Uncorrelated Fluctuations Research Articles

Photocurrent near-field microscopy of Schottky barriers.

We used a combination of internal photoemission and of near-field optical microscopy (SNOM) to study the lateral variations in solid interface properties such as energy barriers and electron-hole recombination. In particular we investigated the fully formed Pt-GaP, Au-GaAs, Au-SiNx-GaAs and PtSi-Si Schottky barriers. Our approach enabled us to measure large lateral variations in the photocurrent with spatial resolution on the nanometric scale. Due to the ability of SNOM to supply parallel topographic information, we observed photocurrent variations from zone to zone that only correlated in a few cases with local variations in surface morphology. We assigned the uncorrelated fluctuations to local variations in the interface stoichiometry, the presence of interface states induced by the metallic overlayer and to defect states at the junction. Furthermore, by tuning the photon energy and applied bias we were able to measure the surface distribution of the diffusion length.

Towards a stochastic model of rainfall for radar hydrology: testing the poisson homogeneity hypothesis

In order to investigate to what extent rainfall fluctuations observed with different types of instruments reflect the properties of the rainfall process itself and to what extent they are merely instrumental artefacts we are in the process of developing a stochastic model of rainfall. The starting point for the development of the model has been the notion that at the spatial and temporal scales associated with many types of surface rainfall measurements, rainfall is a discrete process describing the arrival of raindrops of different sizes at the ground. A fundamental question is whether this raindrop arrival process can be considered a homogeneus (Poisson) process or whether it behaves as a clustering (or possibly even scaling) process, as has recently been proposed in the litereture. We have tested the classical Poisson homogeneity hypothesis in rainfall on a 35 min time series of 10 s raindrop size spectra collected with a 50 cm2 disdrometer. The rain rates calculated from the spectra indicated roughly uncorrelated fluctuations around a constant mean rain rate of about 3.5 mm h−1. Two types of analysis of the drop counts were carried out, a global analysis taking into account all drops regardless of their size and an analysis considering the drop counts in the 16 0.21 mm diameter intervals separately. The first type of analysis revealed that evene for the more or less stationary time series under consideration the total raindrop arrival rate was overdispersed with respect to the homogeneous Poisson process. The second type of analysis demonstrated that this rejection of the homogeneity hypothesis could be attributed entirely to raindrops with diameters smaller than 1.14 mm. Although these drops account for 66% of the raindrop concentration in the air and 55% of the raindrop arrival rate at the ground, they only account for 14% of the rain rate and 2% of the radar reflectivity factor (on the basis of the mean drop size distribution during the experiment). In order words, although clustering may be a significant phenomenon for the smallest raindrops, the analyzed data seem to indicate that for moderate rain rates the arrival rate fluctuations of the raindrops which contribute most to the rain rate and radar reflectivity factor behave according to Poisson statistics.

Freezing of compact random heteropolymers with correlated sequence fluctuations

Random heteropolymers (RHPs) with uncorrelated sequence fluctuations on the segmental scale can undergo a transition wherein, below a certain temperature, the thermodynamics is determined by a few dominant conformations. We study this “freezing” transition for RHPs with correlated sequence fluctuations. Specifically, we apply our theory to the case where the correlations decay with a single correlation length; a pragmatically realizable example is provided by random block copolymers. Our results show that the temperature at which freezing occurs grows with the block length of such polymers. Freezing also occurs on the scale of the correlation length, thus making experimental observation of this phenomenon (a consequence of frustration coupled with quenched disorder) more accessible. The results are rationalized on physical grounds.

