1,509 research outputs found
Exploring an Objective Weighting System for Travel & Tourism Pillars
AbstractThe World Economic Forum employs Travel & Tourism Competitiveness Indexes (TTCI) to measure the travel & tourism (T&T) global competitiveness of a country. The TTCI overall scores are calculated with an arithmetic mean aggregation from the scores of the fourteen composite pillars with a subjective assumption of all the pillars having the same weights. This paper attempts to release such a subjective assumption by proposing a new solution framework to explore an objective weighting system for the pillars. The proposed solution framework employs the Expectation Maximization (EM) clustering algorithm to group the 139 ranked countries into three classes and then performs the Artificial Neural Network (ANN) analysis to explore the objective weighting system for the fourteen pillars. The results show that tourism infrastructure, ground transport infrastructure, air transport infrastructure, cultural resources, health and hygiene, and ICT infrastructure are the six most critical pillars contributing to the TTCI overall scores. Accordingly, the policy makers should allocate limited resources with priority to improve these six pillars to frog leap the T&T global competitiveness
Relating c<0 and c>0 Conformal Field Theories
A `canonical mapping' is established between the c=-1 system of bosonic
ghosts and the c=2 complex scalar theory and, a similar mapping between the
c=-2 system of fermionic ghosts and the c=1 Dirac theory. The existence of this
mapping is suggested by the identity of the characters of the respective
theories. The respective c0 theories share the same space of states,
whereas the spaces of conformal fields are different. Upon this mapping from
their c0) complex scalar and the Dirac theories inherit
hidden nonlocal sl(2) symmetries.Comment: 23 pages, harvma
Combined ion and atom trap for low temperature ion-atom physics
We report an experimental apparatus and technique which simultaneously traps
ions and cold atoms with spatial overlap. Such an apparatus is motivated by the
study of ion-atom processes at temperatures ranging from hot to ultra-cold.
This area is a largely unexplored domain of physics with cold trapped atoms. In
this article we discuss the general design considerations for combining these
two traps and present our experimental setup. The ion trap and atom traps are
characterized independently of each other. The simultaneous operation of both
is then described and experimental signatures of the effect of the ions and
cold-atoms on each other are presented. In conclusion the use of such an
instrument for several problems in physics and chemistry is briefly discussed.Comment: 24 pages, 13 figures. Figures Fixe
Changes in Optical Conductivity due to Readjustments in Electronic Density of States
Within the model of elastic impurity scattering, we study how changes in the
energy dependence of the electronic density of states (EDOS)
around the Fermi energy are reflected in the frequency-dependent
optical conductivity . While conserving the total number of
states in we compute the induced changes in as a
function of and in the corresponding optical scattering rate
. These quantities mirror some aspects of the EDOS
changes but the relationship is not direct. Conservation of optical oscillator
strength is found not to hold, and there is no sum rule on the optical
scattering rate although one does hold for the quasiparticle scattering.
Temperature as well as increases in impurity scattering lead to additional
changes in optical properties not seen in the constant EDOS case. These effects
have their origin in an averaging of the EDOS around the Fermi energy
on an energy scale set by the impurity scattering.Comment: 13 pages, 7 figure
Ring exchange, the Bose metal, and bosonization in two dimensions
Motivated by the high-T_c cuprates, we consider a model of bosonic Cooper
pairs moving on a square lattice via ring exchange. We show that this model
offers a natural middle ground between a conventional antiferromagnetic Mott
insulator and the fully deconfined fractionalized phase which underlies the
spin-charge separation scenario for high-T_c superconductivity. We show that
such ring models sustain a stable critical phase in two dimensions, the *Bose
metal*. The Bose metal is a compressible state, with gapless but uncondensed
boson and ``vortex'' excitations, power-law superconducting and charge-ordering
correlations, and broad spectral functions. We characterize the Bose metal with
the aid of an exact plaquette duality transformation, which motivates a
universal low energy description of the Bose metal. This description is in
terms of a pair of dual bosonic phase fields, and is a direct analog of the
well-known one-dimensional bosonization approach. We verify the validity of the
low energy description by numerical simulations of the ring model in its exact
dual form. The relevance to the high-T_c superconductors and a variety of
extensions to other systems are discussed, including the bosonization of a two
dimensional fermionic ring model
Quantum Maxwell-Bloch equations for spatially inhomogeneous semiconductor lasers
We present quantum Maxwell-Bloch equations (QMBE) for spatially inhomogeneous
semiconductor laser devices. The QMBE are derived from fully quantum mechanical
operator dynamics describing the interaction of the light field with the
quantum states of the electrons and the holes near the band gap. By taking into
account field-field correlations and field-dipole correlations, the QMBE
include quantum noise effects which cause spontaneous emission and amplified
spontaneous emission. In particular, the source of spontaneous emission is
obtained by factorizing the dipole-dipole correlations into a product of
electron and hole densities. The QMBE are formulated for general devices, for
edge emitting lasers and for vertical cavity surface emitting lasers, providing
a starting point for the detailed analysis of spatial coherence in the near
field and far field patterns of such laser diodes. Analytical expressions are
given for the spectra of gain and spontaneous emission described by the QMBE.
These results are applied to the case of a broad area laser, for which the
frequency and carrier density dependent spontaneous emission factor beta and
the evolution of the far field pattern near threshold are derived.Comment: 22 pages RevTex and 7 figures, submitted to Phys.Rev.A, revisions in
abstract and in the discussion of temporal coherenc
The Approach to Ergodicity in Monte Carlo Simulations
The approach to the ergodic limit in Monte Carlo simulations is studied using
both analytic and numerical methods. With the help of a stochastic model, a
metric is defined that enables the examination of a simulation in both the
ergodic and non-ergodic regimes. In the non-ergodic regime, the model implies
how the simulation is expected to approach ergodic behavior analytically, and
the analytically inferred decay law of the metric allows the monitoring of the
onset of ergodic behavior. The metric is related to previously defined measures
developed for molecular dynamics simulations, and the metric enables the
comparison of the relative efficiencies of different Monte Carlo schemes.
Applications to Lennard-Jones 13-particle clusters are shown to match the model
for Metropolis, J-walking and parallel tempering based approaches. The relative
efficiencies of these three Monte Carlo approaches are compared, and the decay
law is shown to be useful in determining needed high temperature parameters in
parallel tempering and J-walking studies of atomic clusters.Comment: 17 Pages, 7 Figure
Antiferromagnetic ordering in a 90 K copper oxide superconductor
Using elastic neutron scattering, we evidence a commensurate
antiferromagnetic Cu(2) order (AF) in the superconducting (SC) high-
cuprate (y=0.013, =93 K). As
in the Co-free system, the spin excitation spectrum is dominated by a magnetic
resonance peak at 41 meV but with a reduced spectral weight. The substitution
of Co thus leads to a state where AF and SC cohabit showing that the CuO
plane is a highly antiferromagnetically polarizable medium even for a sample
where T remains optimum.Comment: 3 figure
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