837 research outputs found
Disordered Topological Insulators via -Algebras
The theory of almost commuting matrices can be used to quantify topological
obstructions to the existence of localized Wannier functions with time-reversal
symmetry in systems with time-reversal symmetry and strong spin-orbit coupling.
We present a numerical procedure that calculates a Z_2 invariant using these
techniques, and apply it to a model of HgTe. This numerical procedure allows us
to access sizes significantly larger than procedures based on studying twisted
boundary conditions. Our numerical results indicate the existence of a metallic
phase in the presence of scattering between up and down spin components, while
there is a sharp transition when the system decouples into two copies of the
quantum Hall effect. In addition to the Z_2 invariant calculation in the case
when up and down components are coupled, we also present a simple method of
evaluating the integer invariant in the quantum Hall case where they are
decoupled.Comment: Added detail regarding the mapping of almost commuting unitary
matrices to almost commuting Hermitian matrices that form an approximate
representation of the sphere. 6 pages, 6 figure
Staying in place during times of change in Arctic Alaska: The implications of attachment,alternatives, and buffering
The relationship between stability and change in social-ecological systems has received considerable attention in recent years, including the expectation that significant environmental changes will drive observable consequences for individuals, communities, and populations. Migration, as one example of response to adverse economic or environmental changes, has been observed in many places, including parts of the Far North. In Arctic Alaska, a relative lack of demographic or migratory response to rapid environmental and other changes has been observed. To understand why Arctic Alaska appears different, we draw on the literature on environmentally driven migration, focusing on three mechanisms that could account for the lack of response: attachment, the desire to remain in place, or the inability to relocate successfully; alternatives, ways to achieve similar outcomes through different means; and buffering, the reliance on subsidies or use of reserves to delay impacts. Each explanation has different implications for research and policy, indicating a need to further explore the relative contribution that each makes to a given situation in order to develop more effective responses locally and regionally. Given that the Arctic is on the front lines of climate change, these explanations are likely relevant to the ways changes play out in other parts of the world. Our review also underscores the importance of further attention to the details of social dynamics in climate change impacts and responses
A mass formula for baryon resonances
Light-baryon resonances with u,d, and s quarks only can be classified using
the non-relativistic quark model. When we assign to baryon resonances with
total angular momenta J intrinsic orbital angular momenta L and spin S we make
the following observations: plotting the squared masses of the light-baryon
resonances against these intrinsic orbital angular momenta L, Delta's with even
and odd parity can be described by the same Regge trajectory. For a given L,
nucleon resonances with spin S=3/2 are approximately degenerate in mass with
Delta resonances of same total orbital momentum L. To which total angular
momentum L and S couple has no significant impact on the baryon mass. Nucleons
with spin 1/2 are shifted in mass; the shift is - in units of squared masses -
proportional to the component in the wave function which is antisymmetric in
spin and flavor. Sequential resonances in the same partial wave are separated
in mass square by the same spacing as observed in orbital angular momentum
excitations. Based on these observations, a new baryon mass formula is proposed
which reproduces nearly all known baryon masses.Comment: 4 pages, 1 figur
Ways to Help and Ways to Hinder: Governance for Effective Adaptation to an Uncertain Climate
This paper compares two case studies in Alaska, one on commercial fishers of the Bering Sea and Aleutian Islands region and the other on moose hunters of Interior Alaska, to identify how governance arrangements and management strategies enhance or limit peopleâs ability to respond effectively to changing climatic and environmental conditions. The two groups face similar challenges regarding the impacts of a changing climate on wild fish and game, but they tell very different stories regarding how and under what conditions these impacts challenge their harvest activities. In both regions, people describe dramatic changes in weather, land, and seascape conditions, and distributions of fish and game. A key finding is that the âcommand-and-controlâ model of governance in the Alaska Interior, as implemented through state and federal management tools such as registration hunts and short open seasons, limits effective local