Incoherent Channels Research Articles

On the Gerasimov-Drell-Hearn sum rule for the deuteron

The Gerasimov-Drell-Hearn sum rule is evaluated for the deuteron by explicit integration up to 550 MeV including contributions from the photodisintegration channel and from coherent and incoherent single pion production as well. The photodisintegration channel converges fast enough in this energy range and gives a large negative contribution, essentially from the 1 S 0 resonant state near threshold. Its absolute value is about the same size as the sum of proton and neutron GDH values. It is only partially cancelled by the single pion production contribution. But the incoherent channel has not reached convergence at 550 MeV.

Interaction Effects and Transport in the Localized Phase

The enhancement of electronic transport in the localized regime by the electron–electron interactions is discussed both in the incoherent channel, leading to transport via inelastic scattering, and in the coherent channel, via delocalization. For the former channel, a new prediction for the temperature dependence of the conductivity in a weak granular insulator is presented. The delocalization by the interactions is considered by treating interblock transport of more than one particle within the scaling theory of localization. Thus, the latter theory can include some of the effects of the interactions. Two-electron delocalization is the first and most straightforward example for that. A simple physical interpretation is given to earlier results of Dorokhov who found from a transfer matrix formulation that a weakly bound pair of particles in a random potential is delocalized with respect to the separate consituents. A very strongly bound pair obviously behaves like a heavier particle and is more strongly localized. Finally, the delocalization of two interacting quasiparticles above the Fermi sea is considered with some recent modifications.

Femtosecond Spectroscopy of Halorhodopsin and Rhodopsin in a Broad Spectral Range of 400−1000 nm

Femtosecond pump−probe spectroscopy of halorhodopsin from Halobacterium halobium and rhodopsin from octopus (paroctopus defleini) has been studied in a wide spectral range extending from 400 to 1000 nm. There are five common features to halorhodopsin and rhodopsin in the transient absorption and gain spectra. A comparative description of the primary photochemistries in the retinal proteins including bacteriorhodopsin is presented to explain the present experimental results systematically together with the previous studies. In the model there is a branching from the Franck−Condon state into coherent and incoherent channels. The former is the rapid process of direct formation of the first intermediate (bathorhodopsin for rhodopsin and hRK intermediate for halorhodopsin), and the latter is the thermalization of the excited state in the retinoid proteins, from which slow formation of the first intermediate takes place.

Near-optimal parallel distributed data detection for page-oriented optical memories

Volume optical storage systems suffer from numerous sources of noise and interference, the effects of which can seriously degrade retrieved data fidelity and produce unacceptable bit-error rates (BERs). We examine the problem of reliable two-dimensional data retrieval in the context of recently developed soft-decision methods for iterative decoding. We describe a novel near-optimal algorithm in which each pixel on the page is treated as a starting point for a simple iterative procedure so that a highly parallel, locally connected, distributed computational model emerges whose operation is well suited to the page-oriented memory (POM) interface format. We study the use of our two-dimensional distributed data detection (2D/sup 4/) algorithm with both incoherent (linear) and coherent (nonlinear) finite-contrast POM channel models. We present BER results obtained using the 2D/sup 4/ algorithm and compare these with three other typical methods [i.e., simple thresholding (THA), differential encoding (DC) and the decision feedback Viterbi algorithm (DFVA)]. The BER improvements are shown to have a direct impact on POM storage capacity and density and this impact is quantified for the special case of holographic POM. In a Rayleigh resolved holographic POM system with infinite contrast, we find that 2D/sup 4/ offers capacity improvements of 84%, 56%, and 8% as compared with DC, THA, and DFVA respectively, with corresponding storage density gains of 85%, 26%, and 9%. In the case of finite contrast (C=4), similar capacity improvements of 93%, 18%, and 4% produce similar density improvements of 98%, 21%, and 6%. Implementational issues associated with the realization of this new distributed detection algorithm are also discussed and parallel neural and focal plane strategies are considered. A 2 cm/sup 2/ /spl lambda/=0.1 /spl mu/m digital VLSI real estate budget will support a 600/spl times/600 pixel 2D/sup 4/ focal plane processor operating at 40 MHz with less than 1.7 W/cm/sup 2/ power dissipation.

Population transfer through the continuum with temporally delayed chirped laser pulses

Coherent population transfer in the laser-induced continuum structure system is investigated using chirped pulsed excitation. It is found that very high efficiency is obtainable if a counterintuitive sequence of pulses is used. The addition of incoherent ionization channels is also investigated.

