Anisotropic Fluctuations Research Articles

Analysis of Local Anisotropy Fluctuations in Compact Objects

Mathematical modeling within the framework of the general theory of relativity has been used to explain the behavior and structure of massive objects as neutron stars, quasars, black holes, pulsars and white dwarfs and requires finding the exact solutions of the Einstein-Maxwell system. In this paper we study the effects induced by fluctuations of local anisotropy in a new family of anisotropic solutions depending on a parameter α, whose value α=2 provides a radial pressure having the same functional dependence on the radial coordinate as the Schwarzschild solution. It is shown the effect the functional dependence on the radial coordinate has in the occurrence of cracking within the sphere when anisotropy fluctuations are allowed.

Functional-structural decoupling in visual network is associated with cognitive decline in patients with type 2 diabetes mellitus: evidence from a multimodal MRI analysis.

Type 2 diabetes mellitus (T2DM) and cognitive dysfunction are highly prevalent disorders worldwide. Although visual network (VN) alteration and functional-structural coupling are potential warning factors for mild cognitive impairment (MCI) in T2DM patients, the relationship between the three in T2DM without MCI is unclear. Thirty T2DM patients without MCI and twenty-nine healthy controls (HC) were prospectively enrolled. Visual components (VC) were estimated by independent component analysis (ICA). Degree centrality (DC), amplitude of low frequency fluctuation (ALFF) and fractional anisotropy (FA) were established to reflect functional and structural characteristics in these VCs respectively. Functional-structural coupling coefficients were further evaluated using combined FA and DC or ALFF. Partial correlations were performed among neuroimaging indicators and neuropsychological scores and clinical variables. Three VCs were selected using group ICA. Deteriorated DC, ALFF and DC-FA coefficients in the VC1 were observed in the T2DM group compared with the HC group, while FA and ALFF-FA coefficients in these three VCs showed no significant differences. In the T2DM group, DC in the VC1 positively correlated with 2 dimensions in the California Verbal Learning Test, including Trial 4 and Total trial 1-5. The impaired DC-FA coefficients in the VC1 markedly affected the Total perseverative responses % of the Wisconsin Card Sorting Test. These findings indicate that DC and DC-FA coefficients in VN may be potential imaging biomarkers revealing early cognitive deficits in T2DM.

Magnetization reversal through an antiferromagnetic state

Magnetization reversal in ferro- and ferrimagnets is a well-known archetype of non-equilibrium processes, where the volume fractions of the oppositely magnetized domains vary and perfectly compensate each other at the coercive magnetic field. Here, we report on a fundamentally new pathway for magnetization reversal that is mediated by an antiferromagnetic state. Consequently, an atomic-scale compensation of the magnetization is realized at the coercive field, instead of the mesoscopic or macroscopic domain cancellation in canonical reversal processes. We demonstrate this unusual magnetization reversal on the Zn-doped polar magnet Fe2Mo3O8. Hidden behind the conventional ferrimagnetic hysteresis loop, the surprising emergence of the antiferromagnetic phase at the coercive fields is disclosed by a sharp peak in the field-dependence of the electric polarization. In addition, at the magnetization reversal our THz spectroscopy studies reveal the reappearance of the magnon mode that is only present in the pristine antiferromagnetic state. According to our microscopic calculations, this unusual process is governed by the dominant intralayer coupling, strong easy-axis anisotropy and spin fluctuations, which result in a complex interplay between the ferrimagnetic and antiferromagnetic phases. Such antiferro-state-mediated reversal processes offer novel concepts for magnetization control, and may also emerge for other ferroic orders.

