Strong Inhomogeneity Research Articles

Electric properties of black arsenic

R(T, B) of the natural black arsenic (b-As) polycrystal was studied. It was shown that the polycrystalline b-As sample contains the b-As phase and also the traces of its oxide, as well as grey arsenic and arsenolite (As2O3). The behavior of the relative magnetoresistance of the b-As crystal was described by the relation MR(B) = bBn + cBm, where the coefficients b and c and the exponents n and m were also affected by the mechanisms of magnetoresistance formation and temperature. At the temperatures below 10 K, MR(B) shows the presence of a competition between negative (with b < 0 and n ≈ 0.5) and positive (with c > 0 and m ≈ 1) contributions. Above 10 K, only the PMR effect was presented. For the PMR effect, at 10 < T < 100 K it is observed that the values of b > 0, n ≈ 1 and c → 0. Above 100 K, it is observed that the values of b, c > 0 and n ≈ 1 and 1.30 < m < 1.47. The observed behavior of the R(T, B) dependences is associated with strong inhomogeneity and/or disorder of the investigated black arsenic crystal.

Ultrafast Nanoscopy of High-Density Exciton Phases in WSe2.

The density-driven transition of an exciton gas into an electron-hole plasma remains a compelling question in condensed matter physics. In two-dimensional transition metal dichalcogenides, strongly bound excitons can undergo this phase change after transient injection of electron-hole pairs. Unfortunately, unavoidable nanoscale inhomogeneity in these materials has impeded quantitative investigation into this elusive transition. Here, we demonstrate how ultrafast polarization nanoscopy can capture the Mott transition through the density-dependent recombination dynamics of electron-hole pairs within a WSe2 homobilayer. For increasing carrier density, an initial monomolecular recombination of optically dark excitons transitions continuously into a bimolecular recombination of an unbound electron-hole plasma above 7 × 1012 cm-2. We resolve how the Mott transition modulates over nanometer length scales, directly evidencing the strong inhomogeneity in stacked monolayers. Our results demonstrate how ultrafast polarization nanoscopy could unveil the interplay of strong electronic correlations and interlayer coupling within a diverse range of stacked and twisted two-dimensional materials.

Statistical properties of pressure-Hessian tensor in a turbulent channel flow

A direct numerical simulation database of a weakly compressible turbulent channel flow with bulk Mach number 1.56 is studied in detail, including the geometrical relationships between the pressure-Hessian tensor and the vorticity/strain-rate tensor, as well as the mechanism of the pressure-Hessian tensor contributing to the evolution of invariants of the velocity gradient tensor. The results show that the geometrical relationships between the pressure-Hessian tensor and the vorticity/strain-rate tensor in the central region of the channel are consistent with that of isotropic turbulence. However, in the buffer layer with relatively stronger inhomogeneity and anisotropy, the vorticity tends to be aligned with the first or second eigenvector of the pressure-Hessian tensor in the unstable focus/compressing topological region, and tends to be aligned with the first eigenvector of the pressure-Hessian tensor in the stable focus/stretching topological region. In the unstable node/saddle/saddle and stable node/saddle/saddle topological regions, the vorticity prefers to lie in the plane of the first and second eigenvectors of the pressure-Hessian tensor. The strain-rate and the pressure-Hessian tensors tend to share their second principal direction. Moreover, for the coupling between the pressure-Hessian tensor and the principal strain rates, we clarify the influence on dissipation, the nonlinear generation of dissipation and the enstrophy generation. The decomposition of the pressure-Hessian tensor further shows that the slow pressure-related term dominates the pressure-Hessian tensor's contribution, and the influence of inhomogeneity and anisotropy mainly originates from the inhomogeneity and anisotropy of the fluctuating velocity. These statistical properties would be instructive in formulating dynamical models of the velocity gradient tensor for wall turbulence.

