624 research outputs found
Electrical properties of nitrogen doped GaP
Journal ArticleThe electrical properties namely, electron concentration and mobility, have been investigated in the temperature range from 53 to 400 K for undoped and nitrogen-doped VPE GaP
Two-band second moment model and an interatomic potential for caesium
A semi-empirical formalism is presented for deriving interatomic potentials
for materials such as caesium or cerium which exhibit volume collapse phase
transitions. It is based on the Finnis-Sinclair second moment tight binding
approach, but incorporates two independent bands on each atom. The potential is
cast in a form suitable for large-scale molecular dynamics, the computational
cost being the evaluation of short ranged pair potentials. Parameters for a
model potential for caesium are derived and tested
A Planck-scale axion and SU(2) Yang-Mills dynamics: Present acceleration and the fate of the photon
From the time of CMB decoupling onwards we investigate cosmological evolution
subject to a strongly interacting SU(2) gauge theory of Yang-Mills scale
eV (masquerading as the factor of the SM at
present). The viability of this postulate is discussed in view of cosmological
and (astro)particle physics bounds. The gauge theory is coupled to a spatially
homogeneous and ultra-light (Planck-scale) axion field. As first pointed out by
Frieman et al., such an axion is a viable candidate for quintessence, i.e.
dynamical dark energy, being associated with today's cosmological acceleration.
A prediction of an upper limit for the duration of the
epoch stretching from the present to the point where the photon starts to be
Meissner massive is obtained: billion years.Comment: v3: consequences of an error in evolution equation for coupling
rectified, only a minimal change in physics results, two refs. adde
Signatures of Supernova Neutrino Oscillations into Extra Dimensions
We consider the mixing of muon and tau neutrinos with sterile fermion fields
propagating in extra dimensions in the context of core collapse supernova
physics, extending the analysis of the electron neutrino case done in a
previous work. We show that the potentially dramatic modifications to the
supernova evolution are prevented by a mechanism of feedback, so that no severe
bounds on the parameters of the extra dimensions need to be imposed.
Nevertheless, the supernova core evolution is significantly modified. We
discuss the consequences on the delayed explosion mechanism and the
compatibility with the SN1987A signal. Then, for the cases of both nu_{mu,tau}
and nu_e mixing with bulk fermions, we analyse the distinctive features of the
signal on Earth.Comment: 16 pages, 7 figures; v2: minor changes, matches published versio
Superhard Phases of Simple Substances and Binary Compounds of the B-C-N-O System: from Diamond to the Latest Results (a Review)
The basic known and hypothetic one- and two-element phases of the B-C-N-O
system (both superhard phases having diamond and boron structures and
precursors to synthesize them) are described. The attention has been given to
the structure, basic mechanical properties, and methods to identify and
characterize the materials. For some phases that have been recently described
in the literature the synthesis conditions at high pressures and temperatures
are indicated.Comment: Review on superhard B-C-N-O phase
Global agricultural intensification during climate change: A role for genomics
Summary: Agriculture is now facing the 'perfect storm' of climate change, increasing costs of fertilizer and rising food demands from a larger and wealthier human population. These factors point to a global food deficit unless the efficiency and resilience of crop production is increased. The intensification of agriculture has focused on improving production under optimized conditions, with significant agronomic inputs. Furthermore, the intensive cultivation of a limited number of crops has drastically narrowed the number of plant species humans rely on. A new agricultural paradigm is required, reducing dependence on high inputs and increasing crop diversity, yield stability and environmental resilience. Genomics offers unprecedented opportunities to increase crop yield, quality and stability of production through advanced breeding strategies, enhancing the resilience of major crops to climate variability, and increasing the productivity and range of minor crops to diversify the food supply. Here we review the state of the art of genomic-assisted breeding for the most important staples that feed the world, and how to use and adapt such genomic tools to accelerate development of both major and minor crops with desired traits that enhance adaptation to, or mitigate the effects of climate change. >
