Atomic Effective Pseudopotentials for Semiconductors

We derive an analytic connection between the screened self-consistent effective potential from density functional theory (DFT) and atomic effective pseudopotentials (AEPs). The motivation to derive AEPs is to address structures with thousands to hundred thousand atoms, as given in most nanostructures. The use of AEPs allows to bypass a self-consistent procedure and to address eigenstates around a certain region of the spectrum (e.g., around the band gap). The bulk AEP construction requires two simple DFT calculations of slightly deformed elongated cells. The ensuing AEPs are given on a fine reciprocal space grid, including the small reciprocal vector components, are free of parameters, and involve no fitting procedure. We further show how to connect the AEPs of different bulk materials, which is necessary to obtain accurate band offsets. We derive a total of 20 AEPs for III-V, II-VI and group IV semiconductors and demonstrate their accuracy and transferability by comparison to DFT calculations of strained bulk structures, quantum wells with varying thickness, and semiconductor alloys.Comment: 10 pages, 5 figures, submitted to PR

Inflation as a response to protect the Holographic Principle

A model where the inflationary phase emerges as a response to protect the Fischler-Susskind holographic bound is described. A two fluid model in a closed universe inflation picture is assumed, and a discussion on conditions under which is possible to obtain an additional exponential expansion phase as those currently observed is given.Comment: 6 pages, 2 figures. Accepted for publication in MPL

Short-term Effects of Gamma Ray Bursts on Earth

The aim of the present work is to study the potential short-term atmospheric and biospheric influence of Gamma Ray Bursts on the Earth. We focus in the ultraviolet flash at the planet's surface, which occurs as a result of the retransmission of the $\gamma$ radiation through the atmosphere. This would be the only important short-term effect on life. We mostly consider Archean and Proterozoic eons, and for completeness we also comment on the Phanerozoic. Therefore, in our study we consider atmospheres with oxygen levels ranging from $10^{-5}$ to 1% of the present atmospheric level, representing different moments in the oxygen rise history. Ecological consequences and some strategies to estimate their importance are outlined.Comment: 13 pp., to appear in Astrophysics and Space Scienc

Emisiones antropogénicas de amoniaco, nitratos y óxido nitroso: compuestos nitrogenados que afectan el medio ambiente en el sector agropecuario colombiano

El cambio climático es un fenómeno global actual que posee diversas causas, tales como el incremento antropogénico en las emisiones de gases de efecto invernadero y su influencia en el planeta. El óxido nitroso (N2O) es un gas con un potencial de calentamiento 298 veces mayor que el CO2; es emitido por diversos sectores económicos, entre ellos la agricultura y la ganadería por el empleo de fertilizantes nitrogenados. Este uso no incrementa únicamente las emisiones de N2O, sino que afecta todos los eslabones del ciclo del nitrógeno, siendo importante reconocer las interacciones entre sus reacciones (amonificación, nitrificación, denitrificación, etc.) para comprender los efectos sobre la lixiviación de nitratos, volatilización de amoniaco y emisiones de N2O, tres compuestos naturales que afectan el medio ambiente. En Colombia, el IDEAM reportó que en 2004 el sector agropecuario nacional produjo 94,91 gigagramos (Gg) de N2O, pues los sistemas de producción agrícolas y bovinos utilizan altas cantidades de fertilizantes nitrogenados con bajos índices de asimilación. Por ello adquiere importancia implementar buenas prácticas que midan y mitiguen estas emisiones en campo, buscando mayor eficiencia en la transformación de componentes nitrogenados dentro del sistema. En este artículo se enfatiza un manejo adecuado del suelo y la utilización de inhibidores de nitrificación naturales (presentes en algunas plantas tropicales) o químicos, como la nitrapyrina, DCD y NBPT (este último como inhibidor de la amonificación), observando su forma de acción y medición, su efecto mitigador y los incrementos productivos. La presente es una revisión bibliográfica que aborda la problemática ambiental de la ganadería bovina

Bilateral Internuclear Ophthalmoplegia in a Young Woman with Vertebral Artery Dissection

Background: Internuclear ophthalmoplegia (INO) is an eye movement disorder caused by a lesion in the medial longitudinal fasciculus (MLF) located in the midbrain. Adduction paralysis of both eyes and bilateral abduction nystagmus are the main features of INO[1]. Case presentation: A 29-year-old Hispanic woman was admitted to the emergency department complaining of an intense holocranial headache lasting 9 days, associated with nausea and vomiting. She was discharged home with resolution of the headache but persistence of symptoms. However, she subsequently developed horizontal diplopia and gait abnormalities. She was readmitted to hospital because of anomalous eye movements and conjugate gaze palsy, manifested as bilateral INO. Magnetic resonance angiography (MRA) findings were consistent with dissection of the left V4 vertebral artery with multiple brain infarcts in the superior cerebellar artery territory, comprising both MLF tracts. Conclusions: In young adults, bilateral INO is normally caused by demyelinating disease. In other patients, common causes include trauma, infections and autoimmune diseases with neurological symptoms. Vascular disease is implicated in over a third of cases

Some Remarks on Oscillating Inflation

In a recent paper Damour and Mukhanov describe a scenario where inflation may continue during the oscillatory phase. This effect is possible because the scalar field spends a significant fraction of each period of oscillation on the upper part of the potential. Such additional period of inflation could push perturbations after the slow roll regime to observable scales. Although in this work we show that the small region of the Damour-Mukhanov parameter q gives the main contribution to oscillating inflation, it was not satisfactory understood until now. Furthermore, it gives an expression for the energy density spectrum of perturbations, which is well behaved in the whole physical range of q .Comment: 4 pages including figures caption, 3 ps-figures. To appear in Phys. Rev.