12,888 research outputs found
Locally Inertial Reference Frames in Lorentzian and Riemann-Cartan Spacetimes
In this paper we scrutinize the concept of locally inertial reference frames
(LIRF) in Lorentzian and Riemann-Cartan spacetime structures. We present
rigorous mathematical definitions for those objects, something that needs
preliminary a clear mathematical distinction between the concepts of observers,
reference frames, naturally adapted coordinate functions to a given reference
frame and which properties may characterize an inertial reference frame (if
any) in the Lorentzian and Riemann-Cartan structures. We hope to have clarified
some eventual obscure issues associated to the concept of LIRF appearing in the
literature, in particular the relationship between LIRFs in Lorentzian and
Riemann-Cartan spacetimes and Einstein's most happy though, i.e., the
equivalence principle.Comment: In this version a new reference has been added, some misprints and
typos have been corrected and some few sentences in two remarks and in the
conclusions have been changed for better intelligibilit
Rigorous Formulation of Duality in Gravitational Theories
In this paper we evince a rigorous formulation of duality in gravitational
theories where an Einstein like equation is valid, by providing the conditions
under which the Hodge duals (with respect to the metric tensor g) of T^a and
R_b^a may be considered as the torsion and curvature 2-forms associated with a
connection D', part of a Riemann-Cartan structure (M,g',D'), in the cases g =
g' and g does not equal g', once T^a and R_b^a are the torsion and curvature
2-forms associated with a connection D part of a Riemann-Cartan structure
(M,g,D). A new form for the Einstein equation involving the dual of the Riemann
tensor of D is also provided, and the result is compared with others appearing
in the literature.Comment: 15 page
Is Small Perfect? Size Limit to Defect Formation in Pyramidal Pt Nanocontacts
We report high resolution transmission electron microscopy and ab initio
calculation results for the defect formation in Pt nanocontacts (NCs). Our
results show that there is a size limit to the existence of twins (extended
structural defects). Defects are always present but blocked away from the tip
axes. The twins may act as scattering plane, influencing contact electron
transmission for Pt NC at room temperature and Ag/Au NC at low temperature.Comment: 4 pages, 3 figure
A Multiple Criteria Framework to Evaluate Bank Branch Potential Attractiveness
Remarkable progress has occurred over the years in the performance evaluation of bank branches. Even though financial measures are usually considered the most important in assessing branch viability, we posit that insufficient attention has been given to other factors that affect the branchesâ potential profitability and attractiveness. Based on the integrated used of cognitive maps and MCDA techniques, we propose a framework that adds value to the way that potential attractiveness criteria to assess bank branches are selected and to the way that the trade-offs between those criteria are obtained. This framework is the result of a process involving several directors from the five largest banks operating in Portugal, and follows a constructivist approach. Our findings suggest that the use of cognitive maps systematically identifies previously omitted criteria that may assess potential attractiveness. The use of MCDA techniques may clarify and add transparency to the way trade-offs are dealt with. Advantages and disadvantages of the proposed framework are also discussed.
Generalization of Dirac Non-Linear Electrodynamics, and Spinning Charged Particles
In this note we generalized the Dirac non-linear electrodynamics, by
introducing two potentials (namely, the vector potential A and the
pseudo-vector potential gamma^5 B of the electromagnetic theory with charges
and magnetic monopoles) and by imposing the pseudoscalar part of the product
omega.omega* to be zero, with omega = A + gamma^5 B. We show that the field
equations of such a theory possess a soliton-like solution which can represent
a priori a "charged particle", since it is endowed with a Coulomb field plus
the field of a magnetic dipole. The rest energy of the soliton is finite, and
the angular momentum stored in its electromagnetic field can be identified
--for suitable choices of the parameters-- with the spin of the charged
particle. Thus this approach seems to yield a classical model for the charged
(spinning) particle, which does not meet the problems met by earlier attempts
in the same direction.Comment: standard LaTeX file; 16 pages; it is a corrected version of a paper
appeared in Found. Phys. (issue in honour of A.O.Barut) 23 (1993) 46
Superconducting charge qubits from a microscopic many-body perspective
The quantised Josephson junction equation that underpins the behaviour of
charge qubits and other tunnel devices is usually derived through cannonical
quantisation of the classical macroscopic Josephson relations. However, this
approach may neglect effects due to the fact that the charge qubit consists of
a superconducting island of finite size connected to a large superconductor.
We show that the well known quantised Josephson equation can be derived
directly and simply from a microscopic many-body Hamiltonian. By choosing the
appropriate strong coupling limit we produce a highly simplified Hamiltonian
that nevertheless allows us to go beyond the mean field limit and predict
further finite-size terms in addition to the basic equation.Comment: Accepted for J Phys Condensed Matte
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