On the possibility of effective attraction between delocalized electrons in organic systems

The attractive interaction between delocalized electrons in organic systems arising from the polarization of both their σ- and neighbouring π-systems is shown to be able to overcompensate the Coulomb repulsion. The crucial role of the asymmetry of the system is emphasized in creating attraction.     

Quantum Monte Carlo study of a proton in an electron gas

The polarization of jellium by a fixed proton impurity at 0 K is determined for an electron density range from metallic to dilute using the quantum Monte Carlo algorithm. Preliminary results show the correct H^– binding limit for the impurity in a dilute electron gas. The screening indicates transitions from two-to one-electron binding and from localized to delocalized electrons as the jellium density increases. The results are compared to density functional calculations. Pair distribution functions, Friedel oscillations, and binding energies are discussed.     

Hund’s Rule and Metallic Ferromagnetism

We study tight-binding models of itinerant electrons in two different bands, with effective on-site interactions expressing Coulomb repulsion and Hunds rule. We prove that, for sufficiently large on-site exchange anisotropy, all ground states show metallic ferromagnetism: They exhibit a macroscopic magnetization, a macroscopic fraction of the electrons is spatially delocalized, and there is no energy gap for kinetic excitations.     

Effect of spin polarization on the optical properties of Co-doped TiO2 thin films

A non-linear variation of bandgap energy with Co doping is observed in sputter deposited Co-doped TiO₂ thin films. This peculiar behavior is explained on the basis of mechanical stress in the films together with spin polarization due to s,p-d exchange interaction between the localized Co 3d electrons and delocalized electrons. Quantitative analyses of mechanical stress and grain boundary barrier potential due to spin polarization are performed from the below bandgap absorption tail. Furthermore, anomalous variations in both the refractive indices and extinction coefficients with Co doping are noted and are explained on the basis of ab-initio calculations based on density functional theory.     

Electron transport in solids and liquids

We review our recent experimental studies of the excess electron states in insulating solids and liquids. We use a muon spin relaxation technique to explore the phenomenon of delayed muonium formation: excess electrons liberated in the μ⁺ ionization track converge upon the positive muons and form Mu (μ⁺e^?) atoms in which the μ⁺ polarization is partially lost. The spatial distribution of such electrons with respect to the muon is shown to be highly anisotropic: the μ⁺ thermalizes well “downstream” from the center of the electron distribution. Measurements in electric fields up to 30 kV/cm allow one to estimate the characteristic muon-electron distance in different insulators: the results range from 10^?6 to 10^?4 cm. Such a microscopic length scale enables the electron to sometimes spend its entire free lifetime in a state which may not be detected by conventional macroscopic techniques. This circumstance illustrates the potential of μ⁺SR techniques in studies of electron transport in matter. The muonium formation process in condensed matter is shown to depend critically upon whether the excess electron forms a polaron or remains in a delocalized state. Different mechanisms of electron transport in insulators are discussed.     

微波电子回旋共振等离子体化学气相淀积法制备非晶氟化碳薄膜的研究

采用电子回旋共振等离子体化学气相淀积(ECR-CVD)法，以C₄F₈和CH₄为源气体制备了非晶氟化碳(a-C：F)薄膜.X射线电子能谱(XPS)和傅里叶变换红外光谱(FTIR)分析表明，a-C：F薄膜退火后厚度减小是由于位于a-C：F薄膜交联结构末端的C—C和CF₃结合态的热稳定性较差，导致退火时容易生成气态挥发物造成的.a-C：F膜介电常数在300℃氮气气氛中退火后由于电子极化增大和薄膜密度增加而上升，界面态陷阱密度从(5—9)×10¹¹eV^-1·cm^-2降至(4—6)×10¹¹eV^-1·cm^-2.a-C：F薄膜导电行为在低场强区域呈现欧姆特性，在高场强区域符合 Poole-Frankel机理.非定域π电子在带尾形成陷阱且陷阱能量在退火后降低，从而使更多陷阱电子在场增强热激发作用下进入导带并引起电流增大. 关键词： a-C：F ECR-CVD 键结构 电学性质     

Dual behavior of excess electrons in rutile TiO2

The behavior of electrons in the conduction band of TiO₂ and other transition‐metal oxides is key to the many applications of these materials. Experiments seem to produce conflicting results: optical and spin‐resonance techniques reveal strongly localized small polarons, while electrical measurements show high mobilities that can only be explained by delocalized free electrons. By means of hybrid functional calculations we resolve this apparent contradiction and show that small polarons can actually coexist with delocalized electrons in the conduction band of TiO₂, the former being energetically only slightly more favorable. We also find that small polarons can form complexes with oxygen vacancies and ionized shallow‐donor impurities, explaining the rich spectrum of Ti³⁺ species observed in electron spin resonance experiments. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Localization in artificial disorder: two coupled quantum dots

Using single electron capacitance spectroscopy, we study electron additions in quantum dots containing two potential minima separated by a shallow barrier. Analysis of the addition spectra in the magnetic field allows us to distinguish between electrons delocalized over the entire dot and those localized in either of the potential minima. We demonstrate that a high magnetic field abruptly splits up a low-density droplet into two smaller fragments, each residing in a potential minimum. An unexplained cancellation of electron repulsion between electrons in these fragments gives rise to paired electron additions.     

