11,031 research outputs found
Once again: Instanton method vs. WKB
A recent analytic test of the instanton method performed by comparing the
exact spectrum of the Lam potential (derived from representations
of a finite dimensional matrix expressed in terms of generators) with
the results of the tight--binding and instanton approximations as well as the
standard WKB approximation is commented upon. It is pointed out that in the
case of the Lam potential as well as others the WKB--related method
of matched asymptotic expansions yields the exact instanton result as a result
of boundary conditions imposed on wave functions which are matched in domains
of overlap.Comment: 10 pages, no figures. References list revised according to JHE
The vortex dynamics of a Ginzburg-Landau system under pinning effect
It is proved that the vortices are attracted by impurities or inhomogeities
in the superconducting materials. The strong H^1-convergence for the
corresponding Ginzburg-Landau system is also proved.Comment: 23page
Region-based Skin Color Detection.
Skin color provides a powerful cue for complex computer vision applications. Although skin color detection
has been an active research area for decades, the mainstream technology is based on the individual pixels.
This paper presents a new region-based technique for skin color detection which outperforms the current
state-of-the-art pixel-based skin color detection method on the popular Compaq dataset (Jones and Rehg,
2002). Color and spatial distance based clustering technique is used to extract the regions from the images,
also known as superpixels. In the first step, our technique uses the state-of-the-art non-parametric pixel-based
skin color classifier (Jones and Rehg, 2002) which we call the basic skin color classifier. The pixel-based skin
color evidence is then aggregated to classify the superpixels. Finally, the Conditional Random Field (CRF)
is applied to further improve the results. As CRF operates over superpixels, the computational overhead is
minimal. Our technique achieves 91.17% true positive rate with 13.12% false negative rate on the Compaq
dataset tested over approximately 14,000 web images
Semiquantum key distribution using entangled states
Recently, Boyer et al. presented a novel semiquantum key distribution
protocol [M. Boyer, D. Kenigsberg, and T. Mor, Phys. Rev. Lett. 99, 140501
(2007)], by using four quantum states, each of which is randomly prepared by Z
basis or X basis. Here we present a semiquantum key distribution protocol by
using entangled states in which quantum Alice shares a secret key with
classical Bob. We also show the protocol is secure against eavesdropping.Comment: 6 page
QCD corrections to polarization of J/\psi and \Upsilon at Fermilab Tevatron and CERN LHC
In this work, we present more detail of the calculation on the NLO QCD
corrections to polarization of direct J/psi production via color singlet at
Tevatron and LHC, as well as the results for Upsilon for the first time. Our
results show that the J/psi polarization status drastically changes from
transverse polarization dominant at LO into longitudinal polarization dominant
in the whole range of the transverse momentum of J/psi when the NLO
corrections are counted. For Upsilon production, the p_t distribution of the
polarization status behaves almost the same as that for J/psi except that the
NLO result is transverse polarization at small p_t range. Although the
theoretical evaluation predicts a larger longitudinal polarization than the
measured value at Tevatron, it may provide a solution towards the previous
large discrepancy for J/psi and Upsilon polarization between theoretical
predication and experimental measurement, and suggests that the next important
step is to calculate the NLO corrections to hadronproduction of color octet
state J/psi^(8) and Upsilon^(8). Our calculations are performed in two ways,
namely we do and do not analytically sum over the polarizations, and then check
them with each other.Comment: 12 pages, 12 figures, two columns, use revtex4; to appear in PR
Real-time motion data annotation via action string
Even though there is an explosive growth of motion capture data, there is still a lack of efficient and reliable methods to automatically annotate all the motions in a database. Moreover, because of the popularity of mocap devices in home entertainment systems, real-time human motion annotation or recognition becomes more and more imperative. This paper presents a new motion annotation method that achieves both the aforementioned two targets at the same time. It uses a probabilistic pose feature based on the Gaussian Mixture Model to represent each pose. After training a clustered pose feature model, a motion clip could be represented as an action string. Then, a dynamic programming-based string matching method is introduced to compare the differences between action strings. Finally, in order to achieve the real-time target, we construct a hierarchical action string structure to quickly label each given action string. The experimental results demonstrate the efficacy and efficiency of our method
Interfacial thermal transport in atomic junctions
We study ballistic interfacial thermal transport across atomic junctions.
Exact expressions for phonon transmission coefficients are derived for thermal
transport in one-junction and two-junction chains, and verified by numerical
calculation based on a nonequilibrium Green's function method. For a
single-junction case, we find that the phonon transmission coefficient
typically decreases monotonically with increasing freqency. However, in the
range between equal frequency spectrum and equal acoustic impedance, it
increases first then decreases, which explains why the Kapitza resistance
calculated from the acoustic mismatch model is far larger than the experimental
values at low temperatures. The junction thermal conductance reaches a maximum
when the interfacial coupling equals the harmonic average of the spring
constants of the two semi-infinite chains. For three-dimensional junctions, in
the weak coupling limit, we find that the conductance is proportional to the
square of the interfacial coupling, while for intermediate coupling strength
the conductance is approximately proportional to the interfacial coupling
strength. For two-junction chains, the transmission coefficient oscillates with
the frequency due to interference effects. The oscillations between the two
envelop lines can be understood analytically, thus providing guidelines in
designing phonon frequency filters.Comment: 10 pages, 13 figures. Accepted by Phys. Rev.
Phonon Hall Effect in Four-Terminal Junctions
Using an exact nonequilibrium Green's function formulism, the phonon Hall
effect for paramagnetic dielectrics is studied in a four-terminal device
setting. The temperature difference in the transverse direction of the heat
current is calculated for two-dimensional models with the magnetic field
perpendicular to the plane. We find a surprising result that the square lattice
does not have the phonon Hall effect while a honeycomb lattice has. This can be
explained by symmetry. The temperature difference changes sign if the magnetic
field is sufficiently large.Comment: 4 pages, 5 figure
- …