Aprendizaje implicito y explícito: ¿dos procesos diferentes o dos niveles de abstracción?

En los estudios centrados en analizar las diferencias entre el aprendizaje implícito y el explícito es frecuente observar definiciones que relacionan estas dos formas de aprendizaje con dos procesos diferentes: procesamiento consciente y no consciente de la inforrnación. En este artículo abordamos la distinción explícito versus implícito desde la perspectiva de distintos niveles de abstracción de reglas. Consideramos que tanto la evidencia experimental como las aportaciones de los modelos conexionistas nos pemiten pensar en la distinción implicito versus explícito relacionada con las diferencias entre representaciones especíjicas basadas en covariaciones o ejemplares y representaciones basadas en reglas más abstractas. Los resultados de algunos experimentos usí como el análisis de sinzulaciones mediante redes neuronales han posibilitado la identifcaciórz de algunas características asociadas a estos niveles de abstracciónRegarding the differences between implicit and explicit leaming reported in the literature, the definitions frequently relate these two kinds of learning with two different processes: conscious and unconscious information processing. In this paper we focus on the distinction between explicit and implicitfrom the point of view of different levels of rule abstraction. We think that evidence from experiments and connectionist models pemits us to relate the explicit-implicit distinction to the differences between specific representations based on covariations or exemplars and those based on more abstract rules. Both experimental results and connectionist simulations have made it possible to identify certain features associated with these levels of abstractio

First-principles transport calculation method based on real-space finite-difference nonequilibrium Green's function scheme

We demonstrate an efficient nonequilibrium Green's function transport calculation procedure based on the real-space finite-difference method. The direct inversion of matrices for obtaining the self-energy terms of electrodes is computationally demanding in the real-space method because the matrix dimension corresponds to the number of grid points in the unit cell of electrodes, which is much larger than that of sites in the tight-binding approach. The procedure using the ratio matrices of the overbridging boundary-matching technique [Phys. Rev. B {\bf 67}, 195315 (2003)], which is related to the wave functions of a couple of grid planes in the matching regions, greatly reduces the computational effort to calculate self-energy terms without losing mathematical strictness. In addition, the present procedure saves computational time to obtain Green's function of the semi-infinite system required in the Landauer-B\"uttiker formula. Moreover, the compact expression to relate Green's functions and scattering wave functions, which provide a real-space picture of the scattering process, is introduced. An example of the calculated results is given for the transport property of the BN ring connected to (9,0) carbon nanotubes. The wave function matching at the interface reveals that the rotational symmetry of wave functions with respect to the tube axis plays an important role in electron transport. Since the states coming from and going to electrodes show threefold rotational symmetry, the states in the vicinity of the Fermi level, whose wave function exhibits fivefold symmetry, do not contribute to the electron transport through the BN ring.Comment: 34 page

Elasto-thermoelectric beam formulation for modeling thermoelectric devices

[EN] The present paper provides a dynamic, non-linear and fully coupled Finite Element (FE) formulation based on the Timoshenko beam theory to study elasto-thermoelectric responses in thermoelectric devices. The two main motivations of this work are: i) to study mechanical responses in thermoelectric devices, which must be taken into account in the design of Peltier cells due to the fragility and relative low strength of the semiconductors, and ii) to provide a numerical tool that decreases the CPU time to allow the introduction of designs based on optimization processes and on sensitivity analyses that could require many evaluations. In order to undertake the objectives of this work, the general three-dimensional governing equations are reduced to one-dimensional ones by means of several assumptions. Then, a set of five multi-coupled partial differential equations is obtained. The resultant expressions are thermodynamically consistent and form a multi-coupled monolithic FE formulation, differently to stagger formulations that require two separated steps to reach the final result. Numerically, this set of multi-coupled equations is discretized using the FE method and implemented into FEAP Taylor, 2010 [1]. For a proper validation of the code, four benchmarks are performed using one- dimensional dynamic analytical solutions developed by the authors. Finally, this formulation is compared with a three-dimensional FE formulation also developed by the authors in Pe ́rez-Aparicio et al., 2015 [2] to model a commercial Peltier cell. This comparison reveals that: i) relative errors are lower than 13% and ii) CPU times decrease significantly, more than one order of magnitude. In conclusion, the beam thermoelectric formulation is an accurate model that reduces CPU time and could be used in future design of thermoelectric devices.Palma, R.; Moliner, E.; Pérez-Aparicio, JL. (2017). Elasto-thermoelectric beam formulation for modeling thermoelectric devices. Finite Elements in Analysis and Design. 129:32-41. doi:10.1016/j.finel.2017.02.001S324112

Magnetization Process of the Classical Heisenberg Model on the Shastry-Sutherland Lattice