The spin response of the deuteron and its implication for the Gerasimov-Drell-Hearn sum rule

The spin response of the deuteron, i.e., the asymmetry of photoabsorption with respect to parallel and antiparallel spins of photon and deuteron, is calculated allowing to explicitly evaluate the Gerasimov-Drell-Hearn sum rule by integration up to a photon energy of 550 MeV including contributions from the photodisintegration channel and from coherent and incoherent single pion production as well. The photodisintegration channel converges fast enough in this energy range and gives a large negative contribution, essentially from the 1 S 0 resonant state near threshold. Its absolute value is about the same size than the sum of proton and neutron GDH values. It is only partially cancelled by the single pion production contribution. But the incoherent channel has not reached convergence at 550 MeV.

Electron Transfer Rates in Bridged Molecular Systems: A Phenomenological Approach to Relaxation

A model for bridge-assisted, long-range electron transfer in a molecule interacting with a dissipative external bath is presented. The effects of the system−bath interaction are included phenomenologically in the evolution of the system density matrix as energy dephasings on the bridge sites. When the bridge dephasings are small, the steady state ET rate in this model is found to be the sum of two competing terms; the first is a McConnell-type rate arising from direct tunneling from donor to acceptor, and the second is a dephasing-dependent, length-independent scattering channel through the bridge sites. In the limit of large dephasings, an incoherent channel dominates the dynamics and leads to ET rates that can become only weakly dependent (kET ∝ 1/N) on the number of bridge sites in the system, for multisite bridges.

Population transfer by delayed pulses via continuum states

This paper analyzes the process of population transfer between two discrete states via a common continuum by using two partially overlapping delayed laser pulses. The incoherent ionization channels, the Stark shifts, and the nonzero Fano parameter are taken into account. An analytic approximation to the final-state population is derived for Gaussian pulse shapes.

State-by-state calculations for all channels of the exotic (μ-,e-) conversion process

The coherent and incoherent channels of the neutrinoless muon to electron conversion in nuclei, ${\ensuremath{\mu}}^{\ensuremath{-}}(A,Z)\ensuremath{\rightarrow}{e}^{\ensuremath{-}}{(A,Z)}^{*}$, are studied throughout the periodic table. The relevant nuclear matrix elements are computed by explicitly constructing all possible final nuclear states in the context of the quasiparticle random phase approximation. The obtained results are discussed in view of the existing at PSI and TRIUMF experimental data for ${}^{48}\mathrm{Ti}$ and ${}^{208}\mathrm{Pb}$ and compared with results obtained by (i) shell model sum-rule techniques, (ii) nuclear matter mapped into nuclei via a local density approximation, and (iii) earlier similar calculations.

Signal-to-noise ratio measurement of a 2.5-Gb/s spectrum-sliced incoherent light channel

We have measured the signal-to-noise ratio (SNR) of a 2.5-Gb/s spectrum-sliced incoherent light channel in the large beat-noise limit. SNR changes strongly in time depending on the receiver electrical filter and the extinction ratio. Except for some minor differences, the SNR behaved quite similar to the theory. In order to obtain a high SNR, the nonreturn-to-zero format should be used rather than the return-to-zero format and the receiver sampling time should be chosen more precisely than that of conventional receivers.

Flux-sensitive correlations of mutually incoherent quantum channels.

A two-particle state can be sensitive to a magnetic flux through exchange even if it is composed of single-particle states which on the average are statistically uncorrelated and exhibit no direct interference. Consequently, two-particle observables can be sensitive to a flux even if all single-particle observables are flux insensitive. We investigate exchange effects in current cross correlations of small conductors subject to a magnetic field.

