Space Charge Research Articles

Selective Control of Electric Charge of Weyl Fermions in Pyrochlore Iridates.

Weyl fermions can exhibit exotic phenomena due to their magnetic charge in momentum space, while Weyl nodes are usually located away from Fermi energy, which forms electron or hole pockets as the electric charges. Previous studies have mostly focused on the magnetic charge, however, the electric charges are rarely explored. Here, the intriguing Hall responses arising from the interplay between magnetic and electric charges of Weyl fermions in pyrochlore iridates are reported. Explicitly, unexpected linear scaling is observed between the anomalous Hall conductivity and Hall carrier density in strained Nd2Ir2O7 thin films, and its slope shows a sign change approaching the Néel temperature of Nd. Theoretical calculations unveil that the cluster magnetic multipoles induce local energy inversion of Weyl nodes, which alters the electric charge of Weyl fermions and accounts for the observed nontrivial Hall responses. Moreover, the correlation between the magnetic and electric charges is further probed by voltage-controlled hydrogenation, which leads to the suppression of the anomalous Hall effect through electron filling. This work not only reveals the essential role of the both magnetic and electric charges of Weyl fermions, but also demonstrates the hydrogenation as an effective tuning knob in exploring correlated topological properties.

Multiphysical description of atmospheric pressure interface chemical ionisation in MION2 and Eisele type inlets

Abstract. Chemical ionisation inlets are fundamental instrument components in chemical ionisation mass spectrometry (CIMS). However, the sample gas and reagent ion trajectories are often understood only in a general and qualitative manner. Here, we evaluate two atmospheric pressure interface chemical ionisation inlets (MION2 and Eisele type inlet) with 3D computational fluid dynamics physicochemical models regarding the reagent ion and sample gas trajectories and estimate their efficiencies of reagent ion production, reagent ion delivery from the ion source volume into the ion–molecule mixing region, and the interaction between reagent ions and target molecules. The models are validated by laboratory measurements and quantitatively reproduce observed sensitivities to tuning parameters, including ion currents and changes in mass spectra. The study elucidates how the different transport and chemical reactions proceed within the studied inlets, where space charge can already be relevant at ion concentrations as low as 107 cm−3, and compares the two investigated inlet models. The models provide insights into how to operate the inlets and will help in the development of future inlets that further enhance the capability of CIMS.

Characterization of the Lithium/Solid Electrolyte Interface in the Presence of Nanometer-thin TiOx Layers for All-Solid-State Batteries.

It is still unclear which role space charge layers (SCLs) play within an all-solid-state battery during operation with high current densities, as well as to which extent they form. Herein, we use a solid electrolyte with a known SCL formation and investigate it in a symmetric cell under non-blocking conditions with Li metal electrodes. Since the used LICGC™ electrolyte is known for its instability against lithium, it is protected from rapid degradation by nanometer-thin layers of TiOx deployed by atomic layer deposition. Close attention is given to the interfacial properties, as now additional Li+ can traverse through the interface depending on the applied bias potential. The interlayer's impedance response shows efficient lithium-ion conduction for low bias potentials and a diffusion-limiting effect towards high positive and negative potentials. SCLs grow up to a thickness of 5.1 μm. Additionally, estimating the apparent rate constant of the charge transfer across the interface indicates that the potentials where kinetics are hindered coincide with the widest SCLs. In conclusion, the investigation under higher steady-state currents was only possible because of the improved stability due to the interlayer. No chemo-physical failure could be observed after 800+ hours of cycling. However, an ex-situ SEM study shows a new phase at the interface, which grows into the electrolyte.

Instantaneous and Retarded Interactions in Coherent Radiation.

In coherent radiation of an ensemble of electrons, the radiation field from electrons resonantly drives the other electrons inside to produce stimulated emission. The radiation reaction force on the electrons accounting for this stimulated radiation loss is classically described by the Liénard-Wiechert potential. Despite its being the foundation of beam physics for decades, we show that using the "acceleration field" in Liénard-Wiechert potential to describe radiative interactions leads to divergences due to its implicit dependence on instantaneous interactions. Here, we propose an alternative theory for electromagnetic radiation by decomposing the interactions into an instantaneous part and retarded part. It is shown that only the retarded part contributes to the irreversible radiation loss and the instantaneous part describes the space charge related effects. We further apply this theory to study the coherent synchrotron radiation energy loss, which hopefully will reshape our understanding of coherent radiation and collective interactions.

