Search Results (24 total)

A model for information spreading in a population of N mobile agents is extended to d-dimensional regular lattices. This model, already studied on two-dimensional lattices, also takes into account the degeneration of information as it passes from one agent to the other. Here, we find that the structure of the underlying lattice strongly affects the time τ at which the whole population has been reached by information. By comparing numerical simulations with mean-field calculations, we show that dimension d=2 is marginal for this problem and mean-field calculations become exact for d>2. Nevertheless, the striking nonmonotonic behavior exhibited by the final degree of information with respect to N and the lattice size L appears to be geometry independent.

We introduce a model for information spreading among a population of N agents diffusing on a square L×L lattice, starting from an informed agent (Source). Information passing from informed to unaware agents occurs whenever the relative distance is ≤1. Numerical simulations show that the time required for the information to reach all agents scales as N^−αL^β, where α and β are noninteger. A decay factor z takes into account the degeneration of information as it passes from one agent to another; the final average degree of information of the population I_av(z) is thus history dependent. We find that the behavior of I_av(z) is nonmonotonic with respect to N and L and displays a set of minima. Part of the results are recovered with analytical approximations.

The use of Bayes theorem in quantum mechanics is discussed. It is shown that the quantum Bayes theorem follows from the ordinary quantum measurement theory, when applied to density operators that represent our a priori knowledge of a system. The examples studied involve measurements on multiple copies of the same (unknown) state. The theorem is used to determine the unknown state by successive measurements on several of the copies of the state. An idealized information-theoretic description of propagating CW laser beams is treated in detail. It is shown how photon detections on part of the beams can be used to determine the phase of the rest of the beams. Also discussed, are the limitations on the accuracy of the phase determination that follow from the fact that it is accomplished by the detection of a finite number of photons. Explicit expressions are derived for the conditional probabilities of detecting photons at different locations, given the numbers of photons detected in the past. The quantitative predictions could be used, in principle, to test proposed quantum states of propagating laser beams.

A complete magnetic phase diagram for exchange-coupled planar hard-soft nanocomposites has been obtained in the frame of a one-dimensional micromagnetic model describing the dependence of the properties along the growth direction. The phase diagram in terms of layer thicknesses provides information on the type of demagnetization processes and the critical fields at which nucleation and reversal take place. The basic criterion to this purpose is the analytical expression we have obtained of the critical susceptibility at the nucleation field. The phase diagram is divided into three regions: the exchange-spring magnet (ES), the rigid composite magnet (RM), and the decoupled magnet (DM). The main boundary line is an U-shaped line corresponding to divergence of the critical susceptibility. The diagram also reports the isocritical field lines both for the nucleation and the reversal field. These lines bifurcate along the RM boundary line. The essential characteristics of the phase diagram are directly connected with the intrinsic properties of the chosen soft and hard materials. With increasing ratio between the anisotropy constants of soft to hard phases the ES region is reduced until it disappears at a critical value. The model includes as limiting cases the classical problems of the planar soft inclusion in a bulk magnet and of the domain-wall depinning at the hard-soft interface.

Amorphous a-CN_x thin films were deposited at room temperature by pulsed laser ablation of graphite targets in a controlled nitrogen atmosphere. By means of reflection electron-energy-loss spectroscopy their dielectric function has been obtained in the 0–45 eV energy range. An appropriate method of analysis has been also proposed which does not take into account surface contributions to the measured spectra in the presence of large electron inelastic mean free paths. The overall results show that the nitrogen introduction in the amorphous carbon matrix induces an increase in the total threefold coordination, i.e., a progressive material graphitization. This finding is also confirmed from x-ray photoelectron spectroscopy results for the N and C 1s core levels.

Transverse beam profiles are observed to broaden with increasing intensity in the Proton Storage Ring at the Los Alamos Neutron Scattering Center. Measured profiles are simulated with an H^- injection model that includes a 2D particle-in-cell space charge calculation. Inclusion of space charge effects in the simulation improves the agreement between the experimentally observed profiles and the calculated profiles. The comparisons are made for a range of injected intensities.

The inductance of the vacuum chamber of the Proton Storage Ring at Los Alamos National Laboratory was intentionally increased by the introduction of ferrite rings to counteract the longitudinal space-charge effect of the intense beam. The magnetic permeability of the ferrite could be adjusted by introducing current into solenoids wound around the ferrite. Results show that the minimum rf voltage necessary to stabilize the beam against e-p instability may be reduced over that previously measured. The injected bunch length was observed to be longer when the ferrite was heavily biased so that its effect was reduced.

