Direct Coulomb Research Articles

ERCS24: An updated version of the ERCS08 program for calculations of the cross sections for atomic electron removal based on the ECPSSR theory and its variants

ERCS24: An updated version of the ERCS08 program for calculations of the cross sections for atomic electron removal based on the ECPSSR theory and its variants

Scalar Breit interaction for molecular calculations.

Variational treatment of the Dirac-Coulomb-Gaunt or Dirac-Coulomb-Breit two-electron interaction at the Dirac-Hartree-Fock level is the starting point of high-accuracy four-component calculations of atomic and molecular systems. In this work, we introduce, for the first time, the scalar Hamiltonians derived from the Dirac-Coulomb-Gaunt and Dirac-Coulomb-Breit operators based on spin separation in the Pauli quaternion basis. While the widely used spin-free Dirac-Coulomb Hamiltonian includes only the direct Coulomb and exchange terms that resemble nonrelativistic two-electron interactions, the scalar Gaunt operator adds a scalar spin-spin term. The spin separation of the gauge operator gives rise to an additional scalar orbit-orbit interaction in the scalar Breit Hamiltonian. Benchmark calculations of Aun (n = 2-8) show that the scalar Dirac-Coulomb-Breit Hamiltonian can capture 99.99% of the total energy with only 10% of the computational cost when real-valued arithmetic is used, compared to the full Dirac-Coulomb-Breit Hamiltonian. The scalar relativistic formulation developed in this work lays the theoretical foundation for the development of high-accuracy, low-cost correlated variational relativistic many-body theory.

The Optical Signatures of Stochastic Processes in Many-Body Exciton Scattering.

We review our recent quantum stochastic model for spectroscopic lineshapes in the presence of a coevolving and nonstationary background population of excitations. Starting from a field theory description for interacting bosonic excitons, we derive a reduced model whereby optical excitons are coupled to an incoherent background via scattering as mediated by their screened Coulomb coupling. The Heisenberg equationsof motion for the optical excitons are then driven by an auxiliary stochastic population variable, which we take to be the solution of an Ornstein-Uhlenbeck process. Here, we present an overview of the theoretical techniques we have developed as applied to predicting coherent nonlinear spectroscopic signals. We show how direct (Coulomb) and exchange coupling to the bath give rise to distinct spectral signatures and discuss mathematical limits on inverting spectral signatures to extract the background density of states. Expected final online publication date for the Annual Review of Physical Chemistry, Volume 74 is April 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Hybrid data-based modeling for the prediction and diagnostics of Li-ion battery thermal behaviors

Lithium-ion battery (LIB) has been deployed for the electrification of the transport sector as a key strategy for climate change mitigation and adaptation. However, it has significant technical challenges such as thermal runaway, requiring a good understanding and accurate prediction of the LIB thermal behavior (heat generation rate). In this study, a novel hybrid approach using an Artificial Neural Network (ANN) is developed for predicting the heat generation rate with discharge current, output voltage, ambient temperature, cell surface temperature, and Depth of Discharge (DOD) as the feature vectors (inputs); where the DOD is estimated with an Extended Kalman Filter (EKF), and direct Coulomb Counting (CC) method, respectively. A shallow neural network utilizing the Marquette-Levenberg algorithm is designed and calibrated using over 8000 cases of the testing data. It is shown that the predicted heat generation rate of LIB agrees well with the experimental results with an accuracy of R > 0.995. Further potential of this hybrid data-based model is evaluated by simulating a thermal management system control and by introducing voltage and current sensor faults for diagnostic purposes. It is shown that, when compared to the experimental value, the relative error of the total heat output generated is less than 2% when there is no sensor fault, and greater than 50% and 25%, respectively, with an induced failure of the current and voltage sensor, demonstrating the ability to build accurate models relying solely on LIB discharge data for sensor diagnostics. This study highlights the combination of using battery thermal behavior with machine learning for real time battery system monitoring, controls and field diagnostics.

