Electron Wave Research Articles

Microwave Imaging of Anisotropic Objects by Artificial Intelligence Technology.

In this paper, we present the microwave imaging of anisotropic objects by artificial intelligence technology. Since the biaxial anisotropic scatterers have different dielectric constant components in different transverse directions, the problems faced by transverse electronic (TE) polarization waves are more complex than those of transverse magnetic (TM) polarization waves. In other words, measured scattered field information can scarcely reconstruct microwave images due to the high nonlinearity characteristic of TE polarization. Therefore, we first use the dominant current scheme (DCS) and the back-propagation scheme (BPS) to compute the initial guess image. We then apply a trained convolution neural network (CNN) to regenerate the microwave image. Numerical results show that the CNN possesses a good generalization ability under limited training data, which could be favorable to deploy in image processing. Finally, we compare DCS and BPS reconstruction images for anisotropic objects by the CNN and prove that DCS is better than BPS. In brief, successfully reconstructing biaxial anisotropic objects with a CNN is the contribution of this proposal.

Strong Laser Field-Driven Coupled Electron-Nuclear Dynamics: Quantum vs Classical Description.

We have performed a coupled electron-nuclear dynamics study of H2+ molecular ions under the influence of an intense few-cycle 4.5 fs laser pulse with an intensity of 4 × 1014 W/cm2 and a central wavelength of 750 nm. Both quantum and classical dynamical methods are employed in the exact similar initial conditions with the aim of head-to-head comparison of two methodologies. A competition between ionization and dissociation channel is explained under the framework of quantum and classical dynamics. The origin of the electron localization phenomena is elucidated by observing the molecular and electronic wave packet evolution pattern. By probing with different carrier envelope phase (CEP) values of the ultrashort pulse, the possibility of electron localization on either of the two nuclei is investigated. The effects of initial vibrational states on final dissociation and ionization probabilities for several CEP values are studied in detail. Finally, asymmetries in the dissociation probabilities are calculated and mutually compared for both quantum and classical dynamical methodologies, whereas Franck-Condon averaging over the initial vibrational states is carried out in order to mimic the existing experimental conditions.

DFT study of SF6 adsorption by Pd-doped hydroxyl-terminal modified Ti3C2Tx MXene.

SF6 gas has a strong greenhouse effect, and how to treat SF6 in an environmentally friendly way has been a hot topic of current research. In this paper, the adsorption behavior of SF6 on the surface of Pd-doped hydroxyl-terminated modified Ti3C2Tx (i.e., Ti3C2(OH)2) was investigated based on the density functional theory using two-dimensional MXene as the catalyst. The structures of different Pd-doped Ti3C2(OH)2 were analyzed and the most structurally stable doped structures were selected as the basis for subsequent calculations. A large number of adsorption configurations were constructed and geometrically optimized, and the adsorption energy, charge transfer, differential charge density, and density of states of the systems were calculated in order to analyze the gas-solid interactions and find the surface active sites; compared with the adsorption performance of undoped Ti3C2(OH)2 on SF6, it was found that Pd doping played a less inhibitory role in the adsorption of SF6 on the Ti3C2(OH)2 surface. The results of this study can provide theoretical support for the use of Pd-doped Ti3C2(OH)2 as a catalyst for the degradation of SF6. In this paper, simulations of SF6 adsorption on Ti3C2Tx surfaces are based on density functional theory and are carried out in the Dmol3 module of Material Studio. To better describe the non-uniform electron density of the actual system, the PBE functional in the generalized gradient approximation (GGA) was chosen for the optimization of the structure of the gas-solid interface system and the calculation of the relevant electronic properties, combined with the Grimme dispersion correction in the DFT-D dispersion correction for the electron exchange correlation term. Because both Pd and Ti are transition metal elements, the mode-conserving pseudopotential DNP basis set containing relativistic effects was chosen for the electronic wave function expansion. In this paper, an all-electron model is used for the inner core treatment of gas molecules and a density generalized semi-nuclear pseudopotential DSSP is used for the solid surface treatment.

