Localized Electron Pairs Research Articles

Electronic paddle-wheels in a solid-state electrolyte

Solid-state superionic conductors (SSICs) are promising alternatives to liquid electrolytes in batteries and other energy storage technologies. The rational design of SSICs and ultimately their deployment in battery technologies is hindered by the lack of a thorough understanding of their ion conduction mechanisms. In SSICs containing molecular ions, rotational dynamics couple with translational diffusion to create a paddle-wheel effect that facilitates conduction. The paddle-wheel mechanism explains many important features of molecular SSICs, but an explanation for ion conduction and anharmonic lattice dynamics in SSICs composed of monatomic ions is still needed. We predict that ion conduction in the classic SSIC AgI involves electronic paddle-wheels, rotational motion of localized electron pairs that couples to and facilitates ion diffusion. The electronic paddle-wheel mechanism creates a universal perspective for understanding ion conductivity in both monatomic and molecular SSICs that will create design principles for engineering solid-state electrolytes from the electronic level up to the macroscale.

HClO4 BONDING CHARACTERISTICS AGAINST FORMAL CHARGE PREDICTION AND HYPERVALENCE CONCEPT. SHOULD THESE CONCEPTS BE TAUGHT?

Chemical bonding is a central topic in general chemistry courses. Lewis (1916) published a fundamental paper where he proposed a model based on localized electron pairs and as a consequence, gave rise to the octet rule. However, the discovery of xenon fluorides and compounds such as SF6 gave rise to the concept of “hypervalence” involving d orbitals in hybridization schemes. Currently, it has been demonstrated that d orbitals have a negligible participation in the bonds of such molecules and, in some cases octet rule is not violated. This article proposes a discussion about the concepts we use in introductory chemistry lectures. Some models are useful and valuable from a pedagogical point of view, nevertheless, others are unnecessary and far from reality causing misconceptions and lack of interest, since it is hard to rationalize inconsistencies found. HClO4 molecule was selected to demonstrate that hypervalence concept, molecular structure selection based on formal charge rules and participation of d orbitals on bonds are not appropriate approximations. Bond orders and atomic charges determined through Natural Bond Orbital calculations indicate bonds between Cl and O atoms are mainly simple with a high ionic character.

How electron delocalization influences the electron-withdrawing properties of isomeric benzobischalcogenadiazoles

The fermionic potential and delocalization indices for benzo-bis-1,2,5-chalcogenadiazoles reveal inhomogeneous electron delocalization in their benzene ring, which results in compactly localized lone electron pairs on the chalcogen atoms. These features of (de)localization are rooted in a local increase in the kinetic component of the electron correlation, which expresses the Fermi hole variability and the kinetic potential response to electron density variations in the benzene ring of benzobis-1,2,5-chalcogenadiazoles. This explains their better electron-withdrawing properties compared to benzobis-1,2,3-chalcogenadiazoles

Electronic State of Arsenic <i>endo</i>-atom and indices of interatomic bonds in[As@Ni<sub>12</sub>As<sub>20</sub>]<sup>3-/0</sup>, As<sub>20</sub>, Ni<sub>12</sub>As<sub>20</sub>, As@C<sub>60</sub>, and As@C<sub>70</sub> Clusters

The DFT PBE0/SDD method was used to calculate bond lengths and bond indices in As20, Ni12As20, [As@Ni12As20]3-, As@C60 and As@C70 clusters. The degrees of oxidation and reduction of the endo -atom and the shell are expressed in terms of the populations of one-electron states localized in these components of the complexes. Each As atom in clusters has a entirely localized lone electron pair. The arsenic atom inside fullerenes retains the electronic configuration and spin of the ground state of the free As atom. Inside the [Ni12As20]6- shell, it has an oxidation state of 3+. There is no covalent bond between the endo -atom and the shell in clusters. The bond indices refute the opinion about the “onion” structure of [As@Ni12@As20]3-: the nickel atoms are not bonded to each other, the As-As bond indices are three times lower than in As20.

Electron delocalization in defect-containing graphene and its influence on tetrel bond formation.

