Low Field Research Articles

Experimental research of ECW pre-ionization and assisted startup in EAST

Experimental research on the electron cyclotron wave (ECW) pre-ionization and assisted start-up was carried out systematically for the first time in EAST tokamak, which is a superconducting device with ITER-like full metal wall. Breakdown and plasma initiation at low toroidal electric fields (<0.3 V m−1) with ECW pre-ionization and startup assistance has been demonstrated. Also, the parameter domain of breakdown is significantly extended towards higher prefill gas pressure. The effect of ECW injection timing, power, toroidal injection angle on breakdown were also investigated. Injecting ECW earlier leads to an earlier breakdown and a higher plasma current ramp rate. The electron cyclotron heating (ECH) power threshold for breakdown in EAST is approximately 0.4 MW. In the range of ECH power tested in this work, higher ECH power is advantageous for achieving earlier and faster breakdown. Furthermore, the breakdown with radial ECW injection occurs earlier compared with oblique injections (co-current and counter-current). During the ECW-assisted startup, the process of burn-through is prolonged by the higher pre-filled gas pressure even though it enhances the ease of breakdown. In addition, compared to the low hybrid wave assistance, the ECW assistance has an effect in averting the generation of runaway electrons and improving the safety of device during startup. Moreover, the ECW assistance exhibits a high tolerance to the impurity and thus ensures a high ramp rate of plasma current even with a high impurity level.

Influence of the electron density on the giant negative magnetoresistance in two-dimensional electron gases

variation of the electron density via a metallic gate can control the disorder potentials in two-dimensional electron gases (2DEGs). This also influences the negative magnetoresistance at low magnetic fields, which is commonly observed in ultrahigh mobility 2DEGs. We investigate the temperature-dependent giant negative magnetoresistance (GNMR) as a function of the electron density for several temperatures and currents. Thereby, we find that the GNMR behavior depends decisively on the electron density. This observation is attributed to a changed disorder potential with electron density. In the case of higher electron densities, a nonlinear current dependency of the GNMR is observed, which could be described within the hydrodynamic regime. Published by the American Physical Society 2024

Magnetic domain structures in ultrathin Bi2Te3/CrTe2 heterostructures

Abstract Chromium tellurium compounds are important two-dimensional van der Waals ferromagnetic materials with high Curie temperature and chemical stability in air, which is promising for applications in spintronic devices. Here, high-quality spin-orbital-torque (SOT) device, Bi2Te3/CrTe2 heterostructure was epitaxially grown on Al2O3 (0001) substrates. Anomalous Hall measurements indicate the existence of strong ferromagnetism in this device with the CrTe2 thickness down to 10 nm. In order to investigate its micromagnetic structure, cryogenic magnetic force microscope (MFM) was utilized to measure the magnetic domain evolutions at various temperatures and magnetic fields. The virgin domain state of the device shows a worm-like magnetic domain structure with the size around 0.6 ~ 0.8 µm. Larger irregular-shape magnetic domains (＞1µm) can be induced and pinned, after the field is increased to coercive field and ramped back to low fields. The temperature-dependent MFM signals exhibit a nice mean-field-like ferromagnetic transition with Curie temperature around 201.5 K, indicating a robust ferromagnetic ordering. Such device can be potentially implemented in future magnetic memory technology.

Low field mobility in bulk GaN and its ternary AlGaN/GaN compounds (quantum kinetic approach)

Analytical expressions for the low-field mobility of charge carrier gases with three-(3D), two-(2D) and one-(1D) dimensionalities are obtained. Multi-ion ionized impurities scattering, acoustic and polar optic phonons are considered as scattering mechanisms. The calculated values of mobility are compared to known experimental data for bulk (3D) n-and p-type wurtzite, n-type zinc-blende GaN crystals and low dimensional (2D and 1D) ternary GaAlN compounds. The resulting analytical expressions give the dependences of mobility on dimensionality of charge carrier gas, its density, effective mass, temperature and confining dimensions. A comparison of the experimental and calculated temperature dependences of the mobility in bulk GaN crystals (3D) and in AlGaN/GaN nanowires (1D) shows that the mobility at 100\\,{\ ext{K}}$?> T>100K is determined by the scattering of charge carriers by polar optical phonons with an energy of 91.2 meV. The temperature dependences of mobility in Al0.25Ga0.75N/GaN heterostructures (2D) at 100\\,{\ ext{K}}$?> T>100K are in consistent with experiment for electron scattering by polar optical phonons with a noticeably higher energy of 160 meV. We associate this fact with the heterointerface, which according to well-known theoretical studies can change both the strength of electron polar optical phonons scattering and the energy of the phonons.

