tree-level mass.Comment: 13 pages, latex file, no figures, some corrections adde
Solutions to the Wheeler-Dewitt Equation Inspired by the String Effective Action
The Wheeler-DeWitt equation is derived from the bosonic sector of the
heterotic string effective action assuming a toroidal compactification. The
spatially closed, higher dimensional Friedmann-Robertson-Walker (FRW) cosmology
is investigated and a suitable change of variables rewrites the equation in a
canonical form. Real- and imaginary-phase exact solutions are found and a
method of successive approximations is employed to find more general power
series solutions. The quantum cosmology of the Bianchi IX universe is also
investigated and a class of exact solutions is found.Comment: 21 pages of plain LaTeX, Fermilab-Pub-93/100-
The trace left by signature-change-induced compactification
Recently, it has been shown that an infinite succession of classical
signature changes (''signature oscillations'') can compactify and stabilize
internal dimensions, and simultaneously leads, after a coarse graining type of
average procedure, to an effective (''physical'') space-time geometry
displaying the usual Lorentzian metric signature. Here, we consider a minimally
coupled scalar field on such an oscillating background and study its effective
dynamics. It turns out that the resulting field equation in four dimensions
contains a coupling to some non-metric structure, the imprint of the
''microscopic'' signature oscillations on the effective properties of matter.
In a multidimensional FRW model, this structure is identical to a massive
scalar field evolving in its homogeneous mode.Comment: 15 pages, LaTeX, no figure
The General Supersymmetric Solution of Topologically Massive Supergravity
We find the general fully non-linear solution of topologically massive
supergravity admitting a Killing spinor. It is of plane-wave type, with a null
Killing vector field. Conversely, we show that all solutions with a null
Killing vector are supersymmetric for one or the other choice of sign for the
Chern-Simons coupling constant \mu. If \mu does not take the critical value
\mu=\pm 1, these solutions are asymptotically regular on a Poincar\'e patch,
but do not admit a smooth global compactification with boundary S^1\times\R. In
the critical case, the solutions have a logarithmic singularity on the boundary
of the Poincar\'e patch. We derive a Nester-Witten identity, which allows us to
identify the associated charges, but we conclude that the presence of the
Chern-Simons term prevents us from making a statement about their positivity.
The Nester-Witten procedure is applied to the BTZ black hole.Comment: Minor correction
- …