Nanowires: A route to efficient thermoelectric devices

Miniaturization of electronic devices aims at manufacturing ever smaller products, from mesoscopic to nanoscopic sizes. This trend is challenging because the increased levels of dissipated power demands a better understanding of heat transport in small volumes. A significant amount of the consumed energy is transformed into heat and dissipated to the environment. Thermoelectric materials offer the possibility to harness dissipated energy and make devices less energy-demanding. Heat-to-electricity conversion requires materials with a strongly suppressed thermal conductivity but still high electronic conduction. Nanowires can meet nicely these two requirements because enhanced phonon scattering at the surface and defects reduces the lattice thermal conductivity while electric conductivity is not deteriorated, leading to an overall remarkable thermoelectric efficiency. Therefore, nanowires are regarded as a promising route to achieving valuable thermoelectric materials at the nanoscale. In this paper, we present an overview of key experimental and theoretical results concerning the thermoelectric properties of nanowires. The focus of this review is put on the physical mechanisms by which the efficiency of nanowires can be improved. Phonon scattering at surfaces and interfaces, enhancement of the power factor by quantum effects and topological protection of electron states to prevent the degradation of electrical conductivity in nanowires are thoroughly discussed

Comunidad de Escarabajos Coprófagos (Coleoptera: Scarabaeidae: Scarabaeinae) en Hábitats bajo Distinta Intensidad de Uso en Yuscarán, Honduras

The Municipality of Yuscarán is under continuous habitat degradation processes due to invasive agriculture and cattle practices. Dung beetles (Coleoptera: Scarabaeidae: Scarabaeinae) are sensitive to habitat disturbance. The objective of this study was to describe how the dung beetle community structure is modified from less to more perturbed habitats throughout three different landscapes, according to the intensity of land use and vegetation structure. The investigation took place from June to December 2012 in a Tropical Evergreen Pine Forest and in a Deciduous Woodland. Each forest type consisted of two different areas divided by 3 collection sites according to land use. Beetles were collected in pitfall traps baited with cow dung. Vegetation structure and abiotic parameters were determined per habitat. A total of 2,749 beetles were captured, belonging to 9 tribes, 11 genera, and 21 species. Mean beetle abundance was reduced by 50% due to increased human intervention. In the Evergreen Forest, the dung beetle community structure was mainly formed by forest specialists and generalists, whereas in the Deciduous Woodland the dung beetle community was formed mainly by generalist and heliophilic species. The study shows how human intervention diminishes the richness, diversity, and abundance of the dung beetle communities in the Evergreen Forest. However, there is an inverse scenario in the Deciduous Woodland where the results indicated that there could be a faunal homogenization of generalist beetles, biased towards degraded and highly perturbed habitats. DOI: http://dx.doi.org/10.5377/ceiba.v52i2.1758Los recursos naturales en el Municipio de Yuscarán se encuentran bajo un continuo proceso de degradación producto de mal manejo del suelo, así como prácticas agrícolas y ganaderas intensivas. Los escarabajos coprófagos (Coleoptera: Scarabaeidae: Scarabaeinae) son insectos muy susceptibles a la perturbación del hábitat; por lo tanto, el objetivo de la investigación fue describir como la estructura de sus comunidades es modificada desde hábitats menos a más intervenidos, a través de diferentes usos del suelo, tomando en cuenta la intensidad de uso y su estructura vegetal. La investigación se realizó de junio a diciembre del 2012 en un Bosque Tropical Siempre Verde y un Arbustal Deciduo de Tierras Bajas, cada uno con dos zonas divididas en tres sitios de muestreo según el uso del suelo. Los escarabajos se recolectaron utilizando trampas de caída cebadas con estiércol de vaca. Se caracterizó la vegetación y se registraron parámetros abióticos por hábitat. Se capturaron 2,749 individuos pertenecientes a 9 tribus, 11 géneros y 21 especies. La abundancia promedio de escarabajos disminuyó más del 50% al intensificarse la intervención humana. En el Bosque Siempre Verde, las comunidades de escarabajos estuvieron conformadas principalmente por especialistas y generalistas forestales; en el Arbustal Deciduo, las comunidades estaban compuestas por especies generalistas y heliofílicas. Este estudio muestra como la intervención humana conlleva a la pérdida de riqueza, diversidad y abundancia de las comunidades de escarabajos del Bosque Siempre Verde. No obstante, la situación es inversa y más precaria en el Ecosistema Deciduo, donde los resultados indican que se puede estar llegando a una homogenización de fauna con hábitos generalistas producto del alto grado de intervención del hábitat.DOI: http://dx.doi.org/10.5377/ceiba.v52i2.175

Cerebellar direct current stimulation enhances on-line motor skill acquisition through an effect on accuracy

