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SUMMARYDysregulated central-energy metabolism is a hallmark of brain aging. Supplying enough energy for neurotransmission relies on the neuron-astrocyte metabolic network. To identify genes contributing to age-associated brain functional decline, we formulated an approach to analyze the metabolic network by integrating flux, network structure and transcriptomic databases of neurotransmission and aging. Our findings support that during brain aging: 1) The astrocyte undergoes a metabolic switch from aerobic glycolysis to oxidative phosphorylation, decreasing lactate supply to the neuron, while the neuron suffers intrinsic energetic deficit by downregulation of Krebs cycle genes, including mdh1 and mdh2 (Malate-Aspartate Shuttle); 2) Branched-chain amino acid degradation genes were downregulated, identifying dld as a central regulator; 3) Ketone body synthesis increases in the neuron, while the astrocyte increases their utilization, in line with neuronal energy deficit in favor of astrocytes. We identified candidates for preclinical studies targeting energy metabolism to prevent age-associated cognitive decline.

Oxygen absorption and the thermal stability of Al147 nanoparticles were studied by means of classical molecular dynamics simulations and Monte Carlo methods. The results suggest that for the studied sizes, oxygen incorporation yields an Al2O3 nanoparticle with a Janus-like morphology, contrary to the expected core–shell nanostructure observed in simulations and experiments of nanometer-size nanoparticles. A simulated annealing, introduced to support this assumption, shows that the Janus-like morphology has a lower energy than that of Al@Al2O3 with a core@shell conformation. Also, the thermal behavior of a Janus-like Al/Al2O3 nanoparticle as a function of oxygen concentration was investigated. It is observed that the partial oxidation reduces the nanoparticle melting temperature because the number of pure aluminum atoms is reduced. In fact, the melting point can be as low as 400 K for an Al147O30 nanoparticle. The melting process leads to a solid alumina region that coexists with liquid-like aluminum nanoparticles. The oxide never adopts a protective shell covering configuration of the aluminum nanoparticle.

Many non-pharmaceutical interventions (NPI) have been implemented in several countries to prevent and mitigate the COVID-19 pandemic. In spite of the emergence of vaccines that mitigate illness and death, no effective measures exist to control the contagion. Testing, tracing, and isolation (TTI) in conjunction with disciplined social distancing were expected to achieve negligible-contagion levels and avoid, for example, superspreading events 1. However, the logic sustaining these strategieshas proven insufficient and a commitment to implementing them leads to controversial ethical challenges. This paper focuses on the effectiveness of contagion control policies that are independent of the ethical challenges that the chosen strategies pose, and of the complex realities of different countries, that are applicable both country-wide and at the local level. Even more relevant is that our performance measure indicators are given by variables that provide an early warning as to when the epidemic might expand, despite some temporal case decrements. We show that if case reduction is not sustained for more than 7-14 days, the risk growth is considerable, warranting immediate attention and action. To draw these conclusions, our research does not only follow the contagion change but its velocity, and applies these indicators to a sample of prototypical countries, finding strong evidence for our conclusions. The latter are independent of different applied NPI and country differences, suggesting that they can be used as "universal" indicators for epidemic control policies. Our main conclusion is that if the spread reduction is lost during one, or at most two weeks, immediate measures have to be implemented to avoid undesirable scenarios.

Mechanical properties of nanomaterials, such as nanowires and nanotubes, are an important feature for the design of novel electromechanical nano-architectures. Since grain boundary structures and surface modifications can be used as a route to modify nanostructured materials, it is of interest to understand how they affect material strength and plasticity. We report large-scale atomistic simulations to determine the mechanical response of nickel nanowires and nanotubes subject to uniaxial compression. Our results suggest that the incorporation of nanocrystalline structure allows completely flexible deformation, in sharp contrast with single crystals. While crystalline structures at high compression are dominated by dislocation pinning and the multiplication of highly localized shear regions, in nanocrystalline systems the dislocation distribution is significantly more homogeneous. Therefore, for large compressions (large strains) coiling instead of bulging is the dominant deformation mode. Additionally, it is observed that nanotubes with only 70% of the nanowire mass but of the same diameter, exhibit similar mechanical behavior up to 0.3 strain. Our results are useful for the design of new flexible and light-weight metamaterials, when highly deformable struts are required.

During the current COVID-19 pandemic it is imperative to give early warnings to reduce mortality. However, non-specialist such as authorities and the general population face several problems to understand the real thread of this pandemic, and under/overestimation of its risk are a commonplace in the press and social media. Here we define an index, which we call the COVID-19 Burden Index, that relates the capacities of the healthcare system of a given country to treat severe and critical cases. Its value is 0 if there is no extra strain in the healthcare system, and it reaches 1.0 when the collapse is imminent. As of 23 March 2020, we show that Chile, the USA, UK, among other countries, must reduce the rate of infections right now, otherwise, in less than 7 days they could be in a catastrophic situation such as Italy, Spain and Iran.

