Pedro García-Mochales

Nanogeometry Matters: Unexpected Decrease of Capillary Adhesion Forces with Increasing Relative Humidity

The sticking effect between hydrophilic surfaces occurring at increasing relative humidity (RH) is an everyday phenomenon with uncountable implications. Here experimental evidence is presented for a counterintuitive monotonous decrease of the capillary adhesion forces between hydrophilic surfaces with increasing RH for the whole humidity range. It is shown that this unexpected result is related to the actual shape of the asperity at the nanometer scale: a model based on macroscopic thermodynamics predicts this decrease in the adhesion force for a sharp object ending in an almost flat nanometer-sized apex, in full agreement with experiments. This anomalous decrease is due to the fact that a significant growth of the liquid meniscus formed at the contact region with increasing humidity is hindered for this geometry. These results are relevant in the analysis of the dynamical behavior of nanomenisci. They could also have an outstanding value in technological applications, since the undesirable sticking effect between surfaces occurring at increasing RH could be avoided by controlling the shape of the surface asperities at the nanometric scale.

Statistical analysis of the breaking processes of Ni nanowires

We have performed a massive statistical analysis on the breaking behaviour of Ni nanowires using molecular dynamic simulations. Three stretching directions, five initial nanowire sizes and two temperatures have been studied. We have constructed minimum cross-section histograms and analysed for the first time the role played by monomers and dimers. The shape of such histograms and the absolute number of monomers and dimers strongly depend on the stretching direction and the initial size of the nanowire. In particular, the statistical behaviour of the breakage final stages of narrow nanowires strongly differs from the behaviour obtained for large nanowires. We have analysed the structure around monomers and dimers. Their most probable local configurations differ from those usually appearing in static electron transport calculations. Their non-local environments show disordered regions along the nanowire if the stretching direction is [100] or [110]. Additionally, we have found that, at room temperature, [100] and [110] stretching directions favour the appearance of non-crystalline staggered pentagonal structures. These pentagonal Ni nanowires are reported in this work for the first time. This set of results suggests that experimental Ni conducting histograms could show a strong dependence on the orientation and temperature.P. Garćıa-Mochales, R. Paredes, 3 S. Peláez, and P. A. Serena Departamento de F́ısica de la Materia Condensada, Facultad de Ciencias, Universidad Autónoma de Madrid, c/ Francisco Tomás y Valiente 7, Campus de Cantoblanco, E-28049-Madrid, Spain∗ Centro de F́ısica, Instituto Venezolano de Investigaciones Cient́ıficas, Apdo 20632, Caracas 1020A, Venezuela Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Cient́ıficas c/ Sor Juana Inés de la Cruz 3, Campus de Cantoblanco, E-28049-Madrid, Spain (Dated: February 2, 2008)

Icosahedral Ni Nanowires Formed from Nanocontacts Breaking: Identification and Characterization by Molecular Dynamics

We present and discuss an algorithm to identify and characterize the long icosahedral structures (staggered pentagonal nanowires with 1-5-1-5 atomic structure) that appear in Molecular Dynamics simulations of metallic nanowires of different species subjected to stretching. The use of this algorithm allows the identification of pentagonal rings forming the icosahedral structure as well as the determination of its number np , and the maximum length of the pentagonal nanowire Lpm. The algorithm is tested with some ideal structures to show its ability to discriminate between pentagonal rings and other ring structures. We applied the algorithm to Ni nanowires with temperatures ranging between 4K and 865K, stretched along the [111], [100] and [110] directions. We studied statistically the formation of pentagonal nanowires obtaining the distributions of length Lpm and number of rings np as function of the temperature. The Lpm distribution presents a peaked shape, with peaks located at fixed distances whose separation corresponds to the distance between two consecutive pentagonal rings.

Statistical molecular dynamics study of (111) and (100) Ni nanocontacts: evidences of pentagonal nanowires

We present molecular dynamics calculations on the evolution of Ni nanowires stretched along the (111) and (100) directions, and at two different temperatures. Using a methodology similar to that required to build experimental conductance histograms, we construct minimum crosssection histograms H(Sm). These histograms are useful to understand the type of favorable atomic configurations appearing during the nanowire breakage. We have found that minimum crosssection histograms obtained for (111) and (100) stretching directions are rather different. When the nanowire is stretched along the (111) direction, monomer and dimer-like configurations appear, giving rise to well-defined peaks in H(Sm). On the contrary, (100) nanowire stretching presents a different breaking pattern. In particular, we have found, with high probability, the formation of staggered pentagonal nanowires, as it has been reported for other metallic species.

