Quantum Mechanical Exchange Research Articles

Inelastic Compton scattering: Tool to validate quantum mechanical potential energies

Inelastic γ-ray scattering (Compton scattering) is a distinguished tool to explore the electronic and magnetic response of various substances and is also an emphatic technique to validate the quantum mechanical exchange and correlation potentials used in the DFT calculations. Charge and magnetic Compton profiles are used to deduce the electron/spin momentum densities in functional materials like dichalcogenides, ferrites, ferroelectrics, chromates, Heusler alloys, oxides, etc. In this review, besides salient features of basic principles of Compton scattering, we have included representative studies on Compton profiles and relative nature of bonding in lead titanate/lead zirconate. Temperature dependency of spin moments using magnetic Compton profiles of Pr doped NiFe2O4 system has also be discussed. Recent work on charge and magnetic Compton scattering of variety of materials is reviewed.

Classical Exchange Polarization: An Anisotropic Variable Polarizability Model.

Polarizability, or the tendency of the electron distribution to distort under an electric field, often depends on the local chemical environment. For example, the polarizability of a chloride ion is larger in gas phase compared to a chloride ion solvated in water. This effect is due to the restriction the Pauli exclusion principle places on the allowed electron states. Because no two electrons can occupy the same state, when a highly polarizable atom comes in close contact with other atoms or molecules, the space of allowed states can dramatically decrease. This constraint suggests that an accurate molecular mechanics polarizability model should depend on the radial distance between neighboring atoms. This paper introduces a variable polarizability model within the framework of the HIPPO (Hydrogen-like Intermolecular Polarizable Potential) force field, by damping the polarizability as a function of the orbital overlap of two atoms. This effectively captures the quantum mechanical exchange polarization effects, without explicit utilization of antisymmetrized wave functions. We show that the variable polarizability model remarkably improves the two-body polarization energies and three-body energies of ion-ion and ion-water systems. Under this model, no manual tuning of atomic polarizabilities for monatomic ions is required; the gas-phase polarizability can be used because an appropriate damping function is able to correct the polarizability at short range.

Azolium Control of the Osmium-Promoted Aromatic C-H Bond Activation in 1,3-Disubstituted Substrates.

The hexahydride complex OsH6(PiPr3)2 promotes the C–H bond activation of the 1,3-disubstituted phenyl group of the [BF4]− and [BPh4]− salts of the cations 1-(3-(isoquinolin-1-yl)phenyl)-3-methylimidazolium and 1-(3-(isoquinolin-1-yl)phenyl)-3-methylbenzimidazolium. The reactions selectively afford neutral and cationic trihydride-osmium(IV) derivatives bearing κ2-C,N- or κ2-C,C-chelating ligands, a cationic dihydride-osmium(IV) complex stabilized by a κ3-C,C,N-pincer group, and a bimetallic hexahydride formed by two trihydride-osmium(IV) fragments. The metal centers of the hexahydride are separated by a bridging ligand, composed of κ2-C,N- and κ2-C,C-chelating moieties, which allows electronic communication between the metal centers. The wide variety of obtained compounds and the high selectivity observed in their formation is a consequence of the main role of the azolium group during the activation and of the existence of significant differences in behavior between the azolium groups. The azolium role is governed by the anion of the salt, whereas the azolium behavior depends upon its imidazolium or benzimidazolium nature. While [BF4]− inhibits the azolium reactions, [BPh4]− favors the azolium participation in the activation process. In contrast to benzimidazolylidene, the imidazolylidene resulting from the deprotonation of the imidazolium substituent coordinates in an abnormal fashion to direct the phenyl C–H bond activation to the 2-position. The hydride ligands of the cationic dihydride-osmium(IV) pincer complex display intense quantum mechanical exchange coupling. Furthermore, this salt is a red phosphorescent emitter upon photoexcitation and displays a noticeable catalytic activity for the dehydrogenation of 1-phenylethanol to acetophenone and of 1,2-phenylenedimethanol to 1-isobenzofuranone. The bimetallic hexahydride shows catalytic synergism between the metals, in the dehydrogenation of 1,2,3,4-tetrahydroisoquinoline and alcohols.