Increasing the Efficiency of Thermoelectric Conversion of Solar Energy

The article considers the latest achievements in the field of thermionic conversion of solar energy. The key aspects of thermionic emission, the problems of reducing the electrode work function and increasing the efficiency of devices are analyzed. It is shown that diamonds doped with phosphorus can be an attractive material for the purposes of thermionic emission. In this case, the presence of donor states can significantly narrow the space charge region, and at the same time reduce the potential barrier in the work function. The studies also found that the most successful and widely used method for overcoming the space charge effect is filling the interelectrode gap with cesium. The efficiency of thermionic conversion devices for solar energy increases by 1.6 times. Particular attention is paid to the use of new materials, such as nanostructures and carbon nanotubes, as well as promising technologies, such as photonic enhancement of thermionic processes. The analysis of the results showed that the photonic enhancement of the electron yield due to illumination is greater in p-type silicon, as predicted by theoretical models. The thermionic current increases by 1.7 times compared to silicon. The distance between the quasi-Fermi level and the Fermi level in n-type silicon is less than this distance in p-type silicon. As a consequence, the number of electrons in the conduction band is smaller. Methods for combating the space charge effect and optimizing the configuration of energy conversion systems are discussed.

High‐Gain and Tunable Linear Photodetection in 2D Tunneling Heterostructures Through Potential Engineering

Abstract2D materials are extensively employed in the fabrication of high‐performance photodetectors owing to their exceptional physical properties. However, most 2D material photodetectors fail to sustain high gain under intense illumination due to the limited intrinsic trap states. Here, an n‐n type Bi2O2Se/SnSe2 van der Waals tunneling heterojunction photodetector with a detection range from visible to near‐infrared (VIS‐NIR) is presented. Under reverse bias, the heterojunction induces a significant electron barrier and hole potential well, ensuring low leakage current and ample hole defect states. Therefore, the photodetector demonstrated a responsivity of 1636.3 AW−1 and a detectivity of 1.39 × 1014 Jones under 660 nm illumination, maintaining a tunable linear dynamic range (LDR) of ≈74.7 dB. This performance is attributed to the hole potential well‐suppressing the recombination of photogenerated carriers, thereby enhancing the device's gain. Furthermore, the tunneling of photogenerated electrons within the heterojunction's space charge region under bias enables rapid response (75.1 and 15.6 µs). In summary, the study introduces a novel strategy to overcome the limited detection capabilities of 2D devices under intense illumination, characterized by outstanding linearity for rapid detection and high‐resolution imaging.

Multiple Energy Transfer Modes From o‐Carborane Dyad Induced by Synergetic Conformational Regulations and Intense Circularly Polarized Luminescence in Self‐Assembly Aggregates

AbstractPhotothermal modulated excited conformations in traditional optoelectronic materials have been extensively investigated for the manipulation of emissive properties. However, photoinduced molecular synergetic conformational changes, which are promising for controlling intramolecular energy transfer modes through molecular orbital breaking, are never been explored. Herein, a novel o‐carborane dyad composed of carbazole units as electron donors and triazine derivatives as electron acceptors is demonstrated to achieve conformation‐dependent emissions and abundant emissions via synergetic conformational changes. Notably, the resulting dyad exhibited multiple conformational changes that induced emissions through different energy transfer modes involved “through space” and “through bond.” Exactly, the electron spin from singlet to triplet state took a flip accompanied by molecular orbitals from π–π molecular orbitals in the space charge transfer style to π and σ molecular orbitals in the bond charge transfer mode. The circularly polarized luminescence (CPL) properties are systematically investigated, exhibiting intense CPL with large glum values. These results provide novel perspectives for elucidating the complicated emission changes in aggregated states, as well as unique perspectives on the relationship between molecular conformations, energy transfer modes, and molecular orbital breaking.

Unraveling Asymmetric Electrochemical Kinetics in Low-Mass-Loading LiNi1/3Mn1/3Co1/3O2 (NMC111) Li-Metal All-Solid-State Batteries

In this study, we fabricated a Li-metal all-solid-state battery (ASSB) with a low mass loading of NMC111 cathode electrode, enabling a sensitive evaluation of interfacial electrochemical reactions and their impact on battery performance, using Li1.3Al0.3Ti1.7(PO4)3 (LATP) as the solid electrolyte. The electrochemical behavior of the battery was analyzed to understand how the solid electrolyte influences charge storage mechanisms and Li-ion transport at the electrolyte/electrode interface. Cyclic voltammetry (CV) measurements revealed the b-values of 0.76 and 0.58, indicating asymmetry in the charge storage process. A diffusion coefficient of 1.5 × 10−9 cm2⋅s−1 (oxidation) was significantly lower compared to Li-NMC111 batteries with liquid electrolytes, 1.6 × 10−8cm2⋅s−1 (oxidation), suggesting that the asymmetric charge storage mechanisms are closely linked to reduced ionic transport and increased interfacial resistance in the solid electrolyte. This reduced Li-ion diffusivity, along with the formation of space charge layers at the electrode/electrolyte interface, contributes to the observed asymmetry in charge and discharge processes and limits the rate capability of the solid-state battery, particularly at high charging rates, compared to its liquid electrolyte counterpart.