Reconstruction of Angstrom resolution exit-waves by the application of drift-corrected phase-shifting off-axis electron holography

Phase-shifting electron holography is an excellent method to reveal electron wave phase information with very high phase sensitivity over a large range of spatial frequencies. It circumvents the limiting trade-off between fringe spacing and visibility of standard off-axis holography. Previous implementations have been limited by the independent drift of biprism and sample. We demonstrate here an advanced drift correction scheme for the hologram series that exploits the presence of an interface of the TEM specimen to the vacuum area in the hologram. It allows to obtain reliable phase information up to 2π/452 at the 1 Å information limit of the Titan 80–300 kV environmental transmission electron microscope used, by applying a moderate voltage of 250 V to a single biprism for a fringe spacing of 1 Å. The obtained phase and amplitude information is validated at a thin Pt sample by use of multislice image simulation with the frozen lattice approximation and shows excellent agreement. The presented method is applicable in any TEM equipped with at least one electron biprism and thus enables achieving high resolution off-axis holography in various instruments including those for in-situ applications. A software implementation for the acquisition, calibration and reconstruction is provided.

Microstructure and properties evolution during annealing in low-carbon Nb containing steel with high strength and electrical conductivity: an experimental and theoretical study

Based on the experiment and first-principles calculation methods, the evolution of microstructure, mechanical properties and electrical conductivity of low-carbon steel were investigated at various annealing conditions. The results show that with the increase of annealing temperature, the morphology of ferrite matrix grain undergoes a transformation from narrow and long fibrous to, accompanied by the continuous growth in size. The strength of α-fibre texture is reduced continuously while the strength of γ-fibre texture increases evidently. The tensile and conductivity testing results indicate that with the increase of annealing temperature, both the yield strength and tensile strength of the steel decrease, while the elongation exhibits an opposite tendency due to the continuous growth of ferrite grains and dissolution of tertiary cementites. Additionally, the electrical conductivity increases because the reduction in the number of grain boundaries weakens their scattering ability to electronic waves. The first-principles calculation results indicate that with increasing distance to the interface, the segregation energy of Nb atom in ferrite matrix is reduced, as well as the interfacial energy of the structure is also decreased. The interface segregation preference of Nb atoms induces the transfer of charge, which is beneficial to enhance the conductivity of the ferrite matrix.

Adiabatic deformations of quantum Hall droplets

We consider area-preserving deformations of the plane, acting on electronic wave functions through “quantomorphisms” that change both the underlying metric and the confining potential. We show that adiabatic sequences of such transformations produce Berry phases that can be written in closed form in terms of the many-body current and density, even in the presence of interactions. For a large class of deformations that generalize squeezing and shearing, the leading piece of the phase is a super-extensive Aharonov-Bohm term (\propto N^2∝N2 for NN electrons) in the thermodynamic limit. Its gauge-invariant subleading partner only measures the current, whose dominant contribution to the phase stems from a jump at the edge in the limit of strong magnetic fields. This results in a finite Berry curvature per unit area, reminiscent of the Hall viscosity. We show that the latter is in fact included in our formalism, bypassing its standard derivation on a torus and suggesting realistic experimental setups for its observation in quantum simulators.

Radio Pulsars Resonantly Accelerating Electrons

Based on the recently demonstrated resonant wave–wave process, it is shown that electrons can be accelerated to ultra-relativistic energies in the magnetospheres of radio pulsars. The energization occurs via the resonant interaction of the electron wave (described by the Klein–Gordon (KG) equation) moving in unison with an intense electromagnetic (EM) wave; the KG wave/particle continuously draws energy from EM. In a brief recapitulation of the general theory, the high-energy (resonantly enhanced) electron states are investigated by solving the KG equation, minimally coupled to the EM field. The restricted class of solutions that propagate in phase with EM radiation (functions only of ζ=ωt−kz) are explored to serve as a possible basis for the proposed electron energization in the radio pulsars. We show that the wave–wave resonant energization mechanism could be operative in a broad class of radio pulsars with periods ranging from milliseconds to normal values (∼1 s); this could drive the magnetospheric electrons to acquire energies from 100 s of TeVs (millisecond pulsars) to 10 ZeVs (normal pulsars).

