Wavelength Fluctuations Research Articles

Diffusive mass transfer by nonequilibrium fluctuations: Fick’s law revisited

Recent experimental and theoretical works have shown that giant fluctuations are present during diffusion in liquid systems. We use linearized fluctuating hydrodynamics to calculate the net mass transfer due to these nonequilibrium fluctuations. Remarkably, the mass flow turns out to coincide with the usual Fick's one. The renormalization of the hydrodynamic equations allows us to quantify the gravitational modifications of the diffusion coefficient induced by the gravitational stabilization of long wavelength fluctuations.

Short wavelength fluctuations and electron transport in TFTR

Correlation between electron heat diffusivity and short wavelength ( kρ i ∼5) fluctuation amplitude was observed in the TFTR tokamak in the core of ERS plasmas (Phys. Lett. A 236 (1997) 339). These fluctuations propagate in the ion diamagnetic drift direction with wave number comparable to ω pe / c. Further analysis of these data yields the ratios χ e / χ i and χ e / D e , and their values are consistent with the picture that the electron transport is mainly induced by the short wavelength fluctuations in the plasma core where the long wavelength ( kρ i ∼1) fluctuations are absent. Although there is not enough information to identify these short wavelength modes, the values of χ e is found to be comparable to theoretical predictions based on the current diffusive ballooning mode theory (Plasma Phys. Control. Fusion 36 (1994) 279).

Spinodal decomposition in high temperature gauge theories

After a rapid increase in temperature across the deconfinement temperature T d , pure gauge theories exhibit unstable long wavelength fluctuations in the approach to equilibrium. This phenomenon is analogous to spinodal decomposition observed in condensed matter physics, and also seen in models of disordered chiral condensate formation. At high temperature, the unstable modes occur only in the range 0≤ k ≤ k c , where k c is on the order of the Debye screening mass m D . Equilibration always occurs via spinodal decomposition for SU(2) at temperatures T> T d and for SU(3) for T≫ T d . For SU(3) at temperatures T≳ T d , nucleation may replace spinodal decomposition as the dominant equilibration mechanism. Monte Carlo simulations of SU(2) lattice gauge theory exhibit the predicted phenomena. The observed value of k c is in reasonable agreement with a value predicted from previous lattice measurements of m D .

Non-Crossing Effect on Long Wavelength Fluctuation of Multilayer Membrane System

We investigate the fluctuation properties of a multilayer membrane system by means of a capillary-wave (CW) picture. We examine the non-crossing effect between membranes. From the CW picture, we derive a novel Hamiltonian expressing the fluctuation properties, which is different from the Hamiltonian proposed by Helfrich [W. Helfrich: Z. Naturforsch 33a (1978) 305]. To verify the new Hamiltonian, we performed a Monte Carlo simulation on the basis of the Gaussian solid-on-solid model for a one-dimensional multilayer membrane system. The result strongly supports the novel Hamiltonian.

Spin time-relaxation within strongly coupled paramagnetic systems exhibiting paramagnetic–ferrimagnetic transitions

The purpose of the present work is a quantitative study of the spin time relaxation within superweak ferrimagnetic materials exhibiting a paramagnetic–ferrimagnetic transition, when the temperature is changed from an initial value T i to a final one T f very close to the critical temperature T c. From a magnetic point of view, the material under investigation is considered to be made of two strongly coupled paramagnetic sublattices of respective moments ϕ and ψ. Calculations are made within a Landau mean-field theory, whose free energy involves, in addition to quadratic and quartic terms in both moments ϕ and ψ, a lowest-order coupling – Cϕψ, where C<0 stands for the coupling constant measuring the interaction between the two sublattices. We first determine the time dependence of the shifts of the order parameters δϕ and δψ from the equilibrium state. We find that this time dependence is completely controlled by two kinds of relaxation times τ 1 and τ 2. The former is a long time and the second a short one, and they are associated, respectively, with long and local wavelength fluctuations. We find that, only the first relaxation time is relevant for physics, since it drives the system to undergo a phase transition. Spatial fluctuations are also taken into account. In this case, we find an explicit expression of the relaxation times, which are functions of temperature T, coupling constant C and wave vector q . We find that the critical mode is that given by the zero scattering-angle limit, i.e. q=0. Finally, we emphasize that the appearance of these two relaxation times is in good agreement with results reported in recent experimental work dealt with the Curie–Weiss paramagnet compound Li x Ni 2− x O 2, where the composition x is very close to 1.

