Presence Of Strong Fluctuations Research Articles

Genetic analysis reveals long-standing population differentiation and high diversity in the rust pathogen Melampsora lini.

A priority for research on infectious disease is to understand how epidemiological and evolutionary processes interact to influence pathogen population dynamics and disease outcomes. However, little is understood about how population adaptation changes across time, how sexual vs. asexual reproduction contribute to the spread of pathogens in wild populations and how diversity measured with neutral and selectively important markers correlates across years. Here, we report results from a long-term study of epidemiological and genetic dynamics within several natural populations of the Linum marginale-Melampsora lini plant-pathogen interaction. Using pathogen isolates collected from three populations of wild flax (L. marginale) spanning 16 annual epidemics, we probe links between pathogen population dynamics, phenotypic variation for infectivity and genomic polymorphism. Pathogen genotyping was performed using 1567 genome-wide SNP loci and sequence data from two infectivity loci (AvrP123, AvrP4). Pathogen isolates were phenotyped for infectivity using a differential set. Patterns of epidemic development were assessed by conducting surveys of infection prevalence in one population (Kiandra) annually. Bayesian clustering analyses revealed host population and ecotype as key predictors of pathogen genetic structure. Despite strong fluctuations in pathogen population size and severe annual bottlenecks, analysis of molecular variance revealed that pathogen population differentiation was relatively stable over time. Annually, varying levels of clonal spread (0–44.8%) contributed to epidemics. However, within populations, temporal genetic composition was dynamic with rapid turnover of pathogen genotypes, despite the dominance of only four infectivity phenotypes across the entire study period. Furthermore, in the presence of strong fluctuations in population size and migration, spatial selection may maintain pathogen populations that, despite being phenotypically stable, are genetically highly dynamic.

Large-Scale Pattern Formation in the Presence of Small-Scale Random Advection.

Despite the presence of strong fluctuations, many turbulent systems such as Rayleigh-Bénard convection and Taylor-Couette flow display self-organized large-scale flow patterns. How do small-scale turbulent fluctuations impact the emergence and stability of such large-scale flow patterns? Here, we approach this question conceptually by investigating a class of pattern forming systems in the presence of random advection by a Kraichnan-Kazantsev velocity field. Combining tools from pattern formation with statistical theory and simulations, we show that random advection shifts the onset and the wave number of emergent patterns. As a simple model for pattern formation in convection, the effects are demonstrated with a generalized Swift-Hohenberg equation including random advection. We also discuss the implications of our results for the large-scale flow of turbulent Rayleigh-Bénard convection.

Control of Excitation Energy Transfer in Condensed Phase Molecular Systems by Floquet Engineering.

Excitation energy transfer (EET) is one of the most important processes in both natural and artificial chemical systems including, for example, photosynthetic complexes and organic solar cells. The EET rate, however, is strongly suppressed when there is a large difference in the excitation energy between the donor and acceptor molecules. Here, we demonstrate both analytically and numerically that the EET rate can be greatly enhanced by periodically modulating the excitation energy difference. The enhancement of EET by using this Floquet engineering, in which the system's Hamiltonian is made periodically time-dependent, turns out to be efficient even in the presence of strong fluctuations and dissipations induced by the coupling with a huge number of dynamic degrees of freedom in the surrounding molecular environments. As an effect of the environment on the Floquet engineering of EET, the optimal driving frequency is found to depend on the relative magnitudes of the system and environment's characteristic time scales with an observed frequency shift when moving from the limit of slow environmental fluctuations (inhomogeneous broadening limit) to that of fast fluctuations (homogeneous broadening limit).

X-ray circular magnetic dichroism in the presence of strong spin fluctuations

The use of X-ray magnetic circular dichroism (XMCD) spectra for determininghe t magnitude of atomic magnetic moments in compounds of rare-earth and transition elements is discussed. The standard sum rule approach often yields a magnitude of moments that is often smaller than values obtained from magnetic measurements. We attribute this to strong spin fluctuations in the surface layers in which XMCD signals form. A way of determining the values of local magnetic moments in the presence of strong fluctuations is proposed and tested.

