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Topological insulators—materials that are insulating in the bulk but allow electrons to flow on their surface—are striking examples of materials in which topological invariants are manifested in robustness against perturbations such as defects and disorder 1 . Their most prominent feature is the emergence of edge states at the boundary between areas with different topological properties. The observable physical effect is unidirectional robust transport of these edge states. Topological insulators were originally observed in the integer quantum Hall effect 2 (in which conductance is quantized in a strong magnetic field) and subsequently suggested 3 – 5 and observed 6 to exist without a magnetic field, by virtue of other effects such as strong spin–orbit interaction. These were systems of correlated electrons. During the past decade, the concepts of topological physics have been introduced into other fields, including microwaves 7 , 8 , photonic systems 9 , 10 , cold atoms 11 , 12 , acoustics 13 , 14 and even mechanics 15 . Recently, topological insulators were suggested to be possible in exciton-polariton systems 16 – 18 organized as honeycomb (graphene-like) lattices, under the influence of a magnetic field. Exciton-polaritons are part-light, part-matter quasiparticles that emerge from strong coupling of quantum-well excitons and cavity photons 19 . Accordingly, the predicted topological effects differ from all those demonstrated thus far. Here we demonstrate experimentally an exciton-polariton topological insulator. Our lattice of coupled semiconductor microcavities is excited non-resonantly by a laser, and an applied magnetic field leads to the unidirectional flow of a polariton wavepacket around the edge of the array. This chiral edge mode is populated by a polariton condensation mechanism. We use scanning imaging techniques in real space and Fourier space to measure photoluminescence and thus visualize the mode as it propagates. We demonstrate that the topological edge mode goes around defects, and that its propagation direction can be reversed by inverting the applied magnetic field. Our exciton-polariton topological insulator paves the way for topological phenomena that involve light–matter interaction, amplification and the interaction of exciton-polaritons as a nonlinear many-body system. A part-light, part-matter exciton-polariton topological insulator is created in an array of semiconductor microcavities.

Air pollution is growing fastest in monsoon-impacted South Asia. During the dry winter monsoon, the fumes disperse toward the Indian Ocean, creating a vast pollution haze. The fate of these fumes during the wet summer monsoon has been unclear. Lelieveld et al. performed atmospheric chemistry measurements by aircraft in the Oxidation Mechanism Observations campaign, sampling the summer monsoon outflow in the upper troposphere between the Mediterranean and the Indian Ocean. The measurements, supported by model calculations, show that the monsoon sustains a remarkably efficient cleansing mechanism in which contaminants are rapidly oxidized and deposited on Earth's surface. However, some pollutants are lofted above the monsoon clouds and chemically processed in a reactive reservoir before being redistributed globally, including to the stratosphere. Science , this issue p. 270 The South Asian monsoon removes some air pollution and disperses the rest. Air pollution is growing fastest in monsoon-affected South Asia. During the dry winter monsoon, the fumes disperse toward the Indian Ocean, creating a vast pollution haze, but their fate during the wet summer monsoon has been unclear. We performed atmospheric chemistry measurements by aircraft in the Oxidation Mechanism Observations campaign, sampling the summer monsoon outflow in the upper troposphere between the Mediterranean and the Indian Ocean. The measurements, supported by model calculations, show that the monsoon sustains a remarkably efficient cleansing mechanism by which contaminants are rapidly oxidized and deposited to Earth’s surface. However, some pollutants are lofted above the monsoon clouds and chemically processed in a reactive reservoir before being redistributed globally, including to the stratosphere.

Forest self-reliance Measurements taken by aircraft flying over the Amazon rain forest reveal unexpectedly high concentrations of hydroxyl radicals in the lower atmosphere. Hydroxyl is the primary atmospheric oxidant, and it was conventional wisdom that large forest emissions of hydrocarbons strongly reduce the atmospheric oxidation capacity. The new data suggest that this is not the case, and that the pristine forest can 'manage' its atmospheric sustainability remarkably well. A possible mechanism is suggested: hydroxyl radicals may be recycling via the natural oxidation of volatile organic compounds, mainly isoprene. In the absence of external influences, the forest seems able to maintain a benign atmosphere. But where deforestation and anthropogenic emissions of NO intervene, photochemical air pollution remains likely. Aircraft measurements of atmospheric trace gases performed over the pristine Amazon forest find unexpectedly high hydroxyl radical concentrations. On the basis of a model study and the results of laboratory experiments, it is proposed that natural volatile organic compounds oxidation, notably of isoprene, recycles hydroxyl radical efficiently through reactions of organic peroxy radicals, which may be able to explain the high hydroxyl radical levels observed. Terrestrial vegetation, especially tropical rain forest, releases vast quantities of volatile organic compounds (VOCs) to the atmosphere 1 , 2 , 3 , which are removed by oxidation reactions and deposition of reaction products 4 , 5 , 6 . The oxidation is mainly initiated by hydroxyl radicals (OH), primarily formed through the photodissociation of ozone 4 . Previously it was thought that, in unpolluted air, biogenic VOCs deplete OH and reduce the atmospheric oxidation capacity 5 , 6 , 7 , 8 , 9 , 10 . Conversely, in polluted air VOC oxidation leads to noxious oxidant build-up by the catalytic action of nitrogen oxides 5 , 6 , 7 , 8 , 9 , 10 (NO x = NO + NO 2 ). Here we report aircraft measurements of atmospheric trace gases performed over the pristine Amazon forest. Our data reveal unexpectedly high OH concentrations. We propose that natural VOC oxidation, notably of isoprene, recycles OH efficiently in low-NO x air through reactions of organic peroxy radicals. Computations with an atmospheric chemistry model and the results of laboratory experiments suggest that an OH recycling efficiency of 40–80 per cent in isoprene oxidation may be able to explain the high OH levels we observed in the field. Although further laboratory studies are necessary to explore the chemical mechanism responsible for OH recycling in more detail, our results demonstrate that the biosphere maintains a remarkable balance with the atmospheric environment.