Comparison of the Mechanism of Optical Amplification in InGaN/GaN Heterostructures Grown by Molecular Beam Epitaxy and MOCVD

We comprehensively studied InGaN/GaN heterostructures grown by molecular beam epitaxy (MBE) and metal-organic vapor deposition epitaxy (MOCVD) using a variety of methods of optical spectroscopy, such as cathodoluminescence microscopy (CL), time-integrated and time-resolved photoluminescence (PL). Micro-photoluminescence and cathodoluminescence results show the variation in emission wavelength at different scales, and this reflects the degree of compositional fluctuations in the samples. We obtain information on the decay times of the main emission lines using time-resolved photoluminescence spectroscopy and models of stretched exponentials, indicating the importance of nanoscale fluctuations for the recombination mechanism. The temperature dependent behavoir of the InGaN emission is explained in terms of a carrier freeze out at local potential fluctuations combined with a thermionic thermalization at elevated temperatures. To correlate the fluctuations in emission wavelength with values for the optical amplification we performed gain measurements in edge-stripe geometry. From all these results we conclude, that localized carriers at a statistical distribution of potential fluctuations act as recombination centers and that the degree of fluctuations determines the efficiency of optical amplification. The threshold values for lasing and the gain values are compared and discussed with respect to the different growth procedures. From all these findings we draw conclusions concerning the influence of differences in the growth conditions and their impact on the optical properties.

Capillary-wave effects at critical wetting in type-I superconductors

We discuss the effect of fluctuations of the superconductor-normal (SC/N) interface on the (short-range) critical wetting transition in type-I superconductors. Functional renormalization of a standard effective interface Hamiltonian shows that the fluctuation regimes found for short-range critical wetting in conventional fluid systems appear in superconductors with slight modifications. Because the fluctuation parameter omega approximately 1/(1-sqrt[2]kappa) depends on the Ginzburg-Landau parameter kappa, strong fluctuation effects would be expected in the limit kappa-->1/sqrt[2]. However, the capillary-wave spectrum of the SC/N interface has an unusual form due to a relevant magnetic field contribution which suppresses long wavelength fluctuations, invalidating conclusions drawn from the standard effective interface Hamiltonian, and validating the results of mean-field theory.

Nonlocal dielectric functions of molecular liquids from computer simulations

The longitudinal dielectric function, χ( k)=1−1/ ε L( k), of both polar and nonpolar liquid environments is a key ingredient to modern theories of solvent effects upon charge transfer reactions in solution and solvation dynamics processes in general. In this work, we present a molecular dynamics simulation study of the dielectric function of several molecular liquids ranging from associating (H 2O, CH 3OH) and nonassociating ((CH 3) 2SO, CH 3CN) polar liquids to nondipolar ones (C 6H 6, CCl 4 and CO 2) aiming to better understand the relationships between the structure of the χ( k) functions to the charge–charge spatial correlations in each liquid. We show that the characteristic main peak of χ( k), located at some wave vector k ∗ , whose value depends on the liquid, arises from spatial charge density fluctuations of wavelength 2 π/k ∗ for all liquids considered, with the exception of CO 2. Further details about the shape of the χ( k) functions for the simulated liquids are discussed in terms of intra and intermolecular charge–charge correlations.

Quantum decay of a superflow through a weak link in a thin tube

We present an analytic calculation for the quantum decay rate of a superflow through a weak link in a one-dimensional (1D) Bose–Einstein condensate (BEC). Deriving an effective action for the phase difference across the weak link, we arrive at the well-known problem of a massive particle with damping in a periodic potential. It follows that the superfluid shows an algebraic flow–pressure characteristic. Short wave length fluctuations lead to a renormalization of the interaction between the bosons. As a consequence, the quantum phase transition observed for a particle with damping is not present in the superfluid system.

