Catálogo de publicaciones - revistas

<jats:title>Superconducting Quantum Optics</jats:title> <jats:p> The coherence properties of superconducting circuits enable them to be developed as qubits in quantum information processing applications. <jats:bold> Astafiev <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="840" related-article-type="in-this-issue" vol="327" xlink:href="10.1126/science.1181918">840</jats:related-article> ) now show that these macroscopic superconducting devices also behave as artificial atoms and can exhibit quantum optical effects. The ability to fabricate and integrate these superconducting devices in electronic circuitry may help toward developing a fully controlled quantum optics system on a chip. </jats:p>

<jats:title>Wave-Particle Duality</jats:title> <jats:p> The dual-wave nature of particles is nowhere more evident than in a confined space, where standing waves are formed with wavelengths that depend on particle energy. This so-called quantum interference has been observed in nanostructures using surface probes such as scanning tunneling microscopy. Now, <jats:bold> Oka <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="843" related-article-type="in-this-issue" vol="327" xlink:href="10.1126/science.1183224">843</jats:related-article> ) use the spin-polarized version of this technique to study spin-dependent quantum interference on a triangular nanoscale cobalt island deposited on a copper surface. They observe the modulation of the magnetization, with the pattern depending on the energy of the interfering electrons. The experimental results are in good agreement with simulations, which indicate that the magnetization at a given energy and position largely depends on which of two electron spin states present dominates. </jats:p>

<jats:title>Many Mixed Linkers in MOFs</jats:title> <jats:p> Crystallization can separate different molecules because different molecules cannot generally be accommodated equally well in the same crystal lattice. However, in metal-organic framework (MOF) compounds, the organic linkers do not pack closely to other parts of the lattice, so it may be possible to mix several linkers that are derivatives of a parent compound with the same end groups. <jats:bold> Deng <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="846" related-article-type="in-this-issue" vol="327" xlink:href="10.1126/science.1181761">846</jats:related-article> ) show that zinc-based MOFs can be made that mix 1,4-benzenedicarboxylate and up to eight of its derivatives in a random fashion. The effects of such mixing on porosity and absorption characteristics is nonlinear; in one case, a mixed-linker compound was four times better for selecting CO <jats:sub>2</jats:sub> versus CO compared with the best MOF bearing only one of the component linkers. </jats:p>

<jats:title>From Steps to Clusters</jats:title> <jats:p> When a flat surface of a single crystal is formed by cutting or cleavage, the atoms may move little from their bulk positions, or the surface may reconstruct as the atoms move to more energetically favorable positions. The adsorption of molecules can also change the energetic landscape and cause reconstruction. <jats:bold> Tao <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="850" related-article-type="in-this-issue" vol="327" xlink:href="10.1126/science.1182122">850</jats:related-article> ; see the Perspective by <jats:bold>Altman</jats:bold> ) examined “stepped” platinum surfaces, the (557) and (332) surfaces in which flat terraces are connected by atomic steps. Scanning tunneling microscopy and x-ray photoelectron spectroscopy revealed a reversible breakup into nanometer-scale clusters when CO surface coverages were very high. Density functional theory calculations suggest that this new morphology increases the number of edge sites for adsorption and relieves unfavorable CO-CO repulsions. </jats:p>

<jats:title>Colliding in the Cold</jats:title> <jats:p> Chemical reactions occur through molecular collisions, which, in turn, are governed by the distributions of energy in each colliding partner. What happens when molecules are cooled so that they no longer have sufficient energy to collide? <jats:bold> Ospelkaus <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="853" related-article-type="in-this-issue" vol="327" xlink:href="10.1126/science.1184121">853</jats:related-article> ; see the Perspective by <jats:bold>Hutson</jats:bold> ) explored this question by preparing a laser-cooled sample of potassium rubidium (KRb) diatomics with barely any residual energy in any form (translational, rotational, vibrational, or electronic). By monitoring heat release over time, evidence was gathered for exothermic atom exchange reactivity through quantum mechanical tunneling. As predicted by theory, these reactions were exquisitely sensitive to the molecular states, with rates changing by orders of magnitude on varying minor factors such as nuclear spin orientation. </jats:p>