Relating microscale rock-fluid interaction to macroscale fluid flow structure

Abstract Borehole logs of rock-property spatial fluctuations in the metre to kilometre scale range have power-law Fourier power-spectra that scale inversely with spatial frequency to a power near unity, S ( k ) ∞ 1/ k α . The spectral scaling is ‘universal’ in the sense that a narrow range of scaling exponents, α v ≈ 1.1 ± 0.1 for vertical wells and α h ≈ 1.34 ± 0.1 for horizontal wells, describes rock density, elastic modulus, porosity and lithology fluctuation spectra of a sample of 50 well logs; for 35 vertical logs the bounds on α v hold for both sedimentary and crystalline rock types. The power-law nature of rock-property fluctuation spectra are modelled as long range spatial correlations arising from short range (grain scale) uncorrelated random fluctuations. Such long range random spatial correlations occur in thermodynamic order-disorder phase transitions. Applying to rock the statistical physics of thermodynamic order-disorder transitions, rock heterogeneity observed in borehole logs formally emerges from grain-scale elastic interactions and long range spatial organization of finite strain induced grain-scale defects associated with fluid percolation. If fluid flow paths in rock are significantly influenced by long range correlated random structures, reservoir management cannot be accurately conducted from flow models constrained by small-scale sampling of the reservoir rock; macro scale measurements of site-specific long range random correlation structures are needed.

Foregrounds Removal and CMB Fluctuations in a Multiband Anisotropy Experiment: ARGO 1993

We present the results of a diffuse radiation survey carried out at millimeter and submillimeter wavelengths in the Aries and Taurus sky regions. A balloon-borne telescope with 086 FWHM resolution and four millimeter-wave bands scanned 147 independent sky directions. From the multiband observations we were able to separate two independent components: thermal emission from cirrus dust and cosmic microwave background (CMB) temperature fluctuations. A statistically significant detection of anisotropies is found in the data of the CMB channel: ΔTsky = (24 ± 7) μK rms (95% CL plus 10% calibration error). Assuming uncorrelated Gaussian temperature fluctuations we find a band averaged estimate of the CMB anisotropy power spectrum Cl = (20 ± 9) μK2 at l ≃ 110. The data are consistent with anisotropies in a "standard" Ωb = 0.05, n = 1 model normalized to the rms anisotropy detected by COBE Differential Microwave Radiometers.

Molecular Dynamics Analysis of Coupling between Librational Motions and Isomeric Jumps in Chain Molecules

The coupling between librational motions and rotational isomeric jumps in polymers is investigated. Librations originate from bond stretching, bond angle distortion, and small-amplitude fluctuations in the bond dihedral angles. Molecular dynamics trajectories for a polymer chain of 100 bonds in vacuo are used for assessing the contribution of librations to the dynamic behavior of chains. Each trajectory is resolved into two sets of components, low and high frequency, representing bond rotameric transitions and librations, respectively. Librational motions are observed to be strongly correlated with the rotational jumps of bonds between isomeric states. In fact, the passage from an isomeric state to another is accompanied by an enhancement in librational motions. Examination of the contribution of librations to the local relaxation behavior of polymers indicates that librations affect the mechanism of motion via two opposing effects. On the one hand, they provide an additional degree of freedom which causes a faster relaxation of the conformational correlation functions associated with the bond dihedral angles. On the other hand, the orientational correlation functions associated with the spatial reorientation of backbone bonds exhibit a slower time decay due to the compensating effect of librational motions. In fact, librational motions are not random but highly directed such that they collectively preserve local chain direction and help in accommodating the local reorientations induced by isomeric jumps, which would otherwise result in larger displacements of chain segments. Finally, it is shown that replacing the librations of a trajectory with random, uncorrelated fluctuations leads to an overestimation of the relaxation rate of configurational correlations.

Interrelationship of breath components in neighboring breaths of normal eupneic subjects.

To determine the fraction of variational activity that is correlated on a breath-to-breath basis from uncorrelated random fluctuations, we performed autocorrelation analysis in 33 normal subjects during resting breathing. A calibrated inductive plethysmograph was used to nonobtrusively record 700 breaths in each subject. The group mean autocorrelation coefficients at a lag of 1 breath for each of the three primary breath components, tidal volume (VT), inspiratory time (TI), and expiratory time (TE), were significantly different from zero (p < 0.001). The autocorrelation coefficients for VT, 0.295 +/- 0.148 (SD), and TE, 0.259 +/- 0.121, were greater than that for TI, 0.201 +/- 0.135 (p < 0.001 and p < 0.01, respectively). The autocorrelation coefficients for each breath component remained significant for approximately 3 consecutive breaths (p < 0.001), indicating the presence of "short-term memory." Cross-correlation analysis revealed significant interrelationships (p < 0.001) for all component irrespective of which component was leading or following, with the exception of the pairing of VT in the leading breath and TI in the subsequent breath. In conclusion, in resting healthy subjects breath components display considerable breath-to-breath variability that is not completely random in nature, but which, instead, has a significant fraction of structured correlated variational activity.

