Antiparallel Spin Electrons Research Articles

Entropy production by thermodynamic currents in ambipolar conductors with identical spin dynamics characteristics between holes and electrons

We have evaluated the role of spin current in the energy dissipation mechanism in ambipolar conductors with identical spin-related characteristics between holes and electrons. Application of the Gibbs–Duhem relation to ambipolar conductors establishes a thermodynamic relation between the spin-dependent chemical potentials of holes and electrons, inducing an asymmetric spin splitting between the hole and electron chemical potentials. This yields two types of spin relaxation so as to allow the antiparallel spin current, where hole and electron spins flow in the opposite direction, to have a large spin diffusion length, but to retain that of the parallel spin current at a standard value.

Entanglement creation in electron-electron collisions at solid surfaces

For spin-polarized low-energy electrons impinging on a crystalline surface, an important reaction channel is the collision with a bound valence electron of opposite spin, followed by the emission of a correlated electron pair with antiparallel spins. While primary and valence electrons are not entangled, the screened Coulomb interaction generates spin entanglement between the two outgoing electrons. As a quantitative measure of this entanglement, we calculated a modified von Neumann entropy in terms of direct and exchange transition matrix elements. For coplanar symmetric setups with equal energies of antiparallel-spin electrons, maximal entanglement is analytically shown to occur quite universally, irrespective of the choice of the primary electron energy, the outgoing electron energy, and polar emission angle, and even of the choice of the surface system. Numerical results for Fe(110) and Cu(111) demonstrate first that strong entanglement can persist for unequal energies and second that an overall entanglement reduction due to nonentangled parallel-spin electrons can be avoided for ferromagnetic and even for nonmagnetic surfaces.

Stray Charge in Quantum-dot Cellular Automata: A Validation of the Intercellular Hartree Approximation

The authors analyze the effect of stray charges near a line of quantum-dot cellular automata (QCA) cells. Considering both the ground-state polarization and the excitation energy of the system, it is determined that there is a 129-nm-wide region surrounding a QCA wire where a stray charge will cause the wire to fail. This calculation is the result of a full-basis-set simulation of a four-cell line. A comparison is made between cells with parallel-spin electrons and those with antiparallel spin electrons, showing that they yield essentially identical results. Therefore, the added complexity of accounting for antiparallel spins does not yield superior simulation results. Finally, a comparison is made between the full-basis calculations and the results of the same calculation using the intercellular Hartree approximation (ICHA). The similarity of these two results demonstrates that the ICHA method is a valid tool for studying the effect of stray charges in larger systems.

Electron localizability indicators ELI–D and ELIA for highly correlated wavefunctions of homonuclear dimers. I. Li2, Be2, B2, and C2

Electron localizability indicators based on the parallel-spin electron pair density (ELI-D) and the antiparallel-spin electron pair density (ELIA) are studied for the correlated ground-state wavefunctions of Li(2), Be(2), B(2), and C(2) diatomic molecules. Different basis sets and reference spaces are used for the multireference configuration interaction method following the complete active space calculations to investigate the local effect of electron correlation on the extent of electron localizability in position space determined by the two functionals. The results are complemented by calculations of effective bond order, vibrational frequency, and Laplacian of the electron density at the bond midpoint. It turns out that for Li(2), B(2), and C(2) the reliable topology of ELI-D is obtained only at the correlated level of theory.

The analysis of the knight shift in the μ+ SR technique

We present an analysis and first-principles calculations of the Knight shifts of a positive muon (μ+) in normal and transition metals using a cellular multiple-scattering technique. The main conclusions are the following: (a) The μ+ induces the segregation of states (one of each spin) from the conduction band to lower energies. This cluster bonding state is occupied with two (antiparallel spin) electrons but in general it has less than 35% of μ+1s character. The screening is completed with the conduction electrons, (b) The observed Knight shift is the sum of a diamagnetic shielding (of the order —20 to —50 ppm arising in almost equal parts from the localized bonding cluster states and from the diamagnetic shielding of the conduction electrons) and a spin-enhanced paramagnetic shielding, large for all metals except for the divalent normal metals having a low density of states at the Fermi level which seats at the separation between the s- and the p-conduction bands. A spin-polarized calculation shows that the local spin density at the μ+ site is definitely smaller or even different in sign than the value computed from the local charge density enhancement. The ferromagnetic transition metals, because of the d-electron polarization, present a negative local magnetization at the μ+ site which gives a dominant negative contribution to the observed values.

