Edge Effects in Finite Elongated Graphene Nanoribbons

We analyze the relevance of finite-size effects to the electronic structure of long graphene nanoribbons using a divide and conquer density functional approach. We find that for hydrogen terminated graphene nanoribbons most of the physical features appearing in the density of states of an infinite graphene nanoribbon are recovered at a length of 40 nm. Nevertheless, even for the longest systems considered (72 nm long) pronounced edge effects appear in the vicinity of the Fermi energy. The weight of these edge states scales inversely with the length of the ribbon and they are expected to become negligible only at ribbons lengths of the order of micrometers. Our results indicate that careful consideration of finite-size and edge effects should be applied when designing new nanoelectronic devices based on graphene nanoribbons. These conclusions are expected to hold for other one-dimensional systems such as carbon nanotubes, conducting polymers, and DNA molecules.Comment: 4 pages, 4 figure

Magnetic Exchange Couplings from Noncollinear Spin Density Functional Perturbation Theory

We propose a method for the evaluation of magnetic exchange couplings based on noncollinear spin-density functional calculations. The method employs the second derivative of the total Kohn-Sham energy of a single reference state, in contrast to approximations based on Kohn-Sham total energy differences. The advantage of our approach is twofold: It provides a physically motivated picture of the transition from a low-spin to a high-spin state, and it utilizes a perturbation scheme for the evaluation of magnetic exchange couplings. The latter simplifies the way these parameters are predicted using first-principles: It avoids the non-trivial search for different spin-states that needs to be carried out in energy difference methods and it opens the possibility of "black-boxifying" the extraction of exchange couplings from density functional theory calculations. We present proof of concept calculations of magnetic exchange couplings in the H--He--H model system and in an oxovanadium bimetallic complex where the results can be intuitively rationalized.Comment: J.Chem. Phys. (accepted

Enhanced Half-Metallicity in Edge-Oxidized Zigzag Graphene Nanoribbons

We present a novel comprehensive first-principles theoretical study of the electronic properties and relative stabilities of edge-oxidized zigzag graphene nanoribbons. The oxidation schemes considered include hydroxyl, carboxyl, ether, and ketone groups. Using screened exchange density functional theory, we show that these oxidized ribbons are more stable than hydrogen-terminated nanoribbons except for the case of the etheric groups. The stable oxidized configurations maintain a spin-polarized ground state with antiferromagnetic ordering localized at the edges, similar to the fully hydrogenated counterparts. More important, edge oxidation is found to lower the onset electric field required to induce half-metallic behavior and extend the overall field range at which the systems remain half-metallic. Once the half-metallic state is reached, further increase of the external electric field intensity produces a rapid decrease in the spin magnetization up to a point where the magnetization is quenched completely. Finally, we find that oxygen containing edge groups have a minor effect on the energy difference between the antiferromagnetic ground state and the above-lying ferromagnetic state.Comment: 5 pages,5 figures, 1 tabl

Rehabilitación edificio para la Colombiana de Capitalización

Trabajo de gradoEl proyecto empieza a partir de un mapeo colectivo realizado en el centro histórico de la ciudad de Bogotá, donde encontramos diferentes falencias con respecto a los componentes sociales y de paisaje, en este caso poco espacio público para la sociedad. Como punto de partida se plantea un diseño urbano destinado netamente a la población que constantemente hace uso de espacios no aptos para presentaciones, para demostraciones de talentos y aptitudes, actualmente el centro tiene un movimiento artístico regados, se quiere lograr juntar estos talentos en un solo lugar en donde tengan el aprecio del resto de sociedad y tengan un reconocimiento mayor. El proyecto a trabajar es el EDIFICIO PARA LA COLOMBIANA DE CAPITALIZACIÓN, ubicado en la Carrera 10 # 15 – 11, el edificio actualmente funciona en su primer nivel como comercio y de segundo en adelante como uso de oficinas, la intervención de este es fundamentada con el amarre directo del espacio público propuesto y desde luego su uso, en primer nivel se eliminara la barrera que existía entre lo privado y público, de acuerdo a esto se genera de cierta manera una planta libre en donde se promocionara y se mostrara el arte de la población que caracteriza el centro.PregradoArquitect

Seed Size, Not Dispersal Syndrome, Determines Potential for Spread of Ricefield Weeds by Gulls

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/plants12071470/s1This research was funded by the Spanish Ministry of Science and Innovation (project Refs CGL2016-76067-P and PID2020-112774GB- I00/AEI/10.13039/501100011033). S.R. was supported by a research contract in the framework of the project CGL2016-76067-P (AEI/FEDER, EU) and by the European Social Fund and Junta de Andalucía (Talento Doctores DOC_01221).Recent field data suggest that migratory gulls disperse many rice field weeds by gut passage (endozoochory), most of which are dry fruited and widely assumed to have no long-distance dispersal mechanisms, except via human activity. We investigated this mechanism with a feeding experiment, in which seeds of five common rice field weeds (in order of increasing seed size: Juncus bufonius, Cyperus difformis, Polypogon monspeliensis, Amaranthus retroflexus, and the fleshy-fruited Solanum nigrum) were fed to seven individuals of lesser black-backed gulls Larus fuscus held in captivity. We quantified seed survival after collecting faeces at intervals for 33 h after ingestion, then extracting intact seeds and running germination tests, which were also conducted for control seeds. All five species showed high seed survival after gut passage, of >70%. Gut retention times averaged 2–4 h, but maxima exceeded 23 h for all species. Germinability after gut passage was 16–54%, and gut passage accelerated germination in J. bufonius and S. nigrum, but slowed it down in the other species. All species had lower germinability after gut passage compared to control seeds (likely due to stratification prior to the experiment), but the loss of germinability was higher in smaller seeds. There was no evidence that the different dispersal syndromes assigned to the five species (endozoochory, epizoochory or barochory) had any influence on our results. In contrast, mean gut retention time was strongly and positively related to seed size, likely because small seeds pass more quickly from the gizzard into the intestines. Non-classical endozoochory of dry-fruited seeds by waterbirds is a major but overlooked mechanism for potential long-distance dispersal, and more research into this process is likely essential for effective weed management.Spanish Ministry of Science and Innovation CGL2016-76067-P, PID2020-112774GB- I00/AEI/10.13039/501100011033CGL2016-76067-P (AEI/FEDER, EU)European Social Fund and Junta de Andalucía (Talento Doctores DOC_01221

Evolutionary support vector machines for time series forecasting

Abstract. Time Series Forecasting (TSF) uses past patterns of an event in order to predict its future values and is a key tool to support decision making. In the last decades, Computational Intelligence (CI) techniques, such as Artificial Neural Networks (ANN) and more recently Support Vector Machines (SVM), have been proposed for TSF. The accuracy of the best CI model is affected by both the selection of input time lags and the model’s hyperparameters. In this work, we propose a novel Evolutionary SVM (ESVM) approach for TSF based on the Estimation Distribution Algorithm to search for the best number of inputs and SVM hyperparameters. Several experiments were held, using a set of six time series from distinct real-world domains. Overall, the proposed ESVM is competitive when compared with an Evolutionary ANN (EANN) and the popular ARIMA methodology, while consuming less computational effort when compared with EANN.The research reported here has been supported by FEDER (program COMPETE and FCT) under project FCOMP-01-0124-FEDER-02267