Electron Migratory Polarization of Interfacial Electric Fields Facilitates Efficient Microwave Absorption

AbstractHigh‐performance microwave absorption materials (MAM) are often accompanied by synergistic effects of multiple loss mechanisms, but the contribution share of various loss mechanisms has been neglected to provide a template and reference for the design of MAM. Here, a highly conductive 2D structure is designed through a functional group‐induced structure modulation strategy, composite L‐Ni@C can reach an effective absorption bandwidth of 6.45 GHz at 15% fill rate, with a maximum absorption efficiency of 99.9999%. Through the layer‐by‐layer analysis of the loss mechanism, it is found that the strong loss originates from the polarization loss at the heterogeneous interface. The movement of space charge between the two‐phase interface forms an interfacial electric field, and the in situ doping of nitrogen is cleverly achieved by the introduction of amino functional groups, which significantly enhances the rate of space charge transfer between the two‐phase interface and greatly facilitates the electron migration polarization. The space charge motion law of the interfacial electric field is also simulated using COMSOL simulation software to illustrate the electron migration polarization mechanism at heterogeneous interfaces. This work fills the gap of functional group‐induced structural modulation and presents new theories into the mechanism of space charge movement at heterogeneous interfaces.

Robust resistance switching performance in a Ca3Ti2O7/La0.67Sr0.33MnO3 heterostructure induced by the interface

Ruddlesden–Popper phase oxides, such as Ca3Ti2O7, have been established as hybrid improper ferroelectrics. However, investigations into Ca3Ti2O7 have primarily concentrated on their structural and ferroelectric properties. In this study, we prepared epitaxial Ca3Ti2O7 thin films via magnetron sputtering. Conducting atomic force microscopy was employed to characterize local current variations under an applied bias voltage. Electron paramagnetic resonance measurements of the Ca3Ti2O7/La0.67Sr0.33MnO3 film were conducted to assess its defect characteristics. Interestingly, the Ca3Ti2O7/La0.67Sr0.33MnO3 stacks exhibited remarkable macroscopic resistance switching, with a resistance on/off ratio reaching 100, alongside robust retention (∼2500 s) and endurance (∼2000 cycles) features. Additionally, density functional theory calculations suggest that the resistance switching is attributable to the interface barrier of the Ca3Ti2O7/La0.67Sr0.33MnO3 interface and the efficacy of space charge limitation. This work proposes an avenue for the utilization of Ruddlesden–Popper phase Ca3Ti2O7 in various applications.

Controlling the digital-to-analog switching in HfO2-based memristors via modulating the oxide thickness

The HfO2-based memristors have been acknowledged as a highly potential candidate for nonvolatile memory and neuromorphic computing applications. However, the analog resistive switching behavior of the HfO2-based memristors still needs improvement. In this work, Cu/HfO2/Pd devices with different oxide layer thicknesses were prepared by the magnetron sputtering process. The resistance switching characteristics related to oxide layer thickness were explored, revealing significant differences in the forming, SET, and RESET characteristics depending on oxide thickness. The conduction mechanism of the Cu/HfO2/Pd memristors is mainly attributed to the space charge limited current (SCLC) model. Thinner devices tend to show a more abrupt SET behavior and larger SET voltage, while thicker devices exhibit more gradual SET behavior and a lower SET voltage. In addition, as the thickness of the oxide layer increases, the Cu/HfO2/Pd memristors transition from digital resistive switching to analog resistive switching. The formation and rupture of oxygen vacancy filaments are predominant in thicker device, and the shape of the conduction filaments has been proposed as responsible for the abrupt and gradual changes in resistive switching. This work demonstrates that the thickness of the oxide layer plays an important role in engineering digital logic and neuromorphic behavior in HfO2-based memristors.