Flow in additively manufactured super-rough channels

Metal additive manufacturing has enabled geometrically complex internal cooling channels for turbine and heat exchanger applications, but the process gives rise to large-scale roughness whose size is comparable to the channel height (which is 500 $\mathrm {\mu }$ m). These super-rough channels pose previously unseen challenges for experimental measurements, data interpretation and roughness modelling. First, it is not clear if measurements at a particular streamwise and spanwise location still provide accurate representation of the mean (time- and plane-averaged) flow. Second, we do not know if the logarithmic layer survives. Third, it is unknown how well previously developed rough-wall models work for these large-scale roughnesses. To answer the above practical questions, we conduct direct numerical simulations of flow in additively manufactured super-rough channels. Three rough surfaces are considered, all of which are obtained from computed tomography scans of additively manufactured surfaces. The roughness’ trough to peak sizes are 0.1 $h$ , 0.3 $h$ and 0.8 $h$ , respectively, where $h$ is the intended half-channel height. Each rough surface is placed opposite a smooth wall and the other two rough surfaces, leading to six rough-wall channel configurations. Two Reynolds numbers are considered, namely $Re_\tau =180$ and $Re_\tau =395$ . We show first that measurements at one streamwise and spanwise location are insufficient due to strong mean flow inhomogeneity across the entire channel, second that the logarithmic law of the wall survives despite the mean flow inhomogeneity and third that the established roughness sheltering model remains accurate.

Trans-dimensional imaging of the random inhomogeneity structure in the southern Ryukyu arc, Japan

SUMMARY The seismic velocity structure of the lithosphere shows various inhomogeneities over a wide range of scales, and such inhomogeneity causes complex seismic waves above a few hertz due to multiple scattering. Medium around active volcanoes and large faults tends to show strong random inhomogeneity in relatively small areas. For a more precise understanding of such random velocity inhomogeneities, it is necessary to estimate their detailed spatial variation without smoothing constraints. This study introduces a trans-dimensional approach for the 3-D imaging of random inhomogeneity using the reversible jump Markov chain Monte Carlo (rjMCMC) method, and set the number of structural parameters and their spatial layout as unknown parameters. Since the scale dependence of the random inhomogeneity is related to the frequency dependence of seismic wave scattering, the covariance matrix of the likelihood function was defined to be non-diagonal so that residuals at different frequencies in each ray path are correlated. A synthetic test showed this covariance matrix worked adequately for estimating parameters of a power-law-type spectrum of random inhomogeneity. Analysis of seismic data at the southern Ryukyu arc in the southwest Japan found anomalies with strong and weak inhomogeneities. A strongly inhomogeneous band with a width of 20–30 km was distributed in the Okinawa Trough at depths of 0–20 km. In part of this area, magma intrusions and associated complex structures have been detected by a seismic reflection survey. The scale of the structures discussed in this study is almost the same with that discussed in the reflection survey. The rjMCMC-based analysis made it possible to compare random inhomogeneities with the structural variations estimated by the deterministic seismic reflection survey. Since analyses of scattered seismic waves can examine much greater depths than seismic reflection surveys, further comparisons between the two methods in the shallow crust could provide useful insights for detailed interpretation of complex structures at deeper depth.

Measurement of two-dimensional electrical potential fields in CFRP using four-probe resistance scans

The carbon fibers in fiber reinforced plastics could be used sensorially to observe the present stress state of the structural component by measuring its electrical resistance. However, the use of resistivity requires a detailed understanding of the electrical potential distribution in a given specimen. Current experimental techniques measure the electrical potential on a limited amount of relatively large electrodes mounted onto the parts surface. This approach suffers from the limited spacial resolution as well as the retroactivity between attached electrodes and observed specimen. This article discusses an experimental method to scan the electrical potential distribution on the surface of a conductor using a typical 4-probe setup and a gantry style robot to move one of the probes. The method is applied to eletrical conduction processes in carbon fiber reinforced plastics (CFRPs) in an effort to visualize the electrical potential distribution near current contacts. This approach allows us to have a much more detailed understanding of electrical conduction processes in CFRP, which can help to better interpret experimental data in the future. A two-dimensional mathematical model based on Laplace equation is derived with appropriate boundary conditions and compared to the measured results. Experimental and numerical results agree well and show the existance of strong current inhomogeneities within the CFRP part. The developed setup is simple to implement, but nevertheless prooved to be effective in measuring the explicit electrical conduction paths in both isotropic and anisotropic conductors.

