16,710 research outputs found
Local correlation functional for electrons in two dimensions
We derive a local approximation for the correlation energy in two-dimensional
electronic systems. In the derivation we follow the scheme originally developed
by Colle and Salvetti for three dimensions, and consider a Gaussian
approximation for the pair density. Then, we introduce an ad-hoc modification
which better accounts for both the long-range correlation, and the
kinetic-energy contribution to the correlation energy. The resulting functional
is local, and depends parametrically on the number of electrons in the system.
We apply this functional to the homogeneous electron gas and to a set of
two-dimensional quantum dots covering a wide range of electron densities and
thus various amounts of correlation. In all test cases we find an excellent
agreement between our results and the exact correlation energies. Our
correlation functional has a form that is simple and straightforward to
implement, but broadly outperforms the commonly used local-density
approximation
Diffusion of wave packets in a Markov random potential
We consider the evolution of a tight binding wave packet propagating in a
time dependent potential. If the potential evolves according to a stationary
Markov process, we show that the square amplitude of the wave packet converges,
after diffusive rescaling, to a solution of a heat equation.Comment: 19 pages, acknowledgments added and typos correcte
Colossal magnetoresistance in an ultra-clean weakly interacting 2D Fermi liquid
We report the observation of a new phenomenon of colossal magnetoresistance
in a 40 nm wide GaAs quantum well in the presence of an external magnetic field
applied parallel to the high-mobility 2D electron layer. In a strong magnetic
field, the magnetoresistance is observed to increase by a factor of ~300 from 0
to 45T without the system undergoing any metal-insulator transition. We discuss
how this colossal magnetoresistance effect cannot be attributed to the spin
degree-of-freedom or localization physics, but most likely emanates from strong
magneto-orbital coupling between the two-dimensional electron gas and the
magnetic field. Our observation is consistent with a field-induced 2D-to-3D
transition in the confined electronic system
Quantum oscillations observed in graphene at microwave frequencies
We have measured the microwave conductance of mechanically exfoliated
graphene at frequencies up to 8.5 GHz. The conductance at 4.2 K exhibits
quantum oscillations, and is independent of the frequency
A different view of the quantum Hall plateau-to-plateau transitions
We demonstrate experimentally that the transitions between adjacent integer
quantum Hall (QH) states are equivalent to a QH-to-insulator transition
occurring in the top Landau level, in the presence of an inert background of
the other completely filled Landau levels, each contributing a single unit of
quantum conductance, , to the total Hall conductance of the system.Comment: 10 pages, 4 figures, Revtex 3.
Dynamic spin-Hall effect and driven spin helix for linear spin-orbit interactions
We derive boundary conditions for the electrically induced spin accumulation
in a finite, disordered 2D semiconductor channel. While for DC electric fields
these boundary conditions select spatially constant spin profiles equivalent to
a vanishing spin-Hall effect, we show that an in-plane ac electric field
results in a non-zero ac spin-Hall effect, i.e., it generates a spatially
non-uniform out-of-plane polarization even for linear intrinsic spin-orbit
interactions. Analyzing different geometries in [001] and [110]-grown quantum
wells, we find that although this out-of-plane polarization is typically
confined to within a few spin-orbit lengths from the channel edges, it is also
possible to generate spatially oscillating spin profiles which extend over the
whole channel. The latter is due to the excitation of a driven spin-helix mode
in the transverse direction of the channel. We show that while finite
frequencies suppress this mode, it can be amplified by a magnetic field tuned
to resonance with the frequency of the electric field. In this case, finite
size effects at equal strengths of Rashba- and Dresselhaus SOI lead to an
enhancement of the magnitude of this helix mode. We comment on the relation
between spin currents and boundary conditions.Comment: 10 pages, 5 figures, added references, corrected typos, extended
section V, VI
Salt Marsh Bacterial Communities Before And After The Deepwater Horizon Oil Spill
Coastal salt marshes along the northern Gulf of Mexico shoreline received varied types and amounts of weathered oil residues after the 2010 Deepwater Horizon oil spill. At the time, predicting how marsh bacterial communities would respond and/or recover to oiling and other environmental stressors was difficult because baseline information on community composition and dynamics was generally unavailable. Here, we evaluated marsh vegetation, physicochemistry, flooding frequency, hydrocarbon chemistry, and subtidal sediment bacterial communities from 16S rRNA gene surveys at 11 sites in southern Louisiana before the oil spill and resampled the same marshes three to four times over 38 months after the spill. Calculated hydrocarbon biomarker indices indicated that oil replaced native natural organic matter (NOM) originating from Spartina alterniflora and marine phytoplankton in the marshes between May 2010 and September 2010. At all the studied marshes, the major class-and order-level shifts among the phyla Proteobacteria, Firmicutes, Bacteroidetes, and Actinobacteria occurred within these first 4 months, but another community shift occurred at the time of peak oiling in 2011. Two years later, hydrocarbon levels decreased and bacterial communities became more diverse, being dominated by Alphaproteobacteria (Rhizobiales), Chloroflexi (Dehalococcoidia), and Planctomycetes. Compositional changes through time could be explained by NOM source differences, perhaps due to vegetation changes, as well as marsh flooding and salinity excursions linked to freshwater diversions. These findings indicate that persistent hydrocarbon exposure alone did not explain long-term community shifts. IMPORTANCE Significant deterioration of coastal salt marshes in Louisiana has been linked to natural and anthropogenic stressors that can adversely affect how ecosystems function. Although microorganisms carry out and regulate most biogeochemical reactions, the diversity of bacterial communities in coastal marshes is poorly known, with limited investigation of potential changes in bacterial communities in response to various environmental stressors. The Deepwater Horizon oil spill provided an unprecedented opportunity to study the long-term effects of an oil spill on microbial systems in marshes. Compared to previous studies, the significance of our research stems from (i) a broader geographic range of studied marshes, (ii) an extended time frame of data collection that includes prespill conditions, (iii) a more accurate procedure using biomarker indices to understand oiling, and (iv) an examination of other potential stressors linked to in situ environmental changes, aside from oil exposure
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