The stochastic dynamics of conserved quantities is an emergent phenomena observed in many complex systems, ranging from social and to biological networks. Using an extension of the Ehrenfest urn model on a complex network, over which a conserved quantity is transported in a random fashion, we study the dynamics of many elementary packets transported through the network by means of a master equation approach and compare with the mean field approximation and stochastic simulations. By use of the mean field theory, it is possible to compute an approximation to the ensemble average evolution of the number of packets in each node which, in the thermodynamic limit, agrees quite well with the results of the master equation. However, the master equation gives a more complete description of the stochastic system and provides a probabilistic view of the occupation number at each node. Of particular relevance is the standard deviation of the occupation number at each node, which is not uniform for a complex network. We analyze and compare different network topologies (small world, scale free, Erdos-Renyi, among others). Given the computational complexity of directly evaluating the asymptotic, or equilibrium, occupation number probability distribution, we propose a scaling relation with the number of packets in the network, that allows to construct the asymptotic probability distributions from the network with one packet. The approximation, which relies on the same matrix found in the mean field approach, becomes increasingly more accurate for a large number of packets.

The bending process of 2D materials, subject to an external force, is investigated, and applied to graphene, molybdenum disulphide (MoS2), and imogolite. For graphene we obtained 3.43 eV A2 per atom for the bending modulus, which is in good agreement with the literature. We found that MoS2 is ∼11 times harder to bend than graphene, and has a bandgap variation of ∼1 eV as a function of curvature. Finally, we also used this strategy to study aluminosilicate nanotubes (imogolite) which, in contrast to graphene and MoS2, present an energy minimum for a finite curvature radius. Roof tile shaped imogolite precursors turn out to be stable, and thus are expected to be created during imogolite synthesis, as predicted to occur by self-assembly theory.

Hollow nanoparticles (hNPs) are of interest because their large cavities and small thickness give rise to a large surface to volume ratio. However, in general they are not in equilibrium and far from their global energy minimum, which often makes them unstable against perturbations. In fact, a temperature increase can induce a structural collapse into a nanoparticle, and consequently a loss of their unique properties. This problem has been studied by means of molecular dynamics (MD) simulations, but without emphasis on the speed of the temperature increase. Here we explore how the temperature variation, and the rate at which it is varied in MD simulations, determines the final conformation of the hNPs. In particular, we show how different temperature ramps determine the final shape of Pt hNPs that initially have an external radius between 0.7 and 24 nm, and an internal radius between 0.19 and 2.4 nm. In addition, we also perform the simulations of other similar metals like Ag and Au. Our results indicate that the temperature ramp strongly modifies the final hNP shape, even at ambient temperature. In fact, a rapid temperature increase leads to the formation of stacking faults and twin boundaries which are not generated by a slower temperature increase. Quantitative criteria are established and they indicate that the stacking fault energy is the dominant parameter. Fil: Reyes, Paula N.. Universidad de Chile; Chile. Centro para el Desarrollo de la Nanociencia y la Nanotecnología; Chile Fil: Valencia, Felipe J.. Universidad de Chile; Chile. Centro para el Desarrollo de la Nanociencia y la Nanotecnología; Chile. Universidad Mayor; Chile Fil: Vega, Hector. Universidad de Chile; Chile Fil: Ruestes, Carlos Javier. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Rogan, José. Universidad de Chile; Chile. Centro para el Desarrollo de la Nanociencia y la Nanotecnología; Chile Fil: Valdivia, J. A.. Universidad de Chile; Chile. Centro para el Desarrollo de la Nanociencia y la Nanotecnología; Chile Fil: Kiwi, Miguel. Universidad de Chile; Chile. Centro para el Desarrollo de la Nanociencia y la Nanotecnología; Chile

The large surface-to-volume ratio of hollow palladium nanoparticles (hNPs) offers room to improve their hydrogen storage capacity as well as their catalytic activity. However, a less explored possibility is to use, in addition, the internal cavity. Here we explore, through classical molecular dynamics, the possibility of boring channels across the hNP wall by collision with solid Pd nanoprojectiles at high velocities, as well as their resilience to maintain their spherical geometry. We choose a stable hNP with an inner diameter of 13 nm and an outer diameter of 15 nm. The projectiles are Pd NPs of 1.5, 2.4, and 3.0 nm, respectively. We consider collision speeds between 3 and 15 km/s, with an impact parameter between 0 to 7 nm. Four different regimes, as a function of kinetic energy and impact parameter of the projectile, are found. For low speeds, the projectile is not able to penetrate the target and only creates surface craters. For a narrow range of intermediate speeds, the projectile enters the target...

