Cold Electron Research Articles

Enhancing Extraction and Suppressing Cooling of Hot Electrons in Lead Halide Perovskites by Dipolar Surface Passivation.

Slowing hot carrier (HC) cooling and improving HC extraction are considered two pivotal factors for enhancing power conversion efficiency in emerging HC photovoltaic applications of perovskites and other materials. Employing ab initio quantum dynamics simulations, we demonstrate the simultaneous slow cooling and efficient extraction of hot electrons at the C60/CsPbI3 interface through dipolar surface passivation with phenethylammonium and 4-fluorophenethylammonium ligands. The passivation effectively suppresses I-Pb lattice vibrations, weakens the hot electron-phonon interaction in CsPbI3, and thus slows down the HC cooling. At the same time, the dipolar surface passivation elevates the LUMO + 1 state in C60 and reduces the energy gap for HC extraction. Concurrently, higher-frequency vibrations of the dipolar layer enhance the coupling between C60 and CsPbI3, promoting efficient HC extraction further. These phenomena are intensified with increased polarity of the dipolar layer. Furthermore, we find that dipolar passivation has the opposite influence on cold electron collection at the band edge, underscoring the fact that the observed improvement in photovoltaic performance stems preferentially from the effective utilization of HCs rather than cold electrons. The work provides a new strategy for achieving high-performance HC perovskite solar cells.

Electron-acoustic solitary waves via suprathermal populations in Saturn's magnetosphere: Bifurcation, sensitivity, and stability analysis

Cassini and Voyager space missions observed non-thermal electron populations (with varying characteristics) in Saturn's magnetosphere, which can be correctly described using kappa distributions. Based on these observations, our objective is to inspect the evolution of electron-acoustic solitary waves (EASWs) within Saturn's magnetosphere. The propagation of weakly nonlinear (EASWs) in a collisional plasma system comprising a cold electron fluid, hot electrons following a kappa distribution, and stationary ions is investigated. By employing the reductive perturbation technique, the Korteweg–de Vries Burgers (KdV–B) equation is derived. An exact solution of the KdV–B equation, with a conformable time-derivative, is found using the unified method. It is observed that plasma current-induced collision between electrons and ions leads to remarkable dissipation, generating EASWs. Furthermore, when studying the sensitivity of the system, the appearance of a positive potential is depicted as external forces vanish, which may be due to stationary ions. Additionally, bifurcation, stability, and significant influence of plasma characteristics are considered.

Signature of conservation laws and solitary wave solution with different dynamics in Thomas–Fermi plasma: Lie theory

We propose a Lie group method to discuss the modified KP equation appearing in Thomas–Fermi (TM) plasma, characterised by cold and hot electrons. The Lie method facilitates the identification of similarity reductions, infinitesimal symmetries, group-invariant solutions, and novel analytical solutions for nonlinear models. The similarity reduction method is carried out to transform the nonlinear partial differential equations (NLPDE) into the nonlinear ordinary differential equations (NLODE). This study focuses on solitary wave profiles due to their usefulness in various engineering applications, including monitoring public transportation systems, managing coastlines, and mitigating disaster risks. It also addresses the conservation laws associated with the modified KP equation. The generalised auxiliary equation (GAEM) scheme is used to compute the new solitary wave patterns of the modified KP equation, which explains the dynamics of nonlinear waves in Thomas–Fermi plasma. The idea of nonlinear self-adjointness is used to compute the conservation laws of the examined model. The graphical behaviour of some solutions is represented by adjusting the suitable value of the parameters involved.

Research on high-strength and rapid-hardening magnesium phosphate as a novel bonding material for limestone artifacts