We investigate classical Heisenberg spins on the Shastry-Sutherland lattice and under an external magnetic field. A detailed study is carried out both analytically and numerically by means of classical Monte-Carlo simulations. Magnetization pseudo-plateaux are observed around 1/3 of the saturation magnetization for a range of values of the magnetic couplings. We show that the existence of the pseudo-plateau is due to an entropic selection of a particular collinear state. A phase diagram that shows the domains of existence of those pseudo-plateaux in the $(h, T)$ plane is obtained.Comment: 9 pages, 11 figure

Hot electron transport in Ballistic Electron Emission Spectroscopy: band structure effects and k-space currents

Using a Green's function approach, we investigate band structure effects in the BEEM current distribution in reciprocal space. In the elastic limit, this formalism provides a 'parameter free' solution to the BEEM problem. At low temperatures, and for thin metallic layers, the elastic approximation is enough to explain the experimental I(V) curves at low voltages. At higher voltages inelastic effects are approximately taken into account by introducing an effective RPA-electron lifetime, much in similarity with LEED theory. For thick films, however, additional damping mechanisms are required to obtain agreement with experiment.Comment: 4 pages, 3 postscript figures, revte

Alcorisa (Teruel) como ejemplo de implicación de una administración local en la divulgación geológica

Las administraciones locales son las más numerosas y también las más próximas a la sociedad de todas las administraciones públicas, por lo que pueden optimizar el capital humano y el material disponible para divulgar y proteger los recursos geológicos de su entorno mediante la realización de actividades de bajo coste pero efectivas, con el asesoramiento de la comunidad científica. Alcorisa es un pequeño pueblo de la provincia de Teruel incluido en el Geoparque del Maestrazgo, que constituye un buen ejemplo de ayuntamiento implicado en la divulgación de los recursos geológicos de su entorno de forma continuada mediante varias iniciativas. Destacan la construcción de un jardín de rocas al que han llamado “Geólogo Juan Paricio”, la organización de las Jornadas de divulgación y defensa del patrimonio geológico turolense que se celebran anualmente, la instalación de un mirador geológico en la discordancia sintectónica, la ruta geológica circular de la “Cuesta del Atajo”, la señalización de cinco puntos de interés geológico local mediante códigos QR, el apoyo al proyecto de recuperación de minería histórica que se inicia con la cantera de arenisca de “Valfonda” y la promoción de paseos geológicos urbanos. Local administrations are the most numerous and also the closest to society of all public administrations, so they can optimize the human and material capital available to disseminate and to protect the geological resources of their surroundings by carrying out low-cost but effective activities, with the advice of the scientific community. Alcorisa is a small town in the province of Teruel included in the Maestrazgo Geopark. It is a good example of a city council involved in the continuous dissemination of the geological resources of its environment throughout several initiatives. It is worth mentioning the construction of the rock garden under the name of Geologist Juan Paricio, the Conference for the Diffusion and Defense of the Geological Heritage of Teruel that is held annually, the observation point of the Synthectonic Unconformity, the circular geological path of the Cuesta del Atajo, the marking of five local geological point of interest with their QR codes, the historic mining recovery project that begins with the Valfonda sandstone quarry and urban geological walks

Magnetic-field effects on transport in carbon nanotube junctions

Here we address a theoretical study on the behaviour of electronic states of heterojunctions and quantum dots based on carbon nanotubes under magnetic fields. Emphasis is put on the analysis of the local density of states, the conductance, and on the characteristic curves of current versus voltage. The heterostructures are modeled by joining zigzag tubes through single pentagon-heptagon pair defects, and described within a simple tight binding calculation. The conductance is calculated using the Landauer formula in the Green functions formalism. The used theoretical approach incorporates the atomic details of the topological defects by performing an energy relaxation via Monte Carlo calculation. The effect of a magnetic field on the conductance gap of the system is investigated and compared to those of isolated constituent tubes. It is found that the conductance gap of the studied CNHs exhibits oscillations as a function of the magnetic flux. However, unlike the pristine tubes case, they are not Aharonov-Bohm periodic oscillations