Anomalous dynamic response of nematic platelets studied by spatially resolved rheo-small angle x-ray scattering in the 1–2 plane

Dispersions of colloidal platelets in the nematic phase display strong wall anchoring, which competes with the reorientational motion of the director when the system is subjected to flow. We show that the mechanical response to large amplitude oscillatory strain and stress depends on the confinement of the system due to this competition. We elucidate the underlying structural response by deflecting a x-ray beam vertically along the vorticity direction of a Couette geometry, such that the structure can be probed throughout the gap with an unprecedented spatial resolution while recording in situ the mechanical response. We observe strong inhomogeneities in terms of the orientation of the nematic director, depending on the extent of the system's yield during an oscillation. At small strain amplitudes, we observe a small region where the director oscillates between wall anchoring and the Leslie angle, while in the bulk, the director tilts out of the flow–flow gradient plane. At large strain amplitudes, the oscillations of the director are symmetric, close to the wall, and propagate into the bulk. Here, a twinning is observed where the director rotates out-of-plane in two opposite directions. Using the sequence of physical process method to analyze the LAOStrain response for both the mechanical and structural response, we locate the yielding in a small time-window around flow reversal and identify that the bulk is the main contributor to the mechanical response. The structural response to LAOStress is much less pronounced even when the stress amplitude causes significant shear thinning.

Axion-photon conversion in neutron star magnetospheres: The role of the plasma in the Goldreich-Julian model

The most promising indirect search for the existence of axion dark matter uses radio telescopes to look for narrow spectral lines generated from the resonant conversion of axions in the magnetospheres of neutron stars. Unfortunately, a large list of theoretical uncertainties has prevented this search strategy from being fully accepted as robust. In this work we attempt to address major outstanding questions related to the role and impact of the plasma, including: $(i)$ does refraction and reflection of radio photons in the magnetosphere induce strong inhomogeneities in the flux, $(ii)$ can refraction induce premature axion-photon de-phasing, $(iii)$ to what extent do photon-plasma interactions induce a broadening of the spectral line, $(iv)$ does the flux have a strong time dependence, and $(v)$ can radio photons sourced by axions be absorbed by the plasma. We present an end-to-end analysis pipeline based on ray-tracing that exploits a state-of-the-art auto-differentiation algorithm to propagate photons from the conversion surface to asymptotically large distances. Adopting a charge symmetric Goldreich-Julian model for the magnetosphere, we show that for reasonable parameters one should expect a strong anisotropy of the signal, refraction induced axion-photon de-phasing, significant line-broadening, a variable time-dependence of the flux, and, for large enough magnetic fields, anisotropic absorption. Our simulation code is flexible enough to serve as the basis for follow-up studies with a large range of magnetosphere models.

B1 inhomogeneity correction of RARE MRI at low SNR: Quantitative in vivo 19 F MRI of mouse neuroinflammation with a cryogenically-cooled transceive surface radiofrequency probe.

Low SNR in fluorine-19 (19 F) MRI benefits from cryogenically-cooled transceive surface RF probes (CRPs), but strong B1 inhomogeneities hinder quantification. Rapid acquisition with refocused echoes (RARE) is an SNR-efficient method for MRI of neuroinflammation with perfluorinated compounds but lacks an analytical signal intensity equation to retrospectively correct B1 inhomogeneity. Here, a workflow was proposed and validated to correct and quantify 19 F-MR signals from the inflamed mouse brain using a 19 F-CRP. In vivo 19 F-MR images were acquired in a neuroinflammation mouse model with a quadrature 19 F-CRP using an imaging setup including 3D-printed components to acquire co-localized anatomical and 19 F images. Model-based corrections were validated on a uniform 19 F phantom and in the neuroinflammatory model. Corrected 19 F-MR images were benchmarked against reference images and overlaid on in vivo 1 H-MR images. Computed concentration uncertainty maps using Monte Carlo simulations served as a measure of performance of the B1 corrections. Our study reports on the first quantitative in vivo 19 F-MR images of an inflamed mouse brain using a 19 F-CRP, including in vivo T1 calculations for 19 F-nanoparticles during pathology and B1 corrections for 19 F-signal quantification. Model-based corrections markedly improved 19 F-signal quantification from errors > 50% to < 10% in a uniform phantom (p < 0.001). Concentration uncertainty maps ex vivo and in vivo yielded uncertainties that were generally < 25%. Monte Carlo simulations prescribed SNR ≥ 10.1 to reduce uncertainties < 10%, and SNR ≥ 4.25 to achieve uncertainties < 25%. Our model-based correction method facilitated 19 F signal quantification in the inflamed mouse brain when using the SNR-boosting 19 F-CRP technology, paving the way for future low-SNR 19 F-MRI applications in vivo.