Hillock formation on nanocrystalline (nc) diamond under swift heavy ion irradiation is studied by means of classical molecular dynamics. The irradiation is simulated by means of a thermal spike model, the nc samples include as many as 5 millions atoms. Our results show that hillocks on nc diamond can be created for stopping powers (SPe) in the range of 12–17 keV/nm, and grain sizes less than 13 nm. For smaller values of the SPe only point defects are observed on the nc surface, while for larger SPe hillocks suffer a transition to crater-rim, because of the increased sputtering that is due to the large energy that the ions deposit. We observe that the sputtering yields depend quadratically on the stopping power, contrary to what has been obtained by simulations for some single crystal solids. In addition, our results show that hillocks are smaller for 5 and 7 nm grain sizes, due to the large free volume that is available on the grain boundaries. Instead, for 10 and 13 nm the hillock is limited only to the amorphization of the grain closest to the surface. No hillock formation is expected for larger grain sizes, because of the transition of the nc to pristine diamond, where no hillock formation has been observed. Fil: Valencia, Felipe J.. Universidad Mayor. Facultad de Ciencias. Núcleo de Matemáticas, Física y Estadística; Chile. Universidad de Chile; Chile. Centro para el Desarrollo de la Nanociencia y la Nanotecnología; Chile Fil: González, Rafael I.. Universidad Mayor; . Centro para el Desarrollo de la Nanociencia y la Nanotecnología; Chile Fil: Bringa, Eduardo Marcial. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza; Argentina Fil: Kiwi, Miguel. Universidad de Chile; Chile. Centro para el Desarrollo de la Nanociencia y la Nanotecnología; Chile

The effects of Na/K substitution during the synthesis of imogolite nanotubes (NTs) were studied using a combination of structural and surface analyses. These were complemented with molecular dynamics (MD) and DFT computational models. Our results provide strong experimental evidence, obtained by various characterization techniques (FT-IR, XRD, IEP, charge measurement, and HR-TEM), showing that K changes the imogolite dimensions. In fact, in the presence of K, the nanotubes become shorter and adopt a larger diameter. Moreover, the presence of the amorphous structures associated with allophane increases, even for low K concentrations. Our results underline the complexity of imogolite synthesis engineering, highlighting their high sensitivity to the chemicals that are used during synthesis.

Amorphous carbon (aC), in particular diamond-like carbon (DLC), is one of the most studied and promising coating materials, but many of the atomic-scale mechanisms involved in their plastic deformation process are not fully understood. The mechanical response of non-hydrogenated DLC films with different sp 3 concentrations is investigated using nanoindentation experiments, ab-initio simulations, and classical molecular dynamics simulations. Experimental characterization is carried out with Raman spectroscopy and Electron Energy Loss Spectroscopy (EELS). Ab-initio and classical simulations show good agreement for sp 1 , sp 2 and sp 3 content of in-silico samples. Elastic modulus and hardness of DLC films increase with sp 3 content, for sp 3 between 10% and 55%, and excellent agreement is obtained between experiments and simulations. Simulated strain distributions are shown to be highly anisotropic, unlike continuum-scale predictions for the use of a perfectly spherical indenter in an amorphous solid. MD simulations also reveal two different plasticity modes depending on the sp 3 level of the indented sample. For films with sp 3 concentrations less than 40%, plasticity is mainly mediated by the sp 2 to sp 3 transition. For larger sp 3 concentrations, DLC plastic deformation is attributed to densification due to bond rearrangement. All in all, our work offers a comprehensive study of DLC, revealing unexpected plastic deformation mechanisms that had not been considered before. Our study might help to the fundamental understanding of amorphous carbon coatings for both scientific and technological purposes.

