177,020 research outputs found
Dark Energy in vector-tensor theories of gravity
We consider a general class of vector-tensor theories of gravity and show
that solutions with accelerated expansion and a future type III singularity are
a common feature in these models. We also show that there are only six
vector-tensor theories with the same small scales behavior as General
Relativity and, in addition, only two of them can be made completely free from
instabilities. Finally, two particular models as candidates for dark energy are
proposed: on one hand, a cosmic vector that allows to alleviate the usual
naturalness and coincidence problems and, on the other hand, the
electromagnetic field is shown to give rise to an effective cosmological
constant on large scales whose value can be explained in terms of inflation at
the electroweak scale.Comment: 4 pages, 1 table. Contribution to the proceedings of Spanish
Relativity Meeting 2009, Bilbao, Spain, 7-11 September 200
Drift-diffusion model for single layer transition metal dichalcogenide field-effect transistors
A physics-based model for the surface potential and drain current for
monolayer transition metal dichalcogenide (TMD) field-effect transistor (FET)
is presented. Taking into account the 2D density-of-states of the atomic layer
thick TMD and its impact on the quantum capacitance, a model for the surface
potential is presented. Next, considering a drift-diffusion mechanism for the
carrier transport along the monolayer TMD, an explicit expression for the drain
current has been derived. The model has been benchmarked with a measured
prototype transistor. Based on the proposed model, the device design window
targeting low-power applications is discussed.Comment: 10 pages, 3 figure
Modelling of last hypothesis of climate change impacts on water resources in Sierra de las Cabras aquifer (Southern Spain)
Having models that reflect the reality of a system operation becomes critical to the effec-tive management of water resources in a watershed, especially considering the different scenes of climate change predicted for the Mediterranean regions by the Intergovernmen-tal Panel on Climate Change (IPCC). The fifth Assessment Report of IPCC (AR5) esti-mates an increase in average temperatures and a precipitation decrease between 10 and 20% for Mediterranean region which could generate drought events in the study area. The present research tries to determinate the potential impacts that climate change may have on water resources. The Sacramento model has been used to model the water resources of an inertial aquifer, Sierra de las Cabras. In turn, IPCC forecasts of climate change for the study area (Mediterranean region) have been simulated taking into account the degree as-sessing of affection to the water resources opposite to eventual episodes of drought, and the social and environmental implications that will result have been also determined.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech
The acoustic cut-off frequency of the Sun and the solar magnetic activity cycle
The acoustic cut-off frequency -the highest frequency for acoustic solar
eigenmodes- is an important parameter of the solar atmosphere as it determines
the upper boundary of the p-mode resonant cavities. At frequencies beyond this
value, acoustic disturbances are no longer trapped but traveling waves.
Interference amongst them give rise to higher-frequency peaks -the pseudomodes-
in the solar acoustic spectrum. The pseudomodes are shifted slightly in
frequency with respect to p modes making possible the use of pseudomodes to
determine the acoustic cut-off frequency. Using data from GOLF and VIRGO
instruments on board the SOHO spacecraft, we calculate the acoustic cut-off
frequency using the coherence function between both the velocity and intensity
sets of data. By using data gathered by these instruments during the entire
lifetime of the mission (1996 till the present), a variation in the acoustic
cut-off frequency with the solar magnetic activity cycle is found.Comment: Paper accepted in ApJ. 26 Pages, 9 figure
Multi-domain ferroelectricity as a limiting factor for voltage amplification in ferroelectric field-effect transistors
We revise the possibility of having an amplified surface potential in
ferroelectric field-effect transistors pointed out by [S. Salahuddin and S.
Datta, Nano Lett. 8, 405 (2008)]. We show that the negative-capacitance regime
that allows for such an amplification is actually bounded by the appearance of
multi-domain ferroelectricity. This imposes a severe limit to the maximum
step-up of the surface potential obtainable in the device. We indicate new
device design rules taking into account this scenario.Comment: 4 pages, 3 figure
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Undulating periodization models for strength training and conditioning
Non-degenerated groundstates in the antiferromagnetic Ising model on triangulations
A triangulation is an embedding of a graph into a closed Riemann surface so
that each face boundary is a 3-cycle of the graph. In this work, groundstate
degeneracy in the antiferromagnetic Ising model on triangulations is studied.
We show that for every fixed closed Riemann surface S, there are
vertex-increasing sequences of triangulations of S with a non-degenerated
groundstate. In particular, we exhibit geometrically frustrated systems with a
non-degenerated groundstate.Comment: 11 pages, 9 figure
Study of the correlation between bicuspid aortic valve and the development of aortic dissection
La disección aórtica (AD) es la condición letal más comúnmente diagnosticada de la arteria aorta y consiste en el redireccionamiento del flujo sanguíneo desde el lumen de la aorta hasta la media de la pared de la aorta a través de una pequeña fisura en la intima. Las causas específicas de la formación de esta fisura, y de la subsecuente dilatación de la pared, todavía no han sido completamente determinadas aunque diversos estudios muestran que puede ser debida o bien a cambios químicos o bien a efectos mecánicos en la pared de la aorta. Este trabajo se centra en el estudio de posibles efectos mecánicos, inducidos por cambios en la hemodinámica de la arteria, que puedan haber conducido al debilitamiento de la pared de la aorta. Válvula aórtica bicúspide (BAV) es la enfermedad congénita del corazón más común y se ha demostrado su importante contribución en el desarrollo de numerosas condiciones cardiovasculares. Esta enfermedad modifica el orificio de salida del corazón, y por tanto el perfil hemodinámico de eyección, del flujo de sangre, lo que podría tener consecuencias en el comportamiento mecánico de la pared de la aorta. Este estudio tiene como objetivo determinar que existe una correlación entre los cambios en la hemodinámica producidos por la presencia de BAV y la formación de AD usando técnicas de análisis de dinámica de fluidos computacional (CFD). Para determinar dicha relación, análisis CFD se han realizado en tres geometrías diferentes: un caso de válvula aórtica tricúspide (TAV) y dos casos distintos de BAV. Todas las geometrías son idealizadas y contemplan la raíz de la aorta, la aorta ascendente y el comienzo del cayado aórtico. Los resultados de los análisis muestran un incremento en la velocidad de eyección de la sangre para ambos casos de BAV debido a la reducción en el área efectiva del orificio. Además, el estudio muestra un incremento en las fuerzas de rozamiento de la pared y en la presión de la pared externa de la aorta. Estos resultados nos llevan a la conclusión de que BAV podría causar hipertensión en la pared externa de la aorta, la cual es una causa mecánica conocida del debilitamiento de vasos sanguíneos
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