Suppression of 1/f noise in graphene due to anisotropic mobility fluctuations induced by impurity motion

Low frequency resistance variations due to mobility fluctuations is one of the key factors of 1/f noise in metallic conductors. According to theory, such noise in a two-dimensional (2D) device can be suppressed to zero at small magnetic fields, implying important technological benefits for low noise 2D devices. In this work, we provide evidence of anisotropic mobility fluctuations by demonstrating a strong field-induced suppression of noise in a high-mobility graphene Corbino disk, even though the device displays only a tiny amount of 1/f noise inherently. The suppression of the 1/f noise depends on charge density, showing less non-uniform mobility fluctuations away from the Dirac point with charge puddles. We model our results using an approach based on impurity clustering dynamics and find our results consistent with the 1/f noise induced by scattering of carriers on mobile impurities forming clusters.

Generalized Hamiltonian drift-fluid and gyrofluid reductions

We provide a procedure for deriving Hamiltonian reduced fluid models for plasmas, starting from a Hamiltonian gyrokinetic system in the approximation. The procedure generalizes, to a considerable extent, previous results. In particular, the evolution of moments with respect to the magnetic moment coordinate is also taken into account, together with background density and magnetic inhomogeneities. In the limit of vanishing Finite Larmor Radius (FLR) effects, an infinite family of reduced electron drift-fluid equations is derived, evolving all the electron moments , with and , where N and M are arbitrary non-negative integers counting the maximum order of the moments taken with respect to the parallel velocity and to the magnetic moment coordinates, respectively. An analogous result is found in the gyrofluid case and applied to the ion species. The gyrofluid result holds for , finite FLR effects and a low ratio β e between electron internal pressure and magnetic guide field pressure. In both the drift and gyrofluid case, the key for the identification of the Hamiltonian structure resides in changes of variables based on orthogonal matrices that diagonalize the Jacobi matrices associated with Hermite and Laguerre polynomials. In terms of the transformed variables, the drift and gyrofluid equations are cast in a simple form, which reduces to advection equations for Lagrangian invariants in the two-dimensional case with homogeneous background. Because the procedure requires to evolve, for a particle species s, all and only the moments , with and , not every choice for the set of moments is admissible, for fixed N and M. This might also explain the scarcity of Hamiltonian reduced fluid models obtained so far which account for anisotropic temperature fluctuations.

Bacterial turbulence in gradient confinement

We investigate a novel form of non-uniform living turbulence at an extremely low Reynolds number using a bacterial suspension confined within a sessile droplet. This turbulence differs from homogeneous active turbulences in two or three-dimensional geometries. The heterogeneity arises from a gradient of bacterial activity due to oxygen depletion along the droplet’s radial direction. Motile bacteria inject energy at individual scales, resulting in local anisotropic energy fluctuations that collectively give rise to isotropic turbulence. We find that the total kinetic energy and enstrophy decrease as distance from the drop contact line increases, due to the weakening of bacterial activity caused by oxygen depletion. While the balance between kinetic energy and enstrophy establishes a characteristic vortex scale depending on the contact angle of the sessile drop. The energy spectrum exhibits diverse scaling behaviors at large wavenumber, ranging from k −1/5 to k −1, depending on the geometric confinement. Our findings demonstrate how spatial regulation of turbulence can be achieved by tuning the activity of driving units, offering insights into the dynamic behavior of living systems and the potential for controlling turbulence through gradient confinements.

Models of interphase drag force from direct numerical simulations of upward turbulent particle-laden channel flows

Data from the interfaced-resolved direct numerical simulations (IR-DNSs) of upward turbulent channel flow laden with finite-size heavy particles at particle concentration ranging from 0.84% to 2.36% are used to develop improved drag correlations which account for the effect of turbulence on drag enhancement. First, the contribution of drag nonlinearity to drag enhancement for the turbulent flow over a fixed sphere is analyzed with the consideration of velocity fluctuation anisotropy. The nonlinear drag enhancement is generally smaller than the actual drag enhancement in our numerical simulations. Second, several models for turbulence correction are presented with different choice of model parameters, and it is found that the turbulent intensity normalized by the mean slip velocity and the particle size normalized by the Kolmogorov scale are most important for modeling the effect of turbulence on particle drag, while the anisotropy of fluctuating velocity and the particle Reynolds number are less important.

