University of Maryland, College Park

A physicist peels back the layers of excitement about graphene.

Graphene is an atom-thick sheet of carbon in which electrons behave as if they have no mass. Atomic carbon layers have been grown epitaxially — that is, perfectly aligned with atoms in an underlying crystal surface — on metals and semiconductors for decades, so why the fuss lately?

Well, in the past few years much work in this field has revolved around graphene obtained by 'exfoliating' or peeling it from graphite. By mounting exfoliated graphene on insulating silicon dioxide, researchers observed a half-integer quantum Hall effect, an anomalous measurement that stems from the existence of a Landau level — the quantized orbit of electrons in a magnetic field — at exactly zero energy, a signature property of massless electrons.

But exfoliated graphene is dirty, lumpy and tiny (the biggest pieces are still a tenth of a millimetre in diameter). I wondered whether the older technique of epitaxial growth could produce a better material. In May, a group led by Joseph Stroscio showed that it could (D. L. Miller et al. Science 324, 924–927; 2009). Using scanning tunnelling spectroscopy to study epitaxial graphene on the surface of silicon carbide in a magnetic field, they showed that epitaxial graphene is extraordinarily clean and flat, and clearly exhibits the zero-energy Landau level. For the first time in any material, epitaxial graphene allows direct observation with atomic resolution of the behaviour of electrons in quantized Landau levels, opening a new window on the quantum Hall effect.

Researchers using other techniques, such as cyclotron resonance and photoemission, are also reporting an astonishingly clean electronic system in epitaxial graphene. Experiments on conductivity remain a challenge, but epitaxial graphene seems to have a bright future.

Posted by Brendan Maher at 08:39 PM | Permalink | Comments (1) | TrackBacks (0)

ETH Zürich, Switzerland

An infection biologist points out an outstanding issue in mucosal immunology.

The gut immune system can distinguish between harmless commensal microorganisms and dangerous pathogens, and attenuates its response to the former to avoid dangerous chronic inflammation. The mechanisms that maintain this hyporesponsiveness are just beginning to be unravelled.

Dendritic cells, the key organizers of appropriate immune responses, actively sample commensal microbes. In organs other than the gut, this would trigger a strong immune response, and the responsiveness of intestinal dendritic cells to microbes is thought to be thwarted by anti-inflammatory molecules released by gut cells. But the situation could be much more complex: hyporesponsiveness might be restricted to certain 'microbe-associated molecular patterns' (MAMPs), such as lipopolysaccharides, large molecules attached to the outer membrane of many bacteria.

Linda Klavinskis of Kings College London and her team have analysed the MAMP-responsiveness of dendritic cells migrating from gut tissue to local lymph nodes. Surprisingly, these cells do respond to harmless Bacillus spores and most MAMPs — but not lipopolysaccharides (V. Cerovic et al. J. Immunol. 182, 2405–2415; 2009). Does this suggest that hyporesponsiveness of intestinal dendritic cells is transient? The maintenance of hyporesponsiveness in the gut mucosa, patterns of MAMP-hyporesponsiveness, and localization and timing of MAMP responses will be important topics for future research.

Unfortunately, unactivated dendritic cells are hard to isolate from the gut mucosa. In situ analysis of dendritic-cell responses to gut microbes in intact tissue holds much promise. Technical advances in multicolour two-photon microscopy, fluorescently tagged microbes, and transgenic mice expressing cell-type and response-specific fluorescent reporter proteins will be instrumental in this key area of biology.

Posted by Brendan Maher at 07:31 PM | Permalink | Comments (0) | TrackBacks (0)

University of Texas, Austin, USA

A geophysicist ponders the mysteries of intraplate earthquakes.

During my first semester at college, I attended a lecture describing plate tectonics, and immediately knew that geophysics would be my major and hopefully my career. Subsequent lectures, textbooks, journal articles and, later, my own research educated me about how elegantly plate tectonics explains the processes that control the locations of most earthquakes.

However, some of the largest-known North American earthquakes — including those of 1811–12 famed for ringing church bells in Boston and changing the course of the Mississippi River — are associated with the New Madrid Seismic Zone (NMSZ) in the Southern and Midwestern United States, far from known plate boundaries. So what causes these events? Eric Calais of Purdue University in Indiana and Seth Stein at Northwestern University in Illinois present some surprising results from an examination of Global Positioning System (GPS) data from the region (E. Calais and S. Stein Science 323, 1442; 2009).

Previous studies found that the NMSZ was moving at a different rate and in a different direction from the North American Plate, implying that strain would steadily accumulate until released by a large-magnitude earthquake. But, incorporating three years' worth of extra GPS data, Calais and Stein found motions indistinguishable from those of the North American Plate, corresponding to extremely low strain rates. It is not clear what the underlying processes causing the NMSZ earthquakes are. Is strain accumulation variable over time in intraplate settings? What are the implications for hazard prediction?

The results leave me perplexed, but oddly comforted — there are plenty of mysteries left for the next generation of geophysicists. And perhaps one day a theory will elegantly explain intraplate seismicity, just as plate tectonics did for interplate seismicity.

Posted by Brendan Maher at 04:39 AM | Permalink | Comments (0) | TrackBacks (0)

Nicholas School of the Environment, Duke University, Durham, North Carolina

An ecologist calls for a citizen-science 'Wiki'.

Where do species occur and why? What happens to ecological communities when species are removed or when alien species invade? And how will the answers shift as climates change? These questions span huge spatial and temporal scales, and involve millions of species. By contrast, ecological field studies are generally of short duration, include few species and cover small areas. This means that getting data for the big questions is a tall order — impossible without harnessing a deeper reserve of people power.

Citizen science — in which qualified scientists oversee volunteers — is not new. The Audubon Society's Christmas Bird Count has run for 108 years and mobilizes about 60,000 volunteers across 1 million square kilometres of North America who count about 58 million individual birds annually. Other citizen-science projects are under way around the world.

Dirk Schmeller at the Helmholtz Centre for Environmental Research in Leipzig, Germany, and his colleagues analysed 395 citizen-science projects across five European countries, involving more than 46,000 participants (D. Schmeller Conserv. Biol. 23, 307–316; 2008). Volunteers donated more than 148,000 person-days per year, a figure inconceivable using professional scientists alone. Schmeller et al. found that volunteer-gathered data are reliable and unbiased, with data quality determined less by 'volunteer' status, and more by survey design and methodology.

The rapid increase in citizen-science data sets can revolutionize what we know about the natural world. Wikipedia has shown that the public is willing to donate time, talent and knowledge, given a sufficient platform. Biodiversity data lack such a platform for input, integration, mapping and dissemination. This is a deficiency that the environmental community should address.

Posted by Brendan Maher at 12:32 AM | Permalink | Comments (1) | TrackBacks (0)