Dielectric behavior and defects of nitrogen-containing single crystal diamond films

Single crystal diamond (SCD) film, as a new substrate material, has attracted growing attention because of its low dielectric loss. The nitrogen was usually introduced into microwave plasma chemical vapor deposition (MPCVD) process in order to enhance the growth rate of SCD films. So SCD films with nitrogen were prepared by MPCVD compared with the undoped film. The impurities and defects of diamond films were characterized by Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR) and photoluminescence spectroscopy (PL). The dielectric behavior of SCD films was measured using the LCR measurement in the frequency range of 1 kHz to 110 MHz. The results indicate that the primary dielectric loss in SCD films is conductivity loss and space charge polarization below 1 MHz, while the main dielectric loss is dipole orientation polarization and electronic relaxation polarization in the frequency range of 1 MHz to 110 MHz. Point defects like substitutional N+, NV− and SiV− exert a significant influence on polarization loss. This is essential to improve the growth of SCD with excellent dielectric properties.

Improved body and interface properties of EPDM insulation for HVDC cable accessory by grafted voltage stabilizer

As a key component in HVDC cable accessory, EPDM insulation is prone to breakdown both in body and along interface. To improve the comprehensive electrical strength of EPDM, voltage stabilizer 2-propenyl-4,6-dibenzoylresorcinol (ADR) was simultaneously grafted onto macromolecules through free radical reaction on its vinyl during crosslinking of EPDM. After modification, the mechanical strength and thermal stability of EPDM are improved. In terms of body properties, the injection and rapid transfer of space charge under normal electric field are weakened, the volume conductivity is reduced, the breakdown strength is improved and the growth of electrical tree is suppressed. In terms of interface properties, the normal electric field at the EPDM/XLPE interface is weakened, hence reduced potential risk of deterioration initiated from interface. Moreover, the surface conductivity of EPDM is reduced, the charge accumulation and migration along the EPDM/XLPE interface are suppressed, and the interface breakdown strengths under different interface pressures are significantly increased under tangential electric field. Molecular simulations and UV absorption characteristics indicate that the grafted ADR molecules not only introduce charge traps into EPDM, but also absorb the high energy from trapped electrons and consume it through multiple excitations, thus preventing the electron avalanche and subsequent degradation of EPDM.

Revealing Local Grain Boundary Chemistry and Correlating it with Local Mass Transport in Mixed-Conducting Perovskite Electrodes.

Grain boundary (GB) mass transport, and chemistry exert a pronounced influence on both the performance and stability of electrodes for solid oxide electrochemical cells. Lanthanum strontium cobalt ferrite (LSCF6428) is applied as a model mixed ionic and electronic conducting (MIEC) perovskite oxide. The cation-vacancy distribution at the GBs is studied at both single and multi-grain scales using high-resolution characterization techniques and computational approaches. The accumulation of oxygen vacancies ( ) in the GB region, rather than necessarily at the GB core, results in an enhancement of the oxygen diffusivity by 3 - 4 orders of magnitude along the GBs (Dgb). At 350°C, the oxygen tracer diffusion coefficient (D*) is measured as 2.5 × 10-14 cm2s-1. The Dgb is determined to be 2.8 × 10-10 cm2s-1 assuming a crystallographic GB width (δcrystal) of 1nm, and 2.5 × 10-11 cm2s-1 using a chemically measured δchem of 11.10nm by atom probe tomography (APT). The origin of the concomitant changes in the cation composition is also investigate. In addition to the host cations, strong Na segregation is detected at all the GBs examined. Despite the low (ppm) level of this impurity, its presence can affect the space charge potential (Φ0). This, in turn, will influence the evolution of GB chemistry.

Characterization and properties of solid-state nanocomposite Acacia gum electrolytes with emphasis on diverse applications

A novel, natural, eco-friendly, solid-state Acacia gum nanocomposite electrolyte has been prepared with different weight ratios of nanoparticle ZrO2 introduced to the standard composition of 70% Acacia gum and 30% ZnSO4·7H2O. Sequential examinations, encompassing XRD, and FTIR, were conducted to evaluate the amorphous characteristics, complexation nature, and interaction of dopant nanoparticles within the acacia-salt electrolyte matrix network. The optimum ionic conductivity of nanocomposite electrolytes was found using electrochemical impedance analysis. The remarkable ionic conductivity was achieved for the electrolyte sample consisting of 10% nanoparticle ZrO2 introduced to the standard composition of 70% of Acacia gum and 30% of ZnSO4·7H2O(AZR4) at 100 °C, 6.1×10−3S/cm, at room temperature (RT) the electrolyte sample exhibited conductivity value of 1.13×10−3 S/cm. Additionally, an examination of the dielectric characteristics dependent on frequency demonstrated the existence of space charge polarization in the electrolyte and a notable tail in the low-frequency area. Through the parameters of the dielectric modulus, the research also investigated the hopping mechanism and relaxation time. Furthermore, nano-composite electrolytes were employed in the construction of solar cell devices. The outstanding results were obtained for electrolyte upon (10 wt%) nanoparticle-introduced, overall power conversion efficiency (ƞ) reached 5.38%, short-circuit photocurrent density (Jsc) 10.92 mA/cm2, open-circuit voltage (Voc) at 735 mV, and fill factor (FF) at 0.63.

