Low Field Region Research Articles

Effect of plasma step gradient modification on surface electrical properties of epoxy resin

In this paper, plasma fluorination is combined with plasma silicon deposition to achieve step gradient modification on an epoxy resin surface. The physicochemical characteristics of samples are investigated and the electrical performances measured. The obtained results show that compared with untreated and single treated samples, the samples treated by step gradient modification significantly improve the flashover performance. According to experiment and simulation, the mechanism explanations are summarized as follows. First, it is found that the step gradient conductivity can effectively optimize the electric field distribution of a needle-needle electrode. Then, step gradient modification suppresses the accumulation of surface charge at the triple junction and makes the charge distribution more uniform. Furthermore, it can accelerate the surface dissipation on a high electrical field region and control the dissipation rate on a low electrical field region. All these results can restrain surface discharge and increase the flashover voltage. The step gradient modification method proposed in this paper provides a new idea for improving the surface insulation performance.

Hybrid evaporative cooling of 133Cs atoms to Bose-Einstein condensation.

The Bose-Einstein condensation (BEC) of 133Cs atoms offers an appealing platform for studying the many-body physics of interacting Bose quantum gases, owing to the rich Feshbach resonances that can be readily achieved in the low magnetic field region. However, it is notoriously difficult to cool 133Cs atoms to their quantum degeneracy. Here we report a hybrid evaporative cooling of 133Cs atoms to BEC. Our approach relies on a combination of the magnetically tunable evaporation with the optical evaporation of atoms in a magnetically levitated optical dipole trap overlapping with a dimple trap. The magnetic field gradient is reduced for the magnetically tunable evaporation. The subsequent optical evaporation is performed by lowering the depth of the dimple trap. We study the dependence of the peak phase space density (PSD) and temperature on the number of atoms during the evaporation process, as well as how the PSD and atom number vary with the trap depth. The results are in excellent agreement with the equation model for evaporative cooling.

Experimental Study on Microwave Generation due to Merged Instability in F-Band Surface Wave Oscillator

A surface wave oscillator (SWO) is driven by an electron beam to generate intense microwaves. The electron beam possesses slow space-charge (SSC) and slow cyclotron (SC) modes that interact with the surface wave leading to microwave generation. The beam current and external magnetic field affect the relationship between SSC and SC modes. The SSC mode gradually approaches the SC mode when the beam current increases. Meanwhile the SC mode gradually approaches the SSC mode when the magnetic field decreases. The two modes merge in a low magnetic field and high beam current. In this work, we experimentally examine the operation of an F-band SWO in the low magnetic field region. The output power decreases with low beam current when magnetic field decreases. Meanwhile, the SWO maintains its power level with high beam current even though the magnetic field decreases to around 0.4 T. The merged instability enables a sustained power level in the low magnetic field region.

Superconducting properties of sintered ex situ MgB2 tapes through ball milling process as a function of crystallite size in the as-milled and sintered states

Ball milled MgB2 powders and ex situ processed tapes using those milled powders were sintered at various temperatures. Critical temperature (Tc) increases moderately with increasing sintering temperature (Tsin) up to 705 – 735 °C, at which best performance in transport critical current density (Jc) at 4.2 K in 10 T for the tape samples is obtained. In contrast, the Tc rapidly increases when sintered above that temperature. Although the transport Jc property is improved with increasing the Tsin up to this optimal Tsin, further increase of the Tsin degrades the Jc property in the high-field region. These behaviors of the superconducting properties are attributed to the crystallite growth. The reduced crystallite size brought about through milling process recovers by sintering, and this recovery is remarkable above the optimal Tsin. With increasing the crystallite size, the Tc increases, whereas both upper critical field (Hc2) and irreversibility field (Hirr) decrease. While MgB2 decomposition is promoted above the optimal Tsin, grain connectivity is improved with increasing the Tsin. Thus, high-temperature sintering not only improves the grain connectivity and hence the Jc property in the low-field region, but also brings about large Jc degradation in the high-field region due to the decrease in Hc2 and Hirr at 4.2 K. Since the crystallite size in the as-milled and sintering states are linked with one another, those superconducting properties of the optimally sintered tapes in which MgB2 decomposition is ignorable are easily deduced from the crystallite size of MgB2 in the filling powders.

