1,156 research outputs found
Coulomb blockade in silicon based structures at temperatures up to 50 K
Coulomb blockade has been observed in the current-voltage characteristics of structures fabricated in silicon germanium delta-doped material at temperatures up to 50 K. This is consistent with the estimated effective tunnel capacitance of 10 aF which is significantly smaller than the reported capacitances of tunnel junctions made from Al or GaAs/AlGaAs heterostructures
Left-Right Symmetric Heterotic-String Derived Models
Recently it was demonstrated that free fermionic heterotic-strings can
produce models with solely the Minimal Supersymmetric Standard Model states in
the low energy spectrum. This unprecedented result provides further strong
evidence for the possibility that the true string vacuum shares some of the
properties of the free fermionic models. Past free fermionic models have
focused on several possible unbroken observable SO(10) subgroups at the string
scale, which include the flipped SU(5) (FSU5), the Pati-Salam (PS) string
models, and the string Standard-like Models (SLM). We extend this study to
include the case in which the SO(10) symmetry is broken to the Left-Right
Symmetric (LRS) gauge group, SO(10) -> SU(3)_C X U(1)_{B-L} X SU(2)_L X
SU(2)_R. We present several models of this type and discuss their
phenomenological features. The most striking new outcome of the LRS string
models, in contrast to the case of the FSU5, the PS, and the SLM string models,
is that they can produce effective field theories that are free of Abelian
anomalies. We discuss the distinction between the two types of free fermionic
models which result in the presence, or absence, of an anomalous U(1). As a
counter example we also present a LRS model that does contain an anomalous
U(1). Additionally, we discuss how in string models the Standard Model spectrum
may arise from the three \mbf 16 representations of SO(10), while the
weak-hypercharge does not have the canonical SO(10) embedding.Comment: 39 pages. Standard Latex. Version to appear in PR
Initial Systematic Investigations of the Landscape of Low Layer NAHE Extensions
The discovery that the number of physically consistent string vacua is on the
order of 10^500 has prompted several statistical studies of string
phenomenology. Contained here is one such study that focuses on the Weakly
Coupled Free Fermionic Heterotic String (WCFFHS) formalism. Presented are
systematic extensions of the well-known NAHE (Nanopoulos, Antoniadis, Hagelin,
Ellis) set of basis vectors, which have been shown to produce
phenomenologically realistic models. Statistics related to the number of
U(1)'s, gauge group factors, non-Abelian singlets, ST SUSYs, as well as the
gauge groups themselve are discussed for the full range of models produced as
well as models containing GUT groups only. Prior results of other large-scale
investigations are compared with these regarding the aforementioned quantities.
Statistical coupling between the gauge groups and the number of ST SUSYs is
also discussed, and it was found that for order-3 extensions there are more
models with enhanced ST SUSY when there is an exceptional group present. Also
discussed are some three-generation GUT models found in the data sets. These
models are unique because they come from basis vectors which still have a
geometric interpretation -- there are no "rank-cuts" in these models.Comment: 65 Pages, 31 Tables, 31 Figure
Phenomenology of A Three-Family Standard-like String Model
We discuss the phenomenology of a three-family supersymmetric Standard-like
Model derived from the orientifold construction, in which the ordinary chiral
states are localized at the intersection of branes at angles. In addition to
the Standard Model group, there are two additional U(1)' symmetries, one of
which has family non-universal and therefore flavor changing couplings, and a
quasi-hidden non-abelian sector which becomes strongly coupled above the
electroweak scale. The perturbative spectrum contains a fourth family of exotic
(SU(2)- singlet) quarks and leptons, in which, however, the left-chiral states
have unphysical electric charges. It is argued that these decouple from the low
energy spectrum due to hidden sector charge confinement, and that anomaly
matching requires the physical left-chiral states to be composites. The model
has multiple Higgs doublets and additional exotic states. The moduli-dependent
predictions for the gauge couplings are discussed. The strong coupling agrees
with experiment for reasonable moduli, but the electroweak couplings are too
small.Comment: 22 pages, 4 figure
Nematic-Isotropic Transition with Quenched Disorder
Nematic elastomers do not show the discontinuous, first-order, phase
transition that the Landau-De Gennes mean field theory predicts for a
