392 research outputs found
Asymmetries in symmetric quantum walks on two-dimensional networks
We study numerically the behavior of continuous-time quantum walks over
networks which are topologically equivalent to square lattices. On short time
scales, when placing the initial excitation at a corner of the network, we
observe a fast, directed transport through the network to the opposite corner.
This transport is not ballistic in nature, but rather produced by quantum
mechanical interference. In the long time limit, certain walks show an
asymmetric limiting probability distribution; this feature depends on the
starting site and, remarkably, on the precise size of the network. The limiting
probability distributions show patterns which are correlated with the initial
condition. This might have consequences for the application of continuous time
quantum walk algorithms.Comment: 9 pages, 12 figures, revtex
Quantum transport on two-dimensional regular graphs
We study the quantum-mechanical transport on two-dimensional graphs by means
of continuous-time quantum walks and analyse the effect of different boundary
conditions (BCs). For periodic BCs in both directions, i.e., for tori, the
problem can be treated in a large measure analytically. Some of these results
carry over to graphs which obey open boundary conditions (OBCs), such as
cylinders or rectangles. Under OBCs the long time transition probabilities
(LPs) also display asymmetries for certain graphs, as a function of their
particular sizes. Interestingly, these effects do not show up in the marginal
distributions, obtained by summing the LPs along one direction.Comment: 22 pages, 11 figure, acceted for publication in J.Phys.
Estudio de pre-factibilidad técnica y económica de la implementación de una biorrefinerÃa para la conversión de residuos de cáscara de naranja
Orange peel is an abundant organic waste in Ecuador that can be used and transformed into products of high added value. Therefore, this work analyzes the technical-economic pre-feasibility of a biorefinery from orange peel to obtain essential oil, pectin and / or bioethanol. First, several scenarios were compared around the combination of products to be obtained, and it was established that the most convenient one would be obtaining essential oil and pectin, with an annual production of 8.7 and 44.4 tons, respectively. Next, a process was designed for this biorefinery that consists of 3 sections: the pretreatment of raw material, extraction of the oil, and the extraction of pectin. Material and energy balances of the process were carried out, and later, the equipment was selected and sized according to specific methodologies. The process considered the recovery and recirculation of ethanol used in the pectin extraction section to reduce production costs. Finally, an economic analysis was carried out based on the theoretical cost estimates and a catalog cost analysis. It was found that the project is profitable, and that the payback time for the investment would be between 5 and 6 years. Therefore, the implementation of the biorefinery would generate a positive economic, environmental and social impact in the country.La cáscara de naranja es un residuo orgánico abundante en el Ecuador que puede ser aprovechado y transformado en productos de alto valor agregado. Por ello, el presente artÃculo analiza la pre-factibilidad técnico-económica de una biorrefinerÃa a partir de cáscara de naranja para la obtención de aceite esencial, pectina y/o bioetanol. Primero, se compararon varios escenarios alrededor de la combinación de productos a ser obtenidos, y se estableció que el más conveniente serÃa la obtención de aceite esencial y pectina, con una producción anual de 8,7 y 44,4 toneladas, respectivamente. A continuación, se diseñó un proceso para esta biorrefinerÃa que consiste en 3 secciones: el pretratamiento de la materia prima, la extracción del aceite, y la extracción de la pectina. Se realizaron los balances de materia y energÃa del proceso, y posteriormente, se seleccionaron y dimensionaron los equipos de acuerdo a metodologÃas especÃficas. El proceso consideró la recuperación y recirculación de etanol empleado en la sección de extracción de pectina para reducir los costos de producción. Finalmente, se realizó un análisis económico a partir de las estimaciones de costos teóricos y un análisis de costos de catálogo. Se encontró que el proyecto es rentable y que el tiempo de recuperación de la inversión estarÃa entre 5 y 6 años. Por lo tanto, la implementación de la biorrefinerÃa generarÃa un impacto positivo a nivel económico, ambiental y social en el paÃs
Remarks on singular Cayley graphs and vanishing elements of simple groups
Let Γ be a finite graph and let A(Γ) be its adjacency matrix. Then Γ is singular if A(Γ) is singular. The singularity of graphs is of certain interest in graph theory and algebraic combinatorics. Here we investigate this problem for Cayley graphs Cay(G,H) when G is a finite group and when the connecting set H is a union of conjugacy classes of G. In this situation, the singularity problem reduces to finding an irreducible character χ of G for which ∑h∈Hχ(h)=0. At this stage, we focus on the case when H is a single conjugacy class hG of G; in this case, the above equality is equivalent to χ(h)=0 . Much is known in this situation, with essential information coming from the block theory of representations of finite groups. An element h∈G is called vanishing if χ(h)=0 for some irreducible character χ of G. We study vanishing elements mainly in finite simple groups and in alternating groups in particular. We suggest some approaches for constructing singular Cayley graphs
Internationalisation and development in East Asian higher education: an introduction
It is important to recognise that comparison is not a method or even an academic technique; rather, it is a discursive strategy… Good comparisons often come from the experience of strangeness and absences. (Benedict Anderson, 2016
Exploring the solid state and solution structural chemistry of the utility amide potassium hexamethyldisilazide (KHMDS)
The structural chemistry of eleven donor complexes of the important Brønsted base potassium 1,1,1,3,3,3-hexamethyldisilazide (KHMDS) has been studied. Depending on the donor, each complex adopted one of four general structural motifs. Specifically, in this study the donors employed were toluene (to give polymeric 1 and dimeric 2), THF (dimeric 3), N,N,N',N'-tetramethylethylenediamine (TMEDA) (dimeric 4), (R,R)-N,N,N',N'-tetramethyl-1,2-diaminocyclohexane [(R,R)-TMCDA] (dimeric 5), 12-crown-4 (dimeric 6), N,N,N',N'-tetramethyldiaminoethyl ether (TMDAE) (tetranuclear dimeric 8 and monomeric 10), N,N,N',N',N''-pentamethyldiethylentriamine (PMDETA) (tetranuclear dimeric 7), tris[2-dimethyl(amino)ethyl]amine (Me6TREN) (tetranuclear dimeric 9) and tris{2-(2-methoxyethoxy)ethyl}amine (TMEEA) (monomeric 11). The complexes were also studied in solution by 1H and 13C NMR spectroscopy as well as DOSY NMR spectroscopy
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