Scale Of Coherence Length Research Articles

Turbulent Noise Sources Reduction by Large-Eddy Breakup Devices and Riblets

An experimental study is presented on the application of large-eddy breakup (LEBU) on a flat plate as an outer layer source-targeting device to mitigate the wall pressure fluctuations and lateral coherence length scale of a turbulent boundary-layer developed at zero pressure gradient. Both represent the prominent noise sources for the trailing-edge noise radiation. When a LEBU is placed strategically at the outer part of a turbulent boundary-layer, the wall pressure spectra can establish a self-similar behavior against s′, which is a normalized separation distance between the LEBU’s trailing-edge and the targeted location for noise mitigation. It is found that s′ has to be greater than 3 to achieve an overall reduction in the wall pressure fluctuations. What appears to be the optimal LEBU configuration for the mitigation of wall pressure fluctuations, however, will not be reciprocated in the lateral coherence length. The combined effect would render the LEBU rather ineffective to reduce the frequency-integrated, overall noise radiation. Our previous study has confirmed that riblets, a near-wall source-targeting device, is more effective in the mitigation of the lateral coherence length. Under the principle of non-interference, a combination of riblets and LEBU demonstrates that reduction of the frequency-integrated, overall noise radiation can be achieved, again at s′>3.

Evolution of current-carrying string networks

Cosmic string networks are expected to form in early Universe phase transitions via the Kibble mechanism and are unavoidable in several Beyond the Standard Model theories. While most predictions of observational signals of string networks assume featureless Abelian-Higgs or Nambu-Goto string networks, in many such extensions the networks can carry additional degrees of freedom, including charges and currents; these are often generically known as superconducting strings. All such degrees of freedom can impact the evolution of the networks and therefore their observational signatures. We report on the results of 20483 field theory simulations of the evolution of a current-carrying network of strings, highlighting the different scaling behaviours of the network in the radiation and matter eras. We also report the first numerical measurements of the coherence length scales for the charge and current and of the condensate equation of state, and show that the latter mainly depends on the expansion rate, with chirality occurring for the matter era. Qualitatively, the fact that the matter era is the optimal expansion rate for these networks to reach scaling is in agreement with recent analytic modelling.

Magnetogenesis with gravitational waves and primordial black hole dark matter

Strongly supercooled first-order phase transitions (FOPTs) can produce primordial black hole (PBH) dark matter (DM) along with observable gravitational waves (GWs) from bubble collisions. Such FOPTs may also produce coherent magnetic fields generated by bubble collisions and by turbulence in the primordial plasma. Here we find that the requirement for PBH DM can produce large primordial magnetic fields which subsequently yield intergalactic magnetic fields in the present Universe (with magnitude ≲20 pG across coherence length scales of ≃0.001–0.01 Mpc, assuming maximally helical magnetic fields) that easily exceed lower bounds from blazar observations. We follow a largely model-independent approach and highlight the possibility of producing DM and observable multimessenger magnetic fields and GW signals visible in next generation experiments. Published by the American Physical Society 2024

Probing coherence in metal absorption towards multiple images of strong gravitationally lensed quasars

ABSTRACT We present a tomographic analysis of metal absorption lines arising from the circumgalactic medium (CGM) of galaxies at z ≈ 0.5–2, using Multi Unit Spectroscopic Explorer (MUSE) observations of two background quasars at z ≈ 2.2 and 2.8, which are two of the few currently known quasars with multiple images due to strong gravitational lensing by galaxy clusters at z ≈ 0.6 and 0.5, respectively. The angular separations between different pairs of quasar multiple images enable us to probe the absorption over transverse physical separations of ≈0.4–150 kpc, which are based on strong lensing models exploiting MUSE observations. The fractional difference in rest-frame equivalent width (ΔWr) of Mg ii, Fe ii, and C iv absorption increases on average with physical separation, indicating that the metal-enriched gaseous structures become less coherent with distance, with a likely coherence length-scale of ≈10 kpc. However, ΔWr for all the ions vary considerably over ≈0.08–0.9, indicating a clumpy CGM over the full range of length-scales probed. At the same time, paired Mg ii absorption is detected across ≈100–150 kpc at similar line-of-sight velocities, which could be probing cool gas clouds within the same halo. No significant dependence of ΔWr is found on the equivalent width and redshift of the absorbing gas and on the galaxy environment associated with the absorption. The high-ionization gas phase traced by C iv shows a higher degree of coherence than the low-ionization gas phase traced by Mg ii, with ≈90 per cent of C iv systems exhibiting ΔWr ≤ 0.5 at separations ≤10 kpc compared to ≈50 per cent of Mg ii systems.