Breaking Space Inversion‐Symmetry to Obtain Asymmetric Spin‐Wave Excitation in Systems with Nonuniform Magnetic Exchange

AbstractThe consequences of non‐uniform exchange in magnetic systems are reported. The quantum mechanical exchange interaction between spins is responsible for the phenomenon of magnetic order, and is generally considered to be uniform across bulk magnetic systems. Partly inspired by the Dzyaloshinskii‐Moriya interaction—also known as antisymmetric exchange—a linearly varying exchange interaction is used along a magnetic strip as a route to spatial inversion symmetry‐breaking. It is found that, in addition to asymmetric modes and localization, spatially varying exchange can be used to design nonreciprocal magnetic signal excitation at frequencies that are tunable. Moreover, the authors’ work predicts nonreciprocity to occur across a vast range of frequencies up to hundreds of GHz. Such spin wave engineering is a key area of ongoing research in the fields of magnonics and spintronics, which are expected to enable the next generation of wireless communication technology and information processing. Analogous nonreciprocity is expected to occur in other wave systems with gradient properties.

Exchange interaction and models of contact generation of disturbances in tribosystems

The physical mechanisms have been investigated that form and transform the corpuscular-vortex-wave thermal complexes of disturbances in contact tribosystems based on the quantum-mechanical exchange interaction. The presence of a contact gap determines the generation of pairs of quasi-particles-disturbances stabilized by wavelength and frequency. Internal instability and collapse processes in such a system of disturbances lead to the formation of defects in a tribopair's material and underlie the emergency friction regimes. This paper gives specific technical examples of the generation of thermal complexes at fretting, during the friction of sliding and rolling, and at cutting. It has been established that the destructive nature of the process of fretting at low values of reverse sliding speeds is caused by the generation and collapse of the corpuscular-vortex-wave thermal complexes. An example of acoustic friction emission in the ultrasonic region of the spectrum has been used to show the quantum nature of the disturbances generated by friction. The high-frequency spectrum of acoustic emission corresponds to the unbalanced composition of the disturbances and leads to the formation of wear particles. The exchange interaction in a tribosystem involving rolling on the plane has been considered. The results of statistical analysis of such rolling showed the existence of the effect of negative friction caused by the quantum generation of longwave disturbances. It has been demonstrated that the collapsed component of the generation of disturbances is significantly increased under the modes of materials destruction, including when cutting the materials. The corpuscular-vortex-wave mechanism of selective transfer and hydrogen wear in tribosystems has been described. It is shown that the properties of a servovite film under the mode of selective transfer are provided by the collapse processes in the system of disturbances. Similar processes at the vortex-wave transfer of hydrogen atoms in metals lead to the wear and destruction of the surface layer of friction

Quantum-mechanical approaches in evaluating the contact interaction of tribosystems

The physical mechanisms of the formation and transformation of corpuscular-vortex-wave thermal complexes in contact tribosystems based on the quantum-mechanical exchange interaction in ensembles of identical quasiparticles are considered. The presence of contact discontinuity of two thermostats with different signs of absolute temperature determines the generation of pairs of quasiparticle perturbations stabilized along the wavelength in cooperation with photons of stimulated thermal radiation, and also in frequency in interaction with phonons. Internal instability and collapse processes in such a system of perturbations lead to the formation of defects in the material of the tribocouple and are the cause of emergency friction conditions. Specific technical examples of thermal complex generation during fretting, sliding and rolling friction, and cutting are given. A corpuscular-vortex-wave mechanism of selective transfer and hydrogen wear in tribosystems is proposed

Enabling Large-Scale Condensed-Phase Hybrid Density Functional Theory Based Ab Initio Molecular Dynamics. 1. Theory, Algorithm, and Performance.

