Catálogo de publicaciones - revistas

<jats:title>Short wavelengths birth shorter ones</jats:title> <jats:p> The shortest laser pulses—with durations measured in attoseconds—arise from a process termed high-harmonic generation (HHG). Essentially, a longer, “driving” pulse draws electrons out of gaseous atoms like a slingshot, and, when they ricochet back, light emerges at shorter wavelengths. Most HHG has been carried out using light near the visible/infrared boundary for the driving pulse. Popmintchev <jats:italic>et al.</jats:italic> used an ultraviolet driving pulse instead, which yielded an unexpectedly efficient outcome. These results could presage a more generally efficient means of creating x-ray pulses for fundamental dynamics studies as well as technological applications. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. 1225 </jats:p>

<jats:title>Mixing interactions and superlattices</jats:title> <jats:p> Under the influence of an external magnetic field, the energies of electrons in two-dimensional systems group into the so-called Landau levels. In the cleanest samples, interactions among electrons lead to fractional quantum Hall (FQH) states. If such a system is then subjected to a superlattice potential, it is unclear whether the fragile FQH states will survive. To address this question, Wang <jats:italic>et al.</jats:italic> sandwiched graphene between two layers of hexagonal boron nitride. Transport measurements on the superlattice showed that some FQH states did survive. Furthermore, the interplay between interactions and the superlattice potential produced additional, anomalous states. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. 1231 </jats:p>

<jats:title>Faster elimination inside a cavity</jats:title> <jats:p> Metals are adept at shuffling molecular bonds. They pry apart two atoms and then pair each one with a different partner. Sometimes the atoms get stuck on the metal, though, and the newly partnered products aren't released. Kaphan <jats:italic>et al.</jats:italic> designed a strategy for accelerating this elimination process (see the Perspective by Yan and Fujita). A hollow supramolecular capsule captured a gold or platinum complex and induced rapid bond formation between the carbon atoms in methyl groups bound to the metal. Generalization of this strategy could open the door to a wide range of chemical transformations that are currently held up by slow eliminations. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. 1235; see also p. 1165 </jats:p>

<jats:title>Electron acceleration in solar flares</jats:title> <jats:p> Magnetic reconnection during a solar flare releases energy into the Sun's atmosphere, some of which is converted into accelerated particles in the plasma. Chen <jats:italic>et al.</jats:italic> combined radio and ultraviolet observations of a solar flare to identify the termination shock region where electrons are accelerated to relativistic speeds. </jats:p> <jats:p>They confirmed these results with magneto-hydrodynamic simulations. This improved knowledge of the mechanism behind flares improves our understanding of the solar wind and space weather.</jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. 1238 </jats:p>

<jats:title>Magnetic fields near the event horizon</jats:title> <jats:p> Astronomers have long sought to examine a black hole's event horizon—the boundary around the black hole within which nothing can escape. Johnson <jats:italic>et al.</jats:italic> used sophisticated interferometry techniques to combine data from millimeter-wavelength telescopes around the world. They measured polarization just outside the event horizon of Sgr A*, the supermassive black hole at the center of our galaxy, the Milky Way. The polarization is a signature of ordered magnetic fields generated in the accretion disk around the black hole. The results help to explain how black holes accrete gas and launch jets of material into their surroundings. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. 1242 </jats:p>

<jats:title>Toward a level playing field?</jats:title> <jats:p>Do free learning resources benefit the disadvantaged and decrease gaps between rich and poor? Hansen and Reich studied the relationships between socioeconomic status (SES) and enrollment in and completion of free Massive Open Online Courses (MOOCs) offered by Harvard and MIT. Students from low-SES backgrounds were less likely to enroll in MOOCs and earn a certificate than their high-SES peers. Thus, although there are many free online learning opportunities, it is not safe to assume that they will “level the playing field.”</jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. 1245 </jats:p>

<jats:title>Local decisions with global consequences</jats:title><jats:p>Some estimates suggest that humanity has already exceeded our sustainable global water footprint: the balance between fresh water use and supply. It seems that the situation may be more unsustainable than we realize. Jaramillo and Destouni analyzed hydroclimatic data for 100 large basins dating back to 1901. Better accounting of local water use revealed larger than anticipated effects on the global water cycle. For example, local regulation of surface water flow and expanded regional irrigation activities have increased global evapotranspiration rates.</jats:p><jats:p><jats:italic>Science</jats:italic>, this issue p. 1248</jats:p>

<jats:title>Maturation of olfactory neurons</jats:title> <jats:p> The sense of smell depends on neurons in the olfactory epithelium to perceive chemical scents. Each neuron specializes with one receptor. Hanchate <jats:italic>et al.</jats:italic> now show that the one-for-one relationship is not as simple as thought. As new neurons develop to replenish the olfactory epithelium, they initially express several different alleles of olfactory receptors. Then, as each neuron matures, they specialize to express a single receptor. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. 1251 </jats:p>

<jats:title>Not enough protection for migrating birds</jats:title> <jats:p> Animals that migrate pass through a varying number of regions. Each of these regions contributes to a different component of their life cycles. Runge <jats:italic>et al.</jats:italic> looked at the degree of protection migratory birds receive, globally, across their breeding and wintering ranges. A remarkably low percentage of migratory birds receive adequate protection across their entire ranges. Given that over half the world's migratory bird populations are declining, these results emphasize the urgency with which we must act to protect birds across their entire migratory cycle. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. 1255 </jats:p>

<jats:title>Broad versus restricted expression</jats:title> <jats:p> Color vision in fruit flies requires the restricted expression of light-sensing rhodopsins with different wavelength sensitivities in subsets of photoreceptors. However, all photoreceptors express factors that transduce and amplify the visual signal. Rister <jats:italic>et al.</jats:italic> found that the distinct expression patterns are determined by a highly tunable regulatory motif. Genes that are broadly expressed have a palindromic variant of the motif. Spatially restricted rhodopsin genes display single-base-pair changes that alter the symmetry of the palindrome and are critical for subtype-specific expression. These findings on the differential regulation of gene expression in fly photoreceptors have implications for the evolution of neuronal subtype diversity. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. 1258 </jats:p>