Deterministic Current‐Induced Perpendicular Switching in Epitaxial Co/Pt Layers without an External Field

AbstractCurrent‐induced spin‐orbit torques (SOTs) have emerged as a powerful tool to control magnetic elements and non‐uniform magnetic textures such as domain walls and skyrmions. SOT‐induced switching of perpendicular magnetization generally requires an external field to break the rotational symmetry of the spin‐orbit effective fields responsible for the deterministic reversal. The proposed mechanisms to eliminate this requirement often rely on complex multilayer structures that necessitate laborious optimization in the material and spin transport properties, making them less attractive for applications. Herein, current‐induced, external field‐free switching of an epitaxial MgO/Pt/Co trilayer with an extremely large perpendicular anisotropy in excess of 3 Tesla is reported. It is found that switching occurs due to the interplay of strong SOTs, local anisotropy fluctuations, and the Dzyaloshinkii‐Moriya interaction inherent to this epitaxial system. Given that these layers constitute the base stack of a magnetic tunnel junction, this switching mechanism offers the most technologically viable path toward devices such as field‐free SOT‐based magnetic random‐access memories.

Anisotropic and Finite Effects on Intermolecular Vibration and Relaxation Dynamics: Low-Frequency Raman Spectroscopy of Water Film and Droplet on Graphene by Molecular Dynamics Simulations.

The structural and dynamical properties of water can be greatly altered by the anisotropic interfacial environment. Here, we study the intermolecular vibration and relaxation dynamics of a water film and a water droplet on a graphene surface based on low-frequency Raman spectra calculated from molecular dynamics simulations. The calculated Raman spectra of the interfacial water systems show a weakened libration peak and an enhanced intermolecular hydrogen bond (HB) stretching peak compared to the spectrum of bulk water, which are attributed to softened orientation motion. We also find that the collective polarizability relaxation in the droplet is much slower than that in the film and bulk, which is completely different from the collective dipole relaxation. The slow relaxation is due to a positive correlation between the induced polarizabilities of distinct molecules caused by the global and anisotropic structural fluctuations of the water droplet. Furthermore, we find that the two-dimensional HB network by the orientation-ordered interfacial water molecules leads to different intermolecular vibration dynamics between the parallel and perpendicular components. The present theoretical study demonstrates that low-frequency Raman spectroscopy can reveal the anisotropic and finite effects on the intermolecular dynamics of the water film and droplet.

Anisotropic flow and flow fluctuations of identified hadrons in Pb–Pb collisions at sqrt{s_{textrm{NN}}} = 5.02 TeV

The first measurements of elliptic flow of π±, K±, textrm{p}+overline{textrm{p}} , {textrm{K}}_{textrm{S}}^0 , Lambda +overline{Lambda} , ϕ, {Xi}^{-}+{overline{Xi}}^{+} , and {varOmega}^{-}+{overline{varOmega}}^{+} using multiparticle cumulants in Pb–Pb collisions at sqrt{s_{textrm{NN}}} = 5.02 TeV are resented. Results obtained with two- (v2{2}) and four-particle cumulants (v2{4}) are shown as a function of transverse momentum, pT, for various collision centrality intervals. Combining the data for both v2{2} and v2{4} also allows us to report the first measurements of the mean elliptic flow, elliptic flow fluctuations, and relative elliptic flow fluctuations for various hadron species. These observables probe the event-by-event eccentricity fluctuations in the initial state and the contributions from the dynamic evolution of the expanding quark–gluon plasma. The characteristic features observed in previous pT-differential anisotropic flow measurements for identified hadrons with two-particle correlations, namely the mass ordering at low pT and the approximate scaling with the number of constituent quarks at intermediate pT, are similarly present in the four-particle correlations and the combinations of v2{2} and v2{4}. In addition, a particle species dependence of flow fluctuations is observed that could indicate a significant contribution from final state hadronic interactions. The comparison between experimental measurements and CoLBT model calculations, which combine the various physics processes of hydrodynamics, quark coalescence, and jet fragmentation, illustrates their importance over a wide pT range.