Giant magneto-photoluminescence at ultralow field in organic microcrystal arrays for on-chip optical magnetometer

Optical detection of magnetic field is appealing for integrated photonics; however, the light-matter interaction is usually weak at low field. Here we observe that the photoluminescence (PL) decreases by > 40% at 10 mT in rubrene microcrystals (RMCs) prepared by a capillary-bridge assembly method. The giant magneto-PL (MPL) relies on the singlet-triplet conversion involving triplet-triplet pairs, through the processes of singlet fission (SF) and triplet fusion (TF) during radiative decay. Importantly, the size of RMCs is critical for maximizing MPL as it influences on the photophysical processes of spin state conversion. The SF/TF process is quantified by measuring the prompt/delayed PL with time-resolved spectroscopies, which shows that the geminate SF/TF associated with triplet-triplet pairs are responsible for the giant MPL. Furthermore, the RMC-based magnetometer is constructed on an optical chip, which takes advantages of remarkable low-field sensitivity over a broad range of frequencies, representing a prototype of emerging opto-spintronic molecular devices.

Whole-body magnetic resonance imaging at 0.05 Tesla

Despite a half-century of advancements, global magnetic resonance imaging (MRI) accessibility remains limited and uneven, hindering its full potential in health care. Initially, MRI development focused on low fields around 0.05 Tesla, but progress halted after the introduction of the 1.5 Tesla whole-body superconducting scanner in 1983. Using a permanent 0.05 Tesla magnet and deep learning for electromagnetic interference elimination, we developed a whole-body scanner that operates using a standard wall power outlet and without radiofrequency and magnetic shielding. We demonstrated its wide-ranging applicability for imaging various anatomical structures. Furthermore, we developed three-dimensional deep learning reconstruction to boost image quality by harnessing extensive high-field MRI data. These advances pave the way for affordable deep learning–powered ultra-low-field MRI scanners, addressing unmet clinical needs in diverse health care settings worldwide.

Low-carbon building evaluation index system based on hierarchical analysis method

Low-carbon building is an unavoidable development trend in the construction industry, especially in the critical moment of global warming, it is necessary to make a comprehensive evaluation of low-carbon buildings. At this stage, low carbon building has become an important direction in the construction field, and whether the low carbon building reaches the corresponding standards and the advantages played by the low carbon field need to be assessed with perfect evaluation indexes. Based on this, this paper constructs a low-carbon building evaluation system from the whole life cycle of the building using the hierarchical analysis method (AHP) and BP neural network method. Firstly, the definition and influencing factors of low-carbon buildings are analyzed, secondly, the evaluation index system of low-carbon buildings is constructed, and then the evaluation index system of low-carbon buildings is verified by using the hierarchical analysis method, and the results show that the evaluation results based on the hierarchical analysis and the BP neural network method are more accurate than those of the traditional hierarchical analysis method. The results show that the evaluation results based on hierarchical analysis and BP neural network are more accurate than the traditional hierarchical analysis method. It shows that the BP neural network method can effectively reduce the influence of subjective factors in the hierarchical analysis method and improve the objectivity of the evaluation results. On this basis, this paper proposes countermeasures to promote the development of low-carbon buildings, in order to provide a certain reference for the long-term development of low-carbon buildings.