Using the advanced analyses of electron density and fermionic potential, we show how electron delocalization influences the ability of defect-containing graphene to form tetrel bonds. The Cg atoms of a vacancy defect can produce one nonpolar interaction, alongside a peculiar polar Cg⋯Cg bond. The latter stems from the presence of a localized electron pair on a vacancy defect Cg atom and the local depletion of electron localization on another Cg atom. This interaction is an example of intralayer tetrel bond. In the presence of an absorbed molecule of bisphenol A diglycidyl ether (DGEBA), graphene is able to form incipient tetrel Cg⋯O bonds with an ether group oxygen. In contrast to an epoxy group oxygen, the disposition of the ether oxygen often causes the orientation of electron-rich π-domains of graphene carbon on the weakly expressed electrophilic region of the oxygen. In the case of graphene with a point Si defect, the Si atom can form quite strong Si⋯C interactions with the DGEBA aryl carbons. In contrast to other noncovalent bonds, this interaction significantly alters the electron (de)localization on the Si atom and in the aryl ring. The reliability of the obtained results is enhanced by the use of multiple 2D periodic models with defects located at different positions along the DGEBA skeleton.

Quantum chemical modeling of electron‐deficient hollow TlkPb12–k and TlkBi20–k shells and related endohedral complexes (k = 1; 2)

AbstractThe equilibrium structural parameters, symmetry groups, and dipole moments of the free hollow electron‐deficient TlBi19, Tl2Bi18, p‐, m‐, o‐Tl2Pb10 shells and the endohedral (He, Ne)@Tl2Pb10, (Y, La, Lu, Tl, Bi, Ce)@TlPb11, Tl@(Y, La, Lu) Pb11, Sr@TlBi19, Tl@SrBi19, Yb@TlBi19, Tl@YbBi19, [Y@TlBi19]+, [Lu@TlBi19]+, Sr@Tl2Bi18, and Yb@Tl2Bi18 complexes are predicted by using DFT (U)PBE0 hybrid method. The isomerization of (Sr, Yb)@TlkBi20–k into Tl@(Sr, Yb) Tlk–1Bi20–k is endothermic. The isodesmic substitution of an endo‐atom in the Tl@TlPb11 cluster for a rare‐earth atom is exothermic. The hollow TlkBi20–k (Sr, Yb) complexes with Tl in the shell and with Sr or Yb exo‐atom are impossible. The endo‐atoms plunge in hollow TlPb11 and hollow TlBi19 shells without delay on its surface. The (C5v)‐TlPb11 and (C3v)‐TlBi19 shells are stabilized by an endo‐atom X with oxidation number ΞX = +3 and +2, respectively. No one cerium IV oxidation state has been predicted. The integer number ΞX is presented as a function of the populations of one‐electron states in the shell surrounding the endo‐atom X. Each Pb or Bi atom in the electron‐deficient clusters retains an entirely localized lone electron pair. The change in the IR spectrum when a metal atom is embedded into its cavity indicates a strong interaction between the components of the endohedral complex. Structural aspects of the β‐decay of a radioactive endo‐atom in the cavity of a metal cluster are discussed.

Efficient Slave-Boson Approach for Multiorbital Two-Particle Response Functions and Superconductivity

We develop an efficient approach for computing two-particle response functions and interaction vertices for multiorbital strongly correlated systems based on fluctuation around rotationally-invariant slave-boson saddle-point. The method is applied to the degenerate three-orbital Hubbard-Kanamori model for investigating the origin of the s-wave orbital antisymmetric spin-triplet superconductivity in the Hund's metal regime, previously found in the dynamical mean-field theory studies. By computing the pairing interaction considering the particle-particle and the particle-hole scattering channels, we identify the mechanism leading to the pairing instability around Hund's metal crossover arises from the particle-particle channel, containing the local electron pair fluctuation between different particle-number sectors of the atomic Hilbert space. On the other hand, the particle-hole spin fluctuations induce the s-wave pairing instability before entering the Hund's regime. Our approach paves the way for investigating the pairing mechanism in realistic correlated materials.

A Critical Look at Linus Pauling's Influence on the Understanding of Chemical Bonding.

The influence of Linus Pauling on the understanding of chemical bonding is critically examined. Pauling deserves credit for presenting a connection between the quantum theoretical description of chemical bonding and Gilbert Lewis’s classical bonding model of localized electron pair bonds for a wide range of chemistry. Using the concept of resonance that he introduced, he was able to present a consistent description of chemical bonding for molecules, metals, and ionic crystals which was used by many chemists and subsequently found its way into chemistry textbooks. However, his one-sided restriction to the valence bond method and his rejection of the molecular orbital approach hindered further development of chemical bonding theory for a while and his close association of the heuristic Lewis binding model with the quantum chemical VB approach led to misleading ideas until today.