Due to natural erosion and human interference, limestone artifacts often suffer from cracks and gradual detachment. There is a critical need for new bonding materials that possess excellent aging resistance and high compatibility with the artifact substrates. This study investigates the effects of varying proportions of citric acid (CA) and borax (B) as composite retarders on the fundamental properties of magnesium phosphate cement (MPC). Through the application of X-ray diffraction techniques and cold field emission scanning electron microscopy, this research elucidates the mechanisms by which the retarders affect MPC. It was found that a 5 % addition of CA and B significantly reduces the hydration temperature during the early stages of MPC setting, enhances the unconfined compressive strength (reaching 44.3 MPa at 28 days), and allows for controllable setting times (17 minutes). Additionally, the presence of CA was found to delay crystal growth, resulting in an increased quantity of the amorphous phase and higher density of MPC. The optimal MPC formulation was selected as a bonding and repair material for limestone artifacts. For comparative analysis, traditional glutinous rice lime mortar (GL) and commercially available epoxy resin (EP) were tested under wet-dry cycling and UV aging conditions. Results indicated that EP materials demonstrated significant aesthetic incompatibilities and progressive cracking and detachment as aging tests progressed. GL materials exhibited significantly lower bonding strength compared to EP and MPC. In contrast, MPC displayed excellent aging resistance, moderate bonding strength, and a Shore hardness value closely matching that of the limestone substrates. Therefore, high-strength, rapid-hardening MPC represents a valuable and forward-looking solution for the conservation of limestone artifacts.

Quantum Dots and Molecular Junctions as Energy Filters for Thermal-to-Electric Energy Conversion

The thermoelectric conversion of heat into electricity requires a form of energy filtering, that allows hot electrons to pass while blocking cold electrons. The energy conversion efficiency is determined by the details of the energy filtering, with an infinitely sharp (delta function) transmission spectrum allowing near ideal (Carnot) efficiency. [1]I will introduce this concept and report on a proof-of-performance experiment based on a quantum dot embedded into a single, semiconductor nanowire. This near-ideal quantum-dot heat engine realized power production at maximum power with Curzon-Ahlborn efficiency as well more than 70% of Carnot efficiency at maximum efficiency settings [2].To further enhance efficiency at maximum power one wishes to shape the transmission function to allow a spectrum of warmer electrons to pass while still completely blocking cold electrons. This may be achieved by exploiting quantum interference phenomena to block low-energy electron transmission by destructive interference, and to thus sculpt the transmission function of quantum dots or molecular junctions.[4] In contrast to quantum dots, molecular junctions have the advantage that they can be used at room temperature and can be produced, in principle, cheaply and scalable. To systematically explore the effect of quantum interferences, we designed and synthesized a new class of porphyrins that are expected to feature destructive quantum interference in single molecule junctions with gold electrodes and may thus show high thermopower, as well as a control that does not. We performed detailed experimental single-molecule break-junction studies of conductance and thermopower on porphyrin molecular junctions. We find a somewhat better thermoelectric performance for the porphyrin where we expect destructive interference. However, the predicted large difference in conductance and thermopower is not supported by our experimental findings.[4]Finally, I will also briefly discuss the potential application of the concepts and systems presented here in hot-carrier photovoltaics, as an approach to increasing photovoltaic energy conversion efficiency beyond current limits.[1] T.E. Humphrey et al., Reversible Quantum Brownian Heat Engines for Electrons. Phys. Rev. Lett., 89, 116801 (2002)[2] M. Josefsson et al. A quantum-dot heat engine operated close to thermodynamic efficiency limits. Nature Nanotechn. 13, 920 (2018)[3] O Karlström et al. Increasing thermoelectric performance using coherent transport. Phys. Rev. B 84, 113415 (2011)[4] H. Xu et al. Electrical Conductance and Thermopower of β-Substituted Porphyrin Molecular Junctions Synthesis and Transport. JACS 145 , 23541 (2023) Figure 1

An experimental investigation on the damage mechanisms of red glutenite in the Mount Wuyi cultural and natural heritage site subject to acid rain and wet-dry cycles: a macro-to-micro approach

The safety of rock landscapes in Mount Wuyi is significantly impacted by acid rain and wet-dry cycles. In this paper, the decay characteristics of the physical–mechanical properties of red glutenite were investigated under acidic wet-dry cycles. A systematic approach, including cold field emission scanning electron microscopy (CFE-SEM), image processing techniques, and X-ray diffraction (XRD), was proposed to investigate the damage mechanism of red glutenite under acidic wet-dry cycles. The results indicate that with increasing solution acidity and wet-dry cycles, dry density (DD), longitudinal wave velocity (LWV), uniaxial compressive strength (UCS), and elastic modulus (EM) of red glutenite significantly decrease. Under different acidic conditions, DD, LWV, and UCS exhibit exponential decay with wet-dry cycles, while EM exhibits linear decay. A regression fitting was employed to establish a prediction model for UCS, which exhibited a better capability in predicting the correlation between UCS, pH, and the number of wet-dry cycles. Microscopic comprehensive analysis reveals that the interaction between rock dissolution and desiccation is the primary factor leading to changes in the microstructure and mineral composition of red glutenite, culminating in the decay of its physical–mechanical properties. This study holds significant guidance implications for the preservation of cultural and natural heritage in Mount Wuyi.

