64 research outputs found
Casmir-Lifshitz Forces and Entropy
It is shown that the violation of the positiveness of the entropy due to the
Casimir-Lifshitz interaction claimed in several papers is an artifact related
to an improper interpretation of the "Casimir entropy", which actually is a
difference of two positive terms. It is explained that at definite condition
this "Casimir entropy" must be negative. A direct derivation of the low
temperature behavior of the surface entropy of a metallic surface in conditions
of the anomalous skin effect is given and singular temperature dependency of
this quantity is discussed. In conclusion a hydrodynamic example of the entropy
of a liquid film is considered. It occurs that the entropy of a film of finite
thickness and a liquid half-space behave differently at T tends to 0.Comment: 4 page
Den allvarliga lÀrarbristen i Sverige - hur kunde detta hÀnda och vad kan vi göra?
I denna artikel redovisas troliga orsaker till den omfattande lÀrarbristen i Sverige och möjligheter att lösa den pÄ lÀngre sikt. Data Àr baserat pÄ en omfattande litteraturöversikt och en webbundersökning. Den teoretiska förankringen Äterfinns i tre styrningsteorier och dess konsekvenser för det studerade omrÄdet. Resultatet visar att lÀrarbristen kan spÄras minst 50 Är tillbaka i tiden och att orsakerna Àr mÄnga och komplexa, exempelvis mÄnga olika reformer, försÀmrade villkor, lÄg status för yrket och New Public Management som styrningsfilosofi. Möjligheterna att lösa problemen pÄ lÄng sikt Àr flera: högre löner, bÀttre arbetsvillkor, stÀrkt professionsidentitet, högre status samt flexibla vÀgar att studera till lÀrare.
New features of the thermal Casimir force at small separations
The difference of the thermal Casimir forces at different temperatures
between real metals is shown to increase with a decrease of the separation
distance. This opens new opportunities for the demonstration of the thermal
dependence of the Casimir force. Both configurations of two parallel plates and
a sphere above a plate are considered. Different approaches to the theoretical
description of the thermal Casimir force are shown to lead to different
measurable predictions.Comment: 5 pages, 3 figures, to appear in Phys. Rev. Let
Uniaxial negative thermal expansion and metallophilicity in Cu3[Co(CN)6]
We report the synthesis and structural characterisation of the molecular framework copper(I)
hexacyanocobaltate(III), Cu3[Co(CN)6], which we find to be isostructural to H3[Co(CN)6] and the colossal
negative thermal expansion material Ag3[Co(CN)6]. Using synchrotron X-ray powder diffraction measurements,
we find strong positive and negative thermal expansion behaviour respectively perpendicular and parallel to the
trigonal crystal axis: α = 25.4(5) MK a
â1 and α = â 43.5(8) MK c
â1. These opposing effects collectively result in a
volume expansivity α = 7.4(11) MK V
â1 that is remarkably small for an anisotropic molecular framework. This
thermal response is discussed in the context of the behaviour of the analogous H- and Ag-containing systems.
We make use of density-functional theory with many-body dispersion interactions (DFT + MBD) to
demonstrate that Cu+âŠCu+ metallophilic (âcuprophilicâ) interactions are significantly weaker in Cu3[Co(CN)6]
than Ag+âŠAg+ interactions in Ag3[Co(CN)6], but that this lowering of energy scale counterintuitively translates
to a more moderateârather than enhancedâdegree of structural flexibility. The same conclusion is drawn from
consideration of a simple GULP model, which we also present here. Our results demonstrate that strong
interactions can actually be exploited in the design of ultra-responsive materials if those interactions are set up
to act in tension
A procedure to correct proxy-reported weight in the National Health Interview Survey, 1976â2002
<p>Abstract</p> <p>Background</p> <p>Data from the National Health Interview Survey (NHIS) show a larger-than-expected increase in mean BMI between 1996 and 1997. Proxy-reports of height and weight were discontinued as part of the 1997 NHIS redesign, suggesting that the sharp increase between 1996 and 1997 may be artifactual.</p> <p>Methods</p> <p>We merged NHIS data from 1976â2002 into a single database consisting of approximately 1.7 million adults aged 18 and over. The analysis consisted of two parts: First, we estimated the magnitude of BMI differences by reporting status (i.e., self-reported versus proxy-reported height and weight). Second, we developed a procedure to correct biases in BMI introduced by reporting status.</p> <p>Results</p> <p>Our analyses confirmed that proxy-reports of weight tended to be biased downward, with the degree of bias varying by race, sex, and other characteristics. We developed a correction procedure to minimize BMI underestimation associated with proxy-reporting, substantially reducing the larger-than-expected increase found in NHIS data between 1996 and 1997.</p> <p>Conclusion</p> <p>It is imperative that researchers who use reported estimates of height and weight think carefully about flaws in their data and how existing correction procedures might fail to account for them. The development of this particular correction procedure represents an important step toward improving the quality of BMI estimates in a widely used source of epidemiologic data.</p
