Fluctuation Regime Research Articles

Enhanced thermoelectric coupling near electronic phase transition: The role of fluctuation Cooper pairs

Thermoelectric energy conversion is a direct but low-efficiency process, which precludes the development of long-awaited wide-scale applications. As a breakthrough permitting a drastic performance increase is seemingly out of reach, we fully reconsider the problem of thermoelectric coupling enhancement. The cornerstone of our approach is the observation that heat engines are particularly efficient when their operation involves a phase transition of their working fluid. We derive and compute the thermoelastic coefficients of various systems, including Bose and Fermi gases, and fluctuation Cooper pairs. Combination of these coefficients yields the definition of the thermodynamic figure of merit, the divergence of which at finite temperature indicates that conditions are fulfilled for the best possible use of the thermoelectric working fluid. Here, this situation occurs in the fluctuation regime only, as a consequence of the increased compressibility of the working fluid near its phase transition. Our results and analysis clearly show that efforts in the field of thermoelectricity can now be productively directed towards systems where electronic phase transitions are possible.

Effects of periodic forcing on the temporally correlated spikes of a semiconductor laser with feedback.

Optical excitable devices that mimic neuronal behavior can be building-blocks of novel, brain-inspired information processing systems. A relevant issue is to understand how such systems represent, via correlated spikes, the information of a weak external input. Semiconductor lasers with optical feedback operating in the low frequency fluctuations regime have been shown to display optical spikes with intrinsic temporal correlations similar to those of biological neurons. Here we investigate how the spiking laser output represents a weak periodic input that is implemented via direct modulation of the laser pump current. We focus on understanding the influence of the modulation frequency. Experimental sequences of inter-spike-intervals (ISIs) are recorded and analyzed by using the ordinal symbolic methodology that identifies and characterizes serial correlations in datasets. The change in the statistics of the various symbols with the modulation frequency is empirically shown to be related to specific changes in the ISI distribution, which arise due to different phase-locking regimes. A good qualitative agreement is also found between simulations of the Lang and Kobayashi model and observations. This methodology is an efficient way to detect subtle changes in noisy correlated ISI sequences and may be applied to investigate other optical excitable devices.

Unscaling Superconducting Parameters with Tc for Bi-2212 and Bi-2223: A Magnetotransport Study in the Superconductive Fluctuation Regime

To investigate the origin of the enhanced Tc (≈ 110 K) of the trilayer cuprate superconductor Bi2Sr2Ca2Cu3O10+δ (Bi-2223), we have performed systematic magnetoresistance (MR) measurements on this superconductor, as well as on the bilayer superconductor, Bi2Sr2CaCu2O8+δ (Bi-2212). The in-plane coherence length, ξab, and the specific-heat jump, ΔC, have been estimated using the theory of renormalized superconductive fluctuations, and the doping dependence of these parameters has been qualitatively explained using the Fermi arc approach. A detailed comparison of the superconducting parameters with Tc for these compounds suggests that an additional superconducting condensation energy exists, due to an increase in the number of stacking CuO2 planes in a unit cell.

Crystallographic analysis and conductivity fluctuations of the Y2Ba5Cu8O17 superconductor

Synthesis of the Y 2 Ba 5 Cu 8 O 17 superconducting material by the standard solid state reaction is reported. DC resistivity measurements reveal a bulk Tc = 104.95 K which was determined by the criterion of the maximum in the numerical temperature derivative of electrical resistivity. Structure characterization was performed by means of the X-ray diffraction (XRD) technique. A Rietveld refinement of XRD patterns shows that the material crystallizes in an orthorhombic structure, space group Pmm2 with cell parameters a = 3.8712(0) Å, b = 3.8481(4) Å and c = 27.1601(4) Å. In order to study the pairing mechanism close to Tc, conductivity fluctuation analysis was performed by the method of logarithmic temperature derivative of the conductivity excess. Close and above Tc, the conductivity fluctuation analysis reveal the occurrence of two fluctuation regimes characterized by the critical exponents λ 3D = 0.52 and λ 1D = 1.51, corresponding to 3D and 1D Gaussian regimes, respectively. The correlations of the critical exponents with the dimensionality of the fluctuation system for each Gaussian regime were performed by using the Aslamazov–Larkin theory.

Resonant tunneling of fluctuation Cooper pairs.