Neutrino masses: From fantasy to facts
Theory suggests the existence of neutrino masses, but little more. Facts are
coming close to reveal our fantasy: solar and atmospheric neutrino data
strongly indicate the need for neutrino conversions, while LSND provides an
intriguing hint. The simplest ways to reconcile these data in terms of neutrino
oscillations invoke a light sterile neutrino in addition to the three active
ones. Out of the four neutrinos, two are maximally-mixed and lie at the LSND
scale, while the others are at the solar mass scale. These schemes can be
distinguished at neutral-current-sensitive solar & atmospheric neutrino
experiments. I discuss the simplest theoretical scenarios, where the lightness
of the sterile neutrino, the nearly maximal atmospheric neutrino mixing, and
the generation of & all follow
naturally from the assumed lepton-number symmetry and its breaking. Although
the most likely interpretation of the present data is in terms of
neutrino-mass-induced oscillations, one still has room for alternative
explanations, such as flavour changing neutrino interactions, with no need for
neutrino mass or mixing. Such flavour violating transitions arise in theories
with strictly massless neutrinos, and may lead to other sizeable flavour
non-conservation effects, such as , conversion in
nuclei, unaccompanied by neutrino-less double beta decay.Comment: 33 pages, latex, 16 figures. Invited Talk at Ioannina Conference,
Symmetries in Intermediate High Energy Physics and its Applications, Oct.
1998, to be published by Springer Tracts in Modern Physics. Festschrift in
Honour of John Vergados' 60th Birthda
Many body physics from a quantum information perspective
The quantum information approach to many body physics has been very
successful in giving new insight and novel numerical methods. In these lecture
notes we take a vertical view of the subject, starting from general concepts
and at each step delving into applications or consequences of a particular
topic. We first review some general quantum information concepts like
entanglement and entanglement measures, which leads us to entanglement area
laws. We then continue with one of the most famous examples of area-law abiding
states: matrix product states, and tensor product states in general. Of these,
we choose one example (classical superposition states) to introduce recent
developments on a novel quantum many body approach: quantum kinetic Ising
models. We conclude with a brief outlook of the field.Comment: Lectures from the Les Houches School on "Modern theories of
correlated electron systems". Improved version new references adde
Search for single top quarks in the tau+jets channel using 4.8 fb of collision data
We present the first direct search for single top quark production using tau
leptons. The search is based on 4.8 fb of integrated luminosity
collected in collisions at =1.96 TeV with the D0 detector
at the Fermilab Tevatron Collider. We select events with a final state
including an isolated tau lepton, missing transverse energy, two or three jets,
one or two of them tagged. We use a multivariate technique to discriminate
signal from background. The number of events observed in data in this final
state is consistent with the signal plus background expectation. We set in the
tau+jets channel an upper limit on the single top quark cross section of
\TauLimObs pb at the 95% C.L. This measurement allows a gain of 4% in expected
sensitivity for the observation of single top production when combining it with
electron+jets and muon+jets channels already published by the D0 collaboration
with 2.3 fb of data. We measure a combined cross section of
\SuperCombineXSall pb, which is the most precise measurement to date.Comment: 12 pages, 5 figure
Measurement of Z/gamma*+jet+X angular distributions in ppbar collisions at sqrt{s}=1.96 TeV
We present the first measurements at a hadron collider of differential cross
sections for Z+jet+X production in delta phi(Z, jet), |delta y(Z, jet)| and
|y_boost(Z, jet)|. Vector boson production in association with jets is an
excellent probe of QCD and constitutes the main background to many small cross
section processes, such as associated Higgs production. These measurements are
crucial tests of the predictions of perturbative QCD and current event
generators, which have varied success in describing the data. Using these
measurements as inputs in tuning event generators will increase the
experimental sensitivity to rare signals.Comment: Published in Physics Letters B 682 (2010), pp. 370-380. 15 pages, 6
figure
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