Past research has provided support for the existence of a negativity bias, the tendency for negativity to have a stronger impact than positivity. Theoretically, the negativity bias provides an ...evolutionary advantage, as it is more critical for survival to avoid a harmful stimulus than to pursue a potentially helpful one. The current paper reviews the theoretical grounding of the negativity bias in the Evaluative Space Model, and presents recent findings using a multilevel approach that further elucidate the mechanisms underlying the negativity bias and underscore the importance of the negativity bias for human functioning.
We assess evidence relevant to Earth's equilibrium climate sensitivity per doubling of atmospheric CO2, characterized by an effective sensitivity S . This evidence includes feedback process ...understanding, the historical climate record, and the paleoclimate record. An S value lower than 2 K is difficult to reconcile with any of the three lines of evidence. The amount of cooling during the Last Glacial Maximum provides strong evidence against values of S greater than 4.5 K. Other lines of evidence in combination also show that this is relatively unlikely. We use a Bayesian approach to produce a probability density (PDF) for S given all the evidence, including tests of robustness to difficult‐to‐quantify uncertainties and different priors. The 66% range is 2.6‐3.9 K for our Baseline calculation, and remains within 2.3‐4.5 K under the robustness tests; corresponding 5‐95% ranges are 2.3‐4.7 K, bounded by 2.0‐5.7 K (although such high‐confidence ranges should be regarded more cautiously). This indicates a stronger constraint on S than reported in past assessments, by lifting the low end of the range. This narrowing occurs because the three lines of evidence agree and are judged to be largely independent, and because of greater confidence in understanding feedback processes and in combining evidence. We identify promising avenues for further narrowing the range in S , in particular using comprehensive models and process understanding to address limitations in the traditional forcing‐feedback paradigm for interpreting past changes.
Colloidal nanoplatelets are atomically flat, quasi-two-dimensional sheets of semiconductor that can exhibit efficient, spectrally pure fluorescence. Despite intense interest in their properties, the ...mechanism behind their highly anisotropic shape and precise atomic-scale thickness remains unclear, and even counter-intuitive for commonly studied nanoplatelets that arise from isotropic crystal structures (such as zincblende CdSe and lead halide perovskites). Here we show that an intrinsic instability in growth kinetics can lead to such highly anisotropic shapes. By combining experimental results on the synthesis of CdSe nanoplatelets with theory predicting enhanced growth on narrow surface facets, we develop a model that explains nanoplatelet formation as well as observed dependencies on time and temperature. Based on standard concepts of volume, surface and edge energies, the resulting growth instability criterion can be directly applied to other crystalline materials. Thus, knowledge of this previously unknown mechanism for controlling shape at the nanoscale can lead to broader libraries of quasi-two-dimensional materials.
Quantum computers hold the promise of solving computational problems that are intractable using conventional methods
. For fault-tolerant operation, quantum computers must correct errors occurring ...owing to unavoidable decoherence and limited control accuracy
. Here we demonstrate quantum error correction using the surface code, which is known for its exceptionally high tolerance to errors
. Using 17 physical qubits in a superconducting circuit, we encode quantum information in a distance-three logical qubit, building on recent distance-two error-detection experiments
. In an error-correction cycle taking only 1.1 μs, we demonstrate the preservation of four cardinal states of the logical qubit. Repeatedly executing the cycle, we measure and decode both bit-flip and phase-flip error syndromes using a minimum-weight perfect-matching algorithm in an error-model-free approach and apply corrections in post-processing. We find a low logical error probability of 3% per cycle when rejecting experimental runs in which leakage is detected. The measured characteristics of our device agree well with a numerical model. Our demonstration of repeated, fast and high-performance quantum error-correction cycles, together with recent advances in ion traps
, support our understanding that fault-tolerant quantum computation will be practically realizable.
InChinese Economic Statecraft, William J. Norris introduces an innovative theory that pinpoints how states employ economic tools of national power to pursue their strategic objectives. Norris shows ...what Chinese economic statecraft is, how it works, and why it is more or less effective. Norris provides an accessible tool kit to help us better understand important economic developments in the People's Republic of China. He links domestic Chinese political economy with the international ramifications of China's economic power as a tool for realizing China's strategic foreign policy interests. He presents a novel approach to studying economic statecraft that calls attention to the central challenge of how the state is (or is not) able to control and direct the behavior of economic actors.
Norris identifies key causes of Chinese state control through tightly structured, substate and crossnational comparisons of business-government relations. These cases range across three important arenas of China's grand strategy that prominently feature a strategic role for economics: China's efforts to secure access to vital raw materials located abroad, Mainland relations toward Taiwan, and China's sovereign wealth funds. Norris spent more than two years conducting field research in China and Taiwan during which he interviewed current and former government officials, academics, bankers, journalists, advisors, lawyers, and businesspeople. The ideas in this book are applicable beyond China and help us to understand how states exercise international economic power in the twenty-first century.
