924 research outputs found
Density Functional Theory of the Hubbard-Holstein Model
We present a density functional theory (DFT) for lattice models with local
electron-electron (e-e) and electron-phonon (e-ph) interactions.
Exchange-correlation potentials are derived via dynamical mean field theory for
the infinite-dimensional Bethe lattice, and analytically for an isolated
Hubbard-Holstein site. These potentials exhibit discontinuities as a function
of the density, which depend on the relative strength of the e-e and e-ph
interactions. By comparing to exact benchmarks, we show that the DFT formalism
gives a good description of the linear conductance and real-time dynamics.Comment: 5 pages, 3 figures, supplemental material provided as pd
Nonequilibrium Green's functions and atom-surface dynamics: Simple views from a simple model system
We employ Non-equilibrium Green's functions (NEGF) to describe the real-time
dynamics of an adsorbate-surface model system exposed to ultrafast laser
pulses. For a finite number of electronic orbitals, the system is solved
exactly and within different levels of approximation. Specifically i) the full
exact quantum mechanical solution for electron and nuclear degrees of freedom
is used to benchmark ii) the Ehrenfest approximation (EA) for the nuclei, with
the electron dynamics still treated exactly. Then, using the EA, electronic
correlations are treated with NEGF within iii) 2nd Born and with iv) a recently
introduced hybrid scheme, which mixes 2nd Born self-energies with
non-perturbative, local exchange-correlation potentials of Density Functional
Theory (DFT). Finally, the effect of a semi-infinite substrate is considered:
we observe that a macroscopic number of de-excitation channels can hinder
desorption. While very preliminary in character and based on a simple and
rather specific model system, our results clearly illustrate the large
potential of NEGF to investigate atomic desorption, and more generally, the non
equilibrium dynamics of material surfaces subject to ultrafast laser fields.Comment: 10 pages, 5 figure
Non-Perturbative Theory for Dispersion Self-Energy of Atoms
We go beyond the approximate series-expansions used in the dispersion theory
of finite size atoms. We demonstrate that a correct, and non-perturbative,
theory dramatically alters the dispersion selfenergies of atoms. The
non-perturbed theory gives as much as 100% corrections compared to the
traditional series expanded theory for the smaller noble gas atoms.Comment: 3 pages, no figures, 1 tabl
Casimir attractive-repulsive transition in MEMS
Unwanted stiction in micro- and nanomechanical (NEMS/MEMS) systems due to
dispersion (van der Waals, or Casimir) forces is a significant hurdle in the
fabrication of systems with moving parts on these length scales. Introducing a
suitably dielectric liquid in the interspace between bodies has previously been
demonstrated to render dispersion forces repulsive, or even to switch sign as a
function of separation. Making use of recently available permittivity data
calculated by us we show that such a remarkable non-monotonic Casimir force,
changing from attractive to repulsive as separation increases, can in fact be
observed in systems where constituent materials are in standard NEMS/MEMS use
requiring no special or exotic materials. No such nonmonotonic behaviour has
been measured to date. We calculate the force between a silica sphere and a
flat surface of either zinc oxide or hafnia, two materials which are among the
most prominent for practical microelectrical and microoptical devices. Our
results explicate the need for highly accurate permittivity functions of the
materials involved for frequencies from optical to far-infrared frequencies. A
careful analysis of the Casimir interaction is presented, and we show how the
change in the sign of the interaction can be understood as a result of multiple
crossings of the dielectric functions of the three media involved in a given
set-up.Comment: 6 pages, 4 figure
Non-Perturbative Theory of Dispersion Interactions
Some open questions exist with fluctuation-induced forces between extended
dipoles. Conventional intuition derives from large-separation perturbative
approximations to dispersion force theory. Here we present a full
non-perturbative theory. In addition we discuss how one can take into account
finite dipole size corrections. It is of fundamental value to investigate the
limits of validity of the perturbative dispersion force theory.Comment: 9 pages, no figure
Blue carbon stocks in Baltic Sea eelgrass (Zostera marina) meadows
Although seagrasses cover only a minor fraction of the ocean seafloor, their
carbon sink capacity accounts for nearly one-fifth of the total oceanic
carbon burial and thus play a critical structural and functional role in
many coastal ecosystems. We sampled 10 eelgrass (<i>Zostera marina</i>) meadows in Finland and 10
in Denmark to explore seagrass carbon stocks (C<sub>org</sub> stock) and carbon
accumulation rates (C<sub>org</sub> accumulation) in the Baltic Sea area. The study
sites represent a gradient from sheltered to exposed locations in both
regions to reflect expected minimum and maximum stocks and accumulation. The
C<sub>org</sub> stock integrated over the top 25 cm of the sediment averaged 627 g C m<sup>−2</sup> in Finland, while in Denmark the average C<sub>org</sub> stock was over
6 times higher (4324 g C m<sup>−2</sup>). A conservative estimate of the total
organic carbon pool in the regions ranged between 6.98 and 44.9 t C ha<sup>−1</sup>.
