Direct composition profiling in III-V nanostructures by cross-sectional STM

Using cross-sectional STM we have studied the local composition in III–V nanostructures such as GaAs/InGaAs quantum wells, InGaNAs/InP quantum wells and quantum dots, and InAs/GaAs self-assembled quantum dots. We are able to determine the local composition by either simply counting the constituent atoms, measuring the local lattice constant or measuring the relaxation of the cleaved surface due to the elastic field of the buried strained nanostructures

Monodromy of Cyclic Coverings of the Projective Line

We show that the image of the pure braid group under the monodromy action on the homology of a cyclic covering of degree d of the projective line is an arithmetic group provided the number of branch points is sufficiently large compared to the degree.Comment: 47 pages (to appear in Inventiones Mathematicae

Magnetic and Mössbauer spectroscopy studies of nanocrystalline iron oxide aerogels

A sol-gel synthesis was used to produce iron oxide aerogels. These nanocrystalline aerogels have a pore-solid structure similar to silica aerogels but are composed entirely of iron oxides. Mössbauer experiments and x-ray diffraction showed that the as-prepared aerogel is an amorphous or poorly crystalline iron oxide, which crystallized as a partially oxidized magnetite during heating in argon. After further heat treatment in air, the nanocrystallites are fully converted to maghemite. The particles are superparamagnetic at high temperatures, but the magnetic properties are strongly influenced by magnetic interactions between the particles at lower temperatures

Cosmological Effects of Radion Oscillations

We show that the redshift of pressureless matter density due to the expansion of the universe generically induces small oscillations in the stabilized radius of extra dimensions (the radion field). The frequency of these oscillations is proportional to the mass of the radion and can have interesting cosmological consequences. For very low radion masses $m_b$ ($m_b\sim10-100 H_0\simeq10^{-32} eV$) these low frequency oscillations lead to oscillations in the expansion rate of the universe. The occurrence of acceleration periods could naturally lead to a resolution of the coincidence problem, without need of dark energy. Even though this scenario for low radion mass is consistent with several observational tests it has difficulty to meet fifth force constraints. If viewed as an effective Brans-Dicke theory it predicts $\omega=-1+\frac{1}{D}$ ($D$ is the number of extra dimensions), while experiments on scales larger than $1mm$ imply $\omega>2500$. By deriving the generalized Newtonian potential corresponding to a massive toroidally compact radion we demonstrate that Newtonian gravity is modified only on scales smaller than $m_b^{-1}$. Thus, these constraints do not apply for $m_b>10^{-3} eV$ (high frequency oscillations) corresponding to scales less than the current experiments ($0.3mm$). Even though these high frequency oscillations can not resolve the coincidence problem they provide a natural mechanism for dark matter generation. This type of dark matter has many similarities with the axion.Comment: Accepted in Phys. Rev. D. Clarifying comments added in the text and some additional references include

Data, ideology, and the developing critical program of social informatics

The rapidly shifting ideological terrain of computing has a profound impact on Social Informatics's critical and empirical analysis of computerization movements. As these movements incorporate many of the past critiques concerning social fit and situational context leveled against them by Social Informatics research, more subtle and more deeply ingrained modes of ideological practice have risen to support movements of computerization. Among these, the current emphasis on the promises of data and data analytics presents the most obvious ideological challenge. In order to reorient Social Informatics in relation to these new ideological challenges, Louis Althusser's theory of ideology is discussed, with its implications for Social Informatics considered. Among these implications, a changed relationship between Social Informatics's critical stance and its reliance on empirical methods is advanced. Addressed at a fundamental level, the practice of Social Informatics comes to be reoriented in a more distinctly reflective and ethical direction

Verificación de la precipitación estimada con información satelital y terrestre en un balance hidrológico