Emission kinetics of HITC laser dye on top of arrays of Ag nanowires

Abstract We have grown arrays of silver nanowires in pores of anodic alumina membranes (metamaterials with hyperbolic dispersion at λ ≥ 615 nm), spin coated them with the dye-doped polymer (HITC:PMMA), and studied the rates of radiative and nonradiative relaxation as well as the concentration quenching (Förster energy transfer to acceptors). The results were compared to those obtained on top of planar Ag films and glass (control samples). The strong spatial inhomogeneity of emission kinetics recorded in different spots across the sample and strong inhibition of the concentration quenching in arrays of Ag nanowires are among the most significant findings of this study.

Numerical modeling of magnetic induction in standard and triple Epstein frames considering cutting and staggering of sensor strips

For magnetic energy loss measurements on electric steel sheets, the Epstein tester (ET) is applied for 100 years. However, it suffers from unsolved problems, such as inhomogeneity of flux in the corners and even more of impact of sample cutting. As a matter of fact, a detailed study of flux distributions—as a pre-condition for improvements—has not been performed so far. In this work, we present the results of an advanced numerical 3D-magnetic anisotropic circuit calculation modeling. It considers most relevant impacts such as anisotropy, non-linearity, method of staggering, and in particular, consequences of sample cutting. According to the literature, the latter may yield material deterioration for edge zones of high width WDET. Here, we set WDET to 2.5 mm as a “coarse-cut” convention for practice, and we compare it with perfect “fine-cut” (W = 0). As a main result, deteriorated edge zones of standard ET strips of low width W = 30 mm yield a distinct concentration of flux along inner axial strip regions as a reason for strong local flux inhomogeneity and distortion. Consistent numerical comparisons predict that higher grades of magnetic homogeneity can be expected from wider sample strips, e.g., from a “Triple ET” with W = 90 mm. As a global conclusion, the Epstein frame represents a system of extreme 3D inhomogeneity, especially in the corners. The grade of inhomogeneity reaches its maximum in instants of high permeability. It may be strongly increased through coarse cutting. On the other hand, consequences of cutting can be decreased by increasing the strip width. This would allow for loss measurements using tangential field coils arranged in the central limb regions of TET.

Distributed Temperature Monitoring Inside Ytterbium DFB and Holmium Fiber Lasers

A distributed temperature monitoring inside a cavity of ytterbium DFB and holmium fiber lasers has been demonstrated with a spatial resolution of 1 and 5 mm, respectively, for the first time to the best of our knowledge. For this, we use an optical backscatter reflectometer, which measures intracore temperature, and compare it with the data of an IR thermographic camera, which measures temperature from the surface of a fiber. In the case of holmium fiber laser pumped at a wavelength of 1125 nm with a power of 6 W, the maximum temperature variation along the ∼3-m active fiber reaches ∼60 °C. In the case of ytterbium DFB laser, we observe a strong inhomogeneity of the temperature along the DFB cavity, which leads to a significant decrease in the lasing efficiency. When pumped by a single-mode laser diode at a wavelength of 976 nm with a power of up to 526 mW, the maximum temperature difference reaches 37 °C for the 37-mm DBF cavity.

Characterization of Er3+:YVO4 for microwave to optical transduction

Rare-earth-ion doped crystals are a promising platform for applications in microwave-to-optical transduction. Erbium is of special interest because of its telecom wavelength optical transitions. This work investigates the optical and electron-spin properties of Er${}^{3+}$:YVO${}_{4}$. An initial transduction of classical microwave signal using this material is demonstrated. The measured strong optical dipole moment and narrow ensemble inhomogeneities make Er${}^{3+}$:YVO${}_{4}$ a promising material for microwave-to-optical transduction.

