Current Polarization Research Articles

Probing Magnetic Exchange Interactions with Helium.

Controlling and sensing spin polarization of electrons forms the basis of spintronics. Here, we report a study of the effect of helium on the spin polarization of the tunneling current and magnetic contrast in spin-polarized scanning tunneling microscopy (SP STM). We show that the magnetic contrast in SP STM images recorded in the presence of helium depends sensitively on the tunneling conditions. From tunneling spectra and their variation across the atomic lattice we establish that the helium can be reversibly ejected from the tunneling junction by the tunneling electrons. The energy of the tunneling electrons required to eject the helium depends on the relative spin polarization of the tip and sample, making the microscope sensitive to the magnetic exchange interactions. We show that the time-averaged spin polarization of the tunneling current is suppressed in the presence of helium and thereby demonstrate voltage control of the spin polarization of the tunneling current across the tip-sample junction.

Modeling the emergence of affective polarization in the social media society.

Rising political polarization in recent decades has hampered and gridlocked policymaking, as well as weakened trust in democratic institutions. These developments have been linked to the idea that new media technology fosters extreme views and political conflict by facilitating self-segregation into “echo chambers” where opinions are isolated and reinforced. This opinion-centered picture has recently been challenged by an emerging political science literature on “affective polarization”, which suggests that current polarization is better understood as driven by partisanship emerging as a strong social identity. Through this lens, politics has become a question of competing social groups rather than differences in policy position. Contrary to the opinion-centered view, this identity-centered perspective has not been subject to dynamical formal modeling, which generally permits hypotheses about micro-level explanations for macro-level phenomena to be systematically tested and explored. We here propose a formal model that links new information technology to affective polarization via social psychological mechanisms of social identity. Our results suggest that new information technology catalyzes affective polarization by lowering search and interaction costs, which shifts the balance between centrifugal and centripetal forces of social identity. We find that the macro-dynamics of social identity is characterized by two stable regimes on the societal level: one fluid regime, in which identities are weak and social connections heterogeneous, and one solid regime in which identities are strong and groups homogeneous. We also find evidence of hysteresis, meaning that a transition into a fragmented state is not readily reversed by again increasing those costs. This suggests that, due to systemic feedback effects, if polarization passes certain tipping points, we may experience run-away political polarization that is highly difficult to reverse.

Electric field control of magnon spin currents in an antiferromagnetic insulator.

Pure spin currents can be generated via thermal excitations of magnons. These magnon spin currents serve as carriers of information in insulating materials, and controlling them using electrical means may enable energy efficient information processing. Here, we demonstrate electric field control of magnon spin currents in the antiferromagnetic insulator Cr2O3. We show that the thermally driven magnon spin currents reveal a spin-flop transition in thin-film Cr2O3. Crucially, this spin-flop can be turned on or off by applying an electric field across the thickness of the film. Using this tunability, we demonstrate electric field–induced switching of the polarization of magnon spin currents by varying only a gate voltage while at a fixed magnetic field. We propose a model considering an electric field–dependent spin-flop transition, arising from a change in sublattice magnetizations via a magnetoelectric coupling. These results provide a different approach toward controlling magnon spin current in antiferromagnets.

Spin–orbit torque switching in a single (Ga,Mn)(As,P) layer with perpendicular magnetic anisotropy

We report the observation of current induced spin–orbit torque (SOT) switching of magnetization in a (Ga,Mn)(As,P) film using perpendicular magnetic anisotropy. Complete SOT switching of magnetization was achieved with current densities as low as 7.4 × 105 A/cm2, which is one to two orders of magnitude smaller than that normally used for SOT switching in ferromagnet/heavy metal bilayer systems. The observed magnetization switching chirality during current scans is consistent with SOT arising from spin polarization caused by the Dresselhaus-type spin–orbit-induced (SOI) fields. The magnitudes of effective SOI fields corresponding to the SOT were obtained from shifts of switching angles in angular dependent Hall measurements observed for opposite current polarities. By measuring effective SOI fields for the [11̄0] and the [110] current directions, we were then able to separate the values of the Dresselhaus-type (HeffD) and Rashba (HeffR) SOI fields. At a current density of 6.0 × 105 A/cm2, these values are HeffD=6.73Oe and HeffR=1.31Oe, respectively. The observed ratio of about 5:1 between Dresselhaus-type and Rashba SOI fields is similar to that observed in a GaMnAs film with an in-plane magnetic anisotropy.