Quench Analysis of the CFETR TF Coil Using the Gandalf Code

The toroidal field (TF) coil is a crucial system in the tokamak, which provides the main magnetic field to confine the plasma. One TF coil of the China Fusion Engineering Test Reactor (CFETR) will be constructed with the support of the Chinese government in the next five years. In order to study the quench behavior of the CFETR TF coil, the assumed quench initiation at the maximum and minimum temperature margin position of the TF coil is analyzed, and the effect of the quench detection threshold on the hot spot temperature is studied. It is found that 0.5 V quench detection voltage leads to hot spot temperature above 250 K in the low-field region. In order to meet the design criteria of the hot spot temperature, the quench detection voltage should be less than 0.2 V.

Band tailoring by annealing and current conduction of Co-doped ZnO transparent resistive switching memory

The switching characteristics of ITO/Zn1-xCoxO/ITO transparent resistive random access memories were studied. 5 mol% cobalt doped ZnO resistive layer improves bipolar switching properties. In addition, the redshift in band energy caused by doping of cobalt (Co) was studied. The doped memory device also showed a change in band energy by 0.1 eV when subjected to annealing of 400 °C. Annealing below 400 °C temperature did not show any characteristic changes. The film morphology analysis suggested the increase in roughness with annealing temperature, which can be seen from FESEM and AFM images. In this study annealing and Co doping effect on ZnO based non-volatile memory device is presented. Moreover, transparent memory devices with 90% transmittance at 550 nm wavelength have been reported. At low field and high field region Schottky emission and ionic conduction are dominated respectively.

1 H-NMR analysis, antioxidant activity, and α-amylase and α-glucosidase inhibitory potential of ten common Thai edible flowers.

Edible flowers have been used in cooking as foods as well as medicine alternatives. The objectives were to identify a major class of organic compounds in edible flowers by applying proton nuclear magnetic resonance (1 H-NMR) metabolite fingerprinting and to evaluate bioactivity including antioxidant and α-amylase and α-glucosidase inhibitory activities. A total of 32 metabolites including five organic acids, eight sugars, 11 amino acids and eight phenolic compounds were identified by using 1 H-NMR at 600 MHz. The aromatic proton of phenolic compounds was observed at the low-field region ranged between δ 5.5ppm and 10.0ppm. Total phenolic content (TPC) of the extracts of edible flowers ranged from 6.0mg GAE g-1 dry weight (DW) to 47.97 mg GAE g-1 DW. Antioxidant activity varied from 140.5 mmol L-1 TE g-1 DW to 985.2 mmol L-1 TE g-1 DW. The extracts from edible flowers showed dose-dependent inhibition against enzyme. The extract of African marigold (Tagetes erecta L.) had the highest half maximal inhibitory concentration (IC50 ) against α-amylase at 0.13mg mL-1 while the extract of flower of Sesban agasta (Sesbania grandiflora Desv.) had the lowest IC50 against α-amylase at 0.63 mg mL-1 . Moderate correlation was found between IC50 values against α-glucosidase and antioxidant activity values (R2 = 0.66) (P< 0.05). 1 H-NMR application provided metabolomic fingerprinting of edible flowers and the in vitro inhibitory potential activity against key enzymes related to diabetes indicated that specific edible flowers have a potential use for dietary management during the early stages of hyperglycaemia. © 2021 Society of Chemical Industry.

Metamagnetic transitions and magnetoelectricity in the spin-1 honeycomb antiferromagnet Ni2Mo3O8