Short-Range Electron Correlations of CO<SUB>2</SUB> Molecule by First Principles <I>T</I>-Matrix Calculations

In this paper, we obtain the double ionization energy spectra and the two-electron distribution functions from the first principles evaluating the ladder diagrams up to the infinite order (T-matrix) and discuss the short-range electron correlations. The T-matrix, which describes the multiple scattering between electrons (or holes), can accurately represent the short-range repulsive Coulomb interaction. The double ionization energy spectra calculated for the CO2 molecule agree with the corresponding experiments very well. And the two-electron distribution function obtained from the T-matrix clearly shows ‘‘Coulomb hole’’ due to the avoidance of the antiparallel spin electrons confined in a small region. [doi:10.2320/matertrans.48.638]

Spin-Component Scaling Methods for Weak and Stacking Interactions.

New scaling parameters are presented for use in the spin-component scaled (SCS) variant of density fitted local second-order Møller-Plesset perturbation theory (DF-LMP2) that have been optimized for use in evaluating the interaction energy between nucleic acid base pairs. The optimal set of parameters completely neglects the contribution from antiparallel-spin electron pairs to the MP2 energy while scaling the parallel contribution by 1.76. These spin-component scaled for nucleobases (SCSN) parameters are obtained by minimizing, with respect to SCS parameters, the rms interaction energy error relative to the best available literature values, over a set of ten stacked nucleic acid base pairs. The applicability of this scaling to a wide variety of noncovalent interactions is verified through evaluation of a larger set of model complexes, including those dominated by dispersion and electrostatics.

Two-electron distribution functions and short-range electron correlations of atoms and molecules by first principles T-matrix calculations

The accurate first principles description of the correlations between electrons has been a topic of interest in molecular physics. We have reported in our previous paper [J. Chem. Phys. 123, 144112 (2005)] that the T matrix, which is the ladder diagrams up to the infinite order, can accurately represent the short-range electron correlations while calculating the double ionization energy spectra of atoms and molecules. In this paper, we calculate the two-electron distribution functions of real systems (Ar, CO, CO(2), and C(2)H(2)) from the eigenvalue equation associated with the Bethe-Salpeter equation for the T matrix by beginning with the local density approximation of the density functional theory and the GW approximation. We found that when the interelectron distance is very small, the Coulomb hole appears between antiparallel spin electrons due to the short-range repulsive Coulomb interaction. The resulting two-electron distribution functions clearly show the Coulomb hole.

Time-dependent electron localization functions for coupled nuclear-electronic motion

We study the quantum dynamics in a model system consisting of two electrons and a nucleus which move between two fixed ions in one dimension. The numerically determined wave functions allow for the calculation of time-dependent electron localization functions in the case of parallel spin and of the time-dependent antiparallel spin electron localization functions for antiparallel spin. With the help of these functions, it becomes possible to illustrate how electronic localization is modified through the vibrational wave-packet motion of the nucleus.

Correlation energies of light atoms related to pairing between antiparallel spinelectrons

Early theoretical work emphasized the gross trend of thetotal correlation energy of neutral atoms with atomic numberZ. Thiswas later improved quantitatively, by focusing on the number of pairings betweenantiparallel-spin electrons in the same main shell (i.e., K, L, M...shells).Here, this viewpoint is pressed, and correlation energies associated with s–s, s–pand p–p pairings are extracted. The s–s energy turns out to be rather insensitiveto a change from K to L shells, but a small increase is found to occur in the Mshell. Also, the s–p and p–p pairing energies show a small increase from the L tothe M shells. The s–s and s–p energies are ≈1 eV, thes–s contribution being the larger. The p–p energy is ≈1/3 eV.The effect of removing or adding one electron to the neutral atom has also beenanalysed. Wavefunction overlap and wavefunction localization appear to controlthe values of the pairing energies.