Impact of processing parameters in plasma electrolytic oxidation on corrosion resistance of magnesium alloy type AZ91

AbstractThis study aims to investigate the effect of the processing parameters in plasma electrolytic oxidation (PEO) on the corrosion resistance of magnesium alloy type AZ91. The PEO coatings were prepared on the samples using alkaline‐based electrolyte. Both unipolar and bipolar, different frequencies and duty cycles were applied. Corrosion tests, using potentiodynamic polarization, linear and cyclic, and electrochemical impedance spectroscopy techniques, were applied on the as‐received and PEO coated samples. Scanning electron microscopy was used to investigate the surface morphology, for example, micropores, as well as to measure the thickness of the coated layer by changing the processing parameters. The results show that the size of micropores is interrelated to the duty cycle percentage and current polarities, as the higher frequency causes thinner coating layers, with fewer micropores, consequently higher corrosion resistance. In addition, increasing the duty cycle, a denser and more compact coating was obtained. The XRD results showed a missing peak of the α‐Mg phase in a PEO coated sample using bipolar, the highest frequency (1666 Hz), and the highest duty cycle (66.6%). The mils per year calculations showed that the PEO coated has a lower corrosion rate by at least 8 times than the as‐received alloy.

Design and Sintering of All-Solid-State Composite Cathodes with Tunable Mixed Conduction Properties via the Cold Sintering Process.

Electrodes for solid-state batteries require the conduction of both ions and electrons for extraction of the energy from the active material. In this study, we apply cold sintering to a model composite cathode system to study how low-temperature densification enables a degree of control over the mixed conducting properties of such systems. The model system contains the NASICON-structured Na3V2(PO4)3 (NVP) active material, NASICON-structured solid electrolyte (Na3Zr2Si2PO12, NZSP), and electron-conducting carbon nanofiber (CNF). Pellets of varying weight fractions of components were cold-sintered to greater than 90% of the theoretical density at 350-375 °C, a 360 MPa uniaxial pressure, and with a 3 h dwell time using sodium hydroxide as the transient sintering aid. The bulk conductivity of the diphasic composites was measured with impedance spectroscopy; the total conductivities of the composites are increased from 3.8 × 10-8 S·cm-1 (pure NVP) to 5.81 × 10-6 S·cm-1 (60 wt % NZSP) and 1.31 × 10-5 S·cm-1 (5 wt % CNF). Complimentary direct current polarization experiments demonstrate a rational modulation in transference number (τ) of the composites; τ of pure NVP = 0.966, 60 wt % NZSP = 0.995, and 5 wt % CNF = 0.116. Finally, all three materials are combined into triphasic composites to serve as solid-state cathodes in a half-cell configuration with a liquid electrolyte. Electrochemical activity of the active material is maintained, and the capacity/energy density is comparable to prior work.

Dissolution of iron in sodium chloride solution during alternating current polarization

Iron compounds are widely used in many industries and engineering, and even in medicine. The existing methods of obtaining iron compounds are multi-stage and complex. The purpose of this work is to obtain iron (II) hydroxide and oxide from metal waste under alternating current action using one and two half-cycles. For the first time, the electrochemical behavior of iron electrode was studied by electrolysis method during alternating current polarization of industrial frequency in sodium chloride solutions. The iron polarization was carried out in pair with titanium, while the current density on the iron electrode varied in the range of 200-1200 A/m2, and on the titanium is in the range of 20-100 kA/m2. It is established that in the anode half-cycle of alternating current, iron is oxidized to form divalent ions. At this moment, the titanium electrode is in the cathode half-cycle, hydrogen is released on it, hydroxyl ions are formed in the cathode space. In the solution, ions interact with iron (II) ions to produce iron hydroxide. At temperatures above 600C, iron (II) hydroxide is dehydrated with the production of iron (II) oxide. The electrolysis was carried out in two electrolyzers connected to each other in parallel with the immersion of pair of “titanium-iron” electrodes into each electrolyzer. The iron dissolution occurs simultaneously in two half-cycles of alternating current and this approach is proposed for the first time. The process productivity increases by more than 1.5 times.

