5,919 research outputs found
Single-file diffusion on self-similar substrates
We study the single file diffusion problem on a one-dimensional lattice with
a self-similar distribution of hopping rates. We find that the time dependence
of the mean-square displacement of both a tagged particle and the center of
mass of the system present anomalous power laws modulated by logarithmic
periodic oscillations. The anomalous exponent of a tagged particle is one half
of the exponent of the center of mass, and always smaller than 1/4. Using
heuristic arguments, the exponents and the periods of oscillation are
analytically obtained and confirmed by Monte Carlo simulations.Comment: 12 pages, 6 figure
Impact of edge shape on the functionalities of graphene-based single-molecule electronics devices
We present an ab-initio analysis of the impact of edge shape and
graphene-molecule anchor coupling on the electronic and transport
functionalities of graphene-based molecular electronics devices. We analyze how
Fano-like resonances, spin filtering and negative differential resistance
effects may or may not arise by modifying suitably the edge shapes and the
terminating groups of simple organic molecules. We show that the spin filtering
effect is a consequence of the magnetic behavior of zigzag-terminated edges,
which is enhanced by furnishing these with a wedge shape. The negative
differential resistance effect is originated by the presence of two degenerate
electronic states localized at each of the atoms coupling the molecule to
graphene which are strongly affected by a bias voltage. The effect could thus
be tailored by a suitable choice of the molecule and contact atoms if edge
shape could be controlled with atomic precision.Comment: 11 pages, 20 figure
Asymptotic dimension and geometric decompositions in dimensions 3 and 4
We show that the fundamental groups of smooth -manifolds that admit
geometric decompositions in the sense of Thurston have asymptotic dimension at
most four, and equal to 4 when aspherical. We also show that closed
-manifold groups have asymptotic dimension at most 3. Our proof method
yields that the asymptotic dimension of closed -dimensional Alexandrov
spaces is at most 3. We thus obtain that the Novikov conjecture holds for
closed -manifolds with such a geometric decomposition and closed
-dimensional Alexandrov spaces. Consequences of these results include a
vanishing result for the Yamabe invariant of certain -surgered geometric
-manifolds and the existence of zero in the spectrum of aspherical smooth
-manifolds with a geometric decomposition.Comment: 22 pages, 2 image
CCS and NH_3 Emission Associated with Low-Mass Young Stellar Objects
In this work we present a sensitive and systematic single-dish survey of CCS emission (complemented with ammonia observations) at 1 cm, toward a sample of low- and intermediate-mass young star-forming regions known to harbor water maser emission, made with NASA's 70 m antenna at Robledo de Chavela, Spain. Out of the 40 star-forming regions surveyed in the CCS (2_(1)-1_(0)) line, only six low-mass sources show CCS emission: one transitional object between the prestellar and protostellar Class 0 phase (GF9-2), three Class 0 protostars (L1448-IRS3, L1448C, and B1-IRS), a Class I source (L1251A), and a young T Tauri star (NGC 2071 North). Since CCS is considered an "early-time" (≲10^5 yr) molecule, we explain these results by either proposing a revision of the classification of the age of NGC 2071 North and L1251A, or suggesting the possibility that the particular physical conditions and processes of each source affect the destruction/production of the CCS. No statistically significant relationship was found between the presence of CCS and parameters of the molecular outflows and their driving sources. Nevertheless, we found a significant relationship between the detectability of CCS and the ammonia peak intensity (higher in regions with CCS), but not with its integrated intensity. This tendency may suggest that the narrower ammonia line widths in the less turbulent medium associated with younger cores may compensate for the differences in ammonia peak intensity, rendering differences in integrated intensity negligible. From the CCS detection rate we derive a lifetime of this molecule of ≃(0.7-3) × 10^4 yr in low-mass star-forming regions
Diseño conceptual de una planta de producción de ácido peracético a partir de peróxido de hidrógeno y ácido acético
El desarrollo de procesos para obtener desinfectantes se ha convertido en un tema de investigación frecuente, con el fin de disminuir su impacto ambiental y asegurar la calidad de los procesos -- Este proyecto se basa en el análisis de alternativas, equipos y metodologías para diseñar conceptualmente un proceso que permita obtener ácido peracético, a partir de peróxido de hidrógeno y ácido acético, ya que es un compuesto que ha venido ganando terreno en el mercado mundial de desinfectantes por sus buenas propiedades y su biodegradabilidad -- En este proyecto se tienen en consideración diferentes variables, las cuales permiten determinar las mejores condiciones para la producción de ácido peracético a escala comercial -- Para encontrar la relevancia de variables como la temperatura, el tiempo de reacción, la relación molar de reactivos y el porcentaje de catalizador (ácido sulfúrico), se utiliza un diseño de experimentos factorial, midiendo el porcentaje de producto obtenido -- La conversión obtenida experimentalmente es de 63% al cabo de 120 minutos de reacción con 3% de catalizador -- Una vez determinadas las condiciones adecuadas, se procede a diseñar el proceso -- Inicialmente se definen tiempos de cada etapa y se realiza una simulación utilizando el software Aspen Batch Plus® con el fin de analizar el comportamiento teórico del proceso -- Luego de analizar los resultados se procede con la selección y el diseño de los equipos adecuados para obtener el ácido peracético al 15%, concentración utilizada comercialmente -- Finalmente se realiza una evaluación económica del proyecto para determinar la factibilidad de desarrollarlo en Colombia -- El resultado es una rentabilidad atractiva del proyecto de la manera como se diseñ
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