The reaction of FePS₃ single crystals with a 1.6M n-butyl lithium solution in n-hexane has been followed through dc electrical conductivity measurements as a function of the intercalation time. In order to investigate the influence of lithium insertion on the FePS₃ electronic properties we have also measured as a function of both temperature and intercalation time the dc conductivity of equilibrated Li_xFePS₃ single crystals. Upon lithium intercalation we have observed the following main effects: (i) an increase in the electrical conductivity, (ii) a simultaneous decrease in the activation energy, and (iii) a degenerate semiconductor behavior at the highest lithium content. In analogy to the Li_xNiPS₃ systems, these results, discussed in terms of both the rigid band model and the so-called transition-metal weakly interacting one, seem to indicate that in the Li_xFePS₃ complexes a new conduction mechanism appears at a different energy level from the beginning of the intercalation process with respect to the pure FePS₃. Aiming at a better understanding of the still uncertain nature of reduction sites in the FePS₃ lattice during lithium intercalation, we have also carried out Fourier transform infrared absorption measurements on single crystals of FePS₃ and their lithium intercalation compounds at room temperature and in the frequency region from 800 to 3000 cm^-1. The results agree well with both the literature and the above conductivity data allowing a better identification of the lithium 2s electron accepting levels. © 1996 The American Physical Society.

Conductivity measurements were made on lithium-intercalation complexes Li_xNiPS₃ both during the intercalation process and after the intercalation has ceased. The intercalation process was characterized by means of the conductivity measurements as a function of the intercalation time. Results for equilibrated samples are presented as a function of both the temperature and the lithium concentration. An overall and quick increase in the electrical conductivity, a contemporaneous decrease in the activation energy, and a degenerate semiconducting behavior at high lithium contents were observed. All these effects suggest that, with respect to the pure NiPS₃, a new conduction mechanism occurs at a different energy level from the very beginning of the intercalation process. Taking the probable nickel reduction into account, a possible identification of the lithium 2s electron-accepting levels is proposed. In the early intercalation times a simple empirical model is formulated to correlate the data as a function of the intercalation time and as a function of temperature.

The dielectric behavior of hydrogenated amorphous carbon samples, prepared by keV-Ar-ion bombardment of a polystyrene matrix, in the energy range 0–100 eV has been studied in connection with the change in hydrogen content. The wide-ranging results obtained by means of electron-energy-loss spectroscopy in reflection mode have been successfully compared with the optical constants available in the 1–6 eV range, resulting from the combined use of transmittance and reflectance experimental spectra. A strong rearrangement of the σ→σ^* transitions is evidenced in the bulk loss function Im(-1/ɛ̃) together with a change in the relative oscillator strengths, on going from pure amorphous carbon to the most hydrogenated (x_H=40%) α-C:H sample.

This paper studies the behavior of the moments of a particle distribution as it is transported through a Hamiltonian system. Functions of moments that remain invariant for an arbitrary linear Hamiltonian system are constructed. These functions remain approximately invariant for Hamiltonian systems that are not strongly nonlinear. Consequently, they can be used to characterize the degree of nonlinearity of the system.

The manganese thiophosphate (MnPS₃) optical-absorption spectrum has been investigated as a function of temperature from 300 down to 10 K in the visible region. With decreasing temperature, the fundamental absorption edge shifts towards higher energies without changing its shape and an additional subband-gap feature appears. This structure has been attributed to transitions between the Mn²⁺ third excited state and the ground state. All the subband-gap structures exhibit a small temperature shift, in good agreement both with the crystal-field theory and with the so-called transition-metal weakly interacting model. The temperature dependence of the crystal-field parameters Dq, B, and C has been determined from the transition energies. These results support the hypothesis that MnPS₃ is to be considered an ioniclike compound, as already suggested in the literature.

We have measured, as a function of temperature, the thermopower, the dc conductivity, and photoconductivity of FePS₃ single crystals. For temperatures below 430 K the thermopower shows an activated behavior, whereas at higher temperatures it is almost temperature independent. The dc dark conductivity and photoconductivity are thermally activated over the entire investigated temperature range. Moreover, ac-conductivity measurements have been carried out as a function of both temperature and frequency from 400 Hz to 100 kHz in the 340–500-K temperature region. The transport mechanisms involved in different temperature ranges have been identified. The results have been interpreted on the basis of a simplified energy-band scheme.