Chiral anomaly, induced current, and vacuum polarization tensor for a Dirac field in the presence of a defect

We evaluate the vacuum polarization tensor (VPT) for a massless Dirac field in 1+1 and 3+1 dimensions, in the presence of a particular kind of defect, which in a special limit imposes bag boundary conditions.We also show that the chiral anomaly in the presence of such a defect is the same as when no defects are present, both in 1+1 and 3+1 dimensions. This implies that the induced vacuum current in 1+1 dimensions due to the lowest order VPT is exact.

A Novel Tin-Bonded Silicon Anode for Lithium-Ion Batteries.

Poor cyclic stability and low rate performance due to dramatic volume change and low intrinsic electronic conductivity are the two key issues needing to be urgently solved in silicon (Si)-based anodes for lithium-ion batteries. Herein, a novel tin (Sn)-bonded Si anode is proposed for the first time. Sn, which has a high electronic conductivity, is used to bond the Si-anode material and copper (Cu) current collector together using a hot-pressed method with a temperature slightly above the melting point of Sn. The cycling performance of the electrode is studied using a galvanostatic method. Nanoindentation and peeling tests are conducted to measure the mechanical strength of the electrodes. Direct current polarization and galvanostatic intermittent titration techniques are applied to assess the conductivity of the composites. Electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy are conducted to evaluate the effect of the coating layer on the cycling ability of the composites. The Sn-bonded Si anodes show superior cycling stability and high rate performance with an improved initial Coulombic efficiency. Analyses reveal that the low-melting-point Sn helps to markedly improve the electronic conductivity of the electrodes and serves as a metallic binder as well to enhance the adhesive strength of the electrode. It is hopeful that this novel Sn-bonded Si anode provides a new insight for the development of advanced Si-based anodes for LIBs.

Independent Upper- and Lower-Arm Switching Scheme Based on Reference Current Polarity for Three-Level NPC Inverters

This study presents a novel switching scheme for three-level neutral point clamped (NPC) inverters. The proposed switching method independently drives the upper- and lower-arm elements of the inverter based on the polarity information of the reference current. The proposed switching scheme does not require the inclusion of dead-time for each switching, except when the current polarity changes. Therefore, unlike the conventional inverter switching method, dead-time compensation is not needed, and the possibility of an arm-short accident is prevented. In this study, a switching procedure is detailed, and the operation mode analysis of the proposed switching scheme is presented. The effectiveness of the proposed switching method is verified experimentally by application to a grid-connected inverter that requires inverter current control.

Cultural complex, death anxiety and individuation during times of populism: a dialogue between Jungian psychology and social psychology.

A trend towards political polarization and radicalization is observable in many countries across the world. The rise of populist movements and an increasingly heated polarized discourse in the face of mounting environmental and cultural challenges calls for a multifaceted psychological engagement with these phenomena. C.G. Jung's vision of individuation remains crucial for understanding, and possibly mitigating, developments towards radicalization and division within and between cultures. This paper considers classic and contemporary depth psychological ideas regarding individuation, shadow work and cultural complexes, and applies these ideas to current socio-political developments. To further elucidate, Jungian thought is brought into conversation with the findings of a body of literature within social psychology called Terror Management Theory (TMT) which points to unconscious death anxiety as amplifying a sense of separation and otherness, as well as judgment and conflict between individuals and groups. This paper aims to deepen the understanding of current cultural polarization phenomena as well as to collective healing and growth by synthesizing ideas from Jung and TMT research, and to rekindle the fire of individuation as a counter to the trend of polarization, alienation and conflict.