Model of multiphase flow, superheat transport, and particle capture in a steel continuous caster

A model of turbulent multiphase flow, heat transfer, and particle entrapment during continuous casting of steel is presented. The model includes the top 7m of the vertical and curved strand, and considers the effects of argon gas injection and thermal buoyancy. RANS model flow results are compared with LES. Lagrangian particle transport through the Eulerian-Eulerian multiphase flow field from the k-ɛ RANS model is based on random walk and features advanced particle capture criteria, improved from previous work by including anisotropic turbulent velocity fluctuations near the walls. Particle capture via 3 mechanisms is included: capture by solidified hooks at the meniscus, entrapment between dendrites, and engulfment by surrounding large particles. The fluid flow and bubble/inclusion capture results are validated with plant measurements, including nail board dipping tests and ultrasonic tests of particle locations, and good agreement is seen. The superheat has negligible effect on flow in the mold region but causes complex flow in the lower strand by creating multiple recirculation zones due to the thermal buoyancy. With high (30 K) superheat, this leads to less penetration, and slightly fewer and shallower capture of particles. Lower (10 K) superheat may enable significant top surface freezing, leading to very large internal defect clusters. Lower superheat also leads to deeper meniscus hooks, and more surface capture. Capture bands occur near the transition from vertical to curved, where downward liquid flow velocity balances the particle terminal velocity.

Tuning the topology of a two-dimensional catenated DNA network

Molecular topology of polymers plays a key role in determining their physical properties. We studied herein the topological effects on the static and dynamic properties of a 2D catenated network of DNA rings called a kinetoplast. Restriction enzymes that cleave DNA at sequence-specific sites are used to selectively cut and remove rings from the network and hence tune the molecular topology while maintaining overall structural integrity. We find that topology has minimal effects over the spatial extension of the 2D network; however, it significantly affects the relaxation behavior. The shape fluctuations of the network are governed by two distinct characteristic time scales attributed to the thermal fluctuations and confinement of the network. The relationship between the time constant of thermal relaxation and the amplitude of anisotropy fluctuations yields a universal scaling. Interestingly, this scaling is independent of the detailed arrangements of rings and/or perforation within the catenated networks. This study provides a route to tune the elastic properties of 2D catenated DNA networks and other polymeric materials by modifying the underlying topology in a rational and highly controllable manner.

Alpha tensor and dynamo excitation in turbulent fluids with anisotropic conductivity fluctuations

AbstractA mean‐field theory of the electrodynamics of a turbulent fluid is formulated under the assumption that the molecular electric conductivity is correlated with the turbulent velocity fluctuation in the (radial) direction, . It is shown that for such homogeneous fluids a strong turbulence‐induced field advection anti‐parallel to arises almost independently of rotation. For rotating fluids, an extra effect appears with the known symmetries and with the expected maximum at the poles. Fast rotation, however, with Coriolis number exceeding unity suppresses this term. Numerical simulations of forced turbulence using the nirvana code demonstrate that the radial advection velocity, , always dominates the term. We show finally with simplified models that dynamos are strongly influenced by the radial pumping: for the solutions become oscillatory, while for they become highly exotic if they exist at all. In conclusion, dynamo models for slow and fast solid‐body rotation on the basis of finite conductivity–velocity correlations are unlikely to work, at least for dynamos without strong shear.

