Isotropic Electron Research Articles

Fullerene-hybridized Fused-ring Electron Acceptor with High Dielectric Constant and Isotropic Charge Transport for Organic Solar Cells.

A novel isotropic fullerene-hybridized fused-ring electron acceptor, designated C60-Y, has been synthesized via a mild [4+2] Diels-Alder cycloaddition reaction with fullerene C60 to enhance the performance of organic solar cells (OSCs). Comparative analysis shows that C60-Y significantly outperforms the control acceptor Me-Y, with a notable increase in the relative dielectric constant from 2.79 to 3.95. This improvement enhances exciton dissociation and reduces non-radiative energy losses. Additionally, the isotropic molecular packing of C60-Y, similar to fullerene, facilitates efficient interface formation with donor polymers and improves charge mobility. As a result, incorporating C60-Y as an electron acceptor increases the power conversion efficiency (PCE) of binary OSCs to 15.02%, surpassing the 13.31% achieved with Me-Y. Moreover, when integrated into a ternary blend system, an impressive PCE of 19.22% is achieved, top-performing among reported ternary OSCs utilizing fullerene derivatives as the third component. These results suggest that fullerene-hybridized acceptors like C60-Y hold great potential for advancing high-efficiency OSCs by enhancing exciton dissociation, reducing energy losses, and improving charge mobility.

Optimization of the Electron Spectrometer Telescope geometry for the PRESET satellite through Monte Carlo simulation

The Electron Spectrometer Telescope (EST) aboard the PRESET satellite mission aims to measure the pitch angle distribution of 0.3–4 MeV electrons in the outer Van Allen belts. The PRESET satellite is planned to launch into a sun-synchronous low Earth orbit in Q2, 2026. We present comprehensive Monte Carlo simulations to optimize the design and response of the EST instrument. The EST consists of a stack of silicon strip detectors and a collimator to take pitch-angle dependent electron spectral measurements with a target angular resolution of 6° while rejecting the proton, heavy ion and gamma detection events. Various collimator designs and detector stacking configurations are investigated using the Geant4 code to deduce an optimal design and configuration. For validation of the Geant4 simulations, a benchmark test spectrometer was set up using a stack of four silicon detectors and a good agreement between the measured and simulated responses was found for a beta spectrum from a 90Sr/90Y source. The collimator design was optimized by adjusting total length, aperture size, number of view-ports and collimator materials. The optimum aperture size and the number of view-ports were determined by varying them and selecting the best cases that meet the angular resolution and the geometric factor requirements. Moreover, to address the under-utilization of the front position-sensitive detector encountered in a typical pin-hole collimator, two offset collimators with tilted axes were employed. Extensive simulations were carried out by varying the number of silicon detectors and the thickness of each detector to optimize the configuration of the silicon detector stack. An optimum thickness of the front detector was found to be 140 µm, which can minimize the perturbation counts caused by the proton detection events. For the other detectors, a stack of four 1.5 mm thick detectors was chosen to achieve a good sensitivity to low energy electrons at a tolerable complexity of the system. The angular response simulated for the optimum design of the EST showed a resolution of 5.5°, which is slightly better than the target resolution of 6°. The response matrices of the final design simulated for isotropic electron and proton fields showed a high geometric factor, which is expected to produce an average electron counting rate of 230 cps within the trapped region with a negligible contamination of the electron spectrum by protons except for a mild contamination by the 1.0–1.1 MeV protons.