<jats:title>Salty Stretch</jats:title> <jats:p> What happens at the molecular level when salt dissolves in water? Much of the data characterizing the geometry and dynamics of ion solvation shells has come from indirect observation of the surrounding water structure. Using a time domain Raman technique based on the interference of four ultrashort polarized light pulses, <jats:bold>Heisler and Meech</jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="857" related-article-type="in-this-issue" vol="327" xlink:href="10.1126/science.1183799">857</jats:related-article> ) have now mapped directly the stretching vibrations associated with the weak hydrogen bonding interactions between bulk water molecules and chloride, bromide, or iodide ions. </jats:p>

<jats:title>Standing High</jats:title> <jats:p> Sea-level rises and falls as Earth's giant ice sheets shrink and grow. It has been thought that sea level around 81,000 years ago—well into the last glacial period—was 15 to 20 meters below that of today and, thus, that the ice sheets were more extensive. <jats:bold> Dorale <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="860" related-article-type="in-this-issue" vol="327" xlink:href="10.1126/science.1181725">860</jats:related-article> ; see the Perspective by <jats:bold>Edwards</jats:bold> ) now challenge this view. A speleothem that has been intermittently submerged in a cave on the island of Mallorca was dated to show that, historically, sea level was more than a meter above its present height. This data implies that temperatures were as high as or higher than now, even though the concentration of CO <jats:sub>2</jats:sub> in the atmosphere was much lower. </jats:p>

<jats:title>Of Mice and Men</jats:title> <jats:p> Just how closely must mouse models replicate the known features of human disorders to be accepted as useful for mechanistic and therapeutic studies? <jats:bold> Soliman <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="863" related-article-type="in-this-issue" vol="327" xlink:href="10.1126/science.1181886">863</jats:related-article> , published online 14 January) compared mice that vary only in their allelic composition at one position within the gene encoding brain-derived neurotrophic factor (BDNF) with humans exhibiting the same range of allelic variation. Individuals (mice and humans) carrying the allele that codes for a methionine-containing variant of BDNF retained a fearful response to a threatening stimulus even after its removal in comparison to those with the valine variant. Furthermore, in both cases, this linkage was mediated by diminished activity in the ventral-medial region of the prefrontal cortex. This deficit in extinction learning may contribute to differential responses to extinction-based therapies for anxiety disorders. </jats:p>

<jats:title>Decoding a Second-Messenger's Message</jats:title> <jats:p> Biofilms are aggregates of bacteria on a surface often associated with increased resistance to antibiotics and stress. In <jats:italic>Vibrio cholerae</jats:italic> , the bacterial species that causes cholera, biofilm formation is promoted by the bacterial second-messenger cyclic diguanylate (c-di-GMP) and involves the transcription regulator, VpsT. <jats:bold> Krasteva <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="866" related-article-type="in-this-issue" vol="327" xlink:href="10.1126/science.1181185">866</jats:related-article> ) show that VpsT is itself a receptor for c-di-GMP and that binding of the small signaling molecule promotes VpsT dimerization, which is required for DNA recognition and transcriptional regulation. As well as activating components of the biofilm pathway, VpsT also down-regulates motility genes in a c-di-GMP–dependent manner. </jats:p>

<jats:title>DNA-less Evolution</jats:title> <jats:p> Prions are proteinaceous infectious elements involved in a variety of neurodegenerative diseases, including scrapie in sheep and so-called mad cow disease in cattle. Now <jats:bold> Li <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="869" related-article-type="in-this-issue" vol="327" xlink:href="10.1126/science.1183218">869</jats:related-article> , published online 31 December) show that, when propagated in tissue culture cells, cloned prion populations become diverse by mutational events and can undergo selective amplification. Thus, even though devoid of a coding genome, prions, when propagated under a particular selection regime, can be subject to rapid evolution. </jats:p>