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2021 ◽  
Vol 8 (1) ◽  
pp. 6
Author(s):  
Jacques Curély

In earlier work, we previously established a formalism that allows to express the exchange energy J vs. fundamental molecular integrals without crystal field, for a fragment A–X–B, where A and B are 3d1 ions and X is a closed-shell diamagnetic ligand. In this article, we recall this formalism and give a physical interpretation: we may rigorously predict the ferromagnetic (J < 0) or antiferromagnetic (J > 0) character of the isotropic (Heisenberg) spin-spin exchange coupling. We generalize our results to ndm ions (3 £ n £ 5, 1 £ m £ 10). By introducing a crystal field we show that, starting from an isotropic (Heisenberg) exchange coupling when there is no crystal field, the appearance of a crystal field induces an anisotropy of exchange coupling, thus leading to a z-z (Ising-like) coupling or a x-y one. Finally, we discuss the effects of a weak crystal field magnitude (3d ions) compared to a stronger (4d ions) and even stronger one (5d ions). In the last step, we are then able to write the corresponding Hamiltonian exchange as a spin-spin one.


2021 ◽  
Vol 16 (12) ◽  
pp. P12041
Author(s):  
D. Flay ◽  
D. Kawall ◽  
T. Chupp ◽  
S. Corrodi ◽  
M. Farooq ◽  
...  

Abstract We present details of a high-accuracy absolute scalar magnetometer based on pulsed proton NMR. The B-field magnitude is determined from the precession frequency of proton spins in a cylindrical sample of water after accounting for field perturbations from probe materials, sample shape, and other corrections. Features of the design, testing procedures, and corrections necessary for qualification as an absolute scalar magnetometer are described. The device was tested at B = 1.45 T but can be modified for a range exceeding 1–3 T. The magnetometer was used to calibrate other NMR magnetometers and measure absolute magnetic field magnitudes to an accuracy of 19 parts per billion as part of a measurement of the muon magnetic moment anomaly at Fermilab.


Author(s):  
S Lang ◽  
L Gan ◽  
C McLennan ◽  
O Monchi ◽  
J Kelly

Background: Tumor treatment fields (TTFields) are an approved adjuvant therapy for glioblastoma. The magnitude of applied electrical field is related to the anti-tumoral response. However, peritumoral edema (ptE) may result in shunting of electrical current around the tumor, thereby reducing the intra-tumoral electric field. In this study, we address this issue with computational simulations. Methods: Finite element models were created with varying amounts of ptE surrounding a virtual tumor. The electric field distribution was simulated using the standard TTFields electrode montage. Electric field magnitude was extracted from the tumor and related to edema thickness. Two patient specific models were created to confirm these results. Results: The inclusion of ptE decreased the magnitude of the electric field within the tumor. In the model considering a frontal tumor and an anterior-posterior electrode configuration, ≥ 6 mm of ptE decreased the electric field by 52%. In the patient specific models, ptE decreased the electric field within the tumor by an average of 26%. The effect of ptE on the electric field distribution was spatially heterogenous. Conclusions: Given the importance of electric field magnitude for the anti-tumoral effects of TTFields, the presence of edema should be considered both in future modelling studies and as a predictor of non-response.


2021 ◽  
Vol 9 ◽  
Author(s):  
Chin-Chun Wu ◽  
R. P. Lepping ◽  
D. B. Berdichevsky

We describe a new NASA website that shows normalized magnetic field (B) magnitude profiles within Wind magnetic clouds (MCs) (i.e., observations versus basic model versus modified model) for 209 MCs observed from launch in late 1994 to July of 2015, where model modification is based on the studies of Lepping et al. (Solar Phys, 2017, 292:27) and Lepping et al. (Solar Phys, 2018, 293:162); the basic force free magnetic cloud parameter fitting model employing Bessel functions (Lepping et al., J. Geophys. Res., 1990, 95:11957) is called the LJB model here. The fundamental principles should be applicable to the B-data from any spacecraft at 1 AU. Earlier (in the LJB study), we justified why the field magnitude can be thought of as decoupled from the field direction within an MC, and further, we justified this idea in terms of actual observations seen over a few decades with examples of MCs from Wind data. The model modification is achieved by adding a correction (“Quad”) value to the LJB model (Bessel function) value in the following manner: B (est)/B0 ≈ [LJB Model + Quad (CA,u)], where B0 is the LJB-estimated field magnitude value on the MC’s axis, CA is the relative closest approach (See Supplementary Appendix A), and u is the distance that the spacecraft travels through the MC from its entrance point. In an average sense, the Quad technique is shown to be successful for 82% of the past modeled MCs, when Quality (Q0) is good or excellent (see Supplementary Appendix A). The Quad technique is successful for 78% of MCs when all cases are considered. So Q0 of the MC LJB-fit is not a big factor when the success of the Quad scheme is considered. In addition, it is found that the Quad technique does not work better for MC events with higher solar wind speed. Yearly occurrence frequency of all MC events (NYearly) and those MC events with ΔσN/σN2 ≥ 0.5 (NΔσN/σN2≥0.5) are well correlated, but there is no solar cycle dependence for normalizing NΔσN/σN2≥0.5 with NYearly.


