One-dimensional moir\'e superlattices and flat bands in collapsed chiral carbon nanotubes

We demonstrate that one-dimensional moir\'e patterns, analogous to those found in twisted bilayer graphene, can arise in collapsed chiral carbon nanotubes. Resorting to a combination of approaches, namely, molecular dynamics to obtain the relaxed geometries and tight-binding calculations validated against ab initio modeling, we find that magic angle physics occur in collapsed carbon nanotubes. Velocity reduction, flat bands and localization in AA regions with diminishing moir\'e angle are revealed, showing a magic angle close to 1$^{\rm o}$. From the spatial extension of the AA regions and the width of the flat bands, we estimate that many-body interactions in these systems are stronger than in twisted bilayer graphene. Chiral collapsed carbon nanotubes stand out as promising candidates to explore many-body effects and superconductivity in low dimensions, emerging as the one-dimensional analogues of twisted bilayer graphene

Universality of moir\'e physics in collapsed chiral carbon nanotubes

We report the existence of moir\'e patterns and magic angle physics in all families of chiral collapsed carbon nanotubes. A detailed study of the electronic structure of all types of chiral nanotubes, previously collapsed via molecular dynamics, has been performed. We find that each family possesses a unique geometry and moir\'e disposition, as well as a characteristic number of flat bands. Remarkably, all kinds of nanotubes behave the same with respect to magic angle tuning, showing a monotonic behavior that gives rise to magic angles in full agreement with those of twisted bilayer graphene. Therefore, magic angle behavior is universally found in chiral collapsed nanotubes with a small chiral angle, giving rise to moir\'e patterns. Our approach comprises first-principles and semi-empirical calculations of the band structure, density of states and spatial distribution of the localized states signaled by flat bands

Provenance of the Variscan Upper Allochthon (Cabo Ortegal complex, NW Iberian Massif)

The NW Iberian Massif shows very good exposures and complete sections of the Variscan orogeny of Western Europe. Located above ophiolitic units which outline the Variscan suture, the Upper Allochthon (Upper Units) of the NW Iberian allochthonous complexes is composed of Early–Middle Cambrian magmatic arc related rocks, with siliciclastic and pelitic sedimentary series connected with basin development in this arc context. This Upper Allochthon can be followed in the same structural position along the Variscan belt, from the Iberian to the Bohemian Massifs. This study focusses on the provenance of the Cariño gneiss formation, a gneissic metasiliciclastic unit located in the uppermost structural position of the allochthonous Cabo Ortegal complex. U-Pb and Lu-Hf zircon (LA-ICPMS) and Sm-Nd whole rock analyses have been performed to document the provenance of this unit and so constrain the paleogeographic evolution of the Upper Allochthon. U-Pb geochronology of 6 samples indicates a Middle Cambrian maximum depositional age, with the following zircon age populations: Paleozoic–Neoproterozoic 36 %, Mesoproterozoic 3.6 %, Paleoproterozoic 46.8 % and Archean 13.6 %. Lu-Hf isotope analyses of these zircons typify Pan-African (Cadomian), Eburnean and Archean orogenic pulses in their respective source areas. Pan-African and Eburnean events entailed abundant input of juvenile material involving broad mixing with older crustal sources. Mesoproterozoic activity is scarce and scattered and so unlikely to represent a major crust generation pulse in the source area of the siliciclastic unit. Sm-Nd whole-rock analyses on 10 samples providean average depleted mantle model age of 1.73 Ga. This information is compatible with provenance from a source area with Archean crust that registered an important Eburnean orogenic cycle followed by a Pan-African–Cadomian event. Nd TDM ages suggest almost direct derivation from these sources. These data are interpreted as indicative of West Africa craton provenance. The siliciclastic series from which the Cariño gneisses were derived were probably deposited in a back-arc type basin where detritus were mostly sourced from the stable continental area instead of the magmatic arc. Pre-existing U-Pb and Sm-Nd data from another Upper Allochthon unit from NW Iberia (Órdenes complex) points to strong isotopic heterogeneity within this terrane, which is indicative of source area variability. This observation is interpreted to reflect either a temporal evolution of the arc or the possibility that the terrane represented by the Upper Units is composed of more than one domain of the arc-related system.Peer Reviewe

Two-stage collision: Exploring the birth of Pangea in the Variscan terranes.

