94,773 research outputs found
A Novel VSWR-Protected and Controllable CMOS Class E Power Amplifier for Bluetooth Applications
This paper describes the design of a differential class-E PA for Bluetooth
applications in 0.18um CMOS technology with load mismatch protection and power
control features. The breakdown induced by load mismatch can be avoided by
attenuating the RF power to the final stage during over voltage conditions.
Power control is realized by means of "open loop" techniques to regulate the
power supply voltage, and a novel controllable bias network with temperature
compensated is proposed, which allows a moderate power control slope (dB/V) to
be achieved. Post-layout Simulation results show that the level of output power
can be controlled in 2dBm steps; especially the output power in every step is
quite insensitive to temperature variations
Evolution of transverse flow and effective temperatures in the parton phase from a multi-phase transport model
I study the space-time evolution of transverse flow and effective
temperatures in the dense parton phase with the string melting version of a
multi-phase transport model. Parameters of the model are first constrained to
reproduce the bulk data on the rapidity density, spectrum and
elliptic flow at low for central and mid-central Au+Au collisions
at GeV and Pb+Pb collisions at GeV. I then calculate the
transverse flow and effective temperatures in volume cells within
mid-spacetime-rapidity . I find that the effective temperatures
extracted from different variables, which are all evaluated in the rest frame
of a volume cell, can be very different; this indicates that the parton system
in the model is not in full chemical or thermal equilibrium locally, even after
averaging over many events. In particular, the effective temperatures extracted
from the parton energy density or number density are often quite different than
those extracted from the parton mean or mean energy. For these
collisions in general, effective temperatures extracted from the parton energy
density or number density are higher than those extracted from the parton mean
in the inner part of the overlap volume, while the opposite occurs
in the outer part of the overlap volume. I argue that this indicates that the
dense parton matter in the inner part of the overlap volume is over-populated;
I also find that all cells with energy density above 1 GeV/fm are
over-populated after a couple of fm/.Comment: Corrected a few typos including one in Eq.(8
DGDFT: A Massively Parallel Method for Large Scale Density Functional Theory Calculations
We describe a massively parallel implementation of the recently developed
discontinuous Galerkin density functional theory (DGDFT) [J. Comput. Phys.
2012, 231, 2140] method, for efficient large-scale Kohn-Sham DFT based
electronic structure calculations. The DGDFT method uses adaptive local basis
(ALB) functions generated on-the-fly during the self-consistent field (SCF)
iteration to represent the solution to the Kohn-Sham equations. The use of the
ALB set provides a systematic way to improve the accuracy of the approximation.
It minimizes the number of degrees of freedom required to represent the
solution to the Kohn-Sham problem for a desired level of accuracy. In
particular, DGDFT can reach the planewave accuracy with far fewer numbers of
degrees of freedom. By using the pole expansion and selected inversion (PEXSI)
technique to compute electron density, energy and atomic forces, we can make
the computational complexity of DGDFT scale at most quadratically with respect
to the number of electrons for both insulating and metallic systems. We show
that DGDFT can achieve 80% parallel efficiency on 128,000 high performance
computing cores when it is used to study the electronic structure of
two-dimensional (2D) phosphorene systems with 3,500-14,000 atoms. This high
parallel efficiency results from a two-level parallelization scheme that we
will describe in detail.Comment: 13 pages, 8 figures in J. Chem. Phys. 2015. arXiv admin note: text
overlap with arXiv:1501.0503
Edge reconstruction in armchair phosphorene nanoribbons revealed by discontinuous Galerkin density functional theory
With the help of our recently developed massively parallel DGDFT
(Discontinuous Galerkin Density Functional Theory) methodology, we perform