Effective interactions between parallel-spin electrons in two-dimensional jellium approaching the magnetic phase transition

We evaluate the effective interactions in a fluid of electrons moving in a plane, on the approach to the quantum phase transition from the paramagnetic to the fully spin-polarized phase that has been reported from Quantum Monte Carlo runs. We use the approach of Kukkonen and Overhauser to treat exchange and correlations under close constraints imposed by sum rules. We show that, as the paramagnetic fluid approaches the phase transition, the effective interactions at low momenta develop an attractive region between parallel-spin electrons and a corresponding repulsive region for antiparallel-spin electron pairs. A connection with the Hubbard model is made and used to estimate the magnetic energy gap and hence the temperature at which the phase transition may become observable with varying electron density in a semiconductor quantum well.

Looking at Chemical Bonding from Coulomb and Exchange Correlations in NAOs

Coulomb and exchange correlations, providing information for the interactions of antiparallel and parallel spin electrons, respectively, are investigated in orbitals appropriate for population analysis, such as the natural atomic orbitals (NAOs). In the proposed analysis, both correlations are treated on an equal footing in configuration interaction (CI) (or Hartree−Fock) levels, but an emphasis is given for coulomb correlations and their physical meaning. It is stressed that the two-center interactions of antiparallel spin electrons can be “repulsive” and “attractive” (as a direct consequence of chemical bonding), but the former are less important than the latter; their relative importance is determined by the magnitude of one-center interactions. These globally attractive two-center interactions are balanced by the repulsive one-center interactions. These conclusions are general and hold for any molecular system, under the only assumption that the one-center interactions are repulsive. Molecular orbital...

Short-range correlations in a two-dimensional electron gas

We construct a model for the value of the pair distribution functiong↑↓(0) between antiparallel-spin electrons at contact ina two-dimensional electron gas with e2/r interactions, as a function of thecoupling-strength parameter rs. The model involves an interpolation betweenthe result of a low-rs expansion, including the second-order direct andexchange contributions to the energy in the paramagnetic state, and the resultof a partial-wave phase-shift analysis near Wigner crystallization. Theinterpolation formula is in excellent agreement with many-body calculationsbased on the ladder approximation. We further show through an STLSself-consistent calculation that g↑↓(0) is essentiallyindependent of the state of spin polarization of the electron gas.

On relations between correlation, fluctuation and localization

The ground states of many-electron systems (atoms, molecules, solids) are considered under the aspect of mutual relations among the phenomena of correlation, fluctuation, and localization. Known measures of the correlation strength are summarized (Collins' information entropy, Kutzelnigg's statistical correlation coefficients, Fulde's correlation strength, Cioslowski's strength of short-range correlation) and new indices are proposed (normalization of the correlation tail of the natural occupation numbers, ‘occupation gap’, on-top properties of the intracule pair densities for parallel- and antiparallel spin electron pairs). In addition to these quantum-kinematical quantities, new reference-free ratios of kinetic and potential energies are proposed. Also the concept of local (spatially distributed) correlation measures is presented. It is shown how correlation suppresses fluctuation (e.g. in Wigner–Seitz cells, Bader's basins, Daudel's loges, etc.) and drives localization. The uniform electron gas and the isoelectronic He series serve as illustrating examples.

The wavefunction when antiparallel spin electrons coincide and its relation to the ground-state electron density in the Hookean atom

For a specific force constant, the spatial form of the ground-state wavefunction Ψ( r 1, r 2) is known analytically for the Hookean atom in which two electrons repel coulombically. Here, it is first shown that the wavefunction Ψ( r, r) for the antiparallel spin electrons at the same position r is sufficient to construct the ground-state density in this model example. An off-diagonal generalization is then effected, which relates the one-particle density matrix to the electron pair correlation function, again for this same model.