Structural characterization for epitaxial Fe/Pt(001) multilayer

The structure for the epitaxial Fe/Pt(001) multilayer deposited in an ultrahigh-vacuum chamber is studied using a four-circle x-ray diffractometer. The Fe layer of the multilayer undergoes a structural transition with varying Fe layer thickness. Specular and nonspecular x-ray-diffraction patterns show a strong correlation between the structural transition and disorder. The linewidth of satellite peaks increases with the distance from fundamental reflection. The behavior of the increase becomes pronounced for the Fe layer thickness beyond 12.6 Å. The lateral linewidth is explained by a mosaic structure and correlated roughness. The correlated fluctuation increases with Fe layer thickness. Amplitudes of uncorrelated fluctuation and cumulative fluctuation are small for the Fe layer thickness less than 8.4 Å and increase with Fe layer thickness.

Fluctuations of excitability in the monosynaptic reflex pathway to lumbar motoneurons in the cat

1. It is well known that the amplitude of successive monosynaptic reflexes (MSR), elicited by afferent stimuli of constant strength, fluctuate from trial to trial. Previous evidence suggests that such excitability fluctuations within the motor pool can be introduced either pre- and/or postsynaptically. Using unanesthetized decerebrate or decerebrate/spinal cats, we attempted to evaluate the relative importance of pre- and postsynaptic mechanisms to MSR variability and the potential contribution of changes in the identities of responding motoneurons to such variability. 2. Comparisons between the MSR amplitude, measured in a severed ventral root, and the probability of firing of up to three individual motoneurons in fine filaments teased from the same root, confirmed that both correlated and uncorrelated fluctuations of motoneuron excitability are involved in MSR variability. Linear regression analysis from concurrent intracellular recordings from homonymous motoneurons showed that the MSR fluctuations were correlated with the variations in membrane potential baseline, as well as with the fluctuations in the monosynaptic excitatory postsynaptic potential peak amplitude. In all 11 cases tested, the former correlation was stronger than the latter. 3. Stimulation of the caudal cutaneous sural nerve (CCS) was used to alter the postsynaptic potential background on which triceps surae (GS) MSRs were generated. The interval chosen between CCS conditioning and the GS stimulation excluded the involvement of presynaptic inhibition. When conditioned by preceding CCS stimulation, GS population MSRs generally (8/9 cases tested) increased in amplitude without much change in their overall variance. However, the individual motoneurons that contributed to the population responses did show changes in both relative excitability and in the uncorrelated component of their response variance. About half of the concurrently recorded motoneurons (6/13) showed a decrease in relative excitability after CCS conditioning, 5/13 showed an increase, and 2/13 were unchanged. Comparison of unit and population responses indicated that the identities of the motoneurons that responded at any given level of population response were quite different with and without CCS conditioning. 4. High-frequency stimulation of Ia fibers was used to alter the state of presynaptic Group Ia-afferents that produced population MSRs. Post tetanic potentiation following high-frequency stimulation did not greatly alter the variance of population MSRs or ratio of correlated and uncorrelated fluctuations in MSR responses among individual motoneurons within the responding population. However, intratetanic depression and posttetanic potentiation of population MSRs were accompanied by marked shifts in individual motoneuron excitability relative to the population response, again indicated that changes in the identities of responding motoneurons contributes to population response fluctuations.(ABSTRACT TRUNCATED AT 400 WORDS)

Fluctuations and the canonical model for fragmentation.

Starting from a parametrized canonical expression for partition weights we investigate effects of uncorrelated stochastic fluctuations in the parameters. A special class of weights which are not affected by uncorrelated fluctuations is isolated and used as a reference for a systematic analysis of fluctuations. The technique is appled to partition weights obtained from a simple bond breaking percolation simulation. Strong evidence for the existence of uncorrelated fluctuations is obtained. The effects of fluctuations and of finite size effects on scaled factorial moments of the mass distribution are analyzed and discussed.