The X-ray magnetic dichroism, the sum rule and spin fluctuations on the surface of manganite (LaPrCa)MnO3

The problem of determining the magnitude of the atomic magnetic moments in compounds with rare-earth and transition elements using the X-ray magnetic circular dichroism (XMCD) is investigated. The standard sum rules approach usually gives the moment values by several times smaller than their bulk values obtained from the direct magnetic measurements. We attribute this to the strong spin fluctuations in the surface layer which actually formed the soft XMCD signal. A method of determining the value of the local magnetic moments in the presence of strong fluctuations is proposed. The magnetic interactions in La0.5Pr0.2Ca0.3MnO3 are investigated by x-ray absorption spectroscopy, XMCD and bulk magnetization measurements. The proposed procedure is used to determine the values of atomic magnetic moments of transition and rare-earth elements as well as net magnetization on the surface and in the bulk from experimental Mn L2,3 and Pr M4,5 XMCD spectra.

Adaptation and Optimization of Biological Transport Networks

It has been hypothesized that topological structures of biological transport networks are consequences of energy optimization. Motivated by experimental observation, we propose that adaptation dynamics may underlie this optimization. In contrast to the global nature of optimization, our adaptation dynamics responds only to local information and can naturally incorporate fluctuations in flow distributions. The adaptation dynamics minimizes the global energy consumption to produce optimal networks, which may possess hierarchical loop structures in the presence of strong fluctuations in flow distribution. We further show that there may exist a new phase transition as there is a critical open probability of sinks, above which there are only trees for network structures whereas below which loops begin to emerge.

Quantitative description of Josephson-like tunneling inνT=1quantum Hall bilayers

At total filling factor ${\ensuremath{\nu}}_{T}=1$, interlayer phase coherence in quantum Hall bilayers can result in a tunneling anomaly resembling the Josephson effect in the presence of strong fluctuations. The most robust experimental signature of this effect is a strong enhancement of the tunneling conductance at small voltages. The height and width of the conductance peak depend strongly on the area and tunneling amplitude of the samples, applied parallel magnetic field, and temperature. We find that the tunneling experiments are in quantitative agreement with a theory that treats fluctuations due to meron excitations phenomenologically and takes tunneling into account perturbatively. We also discuss the qualitative changes caused by larger tunneling amplitudes, and provide a possible explanation for recently observed critical currents in counterflow geometry.

Reply to “Comments on the Gravity Wave Theory of J. Weinstock Concerning Dissipation Induced by Nonlinear Effects”

The goal of Weinstock (1976a, hereafter W76) was to treat analytically a spectrum of nonlinearly interacting atmospheric gravity waves. Specifically, W76 considered the strong-interaction case in which the fluid velocities exhibit a somewhat chaotic and practically unpredictable character; such motions have been referred to as nonlinear complexity (Lorenz 1969). The statistical renormalizations employed in W76 are one method that has been used to tackle strong nonlinearity. In his note, Hines (2002a, hereafter H02) has commented on the treatment of waves in that 1976 paper, and coincidentally, its subsequent use for the parameterization of gravity wave drag by Medvedev and Klaassen (1995, hereafter MK95). We are grateful for the attention given to those papers and for the opportunity to explain our views. In what follows, we show that H02’s claim of a “demonstrable mathematical error” in W76 rests on his misinterpretation of the behavior of the velocity and temperature coefficients of W76, vK and TK, respectively, and on a deficient conception of the role of the nonlinear advection terms in wave–wave interactions. We begin by outlining the asymptotic limits taken in W76 and present a new derivation that confirms the emergence of polarization relations in the presence of strong fluctuations. We shall address other key issues along the way, including H02’s time-lag arguments, Fig. 1 of MK95, the nature of diffusion in wave–wave interactions, as well as H02’s notion that W76 invoked “no instability whatever.” The arguments presented in H02 were motivated by the work of Allen and Joseph (1989, hereafter AJ89), Hines (2001, hereafter H01), and Hines (2002b, hereafter H02b). These authors maintain that the spectral “tail” found at large vertical wavenumbers in Eulerian measurements of atmospheric wave is present even when the wave system is nondissipative in the Lagrangian frame. In sections 9 and 10, we argue that the generation of turbulent eddies and dissipation represent fundamental aspects of nonlinear wave–wave interactions that cannot be neglected. We refer to recent measurements by Sica (1999) and Sica and Russell (1999) in support of our views.

