165 research outputs found
Three-State Feshbach Resonances Mediated By Second-Order Couplings
We present an analytical study of three-state Feshbach resonances induced by
second-order couplings. Such resonances arise when the scattering amplitude is
modified by the interaction with a bound state that is not directly coupled to
the scattering state containing incoming flux. Coupling occurs indirectly
through an intermediate state. We consider two problems: (i) the intermediate
state is a scattering state in a distinct open channel; (ii) the intermediate
state is an off-resonant bound state in a distinct closed channel. The first
problem is a model of electric-field-induced resonances in ultracold collisions
of alkali metal atoms [Phys. Rev. A 75, 032709 (2007)] and the second problem
is relevant for ultracold collisions of complex polyatomic molecules, chemical
reaction dynamics, photoassociation of ultracold atoms, and electron - molecule
scattering. Our analysis yields general expressions for the energy dependence
of the T-matrix elements modified by three-state resonances and the dependence
of the resonance positions and widths on coupling amplitudes for the
weak-coupling limit. We show that the second problem can be generalized to
describe resonances induced by indirect coupling through an arbitrary number of
sequentially coupled off-resonant bound states and analyze the dependence of
the resonance width on the number of the intermediate states.Comment: 27 pages, 4 figures; added a reference; journal reference/DOI refer
to final published version, which is a shortened and modified version of this
preprin
A Factor Graph Approach to Exploiting Cyclic Prefix for Equalization in OFDM Systems
In OFDM systems, cyclic prefix (CP) insertion and removal enables the use of a set of computationally efficient single-tap equalizers at the receiver. Due to the extra transmission time and energy, the CP causes a loss in both spectrum efficiency and power efficiency. On the other hand, as a repetition of part of the data, the CP brings extra information and can be exploited for detection. Therefore, instead of discarding the CP observation as in the conventional OFDM system, we utilize all the received signals in a soft-input soft-output equalizer of a turbo equalization OFDM system. First, the models for both the CP part and the non-CP part of observation are presented in a Forney-style factor graph (FFG). Then based on the computation rules of the FFG and the Gaussian message passing (GMP) technique, we develop an equalization algorithm. With proper approximation, the complexity of the proposed algorithm is reduced to O(2RNlog2N+4RGlog2G+2RG) per data block for R iterations, where N is the length of the data block and G is equal to P+L-1 with P the length of the CP and L the maximum delay spread of the channel. To justify the performance improvement, SNR analysis is provided. Simulation results show that the proposed approach achieves a significant gain over the conventional approach and the turbo equalization system converges within two iterations
Iterative frequency domain equalization with generalized approximate message passing
An iterative frequency domain equalization approach for coded single-carrier block transmissions over frequency selective channels is developed by using the recently proposed generalized approximate message passing (GAMP) algorithm. Compared with the low-complexity iterative frequency domain linear minimum mean square error (FD-LMMSE) equalization, the proposed approach can achieve significant performance gain with slight complexity increase
Nasal localization of a <i>Pseudoterranova decipiens</i> larva in a Danish patient with suspected allergic rhinitis
Incorporation of excluded volume correlations into Poisson-Boltzmann theory
We investigate the effect of excluded volume interactions on the electrolyte
distribution around a charged macroion. First, we introduce a criterion for
determining when hard-core effects should be taken into account beyond standard
mean field Poisson-Boltzmann (PB) theory. Next, we demonstrate that several
commonly proposed local density functional approaches for excluded volume
interactions cannot be used for this purpose. Instead, we employ a non-local
excess free energy by using a simple constant weight approach. We compare the
ion distribution and osmotic pressure predicted by this theory with Monte Carlo
simulations. They agree very well for weakly developed correlations and give
the correct layering effect for stronger ones. In all investigated cases our
simple weighted density theory yields more realistic results than the standard
PB approach, whereas all local density theories do not improve on the PB
density profiles but on the contrary, deviate even more from the simulation
results.Comment: 23 pages, 7 figures, 1 tabl
Statistical Mechanics of Membrane Protein Conformation: A Homopolymer Model
The conformation and the phase diagram of a membrane protein are investigated
via grand canonical ensemble approach using a homopolymer model. We discuss the
nature and pathway of -helix integration into the membrane that results
depending upon membrane permeability and polymer adsorptivity. For a membrane
with the permeability larger than a critical value, the integration becomes the
second order transition that occurs at the same temperature as that of the
adsorption transition. For a nonadsorbing membrane, the integration is of the
first order due to the aggregation of -helices.Comment: RevTeX with 5 postscript figure
Dynamics of heteropolymers in dilute solution: effective equation of motion and relaxation spectrum
The dynamics of a heteropolymer chain in solution is studied in the limit of
long chain length. Using functional integral representation we derive an
effective equation of motion, in which the heterogeneity of the chain manifests
itself as a time-dependent excluded volume effect. At the mean field level, the
heteropolymer chain is therefore dynamically equivalent to a homopolymer chain
with both time-independent and time-dependent excluded volume effects. The
perturbed relaxation spectrum is also calculated. We find that heterogeneity
also renormalizes the relaxation spectrum. However, we find, to the lowest
order in heterogeneity, that the relaxation spectrum does not exhibit any
dynamic freezing, at the point when static (equilibrium) ``freezing''
transition occurs in heteropolymer. Namely, the breaking of
fluctuation-dissipation theorem (FDT) proposed for spin glass dynamics does not
have dynamic effect in heteropolymer, as far as relaxation spectrum is
concerned. The implication of this result is discussed
A Method to Study Relaxation of Metastable Phases: Macroscopic Mean-Field Dynamics
We propose two different macroscopic dynamics to describe the decay of
metastable phases in many-particle systems with local interactions. These
dynamics depend on the macroscopic order parameter through the restricted
free energy and are designed to give the correct equilibrium
distribution for . The connection between macroscopic dynamics and the
underlying microscopic dynamic are considered in the context of a projection-
operator formalism. Application to the square-lattice nearest-neighbor Ising
ferromagnet gives good agreement with droplet theory and Monte Carlo
simulations of the underlying microscopic dynamic. This includes quantitative
agreement for the exponential dependence of the lifetime on the inverse of the
applied field , and the observation of distinct field regions in which the
derivative of the lifetime with respect to depends differently on . In
addition, at very low temperatures we observe oscillatory behavior of this
derivative with respect to , due to the discreteness of the lattice and in
agreement with rigorous results. Similarities and differences between this work
and earlier works on finite Ising models in the fixed-magnetization ensemble
are discussed.Comment: 44 pages RevTeX3, 11 uuencoded Postscript figs. in separate file
Classical Density Functional Study on Interfacial Structure and Differential Capacitance of Ionic Liquids near Charged Surfaces
We have implemented a generic coarse-grained model for the aromatic ionic liquid [CnMIM+][Tf2N-]. Various lengths for the alkyl chain on the cation define a homologous series, whose electric properties are expected to vary in a systematic way. Within the framework of a classical density functional theory, the interfacial structures of members of this series are compared over a range of surface charge densities, alkyl chain lengths, and surface geometries. The differential capacitance of the electric double layer, formed by ionic liquids against a charged electrode, is calculated as a function of the surface electric potential. A comparison of planar, cylindrical, and spherical surfaces confirms that the differential capacitance increases and varies less with surface potential as the surface curvature increases. Our results are in qualitative agreement with recent atomistic simulations
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