Twiston vs. random fluctuation as the source of the rapid motion of polyethylene in the inclusion complex with perhydrotriphenylene

AbstractRecently molecular dynamics simulations were performed for polyethylene in the inclusion complex with perhydrotriphenylene. The system contained ninety molecules of perhydrotriphenylene, arranged in six stacks of fifteen molecules each, and one molecule of n‐tetracontane, C40H82. The internal CH2‐CH2 bonds in n‐tetracontane have a very strong preference for the trans state. Nevertheless, the chain exhibits a high degree of internal flexibility. This motion produces a characteristic pattern in δ|ψ N + i ψi| vs. N, where ψi describes the instantaneous angle of a C‐H bond vector at carbon atom i about the axis defined by the channel, and δ denotes the fluctuation. The pattern expected for δ|ψ N + i ψi| vs. N is derived for the case where the rapid internal motion is produced by a twiston. It is compared with the results from the simulation and from the expectation for the case where the rapid motion arises from uncorrelated internal fluctuations within the trans state at each CH2‐CH2 bond.

Static and kinetic magnetic behaviour of ensembles of nanoscopic iron particles

The peculiar magnetic structure of pure iron particles with diameters between 2.6 and 6.8 nm, as a function of temperature and time, was described in our previous publications. We concluded that iron particles with diameters below 6.8 nm are in a “core+surface layer” inhomogeneous magnetization state. The surface layer posesses a non-colinear spin structure, a pronounced “magnetic rigidity” (as compared with the core), and exchange parameters larger than those corresponding to the core region. A thermodynamic treatment of the core subjected to the built-in magnetic field provided by the surface layer, predicts two metastable states “α” and “ϵ”, corresponding to anti-parallel and parallel coupling between “core” and “surface”. The kinetics of the new phase nucleation within the parent phase of both magnetically isotropic and anisotropic particles is the main topic of the present work. We conclude that the growth process of uncorrelated elementary magnetization fluctuations, together with macroscopic thermal fluctuations which collapse according to a Néel-Arrhenius law, could justify the empirically determined Avrami-type spontaneous nucleation rates for spin systems. Moreover, the excitation of a few correlated elementary fluctuations involved in a domain-wall-like (DWL) screw motion, could account for field-induced orientational phase transitions within nanoscopic particles with mixed magnetic anisotropies. Such an approach allows us to derive the temperature dependence of Néels “reptation phenomenon” in metallic, particulate, magnetic recording media. Due to the itinerant nature of both conduction and magnetic electrons in iron, the possibility of a metal-insulator transition in the same size range where the particles are in a peculiar magnetic state, is discussed.

Kink propagation induced by multiplicative noise

The influence of spatio-temporal external multiplicative fluctuations on a single kink in a bistable distributed system is studied. For this purpose we derive a stochastic dynamic equation for the position of the shifted kink. An analytical estimate for spatio-temporally uncorrelated fluctuations is represented and discussed. We draw the conclusion that multiplicative noise induces a propagation of the most probable kink into the region of larger noise. This effect is demonstrated in numerical simulations.

Polydispersity of colloidal particles: Intensities for static and dynamic light scattering

We propose a semiempirical method for calculating scattering intensities in the long-wavelength limit for polydisperse colloids based on an effective hard-sphere model. A distinction is made between the scattering at constant composition or “compression-dilation” mode and the scattering due to the different particle “exchange” modes. The model should apply to any system of spherosymmetrical particles whose inner structural degrees of freedom are known. Whatever the number of free parameters, the problem reduces to the decomposition of the effective hard-sphere size ensemble fluctuations into correlated and uncorrelated fluctuations relative to the scattering power ensemble fluctuations. New dynamic light scattering data from concentrated α-crystalline protein dispersions are presented. They compare well with the predictions of the effective hard-sphere model.

Acoustic characteristics of confined jets. Part 1: Theory and numerical calculations

A theoretical model was developed to study the wall pressure and vibratory motion of an elastic pipe which is excited by a confined turbulent jet flow resulting from fluid flow through orifices in the pipe. Based on flow field measurements, the blocked surface pressure was calculated using Lighthill's method, and then used to drive the fluid-filled shell. The wall pressure and pipe wall acceleration were determined by solving the coupled fluid-solid interaction problem. The wall pressure was obained by summing the blocked surface pressure and the pressure due to the wall vibration. An amplitude-modulated convecting wave field was used to simulate the moving acoustic sources of the jet. The random nature of the turbulent jet was incorporated into the analytical model. Specifically, the acoustic pressure was assumed to result from hydrodynamic pressure fluctuations which are uncorrelated in the radial direction, but correlated in the axial direction near the jet exit. The uncorrelated pressure fluctuations in the radial direction reflect the random motion of the turbulent jet, whereas the correlated pressure fluctuations in the axial direction reflect the motion of the large-scale coherent structures near the jet exit. It has been demonstrated that the noise model is capable of relating the flow field and the acoustic field of confined jet flows. [Work supported by ONT and ONR.]