First principles study of the electronic structure of antimony (Sb) and samarium (Sm) doped tin dioxide (SnO2) based on big data analysis

Abstract To study the electronic structure of Sb and Sm co-doped SnO2 materials, a lattice model of Sb and Sm co-doped SnO2 is designed in this paper based on a big data analysis algorithm. The physical properties of the SnO2 ground state are described by the particle density function using the density generalized function theory. The interactions between the particles are all subsumed into the exchange-correlation generalized function by the Kohn-Sham equation. A big data analysis algorithm is used to construct the electron wave function to reflect the luminescence mechanism of the spectrum produced by the electron leap between energy levels, which makes the computational effort significantly reduced. The results show that the enthalpy change of Sb and Sm co-doped SnO2 in the design model of this paper is −5.59918, and the energy interval of the density of states of s orbitals is [2.36, 31.45]. It can be seen that the co-doping of Sm and Sb can increase the electron polarization ability and electron leap probability of SnO2 in the infrared band and enhance the infrared reflectivity, and the co-doped system has the highest electron-binding ability reflectivity.

Capturing the non-equilibrium state in light–matter–free-electron interactions through ultrafast transmission electron microscopy

Ultrafast transmission electron microscope (UTEM) with the multimodality of time-resolved diffraction, imaging, and spectroscopy provides a unique platform to reveal the fundamental features associated with the interaction between free electrons and matter. In this review, we summarize the principles, instrumentation, and recent developments of the UTEM and its applications in capturing dynamic processes and non-equilibrium transient states. The combination of the transmission electron microscope with a femtosecond laser via the pump–probe method guarantees the high spatiotemporal resolution, allowing the investigation of the transient process in real, reciprocal and energy spaces. Ultrafast structural dynamics can be studied by diffraction and imaging methods, revealing the coherent acoustic phonon generation and photo-induced phase transition process. In the energy dimension, time-resolved electron energy-loss spectroscopy enables the examination of the intrinsic electronic dynamics of materials, while the photon-induced near-field electron microscopy extends the application of the UTEM to the imaging of optical near fields with high real-space resolution. It is noted that light–free-electron interactions have the ability to shape electron wave packets in both longitudinal and transverse directions, showing the potential application in the generation of attosecond electron pulses and vortex electron beams.

Binding Energy Stark-Shift, Polarizability and Dipole Moment Response of Shallow Donor Impurity in GaAs Quantum Dots

In this study, we have examined, under the influence of an electric field applied along the z-direction, the binding energy Stark-shift, the dipole moment and the polarizability of a confined shallow donor impurity in GaAs conical-shaped quantum dots (CSQD). With square infinite confinement system, the calculations are based on the approximation of the effective mass by using the finite difference method. Our results show that increasing the radius of the CSQD structure and the electric field intensity increases the Stark shift binding energy and it has a mixed behavior as a function of the impurity position. Furthermore, the polarizability and the dipole moment vary in a quasi-linear way as a function of the dot radius and they follow a decreasing function as a function of the electric field intensity. These two physical parameters have a double behavior, they decrease with the position of the impurity in the strong confinement regime and they increase in the top regions of the quantum dot. These results provide a lot of information about the behavior of the electronic wave function which give more interesting ideas for the fabrication of optoelectronic devices.

Atomic screening and e+e− pair photoproduction at low energies

The effect of screening by atomic electrons on the behavior of electron and positron wave functions in the continuous spectrum at small distances is studied. It is shown that these asymptotics are affected not only by the static potential of atomic electrons, but also by the polarization potential, as well as by the exchange interaction, which is essential for nonrelativistic electrons. A simple analytical expression is obtained for the photoproduction cross section of e+e− pair in an atomic field near the threshold. The spectrum and angular distribution of the produced particles are considered. It is shown that screening significantly affects the cross section in the near-threshold region.

Anomalous Laser-Fluence Dependence of Electron Spin Excitation in CdS Colloidal Quantum Dots: Surface Effects.

Electron spin dynamics in CdS quantum dots (QDs) with hole acceptor 1-octanethiol organic molecules are investigated by time-resolved ellipticity spectroscopy. An anomalous dependence of laser fluences on electron spin excitation for the first time is reported. Increasing the laser fluence, the electron spin is switched from one direction to an antiparallel direction (spin direction switching, SDS) when adding enough 1-octanethiol hole acceptors in an air atmosphere. The analysis shows that the electron spin direction changes from heavy hole excitation defined to spin-orbit split hole excitation defined. In as-grown CdS QDs with native ligands, laser-fluence-dependent SDS phenomena are absent. Electron wave function spread into 1-octanethiol molecules is demonstrated to be important for the presence of SDS phenomena. The finding here thus reveals the importance of surface conditions on electron spin excitation processes in semiconductor QDs and that the surface can be used as an important factor to manipulate the spin.

