15,447 research outputs found
Heterotic free fermionic and symmetric toroidal orbifold models
Free fermionic models and symmetric heterotic toroidal orbifolds both
constitute exact backgrounds that can be used effectively for phenomenological
explorations within string theory. Even though it is widely believed that for
Z2xZ2 orbifolds the two descriptions should be equivalent, a detailed
dictionary between both formulations is still lacking. This paper aims to fill
this gap: We give a detailed account of how the input data of both descriptions
can be related to each other. In particular, we show that the generalized GSO
phases of the free fermionic model correspond to generalized torsion phases
used in orbifold model building. We illustrate our translation methods by
providing free fermionic realizations for all Z2xZ2 orbifold geometries in six
dimensions.Comment: 1+49 pages latex, minor revisions and references adde
GRB afterglow light curves in the pre-Swift era - a statistical study
We present the results of a systematic analysis of the world sample of
optical/near-infrared afterglow light curves observed in the pre-Swift era by
the end of 2004. After selecting the best observed 16 afterglows with
well-sampled light curves that can be described by a Beuermann equation, we
explore the parameter space of the light curve parameters and physical
quantities related to them. In addition, we search for correlations between
these parameters and the corresponding gamma-ray data, and we use our data set
to look for a fine structure in the light curves.Comment: accepted for publication in ApJ; Version 2: minor changes, one figure
adde
Depletion-Isolation Effect in Vertical MOSFETs During the Transition From Partial to Fully Depleted Operation
A simulation study is made of floating-body effects (FBEs) in vertical MOSFETs due to depletion isolation as the pillar thickness is reduced from 200 to 10 nm. For pillar thicknesses between 200â60 nm, the output characteristics with and without impact ionization are identical at a low drain bias and then diverge at a high drain bias. The critical drain bias Vdc for which the increased drainâcurrent is observed is found to decrease with a reduction in pillar thickness. This is explained by the onset of FBEs at progressively lower values of the drain bias due to the merging of the drain depletion regions at the bottom of the pillar (depletion isolation). For pillar thicknesses between 60â10 nm, the output characteristics show the opposite behavior, namely, the critical drain bias increases with a reduction in pillar thickness. This is explained by a reduction in the severity of the FBEs due to the drain debiasing effect caused by the elevated body potential. Both depletion isolation and gateâgate coupling contribute to the drainâcurrent for pillar thicknesses between 100â40 nm
On the origin dependence of multipole moments in electromagnetism
The standard description of material media in electromagnetism is based on
multipoles. It is well known that these moments depend on the point of
reference chosen, except for the lowest order. It is shown that this "origin
dependence" is not unphysical as has been claimed in the literature but forms
only part of the effect of moving the point of reference. When also the
complementary part is taken into account then different points of reference
lead to different but equivalent descriptions of the same physical reality.
This is shown at the microscopic as well as at the macroscopic level. A similar
interpretation is valid regarding the "origin dependence" of the reflection
coefficients for reflection on a semi infinite medium. We show that the
"transformation theory" which has been proposed to remedy this situation (and
which is thus not needed) is unphysical since the transformation considered
does not leave the boundary conditions invariant.Comment: 14 pages, 0 figure
Onsager approach to 1D solidification problem and its relation to phase field description
We give a general phenomenological description of the steady state 1D front
propagation problem in two cases: the solidification of a pure material and the
isothermal solidification of two component dilute alloys.
The solidification of a pure material is controlled by the heat transport in
the bulk and the interface kinetics.
The isothermal solidification of two component alloys is controlled by the
diffusion in the bulk and the interface kinetics.
We find that the condition of positive-definiteness of the symmetric Onsager
matrix of interface kinetic coefficients still allows an arbitrary sign of the
slope of the velocity-concentration line near the solidus in the alloy problem
or of the velocity-temperature line in the case of solidification of a pure
material. This result offers a very simple and elegant way to describe the
interesting phenomenon of a possible non-single-value behavior of velocity
versus concentration which has previously been discussed by different
approaches. We also discuss the relation of this Onsager approach to the thin
interface limit of the phase field description.Comment: 5 pages, 1 figure, submitted to Physical Review
Orientation and symmetry control of inverse sphere magnetic nanoarrays by guided self-assembly
Inverse sphere shaped Ni arrays were fabricated by electrodeposition on Si through the guided self-assembly of polystyrene latex spheres in Si/SiO2 patterns. It is shown that the size commensurability of the etched tracks is critical for the long range ordering of the spheres. Moreover, noncommensurate guiding results in the reproducible periodic triangular distortion of the close packed self-assembly. Magnetoresistance measurements on the Ni arrays were performed showing room temperature anisotropic magnetoresistance of 0.85%. These results are promising for self-assembled patterned storage media and magnetoresistance devices
Calculation of shear viscosity using Green-Kubo relations within a parton cascade
The shear viscosity of a gluon gas is calculated using the Green-Kubo
relation. Time correlations of the energy-momentum tensor in thermal
equilibrium are extracted from microscopic simulations using a parton cascade
solving various Boltzmann collision processes. We find that the pQCD based
gluon bremsstrahlung described by Gunion-Bertsch processes significantly lowers
the shear viscosity by a factor of 3-8 compared to elastic scatterings. The
shear viscosity scales with the coupling as 1/(alpha_s^2\log(1/alpha_s)). For a
constant coupling constant the shear viscosity to entropy density ratio has no
dependence on temperature. Replacing the pQCD-based collision angle
distribution of binary scatterings by an isotropic form decreases the shear
viscosity by a factor of 3.Comment: 17 pages, 5 figure
Phase separation of binary fluids with dynamic temperature
Phase separation of binary fluids quenched by contact with cold external
walls is considered. Navier-Stokes, convection-diffusion, and energy equations
are solved by lattice Boltzmann method coupled with finite-difference schemes.
At high viscosity, different morphologies are observed by varying the thermal
diffusivity. In the range of thermal diffusivities with domains growing
parallel to the walls, temperature and phase separation fronts propagate
towards the inner of the system with power-law behavior. At low viscosity
hydrodynamics favors rounded shapes, and complex patterns with different
lengthscales appear. Off-symmetrical systems behave similarly but with more
ordered configurations.Comment: Accepted for publication in Phys. Rev. E, 11 figures, best quality
figures available on reques
Anomalies in field theories with extra dimensions
We give an overview of the issue of anomalies in field theories with extra
dimensions. We start by reviewing in a pedagogical way the computation of the
standard perturbative gauge and gravitational anomalies on non-compact spaces,
using Fujikawa's approach and functional integral methods, and discuss the
available mechanisms for their cancellation. We then generalize these analyses
to the case of orbifold field theories with compact internal dimensions,
emphasizing the new aspects related to the presence of orbifold singularities
and discrete Wilson lines, and the new cancellation mechanisms that are
becoming available. We conclude with a very brief discussion on global and
parity anomalies.Comment: Review article written for Int.J.Mod.Phys. A, 63 pages; v2: mistake
in subsection 4.3 corrected, some comments and references added, a few
misprints fixe
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