Earlier neutron scattering studies suggested the coexistence of complex stripelike and zigzaglike antiferromagnetic orders in polar honeycomb lattice ${\mathrm{Ni}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$, while its magnetoelectric (ME) behavior as an emergent effect is thus of high interest. Here we report our observations of two metamagnetic transitions and novel ME responses of ${\mathrm{Ni}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$ single crystals against high magnetic field $H$ up to \ensuremath{\sim}60 T. The $c$-axis (polar axis) spontaneous electric polarization ${P}_{\mathrm{spin}}$, emerging at the magnetic N\'eel temperature ${T}_{N}\ensuremath{\sim}5.5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and its remarkable response to $H$ applied along the $c$ axis ($H//c$) and $a$ axis ($H//a$), respectively, provide the clear evidence for the magnetism-driven ferroelectricity. While the magnetism exhibits the in-plane anisotropy to some extent, the magnetic field dependencies of magnetization and electric polarization in the low-field region and high-field region are distinctly different. In the low-field region where a weak spin-flop type metamagnetic transition occurs, the electric polarization response shows the parabolic dependence of magnetic field applied along both the $c$ axis and in-plane $a$ axis. The second metamagnetic transition happens when the magnetic field extends up to the high-field region where the magnetization and electric polarization response at low temperature are characterized by an extraordinarily broad plateau for the magnetic field along the $c$ axis but roughly linear dependence for field along the $a$ axis. These unusual phenomena are discussed, based on the symmetry-related local ME tensor analysis, and it is suggested that both the spin current and p-d hybridization mechanisms may contribute to the spontaneous electric polarization and ME responses. The present work demonstrates ${\mathrm{Ni}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$ as a unique multiferroic and promising platform for exploring the rich spin-1 physics and ME phenomena in honeycomb lattice.

Detecting Dissociation Dynamics of Phosphorus Molecular Ions by Atom Probe Tomography.

Dissociation processes involving phosphorus cations were investigated during laser-assisted atom probe tomography of crystalline indium phosphide (InP). This technique not only allows the formation of medium-sized phosphorus cations by means of femtosecond laser pulses under ultrahigh vacuum and high electric field conditions but also allows one to study the time-resolved dissociation dynamics. Data reveal the formation of cations up to P232+ and their subsequent dissociation into two smaller Pk+ cations (k > 2). The use of a time- and position-sensitive detector combined with numerical calculations provided information related to the molecule orientation, decay time, and kinetic energy release during dissociation phenomena. Results suggest that the dissociation processes are most likely due to the emission of Pk2+ cations in excited states and their subsequent decay in low field regions during their flight toward the detector. This study provides operative guidelines to obtain information on dissociation processes using a tomographic atom probe as a reaction microscope and indicates the current capabilities and limitations of such an approach.

Transition from trap-mediated to band-like transport in polycrystalline monolayer molybdenum disulfide memtransistors

Multi-terminal memtransistors using polycrystalline monolayer molybdenum disulfide (MoS2) have recently emerged as novel synaptic devices. Due to the coexistence of disorder and strong Coulomb carrier-carrier interactions in MoS2, localization and delocalization of carriers can come into play successively upon the relative strength of disorder and interactions, which can be tuned by the Fermi level (EF). In this work, we show that the transition from trap-mediated to band-like transport leads to the resistive switching behavior in MoS2 memtransistors, which is driven by the EF shift arising from defect profile redistribution that is facilitated by grain boundaries. In the high resistance state, field-driven hopping conduction can be clearly observed in the high-field region (E&amp;gt;0.05 MV/cm), whereas the linear dependence of ln(I/E) on the square root of the electric field, E1/2, suggests Poole–Frenkel emission in the low-field region (E≤0.05 MV/cm). In the low resistance state, strong interactions prevailed and a substantial amount of thermally activated electrons are excited into the conduction band, leading to band-like transport.

Analysis of multipactor in a rectangular waveguide using Spark3D software

Multipactor is a resonant nonlinear electron multiplication effect that may occur in high power microwave devices at very low pressures, such as those operating in particle accelerators and satellite subsystems. In this research, multipactor of a rectangular waveguide was analyzed using the commercially available, numerical simulation software “Spark3D.” The electromagnetic wave in the simulation was a TE10 mode-2.85 GHz wave of varying power, fed into the impedance transformer waveguide. At the lowest threshold, multipactor is generated in the minimum height region in the impedance transformer and nowhere else. More precisely, the multipacting electrons have a continuous energy distribution since the emitted secondary electrons carry a random initial velocity distribution. We observed that there are cases where the impact electron energy decreases despite an increase in power due to growing non-resonance of the microwave field and electron oscillations, resulting in not only two threshold points where secondary emission yield (SEY) = 1 but several more. As a consequence, it was uncovered that when the average SEY in the highest field region is close to or less than one, multipactor may be caused in a lower field region where the SEY is effectively higher than one. The numerical results are compared with data from the experiment. While there is some deviation between the thresholds obtained from Spark3D and the experiment, the results at higher power levels are consistent with the experiment in the view of the SEY for each power level.