Ion-acoustic solitons in multicomponent plasma with two temperature non-Maxwellian electrons

The study of ion-acoustic solitons (IASs) in an unmagnetised collisionless plasma system having inertial fluid ions and two temperature hot as well as cold electrons obeying Cairns–Tsallis (CT) distribution is presented. Using reductive perturbation method, the KdV equation is derived. Further, introducing higher-order effects of nonlinearity and dispersion, a nonlinear KdV-type inhomogeneous equation is derived that supports the formation of dressed solitons. The combined effects of higher-order corrections and various plasma parameters are analysed on the characteristics of the different kinds of ion-acoustic solitons with implications to Saturn's magnetosphere.

Multifrequency Analysis of Favored Models for the Messier 87* Accretion Flow

The polarized images of the supermassive black hole Messier 87* (M87*) produced by the Event Horizon Telescope (EHT) provide a direct view of the near-horizon emission from a black hole accretion and jet system. The EHT theoretical analysis of the polarized M87* images compared thousands of snapshots from numerical models with a variety of spins, magnetization states, viewing inclinations, and electron energy distributions, and found a small subset consistent with the observed image. In this article, we examine two models favored by EHT analyses: a magnetically arrested disk with moderate retrograde spin and a magnetically arrested disk with high prograde spin. Both have electron distribution functions that lead to strong depolarization by cold electrons. We ray trace five snapshots from each model at 22, 43, 86, 230, 345, and 690 GHz to forecast future very long baseline interferometry (VLBI) observations and examine limitations in numerical models. We find that even at low frequencies where optical and Faraday rotation depths are large, approximately rotationally symmetric polarization persists, suggesting that shallow depths dominate the polarization signal. However, morphology and spectra suggest that the assumed thermal electron distribution is not adequate to describe emission from the jet. We find 86 GHz images show a ringlike shape determined by a combination of plasma and spacetime imprints, smaller in diameter than recent results from the Global mm-VLBI Array. We find that the photon ring becomes more apparent with increasing frequency, and is more apparent in the retrograde model, leading to large differences between models in asymmetry and polarization structure.

Oblique instability of quasi-parallel whistler waves in the presence of cold and warm electron populations

Whistler waves propagating nearly parallel to the ambient magnetic field experience a nonlinear instability due to transverse currents when the background plasma has a population of sufficiently low energy electrons. Intriguingly, this nonlinear process may generate oblique electrostatic waves, including whistlers near the resonance cone with properties resembling oblique chorus waves in the Earth’s magnetosphere. Focusing on the generation of oblique whistlers, earlier analysis of the instability is extended here to the case where low-energy background plasma consists of both a “cold” population with energy of a few eV and a “warm” electron component with energy of the order of 100 eV. This is motivated by spacecraft observations in the Earth’s magnetosphere where oblique chorus waves were shown to interact resonantly with the warm electrons. The main new results are: 1) the instability producing oblique electrostatic waves is sensitive to the shape of the electron distribution at low energies. In the whistler range of frequencies, two distinct peaks in the growth rate are typically present for the model considered: a peak associated with the warm electron population at relatively low wavenumbers and a peak associated with the cold electron population at relatively high wavenumbers; 2) overall, the instability producing oblique whistler waves near the resonance cone persists (with a reduced growth rate) even in the cases where the temperature of the cold population is relatively high, including cases where cold population is absent and only the warm population is included; 3) particle-in-cell simulations show that the instability leads to heating of the background plasma and formation of characteristic plateau and beam features in the parallel electron distribution function in the range of energies resonant with the instability. The plateau/beam features have been previously detected in spacecraft observations of oblique chorus waves. However, they have been attributed to external sources and have been proposed to be the mechanism generating oblique chorus. In the present scenario, the causality link is reversed and the instability generating oblique whistler waves is shown to be a possible mechanism for formation of the plateau and beam features.

