Tunneling between edge states in a quantum spin Hall system

We analyze a quantum spin Hall (QSH) device with a point contact connecting two of its edges. The contact supports a net spin tunneling current that can be probed experimentally via a two-terminal resistance measurement. We find that the low-bias tunneling current and the differential conductance exhibit scaling with voltage and temperature that depend nonlinearly on the strength of the electron-electron interaction.Comment: 4 pages, 3 figures; published versio

Wintertime Arctic Ocean sea water properties and primary marine aerosol concentrations

Sea spray aerosols are an important part of the climate system through their direct and indirect effects. Due to the diminishing sea ice, the Arctic Ocean is one of the most rapidly changing sea spray aerosol source areas. However, the influence of these changes on primary particle production is not known. <br><br> In laboratory experiments we examined the influence of Arctic Ocean water temperature, salinity, and oxygen saturation on primary particle concentration characteristics. Sea water temperature was identified as the most important of these parameters. A strong decrease in sea spray aerosol production with increasing water temperature was observed for water temperatures between −1°C and 9°C. Aerosol number concentrations decreased from at least 1400 cm<sup>−3</sup> to 350 cm<sup>−3</sup>. In general, the aerosol number size distribution exhibited a robust shape with one mode close to dry diameter <i>D</i><sub>p</sub> 0.2 μm with approximately 45% of particles at smaller sizes. Changes in sea water temperature did not result in pronounced change of the shape of the aerosol size distribution, only in the magnitude of the concentrations. Our experiments indicate that changes in aerosol emissions are most likely linked to changes of the physical properties of sea water at low temperatures. The observed strong dependence of sea spray aerosol concentrations on sea water temperature, with a large fraction of the emitted particles in the typical cloud condensation nuclei size range, provide strong arguments for a more careful consideration of this effect in climate models

Long-term trends of black carbon and sulphate aerosol in the Arctic: changes in atmospheric transport and source region emissions

As a part of the IPY project POLARCAT (Polar Study using Aircraft, Remote Sensing, Surface Measurements and Models, of Climate, Chemistry, Aerosols and Transport) and building on previous work (Hirdman et al., 2010), this paper studies the long-term trends of both atmospheric transport as well as equivalent black carbon (EBC) and sulphate for the three Arctic stations Alert, Barrow and Zeppelin. We find a general downward trend in the measured EBC concentrations at all three stations, with a decrease of −2.1±0.4 ng m<sup>−3</sup> yr<sup>−1</sup> (for the years 1989–2008) and −1.4±0.8 ng m<sup>−3</sup> yr<sup>−1</sup> (2002–2009) at Alert and Zeppelin respectively. The decrease at Barrow is, however, not statistically significant. The measured sulphate concentrations show a decreasing trend at Alert and Zeppelin of −15±3 ng m<sup>−3</sup> yr<sup>−1</sup> (1985–2006) and −1.3±1.2 ng m<sup>−3</sup> yr<sup>−1</sup> (1990–2008) respectively, while there is no trend detectable at Barrow. <br><br> To reveal the contribution of different source regions on these trends, we used a cluster analysis of the output of the Lagrangian particle dispersion model FLEXPART run backward in time from the measurement stations. We have investigated to what extent variations in the atmospheric circulation, expressed as variations in the frequencies of the transport from four source regions with different emission rates, can explain the long-term trends in EBC and sulphate measured at these stations. We find that the long-term trend in the atmospheric circulation can only explain a minor fraction of the overall downward trend seen in the measurements of EBC (0.3–7.2%) and sulphate (0.3–5.3%) at the Arctic stations. The changes in emissions are dominant in explaining the trends. We find that the highest EBC and sulphate concentrations are associated with transport from Northern Eurasia and decreasing emissions in this region drive the downward trends. Northern Eurasia (cluster: NE, WNE and ENE) is the dominant emission source at all Arctic stations for both EBC and sulphate during most seasons. In wintertime, there are indications that the EBC emissions from the eastern parts of Northern Eurasia (ENE cluster) have increased over the last decade