Evaluation of Waveform Profiles for Traveling Wave Ion Mobility Separations in Structures for Lossless Ion Manipulations.

Structures for lossless ion manipulations (SLIM) have recently enabled a powerful implementation of traveling wave ion mobility spectrometry (TWIMS) for ultrahigh resolution separations; however, experimental parameters have not been optimized, and potential significant gains may be feasible. Most TWIMS separations have utilized square-shaped waveforms applied by time-dependent voltage stepping across repeating sets of electrodes, but alternative waveforms may provide further improvements to resolution. Here, we characterize five waveforms (including square and sine) in terms of their transmission efficiency, IMS resolution, and resolving power, and explore the effects of TW amplitude and speed on the performance of each. We found, consistent with previous work, separations were generally improved with higher TW amplitudes, moderately improved by lower speeds (limited by ion "surfing" with the waves), and found decreases in signal intensity at the extremes of operating conditions. The triangle and asymmetric "ramp forward" shaped profiles were found to provide modestly greater resolution and resolving power, an observation we tentatively attribute to their relatively uniform fields and minimal low-field regions.

Field‐Induced Ferroelectric Hf1‐xZrxO2 Thin Films for High‐k Dynamic Random Access Memory

AbstractThe field‐induced ferroelectric Hf1–xZrxO2 (FFE–HZO) thin film is investigated for use as the capacitive layer in the future dynamic random access memory (DRAM). Although the dielectric permittivity of FFE–HZO is as high as ≈80, a high electric field (4 MV cm−1) is needed to activate the FFE mechanism and the accompanying high dielectric permittivity value. The Zr content is adjusted, or field cycling is performed, to obtain a high dielectric permittivity at a low electric‐field region for feasible DRAM operation. As a result, a dielectric permittivity value of 60–80 is achieved in the low field region (0–2 MV cm−1), which coincided with an equivalent oxide thickness of 0.47 nm with a stable leakage current at 0.8 V. The involvement of hysteresis in the polarization‐electric field curve of the film, however, caused significant energy loss (40–60%). Hysteresis engineering using various dopants, and improving the deposition process, annealing process, or passive interfacial layer, should be the next step.

Observation of surface dominated topological transport in strained semimetallic ErPdBi thin films

In this Letter, we present experimental observation of surface-dominated transport properties in [110]-oriented strained (∼1.6%) ErPdBi thin films. The resistivity data show typical semi-metallic behavior in the temperature range of 3 K ≤ T ≤ 350 K with a transition from semiconductor- to metal-like behavior below 3 K. The metallic behavior at low temperature disappears entirely in the presence of an external magnetic field &amp;gt;1 T. The weak-antilocalization (WAL) effect is observed in magneto-conductance data in the low magnetic field region and follows the Hikami–Larkin–Nagaoka (HLN) model. HLN fitting estimated single coherent channel, i.e., α ∼ −0.51 at 1.9 K, and the phase coherence length (Lϕ) shows the Lϕ ∼ T−0.52 power law dependence on temperature in the range of 1.9 K–10 K, indicating the observation of 2D WAL. Shubnikov–de Haas (SdH) oscillations are observed in magneto-resistance data below 10 K and are fitted to standard Lifhsitz Kosevich theory. Fitting reveals the effective mass of charge carriers ∼0.15 me and a finite Berry phase of 0.86π ± 0.16. The sheet carrier concentration and mobility of carriers estimated using SdH data are ns ∼ 1.35 × 1012 cm−2 and μs = 1210 cm2 V−1 s−1, respectively, and match well with the data measured using the Hall measurement at 1.9 K to be n ∼ 1.22 × 1012 cm−2, μ = 1035 cm2 V−1 s−1. These findings indicate the non-trivial nature and surface-dominated transport properties of strained (110) ErPdBi thin films at low temperatures.