The cerebellum is involved in the update of motor commands during error-dependent learning. Transcranial direct current stimulation (tDCS), a form of noninvasive brain stimulation, has been shown to increase cerebellar excitability and improve learning in motor adaptation tasks. Although cerebellar involvement has been clearly demonstrated in adaptation paradigms, a type of task that heavily relies on error-dependent motor learning mechanisms, its role during motor skill learning, a behavior that likely involves errordependent as well as reinforcement and strategic mechanisms, is not completely understood. Here, in humans, we delivered cerebellar tDCS to modulate its activity during novel motor skill training over the course of 3 d and assessed gains during training (on-line effects), between days (off-line effects), and overall improvement. We found that excitatory anodal tDCS applied over the cerebellum increased skill learning relative to sham and cathodal tDCS specifically by increasing on-line rather than off-line learning. Moreover, the larger skill improvement in the anodal group was predominantly mediated by reductions in error rate rather than changes in movement time. These results have important implications for using cerebellar tDCS as an intervention to speed up motor skill acquisition and to improve motor skill accuracy, as well as to further our understanding of cerebellar function

Twisted equivariant K-theory, groupoids and proper actions

In this paper we define twisted equivariant K-theory for actions of Lie groupoids. For a Bredon-compatible Lie groupoid, this defines a periodic cohomology theory on the category of finite CW-complexes with equivariant stable projective bundles. A classification of these bundles is shown. We also obtain a completion theorem and apply these results to proper actions of groups.Comment: 26 page

Semi-quantum approach to molecular dynamics simulation of thermal properties of low-dimensional nanostructures

We present a detailed description of semi-quantum molecular dynamics simulation of stochastic dynamics of a system of interacting particles. Within this approach, the dynamics of the system is described with the use of classical Newtonian equations of motion in which the effects of phonon quantum statistics are introduced through random Langevin-like forces with a specific power spectral density (the color noise). The color noise describes the interaction of the molecular system with the thermostat. We apply this technique to the simulation of thermal properties and heat transport in different low-dimensional nanostructures. We describe the determination of temperature in quantum lattice systems, to which the equipartition limit is not applied. We show that one can determine the temperature of such system from the measured power spectrum and temperature- and relaxation-rate-independent density of vibrational (phonon) states. We simulate the specific heat and heat transport in carbon nanotubes, as well as the heat transport in molecular nanoribbons with perfect (atomically smooth) and rough (porous) edges, and in nanoribbons with strongly anharmonic periodic interatomic potentials. We show that the effects of quantum statistics of phonons are essential for the carbon nanotube in the whole temperature range T<500K, in which the values of the specific heat and thermal conductivity of the nanotube are considerably less than that obtained within the description based on classical statistics of phonons.Comment: 19 pages, 15 figures, 2 table

Surface acoustic Bloch oscillations, the Wannier-Stark ladder and Landau-Zener tunneling in a solid

We present the experimental observation of Bloch oscillations, the Wannier-Stark ladder, and Landau-Zener tunneling of surface acoustic waves in perturbed grating structures on a solid substrate. A model providing a quantitative description of our experimental observations, including multiple Landau-Zener transitions of the anticrossed surface acoustic Wannier-Stark states, is developed. The use of a planar geometry for the realization of the Bloch oscillations and Landau-Zener tunneling allows a direct access to the elastic field distribution. The vertical surface displacement has been measured by interferometry

Raman study of self-assembled InAs/InP quantum wire stacks with varying spacer thickness

Self-assembled InAs/InP (001) quantum wire stacks have been investigated by means of Raman scattering. The characteristics of the observed vibrational modes show clear evidence of confinement and atomic intermixing between As and P atoms from the wire and the spacer. The change in the intermixing with spacer layer thickness and growth temperature is investigated. Likewise, the effect of annealing on the exchange of As and P atoms is also studied. Resonance effects in confined and interface phonons are discussed for excitation in the vicinity of the InAs E1 critical point. Finally, the energy of the interface modes is related to the structural characteristics of the wires by comparing the experimental data with a lattice dynamic calculation based on the dielectric continuum [email protected] [email protected] [email protected]

What Does It Drive the Relationship Between Suicides and Economic Conditions? New Evidence from Spain

In this paper we analyse suicides across the 17 Spanish regions over the period 2002?2013. In doing so, we estimate count panel data models considering gender differences taking into account before and during economic crisis periods. A range of aggregate socioeconomic regional-level factors have been considered. Our empirical results show that: (1) a socioeconomic urban?rural suicide differentials exist, (2) there exists a Mediterranean suicide pattern; and (3) unemployment levels have a marked importance during the crisis period. The results of this study may have usefulness for suicide prevention in Spain

Trends in early childhood obesity in a large, urban school district in the Southwest from 2007 to 2014.

Presented at: Experimental Biology 2016; April 2-6, 2016; San Diego, CA.https://digitalrepository.unm.edu/prc-posters-presentations/1022/thumbnail.jp