Large-scale motions in a turbulent natural convection boundary layer immersed in a stably stratified environment

This study investigates the coherence of turbulent fluctuations in a turbulent vertical natural convection boundary layer immersed in a stably stratified medium (turbulent buoyancy layer). A turbulent buoyancy layer of a fluid having a Prandtl number of $0.71$ at a Reynolds number of $800$ is numerically simulated using direct numerical simulation. The two-point correlations reveal that the streamwise velocity fluctuations are coherent over large streamwise distances, with the length scale of the streamwise coherence being greater than the boundary layer thickness. This is due to large-scale motions (LSMs), similar to the LSMs observed in canonical wall-bounded turbulence despite the stark differences in flow dynamics. Both high-speed (positive) and low-speed (negative) streamwise velocity fluctuations form LSMs, with their streamwise length scales increasing with increasing wall-normal distance. High-speed LSMs are composed of upwash flow with high temperatures, while low-speed LSMs are composed of downwash flow with low temperatures. Both high-speed and low-speed LSMs meander appreciably in the streamwise direction, with the degree of meandering being correlated with the sign of the spanwise velocity fluctuations. The LSMs exhibit coherence across significant wall-normal distances and contribute significantly to the turbulence production in the outer layer. Examining the one-dimensional energy spectra of the turbulent buoyancy layer shows that the LSMs are the dominant energy-containing motions, implying that the length scale of the energy-containing range is of the order of boundary layer thickness. Notably, wall-normal velocity, spanwise velocity and buoyancy fluctuations do not form LSMs with streamwise length scales comparable to streamwise velocity fluctuations.

Generation and decay of Higgs mode in a strongly interacting Fermi gas

We investigate the life cycle of the large amplitude Higgs mode in strongly interacting superfluid Fermi gas. Through numerical simulations with time-dependent density functional theory and the technique of the interaction quench, we verify the previous theoretical predictions on the mode’s frequency. Next, we demonstrate that the mode is dynamically unstable against external perturbation and qualitatively examine the emerging state after the mode decays. The post-decay state is characterized by spatial fluctuations of the order parameter and density at scales comparable to the superfluid coherence length scale. We identify similarities with FFLO states, which become more prominent at higher dimensionalities and nonzero spin imbalances.

Néel proximity effect at antiferromagnet/superconductor interfaces

Spin-splitting induced in a conventional superconductor weakens superconductivity by destroying spin-singlet and creating spin-triplet Cooper pairs. We demonstrate theoretically that such an effect is also caused by an adjacent compensated antiferromagnet, which yields no net spin-splitting. We find that the antiferromagnet produces N\'eel triplet Cooper pairs, whose pairing amplitude oscillates rapidly in space similar to the antiferromagnet's spin. The emergence of these unconventional Cooper pairs reduces the singlet pairs' amplitude, thereby lowering the superconducting critical temperature. We develop a quasiclassical Green's functions description of the system employing a two-sublattice framework. It successfully captures the rapid oscillations in the Cooper pairs' amplitude at the lattice spacing scale as well as their smooth variation on the larger coherence length scale. Employing the theoretical framework thus developed, we investigate this N\'eel proximity effect in a superconductor/antiferromagnet bilayer as a function of interfacial exchange, disorder, and chemical potential, finding rich physics. Our findings also offer insights into experiments which have found a larger than expected suppression of superconductivity by an adjacent antiferromagnet.