The Accumulation of Organic Carbon in Mineral Soils by Afforestation of Abandoned Farmland
The afforestation of abandoned farmland significantly influences soil organic carbon (OC). However, the dynamics between OC inputs after afforestation and the original OC are not well understood. To learn more about soil OC dynamics after afforestation of farmland, we measured the soil OC content in paired forest and farmland plots in Shaanxi Province, China. The forest plots had been established on farmland 18, 24, 48, 100, and 200 yr previously. The natural 13C abundance of soil organic matter was also analyzed to distinguish between crop- and forest-derived C in the afforested soils. We observed a nonlinear accumulation of total OC in the 0â80 cm depth of the mineral soil across time. Total soil OC accumulated more rapidly under forest stands aged 18 to 48 yr than under forest stands aged 100 or 200 yrs. The rate of OC accumulation was also greater in the 0â10 cm depth than in the 10â80 cm depth. Forest-derived OC in afforested soils also accumulated nonlinearly across time, with the greatest increase in the 0â20 cm depth. Forest-derived OC in afforest soils accounted for 52â86% of the total OC in the 0â10 cm depth, 36â61% of the total OC in the 10â20 cm depth, and 11â50% of the total OC in the 20â80 cm depth. Crop-derived OC concentrations in the 0â20 cm depth decreased slightly after afforestation, but there was no change in crop-derived OC concentrations in the 20â80 cm depth. The results of our study support the claim that afforestation of farmland can sequester atmospheric CO2 by increasing soil OC stocks. Changes in the OC stocks of mineral soils after afforestation appear to be influenced mainly by the input of forest-derived C rather than by the loss of original OC
Sediment Phosphorus Release at Beaver Reservoir, Northwest Arkansas, USA, 2002â2003: A Preliminary Investigation
The first samples from Almahata Sitta showing contacts between ureilitic and chondritic lithologies: implications for the structure and composition of asteroid 2008 TC 3
Almahata Sitta (AhS), an anomalous polymict ureilite, is the first meteorite observed to originate from a spectrally classified asteroid (2008 TC3). However, correlating properties of the meteorite with those of the asteroid is not straightforward because the AhS stones are diverse types. Of those studied prior to this work, 70â80% are ureilites (achondrites) and 20â30% are various types of chondrites. Asteroid 2008 TC3 was a heterogeneous breccia that disintegrated in the atmosphere, with its clasts landing on Earth as individual stones and most of its mass lost. We describe AhS 91A and AhS 671, which are the first AhS stones to show contacts between ureilitic and chondritic materials and provide direct information about the structure and composition of asteroid 2008 TC3. AhS 91A and AhS 671 are friable breccias, consisting of a C1 lithology that encloses rounded to angular clasts (<10 ÎŒm to 3 mm) of olivine, pyroxenes, plagioclase, graphite, and metalâsulfide, as well as chondrules (~130â600 ÎŒm) and chondrule fragments. The C1 material consists of fineâgrained phyllosilicates (serpentine and saponite) and amorphous material, magnetite, breunnerite, dolomite, fayalitic olivine (Fo 28â42), an unidentified Caârich silicate phase, Fe,Ni sulfides, and minor Caâphosphate and ilmenite. It has similarities to CI1 but shows evidence of heterogeneous thermal metamorphism. Its bulk oxygen isotope composition (ÎŽ18O = 13.53â°, ÎŽ17O = 8.93â°) is unlike that of any known chondrite, but similar to compositions of several CCâlike clasts in typical polymict ureilites. Its Cr isotope composition is unlike that of any known meteorite. The enclosed clasts and chondrules do not belong to the C1 lithology. The olivine (Fo 75â88), pyroxenes (pigeonite of Wo ~10 and orthopyroxene of Wo ~4.6), plagioclase, graphite, and some metalâsulfide are ureilitic, based on mineral compositions, textures, and oxygen isotope compositions, and represent at least six distinct ureilitic lithologies. The chondrules are probably derived from type 3 OC and/or CC, based on mineral and oxygen isotope