Atomistic study of CoCrCuFeNi high entropy alloy nanoparticles: Role of chemical complexity.

High entropy alloy nanoparticles are envisaged as one of the most interesting materials compared to monoatomic materials due to their modulated properties in terms of their convenient surface-to-volume ratio. However, studies are still missing to unveil how composition or nanoparticle size can influence nanoparticle morphology. Based on molecular dynamics simulations, we perform a structural characterization as a function of nanoparticle size and the chemical composition of high entropy alloy nanoparticles subject to multiple annealing cycles. After the multiple thermal loads, we observe a substantial migration of copper atoms towards the np surface, consistent with the experimental results of Cu-based high entropy alloys. The resulting high entropy alloy nanoparticle behaves as a core-shell nanostructure with a rich fcc phase on the surface (50% of Cu) and 5% fcc phase in the nanoparticle core. Inspecting the nanoparticle surface, it is observed that high entropy alloy nanoparticles have a lack of surface facets, leading to a more spherical shape, quite different from mono-metallic nanoparticles with a high number of facets. Performing an average atoms simulation, it showed that nanoparticles are prone to form 111 surface facets independent of the nanoparticle size, suggesting that for high entropy alloy nanoparticles, the chemical complexity avoids the formation of surface facets. The latter can be explained in terms of the lattice distortion inducing tensile/compressive stress that drives the surface reconstruction. All in all our results match extremely well with experimental evidence of FeNiCrCoCu nanocrystalline materials, explaining the Cu segregation in terms of surface energy and mixing enthalpy criteria. We believe that our results provide a detailed characterization of high entropy nanoparticles focusing on how chemical complexity induces morphological changes compared to mono-crystalline nanoparticles. Besides, our findings are valuable for experimental works aimed at designing the shape and composition of multicomponent nanoparticles.

Three-dimensional non-approximate Coulomb interaction between two trapped quantum particles.

The two trapped quantum particles interacting problem is generalized to three dimensions, and the exact Coulomb potential is used. The system is solved by expanding the wavefunction in terms of the isotropic harmonic oscillator eigenfunctions and Hydrogen atom eigenfunctions independently, showing that each one results in a prime approximation for different domains of the normalized coupling constant of the relative interactions, suggesting that the combination of the basis is enough to build a well-suited base for the non-approximate problem. The results are compared to previous works that use a model of approximate finite-rage soft-core interaction model of the problem to give insights into the many-body states of strongly correlated ultracold bosons and fermions. We conclude that the proposed three-dimensional approach facilitates the distinction between bosons and fermions while the solutions given by the expansions better define the behavior of the particles for repulsive potentials. In addition, we discuss the substantial differences between our work and the previous approximate model.

Probing the Mechanical Properties of Porous Nanoshells by Nanoindentation.

In this contribution, we present a study of the mechanical properties of porous nanoshells measured with a nanoindentation technique. Porous nanoshells with hollow designs can present attractive mechanical properties, as observed in hollow nanoshells, but coupled with the unique mechanical behavior of porous materials. Porous nanoshells display mechanical properties that are dependent on shell porosity. Our results show that, under smaller porosity values, deformation is closely related to the one observed for polycrystalline and single-crystalline nanoshells involving dislocation activity. When porosity in the nanoparticle is increased, plastic deformation was mediated by grain boundary sliding instead of dislocation activity. Additionally, porosity suppresses dislocation activity and decreases nanoparticle strength, but allows for significant strain hardening under strains as high as 0.4. On the other hand, Young's modulus decreases with the increase in nanoshell porosity, in agreement with the established theories of porous materials. However, we found no quantitative agreement between conventional models applied to obtain the Young's modulus of porous materials.

Circularly polarized light-induced potentials and the demise of excited states.

In the presence of strong electric fields, the excited states of single-electron molecules and molecules with large transient dipoles become unstable because of anti-alignment, the rotation of the molecular axis perpendicular to the field vector, where bond hardening is not possible. We show how to overcome this problem by using circularly polarized electromagnetic fields. Using a full quantum description of the electronic, vibrational, and rotational degrees of freedom, we characterize the excited electronic state dressed by the field and analyze its dependence on the bond length and angle and the stability of its vibro-rotational eigenstates. Although the dynamics is metastable, most of the population remains trapped in this excited state for hundreds of femtoseconds, allowing quantum control. Contrary to what happens with linearly polarized fields, the photodissociation occurs along the initial molecular axis, not perpendicular to it.

Thermal Sensitivity on Eccentric Gold Hollow Nanoparticles: A Perspective from Atomistic Simulations.

