2,676 research outputs found
Restoring observed classical behavior of the carbon nanotube field emission enhancement factor from the electronic structure
Experimental Fowler-Nordheim plots taken from orthodoxly behaving carbon
nanotube (CNT) field electron emitters are known to be linear. This shows that,
for such emitters, there exists a characteristic field enhancement factor (FEF)
that is constant for a range of applied voltages and applied macroscopic fields
. A constant FEF of this kind can be evaluated for classical CNT
emitter models by finite-element and other methods, but (apparently contrary to
experiment) several past quantum-mechanical (QM) CNT calculations find
FEF-values that vary with . A common feature of most such
calculations is that they focus only on deriving the CNT real-charge
distributions. Here we report on calculations that use density functional
theory (DFT) to derive real-charge distributions, and then use these to
generate the related induced-charge distributions and related fields and FEFs.
We have analysed three carbon nanostructures involving CNT-like nanoprotrusions
of various lengths, and have also simulated geometrically equivalent classical
emitter models, using finite-element methods. We find that when the
DFT-generated local induced FEFs (LIFEFs) are used, the resulting values are
effectively independent of macroscopic field, and behave in the same
qualitative manner as the classical FEF-values. Further, there is fair to good
quantitative agreement between a characteristic FEF determined classically and
the equivalent characteristic LIFEF generated via DFT approaches. Although many
issues of detail remain to be explored, this appears to be a significant step
forwards in linking classical and QM theories of CNT electrostatics. It also
shows clearly that, for ideal CNTs, the known experimental constancy of the FEF
value for a range of macroscopic fields can also be found in appropriately
developed QM theory.Comment: A slightly revised version has been published - citation below -
under a title different from that originally used. The new title is:
"Restoring observed classical behavior of the carbon nanotube field emission
enhancement factor from the electronic structure
The Gravity Dual of the Ising Model
We evaluate the partition function of three dimensional theories of gravity
in the quantum regime, where the AdS radius is Planck scale and the central
charge is of order one. The contribution from the AdS vacuum sector can - with
certain assumptions - be computed and equals the vacuum character of a minimal
model CFT. The torus partition function is given by a sum over geometries which
is finite and computable. For generic values of Newton's constant G and the AdS
radius L the result has no Hilbert space interpretation, but in certain cases
it agrees with the partition function of a known CFT. For example, the
partition function of pure Einstein gravity with G=3L equals that of the Ising
model, providing evidence that these theories are dual. We also present
somewhat weaker evidence that the 3-state and tricritical Potts models are dual
to pure higher spin theories of gravity based on SL(3) and E_6, respectively.Comment: 42 page
Modeling the Field Emission Enhancement Factor for Capped Carbon Nanotubes using the Induced Electron Density
In many field electron emission experiments on single-walled carbon nanotubes
(SWCNTs), the SWCNT stands on one of two well-separated parallel plane plates,
with a macroscopic field FM applied between them. For any given location "L" on
the SWCNT surface, a field enhancement factor (FEF) is defined as
/, where is a local field defined at "L".
The best emission measurements from small-radii capped SWCNTs exhibit
characteristic FEFs that are constant (i.e., independent of ). This
paper discusses how to retrieve this result in quantum-mechanical (as opposed
to classical electrostatic) calculations. Density functional theory (DFT) is
used to analyze the properties of two short, floating SWCNTS, capped at both
ends, namely a (6,6) and a (10,0) structure. Both have effectively the same
height ( nm) and radius ( nm). It is found that apex
values of local induced FEF are similar for the two SWCNTs, are independent of
, and are similar to FEF-values found from classical conductor
models. It is suggested that these induced-FEF values relate to the SWCNT
longitudinal system polarizabilities, which are presumed similar. The DFT
calculations also generate "real", as opposed to ``induced", potential-energy
(PE) barriers for the two SWCNTs, for FM-values from 3 V/m to 2 V/nm. PE
profiles along the SWCNT axis and along a parallel ``observation line" through
one of the topmost atoms are similar. At low macroscopic fields the details of
barrier shape differ for the two SWCNT types. Even for , there
are distinct PE structures present at the emitter apex (different for the two
SWCNTs); this suggests the presence of structure-specific chemically induced
charge transfers and related patch-field distributions
A combined approach for comparative exoproteome analysis of Corynebacterium pseudotuberculosis
Background: Bacterial exported proteins represent key components of the host-pathogen interplay. Hence, we
sought to implement a combined approach for characterizing the entire exoproteome of the pathogenic
bacterium Corynebacterium pseudotuberculosis, the etiological agent of caseous lymphadenitis (CLA) in sheep and
goats.
