Lanthanide contraction and magnetism in the heavy rare earth elements

The heavy rare earth elements crystallize into hexagonally close packed (h.c.p.) structures and share a common outer electronic configuration, differing only in the number of 4f electrons they have. These chemically inert 4f electrons set up localized magnetic moments, which are coupled via an indirect exchange interaction involving the conduction electrons. This leads to the formation of a wide variety of magnetic structures, the periodicities of which are often incommensurate with the underlying crystal lattice. Such incommensurate ordering is associated with a 'webbed' topology of the momentum space surface separating the occupied and unoccupied electron states (the Fermi surface). The shape of this surface-and hence the magnetic structure-for the heavy rare earth elements is known to depend on the ratio of the interplanar spacing c and the interatomic, intraplanar spacing a of the h.c.p. lattice. A theoretical understanding of this problem is, however, far from complete. Here, using gadolinium as a prototype for all the heavy rare earth elements, we generate a unified magnetic phase diagram, which unequivocally links the magnetic structures of the heavy rare earths to their lattice parameters. In addition to verifying the importance of the c/a ratio, we find that the atomic unit cell volume plays a separate, distinct role in determining the magnetic properties: we show that the trend from ferromagnetism to incommensurate ordering as atomic number increases is connected to the concomitant decrease in unit cell volume. This volume decrease occurs because of the so-called lanthanide contraction, where the addition of electrons to the poorly shielding 4f orbitals leads to an increase in effective nuclear charge and, correspondingly, a decrease in ionic radii.     

Transmission resonances through aperiodic arrays of subwavelength apertures

Resonantly enhanced light transmission through periodic subwavelength aperture arrays perforated in metallic films has generated significant interest because of potential applications in near-field microscopy, photolithography, displays, and thermal emission. The enhanced transmission was originally explained by a mechanism where surface plasmon polaritons (collective electronic excitations in the metal surface) mediate light transmission through the grating. In this picture, structural periodicity is perceived to be crucial in forming the transmission resonances. Here we demonstrate experimentally that, in contrast to the conventional view, sharp transmission resonances can be obtained from aperiodic aperture arrays. Terahertz transmission resonances are observed from several arrays in metallic films that exhibit unusual local n-fold rotational symmetries, where n = 10, 12, 18, 40 and 120. This is accomplished by using quasicrystals with long-range order, as well as a new type of 'quasicrystal approximates' in which the long-range order is somewhat relaxed. We find that strong transmission resonances also form in these aperiodic structures, at frequencies that closely match the discrete Fourier transform vectors in the aperture array structure factor. The shape of these resonances arises from Fano interference of the discrete resonances and the non-resonant transmission band continuum related to the individual holes. Our approach expands potential design parameters for aperture arrays that are aperiodic but contain discrete Fourier transform vectors, and opens new avenues for optoelectronic devices.     

Abrupt onset of a second energy gap at the superconducting transition of underdoped Bi2212

The superconducting gap--an energy scale tied to the superconducting phenomena--opens on the Fermi surface at the superconducting transition temperature (T(c)) in conventional BCS superconductors. In underdoped high-T(c) superconducting copper oxides, a pseudogap (whose relation to the superconducting gap remains a mystery) develops well above T(c) (refs 1, 2). Whether the pseudogap is a distinct phenomenon or the incoherent continuation of the superconducting gap above T(c) is one of the central questions in high-T(c) research. Although some experimental evidence suggests that the two gaps are distinct, this issue is still under intense debate. A crucial piece of evidence to firmly establish this two-gap picture is still missing: a direct and unambiguous observation of a single-particle gap tied to the superconducting transition as function of temperature. Here we report the discovery of such an energy gap in underdoped Bi2Sr2CaCu2O8+delta in the momentum space region overlooked in previous measurements. Near the diagonal of Cu-O bond direction (nodal direction), we found a gap that opens at T(c) and has a canonical (BCS-like) temperature dependence accompanied by the appearance of the so-called Bogoliubov quasi-particles, a classical signature of superconductivity. This is in sharp contrast to the pseudogap near the Cu-O bond direction (antinodal region) measured in earlier experiments.     

Structural basis for synthesis of inflammatory mediators by human leukotriene C4 synthase

Cysteinyl leukotrienes are key mediators in inflammation and have an important role in acute and chronic inflammatory diseases of the cardiovascular and respiratory systems, in particular bronchial asthma. In the biosynthesis of cysteinyl leukotrienes, conversion of arachidonic acid forms the unstable epoxide leukotriene A4 (LTA4). This intermediate is conjugated with glutathione (GSH) to produce leukotriene C4 (LTC4) in a reaction catalysed by LTC4 synthase: this reaction is the key step in cysteinyl leukotriene formation. Here we present the crystal structure of the human LTC4 synthase in its apo and GSH-complexed forms to 2.00 and 2.15 A resolution, respectively. The structure reveals a homotrimer, where each monomer is composed of four transmembrane segments. The structure of the enzyme in complex with substrate reveals that the active site enforces a horseshoe-shaped conformation on GSH, and effectively positions the thiol group for activation by a nearby arginine at the membrane-enzyme interface. In addition, the structure provides a model for how the omega-end of the lipophilic co-substrate is pinned at one end of a hydrophobic cleft, providing a molecular 'ruler' to align the reactive epoxide at the thiol of glutathione. This provides new structural insights into the mechanism of LTC4 formation, and also suggests that the observed binding and activation of GSH might be common for a family of homologous proteins important for inflammatory and detoxification responses.     