Eccentricity is a common feature consequence of several synthesis protocols of hollow nanoshells. Despite the crescent interest in these nanoparticles, it is still unclear how an irregular layer on the nanoparticle impacts the macroscopic properties. Here, we study the thermal stability of eccentric hollow nanoparticles (hNPs) for different sizes and eccentricity values by means of classical molecular dynamics simulations. Our results reveal that eccentricity displays a significant role in the thermal stability of hNPs. We attribute this behavior to the irregular shell contour, which collapses due to the thermal-activated diffusive process from the nanoparticle shell's most thin region. The mechanism is driven at low temperature by the nucleation of stacking faults until the amorphization for larger temperature values. Besides, for some particular eccentric hNPs, the shell suffers a surface reconstruction process, transforming the eccentric hNP into a concentric hNP. We believe that our study on thermal effects in eccentric hNPs has relevance because of their outstanding applications for plasmonic and sensing.

Resolving early obesity leads to a cardiometabolic profile within normal ranges at 23 years old in a two-decade prospective follow-up study.

Obesity is the most important predisposing factor for cardiovascular disease and type-2 diabetes. We explored the relationship between the age at onset of obesity and selected cardiometabolic parameters in young adults. Longitudinal study of n = 1,039 participants (48% males) in their early twenties. BMI was measured at birth, 1-5-10-12-14-16-23 years. BMI trajectories were interpolated. Five groups were identified: never obese (never-OB); early childhood obesity transitioning to non-obesity before adolescence (former-OB); obesity starting in preadolescence transitioning to non-obesity as adolescents (transient-OB); obesity from adolescence into early adulthood (recent-onset-OB); participants who were obese in early childhood and remained obese into adulthood (persistent-OB). Waist circumference (WC), blood pressure, lipids, glucose, and insulin were measured at 23 years. HOMA-IR and the Metabolic Syndrome Risk Z-Score were estimated. In the sample, 47% were obese during at least one time-point. Mean obesity duration was 20.7 years, 8.5 years, 6.2 years, and 3.3 years in persistent-OBs, recent-onset-OBs, former-OBs, and transient-OBs, respectively. The cardiometabolic profile was more adverse in recent-onset-OBs (12%) and persistent-OBs (15%) compared to never-OB participants (53%). Although former-OBs (15%) and transient-OBs (4%) had higher WC values than never-OBs, no differences were seen in other biomarkers. Both persistent and recent-onset obesity led to a cardiometabolic profile of risk in early adulthood, as suggested by values of WC, HOMA-IR, and hs-CRP above normal limits and HDL-chol values below normal limits. Participants who had obesity in early childhood or preadolescence but transitioned to a non-obesity status had a cardiometabolic profile similar to participants who were never obese and within normal limits. Obesity leads to risky values in a number of cardiometabolic biomarkers in young adulthood independent of age at obesity onset. Likewise, overcoming obesity during the pediatric age leads to a cardiometabolic profile within normal ranges at 23 years of age.

Multifractal Characteristics of Geomagnetic Field Fluctuations for the Northern and Southern Hemispheres at Swarm Altitude.

This paper explores the spatial variations of the statistical scaling features of low to high latitude geomagnetic field fluctuations at Swarm altitude. The data for this study comes from the vector field magnetometer onboard Swarm A satellite, measured at low resolution (1 Hz) for one year (from 9 March 2016, to 9 March 2017). We estimated the structure-function scaling exponents using the p-leaders discrete wavelet multifractal technique, from which we obtained the singularity spectrum related to the magnetic fluctuations in the North-East-Center (NEC) coordinate system. From this estimation, we retain just the maximal fractal subset, associated with the Hurst exponent H. Here we present thresholding for two levels of the Auroral Electrojet index and almost the whole northern and southern hemispheres, the Hurst exponent, the structure-function scaling exponent of order 2, and the multifractal p-exponent width for the geomagnetic fluctuations. The latter quantifies the relevance of the multifractal property. Sometimes, we found negative values of H, suggesting a behavior similar to wave breaking or shocklet-like propagating front. Furthermore, we found some asymmetries in the magnetic field turbulence between the northern and southern hemispheres. These estimations suggest that different turbulent regimes of the geomagnetic field fluctuations exist along the Swarm path.

Simulations suggest that navigation software may not be as efficient as expected for city traffic.