Quaternary AlInGaN/AlInGaN Polarization-Engineered Green Lights-Emitting Diodes by Using Inverted Trapezoidal Quantum Barriers

In this paper, a structure design using grading quaternary AlInGaN as inverted trapezoidal multiple quantum barriers is shown. This paper proposes an inverted trapezoidal structure in quantum barriers to decelerate the transmission of the electron which will enhance efficient carrier injection into the quantum wells and get a more uniform distribution of carriers. The polarization-matched AlInGaN/AlInGaN structures have less polarization and increase the overlap of electron and hole wave function, effectively improving the internal quantum efficiency and wall-plug efficiency, and making the In composition adjustable. The results show that the Inverted trapezoidal polarization-matched AlInGaN structure significantly improves the phenomenon of “green gap” and “green droop.” As a result, the new structure mitigates the droop ratio from 25.5% to 7%, and the LED power is increased from 59.95 mW to 109.68 mW at 200 mA. Finally, the device exhibits a uniform current distribution and greater radiative recombination while reducing resistance.

A Discrete-Variable Local Diabatic Representation of Conical Intersection Dynamics.

Conical intersections (CIs) are ubiquitous in polyatomic molecules and are responsible for a wide range of phenomena in photochemistry and photophysics. Modeling the conical intersection dynamics with adiabatic electronic states is hindered by the divergence of the first- and second-order derivative couplings at CIs due to electronic degeneracy. We introduce and implement a novel diabatic representation for exact correlated electron-nuclear wave packet dynamics through conical intersections. It directly employs the adiabatic electronic states but avoids the singular first- and second-order derivative couplings and is robust to different gauge choices of the electronic wave function phases. The reference nuclear geometries defining the adiabatic electronic states are determined by a discrete-variable representation of the nuclear coordinates. The nonadiabatic effects are accounted for by the electronic overlap matrix instead of derivative couplings as in the adiabatic representation. Illustrated by a two-mode conical intersection model, this representation captures all nonadiabatic effects, including electronic transitions, electronic coherence, and geometric phases. Thus, this representation provides a singularity-free framework for modeling ab initio conical intersection wave packet dynamics.

Deep Variational Free Energy Approach to Dense Hydrogen.

We developed a deep generative model-based variational free energy approach to the equations of state of dense hydrogen. We employ a normalizing flow network to model the proton Boltzmann distribution and a fermionic neural network to model the electron wave function at given proton positions. By jointly optimizing the two neural networks we reached a comparable variational free energy to the previous coupled electron-ion MonteCarlo calculation. The predicted equation of state of dense hydrogen under planetary conditions is denser than the findings of abinitio molecular dynamics calculation and empirical chemical model. Moreover, direct access to the entropy and free energy of dense hydrogen opens new opportunities in planetary modeling and high-pressure physics research.

Efficient strain-modified improved nonparabolic-band energy dispersion model that considers the effect of conduction band nonparabolicity in mid-infrared quantum cascade lasers.

Intersubband polar-optical-phonon (POP) scattering plays an important role in determining the population inversion and optical gain of mid-infrared (mid-IR) quantum cascade lasers (QCLs). In particular, the nonparabolicity of the conduction band (CB) significantly affects the energy dispersion relation and intersubband POP scattering time. However, the currently used parabolic-band (PB) and nonparabolic-band (NPB) energy dispersion models are not appropriate for mid-IR QCLs because they are unsuitable for high electron wave vectors and do not consider the effect of applied strain on the energy dispersion relation of the CB. The eight-band k·p method can provide a relatively accurate nonparabolic energy dispersion relation for high electron wave vectors but has the disadvantages of high computational complexity and spurious solutions to be discarded. Consequently, we propose a strain-modified improved nonparabolic-band (INPB) energy dispersion model that has no spurious solution and acceptable accuracy, compared to the eight-band k·p method. To demonstrate the accuracy and efficiency of our proposed INPB model compared with those of the PB, NPB, and eight-band k·p models, we calculate the energy dispersion relations and intersubband POP scattering times in a strain-compensated QCL with a lasing wavelength of 3.58 µm. Calculation results reveal that our proposed model is almost as accurate as the eight-band k·p model; however, it enables much faster calculations and is free from spurious solutions.

