Low Electron Density Research Articles

Charged-particle transport models for global models

This work analyses the influence of different charged-particle transport models on the global modeling of low-temperature plasmas. The simulations use the LisbOn KInetics simulation tool, calculating the charged-particle loss frequency due to transport with various formulations, categorized into two large groups: ambipolar-based and h-factor transport models. The models are applied to the description of (i) a DC discharge in oxygen (as example of an electronegative multi-ion plasma), at low to intermediate pressures, adopting a validated kinetic scheme as reference model; (ii) a microwave discharge in helium (as example of an electropositive multi-ion plasma), from low to atmospheric pressures, proposing and validating an updated kinetic scheme that bridges two previous models, adjusted separately for low to intermediate pressures and for atmospheric pressure. The analysis shows that using different charged-particle transport models can result in uncertainties of 20%–60% and 8%–115% in the discharge characteristics of oxygen and helium, respectively, with larger dispersion at low pressure and low electron density. The spreading in the results is observed also in the densities of the main plasma species, corresponding to uncertainties up to and within 50%–150% in oxygen and helium, respectively. Since transport accounts for more than of total charge losses, this mechanism should always be part of the quantitative sensitivity analysis of a kinetic scheme, considering several models with different validity domains, according to the electro-positive/-negative single-/multi-ion plasma under study and the low/high pressure conditions considered.

E–H transitions in Ar/O2 and Ar/Cl2 inductively coupled plasmas: Antenna geometry and operating conditions

Electronegative inductively coupled plasmas (ICPs) are used for conductor etching in the microelectronics industry for semiconductor fabrication. Pulsing of the antenna power and bias voltages provides additional control for optimizing plasma–surface interactions. However, pulsed ICPs are susceptible to capacitive-to-inductive mode transitions at the onset of the power pulse due to there being low electron densities at the end of the prior afterglow. The capacitive (E) to inductive (H) mode transition is sensitive to the spatial configuration of the plasma at the end of the prior afterglow, circuit (matchbox) settings, operating conditions, and reactor configurations, including antenna geometry. In this paper, we discuss results from a computational investigation of E–H transitions in pulsed ICPs sustained in Ar/Cl2 and Ar/O2 gas mixtures while varying operating conditions, including gas mixture, pulse repetition frequency, duty cycle of the power pulse, and antenna geometry. Pulsed ICPs sustained in Ar/Cl2 mixtures are prone to significant E–H transitions due to thermal dissociative attachment reactions with Cl2 during the afterglow which reduce pre-pulse electron densities. These abrupt E–H transitions launch electrostatic waves from the formation of a sheath at the boundaries of the plasma and under the antenna in particular. The smoother E–H transitions observed for Ar/O2 mixture results from the higher electron density at the start of the power pulse due to the lack of thermal electron attaching reactions to O2. Ion energy and angular distributions (IEADs) incident onto the wafer and the dielectric window under the antenna are discussed. The shape of the antenna influences the severity of the E–H transition and the IEADs, with antennas having larger surface areas facing the plasma producing larger capacitive coupling. Validation of the model is performed by comparison of computed electron densities with experimental measurements.

Ferromagnetism by p−d exchange spin-orbit coupling in n -type ferromagnetic semiconductors

The exchange interaction between carrier and localized spin in magnets generates intrinsic spin-orbit coupling induced by the magnetism. Recent studies for the exchange spin-orbit coupling (ESOC) in ferromagnets and antiferromagnets suggest the ESOC and the transport phenomena for spintronics. However, the study on the effects of ESOC on the magnetic phase transition has been still lacking especially for ferromagnets even though the ESOC should be one of the fundamental interactions to stabilize the magnetic phase. In this paper, we study the role of the ferromagnetic properties of the ESOC induced by the $p\text{\ensuremath{-}}d$ exchange interaction of the valence band in $n$-type ferromagnetic semiconductors. Based on the mean-field and $\mathbit{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbit{p}$ methods, our theory explains the spin polarization of localized and carrier spin, and the Curie temperature ${T}_{c}$ for the iron concentration, electron density, and host semiconductor dependence. The theoretical ${T}_{c}$ fits experimental values well in low iron concentration and intermediate electron density regions, and the $p\text{\ensuremath{-}}d$ exchange interaction is essential for ${T}_{c}$. Furthermore, the ${T}_{c}$ is higher for narrower gap host semiconductors to provide insights into higher ${T}_{c}$. The theoretical framework of this paper is not limited to the specific materials and will find potential materials for spintronics and the fundamental aspects.

