Dynamical generalized Hurst exponent as a tool to monitor unstable periods in financial time series

We investigate the use of the Hurst exponent, dynamically computed over a weighted moving time-window, to evaluate the level of stability/instability of financial firms. Financial firms bailed-out as a consequence of the 2007–2008 credit crisis show a neat increase with time of the generalized Hurst exponent in the period preceding the unfolding of the crisis. Conversely, firms belonging to other market sectors, which suffered the least throughout the crisis, show opposite behaviors. We find that the multifractality of the bailed-out firms increase at the crisis suggesting that the multi fractal properties of the time series are changing. These findings suggest the possibility of using the scaling behavior as a tool to track the level of stability of a firm. In this paper, we introduce a method to compute the generalized Hurst exponent which assigns larger weights to more recent events with respect to older ones. In this way large fluctuations in the remote past are less likely to influence the recent past. We also investigate the scaling associated with the tails of the log-returns distributions and compare this scaling with the scaling associated with the Hurst exponent, observing that the processes underlying the price dynamics of these firms are truly multi-scaling.     

Modular system for studying tonal sound excitation in resonators with heat addition and mean flow

An educational experimental system has been developed for studying tonal sound generation in acoustic resonators. Tones are excited by either heat addition or vortex shedding in the presence of mean flow. The system construction is straightforward and inexpensive. Several test arrangements and experimental data are described in this paper. The experimental setups include a modified Rijke tube, a standing-wave thermoacoustic engine, a baffled tube with mean flow, and an acoustic energy harvester with a piezoelement. Simplified mathematical models for interpreting data are discussed, and references are provided to literature with more advanced analyses. The developed system can assist both graduate and undergraduate students in understanding acoustic instabilities via conducting and analyzing interesting experiments.     

Floral scent composition of Plumeria tuberculata analyzed by HS-SPME

The headspace volatile compounds of the flowers of Plumeria tuberculata Lodd. were analyzed by solid phase microextraction coupled with capillary gas chromatography/mass spectrometry. Twenty-five compounds were identified, representing 100% of the total composition. The volatile fraction was characterized by oxygenated monoterpenes (79.6%), oxygenated sesquiterpenes (8.4%), sesquiterpene hydrocarbons (7.6%), and benzenoid esters (2.6%). The major components were geraniol (34.9%), citronelol (21.5%) and geranial (16.2%), and they were found to make the major contribution to the typical scent of this flower.     

A fragmentation model with neighborhood interaction

Several models of fragmentation have been studied that suppose random fracture forces. In this article, we did a numerical study on a dynamic model for fragmentation in which the fracture forces are generated by neighboring fragments and are proportional to the size of the common boundary between two fragments. The following assumptions were also considered: the material defects are represented by a random distribution of point flaws; the total mass is conserved; and the iterative fracture of each fragment is randomly stopped by a condition that considers a constant probability and a minimal fragment size. The motivation for this model was to determine under what circumstances a continuous fragmentation model with fracture forces defined by the neighbors’ interaction produces results that are in agreement with the experimental evidence. The main result of this work establishes that the fragment size distribution follows a power-law for fragments of greater area than the minimal fragment size m_fs. The visualizations present complex fracture patterns that resemble real systems.     

Pupil detection and tracking for analysis of fixational eye micromovements

Even with fixed gaze, the human eye is under steady motion (tremor micosaccades and drifts). Detection of these movements requires of invasive techniques or expensive devices with sophisticated detection methods. In this paper we present a technique for pupil segmentation and contour analysis which will provide valuable information about fixational eye movements. The method is based on fitting an ellipse to the pupil contour. Pupillary hippus, microsaccades and drifts are obtained as well as cyclotorsional movements. The method is simple and the experimental requirements are easily available since just a biomicroscope and a digital videocamera are required.     

Vocal reporting of echolocation targets: dolphins often report before click trains end

Bottlenose dolphins (Tursiops truncatus) wore opaque suction cups over their eyes while stationing behind an acoustically opaque door. This put the dolphins in a known position and orientation. When the door opened, the dolphin clicked to detect targets. Trainers specified that Dolphin S emit a whistle if the target was a 7.5 cm water filled sphere, or a pulse burst if the target was a rock. S remained quiet if there was no target. Dolphin B whistled for the sphere. She remained quiet for rock and for no target. Thus, S had to choose between three different responses, whistle, pulse burst, or remain quiet. B had to choose between two different responses, whistle or remain quiet. S gave correct vocal responses averaging 114 ms after her last echolocation click (range 182 ms before and 219 ms after the last click). Average response for B was 21 ms before her last echolocation click (range 250 ms before and 95 ms after the last click in the train). More often than not, B began her whistle response before her echolocation train ended. The findings suggest separate neural pathways for generation of response vocalizations as opposed to echolocation clicks.     