We can mass-produce metal/carbon nanotube (CNT) composites that show improved radiation tolerance. The 0.5 wt% Al+CNT composite showed improved tensile strength without reduction of tensile ductility before radiation, and reduced void/pore generation and radiation embrittlement at high displacements per atom (DPA). Under helium ion irradiation up to 72 DPA, the 1D carbon nanostructures survive, while sp2 bonded graphene transforms to sp3 tetrahedral amorphous carbon. Self-ion (Al) irradiation converts CNTs to a metastable form of Al4C3, but still as slender 1D nanorods with prolific internal interfaces that catalyze recombination of radiation defects, reducing radiation hardening and porosity generation. The 1D fillers may also form percolating paths of "nano-chimneys" that outgas the accumulated helium and other fission gases, providing an essential solution to the gas accumulation problem. Fil: So, Kang Pyo. Massachusetts Institute of Technology; Estados Unidos Fil: Chen, Di. Texas A&M University; Estados Unidos Fil: Kushima, Akihiro. Massachusetts Institute of Technology; Estados Unidos Fil: Li, Mingda. Massachusetts Institute of Technology; Estados Unidos Fil: Kim, Sangtae. Massachusetts Institute of Technology; Estados Unidos Fil: Yang, Yang. Massachusetts Institute of Technology; Estados Unidos Fil: Wang, Ziqiang. Massachusetts Institute of Technology; Estados Unidos Fil: Park, Jong Gil. Sungkyunkwan University; Corea del Sur Fil: Lee, Young Hee. Sungkyunkwan University; Corea del Sur Fil: Gonzalez, Rafael I.. Universidad de Chile; Chile Fil: Kiwi, Miguel. Universidad de Chile; Chile Fil: Bringa, Eduardo Marcial. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; Argentina Fil: Shao, Lin. Texas A&M University; Estados Unidos Fil: Li, Ju. Massachusetts Institute of Technology; Estados Unidos

We have compared the magnetic properties of well-controlled ultra-short (≤50 nm) atomic iron (Fe) chains embedded in Fe-phthalocyanine films with those in Fe–hydrogen (H2) phthalocyanine superlattices. Surprisingly, we found that the coercivity of the atomic chains with free boundary conditions is independent of the chain length, whereas the one subject to hybridization of the chain ends exhibits an unexpected length dependence. These findings suggest that ferromagnetism in the free-boundary condition system is caused by an intrinsic indirect exchange. On the other hand, controlled boundary conditions produce a helical spin structure due to an extrinsic indirect exchange, which arises from the interaction between iron atoms at the ends of the chain and the hydrogen in the H2 phthalocyanine spacer. As a consequence, during magnetic reversal, ultra-short iron chains subject to boundary clamping develop a helical spin structure, leading to increased coercivity. These findings suggest unique insights and ideas for the design of atomic-scale ultra-dense magnetic storage nanodevices.

An enhanced exchange bias was found in FeF2/Ni interfaces by inserting dusting of Pd and Cu atoms. We have used an ultrathin wedge to investigate systematically the effect of a discontinuous nonmagnetic spacer between the antiferromagnetic and ferromagnetic layers. Negative and positive exchange biases are symmetric and maximized for less than two angstroms of the nonmagnetic layer. Moreover, the dusting reduces the field cooling threshold to switch between the negative and the positive exchange bias. This finding demonstrates that nonmagnetic dusted layers can improve the interfacial exchange energy density in antiferromagnetic/ferromagnetic heterostructures and modify the bulk antiferromagnetic domain structure. Other dusting materials such as Ag, Ti, V, and SiO2 showed no enhancement in FeF2/Ni bilayers, which indicates the atomic sensitivity of this effect.

Imogolite is a fascinating inorganic nanotube that is found in nature or synthesized in a laboratory. The synthesis process is carried out in liquid media, and leads to the formation of almost monodisperse diameter nanotubes. Here we investigate, employing classical molecular dynamics simulations, the interaction of water and imogolite for nanotubes of several radii. We established that water penetrates the pores of N = 9 and larger nanotubes, and adopts a coaxial arrangement in it. Also, while water molecules can diffuse along the center of the nanotube, the molecules next to the inner imogolite walls have very low mobility. At the outer nanotube wall, an increase of water density is observed, this effect extends up to 1 nm, beyond which water properties are bulk-like. Both phenomena are affected by the imogolite curvature.

Polycrystalline hollow nanoparticles present a unique combination of strength and flexibility. However, the exact role displayed by their grain structure in mechanical properties has not been yet fully understood. Here, by means of molecular dynamics simulations, the role of grain boundary structure during the nanoindentation of metallic hollow nanoparticles with a polycrystalline shell was investigated. Our simulations were performed for a range of grain sizes and shell thicknesses, including the large strain regime. Our results show that hNP mechanical properties can be controlled by tuning the grain size of the polycrystalline shell, following an inverse Hall-Petch type dependence with the grain size. Deformation involves dislocation activity, twin hardening, grain boundary sliding, coalescence, and rotation. For single crystal shells at large strain there is hardenning following the closure of the internal cavity. For nanocrystalline shells at large strains a constant flow stress regime is observed even for deformations as high as 80%, thanks to grain boundary activity. Surprisingly, some particular grain size not only leads to an improvement in strength, but also a flow stress higher than the observed in their single-crystalline counterparts. Our work, suggest that grain boundary structure can be employed to improve and tailor desired mechanical properties in hollow nanostructures.