Mitigation of turbulent noise sources by riblets

A feasibility study is presented on the application of riblets to suppress turbulent pressure sources leading to a reduction in the generation of aerofoil self-noise. It is shown that riblets can reduce skin friction as well as the turbulence intensity inside the boundary layer. In addition, near-wall turbulence structures were found to be dissipated quite rapidly when crossing the riblets surface. It is found that riblets (1) slightly reduce the wall fluctuating pressure power spectral density level at the low and high frequency ranges, but cause an increase at the mid frequency range, and (2) reduce the lateral turbulence coherence length scale across a large frequency range. As a result, riblets have the potential to reduce trailing edge noise at the low and high frequency regions. The fundamental mechanism by which riblets reduce the turbulent intensity level inside the boundary layer is investigated by studying the spatio-temporal evolution of turbulent spots, which are commonly regarded as the building blocks of a turbulent boundary layer. In this paper two mechanisms are proposed. First, the enhanced momentum achieved at the becalmed region of each turbulent spot will lead to a reduction in the overall turbulence intensity obtained when turbulent spots merge downstream. Second, the internal turbulence level at the rear part of a turbulent spot can be reduced directly by an enhanced re-laminarisation effect.

Spin fluctuations and superconductivity in Y5Rh6Sn18 doped with Pd and Co: Evidence of peak effect in the superconducting mixed state

We performed a systematic study of the nonmagnetic skutterudite-related ${\mathrm{Y}}_{5}{\mathrm{Rh}}_{6}{\mathrm{Sn}}_{18}$ superconductor, where the lattice disorder on the coherence length scale $\ensuremath{\xi}$ additionally generates a nonhomogeneous, high-temperature superconducting phase with disorder-enhanced critical temperature ${T}_{c}^{★}$. We have previously discussed various possibilities of local atomic disorder; one of the possibilities is doping. Our present studies focus on the series of ${\mathrm{Y}}_{5\ensuremath{-}\ensuremath{\delta}}({\mathrm{Rh}}_{5.5}{M}_{0.5}){\mathrm{Sn}}_{18}$ compounds ($\ensuremath{\delta}\ensuremath{\ll}1$), where the dopants $M=$ Co, Ir, Ru, and Pd, when they are smaller (Co) or larger (Pd) than Rh, generate the peak effect in fields smaller than the critical field ${H}_{c2}$. This phenomenon manifests itself as a weak peak in the real and imaginary parts of ac susceptibility and is more distinct in magnetoresistance. Using a simple theoretical model, we demonstrate that the effectiveness of this mechanism depends not only on the magnitude of the difference between the size of the dopant and the host atom, but also on whether the dopant is smaller or larger. The agreement between this prediction and our experimental data strongly supports the impurity-based scenario of the observed peak effect. Magnetoresistance isotherms of the remaining samples ($M=$ Ir and Ru) with the radius of $M$ being very similar to that of Rh show, however, a weak peak-effect-like behavior, which is mainly due to vacancies $\ensuremath{\delta}$ at Y sites, while the corresponding ac susceptibility isotherms exhibit a distinct peak at $H\ensuremath{\sim}{H}_{c2}$. This field-dependent ${\ensuremath{\chi}}_{\mathrm{ac}}$ anomaly appears to be similar in nature to the peak effect; however, it cannot be attributed to pinning and appears to be an equilibrium property of the system. We also report the coexistence of spin fluctuations and superconductivity for ${\mathrm{Y}}_{5}{\mathrm{Rh}}_{6}{\mathrm{Sn}}_{18}$ doped with Pd and Co.

Yu-Shiba-Rusinov states in two-dimensional superconductors with arbitrary Fermi contours

Magnetic impurities on a superconductor induce subgap Yu-Shiba-Rusinov (YSR) bound states, localized at the impurity site, decaying over distances dependent on microscopical details. Here, the authors present an effective model providing analytical expressions of their spatial distribution for a two-dimensional superconductor with an arbitrary Fermi contour. These results prove that vector nesting in anisotropic Fermi contours focuses YSR states into one-dimensional-like beams, extending to coherence length scales. The authors check the accuracy of the approximation by comparing the results to an exact numerical calculation based on a tight-binding Hamiltonian and to STM results on Mn dimers on a superconductor.