The frequency ratio and time delay of solar radio emissions with fundamental and harmonic components

ABSTRACT Solar radio bursts generated through the plasma emission mechanism produce radiation near the local plasma frequency (fundamental emission) and double the plasma frequency (harmonic). While the theoretical ratio of these two frequencies is close to 2, simultaneous observations give ratios ranging from 1.6 to 2, suggesting either a ratio different from 2, a delay of the fundamental emission, or both. To address this long-standing question, we conducted high-frequency, high-time resolution imaging spectroscopy of type III and type J bursts with fine structures for both the fundamental and harmonic components with LOFAR between 30 and 80 MHz. The short-lived and narrow frequency-band fine structures observed simultaneously at fundamental and harmonic frequencies give a frequency ratio of 1.66 and 1.73, similar to previous observations. However, frequency-time cross-correlations suggest a frequency ratio of 1.99 and 1.95 with a time delay between the F and H emissions of 1.00 and 1.67 s, respectively for each event. Hence, simultaneous frequency ratio measurements different from 2 are caused by the delay of the fundamental emission. Among the processes causing fundamental emission delays, anisotropic radio-wave scattering is dominant. Moreover, the levels of anisotropy and density fluctuations reproducing the delay of fundamental emissions are consistent with those required to simulate the source size and duration of fundamental emissions. Using these simulations we are able to, for the first time, provide quantitative estimates of the delay time of the fundamental emissions caused by radio-wave propagation effects at multiple frequencies, which can be used in future studies.

Active Forces in Confluent Cell Monolayers

We use a computational phase-field model together with analytical analysis to study how intercellular active forces can mediate individual cell morphology and collective motion in a confluent cell monolayer. We explore the regime where intercellular forces dominate the tissue dynamics, and polar forces are negligible. Contractile intercellular interactions lead to cell elongation, nematic ordering, and active turbulence characterized by motile topological defects. Extensile interactions result in frustration, and perpendicular cell orientations become more prevalent. Furthermore, we show that contractile behavior can change to extensile behavior if anisotropic fluctuations in cell shape are considered.

Thickness-dependent oxygen chemisorption behaviors on (1 1 1) surfaces of two-dimensional FCC metals Al and Cu: First-principles study

Oxygen chemisorption is one of the crucial factors to understand the performance of metal materials under special service environments. Here, we investigate the thickness-dependent O chemisorption behaviors on two-dimensional Al(111) and Cu(111) films based on first-principles calculations. It is found that the charge transfer from substrate to adsorbate O atom is mainly contributed by surface atoms in the Al system, but by surface associated with sub-surface atoms in the Cu system. Moreover, the thickness-dependent surface energy, work function, and O adsorption energy show an approximate three-monolayer oscillation for the Al surface, but not for the Cu surface. It results from quantum size effects (QSEs) in Al, which are related not only to z-direction size but also to wavevector: the QSEs reach their maximum at Γ¯ point resulting from strong interlayer coupling interaction between 3pz orbitals. The oscillations of Al_3pz PDOS mainly contribute to that of TDOS around EF, thus giving an oscillatory energy state contribution to periodic surface energies and reactivities. However, because d-band metals like Cu(111) have complex Fermi surfaces causing incremental Fermi wavelength anisotropy and larger-amplitude fluctuation of Cu_t2g at EF, QSEs will be generally canceled. Our findings may shed light on modeling the chemisorption and intrinsic physical properties, and thereby the distinct surface behaviors of low-dimensional metallic films.

Turbulence-induced vibrations prediction through use of an anisotropic pressure fluctuation model

In nuclear fuel rod bundles, turbulence-induced pressure fluctuations caused by an axial flow can create small but significant vibrations in the fuel rods, which in turn can cause structural effects such as material fatigue and fretting wear. Fluid-structure interaction simulations can be used to model these vibrations, but for affordable simulations based on the URANS approach, a model for the pressure fluctuations must be utilised. Driven by the goal to improve the current state-of-the-art pressure fluctuation model, AniPFM (Anisotropic Pressure Fluctuation Model) was developed. AniPFM can model velocity fluctuations based on anisotropic Reynolds stress tensors, with temporal correlation through the convection and decorrelation of turbulence. From these velocity fluctuations and the mean flow properties, the pressure fluctuations are calculated. The model was applied to several test cases and shows promising results in terms of reproducing qualitatively similar flow structures, as well as predicting the root-mean-squared pressure fluctuations. While further validation is being performed, the AniPFM has already demonstrated its potential for affordable simulations of turbulence-induced vibrations in industrial nuclear applications.

