289 research outputs found
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
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
A class of colliding waves in metric-affine gravity, nonmetricity and torsion shock waves
By using our recent generalization of the colliding waves concept to
metric-affine gravity theories, and also our generalization of the advanced and
retarded time coordinate representation in terms of Jacobi functions, we find a
general class of colliding wave solutions with fourth degree polynomials in
metric-affine gravity. We show that our general approach contains the standard
second degree polynomials colliding wave solutions as a particular case.Comment: 13 pages, latex, to appear in J.Math.Phy
Non-Riemannian Gravity and the Einstein-Proca System
We argue that all Einstein-Maxwell or Einstein-Proca solutions to general
relativity may be used to construct a large class of solutions (involving
torsion and non-metricity) to theories of non-Riemannian gravitation that have
been recently discussed in the literature.Comment: 9 pages Plain Tex (No Figures), Letter to Editor Classical and
Quantum Gravit
Scalar-Induced Compactifications in Higher Dimensional Supergravities
We discuss compactifications of higher dimensional supergravities which are
induced by scalars. In particular, we consider vector multiplets coupled to the
supergravity multiplet in the case of D=9, 8 and D=7 minimal supergravities.
These vector multiplets contain scalars, which parametrize coset spaces of the
general form SO(10-D,n)/SO(10-D)xSO(n), where n is the number of vector
multiplets. We discuss the compactification of the supergravity theory to D-2
dimensons, which is induced by non-trivial vacuum scalar field configurations.
There are singular and non-singular solutions, which preserve half of the
supersymmetries.Comment: 25 pages, JHEP
Pleba\'nski-Demia\'nski-like solutions in metric-affine gravity
We consider a (non--Riemannian) metric--affine gravity theory, in particular
its nonmetricity--torsion sector ``isomorphic'' to the Einstein--Maxwell
theory. We map certain Einstein--Maxwell electrovacuum solutions to it, namely
the Pleba\'nski--Demia\'nski class of Petrov type D metrics.Comment: 12 pages of a LaTeX-fil
Exactly Solvable Pairing Model Using an Extension of Richardson-Gaudin Approach
We introduce a new class of exactly solvable boson pairing models using the
technique of Richardson and Gaudin. Analytical expressions for all energy
eigenvalues and first few energy eigenstates are given. In addition, another
solution to Gaudin's equation is also mentioned. A relation with the
Calogero-Sutherland model is suggested.Comment: 9 pages of Latex. In the proceedings of Blueprints for the Nucleus:
From First Principles to Collective Motion: A Festschrift in Honor of
Professor Bruce Barrett, Istanbul, Turkey, 17-23 May 200
Symmetric Teleparallel Gravity: Some exact solutions and spinor couplings
In this paper we elaborate on the symmetric teleparallel gravity (STPG)
written in a non-Riemannian spacetime with nonzero nonmetricity, but zero
torsion and zero curvature. Firstly we give a prescription for obtaining the
nonmetricity from the metric in a peculiar gauge. Then we state that under a
novel prescription of parallel transportation of a tangent vector in this
non-Riemannian geometry the autoparallel curves coincides with those of the
Riemannian spacetimes. Subsequently we represent the symmetric teleparallel
theory of gravity by the most general quadratic and parity conserving
lagrangian with lagrange multipliers for vanishing torsion and curvature. We
show that our lagrangian is equivalent to the Einstein-Hilbert lagrangian for
certain values of coupling coefficients. Thus we arrive at calculating the
field equations via independent variations. Then we obtain in turn conformal,
spherically symmetric static, cosmological and pp-wave solutions exactly.
Finally we discuss a minimal coupling of a spin-1/2 field to STPG.Comment: Accepted for publication in the International Journal of Modern
Physics
Dark Matter Gravitational Interactions
We argue that the conjectured dark mater in the Universe may be endowed with
a new kind of gravitational charge that couples to a short range gravitational
interaction mediated by a massive vector field. A model is constructed that
assimilates this concept into ideas of current inflationary cosmology. The
model is also consistent with the observed behaviour of galactic rotation
curves according to Newtonian dynamics. The essential idea is that stars
composed of ordinary (as opposed to dark matter) experience Newtonian forces
due to the presence of an all pervading background of massive gravitationally
charged cold dark matter. The novel gravitational interactions are predicted to
have a significant influence on pre-inflationary cosmology. The precise details
depend on the nature of a gravitational Proca interaction and the description
of matter. A gravitational Proca field configuration that gives rise to
attractive forces between dark matter charges of like polarity exhibits
homogeneous isotropic eternal cosmologies that are free of cosmological
curvature singularities thus eliminating the horizon problem associated with
the standard big-bang scenario. Such solutions do however admit dense hot
pre-inflationary epochs each with a characteristic scale factor that may be
correlated with the dark matter density in the current era of expansion. The
model is based on a theory in which a modification of Einsteinian gravity at
very short distances can be expressed in terms of the gradient of the Einstein
metric and the torsion of a non-Riemannian connection on the bundle of linear
frames over spacetime. Indeed we demonstrate that the genesis of the model
resides in a remarkable simplification that occurs when one analyses the
variational equations associated with a broad class of non-Riemannian actions.Comment: 40 pages, 4 Postscript figure
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