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

String-Inspired Chern-Simons Modified Gravity In 4-Dimensions

Chern-Simons modified gravity models in 4-dimensions are shown to be special cases of low energy effective string models to first order in the string constant.Comment: To appear in the European Physics Journal

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 $\Lambda$. For $\Lambda<0$, 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

Gravity and Electromagnetism with Y(R)F2Y(R)F^2-type Coupling and Magnetic Monopole Solutions

We investigate $Y(R) F^2$-type coupling of electromagnetic fields to gravity. After we derive field equations by a first order variational principle from the Lagrangian formulation of the non-minimally coupled theory, we look for static, spherically symmetric, magnetic monopole solutions. We point out that the solutions can provide possible geometries which may explain the flatness of the observed rotation curves of galaxies.Comment: 10 page

Black Holes with Weyl Charge and Non-Riemannian Waves

A simple modification to Einstein's theory of gravity in terms of a non-Riemannian connection is examined. A new tensor-variational approach yields field equations that possess a covariance similar to the gauge covariance of electromagnetism. These equations are shown to possess solutions analogous to those found in the Einstein-Maxwell system. In particular one finds gravi-electric and gravi-magnetic charges contributing to a spherically symmetric static Reissner-Nordstr\"om metric. Such Weyl ``charges'' provide a source for the non-Riemannian torsion and metric gradient fields instead of the electromagnetic field. The theory suggests that matter may be endowed with gravitational charges that couple to gravity in a manner analogous to electromagnetic couplings in an electromagnetic field. The nature of gravitational coupling to spinor matter in this theory is also investigated and a solution exhibiting a plane-symmetric gravitational metric wave coupled via non-Riemannian waves to a propagating spinor field is presented.Comment: 18 pages Plain Tex (No Figures), Classical and Quantum Gravit

Canonical wave packets in quantum cosmology

We discuss the construction of wave packets resulting from the solutions of a class of Wheeler-DeWitt equations in Robertson-Walker type cosmologies, for arbitrary curvature. We show that there always exists a ``canonical initial slope" for a given initial wave function, which optimizes some desirable properties of the resulting wave packet, most importantly good classical-quantum correspondence. This can be properly denoted as a canonical wave packet. We introduce a general method for finding these canonical initial slopes which is generalization of our earlier work.Comment: 19 pages, 8 figure

Square Root Actions, Metric Signature, and the Path-Integral of Quantum Gravity

We consider quantization of the Baierlein-Sharp-Wheeler form of the gravitational action, in which the lapse function is determined from the Hamiltonian constraint. This action has a square root form, analogous to the actions of the relativistic particle and Nambu string. We argue that path-integral quantization of the gravitational action should be based on a path integrand $\exp[ \sqrt{i} S ]$ rather than the familiar Feynman expression $\exp[ i S ]$, and that unitarity requires integration over manifolds of both Euclidean and Lorentzian signature. We discuss the relation of this path integral to our previous considerations regarding the problem of time, and extend our approach to include fermions.Comment: 32 pages, latex. The revision is a more general treatment of the regulator. Local constraints are now derived from a requirement of regulator independenc

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-

Coherence lifetimes of excitations in an atomic condensate due to the thin spectrum

We study the quantum coherence properties of a finite sized atomic condensate using a toy-model and the thin spectrum model formalism. The decoherence time for a condensate in the ground state, nominally taken as a variational symmetry breaking state, is investigated for both zero and finite temperatures. We also consider the lifetimes for Bogoliubov quasi-particle excitations, and contrast them to the observability window determined by the ground state coherence time. The lifetimes are shown to exhibit a general characteristic dependence on the temperature, determined by the thin spectrum accompanying the spontaneous symmetry breaking ground state

Finite-dimensional Schwinger basis, deformed symmetries, Wigner function, and an algebraic approach to quantum phase

Schwinger's finite (D) dimensional periodic Hilbert space representations are studied on the toroidal lattice {\ee Z}_{D} \times {\ee Z}_{D} with specific emphasis on the deformed oscillator subalgebras and the generalized representations of the Wigner function. These subalgebras are shown to be admissible endowed with the non-negative norm of Hilbert space vectors. Hence, they provide the desired canonical basis for the algebraic formulation of the quantum phase problem. Certain equivalence classes in the space of labels are identified within each subalgebra, and connections with area-preserving canonical transformations are examined. The generalized representations of the Wigner function are examined in the finite-dimensional cyclic Schwinger basis. These representations are shown to conform to all fundamental conditions of the generalized phase space Wigner distribution. As a specific application of the Schwinger basis, the number-phase unitary operator pair in {\ee Z}_{D} \times {\ee Z}_{D} is studied and, based on the admissibility of the underlying q-oscillator subalgebra, an {\it algebraic} approach to the unitary quantum phase operator is established. This being the focus of this work, connections with the Susskind-Glogower- Carruthers-Nieto phase operator formalism as well as standard action-angle Wigner function formalisms are examined in the infinite-period limit. The concept of continuously shifted Fock basis is introduced to facilitate the Fock space representations of the Wigner function.Comment: 19 pages, no figure