Spontaneous organization and phase separation of skyrmions in chiral active matter.

Skyrmions are topologically protected vortex-like excitations that hold promise for applications such as information processing and electron manipulation. Here we combine theoretical analysis and numerical simulations to show that skyrmions can spontaneously emerge in chiral active matter without external confinements or regulation. Strikingly, these activity-driven skyrmions can either self-organize into a periodic, stable square lattice consisting of half Néel skyrmions and antiskyrmions, where the in-plane flows display an antiferromagnetic vortex array, or undergo phase separation between skyrmions with different topological numbers. We identify that the emerging skyrmion dynamics stems from the competition between the chiral and polar coherence length scales dictated by the interplay of intrinsic chirality, polarity, and elasticity in the system. Our results reveal unanticipated topological excitations, self-organization, and phase separation in non-equilibrium systems and also suggest a potential way towards engineering complicated bespoke skyrmionic structures through manipulating active matter.

Signatures of two-dimensional superconductivity emerging within a three-dimensional host superconductor

Spatial disorder has been shown to drive two-dimensional (2D) superconductors to an insulating phase through a superconductor-insulator transition (SIT). Numerical calculations predict that with increasing disorder, emergent electronic granularity is expected in these materials-a phenomenon where superconducting (SC) domains on the scale of the material's coherence length are embedded in an insulating matrix and coherently coupled by Josephson tunneling. Here, we present spatially resolved scanning tunneling spectroscopy (STS) measurements of the three-dimensional (3D) superconductor BaPb1-x Bi x O3 (BPBO), which surprisingly demonstrate three key signatures of emergent electronic granularity, having only been previously conjectured and observed in 2D thin-film systems. These signatures include the observation of emergent SC domains on the scale of the coherence length, finite energy gap over all space, and strong enhancement of spatial anticorrelation between pairing amplitude and gap magnitude as the SIT is approached. These observations are suggestive of 2D SC behavior embedded within a conventional 3D s-wave host, an intriguing but still unexplained interdimensional phenomenon, which has been hinted at by previous experiments in which critical scaling exponents in the vicinity of a putative 3D quantum phase transition are consistent only with dimensionality d = 2.

Quantum correlations in electron microscopy

Electron microscopes provide a powerful platform for exploring physical phenomena with nanoscale resolution, based on the interaction of free electrons with the excitations of a sample such as phonons, excitons, bulk plasmons, and surface plasmons. The interaction usually results in the absorption or emission of such excitations, which can be detected directly through cathodoluminescence or indirectly through electron energy loss spectroscopy (EELS). However, as we show here, the underlying interaction of a free electron and an arbitrary optical excitation goes beyond what was predicted or measured so far, due to the interplay of entanglement and decoherence of the electron-excitation system. The entanglement of electrons and optical excitations can provide new analytical tools in electron microscopy. For example, it can enable measurements of optical coherence, plasmonic lifetimes, and electronic length scales in matter (such as the Bohr radius of an exciton). We show how these can be achieved using common configurations in electron diffraction and EELS, revealing significant changes in the electron’s coherence, as well as in other quantum information theoretic measures such as purity. Specifically, we find that the purity after interaction with nanoparticles can only take discrete values, versus a continuum of values for interactions with surface plasmons. We quantify the post-interaction density matrix of the combined electron-excitation system by developing a framework based on macroscopic quantum electrodynamics. The framework enables a quantitative account of decoherence due to excitations in any general polarizable material (optical environment). This framework is thus applicable beyond electron microscopy. Particularly in electron microscopy, our work enriches analytical capabilities and informs the design of quantum information experiments with free electrons, allowing control over their quantum states and their decoherence by the optical environment.

Enhancing Superconductivity of the Nonmagnetic Quasiskutterudites by Atomic Disorder.