Multifunctional molecular spintronics device based on neutral π-radicals predicted by first-principles study

Recently, planar neutral π-radicals were synthesized based on the insertion of tricoordinate B atoms in the triphenylmethyl framework (Kushida et al. (2017) [24]). This radical shows well balanced ambipolar carrier transport characteristics and has been used for the preparation of organic Mott-insulator transistors that work at room temperature. Here, using a first-principles method which combined non-equilibrium Green's function with spin-polarized density functional theory, we explore the transport properties of a molecular junction where a neutral π-radical dimer is sandwiched between two Au electrodes. Obvious spin current polarization, dual spin polarized rectification, magnetoresistance and low bias negative differential conductance are obtained. Moreover, the molecular device shows strong magnetoelectric coupling that the bias not only diminishes and eventually eliminates radical's magnetic moment but also manipulates magnetic configuration. Our investigations may help design a new generation of transistors with multifunction, super-miniaturization and low-power consumption.

Insulator:conductor interfacial regions — Li ion dynamics in the nanocrystalline dispersed ionic conductor LiF:TiO2

Lithium fluoride is known to be a very poor ionic conductor. Here, we used it as a model substance to investigate the influence of the insulator TiO2 on ion dynamics in nanocrystalline composites of LiF and TiO2. In such two-phase systems a percolation network of fast Li+ pathways at the conductor:insulator interfaces may lead to enhanced overall, through-going ionic (and electronic) transport. Indeed, we observed such an effect and characterized it by conductivity spectroscopy as well as by variable-temperature 7Li and 19F Nuclear Magnetic Resonance (NMR) line shape measurements and diffusion-induced NMR relaxometry. Compared to nanocrystalline LiF, a remarkable increase in total conductivity of almost four orders of magnitude, that is, from 6×10−11 S cm−1 up to 2×10−7 S cm−1 (100 °C) was observed for LiF:TiO2 containing 40 vol.-% of TiO2. Direct current polarization measurements revealed that principally ionic charge carriers are responsible for this enhancement. As both ions in LiF, Li+ and F−, might be mobile, NMR helped revealing that Li+ is the charge carrier being mainly responsible for the increase in ionic conductivity. Compared to SiO2 and Al2O3 [see, S. Breuer et al. J. Phys. Chem. C, 2019, 123, 5222], with TiO2 the largest increase in conductivity was achieved. Hence the introduction of heterogeneous conductor:insulator contacts turned out to be a highly suitable tool to effectively engineer the interfacial regions in such two-phase systems. In general, such a composite effect is important for ion transport in components for energy storage devices and in the solid electrolyte interphase region that generally passivates the anode material in lithium-ion batteries.

Domains of electrically induced valley polarization in two-dimensional Dirac semiconductors

Electrically induced formation of valley-polarized free-carrier domains is theoretically investigated in two-dimensional (2D) honeycomb lattice systems exhibiting topological valley transport and the valley Hall effect. The results show that under a strong electric field $E$ applied along a nanostrip, the domains can be formed across the strip when this transverse dimension is comparable to the intervalley diffusion length ${L}_{iv}$. Further, these domains are found to be characterized by two distinct length scales dependent on $E$ and ${L}_{iv}$, i.e., the extended diffusion length ${L}_{\mathrm{ext}}$ ($\ensuremath{\propto}{L}_{iv}E$) and the compressed diffusion length ${L}_{\mathrm{com}}$ ($\ensuremath{\propto}{L}_{iv}/E$). The former determines the extension of the domain plateaus, whereas the latter specifies the width of the domain wall. Within each of the domains (excluding a narrow region of the domain wall; ${L}_{\mathrm{ext}}\ensuremath{\gg}{L}_{\mathrm{com}}$), the charge carriers are fully polarized belonging to only one of the valleys, providing a pure bulk valley current inside the considered domain region. The domain properties including the polarization amplitude and domain wall position are analyzed as a function of the applied electric field and intervalley scattering rates at the edges. The position of the domain wall, which determines the relative extension of the plateaus, can be controlled by a transverse current flowing between the Hall-type contacts. The current-voltage characteristic demonstrates a superlinear behavior in the range of electric fields corresponding to the valley-polarization domain formation. This feature is a distinctive signature of the anomalous transport arising from Bloch-band Berry curvature, which enhances the longitudinal conductance and diminishes the channel resistance. By reversing the applied electric field, the valley polarization and localization of the valley-polarized currents can be abruptly switchable. We suggest that the studied scheme of electrically induced domains of valley polarization in the 2D honeycomb lattice systems can be used in novel applications with all-electrical control and manipulation of the valley degree of freedom.