Results: An optimized protocol of three-phase partitioning (TPP) was used to obtain the C. pseudotuberculosis
exoproteins, and a newly introduced method of data-independent MS acquisition (LC-MSE) was employed for
protein identification and label-free quantification. Additionally, the recently developed tool SurfG+ was used for in
silico prediction of sub-cellular localization of the identified proteins. In total, 93 different extracellular proteins of
C. pseudotuberculosis were identified with high confidence by this strategy; 44 proteins were commonly identified
in two different strains, isolated from distinct hosts, then composing a core C. pseudotuberculosis exoproteome.
Analysis with the SurfG+ tool showed that more than 75% (70/93) of the identified proteins could be predicted as
containing signals for active exportation. Moreover, evidence could be found for probable non-classical export of
most of the remaining proteins.
Conclusions: Comparative analyses of the exoproteomes of two C. pseudotuberculosis strains, in addition to
comparison with other experimentally determined corynebacterial exoproteomes, were helpful to gain novel
insights into the contribution of the exported proteins in the virulence of this bacterium. The results presented
here compose the most comprehensive coverage of the exoproteome of a corynebacterial species so far
The quantum Hall effect in graphene samples and the relativistic Dirac effective action
We study the Euclidean effective action per unit area and the charge density
for a Dirac field in a two--dimensional spatial region, in the presence of a
uniform magnetic field perpendicular to the 2D--plane, at finite temperature
and density. In the limit of zero temperature we reproduce, after performing an
adequate Lorentz boost, the Hall conductivity measured for different kinds of
graphene samples, depending upon the phase choice in the fermionic determinant.Comment: Conclusions extended. References added. 9 pages. 1 figur
Corrigendum to âIkkepsilon regulates viral-induced interferon regulatory factor-3 activation via a redox-sensitive pathwayâ [Virology 353 (2006) 155â165]
On the quantum mechanics of how an ideal carbon nanotube field emitter can exhibit a constant field enhancement factor
Measurements of current-voltage characteristics from ideal carbon nanotube
(CNT) field electron emitters of small apex radius have shown that these
emitters can exhibit a linear Fowler-Nordheim (FN) plot [e.g., Dean and
Chalamala, Appl. Phys. Lett., 76, 375, 2000]. From such a plot, a constant
(voltage-independent) characteristic field enhancement factor (FEF) can be
deduced. Over fifteen years later, this experimental result has not yet been
convincingly retrieved from first-principles electronic structure calculations,
or more generally from quantum mechanics (QM). On the contrary, several QM
calculations have deduced that the characteristic FEF should be a function of
the macroscopic field applied to the CNT. This apparent contradiction between
experiment and QM theory has been an unexplained feature of CNT emission
science, and has raised doubts about the ability of existing QM models to
satisfactorily describe experimental CNT emission behavior. In this work we
demonstrate, by means of a density functional theory analysis of single-walled
CNTs "floating" in an applied macroscopic field, the following significant
result. This is that agreement between experiment, classical-conductor CNT
models and QM calculations can be achieved if the latter are used to calculate
(from the "real" total-charge-density distributions initially obtained) the
distributions of charge-density, induced local fields and
induced local FEFs. The present work confirms, more reliably and in
significantly greater detail than in earlier work on a different system, that
this finding applies to the common "post-on-a-conducing plane" situation of CNT
field electron emission. This finding also brings out various further
theoretical questions that need to be explored
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