Stochastic model for the alignment and tumbling of rigid fibers in two-dimensional turbulent shear flow

Non-spherical particles transported by an anisotropic turbulent flow preferentially align with the mean shear and intermittently tumble when the local strain fluctuates. Such an intricate behaviour is here studied for inertialess, rod-shaped particles embedded in a two-dimensional turbulent flow with homogeneous shear. A Lagrangian stochastic model for the rods angular dynamics is introduced and compared to the results of direct numerical simulations. The model consists in superposing a short-correlated random component to the steady large-scale mean shear, and can thereby be integrated analytically. To reproduce the single-time orientation statistics obtained numerically, it is found that one has to properly account for the combined effect of the mean shear, for anisotropic velocity gradient fluctuations, and for the presence of persistent rotating structures in the flow that bias Lagrangian statistics. The model is then used to address two-time statistics. The notion of tumbling rate is extended to diffusive dynamics by introducing the stationary probability flux of the rods unfolded angle. The model is found to reproduce the long-term effects of an average shear on the mean and the variance of the fibres angular increment. Still, it does not reproduce an intricate behaviour observed in numerics for intermediate times: the unfolded angle is there very similar to a L\'evy walk with distributions of increments displaying intermediate power-law tails.

Structural and magnetic ordering properties of TbFexMn12-x (x = 6.0–9.0) intermetallic compounds

In this work, single-phase TbFexMn12-x compounds with the optimized composition (x = 6.0–9.0) were prepared, and their structural and magnetic ordering properties were investigated. It was found that the Tb-(Mn, Fe) coupling strength was significantly enhanced by the introduction of Fe, which finally significantly increased the magnetic ordering temperature of the R sublattice and induced interesting magnetic behaviors. When x ≤ 8.0, negative magnetization was observed in the M-T curves when decreasing the temperature in a constant positive external field. When x = 6.0, the M-T curve seemed the M-T curves of normal ferromagnetic materials because the net magnetization of the (Fe, Mn) magnetic sublattice was too small to induce the Tb magnetization to align in the opposite direction of the external magnetic field. Lateral and longitudinal offsets in the hysteresis loops, namely exchange-bias-like behavior, were observed, and with the increase of the maximum measurement field, the lateral and longitudinal offsets become smaller. The estimated minimum magnetic field required to cancel the offset was 110 kOe. In addition, AC susceptibility measurement results showed that all samples had the characteristics of spin-glass state at temperature just below the (Fe, Mn) sublattice ordering temperature due to the fluctuations in exchange interaction and anisotropy caused by the introduction of Fe in the (Fe, Mn) sublattice.

Numerical study on the pulsating energy evolution in the cavitating flow around a mini Cascade

Cavitation is arguably a highly turbulent phenomenon in the liquid flow system. The cavitating flow around a mini cascade was carried out to investigate the turbulent characteristics and pulsation mechanisms. The results demonstrate that cavitation can significantly affect the turbulence velocity fluctuation and turbulence anisotropy, and intensively alter the local turbulent energy. To better provide an understanding of fundamental mechanisms dictating time-averaged pulsating energy, the inhomogeneity of the local concentration of pulsating energy at the vapor–liquid interface and the turbulent vortex core involves different fundamental mechanisms are expounded thoroughly through the ability of the time-averaged turbulent kinetic energy and the time-averaged pulsating entropy. The pulsating energy of cavitating flow around the mini cascade is basically obtained from the time-averaged flow, while the surrounding dissipative mechanisms are driven by the diffusion and dissipation terms. Further, the new definition of viscous diffusion term is derived based on the resolved turbulent kinetic energy, which can also clearly delineate the diffusion effect of turbulent kinetic energy produced by the molecule viscosity. Finally, the turbulent kinetic energy and pulsating enstrophy transport mechanisms inside the shedding vortex are revealed as significant characteristics of the interaction between vortex dynamics and turbulence–cavitation.