Superconducting fluctuations have proved to be an irreplaceable source of information about microscopic and macroscopic material parameters that could be inferred from the experiment. According to common wisdom, the effect of thermodynamic fluctuations in the vicinity of the superconducting transition temperature, Tc, is to round off all of the sharp corners and discontinuities, which otherwise would have been expected to occur at Tc. Here we report the current spikes due to radiation-induced resonant tunneling of fluctuation Cooper pairs between two superconductors which grow even sharper and more pronounced upon approach to Tc. This striking effect offers an unprecedented tool for direct measurements of fluctuation Cooper pair lifetime, which is key to our understanding of the fluctuation regime, most notably to nature of the pseudogap state in high-temperature superconductors. Our finding marks a radical departure from the conventional view of superconducting fluctuations as a blurring and rounding phenomenon.

Magnetic field induced enlargement of the regime of critical fluctuations in the classical superconductor V3Si from high-resolution specific heat experiments

We present high-resolution specific heat data from a high-purity single crystal of the classical superconductor V3Si, which reveal tiny lambda-shape anomalies at the superconducting transition superimposed onto the BCS specific heat jump in magnetic fields of 2 T and higher. The appearance of these anomalies is accompanied by a magnetic-field-induced broadening of the superconducting transition. We demonstrate, using scaling relations predicted by the fluctuation models of the 3d-XY and the 3d-lowest-Landau-level (3d-LLL) universality class that the effect of critical fluctuations becomes experimentally observable due to of a magnetic field-induced enlargement of the regime of critical fluctuations. The scaling indicates that a reduction of the effective dimensionality due to the confinement of quasiparticles into low Landau levels is responsible for this effect.

Signatures of thermally excited vortices in a superconductor with competing orders

Experimental evidence for the existence of a fluctuating charge-density wave order in the pseudogap regime of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+x}$ has renewed interest in its interplay with superconductivity. Here, we consider the problem within a nonlinear sigma model, which was recently proposed to describe the apparent competition between the two order parameters. In particular, we use a saddle-point approximation to calculate the properties of superconducting vortex excitations within such a model. In addition, we analytically calculate a collection of experimentally observable quantities, which probe both the superconducting and charge-density wave fluctuations, and identify expected signatures of thermally excited vortices. Specifically, we find that the magnetization and Nernst signal decay rapidly with temperature for a range of temperatures above the Kosterlitz-Thouless transition, while the x-ray structure factor increases in the same temperature range. At higher temperatures, the system enters a regime of Gaussian fluctuations where vortices are no longer well defined, and as a result the magnetization, Nernst signal, and x-ray structure factor all decrease moderately with temperature.

Численное моделирование вибраций панели летательного аппарата при воздействии акустических пульсаций давления

Investigation vibration characteristics of thin-walled shell when excitation of random pressure fields defined of wall pressure fluctuations in a turbulent boundary layer at flow regime of maximum dynamic pressure and fluctuations induced by the noise of a jet engine at the start. Is assumed that the noise from the jet engine is described a longitudinal acoustic wave propagating along the product's enclosure and diffuse acoustic field, in particular emerging as a result reflection of acoustic waves from the launch facility. As a thin-walled structure is considered streamlined panel fairing products made of a composite materials. In structure of the panel, except shell, included reinforcing circumferential ribs with two frames. The mathematical model presented for described external effects and algorithms for determining of a random vibration characteristics when exposed to fields random pressure. The calculation of the frequency response of the system is produced by the superposition of forms. The spatial correlation function of wall pressure fluctuations in turbulent boundary layer is determined according to the expression Corcos, in which the coefficients the damping of the spatial correlation function are corrected according to the model Efimtsov. The spectral density of wall pressure fluctuations in the turbulent boundary layer is determined according to the model Efimtsov. The levels sound of acoustic pressure caused by the noise of a jet engine at the start is determined by calculating of gas dynamics, and are given as the initial data. It shows the spectral characteristics of the vibration acceleration thin-walled structure depending on the frequency and distribution of the surface of the structure in a neighborhood of the central frequency at which there is maximum loading. It is assumed that the calculated levels of loads will be used as loading conditions during experimental testing of parts.

