504 research outputs found
Gauge Theory of Gravity Requires Massive Torsion Field
One of the greatest unsolved issues of the physics of this century is to find
a quantum field theory of gravity. According to a vast amount of literature
unification of quantum field theory and gravitation requires a gauge theory of
gravity which includes torsion and an associated spin field. Various models
including either massive or massless torsion fields have been suggested. We
present arguments for a massive torsion field, where the probable rest mass of
the corresponding spin three gauge boson is the Planck mass.Comment: 3 pages, Revte
Time-Varying Fine-Structure Constant Requires Cosmological Constant
Webb et al. presented preliminary evidence for a time-varying fine-structure
constant. We show Teller's formula for this variation to be ruled out within
the Einstein-de Sitter universe, however, it is compatible with cosmologies
which require a large cosmological constant.Comment: 3 pages, no figures, revtex, to be published in Mod. Phys. Lett.
Modeling and Simulation of Multi-Lane Traffic Flow
A most important aspect in the field of traffic modeling is the simulation of
bottleneck situations. For their realistic description a macroscopic multi-lane
model for uni-directional freeways including acceleration, deceleration,
velocity fluctuations, overtaking and lane-changing maneuvers is systematically
deduced from a gas-kinetic (Boltzmann-like) approach. The resulting equations
contain corrections with respect to previous models. For efficient computer
simulations, a reduced model delineating the coarse-grained temporal behavior
is derived and applied to bottleneck situations.Comment: For related work see
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The effect of Ag, Pb and Bi impurities on grain boundary sliding and intergranular decohesion in Copper
We investigate the changes in grain boundary sliding (GBS) and intergranular decohesion in copper (Cu), due to the inclusion of bismuth (Bi), lead (Pb) and silver (Ag) substitutional impurity atoms at a 5 (0 1 2) symmetric tilt grain boundary (GB), using a first-principles concurrent multiscale approach. We first study the segregation behavior of the impurities by determining the impurity segregation energy in the vicinity of the GB. We find that the energetically preferred sites are on the GB plane. We investigate the intergranular decohesion of Cu by Bi and Pb impurities and compare this to the effect of Ag impurities by considering the work of separation, and the tensile strength, . Both and decrease in the presence of Bi and Pb impurities, indicating their great propensity for intergranular embrittlement, whilst the presence of Ag impurities has only a small effect. We consider GBS to assess the mechanical properties in nanocrystalline metals and find that all three impurities strongly inhibit GBS, with Ag having the biggest effect. This suggests that Ag has a strong effect on the mechanical properties of nanocrystalline Cu, even though its effect on the intergranular decohesion properties of coarse-grained Cu is not significant.Department of Energy, SciDac [grant number ER-25788], Intel Corporation, Graduate School of Excellence MAINZ, IDEE project of the Carl Zeiss FoundationThis is the author accepted manuscript. The final version is available from Taylor & Francis via http://dx.doi.org/10.1080/14786435.2016.121736
Thermodynamical Properties of a Spin 1/2 Heisenberg Chain Coupled to Phonons
We performed a finite-temperature quantum Monte Carlo simulation of the
one-dimensional spin-1/2 Heisenberg model with nearest-neighbor interaction
coupled to Einstein phonons. Our method allows to treat easily up to 100
phonons per site and the results presented are practically free from truncation
errors. We studied in detail the magnetic susceptibility, the specific heat,
the phonon occupation, the dimerization, and the spin-correlation function for
various spin-phonon couplings and phonon frequencies. In particular we give
evidence for the transition from a gapless to a massive phase by studying the
finite-size behavior of the susceptibility. We also show that the dimerization
is proportional to for .Comment: 10 pages, 17 Postscript Figure
Prato: The Social Construction of an Industrial City Facing Processes of Cultural Hybridization
This chapter deals with a widely studied case, that is, Prato, a middle-sized city with rooted industrial traditions, in the Centre of Italy. Prato is a textile industrial district embedded in the so-called Third Italy—an area characterized by the presence of small firms spread throughout the territory, linked together in supply and subcontracting relationships—which, in the last twenty years, has undergone a profound transformation as a consequence of the crisis of textile and immigration, leading to the formation of a large Chinese community. The related changes brought with them problems of social cohesion and sustainable development. The authors address these issues by analyzing both academic and public discourses on Prato. Their basic idea is that common stereotypes act as drivers of a public discourse that prevents the city to re-negotiate its identity. The analysis concludes that different forms of hybridization—particularly cultural hybridization—are occurring, which would need further investigations
Spin-Peierls transition of the first order in S=1 antiferromagnetic Heisenberg chains
We investigate a one-dimensional S=1 antiferromagnetic Heisenberg model
coupled to a lattice distortion by a quantum Monte Carlo method. Investigating
the ground state energy of the static bond-alternating chain, we find that the
instability to a dimerized chain depends on the value of the spin-phonon
coupling, unlike the case of S=1/2. The spin state is the dimer state or the
uniform Haldane state depending on whether the lattice distorts or not,
respectively. At an intermediate value of the spin-phonon coupling, we find the
first-order transition between the two states. We also find the coexistence of
the two states.Comment: 7 pages, 12 eps figures embedded in the text; corrected typos,
replaced figure
Deterministic approach to microscopic three-phase traffic theory
Two different deterministic microscopic traffic flow models, which are in the
context of the Kerner's there-phase traffic theory, are introduced. In an
acceleration time delay model (ATD-model), different time delays in driver
acceleration associated with driver behaviour in various local driving
situations are explicitly incorporated into the model. Vehicle acceleration
depends on local traffic situation, i.e., whether a driver is within the free
flow, or synchronized flow, or else wide moving jam traffic phase. In a speed
adaptation model (SA-model), vehicle speed adaptation occurs in synchronized
flow depending on driving conditions. It is found that the ATD- and SA-models
show spatiotemporal congested traffic patterns that are adequate with empirical
results. In the ATD- and SA-models, the onset of congestion in free flow at a
freeway bottleneck is associated with a first-order phase transition from free
flow to synchronized flow; moving jams emerge spontaneously in synchronized
flow only. Differences between the ATD- and SA-models are studied. A comparison
of the ATD- and SA-models with stochastic models in the context of three phase
traffic theory is made. A critical discussion of earlier traffic flow theories
and models based on the fundamental diagram approach is presented.Comment: 40 pages, 14 figure
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