Quadratic moments of a particle distribution being transported through a linear Hamiltonian system are considered. A complete set of kinematic invariants made out of these moments are constructed leading to the discovery of new invariants.

Fluorescence measurements on MnPS₃ layered semiconductor crystals, in the visible region, below their fundamental absorption thresholds, were carried out at room temperature. The resulting emission spectra show four overlapping structures, which fall in the region of intraion transitions for the Mn²⁺ 3d⁵ configuration. The results, deduced from deconvolution of the bands, agree well with those obtained from optical-absorption measurements and furthermore confirm the Mn²⁺ 3d energy-level distribution derived from the transition-metal weakly interacting model. From the analysis of the excitation spectrum for the MnPS₃ fluorescence band centered at 2.23 eV, additional information about the Mn²⁺ 3d excited-energy-state distribution and a verification of the validity of the above model are obtained.

The temperature dependence of conduction mechanisms in pure NiPS₃ has been investigated by means of conductivity, photoconductivity, and thermopower measurements. Evidence for a p-type conduction has been found from the thermoelectric power data. The observed activated behaviors of thermopower, dark conductivities, and photoconductivities have allowed us to deduce more detailed information about the distribution of the electronic density of states in the energy-gap region. The results, fitted by assuming a single conduction path through extended-band states, have been interpreted in terms of an energy-band scheme based on a model regarding as insignificant the contribution of the 3d transition-metal states in bond.

It is shown that, at large N, 〈ψ̅ ψ〉 is independent of the temperature in the confining phase. This implies that the temperature at which chiral symmetry is restored is larger than or equal to the critical temperature for deconfinement. If T_chiral=T_deconfining, then the chiral transition must be first order.

It is pointed out that at N→∞, for finite temperature, the Schwinger-Dyson equations imply that below the deconfining phase transition the Wilson loops are independent of the temperature. This suggests a first-order deconfinement phase transition.

Several topics in the loop-space formulation of non-Abelian gauge theories are considered. The basic objects dealt with are the unrenormalized dimensionally regularized gauge-invariant loop functions W(Cⁱ;g, ε), where Cⁱ is a set of loops, g is the unrenormalized coupling constant, and ε is the deviation from four space-time dimensions. The renormalization-group equations satisfied by the corresponding renormalized loop functions are derived and, using asymptotic freedom, used to determine the exact behavior of the functions when the length L of the loops approaches zero. The result is (-lnLμ)^a(γ), where μ is the subtraction mass and γ represents the cusp and cross-point angles of the loops. The function a(γ) is exactly computable and several examples are given. The equivalent result may be stated as the exact behavior of the renormalization-constant matrix Z^ij(γ, g_R, ε) for ε→0 with fixed renormalized coupling constant g_R, or as the exact behavior of the unrenormalized loop function for ε→0 and g_R fixed. It is shown next that the W(Cⁱ;g, ε) satisfy dimensionally regularized Makeenko-Migdal equations in all orders of perturbation theory. The proof makes detailed use of dimensional regularization, Becchi-Rouet-Stora symmetry, gauge-field combinatorics, and properties of the area functional derivative of path-ordered multiple line integrals. Doubt is cast on the existence of such useful equations when other regularizations are used or when renormalization is performed. The Mandelstam constraints are considered next. Among other things, it is shown that the loop-function renormalization may be performed such that the renormalized functions satisfy a constraint which has the same form as the unrenormalized constraint Σi=1^(N+1)↑a_iW(Cⁱ)=0, for the U(N) gauge group. The paper concludes with illustrations of how observable matrix elements of physical (color singlet, quark bilinear) flavor currents may be expressed in terms of loop functions. Among other topics discussed in the paper are the N→∞ limit, two-dimensional QCD, and normalization conditions on the renormalized loop functions.