responses to environmental conditions, while the more decentralized model of governance created by the Limited Access Privilege systems of the Bering Sea allows fishers great flexibility to respond. We discuss ways to implement aspects of a decentralized decision-making model in the Interior that would benefit hunters by increasing their adaptability and success, while also improving conservation outcomes. Our findings also demonstrate the usefulness of the diagnostic framework employed here for facilitating comparative crossregional analyses of natural resource use and management.Ce document Ă©tablit une comparaison entre deux Ă©tudes de cas effectuĂ©es en Alaska, lâune portant sur les pĂȘcheurs commerciaux de la mer de BĂ©ring et de la rĂ©gion des AlĂ©outiennes et lâautre, sur les chasseurs dâorignaux de lâintĂ©rieur de lâAlaska. Cette comparaison avait pour but de dĂ©terminer comment les ententes de gouvernance et les stratĂ©gies de gestion rehaussent ou restreignent lâaptitude des gens Ă rĂ©agir de maniĂšre efficace au changement climatique et aux conditions environnementales. Dans le cas des deux groupes, les dĂ©fis sont semblables en ce qui a trait aux incidences du changement climatique sur le poisson sauvage et le gibier, mais il nâen reste pas moins que les deux groupes tĂ©moignent dâhistoires trĂšs diffĂ©rentes relativement Ă la façon dont les incidences influencent leurs activitĂ©s de chasse ou de pĂȘche, et les circonstances dans lesquelles les incidences prĂ©sentent des dĂ©fis Ă leurs activitĂ©s de chasse ou de pĂȘche. Dans les deux cas, les individus dĂ©crivent des changements dramatiques sur le plan des conditions mĂ©tĂ©orologiques, du paysage terrestre et du paysage marin, ainsi que sur le plan de la rĂ©partition du poisson et du gibier. Une des grandes observations ayant Ă©manĂ© de cette comparaison, câest que le modĂšle de gouvernance consistant à « commander et contrĂŽler » qui est en vigueur dans lâintĂ©rieur de lâAlaska, tel quâimposĂ© par les outils de gestion de lâĂtat et du gouvernement fĂ©dĂ©ral, et qui se traduit notamment par lâenregistrement des chasses et par des saisons de chasse courtes, se trouve Ă restreindre lâefficacitĂ© des rĂ©actions locales vis-Ă -vis des conditions environnementales, tandis que le mode de gouvernance plus dĂ©centralisĂ© crĂ©Ă© par les systĂšmes de privilĂšge Ă accĂšs limitĂ© de la mer de BĂ©ring donne aux pĂȘcheurs une plus grande souplesse pour rĂ©agir. Nous nous penchons sur diverses façons de mettre en oeuvre les aspects dâun modĂšle de prise de dĂ©cisions dĂ©centralisĂ© dans lâintĂ©rieur de maniĂšre Ă ce que les chasseurs en bĂ©nĂ©ficient en augmentant leur adaptabilitĂ© et leur succĂšs, tout en amĂ©liorant les rĂ©sultats de conservation. Nos constatations dĂ©montrent aussi lâutilitĂ© du cadre diagnostic employĂ© ici pour faciliter les analyses inter-rĂ©gionales en matiĂšre dâutilisation et de gestion des ressources naturelles
Almost commuting unitary matrices related to time reversal
The behavior of fermionic systems depends on the geometry of the system and
the symmetry class of the Hamiltonian and observables. Almost commuting
matrices arise from band-projected position observables in such systems. One
expects the mathematical behavior of almost commuting Hermitian matrices to
depend on two factors. One factor will be the approximate polynomial relations
satisfied by the matrices. The other factor is what algebra the matrices are
in, either the matrices over A for A the real numbers, A the complex numbers or
A the algebra of quaternions.
There are potential obstructions keeping k-tuples of almost commuting
operators from being close to a commuting k-tuple. We consider two-dimensional
geometries and so this obstruction lives in KO_{-2}(A). This obstruction
corresponds to either the Chern number or spin Chern number in physics. We show
that if this obstruction is the trivial element in K-theory then the
approximation by commuting matrices is possible.Comment: 33 pages, 2 figures. In version 2 some formulas have been corrected
and some proofs have been rewritten to improve the expositio
Effects of perturbative exchanges in a QCD-string model
The QCD-string model for baryons derived by Simonov and used for the
calculation of baryon magnetic moments in a previous paper is extended to
include also perturbative gluon and meson exchanges. The mass spectrum of the
baryon multiplet is studied. For the meson interaction either the pseudoscalar
or pseudovector coupling is used. Predictions are compared with the
experimental data. Besides these exchanges the influence of excited quark
orbitals on the baryon ground state are considered by performing a multichannel
calculation. The nucleon-Delta splitting increases due to the mixing of higher
quark states while the baryon magnetic momenta decrease. The multichannel
calculation with perturbative exchanges is shown to yield reasonable magnetic
moments while the mass spectrum is close to experiment.Comment: 37 pages Revtex with 2 figures, to be published in Phys. Atom. Nucl.
