Edge states in graphene quantum dots: Fractional quantum Hall effect analogies and differences at zero magnetic field

We investigate the way that the degenerate manifold of midgap edge states in quasicircular graphene quantum dots with zig-zag boundaries supports, under free-magnetic-field conditions, strongly correlated many-body behavior analogous to the fractional quantum Hall effect (FQHE), familiar from the case of semiconductor heterostructures in high magnetic fields. Systematic exact-diagonalization (EXD) numerical studies are presented for the first time for 5 <= N <= 8 fully spin-polarized electrons and for total angular momenta in the range of N(N-1)/2 <= L <= 150. We present a derivation of a rotating-electron-molecule (REM) type wave function based on the methodology introduced earlier [C. Yannouleas and U. Landman, Phys. Rev. B 66, 115315 (2002)] in the context of the FQHE in two-dimensional semiconductor quantum dots. The EXD wave functions are compared with FQHE trial functions of the Laughlin and the derived REM types. It is found that a variational extension of the REM offers a better description for all fractional fillings compared with that of the Laughlin functions (including total energies and overlaps), a fact that reflects the strong azimuthal localization of the edge electrons. In contrast with the multiring arrangements of electrons in circular semiconductor quantum dots, the graphene REMs exhibit in all instances a single (0,N) polygonal-ring molecular (crystalline) structure, with all the electrons localized on the edge. Disruptions in the zig-zag boundary condition along the circular edge act effectively as impurities that pin the electron molecule, yielding single-particle densities with broken rotational symmetry that portray directly the azimuthal localization of the edge electrons.Comment: Revtex. 14 pages with 13 figures and 2 tables. Physical Review B, in press. For related papers, see http://www.prism.gatech.edu/~ph274cy

Phase-Controlled Force and Magnetization Oscillations in Superconducting Ballistic Nanowires

The emergence of superconductivity-induced phase-controlled forces in the (0.01-0.1) nN range, and of magnetization oscillations, in nanowire junctions, is discussed. A giant magnetic response to applied weak magnetic fields, is predicted in the ballistic Josephson junction formed by a superconducting tip and a surface, bridged by a normal metal nanowire where Andreev states form.Comment: 5 pages, 3 figure

Study of 6^{6}He+12^{12}C Elastic Scattering Using a Microscopic Optical Potential

The $^6$He+$^{12}$C elastic scattering data at beam energies of 3, 38.3 and 41.6 MeV/nucleon are studied utilizing the microscopic optical potentials obtained by a double-folding procedure and also by using those inherent in the high-energy approximation. The calculated optical potentials are based on the neutron and proton density distributions of colliding nuclei established in an appropriate model for $^6$He and obtained from the electron scattering form factors for $^{12}$C. The depths of the real and imaginary parts of the microscopic optical potentials are considered as fitting parameters. At low energy the volume optical potentials reproduce sufficiently well the experimental data. At higher energies, generally, additional surface terms having form of a derivative of the imaginary part of the microscopic optical potential are needed. The problem of ambiguity of adjusted optical potentials is resolved requiring the respective volume integrals to obey the determined dependence on the collision energy. Estimations of the Pauli blocking effects on the optical potentials and cross sections are also given and discussed. Conclusions on the role of the aforesaid effects and on the mechanism of the considered processes are made.Comment: 12 pages, 9 figures, accepted for publication in Physical Review

Calculations of 8^{8}He+p Elastic Cross Sections Using Microscopic Optical Potential

An approach to calculate microscopic optical potential (OP) with the real part obtained by a folding procedure and with the imaginary part inherent in the high-energy approximation (HEA) is applied to study the $^8$He+p elastic scattering data at energies of tens of MeV/nucleon (MeV/N). The neutron and proton density distributions obtained in different models for $^{8}$He are utilized in the calculations of the differential cross sections. The role of the spin-orbit potential is studied. Comparison of the calculations with the available experimental data on the elastic scattering differential cross sections at beam energies of 15.7, 26.25, 32, 66 and 73 MeV/N is performed. The problem of the ambiguities of the depths of each component of the optical potential is considered by means of the imposed physical criterion related to the known behavior of the volume integrals as functions of the incident energy. It is shown also that the role of the surface absorption is rather important, in particular for the lowest incident energies (e.g., 15.7 and 26.25 MeV/nucleon).Comment: 11 pages, 7 figures, accepted for publication in Physical Review