High critical current properties of multi-filamentary MgB2 superconducting wires fabricated by an internal Mg diffusion method

Abstract Although both mass density and grain connectivity of MgB2 superconducting layers can be greatly improved through an internal Mg diffusion (IMD) process, poor structure uniformity and low MgB2 filling factor of IMD wires limit further enhancement of their superconducting performance. Herein we prepared 19-filament and 37-filament IMD-MgB2 superconducting wires by a combination of optimizing the component structure and introducing the intermediate annealing process. The microstructure analysis suggests that well structure uniformity and high layer density have been achieved in multi-filamentary MgB2 wires; and the MgB2 filling factor reaches up to 9.3-11.0%. The magnetic superconducting transition of MgB2 wires is relatively sharp and the onset Tc is around 37 K. Remarkably, there is no magnetic flux jump for 37-filament wires in low field region at 5 K. At 4.2 K and 4 T, the transport layer Jc values of 19-filament and 37-filament MgB2 wires are as high as 1.5×105 A/cm2 and 2.2×105 A/cm2, respectively, and accordingly engineering Je of 1.7×104 A/cm2 and 2.0×104 A/cm2. These results indicate that the performance of multi-filamentary IMD-MgB2 wires can compete with the traditional PIT-MgB2 wires as applied in the industry.

Regulating sulfur migration and transformation in low water-binder ratio cementitious system incorporating phosphogypsum aggregate: Environmentally friendly clean materials

The paper aims to clarify the regulation mechanism of sulfur migration and transformation in a low water-binder ratio system incorporating phosphogypsum aggregate (LWC-PG). Firstly, the combination of S-containing adhesive material and phosphate gypsum aggregate can reduce the migration of sulfate ions (SO42−) in the phosphate gypsum aggregate. The reaction mechanism and macro and micro properties of LWC-PG are characterized by universal testing machine, low field nuclear magnetic field, isothermal calorimeter, XRD diffractometer, scanning electron microscope and nano-indentation. The results show that the calcium sulfoaluminate cementitious system (CSA) is compatible with PG. The results show that CSA is compatible with PG, and the 28d strength of 80 % phosphogypsum is 83.4 MPa, which can maintain the required strength for high-performance concrete. Secondly, the sulfides in PG react with C4A6S in CSA, resulting in the formation of ettringite that coats the surface of the C–S–H gel, thereby retarding the hydration rate of the slurry. However, due to the presence of phosphate gypsum aggregate, the reaction continues, enhancing the hydration of sulfate cement. Moreover, the interfacial transition zone between the PG and matrix exhibits a strong bond and shows no significant signs of deterioration. Lastly, when using 80 % phosphogypsum aggregate instead of natural sand and gravel to produce 100 m3 of concrete, 42 t of phosphogypsum aggregate can be used, which reduces the consumption of natural sand and gravel by 61.4 t, decreases energy consumption by 3070 MJ, reduces CO2 emissions by 178.06 kg, SO2 emissions by 0.37 kg, NOX emissions by 0.25 kg, and dust emissions by 0.087 kg. Life cycle evaluation demonstrates that the production of LWC-PG is associated with low carbon emissions and environmental friendliness, which significantly reduces environmental burden. This is of significant research importance for solid waste management and energy-saving and carbon reduction efforts.

Structural and electronic transport properties of Zn- and Ga-doped Bi2− x Sb x Te3− y Se y topological insulator single crystals

A comprehensive study of structural and magnetotransport properties of pristine Bi2−x Sb x Te3−y Se y (BSTS) single crystals and doped with Zn (BSTS:Zn) and Ga (BSTS:Ga) are presented here. Magnetic field dependent Hall resistivities of the single crystals indicate that the holes are the majority carriers. The field dependent resistivity curves at different temperatures of the crystals display cusp-like characteristics at low magnetic fields, attributed to two-dimensional (2D) weak antilocalization (WAL) effect. We fit the observed low-field WAL effects at low temperatures using 2D and three-dimensional (3D) Hikami-Larkin-Nagaoka (HLN) equations. The 2D HLN equation fits the data more closely than the 3D HLN equation, indicating a 2D nature. The 2D HLN equation fit to the low field WAL effects at various temperatures reveal a phase coherence length (l φ) that decreases as temperature increases. The variation of l φ with temperature follows T −0.41 power law for BSTS:Zn, suggesting that the dominant dephasing mechanism is a 2D electron–electron (e−e) interactions. For pristine BSTS and BSTS:Ga, l φ(T) is described by considering a coexistence of 2D e−e and electron–phonon (e−p) interactions in the single crystals. The temperature variation of the longitudinal resistance in BSTS:Ga is described by 3D Mott variable range hoping model. In contrast, the transport mechanisms of both pristine BSTS and BSTS:Zn are described by a combination of 2D WAL/EEI models and 3D WAL.