dedicated to the 70th Birthday of Yu. A. Simono
Random Walks on a Fluctuating Lattice: A Renormalization Group Approach Applied in One Dimension
We study the problem of a random walk on a lattice in which bonds connecting
nearest neighbor sites open and close randomly in time, a situation often
encountered in fluctuating media. We present a simple renormalization group
technique to solve for the effective diffusive behavior at long times. For
one-dimensional lattices we obtain better quantitative agreement with
simulation data than earlier effective medium results. Our technique works in
principle in any dimension, although the amount of computation required rises
with dimensionality of the lattice.Comment: PostScript file including 2 figures, total 15 pages, 8 other figures
obtainable by mail from D.L. Stei
Selected plasma fatty acid levels in subsistence fed sled dogs along the Yukon River: a pilot study for biomonitoring
The introduction of the âwestern diet' marked a decline in omega-3 fatty acids rich foods and a concurrent increase in saturated and omega-6 fatty acids that persists today. Historically, circumpolar people have had a low incidence of obesity, diabetes and cardiovascular disease and this has been largely attributed to polyphenolic compounds and omega-3 fatty acids offered from subsistence foods. In this report, we studied sled dogs as an Arctic sentinel species for monitoring the effect of a changing diet on lipid profiles along the Yukon River. Subsistence fed village sled dogs along the Yukon River, maintained largely on salmon were compared with a control kennel maintained on commercial food. Profiles showed higher levels for long chain omega-3 fatty acids in village subsistence fed dogs compared to control dogs and an opposite trend for omega-6 fatty acids, establishing baseline levels for follow up studies. A comparison with data for previously published mercury levels from the same cohort of dogs revealed a positive correlation with alpha-linolenic fatty acid and a negative correlation with linoleic fatty acid. Food and nutritional security is a concern in the Arctic as the impacts of climate change and transport of contaminants become obvious. This study supports not only the nutritional value of a subsistence diet but also the utility of sled dogs as a sentinel for human dietary chang
Bi-local baryon interpolating fields with two flavours
We construct bi-local interpolating field operators for baryons consisting of
three quarks with two flavors, assuming good isospin symmetry. We use the
restrictions following from the Pauli principle to derive relations/identities
among the baryon operators with identical quantum numbers. Such relations that
follow from the combined spatial, Dirac, color, and isospin Fierz
transformations may be called the (total/complete) Fierz identities. These
relations reduce the number of independent baryon operators with any given spin
and isospin. We also study the Abelian and non-Abelian chiral transformation
properties of these fields and place them into baryon chiral multiplets. Thus
we derive the independent baryon interpolating fields with given values of spin
(Lorentz group representation), chiral symmetry ( group
representation) and isospin appropriate for the first angular excited states of
the nucleon.Comment: 15 pages, 4 tables, accepted by EPJ
CO2 Utilization and Storage in Shale Gas Reservoirs: Experimental Results and Economic Impacts
AbstractNatural gas is considered a cleaner and lower-emission fuel than coal, and its high abundance from advanced drilling techniques has positioned natural gas as a major alternative energy source for the U.S. However, each ton of CO2 emitted from any type of fossil fuel combustion will continue to increase global atmospheric concentrations. One unique approach to reducing anthropogenic CO2 emissions involves coupling CO2 based enhanced gas recovery (EGR) operations in depleted shale gas reservoirs with long-term CO2 storage operations. In this paper, we report unique findings about the interactions between important shale minerals and sorbing gases (CH4 and CO2) and associated economic consequences. Where enhanced condensation of CO2 followed by desorption on clay surface is observed under supercritical conditions, a linear sorption profile emerges for CH4. Volumetric changes to montmorillonites occur during exposure to CO2. Theory-based simulations identify interactions with interlayer cations as energetically favorable for CO2 intercalation. In contrast, experimental evidence suggests CH4 does not occupy the interlayer and has only the propensity for surface adsorption. Mixed CH4:CO2 gas systems, where CH4 concentrations prevail, indicate preferential CO2 sorption as determined by in situ infrared spectroscopy and X-ray diffraction techniques. Collectively, these laboratory studies combined with a cost-based economic analysis provide a basis for identifying favorable CO2-EOR opportunities in previously fractured shale gas reservoirs approaching final stages of primary gas production. Moreover, utilization of site-specific laboratory measurements in reservoir simulators provides insight into optimum injection strategies for maximizing CH4/CO2 exchange rates to obtain peak natural gas production
- âŠ