quadrupolar ordering in 3D. We attribute this behavior to the presence of
network crosslinks, which act as sources of quenched orientational disorder. We
show that the addition of weak random anisotropy results in a singular
renormalization of the Landau-De Gennes expression, adding an energy term
proportional to the inverse quartic power of order parameter Q. This reduces
the first-order discontinuity in Q. For sufficiently high disorder strength the
jump disappears altogether and the phase transition becomes continuous, in some
ways resembling the supercritical transitions in external field.Comment: 12 pages, 4 figures, to be published on PR
Physics Implications of Flat Directions in Free Fermionic Superstring Models II: Renormalization Group Analysis
We continue the investigation of the physics implications of a class of flat
directions for a prototype quasi-realistic free fermionic string model (CHL5),
building upon the results of the previous paper in which the complete mass
spectrum and effective trilinear couplings of the observable sector were
calculated to all orders in the superpotential. We introduce soft supersymmetry
breaking mass parameters into the model, and investigate the gauge symmetry
breaking patterns and the renormalization group analysis for two representative
flat directions, which leave an additional as well as the SM gauge
group unbroken at the string scale. We study symmetry breaking patterns that
lead to a phenomenologically acceptable hierarchy, and for electroweak and intermediate
scale symmetry breaking, respectively, and the associated mass
spectra after electroweak symmetry breaking. The fermion mass spectrum exhibits
unrealistic features, including massless exotic fermions, but has an
interesting -quark hierarchy and associated CKM matrix in one case. There
are (some) non-canonical effective terms, which lead to a non-minimal
Higgs sector with more than two Higgs doublets involved in the symmetry
breaking, and a rich structure of Higgs particles, charginos, and neutralinos,
some of which, however, are massless or ultralight. In the electroweak scale
cases the scale of supersymmetry breaking is set by the mass, with the
sparticle masses in the several TeV range.Comment: 38 pages, 5 figures, LaTex. Minor correction
Quenched disorder and spin-glass correlations in XY nematics
We present a theoretical study of the equilibrium ordering in a 3D XY nematic
system with quenched random disorder. Within this model, treated with the
replica trick and Gaussian variational method, the correlation length is
obtained as a function of the local nematic order parameter and the effective
disorder strength. These results clarify what happens in the limiting cases of
diminishing order parameter and disorder strength, that is near a phase
transition of a pure system. In particular, it is found that quenched disorder
is irrelevant as the order parameter tends to zero and hence does not change
the character of the continuous XY nematic to isotropic phase transition. We
discuss how these results compare with experiments and simulationsComment: 19 pages, 6 figures, corrected typo
Indications for an Extra Neutral Gauge Boson in Electroweak Precision Data
A new analysis of the hadronic peak cross section at LEP 1 implies a small
amount of missing invisible width in Z decays, while the effective weak charge
in atomic parity violation has been determined recently to 0.6% accuracy,
indicating a significantly negative S parameter. As a consequence of these two
deviations, the data are described well if the presence of an additional Z'
boson, such as predicted in Grand Unified Theories, is assumed. Moreover, the
data are now rich enough to study an arbitrary extra Z' boson and to determine
its couplings in a model independent way. An excellent best fit to the data is
obtained in this case, suggesting the possibility of a family non-universal Z'
with properties similar to ones predicted in a class of superstring theories.Comment: 5 pages of ReVTeX, 2 figure
The ins and outs of participation in a weather information system
In this paper our aim is to show even though access to technology, information or data holds the potential for improved participation, participation is wired into a larger network of actors, artefacts and information practices. We draw on a case study of a weather information system developed and implemented by a non-profit organisation to both describe the configuration of participation, but also critically assess inclusion and exclusion. We present a set of four questions - a basic, practical toolkit - by which we together with the organisation made sense of and evaluated participation in the system
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