Why endophenotype development requires families

Endophenotypes are heritable quantitative traits that are associated with disease liability, can be measured in both affected and unaffected individuals, and provide much greater power to localize and identify risk genes for mental illness than does affection status alone. Traditionally, endophenotypic markers for psychiatric illnesses include in vivo neuroanatomic and functional magnetic resonance imaging measurements and indices of neurocognitive abilities. However, neurocognitive and neuroimaging measures are by no means the only classes of endophenotypes that could be useful for identifying genes for mental illness. Given the advantages of endophenotype-based strategies for elucidating the genetic underpinnings of psychiatric disorders, it would seem prudent to develop a wide range of putative endophenotypes. In order for a measure to be considered a valid endophenotype, it must meet a number of criteria. Specifically, the trait must (1) have moderate to high heritability, (2) be associated with the illness, (3) be independent of clinical state, and (4) impairment must co-segregate with the illness within a family, with non-affected family members showing impairment relative to the general population. While each of these criteria is critical, the heritability and co-segregation requirements are really what differentiate an endophenotype from a simple biomarker. At this time, one requires an experimental design that includes families to demonstrate both heritability and co-segregation. The assertion that novel endophenotypes can not be fully established without family data does not preclude work in unrelated individuals, rather that unrelated samples will only be able to nominate potential candidate endophenotypes that subsequently need to be confirmed in family-based experiments.     

Potential release of hydrogen fluoride from domestic coal in endemic fluorosis area in Guizhou,China

Almost half of the total rural area of Guizhou Province and many regions within the 11 adjacent provinces in southwestern China have a long history(at least 70 years) of endemic fluorosis,including dental fluorosis and osteofluorosis along with its associated deformities and disabilities.Over decades of research,this specific type of endemic fluorosis has been defined as coal-burning fluorosis,which is distinct from drinking-water fluorosis.It is generally acknowledged that indoor burning and combustion of high-fluorine coal leads to food contamination,and fluorine then enters the human body.However,the exact chemical form of fluorine during its release and transfer to the body is still unknown.In the present study,21 domestic coal samples from outcrop and semi-outcrop coal collected in five villages with fluorosis were analyzed by time-of-flight secondary ion mass spectrometry(TOF-SIMS).The total mass fraction of sulfur in the samples ranged from 0.24%-5.58% and total fluorine content ranged 90.2-149.2 mg/kg.H3O+,H2SO4+ and HSO4-were detected in the samples by TOF-SIMS,which indicated that sulfuric acid hydrate(H2SO4.H2O) was present in the samples.F-was detected in all of these,which suggested the samples contain ionic fluorine compounds.Under certain circumstances,such as heating or burning,the prevalence and coexistence of the acid(H2SO4.H2O) and base(F-) would lead to a neutralization reaction producing volatile hydrogen fluoride(HF,bp = 19.5℃).This would be the chemical form of fluorine released from the coal.Further studies using HF and SO2 test tubes on headspace gas over coal samples heated to 200℃ in the laboratory and on headspace gas over stoves or chimney tops at rural residences confirmed the release of HF.     

Increased forest ecosystem carbon and nitrogen storage from nitrogen rich bedrock

Nitrogen (N) limits the productivity of many ecosystems worldwide, thereby restricting the ability of terrestrial ecosystems to offset the effects of rising atmospheric CO(2) emissions naturally. Understanding input pathways of bioavailable N is therefore paramount for predicting carbon (C) storage on land, particularly in temperate and boreal forests. Paradigms of nutrient cycling and limitation posit that new N enters terrestrial ecosystems solely from the atmosphere. Here we show that bedrock comprises a hitherto overlooked source of ecologically available N to forests. We report that the N content of soils and forest foliage on N-rich metasedimentary rocks (350-950?mg?N?kg(-1)) is elevated by more than 50% compared with similar temperate forest sites underlain by N-poor igneous parent material (30-70?mg?N?kg(-1)). Natural abundance N isotopes attribute this difference to rock-derived N: (15)N/(14)N values for rock, soils and plants are indistinguishable in sites underlain by N-rich lithology, in marked contrast to sites on N-poor substrates. Furthermore, forests associated with N-rich parent material contain on average 42% more carbon in above-ground tree biomass and 60% more carbon in the upper 30?cm of the soil than similar sites underlain by N-poor rocks. Our results raise the possibility that bedrock N input may represent an important and overlooked component of ecosystem N and C cycling elsewhere.