It is shown that the vacuum expectation values W(C₁,⋯,C_n) of products of the traces of the path-ordered phase factors P exp[ig∮∫C_iA_μ(x)dx^μ] are multiplicatively renormalizable in all orders of perturbation theory. Here A_μ(x) are the vector gauge field matrices in the non-Abelian gauge theory with gauge group U(N) or SU(N), and C_i are loops (closed paths). When the loops are smooth (i.e., differentiable) and simple (i.e., non-self-intersecting), it has been shown that the generally divergent loop functions W become finite functions W̃ when expressed in terms of the renormalized coupling constant and multiplied by the factors e^-KL(C_i), where K is linearly divergent and L(C_i) is the length of C_i. It is proved here that the loop functions remain multiplicatively renormalizable even if the curves have any finite number of cusps (points of nondifferentiability) or cross points (points of self-intersection). If C_γ is a loop which is smooth and simple except for a single cusp of angle γ, then W_R(C_γ)=Z(γ)W̃(C_γ) is finite for a suitable renormalization factor Z(γ) which depends on γ but on no other characteristic of C_γ. This statement is made precise by introducing a regularization, or via a loop-integrand subtraction scheme specified by a normalization condition W_R(C̅ _γ)=1 for an arbitrary but fixed loop C̅ _γ. Next, if C_β is a loop which is smooth and simple except for a cross point of angles β, then W̃(C_β) must be renormalized together with the loop functions of associated sets Sⁱ_β={Cⁱ₁,⋯,Cⁱ_pi} (i=2,⋯,I) of loops Cⁱ_q which coincide with certain parts of C_β≡C₁¹. Then W_R(Sⁱ_β)=Z^ij(β)W̃(S^j_β) is finite for a suitable matrix Z^ij(β). Finally, for a loop with r cross points of angles β₁,⋯,β_r and s cusps of angles γ₁,⋯,γ_s, the corresponding renormalization matrices factorize locally as Z^i₁j₁(β₁)⋯Z^i_rj_r (β_r)Z(γ₁)⋯Z(γ_s).

We present a semiclassical evaluation of the ground-state decay rate of a string. All computation will be performed in (1 + 1)-dimensional quantum electrodynamics and in the limit of heavy quark mass. We will derive a dispersion relation for the energy of the string and finally briefly discuss the excited states.

We establish the Lorentz invariance of the quantum field theory of electric and magnetic charge. This is a priori implausible because the theory is the second-quantized version of a classical field theory which is inconsistent if the minimally coupled charged fields are smooth functions. For our proof we express the generating functional for the gauge-invariant Green's functions of quantum electrodynamics—with or without magnetic charge—as a path integral over the trajectories of classical charged point particles. The electric-electric and electric-magnetic interactions contribute factors exp(JDJ) and exp(JD^′K), where J and K are the electric and magnetic currents of classical point particles and D is the usual photon propagator. The propagator D^′ involves the Dirac string but exp(JD^′K) depends on it only through a topological integer linking string and classical particle trajectories. The charge quantization condition (e_ig_j-g_ie_j) / 4π=integer then suffices to make the gauge-invariant Green's functions string independent. By implication, our formulation shows that if the Green's functions of quantum electrodynamics are expressed, as usual, as functional integrals over classical charged fields, the smooth field configurations have measure zero and all the support of the Feynman measure lies on the trajectories of classical point particles.

Various aspects of Zwanziger's local Lagrangian formulation of a relativistic quantum field theory of electric and magnetic charge are investigated with a view toward determining the consistency of the theory. A slight generalization of the classical particle theory is first studied and shown to be equivalent to the more-familiar nonlocal formulations due to Yan and Schwinger and to Dirac. The actions for each of these theories must first be altered so that the correct Lorentz force law results even if a charged-particle trajectory intersects a solenoidal "string" attached to each monopole. The resultant actions are then seen to be invariant under combined string rotations and (singular) gauge transformations. The duality invariance (i.e., invariance under interchange of electric and magnetic quantities) of these theories is also discussed. For the quantum field theory, it is shown how the boundary conditions are to be chosen so that one has rotational invariance about the string direction n-^ , boost invariance along n-^ , and duality invariance. The condition for full Lorentz invariance is discussed and it is argued that the Lorentz invariance of the classical and first-quantized theories suggests what is necessary in order that the quantum field theory is also Lorentz invariant. The Feynman rules for the theory are confirmed from the Faddeev-Popov ansatz, and their unitarity is explicitly verified. It is also shown that there are no unphysical intermediate states (if duality invariance is maintained), that the bare electric and magnetic charges are renormalized by the same factor, and that the theory is infrared-free.

The Lorentz invariance of the gauge-invariant Green's functions in the local quantum field theory of electric and magnetic monopoles is formally established. This is accomplished by expressing these Green's functions as functional integrals over the trajectories of classical charged particles.