By including a fraction of exact exchange (EXX), hybrid functionals reduce the self-interaction error in semilocal density functional theory (DFT) and thereby furnish a more accurate and reliable description of the underlying electronic structure in systems throughout biology, chemistry, physics, and materials science. However, the high computational cost associated with the evaluation of all required EXX quantities has limited the applicability of hybrid DFT in the treatment of large molecules and complex condensed-phase materials. To overcome this limitation, we describe a linear-scaling approach that utilizes a local representation of the occupied orbitals (e.g., maximally localized Wannier functions (MLWFs)) to exploit the sparsity in the real-space evaluation of the quantum mechanical exchange interaction in finite-gap systems. In this work, we present a detailed description of the theoretical and algorithmic advances required to perform MLWF-based ab initio molecular dynamics (AIMD) simulations of large-scale condensed-phase systems of interest at the hybrid DFT level. We focus our theoretical discussion on the integration of this approach into the framework of Car-Parrinello AIMD, and highlight the central role played by the MLWF-product potential (i.e., the solution of Poisson's equation for each corresponding MLWF-product density) in the evaluation of the EXX energy and wave function forces. We then provide a comprehensive description of the exx algorithm implemented in the open-source Quantum ESPRESSO program, which employs a hybrid MPI/OpenMP parallelization scheme to efficiently utilize the high-performance computing (HPC) resources available on current- and next-generation supercomputer architectures. This is followed by a critical assessment of the accuracy and parallel performance (e.g., strong and weak scaling) of this approach when AIMD simulations of liquid water are performed in the canonical (NVT) ensemble. With access to HPC resources, we demonstrate that exx enables hybrid DFT-based AIMD simulations of condensed-phase systems containing 500-1000 atoms (e.g., (H2O)256) with a wall time cost that is comparable to that of semilocal DFT. In doing so, exx takes us one step closer to routinely performing AIMD simulations of complex and large-scale condensed-phase systems for sufficiently long time scales at the hybrid DFT level of theory.

Cluster stability driven by quantum fluctuations

By means of an accurate path-integral Monte Carlo we investigate a two-dimensional ensemble of particles interacting via a Lifshitz-Petrich-Gaussian potential. In particular, analysing structures described by a commensurate ratio between the two wave numbers that mark the pattern, the Lifshitz-Petrich-Gaussian boson model may display a stable and well-defined stripe phase lacking any global phase coherence but featuring a superfluid signal along the stripe direction only. Upon increasing quantum fluctuations and quantum-mechanical exchange of bosons, the double-degeneration of the negative minima in the Fourier transform of the potential is removed at the expense of a density modulation peculiar to a cluster triangular crystal. We also show that this last structure possess all features adhering to the definition of a supersolid phase.

Qubit exchange interactions from permutations of classical bits

In order to prepare for the introduction of dynamical many-body and, eventually, field theoretical models, we show here that quantum mechanical exchange interactions in a three-spin chain can emerge from the deterministic dynamics of three classical Ising spins. States of the latter form an ontological basis, which will be discussed with reference to the ontology proposed in the Cellular Automaton Interpretation of Quantum Mechanics by ’t[Formula: see text]Hooft. Our result illustrates a new Baker–Campbell–Hausdorff (BCH) formula with terminating series expansion.

Exchange corrections for inelastic electron scattering rates in condensed matter

A method is presented for quantifying quantum-mechanical exchange effects within the dielectric theory of electron scattering, the prevailing model for electron energy losses in condensed-matter systems. The approach utilizes a uniquely symmetric view of direct and exchange scattering events along with a generalized interference phase to account physically for the reduced scattering rates required by the Pauli exclusion of final-state fermions with corresponding spin, energy, and momentum. It is found that existing implementations either neglect or substantially overestimate the impact of exchange interference for condensed materials with broad optical loss spectra due to mathematical approximations or physical assumptions which are largely avoidable. The results suggest that the impact of exchange may alter inelastic scattering cross sections by between 10% and 20% for incident energies below 200 eV, a critical energy range for current investigations in electron and photoelectron spectroscopies.

Externally predictive quantum-mechanical models for the adsorption of aromatic organic compounds by graphene-oxide nanomaterials

ABSTRACTGraphene oxide is most often chosen as an alternative to graphene in the applications of carbon-based nanomaterials where adsorption is the primary process. However, its adsorption properties are poorly understood. The existing reports on the adsorption mechanism of graphene oxide rely on the linear free-energy/solvation-energy relationship (LFER/LSER) models. This computational work explores the role of quantum mechanical descriptors in the adsorption of aromatic organic compounds by graphene-oxide. For this, externally predictive quantitative models based on quantum-mechanical descriptors are developed and compared with the existing LSERs for the prediction of adsorption coefficients of organic compounds at three different adsorbate concentrations. The predictivity of the models is assessed using an external prediction set of compounds not used for developing the models. Notably, the mean polarizability, but originating from the quantum mechanical exchange interactions (between electrons of parallel spin), is found to be the most significant factor in driving the adsorption on graphene oxide. The present work also proposes quantum-mechanical-LSER models based on a combination of quantum-mechanical and LSER descriptors, which are in fact found to be equally predictive as the existing LSERs. The quantum-mechanical models proposed in this work are further utilized for the prediction of adsorption coefficients of aliphatic compounds.