Picturing Global Substorm Dynamics in the Magnetotail Using Low‐Altitude ELFIN Measurements and Data Mining‐Based Magnetic Field Reconstructions

AbstractA global reconfiguration of the magnetotail characterizes substorms. Current sheet thinning, intensification, and magnetic field stretching are defining features of the substorm growth phase and their spatial distributions control the timing and location of substorm onset. Presently, sparse in‐situ observations cannot resolve these distributions. A promising approach is to use new substorm magnetic field reconstruction methods based on data mining, termed SST19. Here we compare the SST19 reconstructions to low‐altitude electron losses and fields investigation (ELFIN) measurements of energetic particle precipitations to probe the radial profile of the equatorial magnetic field curvature during a 19 August 2022 substorm. ELFIN and SST19 yield a consistent dynamical picture of the magnetotail during the growth phase and capture its key features such as the formation of a thin current sheet and its earthward motion. Furthermore, they resolve a “checkmark” pattern of isotropic electron precipitation boundaries in the time‐energy plane, consistent with earlier observations but now over a broad energy range. It is shown that in the growth phase, the mismatch between SST19 and ELFIN latitudes is much less than one degree, the capability unattainable for any other empirical or first‐principles model.

Probing carrier transport of sphalerite Cadmium Chalcogenide (CdX, X=S, Se) from first-principles

Cadmium chalcogenide (CdX, X=S, Se) traps photons in the visible and near-infrared regions, and it has been widely used in optoelectronic devices such as solar cells and light-emitting diodes. The carrier concentration and intrinsic mobility are prerequisites for the optimized design of optoelectronic devices, while the underlying physics that determines the carrier mobility of sphalerite CdX remains elusive. Herein, we explore the temperature-dependent carrier transport properties and scattering mechanisms of sphalerite CdX from first-principles. The calculation results indicate that the isotropic electron mobility for CdS and CdSe are 180.2 cm2/Vs and 656.8 cm2/Vs at room temperature, respectively. An in-depth analysis reveals that polar optical phonon (POP) scattering is the main scattering mechanism limiting the mobility of sphalerite CdX, and electrons are mainly affected by long-range longitudinal optical phonon scattering. The lower electron mobility of CdS compared to CdSe is attributed to the larger Fröhlich coupling constant, -larger Pauling ionicity, and stronger electron-phonon interactions. As the temperature rises, the electron mobility of CdX consistently decreases due to the enhancement of POP scattering and lattice vibrations. The electron mobility of CdX at high doping concentrations decreases dramatically, attributing to the enhancement of ionized impurity scattering i.e. collisional scattering of electrons and ionized impurity ions. This work provides a systematic study of the electron transport properties of sphalerite cadmium chalcogenide from first-principles, which provides a theoretical guideline for the functional regulation and improvement of optoelectronic devices.

“Cosmic alternator” for the onset of large-scale magnetic fields

Magnetic field configurations extending over macroscopic scale distances are shown to be generated in rarefied collisionless plasmas when non-thermal and spatially inhomogeneous electron distributions in phase space emerge. The analyzed representative case is where, in the presence of a significant spatial gradient of the electron pressure and of a sheared magnetic field configuration, an alternating magnetic field reconnection process can develop associated with the anisotropy of the fluctuating electron temperature. The relevant process is intrinsically different from that known as the “Biermann Battery,” which does not involve magnetic reconnection and requires that the unperturbed spatial gradient of the (isotropic) electron temperature be misaligned relative to that of the density gradient.

Doping dependence and multichannel mediators of superconductivity: calculations for a cuprate model

We study two aspects of the superconductivity in a cuprate model system, its doping dependence and the influence of competing pairing mediators. We first include electron–phonon interactions beyond Migdal’s approximation and solve self-consistently, as a function of doping and for an isotropic electron–phonon coupling, the full-bandwidth, anisotropic vertex-corrected Eliashberg equations under a non-interacting state approximation for the vertex correction. Our results show that such pairing interaction supports the experimentally observed dx2−y2 -wave symmetry of the superconducting gap, but only in a narrow doping interval of the hole-doped system. Depending on the coupling strength, we obtain realistic values for the gap magnitude and superconducting critical temperature Tc close to optimal doping, rendering the electron–phonon mechanism an important candidate for mediating superconductivity in this model system. Second, for a doping near optimal hole doping, we study multichannel superconductivity, by including both vertex-corrected electron–phonon interaction and spin and charge fluctuations as pairing mechanisms. We find that both mechanisms cooperate to support an unconventional d-wave symmetry of the order parameter, yet the electron–phonon interaction is mainly responsible for the Cooper pairing and high critical temperature Tc . Spin fluctuations are found to have a suppressing effect on the gap magnitude and critical temperature due to their repulsive interaction at small coupling wave vectors.