Cyclic migration of DNA in gels: DNA stretching and electrophoretic mobility.

Available data from spectroscopic and microscopy studies of electrophoretic orientation of long DNA (above 40 kbp) in agarose gels is analyzed on the basis of the fact that the migration in constant fields is cyclic in nature. Defining a cycle period as the time between two consecutive compact states, a simple model is used to obtain data on the average time period (< T >) and the step length (< L >) of the migration cycle from spectroscopic measurements of the dynamics of helix orientation and center-of-mass velocity. Furthermore, the degree of orientation is used to analyze tube-orientation and DNA stretching contributions to < L > and < T >. Finally, the average electrophoretic velocity v = < L >/< T > is analyzed in terms of < L > and < T > for different DNA sizes (Lc), field strengths (E), and gel concentrations (A). The main results of the analysis are: (i) the increase and saturation of the electrophoretic mobility with increasing E is mainly governed by < L > via the degree of DNA stretching, (ii) DNA molecules of different sizes migrate with the same velocity because < L > and < T > both increase approximately linearly with Lc, and (iii) migration in a denser gel is slower mainly because < T > increases, while the step length is approximately constant. Assuming the charge Q of DNA is the same as in free solution, these results suggest that the reason the fundamental reptation equation for the electrophoretic mobility mu = (Q/zeta) < (hx/Lt)2 > also applies in the presence of strong fluctuations in the tube length Lt, and end-to-end distance hx, is that the friction coefficient zeta for motion along the tube is lower the more stretched the DNA is.

Thermal Fluctuations in Magnetic Field and the Josephson Supercurrent in Pb–Ag–Pb (SNS) Junction

The effect of thermal fluctuations on the critical current of Pb-Ag-Pb (SNS) Josephson junctions has been studied. When the critical current goes to zero in the magnetic field (around the minimum positions of the Fraunhofer pattern), the effect of thermal fluctuations is significantly enhanced. In the presence of strong fluctuations, the envelope of the maximum in the Fraunhofer pattern is proportional to 1/H2, which is quite different from the form of 1/H in the absence of fluctuations.

Thermal Fluctuations in Pb–Ag–Pb (SNS) Josephson Junctions in Magnetic Fields

A systematic investigation of the thermal fluctuation effects has been carried out on Pb–Ag–Pb (superconductor-normal metal-superconductor) Josephson junctions with special emphasis on the critical current I c ( H ) in magnetic fields. Thick normal-metal barriers (∼15000 A) are prepared in order to suppress the Josephson coupling energy between the two superconductors. Thermal fluctuations can significantly break the superconducting phase coupling and then lead to a decrease of the oscillation amplitude of I c ( H ). In the presence of strong fluctuations, the envelope of the maxima in the Fraunhofer diffraction pattern is proportional to 1/ H 2 and not to 1/ H as in the absence of thermal fluctuations. The experimental results are in agreement with theoretical predictions of Ivanchenko and Zil'berman.

An investigation of noise spectra from sheathed and unsheathed air-acetylene flames

Noise power spectra are obtained for both background (-OH band) and analyte (Na) emission signals for two locations in sheathed and unsheathed air-acetylene flames. For locations low in the flame, a flowing gas sheath serves to best minimize flame instability; for positions high in the flame, a quartz sheath proves superior. Although no useful high frequency minima are observed in any noise spectra, the presence of strong fluctuations at discrete low frequencies (around 25 Hz) high in unsheathed and argon-sheathed flames urges the avoidance of these spectral regions whenever possible. Application of these findings to practical flame spectrometric analysis is discussed.

Microwave response of Josephson junctions in the presence of strong fluctuations

Abstract The response of well matched Nb point-contract junctions, with a relatively small ratio of barrier and noise energy, has been measured at 35 GHz and is compared with theories on Josephson tunneling in the presence of strong fluctuations.