350 research outputs found
An Exact Cosmological Solution of the Coupled Einstein-Majorana Fermion-Scalar Field Equations
We couple a neutral scalar field and a Majorana fermion field to Einstein
gravity represented by the Robertson-Walker metric and find a class of exact
cosmological solutions
Thermal Stability of Metallic Single-Walled Carbon Nanotubes: An O(N) Tight-Binding Molecular Dynamics Simulation Study
Order(N) Tight-Binding Molecular Dynamics (TBMD) simulations are performed to
investigate the thermal stability of (10,10) metallic Single-Walled Carbon
Nanotubes (SWCNT). Periodic boundary conditions (PBC) are applied in axial
direction. Velocity Verlet algorithm along with the canonical ensemble
molecular dynamics (NVT) is used to simulate the tubes at the targeted
temperatures. The effects of slow and rapid temperature increases on the
physical characteristics, structural stability and the energetics of the tube
are investigated and compared. Simulations are carried out starting from room
temperature and the temperature is raised in steps of 300K. Stability of the
simulated metallic SWCNT is examined at each step before it is heated to higher
temperatures. First indication of structural deformation is observed at 600K.
For higher heat treatments the deformations are more pronounced and the bond
breaking temperature is reached around 2500K. Gradual (slow) heating and
thermal equilibrium (fast heating) methods give the value of radial thermal
expansion coefficient in the temperature range between 300K-600K as
0.31x10^{-5}(1/K) and 0.089x10^{-5}(1/K), respectively. After 600K, both
methods give the same value of 0.089x10^{-5}(1/K). The ratio of the total
energy per atom with respect to temperature is found to be 3x10^{-4} eV/K
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
An Exactly Solvable Model of Interacting Bosons
We introduce a class of exactly solvable boson models. We give explicit
analytic expressions for energy eigenvalues and eigenvectors for an sd-boson
Hamiltonian, which is related to the SO(6) chain of the Interacting Boson Model
Hamiltonian.Comment: 8 pages of LATE
Classical and quantum spinor cosmology with signature change
We study the classical and quantum cosmology of a universe in which the
matter source is a massive Dirac spinor field and consider cases where such
fields are either free or self-interacting. We focus attention on the spatially
flat Robertson-Walker cosmology and classify the solutions of the
Einstein-Dirac system in the case of zero, negative and positive cosmological
constant . For , these solutions exhibit signature
transitions from a Euclidean to a Lorentzian domain. In the case of massless
spinor fields it is found that signature changing solutions do not exist when
the field is free while in the case of a self-interacting spinor field such
solutions may exist. The resulting quantum cosmology and the corresponding
Wheeler-DeWitt equation are also studied for both free and self interacting
spinor fields and closed form expressions for the wavefunction of the universe
are presented. These solutions suggest a quantization rule for the energy.Comment: 13 pages, 4 figure
Conformal Black Hole Solutions of Axi-Dilaton Gravity in D-dimensions
Static, spherically symmetric solutions of axi-dilaton gravity in
dimensions is given in the Brans-Dicke frame for arbitrary values of the
Brans-Dicke constant and an axion-dilaton coupling parameter . The
mass and the dilaton and axion charges are determined and a BPS bound is
derived. There exists a one parameter family of black hole solutions in the
scale invariant limit.Comment: 6 PAGES, Rev-tex file, no figures, to appear in Phys-Rev
The geometrical form for the string space-time action
In the present article, we derive the space-time action of the bosonic string
in terms of geometrical quantities. First, we study the space-time geometry
felt by probe bosonic string moving in antisymmetric and dilaton background
fields. We show that the presence of the antisymmetric field leads to the
space-time torsion, and the presence of the dilaton field leads to the
space-time nonmetricity. Using these results we obtain the integration measure
for space-time with stringy nonmetricity, requiring its preservation under
parallel transport. We derive the Lagrangian depending on stringy curvature,
torsion and nonmetricity.Comment: 13 page
On Signature Transition and Compactification in Kaluza-Klein Cosmology
We consider an empty (4+1) dimensional Kaluza-Klein universe with a negative
cosmological constant and a Robertson-Walker type metric. It is shown that the
solutions to Einstein field equations have degenerate metric and exhibit
transitioins from a Euclidean to a Lorentzian domain. We then suggest a
mechanism, based on signature transition which leads to compactification of the
internal space in the Lorentzian region as . With the
assumption of a very small value for the cosmological constant we find that the
size of the universe and the internal scale factor would be related
according to in the Lorentzian region. The corresponding
Wheeler-DeWitt equation has exact solution in the mini-superspace giving rise
to a quantum state which peaks in the vicinity of the classical solutions
undergoing signature transition.Comment: 13 pages, 3 figure
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