The Central Atlantic Magmatic Province (CAMP) is the most aerially extensive magmatic event in Earth’s history, but many questions remain about its origin, volume, and distribution. Despite many observations of CAMP magmatism near Earth’s surface, few constraints exist on CAMP intrusions at depth. Here we present detailed constraints on crustal and upper mantle structure from wide-angle seismic data across the Triassic South Georgia Rift that formed shortly before CAMP. Lower crustal magmatism is concentrated where synrift sedimentary fill is thickest and the crust is thinnest, suggesting that lithospheric thinning influenced the locus and volume of magmatism. The limited distribution of lower crustal intrusions implies modest total CAMP volumes of 85,000 to 169,000 km 3 beneath the South Georgia Rift, consistent with moderately elevated mantle potential temperatures (<1500 °C). These results suggest that CAMP magmatism in the South Georgia Rift is caused by syn-rift decompression melting of a warm, enriched mantle. The Central Atlantic Magmatic Province is the most aerially extensive magmatic event in Earth’s history, yet few constraints exist on the volumes of intrusions at depth. Here, the authors find limited intrusive volumes beneath the South Georgia Rift, consistent with modest potential mantle temperatures (<1500 °C) related to syn-rift decompression melting.

Helicobacter pylori is the principal cause of gastric cancer, the second leading cause of cancer mortality worldwide. However, H. pylori prevalence generally does not predict cancer incidence. To determine whether coevolution between host and pathogen influences disease risk, we examined the association between the severity of gastric lesions and patterns of genomic variation in matched human and H. pylori samples. Patients were recruited from two geographically distinct Colombian populations with significantly different incidences of gastric cancer, but virtually identical prevalence of H. pylori infection. All H. pylori isolates contained the genetic signatures of multiple ancestries, with an ancestral African cluster predominating in a low-risk, coastal population and a European cluster in a high-risk, mountain population. The human ancestry of the biopsied individuals also varied with geography, with mostly African ancestry in the coastal region (58%), and mostly Amerindian ancestry in the mountain region (67%). The interaction between the host and pathogen ancestries completely accounted for the difference in the severity of gastric lesions in the two regions of Colombia. In particular, African H. pylori ancestry was relatively benign in humans of African ancestry but was deleterious in individuals with substantial Amerindian ancestry. Thus, coevolution likely modulated disease risk, and the disruption of coevolved human and H. pylori genomes can explain the high incidence of gastric disease in the mountain population.

Airborne organic particles affect Earth’s climate. Imaging of particles after rain events and experimental irrigation shows that water drop impaction of soils generates solid organic particles, with impacts on clouds and radiation absorption. Airborne organic particles play a critical role in Earth’s climate 1 , public health 2 , air quality 3 , and hydrological and carbon cycles 4 . However, sources and formation mechanisms for semi-solid and solid organic particles 5 are poorly understood and typically neglected in atmospheric models 6 . Laboratory evidence suggests that fine particles can be formed from impaction of mineral surfaces by droplets 7 . Here, we use chemical imaging of particles collected following rain events in the Southern Great Plains, Oklahoma, USA and after experimental irrigation to show that raindrop impaction of soils generates solid organic particles. We find that after rain events, sub-micrometre solid particles, with a chemical composition consistent with soil organic matter, contributed up to 60% of atmospheric particles. Our irrigation experiments indicate that intensive water impaction is sufficient to cause ejection of airborne soil organic particles from the soil surface. Chemical imaging and micro-spectroscopy analysis of particle physico-chemical properties suggest that these particles may have important impacts on cloud formation and efficiently absorb solar radiation. We suggest that raindrop-induced formation of solid organic particles from soils may be a widespread phenomenon in ecosystems such as agricultural systems and grasslands where soils are exposed to strong, episodic precipitation events 8 .