Characterizing and modeling an electoral campaign in the context of Twitter: 2011 Spanish Presidential election as a case study

Transmitting messages in the most efficient way as possible has always been one of politicians' main concerns during electoral processes. Due to the rapidly growing number of users, online social networks have become ideal platforms for politicians to interact with their potential voters. Exploiting the available potential of these tools to maximize their influence over voters is one of politicians' actual challenges. To step in this direction, we have analyzed the user activity in the online social network Twitter, during the 2011 Spanish Presidential electoral process, and found that such activity is correlated with the election results. We introduce a new measure to study political sentiment in Twitter, which we call the relative support. We have also characterized user behavior by analyzing the structural and dynamical patterns of the complex networks emergent from the mention and retweet networks. Our results suggest that the collective attention is driven by a very small fraction of users. Furthermore, we have analyzed the interactions taking place among politicians, observing a lack of debate. Finally, we develop a network growth model to reproduce the interactions taking place among politicians.     

Impurity-related polarizability and photoionization-cross section in double quantum wells under electric fields and hydrostatic pressure

Double quantum well heterostructures are quite important for the exploration of correlated electron states in two-dimensional systems. By using the variational procedure, within the effective-mass and parabolic-band approximations, the effects of both electric field and hydrostatic pressure on the shallow-donor-impurity related polarizability and photoionization cross-section in GaAs–Ga_1−xAl_xAs double asymmetric quantum wells are presented. The electric field is considered to be applied along the growth direction. It is found that the impurity binding energy and polarizability can be tuned by means of an applied external electric field or hydrostatic pressure in asymmetric double quantum wells, a behavior which could be used in the design and construction of semiconductor devices. The photoionization cross-section magnitude increases as the pressure and applied electric field are increased, except beyond the Γ–X crossover in the barrier material, where a decrease of the photoionization cross-section is expected due the smaller confinement of the impurity wave function.     

Nanostructured Au/Si substrate for organic molecule SERS detection

Nanoparticles of noble metals, such as gold and silver, exhibit unique and tunable optical properties on account of their surface plasmon resonance. In particular, gold nanoparticles on silicon substrates are attractive for future nanoscale sensors and optical devices due to their resistance to oxidation and due to their electrical and optical properties. In this study, we developed a nanostructured gold/macroporous silicon (Au/PS) substrate capped with 11-mercaptoundecanoic acid (11-MUA) with ultra-sensitive detection properties achieved in characterization, an approach based on surface-enhanced Raman scattering (SERS). Surface-enhanced Raman scattering allows us to detect substances at a low concentration level and to observe structural details of a thiol molecule bonded to small film thicknesses. Raman measurements were carried out at 514 nm and 785 nm. In order to emphasize the effect of the Si microstructuration on the efficiency of this new substrate (Au/PS) proposed for SERS experiments, the same molecule (11-MUA) was adsorbed on it as well as on gold/atomically flat silicon (Au/Si) and on commercial Klarite (Mesophotonics) substrates. Systematic studies realized by Raman spectroscopy, electron microscopy, and X-ray spectroscopy show the influence of silicon substrate texturing and metallic deposition conditions, including time and temperature on the optical phenomena.     

Long-term structural surface modifications of mixed conducting La<Subscript>2</Subscript>NiO<Subscript>4+δ</Subscript> at high temperatures

Abstract  Long-term annealing of La₂NiO_4+δ single crystals at 1,273 K in air leads to the formation of nickel-rich Ruddlesden–Popper phases at the single-crystal surfaces. Both the composition and the morphology of these phases depend on the surface orientation; whereas only crystallites of La₄Ni₃O_10−δ were observed on (001) surfaces, both La₃Ni₂O_7−δ and La₄Ni₃O_10−δ were formed on (100) surfaces. The formation of the nickel-rich RP phases is believed to be due to surface segregation of nickel or evaporation of a volatile lanthanum species. The crystallographic (001) planes inside the La₃Ni₂O_7−δ and La₄Ni₃O_10−δ crystallites were found to be oriented in the direction of preferential crystallite growth, which indicates that the diffusion of lanthanum and nickel cations is faster along the crystallographic (001) planes than perpendicular to these planes. Graphical abstract