Merohedral disorder and impurity impacts on superconductivity of fullerenes

Local quasiparticle states around impurities provide essential insight into the mechanism of unconventional superconductivity, especially when the candidate materials are proximate to an antiferromagnetic Mott-insulating phase. While such states have been reported in atom-based cuprates and iron-based compounds, they are unexplored in organic superconductors which feature tunable molecular orientation. Here we employ scanning tunneling microscopy and spectroscopy to reveal multiple forms of robustness of an exotic s-wave superconductivity in epitaxial Rb3C60 films against merohedral disorder, non-magnetic single impurities and step edges at the atomic scale. Yu-Shiba-Rusinov (YSR) states, induced by deliberately incurred Fe adatoms that act as magnetic scatterers, have also been observed. The YSR bound states show abrupt spatial decay and vary in energy with the Fe adatom registry. These results and a doping-dependent study of superconductivity point towards local electron pairing in which the multiorbital electronic correlations and intramolecular phonons together drive the high-temperature superconductivity of doped fullerenes.

Unusual electronic state of Sn in AgSnSe2

${\mathrm{AgSnSe}}_{2}$, by formal electron count, should have Sn in a highly unusual $3+$ valence state and was therefore suggested to be a valence-skipping compound with potential for negative-$U$ centers and local electron pairing. It has been proposed that the latter may be the mechanism beyond seemingly conventional superconductivity in this compound. We report NMR measurements and first-principles calculations that agree with each other perfectly, and both indicate that valence skipping does not take place and the highly unusual ${\mathrm{Sn}}^{3+}$ state is realized instead, likely because of geometrical constraint prohibiting a breathing distortion that could screen the on-site Coulomb repulsion.

Connection between the charge density waves and the superconducting state in the two-Fluid model of superconductivity in HTSC

The two-fluid model of superconductivity is used to analyze the physical properties of various HTSCs as a function of different SC gap locations Δ and Δ1 along the energy spectra. In this model, upon doping with oxygen, the charge density waves (CDW) in CuO2 planes, formed in the oxygen anion system, are transformed into local electron pairs (LEPs) by Cu ion oscillations at T*. At Tc < T* the LEPs create a SC state and a gap Δ1. In addition, there is a restructuring of the p- and d-bands. The band gap ΔE decreases. At ΔE ∼ Δ1 the decaying LEPs are transferred from the occupied states of the oxygen anion p-band to the free d-subband of the Cu ions due to thermal excitation. The SC state and the d-type gap Δ in the d-subband can be associated with antiferromagnetic (AF) fluctuations. In tunneling experiments without oxygen anions and oscillating Cu ions, the LEPs undergo strong Coulomb electron repulsion. Therefore, the LEPs and, accordingly, Δ1 will be strongly distorted. This is in contrast to the slight change in the Cooper pairs of conventional superconductors in similar experiments. It seems that the LEPs are transitioning into another state, which can be referred to as the CDW state with a pseudogap Σ. It is also necessary to take into account minor changes in the electron pairs and the second gap Δ created by antiferromagnetic fluctuations.The two-fluid model of superconductivity is used to analyze the physical properties of various HTSCs as a function of different SC gap locations Δ and Δ1 along the energy spectra. In this model, upon doping with oxygen, the charge density waves (CDW) in CuO2 planes, formed in the oxygen anion system, are transformed into local electron pairs (LEPs) by Cu ion oscillations at T*. At Tc < T* the LEPs create a SC state and a gap Δ1. In addition, there is a restructuring of the p- and d-bands. The band gap ΔE decreases. At ΔE ∼ Δ1 the decaying LEPs are transferred from the occupied states of the oxygen anion p-band to the free d-subband of the Cu ions due to thermal excitation. The SC state and the d-type gap Δ in the d-subband can be associated with antiferromagnetic (AF) fluctuations. In tunneling experiments without oxygen anions and oscillating Cu ions, the LEPs undergo strong Coulomb electron repulsion. Therefore, the LEPs and, accordingly, Δ1 will be strongly distorted. This is in contrast to the slight ...