Electromagnetic coupled resonator arrays (CRAs) doped with a quantum two-level system allow for the quantum simulation of a Mott-insulator to superfluid phase transition. We demonstrate that the order of this simulated phase transition depends on the type of dynamics. Thus, a first order like phase transition can be induced by a quench dynamics, while a second order like phase transition is produced by an adiabatic dynamics. In addition, we show that the underlying macroscopic behavior of the phase transition in other many body systems, such as domain nucleation and phase coexistence, can also be observed in CRAs. This universal behavior emerges from the light-matter interaction and the topology of the array. Therefore, the latter can be used to manipulate the photonic transport properties of the simulated super fluid phase.

Strong correlation effects emerge from light-matter interactions in coupled resonator arrays, such as the Mott-insulator to superfluid phase transition of atom-photon excitations. We demonstrate that the quenched dynamics of a finite-sized complex array of coupled resonators induces a first-order like phase transition. The latter is accompanied by domain nucleation that can be used to manipulate the photonic transport properties of the emerging superfluid phase; this in turn leads to an empirical scaling law. This universal behavior emerges from the light-matter interaction and the topology of the array. The validity of our results over a wide range of complex architectures might lead to to a promising device for use in scaled quantum simulations., Comment: 7 pages, 4 figures and supplemental material. Accepted by Scientific Reports

Nanoparticles are ubiquitous in nature and are increasingly important for technology. They are subject to bombardment by ionizing radiation in a diverse range of environments. In particular, nanodiamonds represent a variety of nanoparticles of significant fundamental and applied interest. Here we present a combined experimental and computational study of the behaviour of nanodiamonds under irradiation by xenon ions. Unexpectedly, we observed a pronounced size effect on the radiation resistance of the nanodiamonds: particles larger than 8 nm behave similarly to macroscopic diamond (i.e. characterized by high radiation resistance) whereas smaller particles can be completely destroyed by a single impact from an ion in a defined energy range. This latter observation is explained by extreme heating of the nanodiamonds by the penetrating ion. The obtained results are not limited to nanodiamonds, making them of interest for several fields, putting constraints on processes for the controlled modification of nanodiamonds, on the survival of dust in astrophysical environments, and on the behaviour of actinides released from nuclear waste into the environment., Comment: Supplementray information is at: doi:10.1038/s41598-018-23434-y

The ternary alloy FexCoyNiz nanoparticles are of interest in both theoretical and experimental aspects, particularly due to the possible technological applications and due to the novel structural and magnetic properties in the sub-nanometer region. Here we compute the structural parameters, chemical and magnetic properties of these 13 atom nanoclusters. To the best of our knowledge, this is the first time that such kind of calculation is performed for all the possible compositions of 13-atom ternary nanoclusters. We performed density-functional-theory (DFT) calculations, as implemented in the SIESTA code, for all the possible concentrations (i.e. all x , y and z-values). The seeds for the possible homotops are built using a semi-empirical Gupta potential, and these conformations are thereafter subject to reoptimization by means of the SIESTA code. Based on known results, we focus our attention on just two types of possible nanostructures: icosahedral and compact biplanar. We find that approximately half of the minimum energy conformations are icosahedral, and the other half are biplanar, with most interatomic distances smaller than the bulk values. The binding is strongest for the FeCo rich nanoclusters, and weakest for pristine Ni13. However, a set of highly stable structures, with layer-like ordering, were found close to the Ni-rich region. These nanoalloys are expected to be the most abundant ones in cluster-growth experiments. We conclude that the total magnetic moment varies smoothly over the full composition range, with the large Fe moment being quenched by the addition of Co and/or Ni. By alloying, the magnitude of the moments varies almost continuously, and thus allows for fine tuning magnetism by controlling the composition, which has implications for magnetic recording.

The (110) surface of iron fluoride (FeF2) is especially relevant to the understanding of the exchange bias phenomenon, which has important applications in the sensor industry, and has been extensively explored, both theoretically and experimentally. Here we investigate this FeF2 surface by means of ab initio techniques. We compute the (110) surface reconstruction, energetics, magnetic moments, band structure, charge density and electron localization function, for the two possible terminations (Fe and F). The surface reconstruction modifies the atomic and electronic structure of the free surface, yielding magnetism of a magnitude of 0.1 μ B per surface unit cell. Moreover, the charge density also changes, which alters the bonding in the vicinity of the surface. All these changes are expected to be relevant for exchange bias, that is once a ferromagnetic layer is deposited on the FeF2 surface.