Численное моделирование вибраций панели летательного аппарата при воздействии акустических пульсаций давления

Investigation vibration characteristics of thin-walled shell when excitation of random pressure fields defined of wall pressure fluctuations in a turbulent boundary layer at flow regime of maximum dynamic pressure and fluctuations induced by the noise of a jet engine at the start. Is assumed that the noise from the jet engine is described a longitudinal acoustic wave propagating along the product's enclosure and diffuse acoustic field, in particular emerging as a result reflection of acoustic waves from the launch facility. As a thin-walled structure is considered streamlined panel fairing products made of a composite materials. In structure of the panel, except shell, included reinforcing circumferential ribs with two frames. The mathematical model presented for described external effects and algorithms for determining of a random vibration characteristics when exposed to fields random pressure. The calculation of the frequency response of the system is produced by the superposition of forms. The spatial correlation function of wall pressure fluctuations in turbulent boundary layer is determined according to the expression Corcos, in which the coefficients the damping of the spatial correlation function are corrected according to the model Efimtsov. The spectral density of wall pressure fluctuations in the turbulent boundary layer is determined according to the model Efimtsov. The levels sound of acoustic pressure caused by the noise of a jet engine at the start is determined by calculating of gas dynamics, and are given as the initial data. It shows the spectral characteristics of the vibration acceleration thin-walled structure depending on the frequency and distribution of the surface of the structure in a neighborhood of the central frequency at which there is maximum loading. It is assumed that the calculated levels of loads will be used as loading conditions during experimental testing of parts.

True precision limits in quantum metrology

We show that quantification of the performance of quantum-enhanced measurement schemes based on the concept of quantum Fisher information (QFI) yields results that are asymptotically equivalent to those from the rigorous Bayesian approach, provided generic uncorrelated noise is present in the setup. At the same time, we show that for the problem of decoherence-free phase estimation this equivalence breaks down, and the achievable estimation uncertainty calculated within the Bayesian approach is larger by a factor of π than that predicted from the QFI even in the large prior knowledge (small parameter fluctuation) regime, where the QFI is conventionally regarded as a reliable figure of merit. We conjecture that an analogous discrepancy is present in the arbitrary decoherence-free unitary parameter estimation scheme, and propose a general formula for the asymptotically achievable precision limit. We also discuss protocols utilizing states with an indefinite number of particles, and show that within the Bayesian approach it is legitimate to replace the number of particles with the mean number of particles in the formulas for the asymptotic precision, which as a consequence provides another argument for proposals based on the properties of the QFI of indefinite particle number states leading to sub-Heisenberg precisions not being practically feasible.

Intuitive model for the scintillations of a partially coherent beam.

An intuitive model for the scintillation index of a partially coherent beam is developed in which essentially the only critical parameter is the properly defined Fresnel number equal to the ratio of the "working" aperture area to the area of the Fresnel zone. The model transpired from and is supported by numerical simulations using Rytov method for weak fluctuations regime and Tatarskii turbulence spectrum with inner scale. The ratio of the scintillation index of a partially coherent beam to that of a plane wave displays a characteristic minimum, the magnitude of which and its distance from the transmitter are easily explained using the intuitive model. A theoretical asymptotic is found for the scintillation index of a source with decreasing coherence at this minimum.

Coherent transition induced by Sr doping on the YBa2Cu3O7-δ single crystal magnetoconductivity

We have studied by magnetoresistivity measurements the influence of the low Sr chemical doping on the conductivity thermodynamic fluctuation regimes (TFR) of a YBa1.98Sr0.02Cu3O7-δ single crystal. The low-current and low-frequency resistivity, ρ(T,H) measurements were performed in a PPMS while DC magnetic fields up to 50kOe were simultaneously applied perpendicular to the ab plane of the single crystal and to the measurement current density. The dρ(T,H)/dT data shows that the superconducting transition of the sample occur is two stages that corresponds to the signature of a granular superconductor. At this scenario is possible to define two critical temperature transitions TC(H) and TC0(H) [TC(H) > TC0(H)]. At TC(H) the superconductivity sets in within the grains and at TC0(H) a coherent transition takes place and a long-range superconducting order takes place over the whole sample. Special attention is reserved to the paracoherent region [TC0(H) ≤ T < TC(H)] of the ρ(T,H) data. The field and the temperature dependence of the paracoherent TFR behavior is characterized by a power law represented by a critical exponent S0 ≈ 3. This exponent is an indicative that the coherence transition of the sample belongs to the 3D-XY universality class with relevant disorder that probably is associated to the low Sr doping.