Modeling the Solubility of C70 Fullerenes in Diverse Solvents: Role of Quantum-mechanical Descriptors.

Solubility of fullerenes in diverse solvents is difficult to model owing to the involvement of inherent quantum-mechanical interactions. This work proposes pure quantum mechanical models for predicting the solubility of C70 fullerenes. The models are developed using single-molecule quantum mechanical properties of solvents,which includes electron-correlation based descriptors accounting for the instantaneous inter-electronic interactions responsible for the origin of these properties. The external predictivity of the models was validated using an external prediction set of compounds not exposed to the model during its development. The most influential factor affecting the solubility was found to be electron-correlation contribution to the energy of highest occupied molecular orbital of solvents, which in fact, has been analyzed to suppress the solubility of fullerenes. However, the polarizability of the solvents, particularly that due to the quantum-mechanical exchange interactions between the electrons, is observed to drive the solubility of C70 fullerenes. The present work also found several other quantum mechanical properties affecting the solubility of fullerenes, which pay valuable insights towards the origin of the solubility of carbon-based nano-materials.

Quantum mechanics of a black hole: Effect of correlation energy on Schwarzschild radius and Hawking temperature

Using a single particle density distribution for the self-gravitating quantum particles that form an interacting N-particle system which ultimately forms a black hole, we from a condensed matter point of view derive the expressions of the Schwarzschild radius and the Hawking temperature. Including the quantum mechanical exchange correlation energy for the particles, we find small corrections to the above two physical quantities. The approximate expression of ground state energy and the numerical coefficient of the formula for Hawking temperature provide the hint to enable us to propose an exact expression for the ground state energy of the system.

Calculation of transition matrix element, |MAB|, in a triboelectric nanogenerator (TENG) system

Abstract The aim of this paper is to calculate the transition matrix element (MAB), which is one of the parameters related to quantum mechanical charge exchange to describe the microscopic transition process on the contact interface in a triboelectric nanogenerator (TENG) system. It is well known that |MAB| squared (|MAB|2) is correlated to the transition probability of electrons through the barrier between two materials. In particular, the potential barrier between the metal and the polymer is reflected by the electric field resulting from the tribocharges via triboelectrification. Based on the derived formula for |MAB|, a numerical analysis is performed to understand the electron transfer pattern. As a result, it is determined that |MAB| is reduced exponentially at a fixed distance, d, as the electronic energy state, E, is farther away from the Fermi-energy level (EF). In contrast, |MAB| slowly increases as the triboelectric charge density, σ, increases. That is, electrons near EF can influence |MAB| during the charge transfer process. As |MAB| differs among tribomaterials, further study into material selection could enhance TENG performance.

A fully electric field driven scalable magnetoelectric switching element

A technique for micromagnetic simulation of the magnetoelectric (ME) effect in Cr2O3 based structures has been developed. It has been observed that the microscopic ME susceptibility differs significantly from the experimentally measured values. The deviation between the two susceptibilities becomes more prominent near the Curie temperature, affecting the operation of the device at room temperature. A fully electric field controlled ME switching element has been proposed for use at technologically interesting densities: it employs quantum mechanical exchange at the boundaries instead of the applied magnetic field needed in traditional switching schemes. After establishing temperature dependent physics-based parameters, switching performances have been studied for different temperatures, applied electric fields, and Cr2O3 cross-sections. It has been found that our proposed use of quantum mechanical exchange favors reduced electric field operation and enhanced scalability while retaining reliable thermal stability.

High-temperature fusion of a multielectron leviton

The state of electrons injected onto the surface of the Fermi sea depends on temperature. The state is pure at zero temperature and is mixed at finite temperature. In the case of a single-electron injection, such a transformation can be detected as a decrease in shot noise with increasing temperature. In the case of a multi-electron injection, the situation is more subtle. The mixedness helps the development of quantum-mechanical exchange correlations between injected electrons, even if such correlations are absent at zero temperature. These correlations enhance the shot noise, what in part counteracts the reduction of noise with temperature. Moreover, at sufficiently high temperatures, the correlation contribution to noise predominates over the contribution of individual particles. As a result, in the system of $N$ electrons, the apparent charge (which is revealed via the shot noise) is changed from $e$ at zero temperature to $Ne$ at high temperatures. It looks like the exchange correlations glue up electrons into one particle of total charge and energy. This point of view is supported by both charge noise and heat noise. Interestingly, in the macroscopic limit, $N\to \infty$, the correlation contribution completely suppresses the effect of temperature on noise.