In the Amazon basin, particles containing mixed sodium salts are routinely observed and are attributed to marine aerosols transported from the Atlantic Ocean. Using chemical imaging analysis, we show that, during the wet season, fungal spores emitted by the forest biosphere contribute at least 30% (by number) to sodium salt particles in the central Amazon basin. Hydration experiments indicate that sodium content in fungal spores governs their growth factors. Modeling results suggest that fungal spores account for ~69% (31–95%) of the total sodium mass during the wet season and that their fractional contribution increases during nighttime. Contrary to common assumptions that sodium-containing aerosols originate primarily from marine sources, our results suggest that locally-emitted fungal spores contribute substantially to the number and mass of coarse particles containing sodium. Hence, their role in cloud formation and contribution to salt cycles and the terrestrial ecosystem in the Amazon basin warrant further consideration. Salt particles in the Amazon basin are typically attributed to marine aerosols transported from the Atlantic Ocean. Here the authors show the potential importance of fungal spores as a source of sodium-salt particles in the Amazon rainforest.

PurposeExercise levels often decline following cancer diagnosis despite growing evidence of its benefits. Treatment side effects, older age, lack of confidence and opportunity to exercise with others in similar circumstances influence this. Our study explored the experiences of people attending a cancer-specific community-based exercise programme (CU Fitter™).MethodsA survey distributed to those attending the programme gathered demographic/clinical information, self-reported exercise levels, information provision and barriers to/benefits of exercise.ResultsSixty surveys were evaluable from 65/100 returned (62% female, 68% > 60 years, 66% breast/prostate cancer). Most (68%) were receiving treatment. Sixty-eight percent attended classes once or twice weekly. Fifty-five percent received exercise advice after diagnosis, usually from their hospital doctor/nurse. More (73%) had read about exercising, but less used the Internet to source information (32%). Self-reported exercise levels were higher currently than before diagnosis (p = 0.05). Forty-eight percent said their primary barrier to exercising was the physical impact of cancer/treatment. Improving fitness/health (40%) and social support (16%) were the most important gains from the programme. Many (67%) had made other lifestyle changes and intented to keep (50%) or increase (30%) exercising.ConclusionsThis community-based cancer-specific exercise approach engaged people with cancer and showed physical, psychological, and social benefits.Implications for cancer survivorsCommunity-grown exercise initiatives bring cancer survivors together creating their own supportive environment. Combining this with instructors familiar with the population and providing an open-ended service may prove particularly motivating and beneficial. Further work is required to provide evidence for this.

Purpose Yoga is increasingly used as a complementary therapy to manage disease and treatment-related side effects in patients with cancer and has resulted in an increase in the number of studies exploring the effectiveness of yoga interventions. This systematic review examines whether yoga interventions provide any measurable benefit, both physically and psychologically, for women with breast cancer. The results will inform future research in this field and advance the development of yoga programmes. Methods We performed electronic searches of MEDLINE, PsychINFO, the Cochrane Library, Embase, CINAHL, AMED, Web of Science and Scopus for articles published up to June 2012. Only randomised controlled trials (RCTs) were included and methodological quality rating scores were determined using the PEDro (Physiotherapy Evidence Database) Scale. Results One hundred thirty-two studies were identified through a systematic search of eight electronic databases. Only published manuscripts that employed a RCT design were included ( n  = 18). The sample sizes for these studies varied widely from 18 to 164 participants and the associated PEDro scores ranged from 1 (poor) to 8 (good). All 18 studies reported positive effects for treatment-related side effects in favour of the yoga interventions, with the greatest impact on global quality of life (QoL) scores and emotional well-being. Conclusion Results from the few RCTs suggest there is moderate to good evidence that yoga may be a useful practice for women recovering from breast cancer treatments. Large-scale RCTs using objective measures and patient-reported outcomes with long-term follow-up are needed to substantiate whether the benefits are true and sustainable.

Atmospheric concentrations of nitrous acid (HONO), one of the major precursors of the hydroxyl radical (OH) in the troposphere, significantly exceed the values predicted by the assumption of a photostationary state (PSS) during daytime. Therefore, additional sources of HONO were intensively investigated in the last decades. This study presents budget calculations of HONO based on simultaneous measurements of all relevant species, including HONO and OH at two different measurement heights, i.e. 1 m above the ground and about 2 to 3 m above the canopy (24 m above the ground), conducted in a boreal forest environment. We observed mean HONO concentrations of about 6.5 × 108 molecules cm−3 (26 ppt) during daytime, more than 20 times higher than expected from the PSS of 0.2 × 108 molecules cm−3 (1 ppt). To close the budgets at both heights, a strong additional source term during daytime is required. This unidentified source is at its maximum at noon (up to 1.1 × 106 molecules cm−3 s−1, 160 ppt h−1) and in general up to 2.3 times stronger above the canopy than close to the ground. The insignificance of known gas phase reactions and other processes like dry deposition or advection compared to the photolytic decomposition of HONO at this measurement site was an ideal prerequisite to study possible correlations of this unknown term to proposed HONO sources. But neither the proposed emissions from soils nor the proposed photolysis of adsorbed HNO3 contributed substantially to the unknown source. However, the unknown source was found to be perfectly correlated to the unbalanced photolytic loss of HONO.