Application of a deep learning method for shower axis reconstruction in a 3D imaging calorimeter

We present a deep learning method based on Convolutional Neural Networks (CNN) to reconstruct the shower axis of isotropic electrons in a three-dimensional (3D) imaging calorimeter, which is one of the core instruments of the High Energy cosmic-Radiation Detection (HERD) payload. The CNN method is evaluated against the Principal Component Analysis (PCA) method, utilizing isotropic Monte Carlo (MC) simulation data covering an energy spectrum from 10 GeV to 1000 GeV, as well as beam test data ranging from 50 GeV to 250 GeV. The comparative analysis results indicate that the CNN outperforms the PCA by about 40% at 100 GeV in terms of angular resolution under isotropic conditions. Both methods are further validated using beam test data. The results demonstrate the effectiveness of the CNN method in reconstructing the shower axis, and they underscore the potential of incorporating advanced deep learning techniques into the comprehensive task of calorimeter reconstruction.

Using a multistate mapping approach to surface hopping to predict the ultrafast electron diffraction signal of gas-phase cyclobutanone.

Using the recently developed multistate mapping approach to surface hopping (multistate MASH) method combined with SA(3)-CASSCF(12,12)/aug-cc-pVDZ electronic structure calculations, the gas-phase isotropic ultrafast electron diffraction (UED) of cyclobutanone is predicted and analyzed. After excitation into the n-3s Rydberg state (S2), cyclobutanone can relax through two S2/S1 conical intersections, one characterized by compression of the CO bond and the other by dissociation of the α-CC bond. Subsequent transfer into the ground state (S0) is then achieved via two additional S1/S0 conical intersections that lead to three reaction pathways: α ring-opening, ethene/ketene production, and CO liberation. The isotropic gas-phase UED signal is predicted from the multistate MASH simulations, allowing for a direct comparison to the experimental data. This work, which is a contribution to the cyclobutanone prediction challenge, facilitates the identification of the main photoproducts in the UED signal and thereby emphasizes the importance of dynamics simulations for the interpretation of ultrafast experiments.

Robust in-plane ferroelectricity, high hole mobility, and low thermal conductivity in GeO monolayer: A first-principles study

Motivated by the experimental advancements in 2D crystalline germanium oxides, we investigated the electronic and transport properties of GeO and GeO2 monolayers (MLs) using first-principles calculations. GeO ML exhibits an in-plane ferroelectricity until the melting point of 1100 K. Compressive strain facilitates polarization reversion by lowering the ferroelectric transition barrier. The band edges meet the requirements for photocatalytic water splitting across a wide variety of strains. Meanwhile, GeO ML has a high hole mobility of 4854 cm2/V⋅s along the y-axis, owing to its low deformation potential constant. The large difference in hole and electron mobility promotes electron–hole separation. In addition, GeO ML has a low thermal conductivity of κx = 3.37 W/mK and κy = 12.53 W/mK at 300 K, due to the strong anharmonicity caused by lone-pair electrons. In contrast, GeO2 ML has an isotropic electron mobility of 382 cm2/V⋅s and an κ of 22.60 W/mK at 300 K. At last, we discussed the probable reaction to grow 2D GeO crystal and calculated the Raman intensity to distinguish it in future experiments. Our results show that 2D GeO has potential applications in ferroelectrics, thermoelectrics, and water splitting.

