3,402 research outputs found
Impurity State and Variable Range Hopping Conduction in Graphene
The variable range hopping theory, as formulated for exponentially localized
impurity states, does not necessarily apply in the case of graphene with
covalently attached impurities. We analyze the localization of impurity states
in graphene using the nearest-neighbor, tight-binding model of an
adatom-graphene system with Green's function perturbation methods. The
amplitude of the impurity state wave function is determined to decay as a power
law with exponents depending on sublattice, direction, and the impurity
species. We revisit the variable range hopping theory in view of this result
and find that the conductivity depends as a power law of the temperature with
an exponent related to the localization of the wave function. We show that this
temperature dependence is in agreement with available experimental results
Effective Action of QED in Electric Field Backgrounds II: Spatially Localized Fields
We find the Bogoliubov coefficient from the tunneling boundary condition on a
charged particle coupled to a static electric field and,
using the regularization scheme in Phys. Rev. D 78, 105013 (2008), obtain the
exact one-loop effective action in scalar and spinor QED. It is shown that the
effective action satisfies the general relation between the vacuum persistence
and the mean number of produced pairs. We advance an approximation method for
general electric fields and show the duality between the space-dependent and
time-dependent electric fields of the same form at the leading order of the
effective actions.Comment: RevTex 7 pages, no figure; extension of arXiv:0807.2696 to
space-dependent electric fields; new section added on approximate effective
actions in general electric fields and conclusion shortened; references
added; replaced by the version to be published in Phys. Rev.
Improved Approximations for Fermion Pair Production in Inhomogeneous Electric Fields
Reformulating the instantons in a complex plane for tunneling or transmitting
states, we calculate the pair-production rate of charged fermions in a
spatially localized electric field, illustrated by the Sauter electric field
E_0 sech^2 (z/L), and in a temporally localized electric field such as E_0
sech^2 (t/T). The integration of the quadratic part of WKB instanton actions
over the frequency and transverse momentum leads to the pair-production rate
obtained by the worldline instanton method, including the prefactor, of Phys.
Rev. D72, 105004 (2005) and D73, 065028 (2006). It is further shown that the
WKB instanton action plus the next-to-leading order contribution in spinor QED
equals the WKB instanton action in scalar QED, thus justifying why the WKB
instanton in scalar QED can work for the pair production of fermions. Finally
we obtain the pair-production rate in a spatially localized electric field
together with a constant magnetic field in the same direction.Comment: RevTex, 12 pages, two figures; replaced by the version accepted in
Phys. Rev.
Magnetic Response of Hydrothermally Prepared Self-Assembled Co3O4 Nano-platelets
In the present communication, we report a strong ferrimagnetic behavior of self-assembled Co[subscript 3]O[subscript 4] nano-platelets, which most likely originates from the intrinsic spin structure of the unique Co[subscript 3]O[subscript 4] structure. The microsphere-like structures are composed of nano-platelets that are entangled together to form the organized network. These anomalous ferrimagnetic properties can be rationalized by supposing that one of the Co[superscript 3+] and one of the Co[superscript 2+] ions are switched between the octahedral and tetrahedral sites. The powder sample was also characterized by x-ray diffraction and superconducting quantum interface device magnetometry.National Research Foundation of Korea (No. 2013029776 (Mid-career Researcher Program)) Remove selectedResearch Institute for Solar and Sustainable Energies (Core Technology Development Program
The Significance of Powder Breakdown During Conveying Within Industrial Milk Powder Plants
Instant whole milk powder (IWMP) is designed to rapidly dissolve in water, which depends on the particle size distribution (PSD). The fragile milk powder exiting the dryer has to be conveyed for packing, which can break down the particles, worsening the dissolution properties. This work investigated the importance of in-plant conveying in determining the nal functional properties at the industrial scale. IWMP breakdown was compared between two plants with di erent transport systems; a pneumatic system and bucket elevator. It was expected that the plant with the bucket elevator consistently produced powder with superior dissolution due to lower breakdown during transport. This was evaluated using the change in PSD. It was found that both plants had a similar decrease in the median particle size, and powder with an initially larger particle size showed more breakdown. However, it was not enough to compensate for the initially larger size. Thus powder that started out larger still had a larger particle size after transport. When quanti ed using the change in bulk density, a low initial bulk density compensated for large breakdown during conveying and ameliorated the impact on the functional properties. Thus in order to produce IWMP with the desired functionalities the focus should be on improving the initial agglomeration, as oppose to reducing transport breakdown.
Experimental ionization of atomic hydrogen with few-cycle pulses
We present the first experimental data on strong-field ionization of atomic
hydrogen by few-cycle laser pulses. We obtain quantitative agreement at the 10%
level between the data and an {\it ab initio} simulation over a wide range of
laser intensities and electron energies
On effective superpotentials and Kutasov duality
We derive the effective superpotential for an N=1 SU(N_c) gauge theory with
one massless adjoint field and N_f massless fundamental flavors and cubic
tree-level superpotential for the adjoint field. This is a generalization of
the Affleck-Dine-Seiberg superpotential to gauge theories with one massless
adjoint matter field. Using Kutasov's generalization of Seiberg duality, we
then find the effective superpotential for a related theory with massive
fundamental flavors.Comment: 21 pages, Late
Parker Instability in a Self-Gravitating Magnetized Gas Disk: I. Linear Stability Analysis
To be a formation mechanism of such large-scale structures as giant molecular
clouds (GMCs) and HI superclouds, the classical Parker instability driven by
external gravity has to overcome three major obstacles: The convective motion
accompanying the instability generates thin sheets than large condensations.
The degree of density enhancement achieved by the instability is too low to
make dense interstellar clouds. The time and the length scales of the
instability are significantly longer and larger than the estimated formation
time and the observed mean separation of the GMCs, respectively. This paper
examines whether a replacement of the driving agent from the external to the
self gravity might remove these obstacles by activating the gravitational
instability in the Galactic ISM disk. The self gravity can suppress the
convective motions, and a cooperative action of the Jeans and the Parker
instabilities can remove all the obstacles confronting the classical version of
the Parker instability. The mass and mean separation of the structures
resulting from the odd-parity undular mode solution are shown to agree better
with the HI superclouds than with the GMCs. We briefly discuss how inclusions
of the external gravity and cosmic rays would modify behaviors of the
odd-parity undular mode solution.Comment: 53 pages, 21 figure
Schwinger Pair Production in dS_2 and AdS_2
We study Schwinger pair production in scalar QED from a uniform electric
field in dS_2 with scalar curvature R_{dS} = 2 H^2 and in AdS_2 with R_{AdS} =
- 2 K^2. With suitable boundary conditions, we find that the pair-production
rate is the same analytic function of the scalar curvature in both cases.Comment: RevTex 6 pages, no figure; replaced by the version published in PR
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