Carbon and vacancy centers in hexagonal boron nitride

Creation of defect with predetermined optical, chemical and other characteristics is a powerful tool to enhance the functionalities of materials. Herewith, we utilize density functional theory to understand the microscopic mechanisms of formation of defects in hexagonal boron nitride based on vacancies and substitutional atoms. Through in-depth analysis of the defect-induced band structure and formation energy in varying growth conditions, we uncovered a dominant role of interdefect electron paring in stabilization of defect complexes. The electron reorganization modifies the exchange component of the electronic interactions which dominates over direct Coulomb repulsion or structural relaxation effects making the combination of acceptor- and donor-type defect centers energetically favorable. Based on an analysis of a large number of defect complexes we develop a simple picture of the inheritance of electronic properties when individual defects are combined together to form more complex centers.

Understanding the mechanisms of L-shell x-ray emission from Os atoms bombarded by 4–6 MeV/u fluorine ion

The L-subshell ionization mechanism is studied in an ultra-thin Os target bombarded by 4–6 MeV/u fluorine ions. Multiple ionization effects are considered through the change of fluorescence and Coster-Kronig yields while determining L-subshell ionization cross sections from L x-ray production cross sections. The present experimental values are compared with various theoretical approximations: (i) the relativistic semi-classical approximation (RSCA), (ii) the shellwise local plasma approximation (SLPA), and (iii) the ECUSAR theory. We also take into account the vacancy sharing among the subshells by the coupled-states model (CSM) and the electron capture (EC) by a standard formalism. We find that the ECUSAR-CSM-EC describes the measured excitation function curves the best. However, the theoretical calculations are still about a factor of two smaller than the measured values even though the recent fluorescence and Coster-Kronig yields are considered. Hence, a re-evaluation of these parameters is a challenge for the theoretical works. Whatsoever, this work leads to demonstrate that in the present energy range the heavy-ion induced inner-shell ionization of the heavy atoms can be understood by combining the direct Coulomb ionization, the electron capture, and the vacancy sharing among subshells, together with optimizing the atomic parameters. Optimization of the atomic parameters shows that our experimental results agree with theoretical vacancy production theories if the L1 fluorescence yield is nearly doubled. Such a optimization is validated by the proton induced L-shell ionization data of uranium atoms.

Influence of Long-Range Forces on the Transition States and Dynamics of NaCl Ion-Pair Dissociation in Water.

We study NaCl ion-pair dissociation in a dilute aqueous solution using computer simulations both for the full system with long-range Coulomb interactions and for a well-chosen reference system with short-range intermolecular interactions. Analyzing results using concepts from Local Molecular Field (LMF) theory and the recently proposed AI-based analysis tool "State predictive information bottleneck" (SPIB), we show that the system with short-range interactions can accurately reproduce the transition rate for the dissociation process, the dynamics for moving between the underlying metastable states, and the transition state ensemble. Contributions from long-range interactions can be largely neglected for these processes because long-range forces from the direct interionic Coulomb interactions are almost completely canceled (>90%) by those from solvent interactions over the length scale where the transition takes place. Thus, for this important monovalent ion-pair system, short-range forces alone are able to capture detailed consequences of the collective solvent motion, allowing the use of physically suggestive and computationally efficient short-range models for the dissociation event. We believe that the framework here should be applicable to disentangling mechanisms for more complex processes such as multivalent ion disassociation, where previous work has suggested that long-range contributions may be more important.

K-shell x-ray of Al produced by collisions of ions with near Bohr velocities

X-ray emissionproduced by highly charged ions with the energy range near the Bohr velocity involves complicated atomic process. However, duo to the limitation of experimental conditions, the relevant researches are nearly absent. It is unclear whether the existing theory is applicable in such an energy range. This needs further exploring. In the present work, K X-ray spectra of Al excited by H<sup>+</sup>, He<sup>2+</sup> and highly charged heavy ions I<sup>22+</sup> and Xe<sup>20+</sup> are investigated by using an Si drift X-ray detector in the energy range near the Bohr velocity. The X-ray production cross sections are extracted from the X-ray counts and compared with the theoretical simulations from PWBA, ECPSSR and modified BEA model. It is indicated that the cross section increases with the augment of projectile energy. With the same incident energy per nucleon, the cross section induced by highly charged heavy ions is a factor of about 10<sup>4</sup> larger than that by light ions . With the impact of H<sup>+</sup> and He<sup>2+</sup> ions, the K-shell electrons are mainly knocked off through the direct Coulomb ionization, and the X-ray emission cross section can be well predicted by ECPSSR theory. For the bombardment of highly charged heavy ions I<sup>22+</sup> and Xe<sup>20+</sup>, except for the Coulomb ionization, the orbital electrons can also be excited by electron capture. The BEA simulation after being modified by both Coulomb repulsion and effective charge can well predict the x-ray production cross section.