Current polarity effects on laboratory plasma jets

Plasma jets produced by a pulsed power machine were investigated using Thomson scattering and other diagnostics in order to make detailed comparisons to simulations. These jets were produced from a 15 μm thick disk of Al foil on a 1.2 MA, 100 ns rise time, pulsed power machine. Experiments were performed with both a radially inward and a radially outward current flow in the Al foil to investigate the effects of voltage polarity in the experiments and determine how extended magnetohydrodynamic (XMHD) effects, such as the Hall effect, change the formation of the jet. We recorded Thomson scattering spectra with a low enough laser energy to not perturb the plasma while providing a high enough signal to noise ratio to resolve the scattered features. This enabled the measurement of the electron temperature in the jet region of the plasma, 15.5±4 eV for both current polarities. Jets with a radially outward current flow were heated more from inverse bremsstrahlung when 10 J of laser energy was used, implying that these jets are denser than the ones with a radially inward current. This higher density was confirmed by interferometry measurements. Experimental results were compared with XMHD computer simulations, which predicted electron temperatures 1.5–3 σ above those measured, and significantly higher density than experiments in both polarities. Possible sources of this discrepancy are discussed.

A Novel Highly Durable Carbon/Silver/Silver Chloride Composite Electrode for High-Definition Transcranial Direct Current Stimulation.

High-definition transcranial direct current stimulation (HD-tDCS) is a promising non-invasive neuromodulation technique, which has been widely used in the clinical intervention and treatment of neurological or psychiatric disorders. Sintered Ag/AgCl electrode has become a preferred candidate for HD-tDCS, but its service life is very short, especially for long-term anodal stimulation. To address this issue, a novel highly durable conductive carbon/silver/silver chloride composite (C/Ag/AgCl) electrode was fabricated by a facile cold rolling method. The important parameters were systematically optimized, including the conductive enhancer, the particle size of Ag powder, the C:Ag:PTFE ratio, the saline concentration, and the active substance loading. The CNT/Ag/AgCl-721 electrode demonstrated excellent specific capacity and cycling performance. Both constant current anodal polarization and simulated tDCS measurement demonstrated that the service life of the CNT/Ag/AgCl-721 electrodes was 15-16 times of that of sintered Ag/AgCl electrodes. The much longer service life can be attributed to the formation of the three-dimensional interpenetrating conductive network with CNT doping, which can maintain a good conductivity and cycling performance even if excessive non-conductive AgCl is accumulated on the surface during long-term anodal stimulation. Considering their low cost, long service life, and good skin tolerance, the proposed CNT/Ag/AgCl electrodes have shown promising application prospects in HD-tDCS, especially for daily life scenarios.

Effect of DCRP and DCSP Polarity on SMAW Welding on the Depth of Penetration with E7018 Electrodes using Low Carbon Steel ASTM A36

The study aims to determine the level of penetration depth using SMAW (Shielded Metal Arc Welding) Process based on the polarity type of DCRP (Direct Current Reverse Polarity) and DCSP (Direct Current Straight Polarity). This research used ASTM A36 low carbon steel plate with thickness of 6 mm and length of 200 mm, Electrode E7018 LB-52-18 ∅ 3.2 mm, and with current parameters of 90 A, 100 A, 110 A, 120 A and 130 A and 70° welding arc angle. The method used the experimental research. Data collection techniques applied direct observation techniques and descriptive statistical data analysis techniques. Testing the penetration depth of the welds with a macrographic test was conducted by an optical microscope. The test results show that DCRP has more depth than DCSP when the current is 90 A with a difference of 0.38 mm. When the current is 100 A, DCRP is deeper with a difference of 0.312 mm compared to DCSP. The third experiment, with a current of 110 A DCRP, was deeper with a difference of 0.05 mm compared to DCSP. During the fourth and fifth experiments, DCRP was deeper with a difference of 0.21 mm compared to DCSP at 120 A and DCRP was 0.324 mm deeper than DCSP at 130 A. It can be concluded that the effect of DCRP and DCSP polarity on the depth of penetration using E7018 electrodes and ASTM low carbon steel A36, DCRP polarity has a deeper penetration depth compared to DCSP.