Probing the lower ionospheric electron density modulation by the 2019 solar eclipse with VLF phase studies and ion-chemistry model

During a solar eclipse, the temporary blockage of daytime solar illumination resembles a nighttime situation up to a certain degree. This effectively modulates the lower ionosphere by reducing the ionization rate and hence electron–ion density and the VLF signal responds accordingly. This study investigates the lower ionospheric response to the Annular Solar Eclipse of December 26, 2019 (ASE19) by analyzing VLF phase modulation as observed from four different locations in India during the campaign organized by the Indian Centre for Space Physics (ICSP), Kolkata. The eclipse path crossed the four signal paths between NWC (19.8 kHz) transmitter and the receivers from time to time and produced significant deviations in the detected signal amplitudes and phases. We adopt the analysis of the phase delay mechanism to obtain the perturbed electron density fluctuations over the paths and compute changes in VLF reflection heights during the eclipse. The electron density modulation during the eclipse with an in situ theoretical chemical model using the solar obscuration at five different points over all the signal paths are also calculated and compared with the results from VLF. Both the temporal VLF responses at the receiving points and the changes in derived electron densities are found to exactly follow the obscuration profile at the point of maximum obscuration for all the paths. This article, for the first time successfully compares two distinct independent models related to lower ionosphere during a solar eclipse and demonstrates that though we lack the necessary means to directly explore the lower ionospheric dynamics, our theories are more than adequate and roughly reflect the actual scenario.

Novel borides of the boron filled β-Mn-type structure

The existence of a boron-filled β-Mn-type compound has been observed for the first time in the Pd-Cu-B system. The crystal structure of this phase, its homogeneity field and phase equilibria in the relevant compositional range have been examined by electron probe microanalysis, and single crystal and powder X-ray diffraction of alloys annealed at 650 °C for 240 h. The B-filled β-Mn-type structure has been found to exist within ∼50 at% - ∼57 at% Pd at 60–80% boron population in [Pd6] octahedral voids (B in 4a). Metal atom ordering (Mo3Al2C-type structure, Pd in 12d, Cu in 8c, B in 4a) has been proved from single crystal data in Pd3Cu2B0.8 at 80% boron population of the 4a boron atom sites. The structures of the B-filled β-Mn-type samples in the Pd-Cu-B system exhibit preferential occupation of the 12d Wyckoff position by the more electronegative element with larger atomic radius, i.e. Pd. Metallic behavior of this phase was ascertained by measurements of electrical resistivity. No superconductivity has been observed from electrical resistivity measurements down to 0.3 K. Ab initio electronic structure calculations indicate a rather low electronic density of states at the Fermi level for Pd3Cu2B, with the palladium d-states contributing almost 50% to the total density. The band structure of Pd3Cu2B exhibits multiple bands intersecting the Fermi energy.

Electronic-Based Model of the Optical Nonlinearity of Low-Electron-Density Drude Materials

Low-electron-density Drude (LEDD) materials such as indium tin oxide (ITO) are receiving considerable attention for their combination of CMOS compatibility, unique epsilon-near-zero (ENZ) behavior, and giant ultrafast nonlinear thermo-optic response. However, current understanding of the electronic and optical response of LEDD materials is limited due to the simplistic modeling that extends only the known models of noble metals without considering the interplay among the lower electron density, relatively high Debye energy, and the nonparabolic band structure. We bridge this knowledge gap and provide a complete understanding of the nonlinear electronic-thermal-optical response of LEDD materials. In particular, we rely on state-of-the-art electron dynamics modeling, as well as a time-dependent permittivity model for LEDD materials under optical pumping within the adiabatic approximation. We find the electron temperature may reach values much higher than realized before, even exceeding the Fermi temperature, in which case the effective chemical potential dramatically decreases and even becomes negative, thus, transient giving the material some characteristics of a semiconductor. We further show that the nonlinear optical response of LEDD materials originating from the changes to the real part of the permittivity is associated with changes of the population. This resolves the argument about the rise time of the permittivity, showing that it is instantaneous. In this vein, we show that referring to the LEDD permittivity as having a Kerr or ``saturable'' nonlinearity is unsuitable since its permittivity dynamics is absorptive rather than nonresonant and does not originate from population inversion. Finally, we analyze the probe-pulse dynamics and unlike previous work, we obtain a quantitative agreement with the results of recent experiments.