The discovery of dipole-induced exchange bias (EB), switching from negative to positive sign, is reported in systems where the antiferromagnet and the ferromagnet are separated by a paramagnetic spacer (AFM–PM–FM). The magnitude and sign of the EB is determined by the cooling field strength and the PM thickness. The same cooling field yields negative EB for thin spacers, and positive EB for thicker ones. The EB decay profile as a function of the spacer thickness, and the change of sign, are attributed to long-ranged dipole coupling. Our model, which accounts quantitatively for the experimental results, ignores the short range interfacial exchange interactions of the usual EB theories. Instead, it retains solely the long range dipole field that allows for the coupling of the FM and AFM across the PM spacer. The experiments allow for novel switching capabilities of long range EB systems, while the theory allows description of the structures where the FM and AFM are not in atomic contact. The results provide a new approach to design novel interacting heterostructures.

The bending process of 2D materials, subject to an external force, is investigated, and applied to graphene, molybdenum disulphide (MoS

Imogolite is an attractive inorganic nanotube, formed from weathered volcanic ashes, that also can be synthesized in nearly monodisperse diameters. It has found a variety of uses, among them as an effective arsenic retention agent, as a catalyst support and as a constituent of nanowires. However, long after its successful synthesis, the details of the way it is achieved are not fully understood. Here we develop a model of the synthesis, which starts with a planar aluminosilicate sheet that is allowed to evolve freely, by means of classical molecular dynamics, until it achieves its minimum energy configuration. The minimal structures that the system thus adopts are tubular, scrolled, and more complex conformations as well, depending mainly on temperature. The minimal nanotubular configurations that we obtain are monodispersed in diameter and quite similar in diameter both to those of weathered natural volcanic ashes and to the ones that are synthesized in the laboratory. A tendency toward nanotube agglomer...

The melting and soldering processes of two iron filled carbon nanotubes is explored by means of classical molecular dynamics, in order to develop an understanding of the underlying mechanisms that govern the dynamics of nano-soldering. Molten Fe flows from the open end of the two CNTs, leading to a liquid junction, and eventually to a solid contact. This soldering process is accompanied by partial or total healing of the carbon nanotubes, which after cooling and relaxation form just a single unit which encapsulates the iron, depending on the relative separation, diameters and axial offset of the nanotubes. This makes for a promising scenario for CNT soldering, repairing and healing, and a variety of different tools in the field of nanoelectronics. Fil: Munizaga, Vicente. Pontificia Universidad Católica de Chile. Facultad de Física; Chile. Centro para el Desarrollo de la Nanociencias y Nanotecnología; Chile Fil: García, Griselda Beatriz. Centro para el Desarrollo de la Nanociencias y Nanotecnología; Chile. Pontificia Universidad Católica de Chile. Facultad de Física; Chile Fil: Bringa, Eduardo Marcial. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza; Argentina Fil: Weissmann, Mariana Dorotea. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza; Argentina. Comisión Nacional de Energía Atómica. Gerencia Química. CAC; Argentina Fil: Ramirez, Ricardo Martin. Centro para el Desarrollo de la Nanociencias y Nanotecnología; Chile. Pontificia Universidad Católica de Chile. Facultad de Física; Chile Fil: Kiwi, Miguel. Centro para el Desarrollo de la Nanociencias y Nanotecnología; Chile. Universidad de Chile; Chile

We simulate traffic in a city by means of the evolution of a row of interacting cars, using a cellular automaton model, in a sequence of traffic lights synchronized by a “green wave”. When our initial condition is a small density jammed state, its evolution shows the expected scaling laws close to the synchronization resonance, with a uniform car density along the street. However, for an initial large density jammed state, we observe density variations along the streets, which results in the breakdown of the scaling laws. This spatial disorder corresponds to a different attractor of the system. As we include velocity perturbations in the dynamics of the cars, all these attractors converge to a statistically equivalent system for all initial jammed densities. However, this emergent state shows a stochastic resonance-like behavior in which the average traffic velocity increases with respect to that of the system without noise, for several initial jammed densities. This result may help in the understanding of dynamics of traffic jams in cities.

Exchange bias (EB) has been observed in systems where a paramagnetic (PM) spacer separates an antiferromagnet (AFM) from a ferromagnet. This coupling extends to PM spacer thicknesses of up to 100A in Py/(Ag, Au, Cu)/CoO systems. We develop a quantum fluctuation model in which a dipole field is created at the AFM surface due to the breaking of translational invariance symmetry. The uncompensated spins of the semiinfinite AFM induce a field that extends all the way to the CoO/(Ag, Au, Cu) interface and thus generate EB. The magnitude of the EB field that the present model yields is of the same order of magnitude as the one measured in recent experiments.

We investigate the relevance of the choice of a particular classical potential in the task of finding global and local energy minima of small cluster conformations, in the context of their use as input for quantum refinement. We contrast results obtained using the many body Gupta and Sutton–Chen potentials for small nickel and copper clusters, with those of a Lennard-Jones pair potential. To obtain quantitative results we introduced a modified version of the concept of distance between configurations and color maps to represent these distances. Our main conclusion is that, for the small clusters we studied, all three potentials lead to practically the same results after quantum refinement is implemented.

