2,389 research outputs found
Climate-Vegetation-Feedbacks as a Mechanism for Accelerated Climate Change: The onset of the African Humid Period
Paleo-environmental records and models indicate that the African Humid Period (AHPabruptly ended about 5000-4000 years before present (BP). Some proxies indicate alsan abrupt onset of the AHP between 14,000 and 11,000 BP. How important are local orbitaforcing, ice-sheet forcing, greenhouse gas forcing, and the reorganization of the AtlantiMeridional Overturning Circulation (AMOC) for changes in the African Monsoon/vegetatiosystem? Here we use transient simulations with climate-vegetation models of differencomplexity to identify the factors that control the onset of the African Monsoon/VegetationWe test the following hypothesis:(1) There is no indication for insolation-thresholds for the onset/break of the AHP.(2) Forcing from CO2/ice-sheets significantly controls the climate of North Africa.(3) CO2 fertilization contributes to the vegetation changes over North Africa.(4) A shutdown of the AMOC is as important as orbital insolation for the African Monsoon
Resonant and Kondo tunneling through molecular magnets
Transport through molecular magnets is studied in the regime of strong
coupling to the leads. We consider a resonant-tunneling model where the
electron spin in a quantum dot or molecule is coupled to an additional local,
anisotropic spin via exchange interaction. The two opposite regimes dominated
by resonant tunneling and by Kondo transport, respectively, are considered. In
the resonant-tunneling regime, the stationary state of the impurity spin is
calculated for arbitrarily strong molecule-lead coupling using a
master-equation approach, which treats the exchange interaction perturbatively.
We find that the characteristic fine structure in the differential conductance
persists even if the hybridization energy exceeds thermal energies. Transport
in the Kondo regime is studied within a diagrammatic approach. We show that
magnetic anisotropy gives rise to a splitting of the Kondo peak at low bias
voltages.Comment: 13 pages, 5 figures, version as publishe
Doping dependence of the Neel temperature in Mott-Hubbard antiferromagnets: Effect of vortices
The rapid destruction of long-range antiferromagnetic order upon doping of
Mott-Hubbard antiferromagnetic insulators is studied within a generalized
Berezinskii-Kosterlitz-Thouless renormalization group theory in accordance with
recent calculations suggesting that holes dress with vortices. We calculate the
doping-dependent Neel temperature in good agreement with experiments for
high-Tc cuprates. Interestingly, the critical doping where long-range order
vanishes at zero temperature is predicted to be xc ~ 0.02, independently of any
energy scales of the system.Comment: 4 pages with 3 figures included, minor revisions, to be published in
PR
Impedance of a Rectangular Beam Tube with Small Corrugations
We consider the impedance of a structure with rectangular, periodic
corrugations on two opposing sides of a rectangular beam tube. Using the method
of field matching, we find the modes in such a structure. We then limit
ourselves to the the case of small corrugations, but where the depth of
corrugation is not small compared to the period. For such a structure we
generate analytical approximate solutions for the wave number , group
velocity , and loss factor for the lowest (the dominant) mode
which, when compared with the results of the complete numerical solution,
agreed well. We find: if , where is the beam pipe width and is
the beam pipe half-height, then one mode dominates the impedance, with
( is the depth of corrugation),
, and , which (when replacing by
) is the same scaling as was found for small corrugations in a {\it round}
beam pipe. Our results disagree in an important way with a recent paper of
Mostacci {\it et al.} [A. Mostacci {\it et al.}, Phys. Rev. ST-AB, {\bf 5},
044401 (2002)], where, for the rectangular structure, the authors obtained a
synchronous mode with the same frequency , but with .