Elusive <Emphasis Type="Italic">s-f</Emphasis> intrasite interactions and double exchange in solids: Ferromagnetic versus nonmagnetic ground state

From the theory of many-electron states in atoms, we know that there exists a strong Coulomb repulsion, which results in the electronic term structure of atoms and is responsible for Hund’s rules. By expanding the Coulomb on-site repulsion into a multipolar series, we derive this interaction and show that it is also present in solids as a correlation effect, which means that the interaction requires a multideterminant version of the Hartree-Fock method. Of particular interest is the case where this interaction couples states of localized (f) and delocalized (s) electrons. We show that the interaction is bilinear in the creation/annihilation operators for localized electrons and bilinear in the operators for conduction electrons. To study the coupling, we consider a simple model in the framework of an effective limited configuration interaction method with one localized f-electron and one itinerant s-electron per crystal site. The on-site multipole interaction between the f- and s-electrons is explicitly taken into account. It is shown that depending on the low-lying excitation spectrum imposed by the crystal electric field, the model can lead not only to ferromagnetism but also to a nonmagnetic state. The model is relevant for solids with localized and itinerant electron states.     

Strong-field double ionization of Ar below the recollision threshold

Nonsequential double ionization of Ar by 45 fs laser pulses (800 nm) at (4-7)x10;{13} W/cm;{2} was explored in fully differential measurements. Well below the field-modified recollision threshold we enter the multiphoton regime. Strongly correlated back-to-back emission of the electrons along the polarization direction is observed to dominate in striking contrast to all previous data. No effect of Coulomb repulsion can be found, the predicted cutoff in the sum-energy spectra of two emitted electrons is confirmed, and the potential importance of multiple recollisions is discussed.     

Coupled plasmon and phonon dynamics in embedded Na clusters

We investigate, from a theoretical perspective, the coupled electronic and ionic/atomic dynamics of Na clusters embedded in Ar matrices. The system is described by time-dependent density-functional theory for cluster electrons and classical motion for Na⁺ ions as well as for Ar atoms. The interaction with the surrounding Ar atoms is modelled by polarization potentials plus core repulsion. We use this model to study coupled electronic and ionic/atomic motion in embedded clusters following a very short laser pulse. For excitations in the non-linear regime, we find clear signs for the coherent coupling of the Mie plasmon resonance with ionic vibrations (phonons). In addition, an incoherent line stretching is observed which can be traced back to the turning point of ionic vibrations. The coupling to the atomic motion of the surroundings leads to a slow and far reaching rearrangement of the matrix. PACS 36.40.Gk; 36.40.Vz; 31.15.EW     

Entangled Bell states of two electrons in coupled quantum dots—phonon decoherence

We demonstrate coupling and entangling of quantum states in a pair of vertically aligned self assembled quantum dots by studying the dynamics of two interacting electrons driven by external electric field. The present entanglement involves the spatial degree of freedom for the two electrons system. We show that system of two interacting electrons initially delocalized (localized each in one dot) oscillate slowly in response to electric field, since the strong Coulomb repulsion makes them behaving so. We use an explicit formula for the entanglement of formation of two qubit in terms of the concurrence of the density operator. In ideal situations, entangled quantum states would not decohere during processing and transmission of quantum information. However, real quantum systems will inevitably be influenced by surrounding environments. We discuss the degree of entanglement of this system in which we introduce the decoherence effect caused by the acoustic phonon. In this entangled states proposal, the decohering time depends on the external parameters.     

Theory of the unconventional superconductivity in magnetic materials

Recent experimental observations of superconductivity in the presence of strong paramagnetism of lanthanide and actinide materials are theoretically analysed. The formation of superconducting electron pairs in the system of localized and delocalized electrons interacting through the Heisenberg exchange interaction is derived. Theoretical results show that the superconducting transition in the magnetic materials is due to the spin-triplet pairing of the electrons and exhibits the electronic analogy of the superfluid transition in³He.     