Magnetization switching and deterministic nucleation in Co/Ni multilayered disks induced by spin–orbit torques

We present experimental and numerical results on the magnetization reversal induced by spin–orbit torques of micronic disks of a ferromagnetic multilayer with perpendicular magnetic anisotropy on top of a Pt track: Pt (6 nm)/[Co(0.2 nm)/Ni(0.6 nm)] × 5/Al(5 nm). The current induced magnetization switching process is probed by anomalous Hall effect measurements and Kerr microscopy. The electrical characterization reveals the critical current for the complete reversal to be about 3×1011 A/m2, and Kerr microscopy uncovers a deterministic nucleation that depends on current and field polarity. Through the use of experimental switching phase diagrams coupled to micromagnetic simulations, we evaluated the field-like to damping-like torque ratio to be 0.73 ± 0.05, which is in good agreement with experimental values observed by second harmonic measurements. These measurements emphasize an unexpectedly large field-like contribution in this relatively thick Co/Ni multilayer (4 nm). In light of these experiments and simulations, we discuss the key parameters needed to understand the magnetization reversal, namely, the field and damping-like torques and the Dzyaloshinskii–Moriya interaction.

Conduction properties of Ti‐doped NaTaO3 at intermediate temperature

AbstractIn this work, the ionic conductivity and charge carriers of acceptor‐doped sodium tantalate (NaTaO3) with perovskite structure were investigated at intermediate temperatures. The Ta‐site of NaTaO3 was doped with up to 20% titanium (Ti) with the conventional solid‐state reaction method. After calcination at 900°C, samples nominally doped with 5, 10% Ti show X‐ray diffraction (XRD) pattern of orthorhombic NaTaO3 only, while peaks of Na2Ti3O7 can be observed in those doped with 15, 20% Ti. The conductivity of undoped, 5% Ti and 10% Ti‐doped NaTaO3 at 300°C–700°C was measured with electrochemical impedance spectroscopy (EIS) under dry and wet O2 atmospheres. Ti‐doped NaTaO3 samples have higher conductivity in the wet atmosphere than in the dry atmosphere, reaching 3 × 10−4 S/cm at 700°C (10% Ti‐doped NaTaO3), which confirms the hydration and proton conduction in Ti‐doped NaTaO3. Through the investigation on the dependence of conductivity on oxygen partial pressure, hole conduction in an oxidizing atmosphere, and electron conduction in reducing atmosphere can be verified. Na+ conduction was proven to be negligible with direct current polarization.

Insights into surface alkaline-earth segregation on the surface of La0.3Me0.7Fe0.7Cr0.3O3−δ (Me = Sr, Ca) porous electrodes for oxygen reduction reaction

Surface alkaline-earth segregation is a common problem encountered for the cathodes of solid oxide fuels cells based on perovskites (ABO3) containing alkaline-earth acceptor dopants. In this work, the porous electrodes of La0.3Me0.7Fe0.7Cr0.3O3−δ (Me = Sr, Ca) were employed as the model specimens to look into the surface segregation issue. The two electrodes were cathodically polarized by continuously testing their cathodic current polarization curves. The electrocatalytic activity of the two electrodes was examined in relation with their cathodic polarization history. Moreover, the crystal and surface structures of the cathodically polarized electrodes were analyzed. An appreciable segregation of strontium onto the surface was detected for cathodically polarized La0.3Sr0.7Fe0.7Cr0.3O3−δ electrode, leading to an obvious deceleration of disassociative adsorption of molecular oxygen on the surface and thus degradation of the electrode activity. In contrast, La0.3Ca0.7Fe0.7Cr0.3O3−δ electrode displayed nearly negligible surface calcium segregation and degradation of the electrocatalytic activity after experiencing an identical cathodic polarization history. The disparity of the two electrodes was related to their A-site compositions. This work exemplifies the importance of minimizing A-site cation size mismatch in stabilizing the crystal structure of the perovskite electrodes under cathodic polarization conditions and therefore alleviating the segregation of A-site alkaline-earth cations onto the electrode surface.