PosterEl modelo de Balance Hidrológico Operativo para el Agro (BHOA, Fernández Long et al., 2012) constituye una herramienta fundamental para la decisión de manejo del sector agrícola, principalmente porque contribuye al monitoreo diario del almacenaje de agua del suelo, representando su variabilidad temporal en forma operativa (Fernández Long et al., 2018). Su desempeño fue evaluado como satisfactorio al compararlo con mediciones de humedad del suelo realizadas in situ en varios trabajos anteriores (Veliz et al., 2014; Veliz et al., 2016; Gattinoni et al., 2017). La baja densidad de estaciones meteorológicas en Argentina es una fuerte limitante en la estimación de modelos que utilizan la precipitación como variable de entrada principal (Fernández Long et al., 2018). Esta problemática propone grandes desafíos, orientados al desarrollo de nuevas y mejores técnicas de interpolación de los datos observados. Adicionalmente, en los últimos años, la creciente disponibilidad de información satelital ha contribuido a mejorar el conocimiento de la distribución espacial de la precipitación. Sin embargo, dicha información debe ser suministrada en forma operativa y debe contar con una validación con información de campo, con el objetivo de conocer los sesgos asociados a la estimación para cada región. En el año 2015, la Universidad de California puso a disposición un producto que combina información satelital y climatología de los datos provenientes de las estaciones meteorológicas, conocido como CHIRP (por sus siglas en inglés de Precipitación Infrarroja del Grupo de Amenazas Climáticas). La misma base de datos, con la posterior corrección con datos observados (CHIRPS), constituye una mejora sobre CHIRP pero con la desventaja de no ser operativa (Funk et al., 2015). El objetivo de este trabajo es evaluar la información resultante del BHOA a partir de información CHIRP y CHIRPS, comparándolas con la serie de datos proveniente del sensor de humedad del suelo instalado en la localidad de Venado Tuerto en la provincia de Santa Fe. Se espera que este análisis contribuya a una mejora en la decisión y elección adecuada de la fuente de datos para la estimación del perfil de agua del suelo.Fil: Bontempi, M.E. Servicio Meteorológico Nacional; ArgentinaFil: Rossi Lopardo, M.S. Servicio Meteorológico Nacional; ArgentinaFil: Fernández Long, M.E. Universidad de Buenos Aires. Facultad de Agronomía. Cátedra de Climatología; ArgentinaFil: Spennemann, P.C. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Servicio Meteorológico Nacional; Argentina Universidad Nacional de Tres de Febrero; ArgentinaFil: Gattinoni, Natalia N. Instituto Nacional de Tecnología Agropecuaria (INTA). Instituto de Clima y Agua; Argentin

Discrete symmetries, invisible axion and lepton number symmetry in an economic 3-3-1 model

We show that Peccei-Quinn and lepton number symmetries can be a natural outcome in a 3-3-1 model with right-handed neutrinos after imposing a Z_11 x Z_2 symmetry. This symmetry is suitably accommodated in this model when we augmented its spectrum by including merely one singlet scalar field. We work out the breaking of the Peccei-Quinn symmetry, yielding the axion, and study the phenomenological consequences. The main result of this work is that the solution to the strong CP problem can be implemented in a natural way, implying an invisible axion phenomenologically unconstrained, free of domain wall formation and constituting a good candidate for the cold dark matter.Comment: 17 pages, Revtex

Magnetothermal Conductivity of Highly Oriented Pyrolytic Graphite in the Quantum Limit

We report on the magnetic field (0T$\le B \le 9$T) dependence of the longitudinal thermal conductivity $\kappa(T,B)$ of highly oriented pyrolytic graphite in the temperature range 5 K $\le T\le$ 20 K for fields parallel to the $c-$axis. We show that $\kappa(T,B)$ shows large oscillations in the high-field region (B > 2 T) where clear signs of the Quantum-Hall effect are observed in the Hall resistance. With the measured longitudinal electrical resistivity we show that the Wiedemann-Franz law is violated in the high-field regime.Comment: 4 Figures, to be published in Physical Review B (2003