The Variscan suture exposed in NW Iberia contains a stack of terranes including two allochthonous units with continental affinity and Gondwanan provenance (Upper and Basal Units), separated by an ophiolite belt where the most common units show protolith ages at c. 395 Ma. Recent Lu–Hf zircon data obtained from these ophiolites indicate interaction between the gabbroic magmas and old continental crust. Hence, the ophiolites could not have originated in a deep ocean basin associated with a mature mid-ocean-ridge or intraoceanic subduction. The tectonothermal evolution of the continental terranes bounding the suture zone records two consecutive events of deep subduction. The Upper Units record an initial high-P/ultra-high-P metamorphic event that occurred before 400–390 Ma, while the Basal Units were affected by a second high-P/low-to-intermediate-T metamorphic event dated at c. 370 Ma. Continental subduction affected the most external margin of Gondwana and developed in a setting of dextral convergencewith Laurussia. Development of the two high-P events alternated with the opening of an ephemeral oceanic basin, probably of pull-apart type, in Early Devonian times. This ephemeral oceanic domain is suggested as the setting for the protoliths of the most common ophiolites involved in the Variscan suture. Current ideas for the assembly of Pangea advocate a single collisional event between Gondwana and Laurussia in the Carboniferous. However, the new evidence from the allochthonous terranes of the Variscan belt suggests a more complex scenario for the assembly of the supercontinent, with an interaction between the colliding continental margins that started earlier and lasted longer than previously considered. Based onmodern analogs of continental interaction, the development of complex collisions, as here suggested for Gondwana and Laurussia during the assembly of Pangea, could have been the norm rather than the exception throughout Earth history

A pre-Rodinian ophiolite involved in the Variscan suture of Galicia (Cabo Ortegal Complex, NW Spain)

U–Pb dating of zircons from a metagabbro of the Purrido amphibolitic unit (Cabo Ortegal Complex, NW Iberian Massif) yielded an age of 1159 ± 39 Ma, interpreted to approximate the crystallization age of the gabbroic protolith. Considering the arc affinity of the metagabbroic rocks, the unit is interpreted as a pre-Rodinian ophiolite developed in a back-arc setting. It is suggested that the ophiolite was obducted over the West African terranes during the assembly of Rodinia. There, this terrane remained tectonically stable and facing an ocean for a long time, and eventually became part of the Gondwanan margin. The ophiolite was finally involved in the Variscan suture of Galicia where it is sandwiched between Palaeozoic rocks. The Purrido unit is so far the only example of a Mesoproterozoic ophiolite in the European Variscan belt, where pre-Neoproterozoic rocks are very scarce and restricted to small exposures

A U-Pb Study of Zircons from a Lower Crustal Granulite Xenolith of the Spanish Central System: A Record of Iberian Lithospheric Evolution from the Neoproterozoic to the Triassic

A U-Pb laser ablation inductively coupled plasma mass spectrometry and secondary ion mass spectrometry geochronological study has been performed on zircons from a felsic granulite xenolith from the lower crust under the Variscan belt of Iberia. The ages obtained reveal zircon-forming events that span from the late Neoproterozoic (Cadomian-Avalonian orogeny) to the early stages of the opening of the Atlantic Ocean in the Mesozoic, through Cambro-Ordovician rifting, Devono-Carboniferous Variscan-Alleghenian collision, and Permian-Triassic extension and uplift. The U-Pb age groups found in zircons from this single lower crustal xenolith (ca. 220, 250, 280–310, 460–490, 525, and 550–490 Ma) record at least one complete cycle of closing and opening of oceanic basins and collision of continental masses. Zircons from the felsic granulite xenolith contain a synthesis of the geochronological information found on the surface geology and record most but not all major lithospheric events in the region in a ca. 400-m.yr. period

Form Factor Improvement of Smart-Pixels for Vision Sensors through 3-D Vertically- Integrated Technologies

While conventional CMOS active pixel sensors embed only the circuitry required for photo-detection, pixel addressing and voltage buffering, smart pixels incorporate also circuitry for data processing, data storage and control of data interchange. This additional circuitry enables data processing be realized concurrently with the acquisition of images which is instrumental to reduce the number of data needed to carry to information contained into images. This way, more efficient vision systems can be built at the cost of larger pixel pitch. Vertically-integrated 3D technologies enable to keep the advnatges of smart pixels while improving the form factor of smart pixels.Office of Naval Research N000141110312Ministerio de Ciencia e Innovación IPT-2011-1625-43000

Smart imaging for power-efficient extraction of Viola-Jones local descriptors

In computer vision, local descriptors permit to summarize relevant visual cues through feature vectors. These vectors constitute inputs for trained classifiers which in turn enable different high-level vision tasks. While local descriptors certainly alleviate the computation load of subsequent processing stages by preventing them from handling raw images, they still have to deal with individual pixels. Feature vector extraction can thus become a major limitation for conventional embedded vision hardware. In this paper, we present a power-efficient sensing processing array conceived to provide the computation of integral images at different scales. These images are intermediate representations that speed up feature extraction. In particular, the mixed-signal array operation is tailored for extraction of Haar-like features. These features feed the cascade of classifiers at the core of the Viola-Jones framework. The processing lattice has been designed for the standard UMC 0.18μm 1P6M CMOS process. In addition to integral image computation, the array can be reprogrammed to deliver other early vision tasks: concurrent rectangular area sum, block-wise HDR imaging, Gaussian pyramids and image pre-warping for subsequent reduced kernel filtering.Ministerio de Economía y Competitividad TEC2012-38921-C02-01, IPT-2011-1625-430000, IPC-20111009Naval Research (USA) N00014111031