large-scale Kohn-Sham density functional theory calculations on phosphorene
nanoribbons with armchair edges (ACPNRs) containing a few thousands to ten
thousand atoms. The use of DGDFT allows us to systematically achieve
conventional plane wave basis set type of accuracy, but with a much smaller
number (about 15) of adaptive local basis (ALB) functions per atom for this
system. The relatively small number degrees of freedom required to represent
the Kohn-Sham Hamiltonian, together with the use of the pole expansion the
selected inversion (PEXSI) technique that circumvents the need to diagonalize
the Hamiltonian, result in a highly efficient and scalable computational scheme
for analyzing the electronic structures of ACPNRs as well as its dynamics. The
total wall clock time for calculating the electronic structures of large-scale
ACPNRs containing 1080-10800 atoms is only 10-25 s per self-consistent field
(SCF) iteration, with accuracy fully comparable to that obtained from
conventional planewave DFT calculations. For the ACPNR system, we observe that
the DGDFT methodology can scale to 5,000-50,000 processors. We use DGDFT based
ab-initio molecular dynamics (AIMD) calculations to study the thermodynamic
stability of ACPNRs. Our calculations reveal that a 2 * 1 edge reconstruction
appears in ACPNRs at room temperature.Comment: 9 pages, 5 figure
Projected Commutator DIIS Method for Accelerating Hybrid Functional Electronic Structure Calculations
The commutator direct inversion of the iterative subspace (commutator DIIS or
C-DIIS) method developed by Pulay is an efficient and the most widely used
scheme in quantum chemistry to accelerate the convergence of self consistent
field (SCF) iterations in Hartree-Fock theory and Kohn-Sham density functional
theory. The C-DIIS method requires the explicit storage of the density matrix,
the Fock matrix and the commutator matrix. Hence the method can only be used
for systems with a relatively small basis set, such as the Gaussian basis set.
We develop a new method that enables the C-DIIS method to be efficiently
employed in electronic structure calculations with a large basis set such as
planewaves for the first time. The key ingredient is the projection of both the
density matrix and the commutator matrix to an auxiliary matrix called the
gauge-fixing matrix. The resulting projected commutator-DIIS method (PC-DIIS)
only operates on matrices of the same dimension as the that consists of
Kohn-Sham orbitals. The cost of the method is comparable to that of standard
charge mixing schemes used in large basis set calculations. The PC-DIIS method
is gauge-invariant, which guarantees that its performance is invariant with
respect to any unitary transformation of the Kohn-Sham orbitals. We demonstrate
that the PC-DIIS method can be viewed as an extension of an iterative
eigensolver for nonlinear problems. We use the PC-DIIS method for accelerating
Kohn-Sham density functional theory calculations with hybrid
exchange-correlation functionals, and demonstrate its superior performance
compared to the commonly used nested two-level SCF iteration procedure
Interpolative Separable Density Fitting through Centroidal Voronoi Tessellation With Applications to Hybrid Functional Electronic Structure Calculations
The recently developed interpolative separable density fitting (ISDF)
decomposition is a powerful way for compressing the redundant information in
the set of orbital pairs, and has been used to accelerate quantum chemistry
calculations in a number of contexts. The key ingredient of the ISDF
decomposition is to select a set of non-uniform grid points, so that the values
of the orbital pairs evaluated at such grid points can be used to accurately
interpolate those evaluated at all grid points. The set of non-uniform grid
points, called the interpolation points, can be automatically selected by a QR
factorization with column pivoting (QRCP) procedure. This is the
computationally most expensive step in the construction of the ISDF
decomposition. In this work, we propose a new approach to find the
interpolation points based on the centroidal Voronoi tessellation (CVT) method,
which offers a much less expensive alternative to the QRCP procedure when ISDF
is used in the context of hybrid functional electronic structure calculations.