Exact solution of a coupled spin–electron linear chain composed of localized Ising spins and mobile electrons

Exact solution of a coupled spin–electron linear chain composed of localized Ising spins and mobile electrons is found. The investigated spin–electron model is exactly solvable by the use of a transfer-matrix method after tracing out the degrees of freedom of mobile electrons delocalized over a couple of interstitial (decorating) sites. The exact ground-state phase diagram reveals an existence of five phases with different number of mobile electrons per unit cell, two of which are ferromagnetic, two are paramagnetic and one is antiferromagnetic. We have studied in particular the dependencies of compressibility and specific heat on temperature and electron density.     

Electron spin resonance of donor centers in beryllium oxide

The ESR spectra of two deep centers found in irradiated BeO crystals and powders, respectively, are reported and analyzed. They are due to electrons trapped at substitutional boron (B⁺⁺⁺) and fluorine (F^?) ions as concluded from the hf interaction of the unpaired electron with the respective nuclei. The boron center revealed a static, the fluorine center a dynamic Jahn-Teller effect between 77°K and 300°K. In both cases, the unpaired electron was found to be localized in nonaxial orbitals, which is shown to be consistent with the direction of the polar field of the crystal structure. The strong localisation of the electron is contrasted to the behavior of donor electrons in other II–VI compounds, which are highly delocalized.     

Time-dependent polarization in Mössbauer experiments with synchrotron radiation (II)

The resonant forward scattering of X-rays from⁵⁷Fe nuclei is strongly polarization dependent. The broad band excitation provided by synchrotron radiation (SR) results in an interesting time-dependent polarization mixing. This mixing can be used to substantially reduce the nonresonant (nonrotated) scattering from electrons. The presented technique will allow the full utilization of next-generation synchrotron facilities as a source for Mössbauer experiments.     

Breit and Quantum Electrodynamics Energy Contributions in Multielectron Atoms from the Relativistic Screened Hydrogenic Model

The correction to the Coulomb repulsion between two electrons due to the exchange of a transverse photon, referred to as the Breit interaction, as well as the main quantum electrodynamics contributions to the atomic energies (self-energy and vacuum polarization), are calculated using the recently formulated relativistic screened hydrogenic model. Comparison with the results of multiconfiguration Dirac-Hartree-Fock calculations and experimental X-ray energies is made.     

Electron polarization in elastic ed scattering

The situation in which elastically scattered ultrarelativistic electrons that are partially polarized in the initial state are polarized at oriented deuterons with an axially symmetric mixed spin state is considered; an expression for the resulting polarization is obtained. The role played by the orientation of the target state’s symmetry axis and the structure of²H is studied. In the dependence of the scattered-electron polarization on the momentum transmitted, the conditions of existence of extrema are established; the extremal value may be 1 (when the scattered electrons are completely polarized). Analysis of the polarization of the scattered electrons permits not only the separation of the electromagnetic form factors of²H but also the establishment of the relative phases of all the reduced matrix elements, thereby providing new information on the²H structure. Taras Shevchencko Kiev University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 87–91, July, 1996.     

Ultrafast electron dynamics in amorphous and crystalline D2O layers on Ru(0 0 1)

Femtosecond dynamics of excess electrons photo-injected into amorphous and crystalline D₂O layers on Ru(0 0 1) have been investigated by time-resolved two-photon photoelectron spectroscopy. In the crystalline case, excited electrons are transferred into delocalized states considered as image potential states in the conduction band of ice and relax back to the metal on an ultrafast time scale. The life time of the n = 1 image potential state is <5 fs. In the amorphous case, spectral features arise from delocalized and localized electronic states. Relaxation of delocalized electrons back to the metal is as fast as in the crystalline case. The binding energy of localized electrons, however, is found to increase as a function of time delay by 1 eV/ps, which is attributed to the formation of solvated electrons. Such energetic stabilization starting at the bottom of the conduction band is clearly absent in crystalline layers. This pronounced correlation of electronic structure and electron dynamics with molecular structure is associated with the presence of localized states near the bottom of the conduction band in amorphous ice. Such localized states are absent for perfect periodic crystalline structures but prevail in amorphous systems where they serve as precursor sites for electron solvation.     

Electron delocalization observed in the Mössbauer spectrum of ilvaite

Mössbauer spectroscopy is used to detect species associated with delocalized electrons undergoing Fe²⁺ → Fe³⁺ electron delocation in ilvaite. Mössbauer spectra of a suite of naturally occurring ilvaites were recorded from 80K to 575K and fit to five quadrupole doublets assigned to Fe²⁺ (A), Fe²⁺ (B), Fe³⁺ (A), and Fe²⁺ (A) → Fe³⁺ (A)6c and ⊥c. These assignments disagree with earlier interpretations of temperature dependent quadrupole splitting and isomer shift in ilvaite.