Mixing of Hot Shocked Plasma with Cold Gas in Nova YZ Ret 2020

The origin of bright X-ray emission lines that appear late in a nova eruption remains largely a puzzle. We present two high-resolution X-ray grating spectra of the classical nova YZ Ret, observed 77 and 115 days post-eruption, using XMM-Newton and Chandra, respectively. Both spectra feature resolved emission lines blueshifted by v = –1500 km s−1 and broadened by σ v = 500 km s−1. The two spectra are well described by a collisionally ionized plasma of kT ∼ 70 eV that dimmed by a factor of ∼40 between the two exposures. The spectra also show narrow radiative recombination continua (RRCs) of C+4, C+5, and N+5, indicating the interaction of the hot ionized plasma with cold electrons of kT ∼ 2 eV. The high-n Rydberg series of C+4 is anomalously bright, allowing us to measure the electron density through continuum lowering, which is in agreement with the He-like N+5 density diagnostic of n e = (1.7 ± 0.4) × 1011 cm−3. The high population of these high-n levels constitutes the best evidence to date of charge exchange (CX) with neutral H in an astrophysical ionized plasma. The remarkable fact that the velocity and plasma temperature are the same after 38 days, despite the high density and decreasing flux, is evidence for ongoing heating. We suggest the heating is due to a reverse shock in the nova ejecta, which forms a thin X-ray shell. The narrow RRCs and CX are attributed to direct mixing with cold gas, which overtakes the hot plasma either from the shock front or through the contact discontinuity.

Electron acoustic solitary waves in relativistically quantum magnetoplasmas with electron exchange correlation effects

The propagating of electron acoustic solitary waves (EASWs) in relativistically magnetized quantum plasmas which consists inertial cold electrons, inertialess hot electrons and stationary background ions have been theoretically investigated. The modified Zakharov-Kuznetsov (ZK) equation is derived by adopting the standard reductive perturbation method to describe the nonlinear evolution of the EASWs. After discussing the effects of various parameter values on the solitary structures, it is shown that the solitary structures are very sensitive to the relativistic degeneracy effect, the cold to hot electrons number density ratio and the electron exchange correlation effects. The numerical analysis can be well described by the existing analytical theory. Our results may be useful in explaining the physics behind the formation of the EASWs in both astrophysical and laboratory plasmas.

Coupled Monte Carlo density functional theory calculations of cold and laser-assisted electron field emission from graphene-coated copper cathode

Field emission is an important process with a variety of applications. Quantitative predictions of such electron emission need to include details of the internal potentials that shape the electronic wavefunctions (and hence the tunneling probability), predictive analysis of the work function barrier (ΦB), and knowledge of the electron distribution at the surface that constitutes the supply function. Here, these various factors were all collectively considered based on a combined Monte Carlo-density functional theory approach. Results were obtained for both the field-dependent cold electron emission current density as well as photoemission from a short laser pulse. The method also allows for calculations of field-dependent emittance. The technique is general and could be extended to include plasmon–polariton modes, different thicknesses of coatings, and role of surface adsorbates and defects.

Exact solutions and bifurcations for the (3+1)-dimensional generalized KdV-ZK equation

In this paper, a class of (3+1)-dimensional generalized Korteweg–de Vries-Zakharov-Kuznetsov (KdV-ZK) equation is studied by utilizing the bifurcation theory of the planar dynamical systems and the Fan sub-function method. This model can be used to explain the effects of magnetic fields on weakly nonlinear ion-acoustic waves investigated in plasma fields composed of cold and hot electrons. Under the different parameter conditions, the phase portraits and bifurcations are derived, and new exact solutions including soliton, periodic, kink and breaking wave solutions for the model are constructed. Moreover, some exact solutions, which contain soliton, kink, trigonometric function, hyperbolic function, Jacobi elliptic function solutions, are derived via the improved Fan sub-function method. The types of solutions obtained completely correspond to the types of the orbits acquired above, which verifies the validity of the method. Finally, the physical structures of some exact solutions are analyzed in graphical forms.