A quantum critical Bose gas of magnons in the quasi-two-dimensional antiferromagnet YbCl3 under magnetic fields

AbstractBose–Einstein condensation (BEC) is a quantum phenomenon in which a macroscopic number of bosons occupy the lowest energy state and acquire coherence at low temperatures. In three-dimensional antiferromagnets, a magnetic-field-induced transition has been successfully described as a magnon BEC. For a strictly two-dimensional (2D) system, it is known that BEC cannot take place due to the presence of a finite density of states at zero energy. However, in a realistic quasi-2D magnet consisting of stacked magnetic layers, a small but finite interlayer coupling stabilizes marginal BEC but such that 2D physics is still expected to dominate. This 2D-limit BEC behaviour has been reported in a few materials but only at very high magnetic fields that are difficult to access. The honeycomb S = 1/2 Heisenberg antiferromagnet YbCl3 exhibits a transition to a fully polarized state at a relatively low in-plane magnetic field. Here, we demonstrate the formation of a quantum critical 2D Bose gas at the transition field, which, with lowering the field, experiences a BEC marginally stabilized by an extremely small interlayer coupling. Our observations establish YbCl3, previously a Kitaev quantum spin liquid material, as a realization of a quantum critical BEC in the 2D limit.

Double hysteresis (P-E) loops in the transparent composite containing BaTiO3 at room temperature

A transparent composite sample containing BaTiO3 was synthesized by using the melting quenching method. The presence of a glassy phase in this composite sample was detected through XRD analysis, and this was further confirmed by DSC study. TEM and SAED analyses provided evidence that the BaTiO3 nanoparticles/clusters are embedded within the borate glass matrix, establishing the composite nature of this sample. FTIR spectrum of the present sample revealed that the glass matrix is composed of two structural groups (BO3 with NBO’s, and BO4), along with the distinct groups for BaTiO3. XPS spectra of the present composite sample indicated the presence of more than one type of boron, barium, titanium and oxygen. DSC and dielectric studies of the present composite sample revealed the presence of the phase transition temperature (Tc). Dielectric constant (ɛr) and dielectric loss (tan δ) curves of the present composite sample displayed an anomaly peak in the vicinity of TC. The optical transmission spectrum of the present composite sample exhibit two transmission bands of Ti3+ (3d1) ions in tetragonal distorted sites. At room temperature, the present transparent composite sample exhibited double hysteresis loops for BaTiO3 at low electric fields. The results obtained can be used for the development of lead-free ferroelectric material.

Diamond with Sp2-Sp3 composite phase for thermometry at Millikelvin temperatures

Temperature is one of the seven fundamental physical quantities. The ability to measure temperatures approaching absolute zero has driven numerous advances in low-temperature physics and quantum physics. Currently, millikelvin temperatures and below are measured through the characterization of a certain thermal state of the system as there is no traditional thermometer capable of measuring temperatures at such low levels. In this study, we develop a kind of diamond with sp2-sp3 composite phase to tackle this problem. The synthesized composite phase diamond (CPD) exhibits a negative temperature coefficient, providing an excellent fit across a broad temperature range, and reaching a temperature measurement limit of 1 mK. Additionally, the CPD demonstrates low magnetic field sensitivity and excellent thermal stability, and can be fabricated into probes down to 1 micron in diameter, making it a promising candidate for the manufacture of next-generation cryogenic temperature sensors. This development is significant for the low-temperature physics researches, and can help facilitate the transition of quantum computing, quantum simulation, and other related technologies from research to practical applications.