We investigated the effect of enhancement of superconducting transition temperature by nonmagnetic atom disorder in the series of filled skutterudite-related compounds (LaSn, CaRhSn, YRhSn, LuRhSn; Co, Ru, Rh), where the atomic disorder is generated by various defects or doping. We have shown that the disorder on the coherence length scale in these nonmagnetic quasiskutterudite superconductors additionally generates a non-homogeneous, high-temperature superconducting phase with (dilute disorder scenario), while the strong fluctuations of stoichiometry due to increasing doping can rapidly increase the superconducting transition temperature of the sample even to the value of (dense disorder leading to strong inhomogeneity). This phenomenon seems to be characteristic of high-temperature superconductors and superconducting heavy fermions, and recently have received renewed attention. We experimentally documented the stronger lattice stiffening of the inhomogeneous superconducting phase in respect to the bulk one and proposed a model that explains the behavior in the series of nonmagnetic skutterudite-related compounds.

Coherence of Coronal Mass Ejections in Near-Earth Space

Interplanetary coronal mass ejections (ICMEs) primarily move radially as they propagate away from the Sun, maintaining approximately constant angular width with respect to the Sun. As ICMEs have typical angular widths of around 60^{circ }, plasma elements on opposite flanks of an ICME separate in the non-radial direction at a speed, v_{mathrm{G}}, roughly equal to the ICME radial speed. This rapid expansion is a limiting factor on the propagation of information across an ICME at the local Alfvén speed, v_{mathrm{A}}. In this study, the 1-AU properties of ICMEs are used to compute two measures of ICME coherence. The first is the angular separation for which v_{mathrm{G}} exceeds the local v_{mathrm{A}}. The second measure is the angular extent over which a wavefront can propagate as an ICME travels from a given heliocentric distance to 1 AU. For both measures, ICMEs containing magnetic clouds show greater coherence than non-cloud ICMEs. However, even for magnetic clouds, information is unable to propagate across the full span of the structure. Thus interactions of ICMEs with other solar wind structures in the heliosphere are likely to lead to localised distortion, rather than solid-body like deflection. For magnetic clouds, the coherence length scale is significantly greater near the centre of the spacecraft encounter than at the leading or trailing edges. This suggests that magnetic clouds may be more coherent, and thus less prone to distortion, along the direction of the magnetic flux-rope axis than in directions perpendicular to the axis.

Spatial structure and wavenumber filtering of wall pressure fluctuations on a full-scale cockpit model

Measurements of wall pressure fluctuations have been performed with arrays of MEMS microphones on a full-scale model of a business jet fore part. The boundary layers are characterised in terms of mean and root-mean-square velocity profiles and key parameters are computed to serve for the normalisation of pressure-related quantities. Frequency spectra are compared with classical models and strong discrepancies are highlighted, which are attributed to the non-canonical state of the boundary layers. The spatial structure of the pressure field is first characterised by the study of coherence and associated length scales. The hypothesis of auto-similarity on only one parameter, that is the frequency normalised by the separation to convective velocity ratio, is proven to reach its limits in the present case. In turn, the coherence length scales do not exhibit the classical $$-1$$ power law decay with frequency. The wavenumber–frequency formalism is used to differentiate the acoustic and hydrodynamic components of the pressure fluctuations in the spectral space. In the presence of an external acoustic source, the acoustic components are correctly extracted from the total field by spectral filtering, and the resulting frequency spectra are satisfactorily checked against the purely acoustic reference spectrum.