Electron deficiency modulates hydrogen adsorption strength of Ru single-atomic catalyst for efficient hydrogen evolution

It was found that the Na modification is beneficial to the synthesis of Ru single-atomic catalyst anchored on GONa, which was prepared by one-step hydrothermal process. Importantly, the strong metal-support interaction facilitates the electron transfer from Ru SA to GONa via d-π conjugation, thus lowering the electron density of Ru SA. Experimental results and DFT calculations confirmed that the low electron density of Ru SA can significantly weaken the absorption of H* intermediates and simultaneously accelerate the desorption of generated H2 from catalyst surface. As a result, the Ru SA/GONa displayed exceptional HER activity with an extremely low over-potential of 20 mV at 10 mA cm−2, outperforming the benchmark commercial Pt (21 mV over-potential) and Ru nanoparticles (212 mV over-potential) catalysts. When Eosin Y was employed as a light harvester, this Ru SA/GONa achieves outstanding photocatalytic hydrogen production with a record-high apparent quantum efficiency of 65.2% at 520 nm. Moreover, single-atomic Pt, Pd, Au and Rh were also successfully anchored on the Na-functionalized GO support, suggesting the universality of Na-induced single-atomic synthesis. This work not only provides an effective method for the synthesis of single-atomic metal catalysts but also establishes the connection between the electronic structures of catalyst and performances.

Local wavelength evolution and Landau damping of electrostatic plasma wave driven by an ultra-relativistic electron beam in dense inhomogeneous plasma

Evolution of an electrostatic plasma wave driven by a low-density ultra-relativistic electron beam in dense inhomogeneous plasma is considered. In particular, the wavelength variation as observed at fixed locations in the plasma is analyzed in terms of the wave characteristics. It is shown that for a negative density gradient, the observed local wavelength decreases monotonically with time, but for a positive density gradient, it first increases and then decreases with time, accompanied by reversal of the wave phase. However, in both cases the local wavelength eventually decreases with time since Landau damping becomes significant as the wavelength becomes of the order of the plasma Debye length. Results from particle-in-cell simulations agree well with theoretical analyses of the wavelength variation.

Validation of Swarm Langmuir Probes by Incoherent Scatter Radars at High Latitudes

Electron density measured at high latitudes by the Swarm satellites was compared with measurements by the incoherent scatter radars at Resolute Bay and Poker Flat. Overall, the ratio of Swarm-based electron density to that measured by the radars was about 0.5–0.6. Smaller ratios were observed at larger electron densities, usually during the daytime. At low electron densities less than 3 × 1010 m−3, the ratios were typically above 1, indicating an overestimation effect. The overestimation effect was stronger at night and for Swarm B. It was more evident at lower solar activity when the electron densities in the topside ionosphere were lower.

Experimental investigation of the radially-dependent ignition process in a pulsed capacitively coupled RF discharge: Effects of pressure, voltage and afterglow duration