compositions. Some of the metalâsulfide clasts are derived from EC. AhS 91A and AhS 671 are plausible representatives of the bulk of the asteroid that was lost. Reflectance spectra of AhS 91A are dark (reflectance ~0.04â0.05) and relatively featureless in VNIR, and have an ~2.7 ÎŒm absorption band due to OHâ in phyllosilicates. Spectral modeling, using mixtures of laboratory VNIR reflectance spectra of AhS stones to fit the Fâtype spectrum of the asteroid, suggests that 2008 TC3 consisted mainly of ureilitic and AhS 91Aâlike materials, with as much as 40â70% of the latter, and <10% of OC, EC, and other meteorite types. The bulk density of AhS 91A (2.35 ± 0.05 g cmâ3) is lower than bulk densities of other AhS stones, and closer to estimates for the asteroid (~1.7â2.2 g cmâ3). Its porosity (36%) is near the low end of estimates for the asteroid (33â50%), suggesting significant macroporosity. The textures of AhS 91A and AhS 671 (finely comminuted clasts of disparate materials intimately mixed) support formation of 2008 TC3 in a regolith environment. AhS 91A and AhS 671 could represent a volume of regolith formed when a CCâlike body impacted into already wellâgardened ureilitic + impactorâderived debris. AhS 91A bulk samples do not show a solar wind component, so they represent subsurface layers. AhS 91A has a lower cosmic ray exposure (CRE) age (~5â9 Ma) than previously studied AhS stones (11â22 Ma). The spread in CRE ages argues for irradiation in a regolith environment. AhS 91A and AhS 671 show that ureilitic asteroids could have detectable ~2.7 ÎŒm absorption bands
Does sea level influence mid-ocean ridge magmatism on Milankovitch timescales?
Magma production at mid-ocean ridges is driven by seafloor spreading and decompression melting of the upper mantle. In the special case of Iceland, mantle melting may have been amplified by ice sheet retreat during the last deglaciation, yielding anomalously high rates of subaerial volcanism. For the remainder of the global mid-ocean ridge system, the ocean may play an analogous role, with lowering of sea level during glacial maxima producing greater magma flux to ridge crests. Here we show that the mantle decompression rate associated with changes in sea level is a substantial fraction of that from plate spreading. Modeled peaks in magma flux occur after sea level drops rapidly, including the Marine Isotope Stage (MIS) 5/4 and 3/2 transitions. The minimum in simulated flux occurs during the mid-Holocene, due to the rapid sea level rise at the MIS 2/1 boundary. The model results are highly sensitive to melt migration rate; rates of ~1 m/yr produce small signals, while those >5 m/yr yield substantial anomalies. In the latter case, sea level-driven magma flux varies by 15â100% relative to the long-term average, with the largest effect occurring at slow-spreading ridges. We suggest that sedimentary time series of hydrothermal particle flux, oceanic Os isotopic ratio, and oceanic radiocarbon may serve as proxies for magma-flux variations at mid-ocean ridges. Although well-dated records are rare, preliminary data from the Pacific and Atlantic suggest hydrothermal metal flux was elevated during MIS 2 and 4, broadly consistent with our modeling results
Phosphorus in sediments of high-elevation lakes in the Sierra Nevada (California): implications for internal phosphorus loading
In high-elevation lakes of the Sierra Nevada (California), increases in phosphorus (P) supply have been inferred from changes in phytoplankton growth during summer. To quantify rates of sediment P release to high-elevation Sierran lakes, we performed incubations of sediment cores under ambient and reducing conditions at Emerald Lake and analyzed long-term records of lake chemistry for Emerald and Pear lakes. We also measured concentrations of individual P forms in sediments from 50 Sierra Nevada lakes using a sequential fractionation procedure to examine landscape controls on P forms in sediments. On average, the sediments contained 1,445 ”g P gâ1, of which 5 % was freely exchangeable, 13 % associated with reducible metal hydroxides, 68 % associated with Al hydroxides, and the remaining 14 % stabilized in recalcitrant pools. Multiple linear regression analysis indicated that sediment P fractions were not well correlated with soluble P concentrations. In general, sediments behaved as net sinks for P even under reducing conditions. Our findings suggest that internal P loading does not explain the increase in P availability observed in high-elevation Sierran lakes. Rather, increased atmospheric P inputs and increased P supply via dissolved organic C leaching from soils may be driving the observed changes in P biogeochemistry
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