A variational method for the sheath potential of hypersonic leading edges with space-charge limitations

Electron transpiration cooling for the leading edges (LE) of hypersonic aircraft utilizes thermionic emission; however, space-charge effects limit the electron emission rate, potentially diminishing the efficiency of this cooling mechanism. We develop a variational weak form of the Poisson equation that describes the sheath potential and then numerically solve it using the finite element method. This formulation has two main benefits: (1) the space-charge limit condition can be incorporated as a constraint and (2) it allows for the analysis of three-dimensional geometries with complex boundary conditions. We demonstrate that the current emitted from the surface of an LE is generally a small fraction of the Child–Langmuir limit due to space charge. We then propose several methods to enhance the emitted current from the surface and to boost the cooling effect of thermionic emission. These include increasing the plasma density, applying a negative surface potential, and using fringe fields under suitable geometric conditions. For a LaB6 emitting LE, the total emitted current is shown to be minimal and independent of the temperature of a surface with floating potential. However, when a negative potential is applied and the surface is heated, the emitted current follows the Richardson–Dushman relationship up to a critical temperature, beyond which it remains constant. At an applied surface potential of −5 V, the critical temperature is around 1700 K.

Beam energy recovery in Neutral Beam Injections and related simulations

The Neutral Beam Injectors (NBI) will be used to further increase the plasma temperature for the ignition of the fusion reactions in the ITER project. For future applications of electric energy production, the NBI efficiency is crucial. Since the gas neutralizer gives large residual ion fractions of D+ and D-, adequate energy recovery systems may be necessary. Recently a conceptually simple beam energy recovery based on space charge effects has been proposed; it can also work as an ion dump device, with advantages in removing the residual ions with reduced power dissipation. Preliminary simulation results were evidence of that the proposed device can remove all the residual ions by collecting them at very low energy on proper electrodes. In this contribution, further simulations with more accurate space charge calculations are reported to mostly confirm the high residual ion collection efficiency obtained in the preliminary simulations. The simulated operation voltage is also increased from 20 kV to 500 kV in steps. Higher efficiencies require some increase of the collector length, especially at higher operation voltage, as here discussed. Some design detailed features are also updated.

Relationship Between Prestress Breakdown and Space Charge in Micrometer-Thin Metalized Polymer Films

Relationship Between Prestress Breakdown and Space Charge in Micrometer-Thin Metalized Polymer Films

Numerical study on positive streamer in parallel-rod dielectric barrier discharge in atmospheric air

In this work, a parallel-rod dielectric barrier discharge (DBD) operating in atmospheric air is investigated through the two-dimensional plasma fluid model. The effects of applied voltage (Vp), secondary electron emission coefficient (γ), and photoionization are examined. Photoionization can significantly influence streamer dynamics by accelerating and broadening both volumetric and surface streamers and enhance the impact of the applied voltage. Without photoionization, the propagation distance of the surface streamer along the curved dielectric surface is limited to 0.1–0.2 mm under applied voltages of 8–8.5 kV. In contrast, with photoionization, this distance can extend to 0.3–0.6 mm. Achieving the same distance requires much higher voltages (10–11 kV) if without photoionization. The “double-layer” structure of the surface streamer is investigated, revealing that γ predominantly affects the surface branch with little impact on the volumetric branch. The critical charge density for streamer onset is found to be about 1018 m−3, and the volume-to-surface streamer transition is attributed to the lateral electric field provided by the space charges. This work provides insights into the regulation strategies and underlying mechanisms of streamer dynamics in parallel-rod DBDs in atmospheric air.

CSNS-II multi-harmonic LLRF upgrade and commissioning

The second phase of the China Spallation Neutron Source (CSNS-II) will employ a dual-harmonic RF system to mitigate the significant space charge effects associated with 500 kW beam power. To address this, three magnetic alloy loaded cavities, characterized by their wideband and high-gradient properties, will be integrated as multi-harmonic cavities in the upgrade project. Additionally, the current LLRF control system hardware, based on the CPCI platform, is encountering issues related to discontinuation and performance limitations. This paper presents the testing and verification of hardware upgrades and multi-harmonic control system algorithms for the magnetic alloy loaded cavity LLRF control system, ensuring the effective implementation of the multi-harmonic control algorithm within the new hardware framework.