Biaxial strain effects on electronic, transport, and thermoelectric properties of SnX[formula omitted] (X = Se, Te) and Janus SnSeTe 1T-monolayers

Biaxial strain in two-dimensional materials plays a crucial role in degenerating the valence and conduction bands, leading to energy dispersion in the band structure and causing changes in transport properties, such as carrier mobility, Seebeck coefficient, and electrical conductivity. Herein, we investigated the effects of biaxial strain on SnX2 (X = Se, Te) and the Janus SnSeTe 1T-monolayer using density functional theory, deformation potential, and semiclassical Boltzmann transport theory. Our findings reveal that the studied 1T-monolayers exhibit high and directionally isotropic electron mobility. Biaxial tensile strain has the effect of increasing the bandgap, predominantly reducing the effective mass of electrons while increasing that of holes. This results in an enhanced electron mobility along with a simultaneous reduction or increase in the concentration of electron carriers or holes, respectively. Especifically, in the case of the Janus SnSeTe 1T-monolayer, we observed a remarkable 68% increase in electron mobility, reaching a value of 1588 cm2V−1s−1. This increase contributes to higher thermoelectric performance due to elevated electrical conductivity and a simultaneous rise in the Seebeck coefficient when subjected to biaxial strain. Our study underscores that strain engineering is an effective strategy for achieving improved thermoelectric properties, particularly exemplified by the SnSeTe 1T-monolayer, which achieved a maximum value of 2.25 for n-type due the ultralow thermal conductivity of 0.359 Wm−1K−1 as result of strong phonon anharmonicity on acoustical and optical modes.

Floquet Weyl states at one-photon resonance: An origin of nonperturbative optical responses in three-dimensional materials

Electrons driven coherently by laser light can exhibit nonperturbative geometric effects. Drastic deformation and gap openings of the electrons' Floquet bands occur at one-photon resonances since the electron and hole bands hybridize through their replicas at the lowest-order photon exchange. We study the evolution of Floquet bands in three-dimensional (3D) materials driven by circularly polarized light (CPL) using the Dirac model. We find that the light-induced gap closes at a select few points in the momentum space where Floquet Weyl points are formed. The Weyl points are protected by their monopole charge and can merge, separate, or pair annihilate depending on the anisotropy of electrons and the ellipticity of the incident light. In isotropic 3D Dirac electrons driven by CPL, the Weyl points merge to form Floquet double-Weyl points with topological charge ±2. Our results reveal a universal aspect of light-matter interactions in 3D quantum materials and open a route towards controllable versatile electromagnetic responses associated with light-induced Floquet Weyl states. Published by the American Physical Society 2024

Simulating Compressive Stream Interaction Regions during Parker Solar Probe’s First Perihelion Using Stream-aligned Magnetohydrodynamics

We used the stream-aligned magnetohydrodynamics (SA-MHD) model to simulate Carrington rotation 2210, which contains Parker Solar Probe’s (PSP) first perihelion at 36.5 R ⊙ on 2018 November 6, to provide context to the in situ PSP observations by FIELDS and SWEAP. The SA-MHD model aligns the magnetic field with the velocity vector at each point, thereby allowing for clear connectivity between the spacecraft and the source regions on the Sun, without unphysical magnetic field structures. During this Carrington rotation, two stream interaction regions (SIRs) form, due to the deep solar minimum. We include the energy partitioning of the parallel and perpendicular ions and the isotropic electrons to investigate the temperature anisotropy through the compression regions to better understand the wave energy amplification and proton thermal energy partitioning in a global context. Overall, we found good agreement in all in situ plasma parameters between the SA-MHD results and the observations at PSP, STEREO-A, and Earth, including at PSP’s perihelion and through the compression region of the SIRs. In the typical solar wind, the parallel proton temperature is preferentially heated, except in the SIR, where there is an enhancement in the perpendicular proton temperature. This is further showcased in the ion cyclotron relaxation time, which shows a distinct decrease through the SIR compression regions. This work demonstrates the success of the Alfvén wave turbulence theory for predicting interplanetary magnetic turbulence levels, while self-consistently reproducing solar wind speeds, densities, and overall temperatures, including at small heliocentric distances and through SIR compression regions.