The effects of the gas pressure, the voltage amplitude and the afterglow duration on the ignition process over 300 mm-diameter in a pulse-modulated capacitively coupled radio-frequency argon discharge are experimentally investigated. A home-made optical probe is used to measure the optical emission intensity (OEI) as a function of time since the beginning of each pulse at various radial positions. A voltage and a current probe are used to measure the voltage and current waveforms at the power feeding point and then the time-dependent power deposition is also obtained. It was found that the radial profile of the OEI exhibits complex evolution when changing these external conditions. At lower pressures, the ignition occurs earlier, and the radial-integral OEI and the power deposition overshoot more slightly during the ignition. By increasing the pressure, these two quantities overshoot more significantly, and the OEI gradually evolves from an edge-peaked radial profile to a center-high radial profile for a given time when the OEI increases rapidly during the ignition. When increasing the voltage amplitude, the ignition tends to occur earlier, featuring a more significant overshoot of the OEI and power deposition at a higher voltage. Compared to that at high voltage, the OEI exhibits a significant center-high radial profile at low voltage amplitude during the ignition. By increasing the afterglow duration, T off, the ignition is delayed and the overshoot becomes more significant, due to a lower initial electron density when each pulse is turned on. During the phase when the growth rate of the OEI versus time is maximum, the OEI exhibits different radial distributions, i.e., it appears a center-high profile at short T off, an edge-peaked profile at intermediate T off, and a center-high profile at relatively long T off.

A hierarchical cactus-like nanostructure as a bifunctional catalyst for overall water splitting

In this study, a hierarchical cactus-like nanostructure with cobalt doped iron disulfide (Co-FeS2) needle nanowires as trunks and nickel-iron layered double hydroxide (NiFe-LDH) nanosheets as branches was constructed as a self-supporting electrode for overall water splitting in alkaline electrolytes. A low electron density NiFe-LDH derived from the strong electronic interaction between sulfides and NiFe-LDH is remarkably effective in facilitating the adsorption of H2O and OH−, while the presence of Co atoms in FeS2 facilitates the hydrogen adsorption. Such a synergetic effect improved the HER performance in alkaline electrolytes significantly. In terms of the OER process, the Fe/Co oxyhydroxides produced from in situ phase transition from Co-FeS2 in conjunction with NiFe-LDH endow the electrode with excellent OER activity. Furthermore, the Co-FeS2 needle nanowires with metal-like conductivity allow for rapid electron transfer during electrochemical processes, and the hierarchical structure exposes substantial active sites and facilitates electrolyte penetration. In alkaline media, the electrode demonstrated outstanding HER and OER catalytic activity, requiring small overpotentials of 131 mV and 220 mV to reach current densities of 10 mA cm−2 and 20 mA cm−2, respectively. Moreover, it only needed 1.58 V to drive 10 mA cm−2 when utilized simultaneously as both cathode and anode for overall water splitting.

A novel embedding composition for the evaluation of the internal structure of carbon materials using electron microscopy.

The image contrast obtained in electron microscopy depends on the atomic number of the sample. Therefore, obtaining a clear contrast is challenging when samples composed of light elements (carbon materials and polymers) are embedded in the resin. Herein, a newly developed embedding composition exhibiting low viscosity and high electron density is reported, which can be solidified using physical or chemical methods. When used for carbon materials, this embedding composition allows clear microscopic observation with higher contrast compared to conventional resin embedding. Furthermore, details of the observation of samples such as graphite and carbon black using this embedding composition are reported.

Electron density of a cyclic tetrasaccharide composed of benzoylated galactose units

Abstract The electron density distribution (EDD) of a cyclic tetrasaccharide composed of four benzoylated galactopyranosyl units was calculated from refinement with scattering factors from the invariom library based on X-ray diffraction data downloaded from the Crystal Structural Data Base (CSD). Bond topological and atomic properties have been derived. Benzoyl substituents cause weak intermolecular interactions. These are also visible from low electron density concentrations on the Hirshfeld surface and have an influence on a low X-ray density compared to that of non-substituted carbohydrates, where hydrogen bonds allow a 20% more dense packing. On the electrostatic potential (ESP) surface, a strong polarization between the positive outer belt of 12 benzoyl groups and a negative core region composed of the four galactose sugar molecules exists. The positive benzoyl exterior establishes a repulsive wall between adjacent molecules in the structure.