Effect of Crystallinity on the Field Emission Characteristics of Carbon Nanotube Grown on W-Co Bimetallic Catalyst.

Carbon nanotube (CNT) is an excellent field emission material. However, uniformity and stability are the key issues hampering its device application. In this work, a bimetallic W-Co alloy was adopted as the catalyst of CNT in chemical vapor deposition process. The high melting point and stable crystal structure of W-Co helps to increase the grown CNT diameter uniformity and homogeneous crystal structure. High-crystallinity CNTs were grown on the W-Co bimetallic catalyst. Its field emission characteristics demonstrated a low turn-on field, high current density, stable current stability, and uniform emission distribution. The Fowler-Nordheim (FN) and Seppen-Katamuki (SK) analyses revealed that the CNT grown on the W-Co catalyst has a relatively low work function and high field enhancement factor. The high crystallinity and homogeneous crystal structure of CNT also reduce the body resistance and increase the emission current stability and maximum current. The result provides a way to synthesis a high-quality CNT field emitter, which will accelerate the development of cold cathode vacuum electronic device application.

Spin-wave self-imaging: Experimental and numerical demonstration of caustic and Talbot-like diffraction patterns

Extending the scope of the self-imaging phenomenon, traditionally associated with linear optics, to the domain of magnonics, this study presents the experimental demonstration and numerical analysis of spin-wave (SW) self-imaging in an in-plane magnetized yttrium iron garnet film. We explore this phenomenon using a setup in which a plane SW passes through a diffraction grating, and the resulting interference pattern is detected using Brillouin light scattering. We have varied the frequencies of the source dynamic magnetic field to discern the influence of the anisotropic dispersion relation and the caustic effect on the analyzed phenomenon. We found that at low frequencies and diffraction fields, the caustics determine the interference pattern. However, at large distances from the grating, when the waves of high diffraction order and number of slits contribute to the interference pattern, the self-imaging phenomenon and Talbot-like patterns are formed. This methodological approach not only sheds light on the behavior of SW interference under different conditions but also enhances our understanding of the SW self-imaging process in both isotropic and anisotropic media.

Giant dielectric tunability in ferroelectric ceramics with ultralow loss by ion substitution design

Due to their responsiveness to modulation by external direct current fields, dielectric tunable materials are extensively utilized in integrated components, such as ferroelectric phase shifters. Barium strontium titanate ceramics have been considered the most potential tunable materials for a long time. However, the significant dielectric loss and high voltage drive have limited their further applications. Recently, Bi6Ti5WO22 ceramic has regained attention for its high dielectric tunability with low loss. In this study, we judiciously introduce Nb5+ with a larger ionic radius, replacing Ti4+ and W6+. This successful substitution enables the modulation of the phase transition temperature of Bi6Ti5WO22 ceramics to room temperature, resulting in superior tunable properties. Specifically, the 0.7Bi6Ti5WO22−0.3Bi6Ti4Nb2O22 ceramics exhibit giant tunability (~75.6%) with ultralow loss (<0.002) under a low electric field (1.5 kV/mm). This tunability is twice that of barium strontium titanate ceramics with a similar dielectric constant and only one-tenth of the loss. Neutron powder diffraction and transmission-electron-microscopy illustrate the nanodomains and micro-strains influenced by ion substitution. Density functional theory simulation calculations reveal the contribution of ion substitution to polarization. The research provides an ideal substitute for tunable material and a general strategy for adjusting phase transition temperature to improve dielectric properties.