Suprathermal Electron Transport and Electron Beam Formation in the Solar Corona

Electron beams that are commonly observed in the corona were discovered to be associated with solar flares. These “coronal” electron beams are found ≥300 Mm above the acceleration region and have velocities ranging from 0.1c up to 0.6c. However, the mechanism for producing these beams remains unclear. In this paper, we use kinetic transport theory to investigate how isotropic suprathermal energetic electrons escaping from the acceleration region of flares are transported upwardly along the magnetic field lines of flares to develop coronal electron beams. We find that magnetic focusing can suppress the diffusion of Coulomb collisions and background turbulence and sharply collimate the suprathermal electron distribution into beams with the observed velocity within the observed distance. A higher bulk velocity is produced if energetic electrons have harder energy spectra or travel along a more rapidly expanding coronal magnetic field. By modeling the observed velocity and location distributions of coronal electron beams, we predict that the temperature of acceleration regions ranges from 5 × 106 to 2 × 107 K. Our model also indicates that the acceleration region may have a boundary where the temperature abruptly decreases so that the electron beam velocity can become more than triple (even up to 10 times) the background thermal velocity and produce the coronal type III radio bursts.

Multipole rate coefficients for collisional excitation of Fe XIII by isotropic electrons

Multipole rate coefficients for collisional excitation of Fe XIII by isotropic electrons

GEANT4-DNA simulation of temperature-dependent and pH-dependent yields of chemical radiolytic species

Objective. GEANT4-DNA can simulate radiation chemical yield (G-value) for radiolytic species such as the hydrated electron () with the independent reaction times (IRT) method, however, only at room temperature and neutral pH. This work aims to modify the GEANT4-DNA source code to enable the calculation of G-values for radiolytic species at different temperatures and pH values. Approach. In the GEANT4-DNA source code, values of chemical parameters such as reaction rate constant, diffusion coefficient, Onsager radius, and water density were replaced by corresponding temperature-dependent polynomials. The initial concentration of hydrogen ion (H+)/hydronium ion (H3O+) was scaled for a desired pH using the relationship pH = –log10 [H+]. To validate our modifications, two sets of simulations were performed. (A) A water cube with 1.0 km sides and a pH of 7 was irradiated with an isotropic electron source of 1 MeV. The end time was 1 μs. The temperatures varied from 25 °C to 150 °C. (B) The same setup as (A) was used, however, the temperature was set to 25 °C while the pH varied from 5 to 9. The results were compared with published experimental and simulated work. Main results. The IRT method in GEANT4-DNA was successfully modified to simulate G-values for radiolytic species at different temperatures and pH values. Our temperature-dependent results agreed with experimental data within 0.64%–9.79%, and with simulated data within 3.52%–12.47%. The pH-dependent results agreed well with experimental data within 0.52% to 3.19% except at a pH of 5 (15.99%) and with simulated data within 4.40%–5.53%. The uncertainties were below ±0.20%. Overall our results agreed better with experimental than simulation data. Significance. Modifications in the GEANT4-DNA code enabled the calculation of G-values for radiolytic species at different temperatures and pH values.

Electron Dynamics in the Electron Current Sheet During Strong Guide‐Field Reconnection

AbstractIn this study, we investigate detailed electron dynamics in strong guide‐field reconnection (the normalized guide field is ∼1.5). This reconnection event is observed by the Magnetospheric Multiscale (MMS) spacecraft at the center of a flux rope in the magnetotail. With the presence of a large parallel electric field (E‖) in the electron current sheet, electrons are accelerated when streaming into this E‖ region from one direction, and decelerated from the other direction. Some decelerated electrons can reduce the parallel speed to ∼0 to form relatively isotropic electron distributions at one side of the electron current sheet, as the estimated acceleration potential satisfies the relation eΦ‖ ≥ kTe,‖, where Te,‖ is the electron temperature parallel to the magnetic field. Therefore, a large E‖ is generated to balance the parallel electron pressure gradient across the electron current sheet, since electrons at the other side of the current sheet are still anisotropic. Based on these observations, we further show that the electron beta is an important parameter in guide‐field reconnection, providing a new perspective to solve the large parallel electric field puzzle in guide‐field reconnection.