Low electron density channel revealed in the plasma layer of hypersonic vehicle under a novel magnetic control

The “communication blackout” occurs during the reentry of hypersonic vehicles, that is, the electromagnetic wave cannot pass through the plasma sheath due to high electron density formed around the vehicle, which has drawn much attention but has not yet been solved. In this paper, a novel magnetic control approach is proposed, in which a periodic magnetic field is applied near the head of the aircraft. Based on the magnetohydrodynamic simulation, a low electron density channel across the plasma sheath of vehicle under the magnetic control is revealed for the first time, which probably make Ka-band electromagnetic wave pass through. We also find that the reason for the low electron density channel is the variation of the vortex structure in the plasma flow near the wall, by the electromagnetic force exerted on the flow. Furthermore, the optimal magnetic field frequency for generating the low electron density channel is obtained. These findings will be of significant benefit to the solutions for the communication blackout problems.

Compensation of plasma fluctuation induced shadowing effects in gas puff imaging data

For gas puff imaging (GPI) studies of turbulent processes in the plasma edge, it is often assumed that the emitted light is mainly proportional to the local electron density. This, however, only holds for low electron densities in the far scrape-off layer. As GPI typically only detects one emission line, data interpretation further inside the plasma (i.e. around the separatrix) is complicated by two effects. First, there is the strongly non-linear connection between plasma temperature and density and the measured line emission. This can lead to incorrect data interpretation such as an artificial doubling of the frequency of periodic plasma perturbations. Second, a shadowing effect in the neutral distribution can hamper the localization of turbulent structures and modes. For this study thermal helium beam data of an ASDEX Upgrade EDA H-mode scenario are analyzed. By comparing these measurements to a synthetic diagnostic, we demonstrate the presence of diagnostic effects and illustrate their origins. It is shown that when line intensity ratios are used for the analysis, instead of emission from individual lines, the underlying plasma dynamics can be well recovered. • Gas puff imaging techniques deliver detailed information of turbulent structures and filaments at the plasma edge region. • The analysis of emission data requires detailed information of the underlying atomic processes to prevent false interpretation of diagnostic effects. • This is demonstrated by comparing a synthetic helium beam diagnostic with measurement data. • It is shown that by analyzing ratios of two emission lines, diagnostic effects as shadowing or frequency doubling can be prevented. • This allows the diagnostic to resolve fast modes at the plasma edge region as demonstrated in an EDA H-mode scenario.

Visualization of oscillatory electron dynamics on the surface of liquid helium

We investigate the time traces of currents induced in five segmented electrodes by the motion of electrons on the surface of liquid $^{4}\mathrm{He}$ at $\ensuremath{\sim}0.3$ K, that are placed in a perpendicular magnetic field and exposed to microwave radiation. Nonlinear dynamics methods are utilized to explore the characteristic features of the current oscillations in the electrodes for different electron densities and pressing voltages. The wavelet phase coherence and phase shift are calculated to obtain the coherence relationships between the signals in all five electrodes as functions of the pressing voltage. Coupling analysis of the ridge-extracted instantaneous frequencies revealed the directions of motion of electrons inside the cell and provided evidence of strong phase coupling at a pressing voltage of 4.20 V. These classical methods reveal that the motion is oscillatory, with a varying frequency subject to a constant frequency modulation. High harmonics due to nonlinearity arise at higher frequencies where the resonance condition is satisfied at a pressing voltage of 4.20 V for low-electron density. Our approach provides a platform for investigating these phenomena analytically. We show that slow gravity waves on the helium surface modulate the electronic oscillatory behavior and illustrate that the model in fact produces three-dimensional dynamics. Motion of electrons on the surface of liquid helium is shown to be a paradigmatic example of a chronotaxic system, i.e., a system that undergoes continuous perturbation but is nonetheless capable of maintaining its stability.

Discovery of optical emission associated with the supernova remnant G107.5−1.5

ABSTRACT We present the first results from an imaging and a spectroscopic survey of the optical emission associated with supernova remnant (SNR) G107.5−1.5. We discovered optical diffuse and filamentary emission from G107.5−1.5 using the 1.5- and 1-m telescopes. The optical emissions from the North-East (NE) and North-West (NW) regions show the diffuse structure, while the South-East (SE) and East (E) regions show filamentary structure. From long-slit spectra, we found [S ii]/H α > 0.5 for the SE and E regions, which supports the origin of the emission being from shock-heated gas. The average [S ii]/H α ratio is found to be ∼0.4 and ∼0.3 for the NW and NE regions, respectively, indicating that the optical emission is coming from ionized gas in an H ii region. From the ratios of [S ii]λλ 6716/6731, we estimate an average electron density of ∼2400 cm−3 for the NW region, which can be attributed to the dense ionized gas. The average [S ii]λλ 6716/6731 ratios are ∼1.25 and ∼1.15 for the SE and E regions, respectively, which are indicative of low-electron density.