Beam‐Driven Electron Cyclotron Harmonic and Electron Acoustic Waves as Seen in Particle‐In‐Cell Simulations

AbstractRecent study has demonstrated that electron cyclotron harmonic (ECH) waves can be excited by a low energy electron beam. Such waves propagate at moderately oblique wave normal angles (∼70°). The potential effects of beam‐driven ECH waves on electron dynamics in Earth's plasma sheet is not known. Using two‐dimensional Darwin particle‐in‐cell simulations with initial electron distributions that represent typical plasma conditions in the plasma sheet, we explore the excitation and saturation of such beam‐driven ECH waves. Both ECH and electron acoustic waves are excited in the simulation and propagate at oblique wave normal angles. Compared with the electron acoustic waves, ECH waves grow much faster and have more intense saturation amplitudes. Cold, stationary electrons are first accelerated by ECH waves through cyclotron resonance and then accelerated in the parallel direction by both the ECH and electron acoustic waves through Landau resonance. Beam electrons, on the other hand, are decelerated in the parallel direction and scattered to larger pitch angles. The relaxation of the electron beam and the continuous heating of the cold electrons contribute to ECH wave saturation and suppress the excitation of electron acoustic waves. When the ratio of plasma to electron cyclotron frequency ωpe/ωce increases, the ECH wave amplitude increases while the electron acoustic wave amplitude decreases. Our work reveals the importance of ECH and electron acoustic waves in reshaping sub‐thermal electron distributions and improves our understanding on the potential effects of wave‐particle interactions in trapping ionospheric electron outflows and forming anisotropic (field‐aligned) electron distributions in the plasma sheet.

Enhancing starch functionality through synergistic modification via sequential treatments with cold plasma and electron beam irradiation

The impact of dual sequential modifications using radio-frequency (RF) plasma and electron beam irradiation (EBI) on starch properties was investigated and compared with single treatments within an irradiation dose range of 5–20 kGy. Regardless of sequence, dual treatments synergistically affected starch properties, increasing acidity, solubility, and paste clarity, while decreasing rheological features with increasing irradiation dose. The molecular weight distribution was also synergistically influenced. Amylopectin distribution broadened particularly below 10 kGy. Amylose narrowed its distribution across all irradiation doses. This was due to dominating EBI-induced degradation and molecular rearrangements from RF plasma. With the highest average radiation-chemical yield (G) and degradation rate constant (k) of (2.12 ± 0.14) × 10−6 mol·J−1 and (3.43 ± 0.23) × 10−4 kGy−1, respectively, upon RF plasma pre-treatment, amylose underwent random chain scission. In comparison to single treatments, dual modification caused minor alterations in spectral characteristics and crystal short-range order structure, along with increased granule aggregation and surface irregularities. The synergistic effect was dose-dependent, significant up to 10 kGy, irrespective of treatment sequence. The highest synergistic ratio was observed when RF plasma preceded irradiation, demonstrating the superior efficiency of plasma pre-treatment in combination with EBI. This synergy has the potential to lower costs and extend starch's technological uses by enhancing radiation sensitivity and reducing the irradiation dose.

A study on dust inertial Alfvén solitary waves in a magnetized dust-ion-electron plasma using sagdeev potential method

The propagation characteristics of arbitrary amplitude Dust Inertial Alfvén Solitary Wave (DIASW) has been studied in a three-component plasma consisting of magnetized cold dust, magnetized cold ions and electrons with Boltzmann distribution. The range of Mach number where solitary waves can be formed has been established analytically. To discuss the behaviors of the DIASWs, the Sagdeev potential method is used. Both rarefactive and compressive DIASWs travelling at sub and super-Alfvénic speeds are generated is this plasma model.