Seasonal Characteristics of Atmospheric Boundary Layer and its Associated Dynamics over Central India

Atmospheric boundary-layer studies have applications in lower tropospheric processes, air pollution meteorology, weather prediction and climate monitoring. Despite this, very few studies are available over tropics, especially over central India. Here, we analyse, high resolution vertical profiles of meteorological parameters corresponding to fully developed mixed layer obtained using balloon borne radiosonde to study characteristics of atmospheric boundary layer over Nagpur (21.15 °N, 79.15 °E). Study reveals that, within the mixed layer, variation in RH is very less (within 20%) while above mixed layer, large variations are seen. Back trajectory analysis confirms that the moist days (dry days) are associated with the transport of air masses from the Arabian Sea (from arid locations). Mixed layer heights derived through gradients in Virtual Potential Temperature (VPT) showed highest values during Pre monsoon (2564 ± 867 m) and lowest (788 ± 477 m) in Monsoon season. Specific Humidity varies in the range from 0 to 10 g/kg during pre-monsoon and 0–20 g/kg during monsoon season. D-theta exhibits lowest values in monsoon and highest values during pre-monsoon season while Parcel saturation pressure difference exhibits highest values in monsoon and lowest values during pre-monsoon season. During the monsoon and early post monsoon there is a presence of high moist static energy (Theta-e ~ 365 K) at the surface and moderate during mid-winter. We have examined, for the first time, the spatial (vertical) coherence of VPT using a novel technique of auto correlation analysis. Coherence length scales, derived with this method are found to be Maximum (1129 ± 48 m) in winter and minimum (694 ± 281 m) in Pre monsoon season.

Coherence analysis of the simulated sound field of a highly-heated laboratory-scale jet

Large eddy simulations (LES) have successfully reproduced the flow and acoustic fields generated by laboratory-scale jets. However, laboratory-scale test conditions ordinarily do not match those of full-scale military aircraft. Recently, Liu et al. (AIAA 2016–2125) showed that accounting for a variable ratio of specific heats leads to increased accuracy in LES simulations of heated jets. As a step towards modeling operating parameters similar to military aircraft, they produced a simulation of a highly-heated jet with a temperature ratio of seven. This paper shows the levels in the field as well as the axial and azimuthal coherence trends of this simulated jet. Large axial coherence lengths are found in the direction of maximum radiation with decreased coherence towards the sideline. Azimuthally, coherence length scales are greatest in the region of maximum radiation and generally decrease with increasing frequency. Additionally, evidence is seen for multiple self-coherent and mutually-incoherent radiation lobes within the region of maximum radiation. [Work supported by USAFRL through ORISE.]

Field Observations of Alongshore Runup Variability Under Dissipative Conditions in the Presence of a Shoreline Sandwave

AbstractVideo measurements of runup were collected at low tide along several profiles covering an alongshore distance of 500 m. The morphology displayed a complex shape with a shoreline sandwave in the lower beach face of about 250 m long mirrored in the inner sandbar. Wave conditions were stationary and moderate (offshore height of 2 m and peak period of nearly 13 s) but yet dissipative. Runup energy was dominated by infragravity frequencies. Alongshore variations in runup (by a factor up to 3) observed both in the incident and infragravity bands were much higher than reported previously (e.g., Guedes et al., 2012, https://doi.org/10.1016/j.csr.2012.08.022; Ruggiero et al., 2004, https://doi.org/10.1029/2003JC002160) while the alongshore variations in other environmental parameters (e.g., foreshore beach slope) appear to be much lower. Our data suggest that the beach morphology in the inner surf zone plays a crucial role by inducing rapid and significant modification in the incident wave pattern and the alongshore coherence length scales were consistent with the typical alongshore length scale of the morphology.

Coherence of Bessel-Gaussian Beams Propagating in a Turbulent Atmosphere

Coherent properties of vortex Bessel-Gaussian beams propagating in a turbulent atmosphere are theoretically studied based on the analytical solution of the equation for the transverse second-order mutual coherence function of optical radiation field. The behavior of coherence degree, coherence length, and integral scale of coherence degree of vortex Bessel-Gaussian beams depending on beam parameters and characteristics of the turbulent atmosphere is analyzed. It is shown that the coherence length and integral scale of coherence degree of a vortex Bessel-Gaussian beam essentially depend on the topological charge of the beam. When the topological charge of a vortex beam increases, additional decreases in the above parameters become less. The given effect is small under weak and strong fluctuations of optical radiation, and it is maximal in the transition region between them.