2021 ◽  
Author(s):  
Liudmila Rakhmanova ◽  
Maria Riazantseva ◽  
Georgy Zastenker ◽  
Yuri Yermolaev

&lt;p&gt;Development of the turbulent cascade inside the magnetosheath is known to be affected by the bow shock. Recently a number of studies showed various scenario of turbulent cascade modification at the bow shock including deviation from Kolmogorov scaling and additional damping of the kinetic-scale compressive fluctuations. Also, properties of probability distribution function may be modified behind the bow shock. However, factors which govern turbulence development in the magnetosheath remain unclear. Present study focuses on experimental analysis of the solar wind parameters which influence turbulence inside the magnetosheath. Analyzed data involves the combination of the solar wind parameters measured in L1 point by WIND spacecraft and Themis, Cluster and Spektr-R measurements behind the bow shock. Parameters of the frequency spectra of ion flux and/or magnetic field magnitude at frequency band from 0.01 to 2-10 Hz are considered such as slopes at magnetohydrodynamic and kinetic scales and the break frequency. Parameters of spectra are considered behind the bow shock of various topology i.e. for different mutual orientation of the interplanetary magnetic field and the local bow shock normal. Also, distance from the analyzed point to the bow shock nose is taken to the account. Obtained results point out that modification of the turbulent cascade at the bow shock is controlled not only by the bow shock topology but also by variability of the upstream solar wind plasma parameters and direction of the interplanetary magnetic field. In particular, Kolmogorov scaling often survives across the bow shock during periods of high-amplitude variations of plasma density and magnetic field magnitude in the solar wind. Also, increasing amplitude of northern interplanetary magnetic field results in steepening of spectra behind the bow shock. &amp;#160;&lt;/p&gt;


Nanomaterials ◽  
2020 ◽  
Vol 10 (12) ◽  
pp. 2373
Author(s):  
Filippos Sofos ◽  
Theodoros Karakasidis ◽  
Ioannis E. Sarris

The present paper employs Molecular Dynamics (MD) simulations to reveal nanoscale ion separation from water/ion flows under an external electric field in Poiseuille-like nanochannels. Ions are drifted to the sidewalls due to the effect of wall-normal applied electric fields while flowing inside the channel. Fresh water is obtained from the channel centerline, while ions are rejected near the walls, similar to the Capacitive DeIonization (CDI) principles. Parameters affecting the separation process, i.e., simulation duration, percentage of the removal, volumetric flow rate, and the length of the nanochannel incorporated, are affected by the electric field magnitude, ion correlations, and channel height. For the range of channels investigated here, an ion removal percentage near 100% is achieved in most cases in less than 20 ns for an electric field magnitude of E = 2.0 V/Å. In the nutshell, the ion drift is found satisfactory in the proposed nanoscale method, and it is exploited in a practical, small-scale system. Theoretical investigation from this work can be projected for systems at larger scales to perform fundamental yet elusive studies on water/ion separation issues at the nanoscale and, one step further, for designing real devices as well. The advantages over existing methods refer to the ease of implementation, low cost, and energy consumption, without the need to confront membrane fouling problems and complex electrode material fabrication employed in CDI.


2020 ◽  
Vol 497 (4) ◽  
pp. 5041-5060
Author(s):  
Brandon Buncher ◽  
Matias Carrasco Kind

ABSTRACT We present a novel method of robust probabilistic cosmic web particle classification in three dimensions using a supervised machine learning algorithm. Training data were generated using a simplified ΛCDM toy model with pre-determined algorithms for generating haloes, filaments, and voids. While this framework is not constrained by physical modelling, it can be generated substantially more quickly than an N-body simulation without loss in classification accuracy. For each particle in this data set, measurements were taken of the local density field magnitude and directionality. These measurements were used to train a random forest algorithm, which was used to assign class probabilities to each particle in a ΛCDM, dark matter-only N-body simulation with 2563 particles, as well as on another toy model data set. By comparing the trends in the ROC curves and other statistical metrics of the classes assigned to particles in each data set using different feature sets, we demonstrate that the combination of measurements of the local density field magnitude and directionality enables accurate and consistent classification of halo, filament, and void particles in varied environments. We also show that this combination of training features ensures that the construction of our toy model does not affect classification. The use of a fully supervised algorithm allows greater control over the information deemed important for classification, preventing issues arising from arbitrary hyperparameters and mode collapse in deep learning models. Due to the speed of training data generation, our method is highly scalable, making it particularly suited for classifying large data sets, including observed data.


2020 ◽  
Vol 2020 ◽  
pp. 1-6
Author(s):  
Mamoudou Saria ◽  
Bernard Zouma ◽  
Bruno Korgo ◽  
Vinci de Dieu Bokoyo Barandja ◽  
Martial Zoungrana ◽  
...  

This paper investigated, by one-dimensional modelling, the effects of reverse polarisation of an electromagnetic field, generated by an amplitude modulation radio antenna, on the efficiency of a silicon PV cell. Through a simulation, the effects of both the incidence angle and the electromagnetic field magnitude on the power output of the PV cell are analyzed. The power output curves against the junction dynamic velocity are used to find the junction dynamic velocity at the equilibrium, the maximum power output, and the efficiency of the PV cell. The results have shown that the presence of important electromagnetic fields in the neighborhood of a silicon PV cell decreases its performance.


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