Bed load transport over a broad range of timescales: Determination of three regimes of fluctuations

This paper describes the relationship between the statistics of bed load transport flux and the timescale over which it is sampled. A stochastic formulation is developed for the probability distribution function of bed load transport flux, based on the Ancey et al. (2008) theory. An analytical solution for the variance of bed load transport flux over differing sampling timescales is presented. The solution demonstrates that the timescale dependence of the variance of bed load transport flux reduces to a three-regime relation demarcated by an intermittency timescale (tI) and a memory timescale (tc). As the sampling timescale increases, this variance passes through an intermittent stage (≪tI), an invariant stage (tI < t < tc), and a memoryless stage (≫ tc). We propose a dimensionless number (Ra) to represent the relative strength of fluctuation, which provides a common ground for comparison of fluctuation strength among different experiments, as well as different sampling timescales for each experiment. Our analysis indicates that correlated motion and the discrete nature of bed load particles are responsible for this three-regime behavior. We use the data from three experiments with high temporal resolution of bed load transport flux to validate the proposed three-regime behavior. The theoretical solution for the variance agrees well with all three sets of experimental data. Our findings contribute to the understanding of the observed fluctuations of bed load transport flux over monosize/multiple-size grain beds, to the characterization of an inherent connection between short-term measurements and long-term statistics, and to the design of appropriate sampling strategies for bed load transport flux.

High-field fluctuation magnetoconductivity and Hall reversal response in the Hg(Re)Ba2Ca2Cu3O8+δ superconductor

Systematic experiments of high-field (up to 50 kOe) fluctuation magnetoconductivity and Hall magnetoresistivity in Hg1-x Rex Ba2 CaCu3O8+d (x=0.18) polycrystalline samples growth by means the quartz tube technique are reported. The analysis of the experimental data was performed by using the recognized Kouvel-Fisher method, which is frequently applied to study of critical phenomena. Very close to the critical temperature TC, a genuinely critical regime of fluctuations characterized by the critical exponent λc =0.32±0.01 was identified in absence of magnetic fields. This result is consistent with the full dynamic 3D-XY universality class predicted by the model E of Hohenberg-Halperin with a dynamic critical exponent z = 3/2. The genuine critical regime become be unstable on the application of external magnetic fields H≈0.1 kOe. Near above the critical temperature TC, the determined exponent λG3=0.52±0.02 was interpreted as corresponding to homogeneous fluctuations, which develop in a space with three-dimensional geometry. This region is destroyed upon the application of magnetic fields above 0.5 kOe. Increasing the temperature, evidences of a homogeneous two-dimensional behavior are observed by means the identification of a λG2=1.02±0.04. Applied fields H&gt;20 kOe destroy this fluctuation regime. Far above TC , effects of disorder and planar anisotropy produce a fluctuation spectrum characterized by a fractal topology with a critical exponent λG2-G1=1.32±0.04. At last, very far TC, a temperature region with λG1=1.52±0.04 was experimentally identified. This corresponds to the confinement of the quasi-particles into the Lowest-Landau-Level, due to the quantization of the electronic states around the axe of application of the external field. Measurements of Hall were performed. In the normal phase, the Hall resistivity is hole-like and inversely proportional to the temperature. In the mixed phase and when the applied field is below μ0H = 2 T, the Hall resistivity shows a double sign reversal. For fields above 2 T, the Hall resistivity remains positive, although qualitatively showing the trends observed at low fields. We attribute this behavior to two independent contributions with opposite sign. A negative term due to thermal fluctuations is relevant near TC, whereas a positive contribution related to vortex motion dominates at lower temperatures. Near the zero resistance state, the Hall resistivity varies as a power law of the longitudinal resistivity, with a field independent exponent β=1.41. PACS: 74.40.+k; 74.25.Bt; 74.60.Ec; 74.72.Bk. © 2014. Acad. Colomb. Cienc. Ex. Fis. Nat. All rights reserved.

Giant Nernst and Hall effects due to chiral superconducting fluctuations

We consider the Nernst and Hall effects in fluctuation regime of chiral superconductors above transition temperatures, that are raised not by conventional Lorentz force, but by asymmetric scattering due to fluctuations of the Berry phase of the Bogoliubov-de Gennes Hamiltonian. It is found that these effects can be more significant than conventional ones for cleaner samples, exhibiting qualitatively distinct behaviors. The results provide systematic and comprehensive understanding for recent experimental observations of the Nernst effect in a clean URu$_2$Si$_2$ sample, which is suggested to be a chiral superconductor.

Charge density wave and metallic state coexistence in the multiband conductorTTF[Ni(dmit)2]2

