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

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

Markoff-Rosenberger triples in geometric progression

Solutions of the Markoff-Rosenberger equation ax^2+by^2+cz^2 = dxyz such that their coordinates belong to the ring of integers of a number field and form a geometric progression are studied.Comment: To appear in Acta Mathematica Hungaric