Simulation Study for Designing a Dedicated Cardiac TOF-PET System

[EN] The development of dedicated positron emission tomography scanners is an active area of research, especially aiming at the improvement of lesion detection and in support of cancer treatment and management. Recently, dedicated Positron Emission Tomography (PET) systems with different configurations for specific organs have been developed for improving detection effectiveness. Open geometries are always subject to distortion and artifacts in the reconstructed images. Therefore, the aim of this work is to determine the optimal geometry for a novel cardiac PET system that will be developed by our team, and determine the time resolution needed to achieve reasonable image quality for the chosen geometry. The proposed geometries consist of 36 modules. These modules are arranged in two sets of two plates, each one with different configurations. We performed Monte Carlo simulations with different TOF resolutions, in order to test the image quality improvement in each case. Our results show, as expected, that increasing TOF resolution reduces distortion and artifact effects. We can conclude that a TOF resolution of the order of 200 ps is needed to reduce the artifacts, to acceptable levels, generated in the simulated cardiac-PET open geometries.This project has been co-financed by the Spanish Government Grants TEC2016-79884-C2 and RTC-2016-5186-1, by the European Union through the European Regional Development Fund (ERDF) and by the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (Grant Agreement No. 695536). The work of V.I. was supported by the Generalitat Valenciana APOSTD/2019/086 fellowship.Oliver-Gil, S.; Moliner, L.; Ilisie, V.; Benlloch Baviera, JM.; Rodríguez-Álvarez, M. (2020). Simulation Study for Designing a Dedicated Cardiac TOF-PET System. Sensors. 20(5):1-16. https://doi.org/10.3390/s20051311S116205Gaemperli, O., & Kaufmann, P. A. (2011). PET and PET/CT in cardiovascular disease. 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Quantification of Myocardial Blood Flow: What is the Clinical Role? Cardiology Clinics, 27(2), 277-289. doi:10.1016/j.ccl.2008.12.009Knuuti, J., Kajander, S., Mäki, M., & Ukkonen, H. (2009). Quantification of myocardial blood flow will reform the detection of CAD. Journal of Nuclear Cardiology, 16(4), 497-506. doi:10.1007/s12350-009-9101-1Peng, H. (2015). Design study of a cardiac-dedicated PET system. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 779, 39-46. doi:10.1016/j.nima.2015.01.042Gonzalez, A. J., Sanchez, F., & Benlloch, J. M. (2018). Organ-Dedicated Molecular Imaging Systems. IEEE Transactions on Radiation and Plasma Medical Sciences, 2(5), 388-403. doi:10.1109/trpms.2018.2846745Moliner, L., Rodríguez-Alvarez, M. J., Catret, J. V., González, A., Ilisie, V., & Benlloch, J. M. (2019). NEMA Performance Evaluation of CareMiBrain dedicated brain PET and Comparison with the whole-body and dedicated brain PET systems. Scientific Reports, 9(1). doi:10.1038/s41598-019-51898-zAhmed, A. M., Tashima, H., Yoshida, E., Nishikido, F., & Yamaya, T. (2017). Simulation study comparing the helmet-chin PET with a cylindrical PET of the same number of detectors. Physics in Medicine and Biology, 62(11), 4541-4550. doi:10.1088/1361-6560/aa685cCho, Z.-H., Son, Y.-D., Kim, H.-K., Kwon, D.-H., Joo, Y.-H., Ra, J. B., … Kim, Y.-B. (2019). Development of Positron Emission Tomography With Wobbling and Zooming for High Sensitivity and High-Resolution Molecular Imaging. IEEE Transactions on Medical Imaging, 38(12), 2875-2882. doi:10.1109/tmi.2019.2916326Surti, S., & Karp, J. S. (2008). Design considerations for a limited angle, dedicated breast, TOF PET scanner. Physics in Medicine and Biology, 53(11), 2911-2921. doi:10.1088/0031-9155/53/11/010Surti, S., & Karp, J. S. (2016). Advances in time-of-flight PET. Physica Medica, 32(1), 12-22. doi:10.1016/j.ejmp.2015.12.007Grant, A. M., Deller, T. W., Khalighi, M. M., Maramraju, S. H., Delso, G., & Levin, C. S. (2016). NEMA NU 2-2012 performance studies for the SiPM-based ToF-PET component of the GE SIGNA PET/MR system. Medical Physics, 43(5), 2334-2343. doi:10.1118/1.4945416Van Sluis, J., de Jong, J., Schaar, J., Noordzij, W., van Snick, P., Dierckx, R., … Boellaard, R. (2019). Performance Characteristics of the Digital Biograph Vision PET/CT System. Journal of Nuclear Medicine, 60(7), 1031-1036. doi:10.2967/jnumed.118.215418Ito, M., Lee, M. S., & Lee, J. S. (2013). Continuous depth-of-interaction measurement in a single-layer pixelated crystal array using a single-ended readout. Physics in Medicine and Biology, 58(5), 1269-1282. doi:10.1088/0031-9155/58/5/1269Bugalho, R., Di Francesco, A., Ferramacho, L., Leong, C., Niknejad, T., Oliveira, L., … Varela, J. (2018). Experimental results with TOFPET2 ASIC for time-of-flight applications. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 912, 195-198. doi:10.1016/j.nima.2017.11.034Gundacker, S., Auffray, E., Frisch, B., Jarron, P., Knapitsch, A., Meyer, T., … Lecoq, P. (2013). Time of flight positron emission tomography towards 100ps resolution with L(Y)SO: an experimental and theoretical analysis. Journal of Instrumentation, 8(07), P07014-P07014. doi:10.1088/1748-0221/8/07/p07014A Code System for Monte Carlo Simulation of Electron and Photon Transporthttp://www.oecd-nea.org/lists/penelope.htmlStrydhorst, J., & Buvat, I. (2016). Redesign of the GATE PET coincidence sorter. Physics in Medicine and Biology, 61(18), N522-N531. doi:10.1088/0031-9155/61/18/n522Baró, J., Sempau, J., Fernández-Varea, J. M., & Salvat, F. (1995). PENELOPE: An algorithm for Monte Carlo simulation of the penetration and energy loss of electrons and positrons in matter. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 100(1), 31-46. doi:10.1016/0168-583x(95)00349-5Sempau, J., Acosta, E., Baro, J., Fernández-Varea, J. M., & Salvat, F. (1997). An algorithm for Monte Carlo simulation of coupled electron-photon transport. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 132(3), 377-390. doi:10.1016/s0168-583x(97)00414-xSempau, J., Fernández-Varea, J. M., Acosta, E., & Salvat, F. (2003). Experimental benchmarks of the Monte Carlo code penelope. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 207(2), 107-123. doi:10.1016/s0168-583x(03)00453-1Reader, A. J., Ally, S., Bakatselos, F., Manavaki, R., Walledge, R. J., Jeavons, A. P., … Zweit, J. (2002). One-pass list-mode EM algorithm for high-resolution 3-D PET image reconstruction into large arrays. IEEE Transactions on Nuclear Science, 49(3), 693-699. doi:10.1109/tns.2002.1039550Spanoudaki, V. C., & Levin, C. S. (2010). Photo-Detectors for Time of Flight Positron Emission Tomography (ToF-PET). Sensors, 10(11), 10484-10505. doi:10.3390/s101110484Siddon, R. L. (1985). Fast calculation of the exact radiological path for a three-dimensional CT array. Medical Physics, 12(2), 252-255. doi:10.1118/1.595715Vandenberghe, S., Daube-Witherspoon, M. E., Lewitt, R. M., & Karp, J. S. (2006). Fast reconstruction of 3D time-of-flight PET data by axial rebinning and transverse mashing. Physics in Medicine and Biology, 51(6), 1603-1621. doi:10.1088/0031-9155/51/6/017Performance Measurements of Positron Emission Tomographshttps://www.nema.org/Standards/ComplimentaryDocuments/Contents%20and%20Scope%20NEMA%20NU%202%202012.pdfYu, W., & Zeng, L. (2014). A Novel Weighted Total Difference Based Image Reconstruction Algorithm for Few-View Computed Tomography. PLoS ONE, 9(10), e109345. doi:10.1371/journal.pone.0109345Tashima, H., Yamaya, T., Yoshida, E., Kinouchi, S., Watanabe, M., & Tanaka, E. (2012). A single-ring OpenPET enabling PET imaging during radiotherapy. Physics in Medicine and Biology, 57(14), 4705-4718. doi:10.1088/0031-9155/57/14/4705Yamaya, T., Inaniwa, T., Minohara, S., Yoshida, E., Inadama, N., Nishikido, F., … Murayama, H. (2008). A proposal of an open PET geometry. Physics in Medicine and Biology, 53(3), 757-773. doi:10.1088/0031-9155/53/3/015Miyake, K. K., Matsumoto, K., Inoue, M., Nakamoto, Y., Kanao, S., Oishi, T., … Togashi, K. (2014). Performance Evaluation of a New Dedicated Breast PET Scanner Using NEMA NU4-2008 Standards. Journal of Nuclear Medicine, 55(7), 1198-1203. doi:10.2967/jnumed.113.131565Yamamoto, S., Honda, M., Oohashi, T., Shimizu, K., & Senda, M. (2011). Development of a Brain PET System, PET-Hat: A Wearable PET System for Brain Research. IEEE Transactions on Nuclear Science, 58(3), 668-673. doi:10.1109/tns.2011.2105502Garibaldi, F., Capuani, S., Colilli, S., Cosentino, L., Cusanno, F., Leo, R. D., … Tamma, C. (2013). TOPEM: A PET-TOF endorectal probe, compatible with MRI for diagnosis and follow up of prostate cancer. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 702, 13-15. doi:10.1016/j.nima.2012.09.020González-Montoro, A., Sánchez, F., Martí, R., Hernández, L., Aguilar, A., Barberá, J., … González, A. J. (2018). Detector block performance based on a monolithic LYSO crystal using a novel signal multiplexing method. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 912, 372-377. doi:10.1016/j.nima.2017.10.09