Slowing out of equilibrium near the QCD critical point

The QCD phase diagram may feature a critical end point at a temperature $T$ and baryon chemical potential $\ensuremath{\mu}$ which is accessible in heavy-ion collisions. The universal long wavelength fluctuations which develop near this Ising critical point result in experimental signatures which can be used to find the critical point. The magnitude of the observed effects depends on how large the correlation length $\ensuremath{\xi}$ becomes. Because the matter created in a heavy-ion collision cools through the critical region of the phase diagram in a finite time, critical slowing down limits the growth of $\ensuremath{\xi},$ preventing it from staying in equilibrium. This is the fundamental nonequilibrium effect which must be calculated in order to make quantitative predictions for experiment. We use universal nonequilibrium dynamics and phenomenologically motivated values for the necessary nonuniversal quantities to estimate how much the growth of $\ensuremath{\xi}$ is slowed.

Eta Carinae: Testing a Binary Orbit Model with the Hubble Space Telescope/Space Telescope Imaging Spectrograph.

Ground-based spectroscopy of eta Car shows periodic changes in some emission-line wavelengths. These variations have been cited as strong evidence that this object is a 5.5 yr binary system and have been used to produce specific orbit models. High spatial resolution data obtained with the Hubble Space Telescope/Space Telescope Imaging Spectrograph, however, do not confirm the predicted velocity behavior; therefore, the published orbit models are almost certainly invalid. Wavelength fluctuations seen at ground-based spatial resolution most likely result from other effects, which we describe. If this object is a binary system (which has not been proven), then the parameters of the secondary star and of the orbit remain largely unknown.

Correlation between structural properties and optical amplification in InGaN/GaN heterostructures grown by molecular beam epitaxy

We comprehensively studied InGaN/GaN heterostructures grown by molecular beam epitaxy (MBE) using a variety of methods of optical spectroscopy, such as cathodoluminescence microscopy (CL), time-integrated and time-resolved photoluminescence. To correlate the fluctuations in emission wavelength with values for the optical amplification we performed gain measurements in edge-stripe geometry. The lateral homogeneity can be drastically improved using a template of GaN grown on the sapphire substrate by metal-organic vapor phase epitaxy (MOVPE). Gain values up to 62 cm−1 were found in samples with low indium fluctuations, which is comparable to values for high-quality InGaN/GaN heterostructures grown by MOVPE.

Evolution of vibrational excitations in glassy systems

The equations of the mode-coupling theory (MCT) for ideal liquid-glass transitions are used for a discussion of the evolution of the density-fluctuation spectra of glass-forming systems for frequencies within the dynamical window between the band of high-frequency motion and the band of low-frequency-structural-relaxation processes. It is shown that the strong interaction between density fluctuations with microscopic wavelength and the arrested glass structure causes an anomalous-oscillation peak, which exhibits the properties of the so-called boson peak. It produces an elastic modulus which governs the hybridization of density fluctuations of mesoscopic wavelength with the boson-peak oscillations. This leads to the existence of high-frequency sound with properties as found by x-ray-scattering spectroscopy of glasses and glassy liquids. The results of the theory are demonstrated for a model of the hard-sphere system. It is also derived that certain schematic MCT models, whose spectra for the stiff-glass states can be expressed by elementary formulas, provide reasonable approximations for the solutions of the general MCT equations.

Spatially Resolved Electroluminescence of InGaN-MQW-LEDs

Electroluminescence (EL) is the most significant measure for light-emitting diodes since it probes the most relevant properties of the fully processed device during operation. In addition to the information gained by conventional spectrally resolved EL, scanning micro-EL provides spatially resolved information. The devices under investigation are InGaN/GaN-LEDs with single peak band-band emission at about 400 nm grown by MOVPE on sapphire substrates.The µ-EL-characterization is performed as a function of injection current densities and the emission is investigated from the epitaxial layer as well as from substrate side. Spatially resolved wavelength images reveal emission peaks between 406 nm and 417 nm, corresponding either to In fluctuations of 1 %−1.5 % or local fluctuations of piezo electric fields. Beside the information on the emission wavelength fluctuations µ-EL is used to determine the temperature distribution in the LEDs and to investigate transparent contacts.

Elastic Constant Inhomogeneity and the Broadening of the Dynamic Structure Factor in One-Dimensional Disordered Systems

The dynamical structure factor of a model disordered system consisting of spring-disordered harmonic linear chains is studied both analytically and numerically. The width of the acoustic excitation peak is observed to grow quadratically with the exchanged momentum $Q$; such behavior is shown to be due to spatial fluctuations of the local wavelength of the collective vibrational modes, which in turn is produced by the inhomogeneity of the interparticle elastic constants. The extent to which this mechanism can explain the ${Q}^{2}$-dependent broadening recently observed in many glasses both experimentally and numerically is discussed.

