15,524 research outputs found
Quantum phase transition and engineering in two-component BEC in optical lattices
In this paper we review recent progress in studying quantum phase transitions
in one- and two-component Bose-Einstein condensates (BEC) in optical lattices.
These phase transitions involve the emergence and disappearance of quantum
coherence over whole optical lattice and of linear superposition of macroscopic
quantum states. The latter may provide new means to engineer and to manipulate
novel macroscopic quantum states and novel coherent atomic beams for quantum
information processing, quantum computing etc.Comment: Format: LaTex2e. 7 pages, no figure. Talk at the Yang Symposium (in
honor of C.N. Yang's 80th birthday), Beijing, China, June 2002. To appear in
the Proceeding
Distributive Power Control Algorithm for Multicarrier Interference Network over Time-Varying Fading Channels - Tracking Performance Analysis and Optimization
Distributed power control over interference limited network has received an
increasing intensity of interest over the past few years. Distributed solutions
(like the iterative water-filling, gradient projection, etc.) have been
intensively investigated under \emph{quasi-static} channels. However, as such
distributed solutions involve iterative updating and explicit message passing,
it is unrealistic to assume that the wireless channel remains unchanged during
the iterations. Unfortunately, the behavior of those distributed solutions
under \emph{time-varying} channels is in general unknown. In this paper, we
shall investigate the distributed scaled gradient projection algorithm (DSGPA)
in a pairs multicarrier interference network under a finite-state Markov
channel (FSMC) model. We shall analyze the \emph{convergence property} as well
as \emph{tracking performance} of the proposed DSGPA. Our analysis shows that
the proposed DSGPA converges to a limit region rather than a single point under
the FSMC model. We also show that the order of growth of the tracking errors is
given by \mathcal{O}\(1 \big/ \bar{N}\), where is the \emph{average
sojourn time} of the FSMC. Based on the analysis, we shall derive the
\emph{tracking error optimal scaling matrices} via Markov decision process
modeling. We shall show that the tracking error optimal scaling matrices can be
implemented distributively at each transmitter. The numerical results show the
superior performance of the proposed DSGPA over three baseline schemes, such as
the gradient projection algorithm with a constant stepsize.Comment: To Appear on the IEEE Transaction on Signal Processin
Extended Schmidt law holds for faint dwarf irregular galaxies
The extended Schmidt law (ESL) is a variant of the Schmidt law which relates
the surface densities of gas and star formation, with the surface density of
stellar mass added as an extra parameter. We empirically investigate for the
first time whether low metallicity faint dwarf irregular galaxies (dIrrs)
follow the ESL. Here we consider the `global' law where surface densities are
averaged over the galactic discs. dIrrs are unique not only because they are at
the lowest end of mass and star formation scales for galaxies, but also because
they are metal-poor compared to the general population of galaxies. Our sample
is drawn from the Faint Irregular Galaxy GMRT Survey (FIGGS) which is the
largest survey of atomic hydrogen in such galaxies. The gas surface densities
are determined using their atomic hydrogen content. The star formation rates
are calculated using GALEX far ultraviolet fluxes after correcting for dust
extinction, whereas the stellar surface densities are calculated using Spitzer
3.6 m fluxes. All surface densities are calculated over stellar discs
defined by the 3.6 m images. We find dIrrs indeed follow the extended
Schmidt law. The mean deviation of the FIGGS galaxies from the relation is 0.01
dex, with a scatter around the relation of less than half that seen in the
original relation. In comparison, we also show that the FIGGS galaxies are much
more deviant when compared to the `canonical' Kennicutt-Schmidt relation. Our
results help strengthen the universality of the extended Schmidt law,
especially for galaxies with low metallicities. We suggest that models of star
formation in which feedback from previous generations of stars set the pressure
in the ISM, are promising candidates for explaining the ESL. We also confirm
that ESL is an independent relation and not a form of a relation between star
formation efficiency and metallicity.Comment: Accepted for publication in Astronomy & Astrophysics. Figure 2 on
Page 5 shows the main resul
Lower dimensional volumes and the Kastler-Kalau-Walze type theorem for Manifolds with Boundary
In this paper, we define lower dimensional volumes of spin manifolds with
boundary. We compute the lower dimensional volume for
5-dimensional and 6-dimensional spin manifolds with boundary and we also get
the Kastler-Kalau-Walze type theorem in this case
Distributive Network Utility Maximization (NUM) over Time-Varying Fading Channels
Distributed network utility maximization (NUM) has received an increasing
intensity of interest over the past few years. Distributed solutions (e.g., the
primal-dual gradient method) have been intensively investigated under fading
channels. As such distributed solutions involve iterative updating and explicit
message passing, it is unrealistic to assume that the wireless channel remains
unchanged during the iterations. Unfortunately, the behavior of those
distributed solutions under time-varying channels is in general unknown. In
this paper, we shall investigate the convergence behavior and tracking errors
of the iterative primal-dual scaled gradient algorithm (PDSGA) with dynamic
scaling matrices (DSC) for solving distributive NUM problems under time-varying
fading channels. We shall also study a specific application example, namely the
multi-commodity flow control and multi-carrier power allocation problem in
multi-hop ad hoc networks. Our analysis shows that the PDSGA converges to a
limit region rather than a single point under the finite state Markov chain
(FSMC) fading channels. We also show that the order of growth of the tracking
errors is given by O(T/N), where T and N are the update interval and the
average sojourn time of the FSMC, respectively. Based on this analysis, we
derive a low complexity distributive adaptation algorithm for determining the
adaptive scaling matrices, which can be implemented distributively at each
transmitter. The numerical results show the superior performance of the
proposed dynamic scaling matrix algorithm over several baseline schemes, such
as the regular primal-dual gradient algorithm
Disordered Quantum Spin Chains with Long-Range Antiferromagnetic Interactions
We investigate the magnetic susceptibility of quantum spin chains
of spins with power-law long-range antiferromagnetic coupling as a
function of their spatial decay exponent and cutoff length . The
calculations are based on the strong disorder renormalization method which is
used to obtain the temperature dependence of and distribution
functions of couplings at each renormalization step. For the case with only
algebraic decay () we find a crossover at
between a phase with a divergent low-temperature susceptibility
for to a phase with a vanishing
for . For finite cutoff lengths
, this crossover occurs at a smaller . Additionally we
study the localization of spin excitations for by evaluating
the distribution function of excitation energies and we find a delocalization
transition that coincides with the opening of the pseudo-gap at
.Comment: 6 pages, 7 figure
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