Using the localized pair model to describe the nature of the hydrogen bond

Herein, we present a novel study of hydrogen bonding using the localized pair model. Using localized molecular orbitals, we examine how the two localized electron pairs involved in hydrogen bonding change upon the formation of the interaction, as well as while the distance between the two species within the complex change. This is achieved through the use of extracule densities, which describe the probability associated with the centre of mass of an electron pair. Quantitative assessments are performed by analyzing the extracule density demonstrating that this method is on par with other density-based tools for the analysis of hydrogen bonds and their strengths.

Structural and electronic properties of the alkali metal incommensurate phases

Alkali elements form extremely complicated structures under pressure, involving two interpenetrating, incommensurate lattices. The mechanism that stabilizes these structures is not known. In this work, the authors use density functional theory calculations to examine several possible options. After demonstrating that the calculations can capture quite subtle structural changes across the different elements, they show that in all cases the respective host structures combine two types of electride behavior: fully localized electron pairs and electrons partially localized along 1D wells. The guest atoms are seen to aid the structural formation by providing additional electronic charge and optimizing the overall packing density

Transport and excitations in a negative-U quantum dot at the LaAlO3/SrTiO3 interface

In a solid-state host, attractive electron–electron interactions can lead to the formation of local electron pairs which play an important role in the understanding of prominent phenomena such as high Tc superconductivity and the pseudogap phase. Recently, evidence of a paired ground state without superconductivity was demonstrated at the level of single electrons in quantum dots at the interface of LaAlO3 and SrTiO3. Here, we present a detailed study of the excitation spectrum and transport processes of a gate-defined LaAlO3/SrTiO3 quantum dot exhibiting pairing at low temperatures. For weak tunneling, the spectrum agrees with calculations based on the Anderson model with a negative effective charging energy U, and exhibits an energy gap corresponding to the Zeeman energy of the magnetic pair-breaking field. In contrast, for strong coupling, low-bias conductance is enhanced with a characteristic dependence on temperature, magnetic field and chemical potential consistent with the charge Kondo effect.

Correlation effects on the interelectronic distributions of localized electron pairs

It has been shown previously that a wealth of chemical insight may be drawn from studying the interelectronic distribution function for localized electron pairs within a molecular system (Hennessey et al. in Phys Chem Chem Phys 16(46):25548–25556, 2014); however, these accounts have been limited due to the relative inaccuracy of the underlying Hartree–Fock electronic structure model that has been employed. In the current work, we study how the interelectronic distribution function of single electron pairs, represented by localized molecular orbitals, changes between a Hartree–Fock and density functional electronic structure model. We find that localized electron pairs expand, relative to a Hartree–Fock model, in a small but significant way when modelled with density functional theory. More specifically, we generally find that compact electron pairs (i.e. those pairs having narrow distribution functions near small interelectronic distances) result in a smaller change due to correlation than electron pairs more broadly distributed. This counterintuitive effect is attributed to the fact that compact electron pairs are generally held close together by a relatively rigid confining potential (i.e. attraction from local nuclei and repulsion from neighbouring electrons). However, in cases where a series of species all have nearly identical nuclear structure in the vicinity of the LMO of interest, more compact electron pairs did in fact experience the greatest change in interelectronic separation and electron repulsion upon the incorporation of a correlated model. Also, if one allows the geometry of the molecular species to relax, bonds associated with LMOs having compact interelectronic distributions tend to elongate the most with a correlated model (relative to Hartree–Fock) and the resultant deformation of the interelectronic distribution functions then show the most significant changes due to correlation. The results presented herein demonstrate the utility of Kohn–Sham density functional theory for the description of localized chemical space within the scope of the localized pair model.

Using the centre-of-mass of localized electron pairs to quantify electronegativity

Localized electron pair densities are explored with the purpose of developing a relationship between properties of the electron pair centre-of-mass and atomic electronegativities. These electron pair centre-of-mass densities are determined for the localized molecular orbital representing the A–H bond in a compound AHn (where n = 1–4) for each of the first and second row atoms with available valence sites. By analyzing the topography of these densities, we observe the migration of the electron pair as the electronegativity of atom A changes. We demonstrate strong linear correlations between features of these centre-of-mass densities and various empirically based electronegativity scales, all of which are defined in significantly different ways. Based on strong agreement with past electronegativity scales, we propose the use of localized electron pair centre-of-mass densities as a simple and intuitive theoretical model for electronegativity. We have presented our own electronegativity scale, employing Pauling units as is common amongst many existing scales.