Group 10 Bis(iminosemiquinone) Complexes: Measurement of Singlet-Triplet Gaps and Analysis of the Effects of Metal and Geometry on Electronic Structure.

Bis(iminosemiquinone) complexes of divalent group 10 metals have been described as having open-shell singlet ground states characteristic of very strong coupling between the two ligand radicals. By using the nonlinear temperature dependence of the chemical shifts of the 1H NMR spectra, the singlet-triplet gaps in seven of these compounds have been measured, with the nickel compounds having gaps of about 2400 cm-1 and the palladium compounds about 1800 cm-1. Bis(iminosemiquinone)platinum complexes have singlet-triplet gaps too large to measure by this technique (over 2800 cm-1, estimated to be about 3000 cm-1), though bis(3,5-di- tert-butylbenzosemiquinonato)platinum(II) has a measurable singlet-triplet gap of 1850 cm-1. In combination with near-IR absorption data of the neutral, cationic, and anionic bis(iminosemiquinone) complexes, a simplified two-electron, two-orbital bonding model describing these compounds can be fully parametrized based on experimental data. The identity of the central metal principally affects the difference in energy between metal-ligand π nonbonding and metal-ligand π antibonding orbitals, with the strength of the bonding interactions increasing in the order Pd < Ni < Pt. Twisting the ligands out of planarity (by using a 2,2'-biphenylenediyl linker) has a marked effect on the optical spectra of the compounds but not on their singlet-triplet gaps; this indicates that the effect is not due to changes in bonding interactions but rather due to a decrease in the magnitude of the quantum mechanical exchange interactions in the twisted compared to the flat compounds.

Quantum Two-Stream Instability with Exchange Interaction

Quantum plasmas can be described in the framework of the self-consistent field approach by a set of time-dependent Schrödinger equations (TDSE) for the electrons in a self-consistent potential. This approach is also well-known as the multistream model of quantum plasmas. The self-consistent field is commonly calculated in the Vlasov approximation from the average charge density of all electrons thereby neglecting the quantum-mechanical exchange interaction. In the present work, we consider a linear Hartree-Fock model with screened Coulomb interactions to examine the role of the exchange interaction. The two-stream instability is analysed and results for the Vlasov, Hartree and Hartree-Fock approximations are compared in detail. It is found that the exchange interaction considerably modifies the stability behavior for weakly screened Coulomb interactions. In particular, crossings of the two branches of the dispersion relation lead to oscillating unstable solutions that are not present in the absence of the exchange interaction.

Superexchange-mediated magnetization dynamics with ultracold alkaline-earth atoms in an optical lattice**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFA0301504).

Superexchange and inter-orbital spin-exchange interactions are key ingredients for understanding (orbital) quantum magnetism in strongly correlated systems and have been realized in ultracold atomic gases. Here we study the spin dynamics of ultracold alkaline-earth atoms in an optical lattice when the two exchange interactions coexist. In the superexchange interaction dominating regime, we find that the time-resolved spin imbalance shows a remarkable modulated oscillation, which can be attributed to the interplay between local and nonlocal quantum mechanical exchange mechanisms. Moreover, the filling of the long-lived excited atoms affects the collapse and revival of the magnetization dynamics. These observations can be realized in state-dependent optical lattices combined with the state-of-the-art advances in optical lattice clock spectroscopy.

High-coercivity magnetism in nanostructures with strong easy-plane anisotropy

We report the fabrication of a rare-earth-free permanent-magnet material Co3Si in the form of nanoparticles and investigate its magnetic properties by experiments and density-functional theory (DFT). The DFT calculations show that bulk Co3Si has an easy-plane anisotropy with a high K1 ≈ −64 Merg/cm3 (−6.4 MJ/m3) and magnetic polarization of 9.2 kG (0.92 T). In spite of having a negative anisotropy that generally leads to negligibly low coercivities in bulk crystals, Co3Si nanoparticles exhibit high coercivities (17.4 kOe at 10 K and 4.3 kOe at 300 K). This result is a consequence of the unique nanostructure made possible by an effective easy-axis alignment in the cluster-deposition method and explained using micromagnetic analysis as a nanoscale phenomenon involving quantum-mechanical exchange interactions.