608 research outputs found
Reduction of the Casimir force using aerogels
By using silicon oxide based aerogels we show numerically that the Casimir
force can be reduced several orders of magnitude, making its effect negligible
in nanodevices. This decrease in the Casimir force is also present even when
the aerogels are deposited on metallic substrates. To calculate the Casimir
force we model the dielectric function of silicon oxide aerogels using an
effective medium dielectric function such as the Clausius-Mossotti
approximation. The results show that both the porosity of the aerogel and its
thickness can be use as control parameters to reduce the magnitude of the
Casimir force.Comment: to appear J. Appl. Phy
Casimir force calculations near the insulator-conductor transition in gold thin films
We present theoretical calculations of the Casimir force for Au thin films
near the insulator-conductor transition that has been observed experimentally.
The dielectric function of the Au thin films is described by the Drude-Smith
model. The parameters needed to model the dielectric function such as the
relaxation time, plasma frequency and the backscattering constant depend on the
thickness of the film. The Casimir force decreases as the film thickness
decreases until it reaches a minimum after which the force increases again. The
minimum of the force coincides with the critical film thickness where a
percolation conductor-insulator occurs.Comment: 5 figures, 1 tabl
Nonlocal impedances and the Casimir entropy at low temperatures
The problem with the temperature dependence of the Casimir force is
investigated. Specifically, the entropy behavior in the low temperature limit,
which caused debates in the literature, is analyzed. It is stressed that the
behavior of the relaxation frequency in the limit does not play a
physical role since the anomalous skin effect dominates in this range. In
contrast with the previous works, where the approximate Leontovich impedance
was used for analysis of nonlocal effects, we give description of the problem
in terms of exact nonlocal impedances. It is found that the Casimir entropy is
going to zero at only in the case when polarization does not
contribute to the classical part of the Casimir force. However, the entropy
approaching zero from the negative side that, in our opinion, cannot be
considered as thermodynamically satisfactory. The resolution of the negative
entropy problem proposed in the literature is analyzed and it is shown that it
cannot be considered as complete. The crisis with the thermal Casimir effect is
stressed.Comment: Accepted in Phys. Rev.
Solar slow magneto-acoustic-gravity waves: an erratum correction and a revisited scenario
Slow waves are commonly observed on the entire solar atmosphere. Assuming a
thin flux tube approximation, the cut-off periods of slow-mode
magneto-acoustic-gravity waves that travel from the photosphere to the corona
were obtained in Costa et al. (2018). In that paper, however, a typo in the
specific heat coefficient at constant pressure value led to an
inconsistency in the cut-off calculation, which is only significant at the
transition region. Due to the abrupt temperature change in the region, a change
of the mean atomic weight (by a factor of approximately two) also occurs, but
is often overlooked in analytical models for simplicity purposes. In this
paper, we revisit the calculation of the cut-off periods of
magneto-acoustic-gravity waves in Costa et al. (2018) by considering an
atmosphere in hydrostatic equilibrium with a temperature profile, with the
inclusion of the variation of the mean atomic weight and the correction of the
inconsistency aforementioned. In addition, we show that the cut-off periods
obtained analytically are consistent with the corresponding periods measured in
observations of a particular active region.Comment: 12 pages, 7 figures. MNRA
Correction to the Casimir force due to the anomalous skin effect
The surface impedance approach is discussed in connection with the precise
calculation of the Casimir force between metallic plates. It allows to take
into account the nonlocal connection between the current density and electric
field inside of metals. In general, a material has to be described by two
impedances and corresponding to two
different polarization states. In contrast with the approximate Leontovich
impedance they depend not only on frequency but also on the wave
vector along the plate . In this paper only the nonlocal effects happening
at frequencies (plasma frequency) are analyzed. We refer to
all of them as the anomalous skin effect. The impedances are calculated for the
propagating and evanescent fields in the Boltzmann approximation. It is found
that significantly deviates from the local impedance as a result of the
Thomas-Fermi screening. The nonlocal correction to the Casimir force is
calculated at zero temperature. This correction is small but observable at
small separations between bodies. The same theory can be used to find more
significant nonlocal contribution at due to the plasmon
excitation.Comment: 29 pages. To appear in Phys. Rev.
Frequency-dependent Drude damping in Casimir force calculations
The Casimir force is calculated between Au thin films that are described by a
Drude model with a frequency dependent damping function. The model parameters
are obtained from available experimental data for Au thin films. Two cases are
considered; annealed and nonannealed films that have a different damping
function. Compared with the calculations using a Drude model with a constant
damping parameter, we observe changes in the Casimir force of a few percent.
