20,859 research outputs found
Low redshift constraints on energy-momentum-powered gravity models
There has been recent interest in the cosmological consequences of
energy-momentum-powered gravity models, in which the matter side of Einstein's
equations is modified by the addition of a term proportional to some power,
, of the energy-momentum tensor, in addition to the canonical linear term.
In this work we treat these models as phenomenological extensions of the
standard CDM, containing both matter and a cosmological constant. We
also quantitatively constrain the additional model parameters using low
redshift background cosmology data that are specifically from Type Ia
supernovas and Hubble parameter measurements. We start by studying specific
cases of these models with fixed values of which lead to an analytic
expression for the Friedmann equation; we discuss both their current
constraints and how the models may be further constrained by future
observations of Type Ia supernovas for WFIRST complemented by measurements of
the redshift drift by the ELT. We then consider and constrain a more extended
parameter space, allowing to be a free parameter and considering scenarios
with and without a cosmological constant. These models do not solve the
cosmological constant problem per se. Nonetheless these models can
phenomenologically lead to a recent accelerating universe without a
cosmological constant at the cost of having a preferred matter density of
around instead of the usual . Finally we
also briefly constrain scenarios without a cosmological constant, where the
single component has a constant equation of state which needs not be that of
matter; we provide an illustrative comparison of this model with a more
standard dynamical dark energy model with a constant equation of state.Comment: 13+2 pages, 12+1 figures; A&A (in press
Effects of Poverty on Children Educational Attainment in Isua, Akoko South East Local Government, Ondo State
The overall objective of this study is to provide an insight into the influence of poverty on the children’s educational attainment in Isua-Akoko. Poverty is said to exist when people lacks the means to satisfy their basic needs. Thus, improving the educational attainment of the population is an important requirement to reduce poverty amid the populace and foster development. However, the study employed 202 questionnaire to gather information and data from household parents, teachers and students (randomly selected from different social background) in which 146 were reported poor. Binary probit and logit method were used. The result obtained indicates that parent level of education, role model and parent’s autonomous income greatly influence children’s educational attainment in Isua-Akoko. While parent level of income and government policy did not have any influence on the children’s education. The researchers thus suggest that to increase the rate of educational enrolment/completion the following measures should be put in place which include: heavy investment in formal and non-formal education (vocational training), good governance in the delivery of education incentives and involvement of the people at the local levels in education development decisions. Keywords: Poverty, Educational attainment, Isua-Akoko, Probit and Logi
New results for the t-J model in ladders: Changes in the spin liquid state with applied magnetic field. Implications for the cuprates
Exact Diagonalization calculations are presented for the t-J model in the
presence of a uniform magnetic field. Results for 2xL ladders (L=8,10,12) and
4x4 square clusters with 1 and 2 holes indicate that the diamagnetic response
to a perpendicular magnetic field tends to induce a spin liquid state in the
spin background. The zero-field spin liquid state of a two-leg ladder is
reinforced by the magnetic field: a considerable increase of rung
antiferromagnetic correlations is observed for J/t up to 0.6, for 1 and 2
holes. Pair-breaking is also clearly observed in the ladders and seems to be
associated in part with changes promoted by the field in the spin correlations
around the zero-field pair. In the 4x4 cluster, the numerical results seem to
indicate that the field-induced spin liquid state competes with the zero-field
antiferromagnetic short-range-order, the spin liquid state being favored by
higher doping and smaller values of J/t. It is interesting to note that the
field-effect can also be observed in a 2x2 plaquette with 1 and 2 holes. This
opens up the possibility of gaining a qualitative understanding of the effect.Comment: 16 pages, 7 figures, latex New results adde
Magnetic control of particle-injection in plasma based accelerators
The use of an external transverse magnetic field to trigger and to control
electron self-injection in laser- and particle-beam driven wakefield
accelerators is examined analytically and through full-scale particle-in-cell
simulations. A magnetic field can relax the injection threshold and can be used
to control main output beam features such as charge, energy, and transverse
dynamics in the ion channel associated with the plasma blowout. It is shown
that this mechanism could be studied using state-of-the-art magnetic fields in
next generation plasma accelerator experiments.Comment: 10 pages, 3 figure
TRANSIENT KNICKPOINTS” NO LEITO DOS RIOS, SIGNIFICADO NA EVOLUÇÃO DA PAISAGEM
Neste trabalho apresenta-se uma síntese sobre a relevância dos knickpoints na estruturação do perfil longitudinal dos rios e indirectamente, na evolução da paisagem. Aborda-se o significado dos knickpoints relativamente aos conceitos de equilíbrio e desequilíbrio do perfil longitudinal, difundidos na literatura. Leis que regem a incisão fluvial, testadas em modelos físico-matemáticos, estabelecidos nas duas últimas décadas, são também referidas. Salientam-se as potencialidades que decorrem deste tipo de análise quantitativa dos perfis longitudinais dos rios, nomeadamente: a identificação de troços relíquia e troços ajustados a novas condições de equilíbrio; a reconstituição de antigos leitos (erodidos) a jusante de knickpoints transitórios e a correlação com outras unidades geomorfológicas, como sejam os terraços fluviais. A relação entre o comportamento das vagas de erosão com a morfologia dos knickpoints e com certos modelos de evolução da paisagem é salientada neste trabalho
Scanning Gate Spectroscopy of transport across a Quantum Hall Nano-Island
We explore transport across an ultra-small Quantum Hall Island (QHI) formed
by closed quan- tum Hall edge states and connected to propagating edge channels
through tunnel barriers. Scanning gate microscopy and scanning gate
spectroscopy are used to first localize and then study a single QHI near a
quantum point contact. The presence of Coulomb diamonds in the spectroscopy
con- firms that Coulomb blockade governs transport across the QHI. Varying the
microscope tip bias as well as current bias across the device, we uncover the
QHI discrete energy spectrum arising from electronic confinement and we extract
estimates of the gradient of the confining potential and of the edge state
velocity.Comment: 13 pages, 3 figure
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