Structural, magnetic and dielectric studies on half-doped Nd0.5Ba0.5CoO3 perovskite

We have investigated in detail the structural, magnetic, and dielectric properties of half-doped Nd0.5Ba0.5CoO3 perovskite cobaltite synthesized by the auto-combustion method. The Rietveld structure refinement from X-ray diffraction pattern reveals that a polycrystalline sample of Nd0.5Ba0.5CoO3 crystallizes into an orthorhombically distorted crystal structure with space group Pmmm along with a minor distorted tetragonal phase with P4/mmm space group. The temperature dependence of magnetization measurements show paramagnetic to ferromagnetic phase transition at Curie temperature TC around 147 K for H = 200 Oe. The value of TC strongly depends on the applied magnetic bias field, and TC decreases with an increasing magnetic field. The polycrystalline sample of Nd0.5Ba0.5CoO3 cobaltite exhibits Griffiths’ phase in the temperature range 147 K ≤ T ≤ 235 K for H = 200 Oe. Investigation of the isothermal magnetic field measurements on Nd0.5Ba0.5CoO3 reveals that it exhibit antiferromagnetic, ferromagnetic and paramagnetic behaviors, at T = 2 K, 100 K and 150 K, respectively. The negative slope of the isothermal Arrott’s plots (i.e., H/M versus M2) in the low magnetic field region suggests a first-order magnetic phase transition while the positive slope in high magnetic field region reveals a second-order magnetic transition. The dielectric constant and electrical resistivity of the synthesized Nd0.5Ba0.5CoO3 cobaltite material decrease with increasing temperature as well as frequency. Bulk Nd0.5Ba0.5CoO3 cobaltite shows insulator/semiconductor to metal transition at 76 °C.

Evaluation of Hematite Nanoparticles Weak Ferromagnetism

Nanoparticles of hematite were prepared by the precipitation method. Structural, hysteresis, moment-temperature, ferromagnetic resonance tests were performed. All hematite peaks were appeared in the XRD pattern with corundum phase of crystallite size of 15 nm. The SEM images showed that the average particle size is around 21 nm with ribbed shape. Different Fe–O bond peaks were registered in the FTIR test. The sample showed weak ferromagnetic state as was checked by M-H and M-T tests. The M-H test showed opened loop at low-field region and linear behavior at high-field region with susceptibilities of 1.4 × 10−5 emu/g Oe and 7.5 × 10−6 emu/g Oe respectively. Fitting by Langevin equation is conducted to find the particle moment. Morin temperature was found to be 17.77 K, while Neel temperature was about 614 K. A comparison with literature was given. FMR test showed a shift in the resonance field with broadened line having Lande g factor of 1.90498 at 9.3 GHz.

Ultrasonic attenuation in ferromagnet under an applied magnetic field

Effect of an external magnetic field on the sound attenuation coefficient is investigated using the Landau-Ginzburg model incorporating a tricritical to mean-field crossover. The dynamics is treated within the framework of the Onsager theory. Temperature variation of the attenuation coefficient has been obtained for arbitrary values of the magnetic field. For non-zero sixth order term in the Landau-Ginzburg expansion and for sufficiently high magnetic field the maximum in attenuation occurs above the Curie point and is displaced towards the higher temperatures as the magnetic field is increased. In the high temperature region a very rapid increase of attenuation with reduced temperature is demonstrated before the maximum is attained. This increase is much steeper than in the analogous hydrodynamic regime below the Curie temperature. At constant temperature the quadratic field dependence of attenuation on magnetic field is found in the low-field region until a maximum at a certain magnetic field is attained. This maximum is followed by a power-law decay to zero in the high-field region. A quantitative comparison between obtained results and available ultrasonic measurements in MnP is presented. Good agreement is achieved for moderate and large magnetic fields and temperatures not very close to the Curie temperature.