Condensed-matter physics: Superconducting electrons go missing.

'Overdoped' high-temperature superconductors, which have a high density of charge carriers, were thought to be well understood. An experiment challenges what we know about quantum physics in such systems. See Letter p.309 Ivan Božovic et al. present a comprehensive study of the key physical properties of the overdoped copper oxide superconductor La2−xSrxCuO4. Their results run counter to the common assumption that strongly correlated fermion physics evolves smoothly into conventional Bardeen–Cooper–Schrieffer (BCS) behaviour in overdoped copper oxide superconductors. Rather, in La2−xSrxCuO4 the scaling law for the critical superconducting temperature and zero-temperature phase stiffness does not conform to standard BCS physics. The authors speculate that the high critical temperature derives from local electron pairing and unusual kinematics.

Nonlocal quantum state engineering with the Cooper pair splitter beyond the Coulomb blockade regime

A Cooper pair splitter consists of two quantum dots side-coupled to a conventional superconductor. Usually, the quantum dots are assumed to have a large charging energy compared to the superconducting gap, in order to suppress processes other than the coherent splitting of Cooper pairs. In this work, in contrast, we investigate the limit in which the charging energy is smaller than the superconducting gap. This allows us, in particular, to study the effect of a Zeeman field comparable to the charging energy. We find analytically that in this parameter regime the superconductor mediates an inter-dot tunneling term with a spin symmetry determined by the Zeeman field. Together with electrostatically tunable quantum dots, we show that this makes it possible to engineer a spin triplet state shared between the quantum dots. Compared to previous works, we thus extend the capabilities of the Cooper pair splitter to create entangled non local electron pairs.

Fermi-Bose Mixture in Ba(K)BiO3 Superconducting Oxide

We have demonstrated a new description of local electronic structure in the perovskite-like bismuthates Ba1−x K x BiO3 (BKBO) based on existence of the spatially separated Fermi-Bose mixture. We have shown that two types of charge carriers: the local electron pairs (real-space bosons) and the itinerant electrons exist in metallic compound Ba1−x K x BiO3 (x ≥ 0.37). The real-space bosons are responsible for both charge transport in semiconducting BaBiO3 and superconductivity in metallic BKBO, while the fermionic subsystem is responsible for the observed metal-insulator phase transition and appearance of the Fermi-liquid state when the percolation threshold is overcome (x ≥ 0.37). Bosons and fermions occupy different types of the octahedral BiO6 complexes, so they are separated in real space. This scenario fits well into a new quantum state of pair-density wave (PDW), actively discussed for copper-based HTSC.

Dynamical Local Lattice Instability Triggered High TcSuperconductivity

High Tc cuprate superconductors are characterized by two robust features: their strong electronic correlations and their intrinsic dynamical local lattice instabilities. Focusing on exclusively that latter, we picture their parent state in form of a quantum vacuum representing an electronic magma in which bound diamagnetic spin-singlet pairs pop in and out of existence in a Fermi sea of itinerant electrons. The mechanism behind that resides in the structural incompatibility of two stereo-chemical con gurations CuO4 and Cu O4 which compose the CuO2 planes. It leads to spontaneously uctuating Cu O Cu valence bonds which establish a local Feshbach resonance exchange coupling between bound and unbound electron pairs. The coupling, being the only free parameter in this scenario, the hole doping of the parent state is monitored by varying the total number of unpaired and paired electrons, in chemical equilibrium with each other. Upon lowering the temperature to below a certain T ∗, bound and unbound electron pairs lock together in a local quantum superposition, generating transient localized bound electron pairs and a concomitant opening of a pseudo-gap in the single-particle density of states. At low temperature, this pseudo-gap state transits via a rst order hole doping induced phase transition into a superconducting state in which the localized transient bound electron pairs get spatially phase correlated. The mechanism driving that transition is a phase separation between two phases having di erent relative densities of bound and unbound electron pairs, which is reminiscent of the physics of He He mixtures.