Self-diffusivity of dense confined fluids

Molecular transport through tight porous media is crucial to shale gas exploration, but deeper insights of the elemental physics are still required, particularly under high pressures and nanoscale confinements, where Navier–Stokes and Boltzmann solutions are no longer valid. In this work, we carry out a fundamental and systematic study of self-diffusion using event-driven molecular dynamics simulations, varying fluid rarefaction, confinement, and surface friction. We differentiate between fluid–fluid and fluid-wall collisions to identify the interplay of the underpinning diffusive mechanisms, namely, molecular and Knudsen diffusion. We find that the Bosanquet formula, which has been used for describing rarefied gases, is also able to provide a good semi-analytical description of self-diffusivities in confined dense fluids, as long as the pore height is not smaller than five molecular diameters. Importantly, this allows us to predict the self-diffusion coefficient, regardless of the fluid rarefaction, confinement state, and surface roughness, in a wide range of Knudsen numbers that were not possible before. Often as a source of debate, we prove here that despite strong fluid inhomogeneities arising in these conditions, the Einstein self-diffusivity can still be used within Fick's law, provided boundary effects are considered when using Fick's setup. Finally, we notice that a previously identified linear scaling of self-diffusivities with confinement is only valid in the limit of low densities and frictionless walls, which is not representative of shale reservoirs. This work will serve as a foundation for investigating the anomalous gas transport behavior observed in the recent work of dense, confined fluids.

Inhomogeneous time-reversal symmetry breaking in Sr2RuO4

We show that the observed time-reversal symmetry breaking (TRSB) of the superconducting state in $\mathrm{Sr}_{2}\mathrm{Ru}\mathrm{O}_{4}$ can be understood as originating from inhomogeneous strain fields near edge dislocations of the crystal. Specifically, we argue that, without strain inhomogeneities, $\mathrm{Sr}_{2}\mathrm{Ru}\mathrm{O}_{4}$ is a single-component, time-reversal symmetric superconductor, likely with $d_{x^{2}-y^{2}}$ symmetry. However, due to the strong strain inhomogeneities generated by dislocations, a slowly-decaying sub-leading pairing state contributes to the condensate in significant portions of the sample. As it phase winds around the dislocation, time-reversal symmetry is locally broken. Global phase locking and TRSB occur at a sharp Ising transition that is not accompanied by a change of the single-particle gap and yields a very small heat capacity anomaly. Our model thus explains the puzzling absence of a measurable heat capacity anomaly at the TRSB transition in strained samples, and the dilute nature of the time-reversal symmetry broken state probed by muon spin rotation experiments. We propose that plastic deformations of the material may be used to manipulate the onset of broken time-reversal symmetry.

Direct measurement and imaging of magnetocaloric effect inhomogeneities at the microscale in Ni44Co6Mn30Ga20 with infrared thermography

To study the MCE at the microscale with infrared thermography, a bulk Ni44Co6Mn30Ga20 sample was investigated. Direct measurements of the MCE were performed for temperatures around the reverse and direct martensitic transformation, by applying and removing a ~1.2 T magnetic field to the sample and recording the changes in its surface temperature with an IR camera. This was done for different sample temperatures on heating (from 273 K to 327 K) and cooling (from 327 K to 267 K).For sample temperatures around the reverse martensitic transformation, the inverse MCE is observed, with ΔT between −0.04 K and −0.54 K. The effect displays strong spatial inhomogeneity at the microscopic scale, which varies considerably with the progress of phase transformation. This behavior is not observed for the direct martensitic transformation (on cooling).The study reveals substantial novelty in the manifestation of the MCE at the microscopic scale, associated with phase dynamics, showing a different and inhomogeneous behavior on heating and cooling of a FSMA around the martensitic transition.

Archaeometrical Characterization of Romanian Late Bronze Age Ceramic Fragments

Knowledge of the past can provide information to protect the future and the potential of the technological development in the analytical sciences can be successfully applied for the study and conservation of cultural objects. In this context, in the present paper we propose an analytical methodology to characterize seven samples of ancient ceramic objects (dating to the Late Bronze Age). The samples were analyzed using optical microscopy, and all the samples presented a strong inhomogeneity on the surface, as suggested by the different colors of the ceramics. X-ray fluorescence (XRF) results reveal a relatively heterogenous composition of the samples, as well as strong differences between the different surfaces of each sample. By comparative analysis of the diffractograms recorded for both sides of the same samples were observed some differences, especially in terms of relative concentration of the component minerals, and, in lesser content, in terms of new phases present in the samples. Corroborated results obtained by XRF and X-ray diffraction (XRD) offered information regarding mineralogical composition of the samples: for some of them illite/muscovite and plagioclase phases are present in higher quantities or a lower quartz content. The presence of these components was confirmed by Fourier-transform infrared spectroscopy (FTIR) analysis. The thermal analysis completes the analytical investigation of the ceramic samples. The thermal behavior of the sample conducted to some explanation regarding the observed differences, due to the raw materials (that the major clay mineral in the samples is represented by illite) or to environmental factors during their burial in the soil.