K-shell X-ray of Al produced by collisions of ions with near Bohr velocities

X-ray emissionproduced by highly charged ions with the energy range near the Bohr velocity involves complicated atomic process. However, duo to the limitation of experimental conditions, the relevant researches are nearly absent. It is unclear whether the existing theory is applicable in such an energy range. This needs further exploring. In the present work, K X-ray spectra of Al excited by H<sup>+</sup>, He<sup>2+</sup> and highly charged heavy ions I<sup>22+</sup> and Xe<sup>20+</sup> are investigated by using an Si drift X-ray detector in the energy range near the Bohr velocity. The X-ray production cross sections are extracted from the X-ray counts and compared with the theoretical simulations from PWBA, ECPSSR and modified BEA model. It is indicated that the cross section increases with the augment of projectile energy. With the same incident energy per nucleon, the cross section induced by highly charged heavy ions is a factor of about 10<sup>4</sup> larger than that by light ions . With the impact of H<sup>+</sup> and He<sup>2+</sup> ions, the K-shell electrons are mainly knocked off through the direct Coulomb ionization, and the X-ray emission cross section can be well predicted by ECPSSR theory. For the bombardment of highly charged heavy ions I<sup>22+</sup> and Xe<sup>20+</sup>, except for the Coulomb ionization, the orbital electrons can also be excited by electron capture. The BEA simulation after being modified by both Coulomb repulsion and effective charge can well predict the X-ray production cross section.

Electroosmotic Flow in Polarizable Charged Cylindrical Nanopores.

We present a simulation method to study electroosmotic flow in charged nanopores with dielectric contrast between their interior and the surrounding medium. To perform simulations, we separate the electrostatic energy into the direct Coulomb and the polarization contributions. The polarization part is obtained using periodic Green functions and can be expressed as a sum of fast converging modified Bessel functions. On the other hand, the direct Coulomb part of the electrostatic energy is calculated using fast converging three-dimensional (3D) Ewald summation method, corrected for a pseudo one-dimensional (1D) geometry. The effects of polarization are found to be particularly important for systems with multivalent counterions and narrow nanopores. Depending on the surface charge density, polarization can increase the volumetric flow rate by 200%. For systems with 3:1 electrolyte, we observe that there is a saturation of the volumetric flow rate. In this case, for polarizable pores, the flow rate is 100% higher than for nonpolarizable pores.

Multicritical points in a model for 5f-electron systems under pressure and magnetic field

We investigate the evolution of multicritical points under pressure and magnetic field in a model described by two 5f bands (called α and β) that hybridize with a single itinerant conduction band. The interaction is given by the direct Coulomb and the Hund’s rule exchange terms. Three types of orderings are considered: two conventional spin density waves (SDWs) and an exotic SDW, i.e., with no magnetic moment formation. The conventional SDWs phases, are characterized by and , respectively, where and are the intraband staggered magnetizations. The exotic SDW, which has time reversal symmetry, is described by a purely imaginary order parameter. This phase is related to a band mixing given by the spin-flip part of the Hund’s rule exchange interaction. As result, without magnetic field, the phase diagrams of temperature (T) versus pressure (given by the variation of the bandwidth (W)) shows a sequence of phase transitions involving the three phases which gives rise to multicritical points. The presence of the magnetic field (h z ) has drastic effects on part of the phase diagram and the location of the multicritical points.

Solving the Bethe-Salpeter equation on a subspace: Approximations and consequences for low-dimensional materials

It is well known that the ambient environment can dramatically renormalize the quasiparticle gap and exciton binding energies in low-dimensional materials, but the effect of the environment on the energy splitting of the spin-singlet and spin-triplet exciton states is less understood. A prominent effect is the renormalization of the exciton binding energy and optical strength (and hence the optical spectrum) through additional screening of the direct Coulomb term describing the attractive electron-hole interaction in the kernel of the Bethe-Salpeter equation (BSE). The repulsive exchange interaction responsible for the singlet-triplet slitting, on the other hand, is unscreened within formal many-body perturbation theory. However, Loren Benedict argued that in practical calculations restricted to a subspace of the full Hilbert space, the exchange interaction should be appropriately screened by states outside of the subspace, the so-called $S$ approximation \cite{Benedict2002}. Here, we systematically explore the accuracy of the $S$ approximation for different confined systems, including a molecule and heterostructures of semiconducting and metallic layered materials. We show that the $S$ approximation is actually exact in the limit of small exciton binding energies (i.e., small direct term) and can be used to significantly accelerate convergence of the exciton energies with respect to the number of empty states, provided that a particular effective screening consistent with the conventional Tamm-Dancoff approximation is employed. We further find that the singlet-triplet splitting in the energy of the excitons is largely unaffected by the external dielectric environment for most quasi-two-dimensional materials.