The potential and properties of palladium hollow nanoparticles (hNPs) as a possible H storage material are explored by means of classical molecular dynamics (MD) simulations. First, we study the stability of pure Pd hNPs for different sizes and thicknesses, obtaining good agreement with experimental results for nanometer size Pd hNP. Next we add, every 100 fs, single H atoms into the NP cavity. During the first stages of the simulation, our results show hydride formation on the inner surface, similar to what has been observed in experiments on Pd surfaces and NPs. Formation of the Pd hydride decreases the absorption rate, and H gas is formed inside the cavity. The maximum H gas pressure that is reached is of 7 GPa, before fractures appear in the hNP, and consequently the hNP breaks up. We obtain a maximum H/Pd ratio of 1.21 when H is introduced only inside the cavity. However, when H is deposited both on the inside and outside surfaces, this ratio reaches 1.70, which is 25% larger than previous reports. Beyond this ratio, the hNP breaks up, and the H gas is ejected from the hNP cavity. Fil: Valencia, Felipe J.. Universidad de Chile; Chile. Centro para el Desarrollo de la Nanociencia y la Nanotecnología; Chile Fil: González, Rafael I.. Universidad de Chile; Chile. Centro para el Desarrollo de la Nanociencia y la Nanotecnología; Chile Fil: Tramontina Videla, Diego Ramiro. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; Argentina. Universidad de Mendoza; Argentina Fil: Rogan, José. Universidad de Chile; Chile. Centro para el Desarrollo de la Nanociencia y la Nanotecnología; Chile Fil: Valdivia, Juan Alejandro. Universidad de Chile; Chile. Centro para el Desarrollo de la Nanociencia y la Nanotecnología; Chile Fil: Kiwi, Miguel. Universidad de Chile; Chile. Centro para el Desarrollo de la Nanociencia y la Nanotecnología; Chile Fil: Bringa, Eduardo Marcial. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; Argentina

The improvement of radiation resistance in nanocomposite materials is investigated by means of classical reactive molecular dynamics simulations. In particular, we study the influence of carbon nanotubes (CNTs) in an Ni matrix on the trapping and possible outgassing of He. When CNTs are defect-free, He atoms diffuse alongside CNT walls and, although there is He accumulation at the metal-CNT interface, no He trespassing of the CNT wall is observed, which is consistent with the lack of permeability of a perfect graphene sheet. However, when vacancies are introduced to mimic radiation-induced defects, He atoms penetrate CNTs, which play the role of nano-chimneys, allowing He atoms to escape the damaged zone and reduce bubble formation in the matrix. Consequently, composites made of CNTs inside metals are likely to display improved radiation resistance, particularly when radiation damage is related to swelling and He-induced embrittlement. Fil: González, Rafael I.. Universidad de Chile; Chile Fil: Valencia, Felipe. Universidad de Chile; Chile Fil: Mella, Jose Manuel. Universidad de Chile; Chile Fil: Van Duin, Adri C. T.. State University of Pennsylvania; Estados Unidos Fil: So, Kang Pyo. Massachusetts Institute of Technology; Estados Unidos Fil: Li, Ju. Massachusetts Institute of Technology; Estados Unidos Fil: Kiwi, Miguel. Universidad de Chile; Chile Fil: Bringa, Eduardo Marcial. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; Argentina

We characterize, by means of the definition of a generalized distance, the differences and similarities between binary nanoclusters. To define analytically, and to compute numerically this distance, we have generalized an original concept that was introduced for pure clusters. Since the diversity of cluster conformations grows exponentially with their size, and becomes even larger when the cluster atoms are of more than one species, we limit our attention to small ones. Thus, to illustrate and analyze our distance definition we characterize the Lennard-Jones (U) minimum energy conformations of two- and threedimensional (2D and 3D) binary clusters, for 5, This work has been founded by the Fondo Nacional de Investigaciones Científicas y Tecnológicas (FONDECYT, Chile) under Grants #1110135 (JAV), #1120399 and #1130272 (MK and JR), and Financiamiento Basal para Centros Científicos y Tecnológicos de Excelencia (CEDENNA, Chile) No. FB0807.

Collision processes between a single gold atom and a gold cluster are investigated by means of ab initio techniques. The targets we consider are minimum energy 13 gold atom clusters. The kinetic energy of the projectile and its impact parameter are chosen within a range such that the three regimes we are mainly interested in studying (fusion, scattering and fragmentation) are realized. The results of the collision processes are treated using density functional theory molecular dynamics (DFT-MD), analyzed in detail, and compared with previous work, which was carried out using phenomenological potentials and classical molecular dynamics. The differences between classical MD and DFT-MD are quite significant.