Food Storage in Permafrost and Seasonally Frozen Ground in Chukotka and Alaska Communities

Food cellars, otherwise referred to as ice or meat cellars, (lednik in Russian, k’aetyran in Chukchi, siġļuaq in Iñupiaq, and siqlugaq in Yupik) are a natural form of refrigeration in permafrost or seasonally frozen ground used to preserve, age, and ferment foods harvested for subsistence, including marine mammals, birds, fish, and plants. Indigenous peoples throughout the Arctic have constructed cellars in frozen ground for millennia. This paper focuses on cellars in Russian and American coastal and island communities of the Bering Strait, the region otherwise known as Beringia. This area has a unique, culturally rich, and politically dynamic history. Many traditions associated with cellars are threatened in Chukchi communities in Russia because of the impacts of climate change, relocation, dietary changes, and industrial development. However, even with warmer temperatures, cellars still provide a means to age and ferment food stuffs following traditional methods. In cooperation with local stakeholders, we measured internal temperatures of 18 cellars in 13 communities throughout the Bering Strait region and northern Alaska. Though cellars are widely used in permafrost regions, their structure, usage, and maintenance methods differ and exhibit influences of local climates, traditions, and economic activities. Monitoring internal temperatures and recording structural descriptions of cellars is important in the face of climate change to better understand the variety and resilience of living adaptations in different cold regions.Les caves à denrées, aussi connues sous le nom de caves à glace ou de caves à viande (lednik en russe, k’aetyran en tchouktche, siġļuaq en iñupiaq, et siqlugaq en yupik) constituent une forme de réfrigération naturelle dans le pergélisol ou dans le gélisol saisonnier permettant de conserver, de maturer et de fermenter les denrées récoltées à des fins de subsistance, dont les mammifères marins, les oiseaux, les poissons et les plantes. Cela fait des millénaires que les peuples autochtones de l’Arctique construisent des caves dans le gélisol. Cet article porte sur les caves se trouvant dans les localités côtières et insulaires russes et américaines du détroit de Béring, région qui porte également le nom de Béringie. L’histoire de cette région est unique, culturellement riche et politiquement dynamique. De nombreuses traditions liées aux caves des localités tchouktches de la Russie sont menacées en raison des incidences du changement climatique, de la délocalisation, du changement des régimes alimentaires et de l’expansion industrielle. Cependant, malgré les températures plus élevées, les caves constituent toujours un moyen de maturer et de fermenter les denrées alimentaires selon les méthodes traditionnelles. En collaboration avec les parties prenantes de la région, nous avons mesuré les températures internes de 18 caves situées dans 13 localités de la région du détroit de Béring et du nord de l’Alaska. Bien que les caves soient courantes dans les régions de pergélisol, leur structure, leur usage et les méthodes d’entretien diffèrent, et elles sont à l’image des influences des traditions, des activités économiques et des climats locaux. La surveillance des températures internes et l’enregistrement des descriptions structurales des caves revêtent de l’importance à la lumière du changement climatique, car elles permettent de mieux comprendre la variété et la résilience des adaptations de vie dans différentes régions froides.называемые иногда мясными ямами (лéдник по-русски, к’этыран по-чукотски, Siġļuaq или Siqlugaq по-эскимосски), обустроены как в вечномёрзлых породах, так и в сезонноталом слое и являются естественной формой заморозки для сохранения, выдержки и ферментации пищевых продуктов, добытых для пропитания: мясо морских млекопитающих, дичь, рыба, растения и др. Коренные жители Арктики обустраивали хранилища в мерзлоте на протяжении тысячелетий. Данная статья посвящена подземным хранилищам в российских и американских поселениях на берегах Берингова пролива – региона, также называемого Берингией. Эта территория имеет уникальную, богатую культурой и политически динамичную историю. Многие традиции, связанные с хранилищами в поселениях Чукотки, находятся под угрозой исчезновения из-за климатических изменений, миграции жителей, изменений в рационе и промышленного освоения территории. Однако даже при повышении температуры воздуха в хранилищах по-прежнему можно выдерживать и ферментировать пищу традиционными способами. При сотрудничестве с местным населением мы измерили температуры внутреннего воздуха в 18 лéдниках в 13 поселениях в регионе Берингова пролива и на севере Аляски. Несмотря на широкое использование таких хранилищ в криолитозоне, их структура, использование и методы обслуживания различаются под влиянием климатических условий, традиций и особенностей промысла. Мониторинг внутренней температуры воздуха в лéдниках и описание их конструкций важны в контексте изменения климата для лучшего понимания разнообразия и эффективности различных способов адаптации к жизни в холодных регионах