Identification and characterization of icosahedral metallic nanowires

We present and discuss an algorithm to identify ans characterize the long icosahedral structures (staggered pentagonal nanowires with 1-5-1-5 atomic structure) that appear in Molecular Dynamics simulations of metallic nanowires of different species subjected to stretching. The use of the algorithm allows the identification of pentagonal rings forming the icosahedral structure as well as the determination of its number np, and the maximum length of the pentagonal nanowire Lpm. The algorithm is tested with some ideal structures to show its ability to discriminate between pentagonal rings and other ring structures. We applied the algorithm to Ni nanowires with temperatures ranging between 4 K and 865 K, stretched along the [100] direction. We studied statistically the formation of pentagonal nanowires obtaining the distributions of length Lpm and number of rings np as function of the temperature. The Lpm distribution presents a peaked shape, with peaks locate at fixes distances whose separation corresponds to the distance between two consecutive pentagonal rings. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Negative dissipation gradients in hysteretic materials

Measuring energy dissipation on the nanoscale is of great interest not only for nanomechanics but also to understand important energy transformation and loss mechanisms that determine the efficiency of energy of data storage devices. Fully understanding the magnetic dynamics and dissipation processes in nanomagnets is of major relevance for a number of basic and applied issues from magnetic recording to spin-based sensor devices to biomedical magnetic-based hyperthermia treatments. Here we present experimental evidence for a counter-intuitive monotonical reduction of energy dissipation as the interaction between two nanomagnets is enhanced. This behavior, which takes place when spins are parallel, can be understood in terms of hysteresis phenomena involved in the reorientation of these spins. The measured magnetic losses of about a few femtowatts are in agreement with quasi-static micromagnetic numerical simulations.

Modelling Metallic Nanowires Breakage for Statistical Studies: Ni Case as Example

During the last two decades, the study of the properties of nanowires has been one of the keystones for the development of nanotechnology since these nanometric size objects exhibit electrical and mechanical properties of interest in fundamental knowledge as well as for the future development of technological applications. In particular, many experimental studies have afforded the creation, stabilization and deformation of metallic nanowires in order to describe their mechanical properties and their electronic transport quantum features appearing due to electron transversal confinement (Serena & Garcia, 1997; Agrait et al., 2003). The standard approximation for the experimental study of such metallic nanowires includes the formation, elongation and breakage of ultranarrow nanocontacts, as for instance, those formed between an STM tip and a metallic surface. Since the atomic arrangement formed during every indentation-breakage cycle of the STM tip is different, these experimental studies require the accumulation and analysis of a huge amount of data to characterize the electron transport properties of the resulting nanocontacts. With the advent of powerful computational resources and the achievement of realistic descriptions of the atomic interactions, it has been possible to reproduce “in silicon” many of such formation-breaking experiments. An important part of these simulation studies has been done using Molecular Dynamics (MD) algorithms, allowing to elucidate how this formation-elongation-breakage occurs. Furthermore, MD simulations allow the accurate determination of the different structures that appear during the final stages of the breaking process. Getting insight of such structures is a crucial matter since they control the electron transport through the nanowire, allowing a comparison with the experimental data. However, the comparison between experimental results and MD computational simulations requires an extra ingredient: the use of statistics. In general, the study of nanoscale systems demands a statistical approach. This becomes more evident in electron transport studies, since little modifications of the atomic positions results in large conductance variations. It is

Spatial and angular intensity correlations of waves in disordered media

Spatial intensity correlations between waves transmitted through random media are analyzed within the framework of the random matrix theory of transport. Assuming that the statistical distribution of transfer matrices is isotropic, we found that the the spatial correlation function of the normalized intensity can be expressed as the sum of three terms, with distinctive spatial dependences. This result, that coincides with the one obtained from microscopic perturbative calculations valid in the diffusive regime, holds all the way from quasi‐ballistic transport to localization. While correlations are positive in the diffusive regime, we predict a transition to negative correlations for both angular and spatial correlations as the length of the system decreases.

Light transport through Photonic Liquids

Colloidal suspensions, in the presence of long‐range electrostatic repulsion, have been shown to present an unexpected optical behavior: Structural order enhance the scattering strength while at the same time the total light transmission shows strong wavelength dependence, reminiscent of a photonic crystal. In this work we will present the results of extensive numerical calculations of light transport through Photonic Liquids and other systems with strong short range correlations for both two‐dimensional and three‐dimensional systems. The electromagnetic density of states of those systems is analyzed in detail. As we will show, the interplay between order and disorder and the scattering properties of these systems are strikingly similar to those discussed in an early proposal for strong localization of light.

Transmittances Distributions at the Diffusive‐Localized Crossover in Disordered Wave‐guides with Absorption

An analysis of the behavior of transmittances distributions with the length in disordered wave‐guides with absorption is presented. The study is focused on the crossover between diffusive and localize regimens. Distributions are obtained from numerical calculations from the well‐known “tight‐binding” model. Absorption in this model produces also an increase of the scattering rate (and a decrease of the mean free path l and localization length ξ). Distributions of the transmission P(T) on the crossover show a smooth crossover between Gaussian and lognormal distribution (unlike the no‐absorbing case). The ratio var(sa) ξ/L has in the ballistic regime a value similar to the no‐absorbing case, decreases as L rises in the diffusive regime, showing a minimum around ξ increasing again in the crossover. This behavior resembles the results found in microwaves wave‐guide experiments.