Finally, we find that if is large compared to then many nearby modes
contribute to the impedance, resulting in a wakefield that Landau damps.Comment: 18 pages, 6 figures, 1 bibliography fil
Random transition-rate matrices for the master equation
Random-matrix theory is applied to transition-rate matrices in the Pauli
master equation. We study the distribution and correlations of eigenvalues,
which govern the dynamics of complex stochastic systems. Both the cases of
identical and of independent rates of forward and backward transitions are
considered. The first case leads to symmetric transition-rate matrices, whereas
the second corresponds to general, asymmetric matrices. The resulting matrix
ensembles are different from the standard ensembles and show different
eigenvalue distributions. For example, the fraction of real eigenvalues scales
anomalously with matrix dimension in the asymmetric case.Comment: 15 pages, 12 figure
Stochastic Assembly of Bacteria in Microwell Arrays Reveals the Importance of Confinement in Community Development
Citation: Hansen, R. H., Timm, A. C., Timm, C. M., Bible, A. N., Morrell-Falvey, J. L., Pelletier, D. A., . . . Retterer, S. T. (2016). Stochastic Assembly of Bacteria in Microwell Arrays Reveals the Importance of Confinement in Community Development. Plos One, 11(5), 18. doi:10.1371/journal.pone.0155080The structure and function of microbial communities is deeply influenced by the physical and chemical architecture of the local microenvironment and the abundance of its community members. The complexity of this natural parameter space has made characterization of the key drivers of community development difficult. In order to facilitate these characterizations, we have developed a microwell platform designed to screen microbial growth and interactions across a wide variety of physical and initial conditions. Assembly of microbial communities into microwells was achieved using a novel biofabrication method that exploits well feature sizes for control of innoculum levels. Wells with incrementally smaller size features created populations with increasingly larger variations in inoculum levels. This allowed for reproducible growth measurement in large (20 mu m diameter) wells, and screening for favorable growth conditions in small (5, 10 mu m diameter) wells. We demonstrate the utility of this approach for screening and discovery using 5 mu m wells to assemble P. aeruginosa colonies across a broad distribution of innoculum levels, and identify those conditions that promote the highest probability of survivial and growth under spatial confinement. Multi-member community assembly was also characterized to demonstrate the broad potential of this platform for studying the role of member abundance on microbial competition, mutualism and community succession
A theoretical study of the tautomerism and vibrational spectra of 4,5-diamine-2,6-dimercaptopyrimidine
The 4,5-diamine-2,6-dimercaptopyrimidine (DADMcP) compound is an interesting multifunctional species exhibiting a rather complex tautomerism, encompassing nine tautomeric forms. Investigation of tautomerism in this compound has been carried out by means of FTIR spectroscopy, in association with ab-initio HF/SCF and DFT calculations. According to this study three tautomers are energetically favored; the thione form being the most stable one. The theoretical vibrational spectra of such tautomeric forms have been successfully simulated by means of DFT calculations, allowing the elucidation and assignment of the complex composition of the vibrational bands observed for the mixture of isomers.A 4,5-diamina-2,6-dimercaptopirimidina (DADMcP) constitui uma molécula multifuncional que apresenta uma tautomeria bastante complexa, com nove formas isoméricas possíveis. O estudo da tautomeria nesse composto foi realizado por meio da espectroscopia FTIR em associação com cálculos ab-initio HF/SCF e DFT. De acordo com os resultados teóricos, três tautômeros são favorecidos energeticamente, sendo a tiona a forma a mais estável. Os espectros vibracionais das formas tautoméricas mais estáveis foram simulados através de cálculos de DFT, permitindo a atribuição e elucidação do quadro complexo de bandas vibracionais observado experimentalmente, envolvendo a mistura de isômeros
Water potential in peach branches as a function of soil water storage and evaporative demand of the atmosphere¹.
Disorder-Induced Resistive Anomaly Near Ferromagnetic Phase Transitions
We show that the resistivity rho(T) of disordered ferromagnets near, and
above, the Curie temperature T_c generically exhibits a stronger anomaly than
the scaling-based Fisher-Langer prediction. Treating transport beyond the
Boltzmann description, we find that within mean-field theory, d\rho/dT exhibits
a |T-T_c|^{-1/2} singularity near T_c. Our results, being solely due to
impurities, are relevant to ferromagnets with low T_c, such as SrRuO3 or
diluted magnetic semiconductors, whose mobility near T_c is limited by
disorder.Comment: 5 pages, 3 figures; V2: with a few clarifications, as publishe
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