Climate Change Drives Widespread and Rapid Thermokarst Development in Very Cold Permafrost in the Canadian High Arctic

Climate warming in regions of ice‐rich permafrost can result in widespread thermokarst development, which reconfigures the landscape and damages infrastructure. We present multisite time series observations which couple ground temperature measurements with thermokarst development in a region of very cold permafrost. In the Canadian High Arctic between 2003 and 2016, a series of anomalously warm summers caused mean thawing indices to be 150–240% above the 1979–2000 normal resulting in up to 90 cm of subsidence over the 12‐year observation period. Our data illustrate that despite low mean annual ground temperatures, very cold permafrost (<−10 °C) with massive ground ice close to the surface is highly vulnerable to rapid permafrost degradation and thermokarst development. We suggest that this is due to little thermal buffering from soil organic layers and near‐surface vegetation, and the presence of near‐surface ground ice. Observed maximum thaw depths at our sites are already exceeding those projected to occur by 2090 under representative concentration pathway version 4.5

Cellular and Molecular Bases of the Initiation of Fever

All phases of lipopolysaccharide (LPS)-induced fever are mediated by prostaglandin (PG) E(2). It is known that the second febrile phase (which starts at ~1.5 h post-LPS) and subsequent phases are mediated by PGE(2) that originated in endotheliocytes and perivascular cells of the brain. However, the location and phenotypes of the cells that produce PGE(2) triggering the first febrile phase (which starts at ~0.5 h) remain unknown. By studying PGE(2) synthesis at the enzymatic level, we found that it was activated in the lung and liver, but not in the brain, at the onset of the first phase of LPS fever in rats. This activation involved phosphorylation of cytosolic phospholipase A(2) (cPLA(2)) and transcriptional up-regulation of cyclooxygenase (COX)-2. The number of cells displaying COX-2 immunoreactivity surged in the lung and liver (but not in the brain) at the onset of fever, and the majority of these cells were identified as macrophages. When PGE(2) synthesis in the periphery was activated, the concentration of PGE(2) increased both in the venous blood (which collects PGE(2) from tissues) and arterial blood (which delivers PGE(2) to the brain). Most importantly, neutralization of circulating PGE(2) with an anti-PGE(2) antibody both delayed and attenuated LPS fever. It is concluded that fever is initiated by circulating PGE(2) synthesized by macrophages of the LPS-processing organs (lung and liver) via phosphorylation of cPLA(2) and transcriptional up-regulation of COX-2. Whether PGE(2) produced at the level of the blood–brain barrier also contributes to the development of the first phase remains to be clarified

Seasonal abundance of small cladocerans in Lake Mangakaware, Waikato, New Zealand

The seasonal changes in the dynamics and life histories of the Cladocera in Lake Mangakaware, North Island, New Zealand, were studied over 19 months by sampling at weekly or 2-weekly intervals. Lake Mangakaware is a 13.3 ha polymictic lake with high nutrient status, low Secchi disc transparencies, and an unstable thermal regime. The four planktonic cladoceran species (Bosmina longirostris, B. meridionalis, Ceriodaphnia pulchella, and C. dubia) exhibited disjunct population maxima. Only B. longirostris was perennially present. All species exhibited low fecundities and low lipid content, indicating that food resources were limited and that competitive interactions and resistance to starvation were probably important in determining species success. Increases in body size in cooler seasons were unrelated to clutch size, giving further support for the view that available food was limited. These results are consistent with previous experimental findings that subtle differences in life history can determine seasonal success and the outcome of competition between similar species