Nitrate dry deposition in Svalbard

This is the final version of the article. Available from Taylor & Francis via the DOI in this record.Arctic regions are generally nutrient limited, receiving an extensive part of their bio-available nitrogen from the deposition of atmospheric reactive nitrogen. Reactive nitrogen oxides, as nitric acid (HNO3) and nitrate aerosols (p-NO3), can either be washed out from the atmosphere by precipitation or dry deposited, dissolving to nitrate (). During winter, is accumulated in the snowpack and released as a pulse during spring melt. Quantification of deposition is essential to assess impacts on Arctic terrestrial ecology and for ice core interpretations. However, the individual importance of wet and dry deposition is poorly quantified in the high Arctic regions where in-situ measurements are demanding. In this study, three different methods are employed to quantify dry deposition around the atmospheric and ecosystem monitoring site, Ny-Ålesund, Svalbard, for the winter season (September 2009 to May 2010): (1) A snow tray sampling approach indicates a dry deposition of –10.27±3.84 mg m−2 (± S.E.); (2) A glacial sampling approach yielded somewhat higher values –30.68±12.00 mg m−2; and (3) Dry deposition was also modelled for HNO3 and p-NO3 using atmospheric concentrations and stability observations, resulting in a total combined nitrate dry deposition of –10.76±1.26 mg m−2. The model indicates that deposition primarily occurs via HNO3 with only a minor contribution by p-NO3. Modelled median deposition velocities largely explain this difference: 0.63 cm s−1 for HNO3 while p-NO3 was 0.0025 and 0.16 cm s−1 for particle sizes 0.7 and 7 µm, respectively. Overall, the three methods are within two standard errors agreement, attributing an average 14% (total range of 2–44%) of the total nitrate deposition to dry deposition. Dry deposition events were identified in association with elevated atmospheric concentrations, corroborating recent studies that identified episodes of rapid pollution transport and deposition to the Arctic.As a part of the international project ‘Sources, sinks and impacts of atmospheric nitrogen deposition in the Arctic’ (NSINK), this project received financial support from an EU Marie Curie Initial Stage Training Network Award NSINK (FP7 215503) and fieldwork were supported by an Arctic Field Grant, Svalbard Science Forum. NILU's atmospheric monitoring program at the Zeppelin Station is financed by the Norwegian Climate and Pollution Agency and ITM's research at the same station is supported by the Swedish Environmental Protection Agency. This is also a contribution to ‘Crysophere-Atmosphere interaction in a changing Arctic climate (CRAICC)’ a Top-Level Research Initiative (TRI). Logistic support was provided by the Norwegian Polar Institute staff at the Sverdrup Station, Ny-Ålesund, while A. Nowak-Zwierz, K. A. Kozio and C. L. Chakrabarti contributed to the analytical work in Sheffield, and J. Kohler kindly provided snow accumulation data from Austre Brøggerbreen

Graded Contractions of Affine Kac-Moody Algebras

The method of graded contractions, based on the preservation of the automorphisms of finite order, is applied to the affine Kac-Moody algebras and their representations, to yield a new class of infinite dimensional Lie algebras and representations. After the introduction of the horizontal and vertical gradings, and the algorithm to find the horizontal toroidal gradings, I discuss some general properties of the graded contractions, and compare them with the In\"on\"u-Wigner contractions. The example of $\hat A_2$ is discussed in detail.Comment: 23 pages, Ams-Te

Correlated local distortions of the TlO layers in Tl2_2Ba2_2CuOy_{y}: An x-ray absorption study