This behavior is only observed in films of no more than 300 thick.Comment: Proceedings of the meeting "60 years of Casimir effect", Brasilia,
200
Computation of Casimir forces for dielectrics or intrinsic semiconductors based on the Boltzmann transport equation
The interaction between drifting carriers and traveling electromagnetic waves
is considered within the context of the classical Boltzmann transport equation
to compute the Casimir-Lifshitz force between media with small density of
charge carriers, including dielectrics and intrinsic semiconductors. We expand
upon our previous work [Phys. Rev. Lett. {\bf 101}, 163203 (2008)] and derive
in some detail the frequency-dependent reflection amplitudes in this theory and
compute the corresponding Casimir free energy for a parallel plate
configuration. We critically discuss the the issue of verification of the
Nernst theorem of thermodynamics in Casimir physics, and explicity show that
our theory satisfies that theorem. Finally, we show how the theory of drifting
carriers connects to previous computations of Casimir forces using spatial
dispersion for the material boundaries.Comment: 9 pages, 2 figures; Contribution to Proceedings of "60 Years of the
Casimir Effect", Brasilia, June 200
Evaluation of multiple protein docking structures using correctly predicted pairwise subunits
<p>Abstract</p> <p>Background</p> <p>Many functionally important proteins in a cell form complexes with multiple chains. Therefore, computational prediction of multiple protein complexes is an important task in bioinformatics. In the development of multiple protein docking methods, it is important to establish a metric for evaluating prediction results in a reasonable and practical fashion. However, since there are only few works done in developing methods for multiple protein docking, there is no study that investigates how accurate structural models of multiple protein complexes should be to allow scientists to gain biological insights.</p> <p>Methods</p> <p>We generated a series of predicted models (decoys) of various accuracies by our multiple protein docking pipeline, Multi-LZerD, for three multi-chain complexes with 3, 4, and 6 chains. We analyzed the decoys in terms of the number of correctly predicted pair conformations in the decoys.</p> <p>Results and conclusion</p> <p>We found that pairs of chains with the correct mutual orientation exist even in the decoys with a large overall root mean square deviation (RMSD) to the native. Therefore, in addition to a global structure similarity measure, such as the global RMSD, the quality of models for multiple chain complexes can be better evaluated by using the local measurement, the number of chain pairs with correct mutual orientation. We termed the fraction of correctly predicted pairs (RMSD at the interface of less than 4.0Å) as <it>fpair </it>and propose to use it for evaluation of the accuracy of multiple protein docking.</p
Functional genomics with a comprehensive library of transposon mutants for the sulfate-reducing bacterium Desulfovibrio alaskensis G20.
UnlabelledThe genomes of sulfate-reducing bacteria remain poorly characterized, largely due to a paucity of experimental data and genetic tools. To meet this challenge, we generated an archived library of 15,477 mapped transposon insertion mutants in the sulfate-reducing bacterium Desulfovibrio alaskensis G20. To demonstrate the utility of the individual mutants, we profiled gene expression in mutants of six regulatory genes and used these data, together with 1,313 high-confidence transcription start sites identified by tiling microarrays and transcriptome sequencing (5' RNA-Seq), to update the regulons of Fur and Rex and to confirm the predicted regulons of LysX, PhnF, PerR, and Dde_3000, a histidine kinase. In addition to enabling single mutant investigations, the D. alaskensis G20 transposon mutants also contain DNA bar codes, which enables the pooling and analysis of mutant fitness for thousands of strains simultaneously. Using two pools of mutants that represent insertions in 2,369 unique protein-coding genes, we demonstrate that the hypothetical gene Dde_3007 is required for methionine biosynthesis. Using comparative genomics, we propose that Dde_3007 performs a missing step in methionine biosynthesis by transferring a sulfur group to O-phosphohomoserine to form homocysteine. Additionally, we show that the entire choline utilization cluster is important for fitness in choline sulfate medium, which confirms that a functional microcompartment is required for choline oxidation. Finally, we demonstrate that Dde_3291, a MerR-like transcription factor, is a choline-dependent activator of the choline utilization cluster. Taken together, our data set and genetic resources provide a foundation for systems-level investigation of a poorly studied group of bacteria of environmental and industrial importance.ImportanceSulfate-reducing bacteria contribute to global nutrient cycles and are a nuisance for the petroleum industry. Despite their environmental and industrial significance, the genomes of sulfate-reducing bacteria remain poorly characterized. Here, we describe a genetic approach to fill gaps in our knowledge of sulfate-reducing bacteria. We generated a large collection of archived, transposon mutants in Desulfovibrio alaskensis G20 and used the phenotypes of these mutant strains to infer the function of genes involved in gene regulation, methionine biosynthesis, and choline utilization. Our findings and mutant resources will enable systematic investigations into gene function, energy generation, stress response, and metabolism for this important group of bacteria
High-multipolar effects on the Casimir force: the non-retarded limit
We calculate exactly the Casimir force or dispersive force, in the
non-retarded limit, between a spherical nanoparticle and a substrate beyond the
London's or dipolar approximation. We find that the force is a non-monotonic
function of the distance between the sphere and the substrate, such that, it is
enhanced by several orders of magnitude as the sphere approaches the substrate.
Our results do not agree with previous predictions like the Proximity theorem
approach.Comment: 7 pages including 2 figures. Submitted to Europjysics Letter
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