Our results suggest that the Finnish eelgrass meadows are minor carbon sinks
compared to the Danish meadows, and that majority of the C<sub>org</sub> produced in
the Finnish meadows is exported. Our analysis further showed that
> 40 % of the variation in the C<sub>org</sub> stocks was explained by
sediment characteristics, i.e. dry density, porosity and silt content. In
addition, our analysis show that the root : shoot ratio of <i>Z. marina</i> explained
> 12 % and the contribution of <i>Z. marina</i> detritus to the sediment surface
C<sub>org</sub> pool explained > 10 % of the variation in the C<sub>org</sub> stocks.
The mean monetary value for the present carbon storage and carbon sink
capacity of eelgrass meadows in Finland and Denmark, were 281 and 1809 EUR ha<sup>−1</sup>, respectively. For a more comprehensive picture of
seagrass carbon storage capacity, we conclude that future blue carbon
studies should, in a more integrative way, investigate the interactions
between sediment biogeochemistry, seascape structure, plant species
architecture and the hydrodynamic regime
Role of food web interactions in promoting resilience to nutrient enrichment in a brackish water eelgrass (Zostera marina) ecosystem
Understanding the ecological interactions that enhance the resilience of threatened ecosystems is essential in assuring their conservation and restoration. Top-down trophic interactions can increase resilience to bottom-up nutrient enrichment, however, as many seagrass ecosystems are threatened by both eutrophication and trophic modifications, understanding how these processes interact is important. Using a combination of approaches, we explored how bottom-up and top-down processes, acting individually or in conjunction, can affect eelgrass meadows and associated communities in the northern Baltic Sea. Field surveys along with fish diet and stable isotope analyses revealed that the eelgrass trophic network included two main top predatory fish species, each of which feeds on a separate group of invertebrate mesograzers (crustaceans or gastropods). Mesograzer abundance in the study area was high, and capable of mitigating the effects of increased algal biomass that resulted from experimental nutrient enrichment in the field. When crustacean mesograzers were experimentally excluded, gastropod mesograzers were able to compensate and limit the effects of nutrient enrichment on eelgrass biomass and growth. Our results suggest that top-down processes (i.e., suppression of algae by different mesograzer groups) may ensure eelgrass resilience to nutrient enrichment in the northern Baltic Sea, and the existence of multiple trophic pathways can provide additional resilience in the face of trophic modifications. However, the future resilience of these meadows is likely threatened by additional local stressors and global environmental change. Understanding the trophic links and interactions that ensure resilience is essential for managing and conserving these important ecosystems and the services they provide
Violation of the Nernst heat theorem in the theory of thermal Casimir force between Drude metals
We give a rigorous analytical derivation of low-temperature behavior of the
Casimir entropy in the framework of the Lifshitz formula combined with the
Drude dielectric function. An earlier result that the Casimir entropy at zero
temperature is not equal to zero and depends on the parameters of the system is
confirmed, i.e. the third law of thermodynamics (the Nernst heat theorem) is
violated. We illustrate the resolution of this thermodynamical puzzle in the
context of the surface impedance approach by several calculations of the
thermal Casimir force and entropy for both real metals and dielectrics.
Different representations for the impedances, which are equivalent for real
photons, are discussed. Finally, we argue in favor of the Leontovich boundary
condition which leads to results for the thermal Casimir force that are
consistent with thermodynamics.Comment: 24 pages, 3 figures, accepted for publication in Phys. Rev.
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