We suggest a theoretical framework to study the dynamics of an open city, with cars entering at a certain rate and leaving as they reach their destinations. In particular, we assess through simulations some unexpected consequences of the massive use of GPS (global positioning system) navigation systems in the overall dynamics. One of our main interest is to identify what type of measurements would be the most relevant for an experimental study of this system, specifically, the ones useful for city traffic administrators. To do so, we solve the microdynamics using a cellular automaton model considering three different navigation strategies based on the minimization of the individual paths (unweighted strategy) or travel times (weighted strategies). Although the system is inherently stochastic, we found in our simulations an equivalent saddle-node bifurcation for all strategies where the input rate acts as a bifurcation parameter. There is also evidence of additional bifurcations for travel time minimization based strategies. Although we found that weighted strategies are more efficient in terms of car motion, there is a destabilization phenomenon that makes, in an unexpected way, a variation of the unweighted strategy more optimal at certain densities from the fuel efficiency of the overall city traffic point of view. These results bring new insight into the intrinsic dynamics of cities and the perturbations that individual traffic routing can produce on the city as a whole.

Anti-alignment driven dynamics in the excited states of molecules under strong fields.

We develop two novel models of the H2+ molecule and its isotopes from which we assess quantum-mechanically and semi-classically whether the molecule anti-aligns with the field in the first excited electronic state. The results from both models allow us to predict anti-alignment dynamics even for the HD+ isotope, which possesses a permanent dipole moment. The molecule dissociates at angles perpendicular to the field polarization in both the excited and the ground electronic state, as the population is exchanged through a conical intersection. The quantum mechanical dispersion of the initial state is sufficient to cause full dissociation. We conclude that the stabilization of these molecules in the excited state through bond-hardening under a strong field is highly unlikely.

Obesity and impairment of pancreatic ß-cell function in early adulthood, independent of obesity age of onset: The Santiago Longitudinal Study.

AIM: We investigated the relation of time of onset and length of obesity with biomarkers of ß-cell function in early adulthood in an infancy cohort. MATERIAL AND METHODS: In 1039 23-year-olds, body-mass index (BMI) was measured at multiple time-points from enrollment. BMI trajectories were interpolated with cubic polynomials. Fasting glucose, insulin and adiponectin were measured at 23 years. Homeostatic model assessment-insulin resistance (HOMA-IR), HOMA-S, HOMA-ß, HOMA-adiponectin (AD) and disposition index (DI) were estimated. IR and non-alcoholic fatty liver (NAFL) were diagnosed. According to the BMI trajectory, five groups were defined: participants who were never obese (NOB); participants with obesity starting in adolescence and remained obese into adulthood (recent-onset obesity, ROB); participants who were obese in early childhood but transitioned to non-obesity as preadolescents (former obesity, FOB); participants who were obese in early childhood and remained obese into adulthood (persistent obesity, POB); participants with obesity starting in preadolescence and transitioned to non-obesity as adolescents (transient obesity; TOB). RESULTS: Obesity was present in 47% of participants during at least one time-point. ROBs and POBs had higher insulin, HOMA-IR and HOMA-ß, lower HOMA-S and DI, and higher prevalence of IR and NAFL at 23 years than NOBs, TOBs and FOBs. No differences were found in the ß-cell functionality of NOBs, TOBs and FOBs. CONCLUSIONS: Persistent and recent obesity are both related to IR, NAFL and a decline of ß-cell function in emerging adulthood. Defeating obesity in childhood or adolescence allows reaching emerging adulthood with ß-cell functioning similar to that of subjects who were NOB.

Thermal Stability of Hollow Porous Gold Nanoparticles: A Molecular Dynamics Study.

Hollow nanoparticle structures play a major role in nanotechnology and nanoscience since their surface to volume ratio is significantly larger than that of filled ones. While porous hollow nanoparticles offer a significant improvement of the available surface area, there is a lack of theoretical understanding, and scarce experimental information, on how the porosity controls or dominates the stability. Here we use classical molecular dynamics simulations to shed light on the particular characteristics and properties of gold porous hollow nanoparticles and how they differ from the nonporous ones. Adopting gold as a prototype, we show how, as the temperature increases, the porosity introduces surface stress and minor transitions that lead to various scenarios, from partial shrinkage for small filling factors to abrupt compression and the loss of spherical shape for large filling. Our work provides new insights into the stability limits of porous hollow nanoparticles, with important implications for the design and practical use of these enhanced geometries.

Stability and robustness of asymptotic autocatalytic systems.

Here, we address the consequences of the extension in the space of a simple model of a system that is closed to efficient causation: the (M,R)-system model. To do so, we use a diffusion term to describe the collective motion of the nutrients' concentration across the compartmentalized space that defines the organism. We show that the non-trivial stable steady state remains despite such generalization, as long as the system is small enough to deal with the transport of the precursors to feed the entire protocell and dispose of a sufficient concentration of it in its surroundings. Such consideration explains the emergence of a bifurcation with two parameters that we characterize. Finally, we show that the robustness of the system under catastrophic losses of catalysts also remains, preserving the original's model character.