Predicting the compressive strength of tight sandstone based on the low field NMR and pseudo-triaxial compression measurements

The compressive strength is very important for petroleum and other engineering studies. However, the effect of pore size and fluid distribution on the rock’s strength is not fully understood. We developed comprehensive research to study the controlling factors of the compressive strength based on low field nuclear magnetic resonance (NMR) measurements and pseudo-triaxial compression test for tight sandstones. The relationship between the compressive strength and the NMR obtained parameters are investigated completely, aiming for a better estimation of the compressive strength using the NMR data. The result shows that the rock’s strength is strongly controlled by the pore size distribution and the fluid existing state. Generally, the compressive strength is negatively correlated with the average transversal relaxation time, the movable water saturation, and the porosity, but positively correlated with the irreducible water saturation. The result reveals that the rock with larger pore radius and higher percentage of movable fluid is easier to reach the failure state. Further, the precision of the empirical model by multiple regression of the geometric mean of the relaxation time and the porosity is greatly improved compared with the model established by the brittle minerals, which is potentially to be use for geophysical prospecting when the NMR logging data is available.

Nanostructured tungsten trioxide developed from environmentally friendly green process as a promising cathode for excellent field emission

Extensively explored as cathodes for field emission, nanostructured metal oxides play a crucial role in diverse applications such as microwave amplifiers, renewable energy systems, flat panel displays, and micro-electronics. The quest for high current density, low threshold field, and stable field emission persists. In present study, we introduce a green synthesis approach using vegetable oil as a reaction medium to produce hexagonal tungsten trioxide (h-WO3). The structural analysis confirmed the formation of h-WO3 with a nanorod-like morphology. The spin-coated h-WO3 film on a silicon substrate exhibited promising field emission properties, including a field enhancement factor (β) of 1596 and a turn-on field of 4.60 V/μm, comparable to reported values. This research opens avenues for the cost-effective and scalable utilization of h-WO3 as an electron emitter in advanced optical and electrical devices.

Significantly improved ferroelectric properties of (Zn,Co) co-doped BiFeO3 thin films prepared by sol-gel method

BiFe1-2xZnxCoxO3 (BFZCO) thin films (x = 0 %, 1 %, 2 %, 3 %, 4 %, 5 %) were deposited on FTO/glass substrates by sol-gel method. We systematically investigated the effects of equal doping of (Zn, Co) on the crystal microstructure, leakage current, leakage mechanism, and ferroelectric properties of BFZCO thin films. The XRD patterns indicate that all samples matched the chalcogenide structure without any impurity phases. The BiFe0.96Zn0.02Co0.02O3 (BFZCO-2) film exhibited a large residual polarization (2Pr = 339.9 μC/cm2) at an electric field strength of approximately 2000 kV/cm. The curve of leakage current density shows that equal doping of (Zn, Co) can considerably decrease the leakage current of BFO thin films at low electric fields. Furthermore, the mechanism of leakage conduction has shifted from Ohmic conduction at low electric fields to SCLC conduction at high electric fields.

Thresholds and mechanisms of human magnetophosphene perception induced by low frequency sinusoidal magnetic fields

BackgroundVirtually everyone is exposed to power-frequency MF (50/60 Hz), inducing in our body electric fields and currents, potentially modulating brain function. MF-induced electric fields within the central nervous system can generate flickering visual perceptions (magnetophosphenes), which form the basis of international MF exposure guidelines and recommendations protecting workers and the general public. However, magnetophosphene perception thresholds were estimated 40 years ago in a small, unreplicated study with significant uncertainties and leaving open the question of the involved interaction site. MethodsWe used a stimulation modality termed transcranial alternating magnetic stimulation (tAMS), delivering in situ sinusoidal electric fields comparable to transcranial alternating current stimulation (tACS). Magnetophosphene perception was quantified in 81 volunteers exposed to MF (eye or occipital exposure) between 0 and 50 mT at frequencies of 20, 50, 60 and 100 Hz. ResultsReliable magnetophosphene perception was induced with tAMS without any scalp sensation, a major advantage as compared to tACS. Frequency-dependent thresholds were quantified using binary logistic regressions hence allowing to establish condition dependent probabilities of perception. Results support an interaction between induced current density and retinal rod cells. ConclusionBeyond fundamental and immediate implications for international safety guidelines, and for identifying the interaction site underlying phosphene perception (ubiquitous in tACS experiments), our results support exploring the potential of tAMS for the differential diagnosis of retinal disorders and neuromodulation therapy.