EFFECTS OF MATTER DENSITY FLUCTUATION IN LONG BASELINE NEUTRINO OSCILLATION EXPERIMENTS

The effects of matter density fluctuation in long baseline neutrino oscillation experiments are studied. Effects of short wavelength fluctuations are irrelevant. Effects of long wavelength fluctuations must be checked on a case-by-case basis. As an example we checked the fluctuation effects and showed its irrelevance in a case of K2K experiments.

Theory of variable-ratio power splitters using multimode interference couplers

In this letter, we analyze the integrated optical N/spl times/N generalized Mach-Zehnder interferometer (GMZI) employing multimode interference (MMI) couplers. A transfer matrix is used to derive analytic expressions for the intensity distribution at the output ports of the GMZI as a function of the phase shifts applied to the arms. These expressions form a system of nonlinear equations which are numerically solved for the phase shifts, enabling a new interpretation of the GMZI as a variable-ratio power splitter. The performance of the GMZI has been simulated, and it is found that the power splitting ratio is tolerant to both the accuracy of the applied phase shifts and to fluctuations in wavelength.

Change of wandering pattern with anisotropy in step kinetics

We study the effect of anisotropy in step kinetics on the wandering instability of an isolated step. With the asymmetry of the step kinetics, a straight step becomes unstable for long wavelength fluctuations and wanders when the step velocity exceeds a critical value. Near the threshold of the instability, an isotropic step obeys the Kuramoto–Sivashinsky equation, H T =− H XX − H XXXX +( H 2 X /2), and shows a chaotic pattern. A step with anisotropic kinetics obeys the Benney equation, H T =− H XX − δH XXX − H XXXX +( H 2 X /2), and the wandering pattern changes: when the anisotropy is strong, δ≫1, the step shows a regular pattern. Near the threshold of the instability, the anisotropy effect becomes strong while that of the step stiffness becomes weak.

Laser Doppler anemometry and conditional sampling

Abstract This paper reviews a series of conditional sampling techniques developed at Oxford that allow pseudo-instantaneous velocity field pictures to be synthesised from a succession of single-point measurements made with a two-component laser Doppler anemometer (LDA). The first attempt to apply this technique was by Marwood in his study of roof-edge vortices. This pressure conditional velocity method involved extracting appropriate velocity values after examination of pre-recorded simultaneous pressure and velocity time series. In order to obtain the required data from a reasonably short experiment, Marwood accepted velocities within a small range of pressure trigger levels and calculated the flow field mean values of the samples obtained. Subsequently, bending-moment-conditional velocity sampling has been applied to the detection of identifiable coherent flow structures over a forest canopy. In this application, a significant improvement in trigger detection is under investigation. Instead of setting a trigger based upon a single instantaneous bending moment, the bending moment time series is being examined using the relatively new technique of wavelet analysis. In this case, a wavelet function is chosen to detect the short-term fluctuations in bending moment which might be expected to follow the impact of a discrete canopy-penetrating gust. Within a bending moment event the wavelet analysis will give a precise marker on the velocity time series, and once again, repeated experiments with velocity components measured by the LDA at a sequence of locations provide the necessary information to build a pseudo-instantaneous picture of the gust velocity field above and within the forest canopy. This paper reports that preliminary experiments using this method have been unsuccessful because of the inability of the wavelet analysis to specifically detect coherent gusts in a bending moment time series. However, when an inverted analysis is carried out (using velocity-conditional bending-moment wavelet analysis) clear bending moment events can be recognised to coincide with gust fluctuations of similar wavelength as the tree natural frequency. This suggests that the method is viable, but not set up appropriately. More interestingly it leads to the question of whether coherent gusts produced by trees in a forest can lead to resonance damage to the trees themselves.

From the Kondo–Hubbard model to the spin brush model

In the regime of large Hubbard coupling and at half-filling, the Kondo–Hubbard model is mapped onto a new effective model. This is a model of spins interacting antiferromagnetically on each site of a brush (spin brush model). A phase diagram is established. Two phases are found, an antiferromagnetic phase and a phase of local singlet with a band of spin-triplet excitations. Using a recent kind of coherent state, the long wavelength fluctuations are described by the nonlinear sigma model. The quantum critical transition is found and is shifted in comparison to the classical one.