Speeding up maximum population transfer in periodically driven multi-level quantum systems.

A fast and robust approach to controlling the quantum state of a multi-level quantum system is investigated using a twofrequency time-varying potential. A comparison with other related approaches in the context of a two-level system is also presented, showing similar times and fidelities. As a concrete example, we study the problem of a particle in a box with a periodically oscillating infinite square-well potential, from which we obtain results that can be applied to systems with periodically oscillating boundary conditions. We show that the transition between the ground and first excited state is about 20 times faster than the one performed using a single frequency, both with fidelity of 99.97%. The transition time is about 3.5 times the minimum allowed by quantum mechanics. A test of the robustness of the approach is presented, concluding that, counter-intuitively, it is not only faster but also easier to tune up two frequencies than one. This robustness makes the approach suitable for real applications.

Modeling interacting city traffic with finite acceleration and braking capacities.

Understanding the fundamental interactions in the complex behavior of one car moving in a sequence of traffic lights necessarily implies the inclusion of finite braking and accelerating capabilities. This characteristic is usually not considered in the standard cellular automaton models, where car interactions are the main concern. Therefore, here we develop a model which includes interactions and finite braking and accelerating capabilities, filling the gap between a standard cellular automaton model that considers car interactions but infinite braking and accelerating capabilities and the continuous one car model that includes finite braking and accelerating capabilities but does not consider, as the name indicates, car interactions. The proposed new model bridge these two seemingly different approaches in an effort to investigate how the traffic jams are produced. We found that, in the appropriate limits, we can reproduce the complex behavior of the one car continuous model and the dynamics close to the resonance induced by the interacting cars, forced by the traffic lights. In the processes of introducing car interactions, we observe how the average velocity decreases to finally obtain traffic jams, which are an emergent state in which the traffic lights control the generation of pulses of cars but do not control its average speed. This model is expected to improve our understanding of the complexity that appears in city traffic situations, as the finite braking and accelerating capabilities are necessary to describe the vehicle dynamics, the control strategy of traffic light synchronization, the motion of buses in segregated lights, and the whole urban design.

Long-term vs. recent-onset obesity: their contribution to cardiometabolic risk in adolescence.

BACKGROUND: The contribution of long-term vs. recent-onset obesity to cardiometabolic risk in adolescence remains controversial. Here, we aimed to investigate the association of time of onset and length of obesity with the cardiometabolic profile of adolescence. METHODS: Prospective study in 678 16-year-olds. BMI was measured at birth-1-5-10-16 years and BMI trajectories were interpolated using cubic splines. BMI > 2 SD at <6 years was defined as early obesity. Waist circumference (WC), blood pressure, lipid and glucose profiles were measured at 16 years. A cardiometabolic risk score was computed (MetS_score). According to the BMI trajectory, four groups were defined: participants who were never obese (NOB), participants with obesity during adolescence (recent-onset obese (ROB)), participants who were obese in early childhood but transitioned to normal/overweight as preadolescents (formerly obese (FOB)), and participants who were obese in early childhood and remained obese (persistently obese (POB)). RESULTS: ROBs and POBs had significantly unhealthier cardiometabolic profile than NOBs. No differences were observed in the cardiometabolic profile of ROBs compared to POBs. Although FOBs had higher WC and MetS_score than NOBs, no differences were found in other biomarkers. FOBs were in healthier cardiometabolic condition than ROBs and POBs. CONCLUSIONS: Both long-term and recent-onset obesity increase the cardiometabolic risk in adolescents.

Author Correction: Nucleation of superfluid-light domains in a quenched dynamics.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has not been fixed in the paper.

The Stochastic Transport Dynamics of a Conserved Quantity on a Complex Network.

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.

Nucleation of superfluid-light domains in a quenched dynamics.

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.

Bending energy of 2D materials: graphene, MoS2 and imogolite.

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 Å2 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.

Controlling the Quantum State with a time varying potential.

The problem of controlling the quantum state of a system is investigated using a time varying potential. As a concrete example we study the problem of a particle in a box with a periodically oscillating infinite square-well potential, from which we obtain results that can be applied to systems with periodically oscillating boundary conditions. We derive an analytic expression for the frequencies of resonance between states, and against standard intuition, we show how to use this behavior to control the quantum state of the system at will. In particular, we offer as an example the transition from the ground state to the first excited state of the square well potential. At first sight, it may be counter intuitive that we can control the state of such a quantum Hamiltonian, as the Schrödinger equation conserves the norm of the wave function. In this manuscript, we show how that can be achieved.