8,435 research outputs found
Lattice structures for optimal design and robust implementation of two-channel perfect-reconstruction QMF banks
A lattice structure and an algorithm are presented for the design of two-channel QMF (quadrature mirror filter) banks, satisfying a sufficient condition for perfect reconstruction. The structure inherently has the perfect-reconstruction property, while the algorithm ensures a good stopband attenuation for each of the analysis filters. Implementations of such lattice structures are robust in the sense that the perfect-reconstruction property is preserved in spite of coefficient quantization. The lattice structure has the hierarchical property that a higher order perfect-reconstruction QMF bank can be obtained from a lower order perfect-reconstruction QMF bank, simply by adding more lattice sections. Several numerical examples are provided in the form of design tables
CPA for charge ordering in the extended Hubbard model
We study charge ordering in the extended Hubbard model with both on-site and
nearest neighbor Coulomb repulsion (U and V, respectively) within the Coherent
potential approximation (CPA). The phase boundary between the homogeneous and
charge ordered phase for the square lattice is obtained for different values of
U. It is shown that at quarter filling for all values of U the charge ordering
exists only if the inter-site Coulomb repulsion V exceeds certain critical
value which is of the order of the kinetic energy t. At finite temperature a
reentrant transition is found in some region of V.Comment: 5 pages, 4 figure
Electroweak non-resonant NLO corrections to e+ e- -> W+ W- b bbar in the t tbar resonance region
We analyse subleading electroweak effects in the top anti-top resonance
production region in e+ e- collisions which arise due to the decay of the top
and anti-top quarks into the W+ W- b bbar final state. These are NLO
corrections adopting the non-relativistic power counting v ~ alpha_s ~
sqrt(alpha_EW). In contrast to the QCD corrections which have been calculated
(almost) up to NNNLO, the parametrically larger NLO electroweak contributions
have not been completely known so far, but are mandatory for the required
accuracy at a future linear collider. The missing parts of these NLO
contributions arise from matching coefficients of non-resonant production-decay
operators in unstable-particle effective theory which correspond to off-shell
top production and decay and other non-resonant irreducible background
processes to t tbar production. We consider the total cross section of the e+
e- -> W+ W- b bbar process and additionally implement cuts on the invariant
masses of the W+ b and W- bbar pairs.Comment: LaTeX, 33 pages, 6 figure
An isogeometric analysis for elliptic homogenization problems
A novel and efficient approach which is based on the framework of
isogeometric analysis for elliptic homogenization problems is proposed. These
problems possess highly oscillating coefficients leading to extremely high
computational expenses while using traditional finite element methods. The
isogeometric analysis heterogeneous multiscale method (IGA-HMM) investigated in
this paper is regarded as an alternative approach to the standard Finite
Element Heterogeneous Multiscale Method (FE-HMM) which is currently an
effective framework to solve these problems. The method utilizes non-uniform
rational B-splines (NURBS) in both macro and micro levels instead of standard
Lagrange basis. Beside the ability to describe exactly the geometry, it
tremendously facilitates high-order macroscopic/microscopic discretizations
thanks to the flexibility of refinement and degree elevation with an arbitrary
continuity level provided by NURBS basis functions. A priori error estimates of
the discretization error coming from macro and micro meshes and optimal micro
refinement strategies for macro/micro NURBS basis functions of arbitrary orders
are derived. Numerical results show the excellent performance of the proposed
method
Strong efficiency improvement of SOI-LEDs through carrier confinement
Contemporary silicon light-emitting diodes in silicon-on-insulator (SOI) technology suffer from poor efficiency compared to their bulk-silicon counterparts. In this letter, we present a new device structure where the carrier injection takes place through silicon slabs of only a few nanometer thick. Its external quantum efficiency of 1.4 • 10−4 at room temperature, with a spectrum peaking at 1130 nm, is almost two orders higher than reported thus far on SOI. The structure diminishes the dominant role of nonradiative recombination at the n+ and p+contacts, by confining the injected carriers in an SOI peninsula.\ud
With this approach, a compact infrared light source can be fabricated using standard semiconductor processing steps.\u
Case records of the Massachusetts General Hospital. Case 1-2014. A 32-year-old man with loss of vision and a rash
Implementing invariant mass cuts and finite lifetime effects in top-antitop production at threshold
The effects of the finite top quark width in the top pair production cross
section close to the threshold are discussed in this talk. We introduce a
cross section with a cut on the invariant masses of the top and
antitop that can be calculated theoretically with effective field theory (EFT)
methods. The matching procedure to implement the physical phase-space
boundaries in the NRQCD framework (``phase-space matching'') is briefly
outlined.Comment: 4 pages, 4 figures. Talk given at the 14th International QCD
Conference (Montpellier 7-12th July 2008
- …