9,395 research outputs found
Cosmic Evolution in Generalised Brans-Dicke Theory
We have studied the Generalised Brans-Dicke theory and obtained exact
solutions of a(t),phi(t),and omega(t) for different epochs of the cosmic
evolution .We discuss how inflation,decceleration,cosmic acceleration can
result from this solution.The time variation of G(t) is also examined.Comment: 12 pages, no figure
Application of Relativistic Coupled-cluster Theory to Electron Impact Excitations of Mg in the Plasma Environment
A relativistic coupled-cluster (RCC) theory is implemented to study electron
impact excitations of atomic species. As a test case, the electron impact
excitations of the resonance transitions
are investigated in the singly charged magnesium (Mg) ion using this
theory. Accuracies of wave functions of Mg are justified by evaluating its
attachment energies of the relevant states and compared with the experimental
values. The continuum wave function of the projectile electron are obtained by
solving Dirac equations assuming distortion potential as static potential of
the ground state of Mg. Comparison of the calculated electron impact
excitation differential and total cross-sections with the available
measurements are found to be in very good agreements at various incident
electron energies. Further, calculations are carried out in the plasma
environment in the Debye H\"uckel model framework, which could be useful in the
astrophysics. Influence of plasma strength on the cross-sections as well as
linear polarization of the photon emission in the transition is investigated for different incident electron energies.Comment: 9 pages, 1 table and 3 figure
Gravitational Waves in Generalised Brans-Dicke Theory
We have solved cosmological gravitational Wave(GW)equation in the frame work
of Generalised Brans-Dicke(GBD) theory for all epochs of the Universe.The
solutions are expressed in terms of the present value of the Brans-Dicke
coupling parameter .It is seen that the solutions represent
travelling growing modes for negative values of for all epochs of
the Universe.Comment: 7Pages,no figure
Cotunneling through quantum dots coupled to magnetic leads: zero-bias anomaly for non-collinear magnetic configurations
Cotunneling transport through quantum dots weakly coupled to non-collinearly
magnetized leads is analyzed theoretically by means of the real-time
diagrammatic technique. The electric current, dot occupations, and dot spin are
calculated in the Coulomb blockade regime and for arbitrary magnetic
configuration of the system. It is shown that an effective exchange field
exerted on the dot by ferromagnetic leads can significantly modify the
transport characteristics in non-collinear magnetic configurations, in
particular the zero-bias anomaly found recently for antiparallel configuration.
For asymmetric Anderson model, the exchange field gives rise to precession of
the dot spin, which leads to a nonmonotonic dependence of the differential
conductance and tunnel magnetoresistance on the angle between magnetic moments
of the leads. An enhanced differential conductance and negative TMR are found
for certain non-collinear configurations.Comment: 12 pages, 9 figgure
Time Dependence of Brans-Dicke Parameter w for an Expanding Universe
We have studied the time dependence of w for an expanding universe in the
generalised B-D theory and have obtained its explicit dependence on the nature
of matter contained in the universe,in different era.Lastly we discuss how the
observed accelerated expansion of the present universe can be accomodated in
the formalism.Comment: 10 pages,No figure
Mixing quantum and classical mechanics and uniqueness of Planck's constant
Observables of quantum or classical mechanics form algebras called quantum or
classical Hamilton algebras respectively (Grgin E and Petersen A (1974) {\it J
Math Phys} {\bf 15} 764\cite{grginpetersen}, Sahoo D (1977) {\it Pramana} {\bf
8} 545\cite{sahoo}). We show that the tensor-product of two quantum Hamilton
algebras, each characterized by a different Planck's constant is an algebra of
the same type characterized by yet another Planck's constant. The algebraic
structure of mixed quantum and classical systems is then analyzed by taking the
limit of vanishing Planck's constant in one of the component algebras. This
approach provides new insight into failures of various formalisms dealing with
mixed quantum-classical systems. It shows that in the interacting mixed
quantum-classical description, there can be no back-reaction of the quantum
system on the classical. A natural algebraic requirement involving restriction
of the tensor product of two quantum Hamilton algebras to their components
proves that Planck's constant is unique.Comment: revised version accepted for publication in J.Phys.A:Math.Phy
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