Electron cascade for distant spin readout

The spin of a single electron in a semiconductor quantum dot provides a well-controlled and long-lived qubit implementation. The electron charge in turn allows control of the position of individual electrons in a quantum dot array, and enables charge sensors to probe the charge configuration. Here we show that the Coulomb repulsion allows an initial charge transition to induce subsequent charge transitions, inducing a cascade of electron hops, like toppling dominoes. A cascade can transmit information along a quantum dot array over a distance that extends by far the effect of the direct Coulomb repulsion. We demonstrate that a cascade of electrons can be combined with Pauli spin blockade to read out distant spins and show results with potential for high fidelity using a remote charge sensor in a quadruple quantum dot device. We implement and analyse several operating modes for cascades and analyse their scaling behaviour. We also discuss the application of cascade-based spin readout to densely-packed two-dimensional quantum dot arrays with charge sensors placed at the periphery. The high connectivity of such arrays greatly improves the capabilities of quantum dot systems for quantum computation and simulation.

Direct Coulomb explosion of N2O2+ induced by monochromatic extreme ultraviolet photons at 38.5 eV**Project supported by the National Key Research and Development Program of China (Grant No. 2017YFA0402300).

The direct Coulomb explosion of N2O2+ has been investigated experimentally after double-ionization by a single extreme ultraviolet (EUV) photon with an energy of ∼ 38.5 eV. From the ion–ion time-of-flight coincidence spectrum, the de-nitrogenation (N2O2+ → N+ + NO+) and de-oxygenation (N2O2+→ O+ + N2+) photodissociation channels of N2O2+ are unequivocally identified. The measured kinetic energy release (KER) distribution of the de-nitrogenation channel presents a major peak accompanied by a shoulder structure. We find that the major peak can be attributed to the direct photodissociation of the 11Δ and 11Σ+ states, while the shoulder structure should be ascribed to the predissociation of the 11Σ and 11Σ+ states via 13Π state.

QED theory of elastic electron scattering on hydrogen-like ions involving formation and decay of autoionizing states

We develop {\it ab initio} relativistic QED theory for elastic electron scattering on hydrogen-like highly charged ions for impact energies where, in addition to direct (Coulomb) scattering, the process can also proceed via formation and consequent Auger decay of autoionizing states of the corresponding helium-like ions. Even so the primary goal of the theory is to treat electron scattering on highly charged ions, a comparison with experiment shows that it can also be applied for relatively light ions covering thus a very broad range of the scattering systems. Using the theory we performed calculations for elastic electron scattering on B$^{4+}$, Ca$^{19+}$, Fe$^{25+}$, Kr$^{35+}$, and Xe$^{53+}$. The theory was also generalized for collisions of hydrogen-like highly charged ions with atoms considering the latter as a source of (quasi-) free electrons.

Insights into the Mechanisms of K+ Permeation in K+ Channels from Computer Simulations

Ion permeation, selectivity, and the behavior of the K+ channel selectivity filter have been studied intensively in the previous two decades. The agreement among multiple approaches used to study ion flux in K+ channels suggests a consensus mechanism of ion permeation across the selectivity that has been put to the test in recent years with the proposal of an alternative way by which ions can cross the selectivity filter of K+ channels via direct Coulomb repulsion between contacting cations. Past experimental work by Zhou and MacKinnon (J. Mol. Biol. 2004, 338, 839) showed that mutation of the site S4 reduces the total occupancy of the selectivity filter to less than two ions on average by lowering the occupancy of the S2-S4 configuration without changing the S1-S3 configuration much, and this reduction of occupancy means that ion configurations different from the ones involved in the canonical mechanism are likely to be involved. At that time, calculations using complicated kinetic networks to relate occupancy to conduction did not provide deeper insight into the conduction mechanism. Here, to help solve this enigma, umbrella sampling simulations have been performed to evaluate the potential of mean force of two KcsA mutant channels where the S4 site is substituted. Our new results provide insights into the significance of threonine in this position, revealing the effect of substitution on the alternate mechanisms of conduction proposed, involving either water or vacant sites.