Mesh-free micromechanical modeling of inverse opal structures

The application of mesh-free discrete element method (DEM) for modeling of macroporous inverse opals is proposed. The developed and implemented DEM-based strategy is applied to analyze stiffness and strength of inverse opals made of pure silica and inverse opals coated with titania layers of various thicknesses. Also, the influence of the porosity of the initial packing on the properties of the material is investigated. Simulation results have shown that due to the heterogeneous macroscopic structure and unequal material distribution in nodes, struts or near interstitial pores, strong inhomogeneities of the stress distribution arise. Spots of localized stresses located near the interpore openings are the sources of initial defects and small interpore cracks. Reducing the overall porosity or an additional coating of the structure with a titania layer allows to partially homogenize and improve its mechanical properties.It was shown that the obtained simulation results are in very good agreement with previous studies. Despite large computational effort, the performed analysis confirms the high efficiency of the proposed DEM-based approach and demonstrates its great potential for use in problems of a similar class: for modelling of complex-structured multicomponent materials.

Improved susceptibility weighted imaging at ultra-high field using bipolar multi-echo acquisition and optimized image processing: CLEAR-SWI

PurposeSusceptibility Weighted Imaging (SWI) has become established in the clinical investigation of stroke, microbleeds, tumor vascularization, calcification and iron deposition, but suffers from a number of shortcomings and artefacts. The goal of this study was to reduce the sensitivity of SWI to strong B1 and B0 inhomogeneities at ultra-high field to generate homogeneous images with increased contrast and free of common artefacts. All steps in SWI processing have been addressed – coil combination, phase unwrapping, image combination over echoes, phase filtering and homogeneity correction – and applied to an efficient bipolar multi-echo acquisition to substantially improve the quality of SWI. Principal resultsOur findings regarding the optimal individual processing steps lead us to propose a Contrast-weighted, Laplace-unwrapped, bipolar multi-Echo, ASPIRE-combined, homogeneous, improved Resolution SWI, or CLEAR-SWI. CLEAR-SWI was compared to two other multi-echo SWI methods and standard, single-echo SWI with the same acquisition time at 7 T in 10 healthy volunteers and with single-echo SWI in 13 patients with brain tumors. CLEAR-SWI had improved contrast-to-noise and homogeneity, reduced signal dropout and was not compromised by the artefacts which affected standard SWI in 10 out of 13 cases close to tumors (as assessed by expert raters), as well as generating T2* maps and phase images which can be used for Quantitative Susceptibility Mapping. In a comparison with other multi-echo SWI methods, CLEAR-SWI had the fewest artefacts, highest SNR and generally higher contrast-to-noise. Major conclusionsCLEAR-SWI eliminates the artefacts common in standard, single-echo SWI, reduces signal dropouts and improves image homogeneity and contrast-to-noise. Applied clinically, in a study of brain tumor patients, CLEAR-SWI was free of the artefacts which affected standard, single-echo SWI.

Crossover of the relative heat transport contributions of plume ejecting and impacting zones in turbulent Rayleigh-Bénard convection (a) (a)Contribution to the Focus Issue Turbulent Thermal Convection edited by Mahendra Verma and Jörg Schumacher.

Turbulent thermal convection is characterized by the formation of large-scale structures and strong spatial inhomogeneity. This work addresses the relative heat transport contributions of the large-scale plume ejecting vs. plume impacting zones in turbulent Rayleigh-Bénard convection. Based on direct numerical simulations of the two dimensional (2-D) problem, we show the existence of a crossover in the wall heat transport from initially impacting dominated to ultimately ejecting dominated at . This is consistent with the trends observed in 3-D convection at lower Ra, and we therefore expect a similar crossover to also occur there. We identify the development of a turbulent mixing zone, connected to thermal plume emission, as the primary mechanism for the takeover. The mixing zone gradually extends vertically and horizontally, therefore becoming more and more dominant for the overall heat transfer.