We have established a pressure-temperature phase diagram of $\mathrm{TTF}{[\mathrm{Ni}{(\mathrm{dmit})}_{2}]}_{2}$ based on spin susceptibility at ambient pressure as well as longitudinal and transverse resistivity measurements under pressure up to 30 kbar. The data were analyzed on the basis of first-principles density functional theory calculations. We were able to find several phase transitions and identify three different charge density wave (CDW) states which all coexist with a metallic state in a wide temperature range and superconductivity at the lowest temperatures. This metallicity arises from the development, upon cooling, of a two-dimensional band associated to the $\mathrm{Ni}{(\mathrm{dmit})}_{2}$ chains. At low pressure, two successive CDW transitions have been clearly identified and are associated to the successive nesting of two strongly one-dimensional bands: The LUMO and ${\mathrm{HOMO}}_{\mathrm{I}}$ of the $\mathrm{Ni}{(\mathrm{dmit})}_{2}$ chains. These two transitions merge into a single one at 12 kbar which probably corresponds to the partial nesting of a bunch of LUMOs into the ${\mathrm{HOMO}}_{\mathrm{I}}$'s. A maximum of this unique CDW transition temperature is observed at 19 kbar. The CDW instability associated to the LUMO band is announced by an important regime of Peierls-like fluctuations in the metallic state which give rise to the progressive development of a pseudogap in the spin susceptibility which has been quantitatively analyzed using the Lee-Rice-Anderson theory.

Spin density wave order and fluctuations inMn3Si: A transport study

We present a comprehensive transport investigation of the itinerant antiferromagnet Mn3Si which undergoes a spin density wave (SDW) order below T_N~21.3K. The electrical resistivity, the Hall-, Seebeck and Nernst effects exhibit pronounced anomalies at the SDW transition, while the heat conductivity is phonon dominated and therefore is insensitive to the intrinsic electronic ordering in this compound. At temperatures higher than T_N our data provide strong evidence for a large fluctuation regime which extends up to ~200K in the resistivity, the Seebeck effect and the Nernst effect. From the comparison of our results with other prototype SDW materials, viz. LaFeAsO and Chromium, we conclude that many of the observed features are of generic character.

TEC retrieval from spaceborne SAR data and its applications

AbstractIt is well known that the spaceborne synthetic aperture radar (SAR) at VHF‐UHF band can be seriously affected by the ionosphere. Thus, the geophysical information of the ionosphere will be embedded in the low‐frequency SAR echoes after they transverse the ionosphere. Correspondingly, the total electron content (TEC), a typical ionospheric information parameter, can be retrieved from the spaceborne SAR data. However, the existing dual‐band techniques for TEC retrieval usually do not include consideration of multiple scattering effects caused by turbulent ionosphere, which plays an important role in the total path delay of signal under the strong fluctuation regimes. The result of TEC retrieval is therefore inaccurate and not applicable. Aiming at this issue, first, this paper analyzes the effects of regular background and the irregularity of electron density on SAR at L‐band, and the theoretical formulation is given. Then, a triband path delay technique of TEC retrieval based on the SAR data is proposed. By using three path delays corresponding to three specific frequencies within the signal bandwidth, this technique can remove the errors of multiple scattering due to the irregularity, and a high accuracy resolution of TEC value therefore can be obtained. Meanwhile, the sensitivity of this technique is analyzed. Finally, compared with traditional dual‐band technique, the numerical simulations show that the correction of SAR imaging based on triband technique is improved significantly. In addition, the resolution of reconstruction imaging using computerized ionospheric tomography performs significantly better based on the triband technique.

On the concentration of large deviations for fat tailed distributions, with application to financial data

Large deviations for fat tailed distributions, i.e. those that decay slower than exponential, are not only relatively likely, but they also occur in a rather peculiar way where a finite fraction of the whole sample deviation is concentrated on a single variable. The regime of large deviations is separated from the regime of typical fluctuations by a phase transition where the symmetry between the points in the sample is spontaneously broken. For stochastic processes with a fat tailed microscopic noise, this implies that, while typical realizations are well described by a diffusion process with continuous sample paths, large deviation paths are typically discontinuous. For eigenvalues of random matrices with fat tailed distributed elements, a large deviation where the trace of the matrix is anomalously large concentrates on just a single eigenvalue, whereas in the thin tailed world the large deviation affects the whole distribution.These results find a natural application to finance. Since the price dynamics of financial stocks are characterized by fat tailed increments, large fluctuations in stock prices are expected to be realized by discrete jumps. Interestingly, we find that large excursions of prices are more likely realized by continuous drifts rather than by discontinuous jumps. Indeed, auto correlations suppress the concentration of large deviations. Financial covariance matrices also exhibit an anomalously large eigenvalue, the market mode, as compared to the prediction of random matrix theory. We show that this is explained by a large deviation with excess covariance rather than by one with excess volatility.

Correlation and spectral aspects of giant optical fluctuations of 2D electrons

The study of a system of interacting 2D electrons in the giant optical fluctuations regime is carried out by means of a spectral-correlation analysis. The possible scenario of formation of various phase states of the 2D system in a vicinity of a filling factor ν=2 is offered.