Anomalous transmission of Mossbauer radiation in pure nuclear Laue diffraction. III. Anisotropic scattering

For pt.II, see ibid., vol.19, p.2567 (1986). The angular dependence of the anomalous transmission of polarised Mossbauer radiation through a perfect single-crystal platelet of 57FeBO3 was studied. Rocking curves of the Laue reflected and of the Laue transmitted beams were simultaneously recorded for the pure nuclear reflection (001). Striking asymmetries were observed between the transmitted and the reflected intensities for the two polarisation components. The asymmetries were caused by the strong anisotropy of the nuclear resonance scattering. The asymmetry between the total intensities (transmitted plus reflected beam) for the two polarisation components was related to a strongly asymmetric division of the incident radiation into the two wavefields built up inside the crystal. The asymmetry between the intensities of the transmitted and reflected beams revealed a strongly asymmetric composition of the wavefield experiencing anomalous low resonance absorption. The observed anomalous transmission of such a strongly asymmetric wavefield uniquely demonstrated the origin of the suppression of the incoherent reaction channels, i.e. the cancellation of the total amplitude for the individual nuclear excitation (the Kagan-Afanasev rule) rather than the cancellation of the total amplitude of the radiation field at the nuclear position.

Coherent effects in resonant diffraction: theory

The main results of the kinematical theory and of the dynamical theory for resonant diffraction are summarised, with special emphasis given to the coherent enhancement of the radiative channel and to the suppression of the incoherent channels.

Mossbauer transmission spectra at pure nuclear dynamical diffraction in Bragg geometry

Mossbauer spectra of the reflection from and the transmission through a thin nearly perfect 57FeBO3 single-crystal platelet were measured at different angular positions in the vicinity of a Bragg angle. In the transmission spectra an angular-dependent shift of the nuclear resonance dips was observed in common with a shift of the resonance peaks in the reflection spectra. In addition, a pronounced asymmetry of the transmitted intensity above and below the resonances was observed. The angular dependence of the transmission spectra reveals that in transmission under Bragg reflection conditions the coherent channel (diffraction) becomes as important as the incoherent channels (internal conversion and incoherent scattering). The Borrmann effect induced by nuclear resonance diffraction was observed for the first time.

Comments on "Envelope Detector Performance for a Partially Coherent-Fading Sinewave in Noise"

The basic premise in a recent paper that the matchedfilter square-law envelope detector is generally appropriate for fastfading as well as slow-fading incoherent channels is questioned. Its inappropriateness for a fast-fading incoherent Gaussian channel is demonstrated by evaluation of probability of error for this receiver and for the optimum receiver. As a result, modifications to some of the conclusions drawn regarding attainable performance gain with increasing observation time are proposed. In addition, it is pointed out that the performance predictor adopted, viz., deflection, exhibits anomalous behavior (for at least some incoherent fading channels), thereby rendering the other conclusions questionable.

Nonradiative heterofission of guest molecule electronic excitations in doped organic molecular crystals

Formulae for the rate constants of the nonradiative heterofission of a guest molecule electronic excitation into one host molecule electronic excitation and one guest molecule electronic excitation in the doped organic molecular crystals are derived. The coherent and incoherent host exciton heterofission channels are considered. Formulae derived for the singlet host exciton heterofission channels are related to the measurable optical properties of the host and guest molecule of the crystal. The effects of the intramolecular vibration are discussed.

A gradient method for systems with time delays and its application to waveform design

An optimal control program based on steepest descent techniques is developed for systems described by differential-difference equations. It is shown that the design of signals for pulse-position modulation over the incoherent Gaussian white noise channel leads to systems of the above type. Numerical results are given for a practical design problem.

The asymptotic error probability for transmission of orthogonal signals over the generalized incoherent channel

The probability of error for the transmission of one of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</tex> -orthogonal, equally likely, equal-energy signals over the white Gaussian noise channel is known. For large values of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</tex> it is shown that this result is asymptotically equivalent to the error probability for the same signal set transmitted over a different channel, termed here the generalized incoherent channel of order <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">v</tex> . For <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">v = 2</tex> this channel is identical to the phase incoherent channel. This problem, for larger values of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">v</tex> , also arises in a number of other applications, some of which are given.

The semiclassical treatment of some quantum problems

The semiclassical treatment of quantummechanical problems is discussed making use of a generalization of the Hamilton-Jacobi partial differential equation to cases in which only some of the variables of the problem are treated classically. The quantum equations are put into a form in which the effects not covered by the semiclassical approximation are concentrated into one set of terms. This enables one to calculate corrections to the semiclassical theory and to demonstrate that compensations of the decrease of scattering in the coherent (“elastic”) channel by the increase caused by the presence of incoherent (“inelastic”) channels which is exact in the semiclassical approximation holds in a certain approximation quantum-mechanically. By means of these relationships it is shown that some and presumably the main contributions to the dynamical effects of molecular electrons interacting with protons in experiments on proton-proton and proton-neutron scattering are negligibly small.