Theoretically Calculated Properties of a MOS Device on Gallium Oxide

The properties of a MOS device on (100) oriented β-Ga2O3 semiconductor are determined theoretically utilizing the concept of physics that the carrier effective masses in materials are related to the intrinsic Fermi energy levels in materials by the universal mass-energy equivalence equation given as dE/E = dm/m, where E is the energy and m is the free electron mass. The known parameters of isotropic electron effective mass of 0.27m and the bandgap of 4.8 eV for β-Ga2O3 semiconductor are utilized to determine the properties of the MOS device along with the parameter of threshold for electron heating in SiO2 as 2 MV/cm-eV.

Multi-dimensional incoherent Thomson scattering system in PHAse Space MApping (PHASMA) facility.

A multi-dimensional incoherent Thomson scattering diagnostic system capable of measuring electron temperature anisotropies at the level of the electron velocity distribution function (EVDF) is implemented on the PHAse Space MApping facility to investigate electron energization mechanisms during magnetic reconnection. This system incorporates two injection paths (perpendicular and parallel to the axial magnetic field) and two collection paths, providing four independent EVDF measurements along four velocity space directions. For strongly magnetized electrons, a 3D EVDF comprised of two characteristic electron temperatures perpendicular and parallel to the local magnetic field line is reconstructed from the four measured EVDFs. Validation of isotropic electrons in a single magnetic flux rope and a steady-state helicon plasma is presented.

Investigation of Aluminum Arsenide Honeycomb Monolayer via Density Functional Theory

First principle calculations are performed to theoretically predict the physical properties of hexagonal aluminium arsenide planar and buckled monolayers. The structural characteristics showed that the buckled parameter is about 0.32 A°. Cohesive energies have favourable values and it indicates the fabrication possibility. Phonon dispersion properties indicated that the planar aluminium arsenic monolayers are dynamically unstable, while the buckled is less dynamically unstable. The elastic constant parameters achieved the required characteristics of stable hexagonal monolayer structures. The study of electronic band structure prefers to indirect semiconductor band gaps, and the density of states showed strong orbital hybridization in the conduction band. Planar structure has isotropic light electron effective mass and anisotropic heavy hole effective mass. The buckled structure has isotropic light electron effective mass and isotropic heavy hole effective mass. The absorption spectra have high absorption coefficient in various visible and ultraviolet wavelength. The absorption coefficient levels off at about direct and indirect band gaps.

Triggering of Whistler‐Mode Rising and Falling Tone Emissions in a Homogeneous Magnetic Field

AbstractWe perform a self‐consistent one‐dimensional electromagnetic particle simulation with a uniform magnetic field and open boundaries. The plasma environment consists of cold isotropic electrons, energetic electrons, and immobile ions. The energetic electrons are initialized with a subtracted‐Maxwellian distribution with temperature anisotropy. By oscillating external currents with a constant frequency 0.2 fce, where fce is the electron cyclotron frequency, a whistler‐mode wave is injected as a triggering wave from the center of the simulation system, and we investigated the process of interactions between the triggering wave and energetic electrons. We find that both rising‐tone and falling‐tone emissions are triggered through the formation of an electron hole and an electron hill in the velocity phase space consisting of a parallel velocity and the gyro‐phase angle of the perpendicular velocities. The rising‐tone emission varies from 0.2 fce to 0.4 fce, while the falling‐tone varies from 0.2 fce to 0.15 fce. The generation region of the rising‐tone triggered emission starts near the injection point of the triggering wave and moves upstream generating new subpackets. The generation region of the falling‐tone triggered emission also moves upstream generating new subpackets. The simultaneous formation of the electron hole and hill is identified by separating small and large wavenumber components corresponding to lower and higher frequencies, respectively, by applying the discrete Fourier transformation to the waveforms in space. Based on the simulation results of the whistler‐mode triggered emissions, we conclude that the mechanism of frequency variation of whistler‐mode chorus emissions works even in a uniform magnetic field.