Influence of nuclear dynamics on molecular attosecond photoelectron interferometry.

In extreme ultraviolet spectroscopy, the photoionization process occurring in a molecule due to the absorption of a single photon can trigger an ultrafast nuclear motion in the cation. Taking advantage of attosecond photoelectron interferometry, where the absorption of the extreme ultraviolet photon is accompanied by the exchange of an additional infrared quantum of light, one can investigate the influence of nuclear dynamics by monitoring the characteristics of the photoelectron spectra generated by the two-color field. Here, we show that attosecond photoelectron interferometry is sensitive to the nuclear response by measuring the two-color photoionization spectra in a mixture of methane (CH4) and deuteromethane (CD4). The effect of the different nuclear evolution in the two isotopologues manifests itself in the modification of the amplitude and contrast of the oscillations of the photoelectron peaks. Our work indicates that nuclear dynamics can affect the coherence properties of the electronic wave packet emitted by photoionization on a time scale as short as a few femtoseconds.

Effect of magnetic field on recombination dynamics in random electron systems

The evolution of localized electron states with increasing magnetic field is studied using time-resolved photoluminescence in GaAs/AlGaAs short-period superlattices, in which electrons are localized due to quantum interference between electron waves multiply scattered by the short-range potential of a random interface roughness. The nature of electronic states, extended or localized, is fundamentally related to the rate of their recombination, which is determined by the exciton coherence volume. Localization reduces the volume of exciton coherence, thereby decreasing the recombination rate. Correspondingly, the recombination rate in insulating samples turned out to be much lower than in metallic ones. Moreover, in insulating samples, the recombination rate increased with increasing temperature and magnetic field, which was found consistent with the temperature and magnetic field dependences of the electrical resistance. The observed increase in the recombination rate is attributed to the break-down of the quantum interference, leading to delocalization.

Multifractal Conductance Fluctuations of Helical Edge States.

Two-dimensional topological insulators are characterized by the bulk gap and one-dimensional helical states running along the edges. The theory predicts the topological protection of the helical transport from coherent backscattering. However, the unexpected deviations of the conductance from the quantized value and localization of the helical modes are generally observed in long samples. Moreover, at millikelvin temperatures significant mesoscopic fluctuations are developed as a function of the electron energy. Here we report the results of an experimental study of the transport in a HgTe quantum well with an inverted energy spectrum that reveal a multifractality of the conductance fluctuations in the helical edge state dominated transport regime. We attribute observed multifractality to mesoscopic fluctuations of the electron wave function or local density of states at the spin quantum Hall transition. We have shown that the mesoscopic two-dimensional topological insulator provides a highly tunable experimental system in which to explore the physics of the Anderson transition between topological states.

On the specialization of Gaussian basis sets for core-dependent properties.

Despite the fact that most quantum chemistry basis sets are designed for accurately modeling valence chemistry, these general-purpose basis sets continue to be widely used to model core-dependent properties. Core-specialized basis sets are designed with specific features to accurately represent the behavior of the core region. This design typically incorporates Gaussian primitives with higher exponents to capture core behavior effectively, as well as some decontraction of basis functions to provide flexibility in describing the core electronic wave function. The highest Gaussian exponent and the degree of contraction for both s- and p-basis functions effectively characterize these design aspects. In this study, we compare the design and performance of general-purpose basis sets against several literature-based basis sets specifically designed for three core-dependent properties: J coupling constants, hyperfine coupling constants, and magnetic shielding constants (used for calculating chemical shifts). Our findings consistently demonstrate a significant reduction in error when employing core-specialized basis sets, often at a marginal increase in computational cost compared to the popular 6-31G** basis set. Notably, for expedient calculations of J coupling, hyperfine coupling, and magnetic shielding constants, we recommend the use of the pcJ-1, EPR-II, and pcSseg-1 basis sets, respectively. For higher accuracy, the pcJ-2, EPR-III, and pcSseg-2 basis sets are recommended.