Spin contamination in quantum Monte Carlo wave functions

The wave function usually employed in quantum Monte Carlo (QMC) electronic structure calculations is the product of a Jastrow factor and a sum of products of up-spin and down-spin determinants. Typically, a different Jastrow factor is used for parallel- and antiparallel-spin electrons in order to satisfy the cusp conditions and thereby ensure that the local energy at electron–electron coincidence points is finite. However, when the Jastrow factor is not completely symmetric under interchange of the spatial coordinates of the electrons, the resulting wave function may not be an eigenstate of the spin operator Ŝ2. For the Li and Be atoms, we evaluate the spin contamination in a variety of wave functions with progressively higher body-order correlations in the Jastrow factor. The spin contamination is found to be small for all the wave functions (10−3–10−5), the smaller values being obtained for the more accurate wave functions. On the other hand, if we eliminate the spin contamination by employing a symmetric Jastrow (and sacrificing the parallel-spin cusp condition), the resulting wave functions typically have about 20%–40% larger root mean square fluctuations in the local energy. A wave function which both satisfies the cusp conditions and has definite spin can be constructed, but for systems with many electrons, its computational cost is higher than for the commonly used QMC wave functions.

Variation of the ground-state correlation energy of atoms with atomic number

The empirical correlation energy of atoms and singly charged positive and negative ions in their ground state shows a roughly linear dependence with respect to the number of electrons. However, the deviations from this linear behaviour follow a well defined trend. A formulation based on density functional theory allows us to understand these features. This formulation focuses on the number of pairings between antiparallel-spin electrons in the same main shell.

Correlation of two anti-parallel spin electrons in the same orbital state

Correlation of two anti-parallel spin electrons in the same orbital state for strong correlation systems is introduced. For the energy spectrum taking into account this kind of correlation in the mean field approximation of the Hamiltonian is equivalent to the cut-off at the third order of the Green function chain of the Zubarev method. For cuprate oxides four sub-bands are obtained from a two-band Hamiltonian, which is in agreement with the relation between carrier density and doping density found in optical reflectivity experiments.

Simulation of Electrons in Molten Salts

Metal molten salt solutions exhibit a number of interesting properties with variation of the metal concentration. In the dilute limit, the metal valence electrons are released to form F-center-like localized states. At higher concentrations, metallic behavior sets in.A theoretical approach to the properties of these systems obviously requires a correct quantum-mechanical treatment of the solvated electrons. We show here that an approach that uses a local spin density description of many electron interactions, is a powerful and efficient method for the study of the adiabatic dynamics of such systems.We apply our approach to the case of one and two solvated electrons. In the first case we confirm the F-center model and elucidate the mechanism of electron diffusion. In the case of two solvated electrons, we find that parallel spin electrons repel each other and form separate F-center-like states. Antiparallel spin electrons, instead, attract each other and coalesce into a single bipolaronic complex. The diffusion of the bipolaron, while bound, occurs on a ionic time scale. However, dissociation process occur during which the electrons can acquire a high mobility leading to a large electronic diffusion.At even higher concentrations preliminary indication of clustering and of metallic behavior are observed.

Mean free paths of very-low-energy electrons: The effects of exchange and correlation

The mean free paths of low-energy electrons in a free-electron-like material are calculated using a screened electron-electron interaction which is antisymmetrized for the case of parallel-spin electrons. Calculations are carried out for several different approximations of the screening function: Fermi-Thomas, Lindhard, Singwi, and Kukkonen-Overhauser. The first three yield mean free paths that agree to within 10%, while the Kukkonen-Overhauser screening yields mean free paths that are roughly half those given by the other approximations. The effect of the Pauli principal in all cases is that the scattering between antiparallel-spin electrons is roughly three to ten times stronger than between parallel-spin electrons.