We have used the XAFS (x-ray-absorption fine structure) technique to investigate the local structure about the Cu, Ba, and Tl atoms in orthorhombic Tl-2201 with a superconducting transition temperature T$_c$=60 K. Our results clearly show that the O(1), O(2), Cu, and Ba atoms are at their ideal sites as given by the diffraction measurements, while the Tl and O(3) atoms are more disordered than suggested by the average crystal structure. The Tl-Tl distance at 3.5 \AA{ } between the TlO layers does not change, but the Tl-Tl distance at 3.9 \AA{ } within the TlO layer is not observed and the Tl-Ba and Ba-Tl peaks are very broad. The shorter Tl-O(3) distance in the TlO layer is about 2.33 \AA, significantly shorter than the distance calculated with both the Tl and O(3) atoms at their ideal $4e$ sites ( $x=y=$0 or $\frac{1}{2}$). A model based on these results shows that the Tl atom is displaced along the directions from its ideal site by about 0.11 \AA; the displacements of neighboring Tl atoms are correlated. The O(3) atom is shifted from the $4e$ site by about 0.53 \AA{ } roughly along the directions. A comparison of the Tl L$_{III}$-edge XAFS spectra from three samples, with T$_c$=60 K, 76 K, and 89 K, shows that the O environment around the Tl atom is sensitive to T$_c$ while the Tl local displacement is insensitive to T$_c$ and the structural symmetry. These conclusions are compared with other experimental results and the implications for charge transfer and superconductivity are discussed. This paper has been submitted to Phys. Rev. B.Comment: 20 pages plus 14 ps figures, REVTEX 3.

Electromagnetic properties of non-Dirac particles with rest spin 1/2

We resolve a number of questions related to an analytic description of electromagnetic form factors of non-Dirac particles with the rest spin 1/2. We find the general structure of a matrix antisymmetric tensor operator. We obtain two recurrence relations for matrix elements of finite transformations of the proper Lorentz group and explicit formulas for a certain set of such elements. Within the theory of fields with double symmetry, we discuss writing the components of wave vectors of particles in the form of infinite continued fractions. We show that for $Q^{2} \leq 0.5$ (GeV/c)$^{2}$, where $Q^{2}$ is the transferred momentum squared, electromagnetic form factors that decrease as $Q^{2}$ increases and are close to those experimentally observed in the proton can be obtained without explicitly introducing an internal particle structure.Comment: 18 pages, 2 figure

Ferromagnetic Domain Distribution in Thin Films During Magnetization Reversal

We have shown that polarized neutron reflectometry can determine in a model-free way not only the mean magnetization of a ferromagnetic thin film at any point of a hysteresis cycle, but also the mean square dispersion of the magnetization vectors of its lateral domains. This technique is applied to elucidate the mechanism of the magnetization reversal of an exchange-biased Co/CoO bilayer. The reversal process above the blocking temperature is governed by uniaxial domain switching, while below the blocking temperature the reversal of magnetization for the trained sample takes place with substantial domain rotation

Cloud droplet activation of black carbon particles coated with organic compounds of varying solubility

Atmospheric black carbon (BC) particles are a concern due to their impact on air quality and climate. Their net climate effect is, however, still uncertain. This uncertainty is partly related to the contribution of coated BC particles to the global cloud condensation nuclei (CCN) budgets. In this study, laboratory measurements were performed to investigate CCN activity of BC (REGAL 400R pigment black) particles, in pure state or coated through evaporating and subsequent condensation of glutaric acid, levoglucosan (both water-soluble organics) or oleic acid (an organic compound with low solubility). A combination of soot particle aerosol mass spectrometer (SP-AMS) measurements and size distribution measurements with a scanning mobility particle sizer (SMPS) showed that the studied BC particles were nearly spherical agglomerates with a fractal dimension of 2.79 and that they were coated evenly by the organic species. The CCN activity of BC particles increased after coating with all the studied compounds and was governed by the fraction of organic material. The CCN activation of the BC particles coated by glutaric acid and levoglucosan were in good agreement with the theoretical calculations using the shell-and-core model, which is based on a combination of the CCN activities of the pure compounds. The oleic acid coating enhanced the CCN activity of the BC particles, even though the pure oleic acid particles were CCN inactive. The surprising effect of oleic acid might be related to the arrangement of the oleic acid molecules on the surface of the BC cores or other surface phenomena facilitating water condensation onto the coated particles. Our results show that present theories have potential for accurately predicting the CCN activity of atmospheric BC coated with organic species, given that the identities and amounts of the coating species are known. Furthermore, our results suggest that even relatively thin soluble coatings (around 2&thinsp;nm for the compounds studied here) are enough to make the insoluble BC particles CCN active at typical atmospheric supersaturations and thus be efficiently taken up by cloud droplets. This highlights the need for an accurate description of the composition of atmospheric particles containing BC to unravel their net impact on climate.</p