258 research outputs found
Boom and Bust Carbon-Nitrogen Dynamics during Reforestation
Legacies of historical land use strongly shape contemporary ecosystem dynamics. In old-field secondary forests, tree growth embodies a legacy of soil changes affected by previous cultivation. Three patterns of biomass accumulation during reforestation have been hypothesized previously, including monotonic to steady state, non-monotonic with a single peak then decay to steady state, and multiple oscillations around the steady state. In this paper, the conditions leading to the emergence of these patterns is analyzed. Using observations and models, we demonstrate that divergent reforestation patterns can be explained by contrasting time-scales in ecosystem carbon-nitrogen cycles that are influenced by land use legacies. Model analyses characterize non-monotonic plant-soil trajectories as either single peaks or multiple oscillations during an initial transient phase controlled by soil carbon-nitrogen conditions at the time of planting. Oscillations in plant and soil pools appear in modeled systems with rapid tree growth and low initial soil nitrogen, which stimulate nitrogen competition between trees and decomposers and lead the forest into a state of acute nitrogen deficiency. High initial soil nitrogen dampens oscillations, but enhances the magnitude of the tree biomass peak. These model results are supported by data derived from the long-running Calhoun Long-Term Soil-Ecosystem Experiment from 1957 to 2007. Observed carbon and nitrogen pools reveal distinct tree growth and decay phases, coincident with soil nitrogen depletion and partial re-accumulation. Further, contemporary tree biomass loss decreases with the legacy soil C:N ratio. These results support the idea that non-monotonic reforestation trajectories may result from initial transients in the plant-soil system affected by initial conditions derived from soil changes associated with land-use history
Few-body spin couplings and their implications for universal quantum computation
Electron spins in semiconductor quantum dots are promising candidates for the
experimental realization of solid-state qubits. We analyze the dynamics of a
system of three qubits arranged in a linear geometry and a system of four
qubits arranged in a square geometry. Calculations are performed for several
quantum dot confining potentials. In the three-qubit case, three-body effects
are identified that have an important quantitative influence upon quantum
computation. In the four-qubit case, the full Hamiltonian is found to include
both three-body and four-body interactions that significantly influence the
dynamics in physically relevant parameter regimes. We consider the implications
of these results for the encoded universality paradigm applied to the
four-electron qubit code; in particular, we consider what is required to
circumvent the four-body effects in an encoded system (four spins per encoded
qubit) by the appropriate tuning of experimental parameters.Comment: 1st version: 33 pages, 25 figures. Described at APS March Meeting in
2004 (P36.010) and 2005 (B17.00009). Most figures made uglier here to reduce
file size. 2nd version: 19 pages, 9 figures. Much mathematical detail chopped
away after hearing from journal referee; a few typos correcte
Exchange Interaction Between Three and Four Coupled Quantum Dots: Theory and Applications to Quantum Computing
Several prominent proposals have suggested that spins of localized electrons
could serve as quantum computer qubits. The exchange interaction has been
invoked as a means of implementing two qubit gates. In this paper, we analyze
the strength and form of the exchange interaction under relevant conditions. We
find that, when several spins are engaged in mutual interactions, the
quantitative strengths or even qualitative forms of the interactions can
change. It is shown that the changes can be dramatic within a Heitler-London
model. Hund-Mulliken calculations are also presented, and support the
qualititative conclusions from the Heitler-London model. The effects need to be
considered in spin-based quantum computer designs, either as a source of gate
error to be overcome or a new interaction to be exploited.Comment: 16 pages, 16 figures. v3: Added Hund-Mulliken calculations in 3-dots
case. A few small corrections. This version submitted to PR
Protecting Quantum Information Encoded in Decoherence Free States Against Exchange Errors
The exchange interaction between identical qubits in a quantum information
processor gives rise to unitary two-qubit errors. It is shown here that
decoherence free subspaces (DFSs) for collective decoherence undergo Pauli
errors under exchange, which however do not take the decoherence free states
outside of the DFS. In order to protect DFSs against these errors it is
sufficient to employ a recently proposed concatenated DFS-quantum error
correcting code scheme [D.A. Lidar, D. Bacon and K.B. Whaley, Phys. Rev. Lett.
{\bf 82}, 4556 (1999)].Comment: 7 pages, no figures. Discussion in section V.A. significantly
expanded. Several small changes. Two authors adde
Obscured star formation in intermediate-density environments:A Spitzer study of the Abell 901/902 supercluster
We explore the amount of obscured star formation as a function of environment in the Abell 901/902 (A901/902) supercluster at z = 0.165 in conjunction with a field sample drawn from the A901 and CDFS fields, imaged with the Hubble Space Telescope as part of the Space Telescope A901/902 Galaxy Evolution Survey and Galaxy Evolution from Morphology and Spectral Energy Distributions (SEDs) Survey. We combine the combo-17 near-UV/optical SED with Spitzer 24 mu m photometry to estimate both the unobscured and obscured star formation in galaxies with M-* > 10(10) M-circle dot. We find that the star formation activity in massive galaxies is suppressed in dense environments, in agreement with previous studies. Yet, nearly 40% of the star-forming (SF) galaxies have red optical colors at intermediate and high densities. These red systems are not starbursting; they have star formation rates (SFRs) per unit stellar mass similar to or lower than blue SF galaxies. More than half of the red SF galaxies have low infrared-to-ultraviolet (IR-to-UV) luminosity ratios, relatively high Sersicindices, and they are equally abundant at all densities. They might be gradually quenching their star formation, possibly but not necessarily under the influence of gas-removing environmental processes. The other greater than or similar to 40% of the red SF galaxies have high IR-to-UV luminosity ratios, indicative of high dust obscuration. They have relatively high specific SFRs and are more abundant at intermediate densities. Our results indicate that while there is an overall suppression in the SF galaxy fraction with density, the small amount of star formation surviving the cluster environment is to a large extent obscured, suggesting that environmental interactions trigger a phase of obscured star formation, before complete quenching
Optically-passive spirals: The missing link in gradual star formation suppression upon cluster infall
Galaxies migrate from the blue cloud to the red sequence when their star
formation is quenched. Here, we report on galaxies quenched by environmental
effects and not by mergers or strong AGN as often invoked: They form stars at a
reduced rate which is optically even less conspicuous, and manifest a
transition population of blue spirals evolving into S0 galaxies. These
'optically passive' or 'red spirals' are found in large numbers in the STAGES
project (and by Galaxy Zoo) in the infall region of clusters and groups.Comment: Proceedings of "The Starburst-AGN connection" conference held in
Shanghai, Oct 27-31, 200
Policy Feedback and the Politics of the Affordable Care Act
There is a large body of literature devoted to how âpolicies create politicsâ and how feedback effects from existing policy legacies shape potential reforms in a particular area. Although much of this literature focuses on selfâreinforcing feedback effects that increase support for existing policies over time, Kent Weaver and his colleagues have recently drawn our attention to selfâundermining effects that can gradually weaken support for such policies. The following contribution explores both selfâreinforcing and selfâundermining policy feedback in relationship to the Affordable Care Act, the most important healthâcare reform enacted in the United States since the midâ1960s. More specifically, the paper draws on the concept of policy feedback to reflect on the political fate of the ACA since its adoption in 2010. We argue that, due in part to its sheer complexity and fragmentation, the ACA generates both selfâreinforcing and selfâundermining feedback effects that, depending of the aspect of the legislation at hand, can either facilitate or impede conservative retrenchment and restructuring. Simultaneously, through a discussion of partisan effects that shape Republican behavior in Congress, we acknowledge the limits of policy feedback in the explanation of policy stability and change
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