Protein dynamics and distance determination by NOE measurements

Present analysis procedures for NMR structure determination of macromolecules presuppose fixed internuclear distances. Improvement of the precision of the requisite NOE information has stimulated the use of more quantitative distance constraints thus necessitating examination as to whether the assumption of a rigid model systematically biases the distance estimates. Analysis using the simple < r −6> dependence of NOE buildup rates seriously underestimates the correct distance for spatially proximal proton pairs having fluctuations comparable to those observed in X-ray temperature factor analysis. However, by calculating the proper generalized order parameter it is shown that for nuclei undergoing rapid isotropic uncorrelated fluctuations the effective distance is identical to the distance between the mean positions of the nuclei. Similar analysis of molecular dynamics simulation data from bovine pancreatic trypsin inhibitor indicates that the distance obtained from the generalized order parameter predicts the distance between the mean positions to within a few percent regardless of the degree of correlation of the pairwise motion for virtually all main chain and dynamically constrained side chain protons.

Low-frequency intensity noise in semiconductor lasers

The observed 1/f noise in the light-output power S/sub p/ of four different types of heterostructure lasers is explained in terms of spatially uncorrelated gain fluctuations and spontaneous emission fluctuations. Two possible noise sources are suggested: fluctuations in the absorption coefficient and fluctuations in the number of free carriers. Both models are in agreement with the experimental results obtained from index-guided and gain-guided diodes at wavelengths of 1.3 and 0.8 mu m. The dependence S/sub p/ varies as P/sup m/ has been observed with P the average light-output power and m=3/2 under spontaneous emission, a small transition region with m=5/2, m=4 in the superradiation region, and 0 >

Diffusive traversal time: Effective area in magnetically induced interference.

Classical diffusion through a disordered region, connected to leads with unscattered carrier motion, can be characterized by reflection and transmission probabilities and by the time it takes a carrier to diffuse through the sample. This traversal time is found, following a suggestion by Pippard, by letting carriers be absorbed along the way and calculating the resulting reduction in transmission. The traversal time for diffusion is used to calculate the mean square area enclosed between two randomly selected paths in a disordered conductor. In the presence of a magnetic field, this area determines the field scale over which the magnetoresistance exhibits uncorrelated random fluctuations.

Extinction Thresholds in Demographic Models of Territorial Populations

A basic demographic model is constructed for territorial species in a region where patches of habitat suitable for survival and reproduction are randomly (or evenly) interspersed with patches of unsuitable habitat. The model predicts the equilibrium occupancy of suitable habitat as a function of the proportion of the region composed of suitable habitat, h, and of the demographic potential of the population, k, which is determined by parameters of the life history and dispersal behavior of individuals. If 0 < k < 1, the demographic potential gives the equilibrium occupancy in a completely suitable region, and the population will become extinct in the region if h $\leq$ 1 - k. Difficulty in finding a mate, the finite extent of the region containing suitable habitat, and serially uncorrelated fluctuations in life history parameters all increase the minimum value of h necessary to sustain a population. Models of this type should be useful for predicting the effects of habitat destruction and fragmentation, or habitat improvement, on the population sizes of rare or endangered species with territorial behavior.

Noise and correlation effects in GaAlAs broad-band sources

Intensity fluctuations in GaAlAs superluminescent diodes have been measured at low frequencies ( \sim-1 kHz). A monotonic increase in the relative noise with increasing dc bias current was observed. The correlation in noise for light emitted from the two facets was also investigated. The coherence function for the emission from the two facets was found to be near zero at low current levels, and to increase to a maximum value near 0.7 at higher currents. A model which hypothesizes uncorrelated local current fluctuations in different parts of the active region explains the main features of the measured results.