Exploring travelling wave solutions, bifurcation, chaos, and sensitivity analysis in the (3+1)-dimensional gKdV-ZK model: A comprehensive study using Lie symmetry methodology

This article presents a study on the generalized Korteweg-de Vries-Zakharov-Kuznetsov (gKdV-ZK) model, which is a nonlinear system that demonstrates the effect of magnetic fields on weak ion-acoustic waves in plasma consisting of cold and hot electrons. The research entails investigating the reduction of symmetry through Lie group analysis, scrutinizing the characteristics of the dynamic structure using bifurcation phase diagrams, and examining the dynamic behaviour of the perturbed dynamical system employing chaos theory. Methods such as 3D and 2D phase portraits, time series analysis, Poincaré maps, exploration of multistability in the autonomous structure across various initial conditions, Lyapunov exponents, and bifurcation diagrams are exercised to demonstrate chaotic behaviour. Additionally, the research establishes general forms of solitary wave solutions, encompassing hyperbolic, trigonometric, and rational soliton solutions, through the utilization of a modified auxiliary equation approach to analytically address the examined problem. These findings are visually depicted as 2D and 3D graphs with carefully selected parameters, accompanied by their corresponding constraint conditions. Furthermore, the sensitivity analysis of the studied equation is deliberated upon and visually illustrated. The uncovered findings are captivating, innovative, and potentially beneficial for comprehending various physical phenomena in engineering and science.

Permanent helical micro-undulators for x-ray free electron lasers

Helical Halbach undulators, formed by four alternately axially and radially magnetized rare-earth helices, produce a stronger field than two planar Halbach undulators with perpendicular polarization. Hybrid undulators of two longitudinally premagnetized rare-earth helices and two initially unmagnetized steel (or vanadium permendur) helices produce an equal or higher field and are easier to implement. We made sure that the required helices with a period of down to 3 mm or even less could be manufactured with high precision using Wire Electric Discharge Machining. According to the simulation results, when using identical electron bunches with moderate initial spreads in energy and angles, the gain in x-ray free electron laser (XFEL) power from replacing planar micro-undulators with the helical micro-undulators under consideration can be significantly greater than predicted by the simplest 1D theory with a cold electron beam. This makes such micro-undulators promising for creating compact XFELs of various types, both based on modern linear accelerators and plasma wakefield accelerators.

Direct observations of X-rays produced by upward positive lightning

X-rays have been observed in natural downward cloud-to-ground lightning for over 20 years and in rocket-triggered lightning for slightly less. In both cases, this energetic radiation has been detected during the stepped and dart leader phases of downward negative flashes. More recently, X-rays have also been reported during the dart leader phase of upward negative flashes. In this study, we present the observations of four upward positive lightning flashes from the Säntis Tower (2.5 km ASL) in Switzerland. These consist of the simultaneous records of electric current passing through the tower, and electric field strength and X-ray flux 20 m from the tower base. One of the flashes was captured by a high-speed camera operating at 24,000 frames per second, stills from which are also presented. We detected X-rays during the initial phase of upward negative leader propagation, which can be associated with the leader-stepping process from electric field and current waveforms. To the best of our knowledge, this is the first time that such measurements are reported in the literature. The obtained time-synchronised data confirm that the X-ray emissions detected are associated with the initial steps of the upward negative leader. The frequency and energy of X-ray pulses appear to decrease as functions of time, with pulses disappearing altogether within the first millisecond of the leader initiation. X-ray emission also appears to be correlated with the maximum current-derivative and the electric field change of leader steps, consistent with cold electron runaway. These observations contribute to improving our understanding of upward lightning, which is a primary source of damage to tall structures such as wind turbines and telecommunications towers, as well as aircraft during takeoff and landing.

Effect of beam and gate offset size on x-ray focal spot resolution of a cold cathode electron beam

In x-ray imaging, high resolution is essential, particularly in sectors such as medical and industries where the need for nondestructive defect detection is required. Previous research has shown that altering beam design and the number of gates offset holes has an impact on focal spot size (FSS). However, the specific effects of beam size and offset size were not thoroughly assessed. In the present study, the influence of beam size and gate offset size was evaluated by utilizing a cold cathode electron beam. Various beam sizes were employed to achieve a small FSS, and subsequently, the smallest beam was utilized to examine the impact of gate offset size. In doing so, the smallest FSS of 0.25 mm vertical and 0.33 mm horizontal was attained without the utilization of any additional focusing lens. This illustrates that by adjusting the beam size and gate offset size, it is possible to attain a small FSS, facilitating the development of an economically viable x-ray imaging beam.