Fluctuation-driven Coulomb drag in interacting quantum dot systems

Coulomb drag between nanoscale conductors is of both fundamental and practical interest. Here, we theoretically study drag in a double quantum-dot (QD) system consisting of a biased drive QD and an unbiased drag QD coupled via a direct interdot Coulomb interaction. We demonstrate that the Coulomb drag is driven by the charge fluctuations in the drive QD, and show how the properties of the associated quantum noise allow to distinguish it from, e.g., shot-noise driven drag in circuits of weakly interacting quantum conductors. In the strong-interaction regime exhibiting an orbital ("pseudospin") Kondo effect, the drag is governed by charge fluctuations induced by pseudospin-flip cotunneling processes. The quenching of pseudospin-flip processes by Kondo correlations are found to suppress the drag at low bias and introduce a zero-bias anomaly in the second-order differential transconductance. Finally, we show that the drag is maximized for values of the interdot interaction matching the lead couplings. Our findings are relevant for the understanding of drag in QD systems and provide experimentally testable predictions in different transport regimes.

Nonradiative resonance energy transfer in the quantum dot system

Microscopic theory of the nonradiative resonance energy transfer between semiconductor quantum dots is elaborated. The energy transfer due to both direct Coulomb and due to exchange interactions between two quantum dots is considered. An analysis of energy transfer process is performed accordingly the Kane model. For the exchange energy transfer the detailed analytical consideration is performed for the first time. The energy transfer rate is analyzed both in the weak-interaction and strong-interaction approximation. The numerical calculations showed that at nearly contact distances between two quantum dots the rate of the energy transfer due to the direct Coulomb interaction, as well as due to exchange interaction, can reach saturation. At the small distances, these two contributions can be of the same order, and have the same value in the saturation range. It is revealed that the exchange energy transfer should be taken into consideration in qualitative description of the energy transfer at small distances between the quantum dots. In particular, it may be of importance for investigation of the structure and dynamics of biomolecules and their complexes, and consideration of the energy transfer in densely packed thin QD films.

Electrospray molecular dynamics simulations using an octree-based Coulomb interaction method.

A new octree-based Coulomb interaction model is developed to model the electrospray of ionic liquids (ILs) in molecular dynamics. Using an octree-based method, Coulomb interactions are categorized as intra- and interleaf Coulomb interactions based on a criterion related to the Bjerrum length of the IL. The octree-based method is found capable of reproducing Coulomb energy in agreement with established and computationally more expensive models, such as the direct Coulomb and the damped shifted force (DSF) method in the absence of an external electric field. In the presence of an external electric field, the octree-based method produces distinctly different results compared to that obtained by the direct Coulomb method. The time required to form Taylor's cone was shorter for the octree method compared to the direct Coulomb approach. While no emission larger than monomers was observed from the direct Coulomb simulation, emission of larger species such as dimers and trimers was observed when the octree-based Coulomb interaction model was used. Furthermore, the octree-based model forms a smaller ion emission cone compared to that from the direct Coulomb method.

Making Sense of Coulomb Explosion Imaging.

A multifaceted agreement between ab initio theoretical predictions and experimental measurements, including branching ratios, channel-specific kinetic energy release, and three-body momentum correlation spectra, leads to the identification of new mechanisms in Coulomb-explosion (CE) induced two- and three-body breakup processes in methanol. These identified mechanisms include direct nonadiabatic Coulomb explosion responsible for CO bond-breaking, a long-range " inverse harpooning" dominating the production of H2+ + HCOH+, a transient proton migration leading to surprising energy partitioning in three-body fragmentation and other complex dynamics forming products such as H2O+ and H3+. These mechanisms provide general concepts that should be useful for analyzing future time-resolved Coulomb explosion imaging of methanol as well as other molecular systems. These advances are enabled by a combination of recently developed experimental and computational techniques, using weak ultrafast EUV pulses to initiate the CE and a high-level quantum chemistry approach to follow the resulting field-free nonadiabatic molecular dynamics.