The CVT method only uses information from the electron density, and can be
efficiently implemented using a K-Means algorithm. We find that this new method
achieves comparable accuracy to the ISDF-QRCP method, at a cost that is
negligible in the overall hybrid functional calculations. For instance, for a
system containing silicon atoms simulated using the HSE06 hybrid
functional on computational cores, the cost of QRCP-based method for
finding the interpolation points costs seconds, while the CVT procedure
only takes seconds. We also find that the ISDF-CVT method also enhances
the smoothness of the potential energy surface in the context of \emph{ab
initio} molecular dynamics (AIMD) simulations with hybrid functionals
D Wave Heavy Mesons
We first extract the binding energy and decay constants of the
D wave heavy meson doublets and with QCD sum
rule in the leading order of heavy quark effective theory. Then we study their
pionic couplings using the light cone sum rule, from which the
parameter can also be extracted. We then calculate the pionic
decay widths of the strange/non-strange D wave heavy mesons and discuss
the possible candidates for the D wave charm-strange mesons. Further
experimental information, such as the ratio between and modes,
will be very useful to distinguish various assignments for .Comment: 10 pages, 3 figuers, 3 tables. Some descriptions changed, typos
corrected. Published version in PR
Distributed Adaptive Gradient Optimization Algorithm
In this paper, a distributed optimization problem with general differentiable
convex objective functions is studied for single-integrator and
double-integrator multi-agent systems. Two distributed adaptive optimization
algorithm is introduced which uses the relative information to construct the
gain of the interaction term. The analysis is performed based on the Lyapunov
functions, the analysis of the system solution and the convexity of the local
objective functions. It is shown that if the gradients of the convex objective
functions are continuous, the team convex objective function can be minimized
as time evolves for both single-integrator and double-integrator multi-agent
systems. Numerical examples are included to show the obtained theoretical
results.Comment: 12 pages, 3 figure
Superfluids Passing an Obstacle and Vortex Nucleation
We consider a superfluid described by the Gross-Pitaevskii equation passing
an obstacle \epsilon^2 \Delta u+ u(1-|u|^2)=0 \ \mbox{in} \ {\mathbb R}^d
\backslash \Omega, \ \ \frac{\partial u}{\partial \nu}=0 \ \mbox{on}\ \partial
\Omega where is a smooth bounded domain in
(), which is referred as the obstacle and is
sufficiently small. We first construct a vortex free solution of the form with
where is the unique solution for the subsonic irrotational
flow equation \nabla ( (1-|\nabla \Phi|^2)\nabla \Phi )=0 \ \mbox{in} \
{\mathbb R}^d \backslash \Omega, \ \frac{\partial \Phi}{\partial \nu} =0 \
\mbox{on} \ \partial \Omega, \ \nabla \Phi (x) \to \delta \vec{e}_d \ \mbox{as}
\ |x| \to +\infty and (the sound speed).
In dimension , on the background of this vortex free solution we also
construct solutions with single vortex close to the maximum or minimum points
of the function (which are on the boundary of the
obstacle). The latter verifies the vortex nucleation phenomena (for the steady
states) in superfluids described by the Gross-Pitaevskii equations. Moreover,
after some proper scalings, the limits of these vortex solutions are traveling
wave solution of the Gross-Pitaevskii equation. These results also show
rigorously the conclusions drawn from the numerical computations in
\cite{huepe1, huepe2}.
Extensions to Dirichlet boundary conditions, which may be more consistent
with the situation in the physical experiments and numerical simulations (see
\cite{ADP} and references therein) for the trapped Bose-Einstein condensates,
are also discussed.Comment: 21 pages; comments are very welcom
Stability Analysis of Multi-Period Electricity Market with Heterogeneous Dynamic Assets
Market-based coordination of demand side assets has gained great interests in
recent years. In spite of its efficiency, there is a risk that the interaction
between the dynamic assets through the price signal could result in an unstable
closed-loop system. This may cause oscillating power consumption profiles and
high volatile energy price. This paper proposes an electricity market model
which explicitly considers the heterogeneous dynamic asset models. We show that
the market dynamics can be modeled by a discrete nonlinear system, and then
derive analytical conditions to guarantee the stability of the market via
contraction analysis. These conditions imply that the market stability can be
guaranteed by choosing bidding functions with relatively shallower slopes in
the linear region. Finally, numerical examples are provided to demonstrate the
application of the derived stability conditions
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