Evidence of the Coulomb gap in the density of states of MoS2

${\mathrm{MoS}}_{2}$ is an emergent van der Waals material that shows promising prospects in semiconductor industry and optoelectronic applications. However, its electronic properties are not yet fully understood. In particular, the nature of the insulating state at low carrier density deserves further investigation, as it is important for fundamental research and applications. In this study we investigate the insulating state of a dual-gated exfoliated bilayer ${\mathrm{MoS}}_{2}$ field-effect transistor by performing magnetotransport experiments. We observe a positive and nonsaturating magnetoresistance, in a regime where only one band contributes to electron transport. At low electron density ($\ensuremath{\sim}1.4\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$) and a perpendicular magnetic field of 7 Tesla the resistance exceeds by more than one order of magnitude the zero field resistance and exponentially drops with increasing temperature. We attribute this observation to strong electron localization. Both temperature and magnetic field dependence can, at least qualitatively, be described by the Efros-Shklovskii law, predicting the formation of a Coulomb gap in the density of states due to Coulomb interactions. However, the localization length obtained from fitting the temperature dependence exceeds by more than one order of magnitude the one obtained from the magnetic field dependence. We attribute this discrepancy to the presence of a nearby metallic gate, which provides electrostatic screening and thus reduces long-range Coulomb interactions. The result of our study suggests that the insulating state of ${\mathrm{MoS}}_{2}$ originates from a combination of disorder-driven electron localization and Coulomb interactions.

High-frequency phonons drive large phonon-drag thermopower in semiconductors at high carrier density

It has been well established that (i) the thermopower of semiconductors can be enhanced through a phenomenon known as the drag effect, and (ii) the drag enhancement involves only low-frequency acoustic phonons and benefits from low electron densities and low temperatures. Using first-principles calculations we show that large drag enhancements to the thermopower are possible at high carrier density even at room temperature and arise from high-frequency acoustic phonons. A fascinating example is cubic boron arsenide (BAs) for which the calculated room temperature drag enhancement of the thermopower exceeds an order of magnitude at a high hole density of ${10}^{21}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$. This remarkable behavior stems from the simultaneously weak phonon-phonon and phonon-hole scattering of the high-frequency phonons in BAs that become drag active at high carrier densities through electron-phonon interactions. This work advances our understanding of coupled electron-phonon nanoscale transport and introduces an unexpected paradigm for achieving large thermopowers.

Trends in runaway electron plateau partial recombination by massive H2 or D2 injection in DIII-D and JET and first extrapolations to ITER and SPARC

Experimental trends in thermal plasma partial recombination resulting from massive injection into high-Z (Ar) containing runaway electron (RE) plateaus in DIII-D and JET are studied for the purpose of achieving sufficiently low electron density () to increase RE final loss MHD levels. In both DIII-D and JET, thermal electron density is found to drop by ∼100 when the thermal plasma partially recombines, with a minimum at a vacuum vessel-averaged density in the range . RE effective resistivity also drops after partial recombination, indicating expulsion of the Ar content. The level after partial recombination is found to increase as RE current is increased. The amount of initial Ar in the RE plateau is not observed to have a strong effect on partial recombination. Partial recombination timescales of order 5 ms in DIII-D and 15 ms in JET are observed. These basic trends and timescales are matched with a 1D diffusion model, which is then used to extrapolate to ITER and SPARC tokamaks. Within the approximations of this model, it is predicted that ITER will be able to achieve sufficiently low values on time scales faster than expected RE plateau vertical drift timescales (of order 100 ms), provided sufficient or is injected. In SPARC, it is predicted that achieving significant recombination will be challenging, due to the very high RE current density. In both ITER and SPARC, it is predicted that achieving low will be easier with Ar as a background impurity (rather than Ne).