The magnitude of the exchange bias field of patterned systems exhibits a notable increase in relation to the usual bilayer systems, where a continuous ferromagnetic film is deposited on an antiferromagnet insulator. Here we develop a model, and implement a Monte Carlo calculation, to interpret the experimental observations which is consistent with experimental results, on the basis of assuming a small fraction of spins pinned ferromagnetically in the antiferromagnetic interface layer.

Strong correlation effects emerge from light-matter interactions in coupled resonator arrays, such as the Mott-insulator to superfluid phase transition of atom-photon excitations. We demonstrate that the quenched dynamics of a finite-sized complex array of coupled resonators induces a first-order like phase transition. The latter is accompanied by domain nucleation that can be used to manipulate the photonic transport properties of the simulated superfluid phase; this in turn leads to an empirical scaling law. This universal behavior emerges from the light-matter interaction and the topology of the array. The validity of our results over a wide range of complex architectures might lead to a promising device for use in scaled quantum simulations.

We discuss the scientific impact of Latin American scientists in the field of materials science. The analysis is based on the h-index as the scientometric index used to quantify the scientific productivity of an individual. In particular, we focus our analysis in Mexico, Chile and Colombia. We compare the level of productivity between all these countries. We also analyzed the h-index as function of the biological age, by using the first year of publication of a given scientists as a reference and discussed the general distribution of its quantification. We do not find a clear relationship between these two quantities. Based in our results we propose some political measures that these countries could implement to improve productivity as well as scientific development in this field. c

Swift heavy ion irradiation does not generate amorphous tracks in diamond, contrary to what happens in graphite or in diamond-like carbon. Since nanocrystalline diamond is of interest for several technological applications we investigate the reason for this difference, by means of large scale atomistic simulations of ion tracks in nanocrystalline diamond, using a thermal spike model, with up to 2.5 million atoms, and grain sizes in the range 5-10 nm. We conclude that tracking can be achieved under these conditions, when it is absent in single crystal diamond: for 5 nm samples the tracking threshold is below 15 keV/nm. Point defects are observed below this threshold. As the energy loss increases the track region becomes amorphous, and graphitic-like, with predominant sp2 hybridization. This higher sensitivity to irradiation can be related to a very large decrease in thermal conductivity of nanocrystalline diamond, due to grain boundary confinement of the heat spike which enhances localized heating of the lattice. Fil: Valencia, Felipe. Centro Para El Desarrollo de la Nanociencia y Nanotecnologia; Chile. Universidad de Chile; Chile Fil: Mella, José D.. Centro Para El Desarrollo de la Nanociencia y Nanotecnologia; Chile. Universidad de Chile; Chile Fil: González, Rafael I.. Universidad de Chile; Chile. Centro Para El Desarrollo de la Nanociencia y Nanotecnologia; Chile Fil: Kiwi, Miguel. Centro Para El Desarrollo de la Nanociencia y Nanotecnologia; Chile. Universidad de Chile; Chile Fil: Bringa, Eduardo Marcial. Universidad Nacional de Cuyo; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza; Argentina

The Ehrenfest urn model is extended to a complex directed network, over which a conserved quantity is transported in a random fashion. The evolution of the conserved number of packets in each urn, or node of the network, is illustrated by means of a stochastic simulation. Using mean-field theory we were able to compute an approximation to the ensemble-average evolution of the number of packets in each node which, in the thermodynamic limit, agrees quite well with the results of the stochastic simulation. Using this analytic approximation we are able to find the asymptotic dynamical state of the system and the time scale to approach the equilibrium state, for different networks. The study is extended to large scale-free and small-world networks, in which the relevance of the connectivity distribution and the topology of the network for the distribution of time scales of the system is apparent. This analysis may contribute to the understanding of the transport properties in real networks subject to a perturbation, e.g., the asymptotic state and the time scale required to approach it.

International audience; Multiferroic materials are mostly antiferromagnets, often containing incommensurate magnetic arrangements stemming from the magnetoelectric interaction. Using soft x-ray resonant magnetic scattering, we show that these long-range structures induce a magnetization wriggle in cobalt layers deposited on top of BiFeO3 single crystals. This is understood using a simple interface exchange interactions model. It leads to the appearance of a magnetic anisotropy axis, which, in the particular BiFeO3/Co system, can be manipulated using an electric field. More generally, it is demonstrated here that through interfacial magnetic exchange, antiferromagnets can leave an imprint revealing some of their hidden properties, thus providing much richer effects than mere exchange bias.