Trivalent gadolinium doped SrZrO3 perovskite ceramic: Sol-gel synthesis, paramagnetic centres, and luminescence studies

Trivalent gadolinium (Gd3+) doped SrZrO3 perovskite ceramic has been prepared and their X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), electron paramagnetic resonance (EPR), and photoluminescence (PL) spectra are recorded. The existence of divalent and tetravalent cations in SrZrO3 along with a trivalent dopant ion provides an opportunity to study the effect of charge compensators on the system properties. The cubic phase is evident from XRD diffraction patterns of the product. The SEM analysis suggests the formation of voids and pores in the ceramics caused by gases those liberated during the process of sintering. The EPR technique has been used to investigate the un-doped and Gd3+-doped SrZrO3 ceramics. The un-doped SrZrO3 shows an axially symmetric EPR line with g∥ = 1.94 and g⊥ = 1.99, which is attributed to the Zr3+ ion. The Gd3+-doped SrZrO3 shows dominant EPR lines with effective g-values at 1.93, 2.04, and 2.35. Weak lines are observed at 2.72, 3.64, 6.08, and 15.5. At higher dopant concentrations, a broad line with geff ~2.1 is seen, together with low field lines. In the low field region of the EPR spectrum, lines appear because of immediate environment distortions. Under the excitation of 273 nm, the emission spectrum shows band 6P7/2 → 8S7/2 (313 nm). The emission at 313 nm suggests that the prepared sample can be used as a narrow band UV radiation source. Emission and EPR analysis of the sample confirm that Gd3+ is distributed in both Sr2+ and Zr4+ sites in the SrZrO3 matrix.

Low temperature specific heat of 12442-type KCa2Fe4As4F2 single crystals

Low-temperature specific heat (SH) is measured for the 12442-type KCa$_2$Fe$_4$As$_4$F$_2$ single crystal under different magnetic fields. A clear SH jump with the height of $\Delta C/T|_{T_c}$ = 130 mJ/mol K$^2$ is observed at the superconducting transition temperature $T_c$. It is found that the electronic SH coefficient $\Delta\gamma (H)$ quickly increases when the field is in the low-field region below 3 T and then considerably slows down the increase with a further increase in the field, which indicates a rather strong anisotropy or multi-gap feature with a small minimum in the superconducting gap(s). The temperature-dependent SH data indicates the presence of the $T^2$ term, which supplies further information and supports the picture with a line-nodal gap structure. Moreover, the onset point of the SH transition remains almost unchanged under the field as high as 9 T, which is similar to that observed in cuprates, and placed this system in the middle between the BCS limit and the Bose-Einstein condensation.

Assessment of alternative divertor configurations as an exhaust solution for DEMO

Plasma exhaust has been identified as a major challenge towards the realisation of magnetic confinement fusion. To mitigate the risk that the single null divertor (SND) with a high radiation fraction in the scrape-of-layer (SOL) adopted for ITER will not extrapolate to a DEMO reactor, the EUROfusion consortium is assessing potential benefits and engineering challenges of alternative divertor configurations. Alternative configurations that could be readily adopted in a DEMO design include the X divertor (XD), the Super-X divertor (SXD), the Snowflake divertor (SFD) and the double null divertor (DND). The flux flaring towards the divertor target of the XD is limited by the minimum grazing angle at the target set by gaps and misalignments. The characteristic increase of the target radius in the SXD is a trade-off with the increased TF coil volume, but, ultimately, also limited by forces onto coils. Engineering constraints also limit XD and SXD characteristics to the outer divertor leg with a solution for the inner leg requiring up-down symmetric configurations. Capital cost increases with respect to a SND configuration are largest for SXD and SFD, which require both significantly more poloidal field coil conductors and in the case of the SXD also more toroidal field coil conductors. Boundary models with increasing degrees of complexity have been used to predict the beneficial effect of the alternative configurations on exhaust performance. While all alternative configurations should decrease the power that must be radiated in the outer divertor, only the DND and possibly the SFD also ease the radiation requirements in the inner divertor. These decreases of the radiation requirements are however expected to be small making the ability of alternative divertors to increase divertor radiation without excessive core performance degradation their main advantage. Initial 2D fluid modeling of argon seeding in XD and SFD configurations indicate such advantages over the SND, while results for SXD and DND are still pending. Additional improvements, expected from increased turbulence in the low poloidal field region of the SFD also remain to be verified. A more precise comparison with the SND as well as absolute quantitative predictions for all configurations requires more complete physics models that are currently only being developed.