The planar to three dimensional transition of Au_13−nAg_n clusters is investigated.

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.

The determination of the spatial distributions that atoms adopt to form condensed matter is a problem of crucial importance, since most physical properties depend on the atomic arrangement. This is especially relevant for clusters, where periodicity is nonexistent. Several optimization procedures have been implemented to tackle this problem, with ever increasing success. Here we put forward a search scheme which preserves as large a diversity as allowed by the use of phenomenological potentials, generating in an unbiased fashion a bank of configurations to be explored; a procedure we denominate diversity driven unbiased search (DDUS). It consists in the generation, using phenomenological potentials, of a data bank of putative minima rather than a single, or just a few, configurations which are based on the conformational space annealing method (CSA). All of the configurations in the bank are thereafter refined by means of DFT computations. Certainly, in spite of our efforts to generate a bank as diverse as possible, not all relevant structures might be included in it, since quantum effects are ignored. The procedure is applied to several examples of rhodium, palladium, silver, platinum and gold clusters, between 5 and 23 atoms in size. The main conclusion we reach is that unbiased search, among a significant number of candidates, quite often leads to rather unexpectedly low symmetry configurations, which turn out to be the lowest energy ones within our scheme.

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.

High entropy alloy (HEA) nanoparticles (NPs), finite-size nanostructures that combine multiple elements coexistent with a nanoscale surface, have emerged as a new material due to their non-trivial properties. While there exist several studies addressing the behavior of body-centered cubic (BCC) HEAs, there is still missing information devoted to predicting the structural and morphological configurations that HEA NPs can adopt. In this work, using molecular dynamic simulations and Monte Carlo algorithms, HfNbTaZr HEA NPs of 2 and 3 nm were studied. For all simulated scenarios, HEA NPs showed a significant precipitation of elements while maintaining their BCC crystalline structure. The presence of the surface drives the diffusion of some elements, leading to the formation of a core-shell structure with an HfZr rich shell and a near equiatomic HEA core. A topological change in the typical short-range order of HfNbTaZr is induced by the surface, since the formation of HfZr bands is replaced by the formation of an HfZr rich surface. It was observed that BCC HEA NPs have an irregular shear stress, a consequence of the chemical complexity, which at the same time induces surface reconstruction favoring the formation of highly faceted nanoparticles. The {111} planes were identified as the most probable surface facets on the NP, mostly due to the enthalpy of mixing and cohesive energy. Finally, a phase separation of the Hf and Zr surfaces is observed, which is also in accordance with the positive enthalpy of mixing that favors the repulsion between both elements.