The recent association of several short gamma-ray bursts (GRBs) with early-type galaxies with low star formation rates demonstrates that short bursts arise from a different progenitor mechanism than ...long bursts. However, since the duration distributions of the two classes overlap, membership is not always easily established. The picture is complicated by occasional softer, extended emission lasting tens of seconds after the initial spikelike emission comprising an otherwise short burst. Using the large BATSE sample with time-tagged event (TTE) data, we show that the fundamental defining characteristic of the short-burst class is that the initial spike exhibits negligible spectral evolution at energies above 625 keV. The behavior is nearly ubiquitous for the 260 bursts with T sub(90) < 2 s for which the BATSE TTE data type completely included the initial spike. We find this same signature--negligible spectral lag--for six Swift BAT short bursts and one HETE-2 short burst. We also analyze a small sample of "short" BATSE bursts--those with the most fluent, intense extended emission. The same lack of evolution on the pulse timescale obtains for the extended emission in the brighter bursts for which significant measurements can be made. We also show that the dynamic range in the ratio of peak intensities, spike:extended, is 6 10 super(4). However, for our BATSE sample the total counts fluence of the extended component equals or exceeds that in the spike by a factor of several. A high Lorentz factor, 6500-1000, might explain the negligible lags.
Gamma-ray bursts (GRBs) are flashes of high-energy radiation arising from energetic cosmic explosions. Bursts of long (greater than two seconds) duration are produced by the core-collapse of massive ...stars
, and those of short (less than two seconds) duration by the merger of compact objects, such as two neutron stars
. A third class of events with hybrid high-energy properties was identified
, but never conclusively linked to a stellar progenitor. The lack of bright supernovae rules out typical core-collapse explosions
, but their distance scales prevent sensitive searches for direct signatures of a progenitor system. Only tentative evidence for a kilonova has been presented
. Here we report observations of the exceptionally bright GRB 211211A, which classify it as a hybrid event and constrain its distance scale to only 346 megaparsecs. Our measurements indicate that its lower-energy (from ultraviolet to near-infrared) counterpart is powered by a luminous (approximately 10
erg per second) kilonova possibly formed in the ejecta of a compact object merger.
Nanostructured semiconductors emit light from electronic states known as excitons. For organic materials, Hund's rules state that the lowest-energy exciton is a poorly emitting triplet state. For ...inorganic semiconductors, similar rules predict an analogue of this triplet state known as the 'dark exciton'. Because dark excitons release photons slowly, hindering emission from inorganic nanostructures, materials that disobey these rules have been sought. However, despite considerable experimental and theoretical efforts, no inorganic semiconductors have been identified in which the lowest exciton is bright. Here we show that the lowest exciton in caesium lead halide perovskites (CsPbX
, with X = Cl, Br or I) involves a highly emissive triplet state. We first use an effective-mass model and group theory to demonstrate the possibility of such a state existing, which can occur when the strong spin-orbit coupling in the conduction band of a perovskite is combined with the Rashba effect. We then apply our model to CsPbX
nanocrystals, and measure size- and composition-dependent fluorescence at the single-nanocrystal level. The bright triplet character of the lowest exciton explains the anomalous photon-emission rates of these materials, which emit about 20 and 1,000 times faster than any other semiconductor nanocrystal at room and cryogenic temperatures, respectively. The existence of this bright triplet exciton is further confirmed by analysis of the fine structure in low-temperature fluorescence spectra. For semiconductor nanocrystals, which are already used in lighting, lasers and displays, these excitons could lead to materials with brighter emission. More generally, our results provide criteria for identifying other semiconductors that exhibit bright excitons, with potential implications for optoelectronic devices.
Doped Nanocrystals Norris, David J; Efros, Alexander L; Erwin, Steven C
Science (American Association for the Advancement of Science),
03/2008, Letnik:
319, Številka:
5871
Journal Article
Recenzirano
The critical role that dopants play in semiconductor devices has stimulated research on the properties and the potential applications of semiconductor nanocrystals, or colloidal quantum dots, doped ...with intentional impurities. We review advances in the chemical synthesis of doped nanocrystals, in the theoretical understanding of the fundamental mechanisms that control doping, and in the creation of highly conducting nanocrystalline films. Because impurities can be used to alter the properties of nanoscale materials in desirable and controllable ways, doped nanocrystals can address key problems in applications from solar cells to bioimaging.
The flagellum is one of the most sophisticated self-assembling molecular machines in bacteria. Powered by the proton-motive force, the flagellum rapidly rotates in either a clockwise or ...counterclockwise direction, which ultimately controls bacterial motility and behavior. Escherichia coli and Salmonella enterica have served as important model systems for extensive genetic, biochemical, and structural analysis of the flagellum, providing unparalleled insights into its structure, function, and gene regulation. Despite these advances, our understanding of flagellar assembly and rotational mechanisms remains incomplete, in part because of the limited structural information available regarding the intact rotor–stator complex and secretion apparatus. Cryo-electron tomography (cryo-ET) has become a valuable imaging technique capable of visualizing the intact flagellar motor in cells at molecular resolution. Because the resolution that can be achieved by cryo-ET with large bacteria (such as E. coli and S. enterica) is limited, analysis of small-diameter bacteria (including Borrelia burgdorferi and Campylobacter jejuni) can provide additional insights into the in situ structure of the flagellar motor and other cellular components. This review is focused on the application of cryo-ET, in combination with genetic and biophysical approaches, to the study of flagellar structures and its potential for improving the understanding of rotor–stator interactions, the rotational switching mechanism, and the secretion and assembly of flagellar components.
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