Electron Exchange Interaction Research Articles

In English

In review, deals with the theory of exciton quasimolecules in a nanoheterostructures. It has been found that the formation of a exciton quasimolecule in a nanoheterostructures made up of aluminum oxide quantum dots synthesized in a dielectric matrix is of threshold character and can occur in a nanosystem where the distance D between the surfaces of quantum dots is given by the condition  . The existence of such distance arises from quantum size effects in which the decrease in the energies of interaction of the electrons and holes entering into the Hamiltonian of the “exciton molecule” with decrease of the distance D between the surfaces of the QD cannot compensate for the increase in the kinetic energy of the electrons and holes. At larger distances D between the surfaces of quantum dots: the biexciton breaks down into two excitons (consisting of spatially separated electrons and holes), localized over QD surfaces.          It was shown that the convergence of two quantum dots up to a certain critical value   between surfaces of quantum dot lead to overlapping of electron orbitals of superatoms and the emergence of exchange interactions. In this case the overlap integral of the electron wave functions takes a significant value. As a result, the conditions for the formation of quasi-molecules from quantum dots can be created.          We have shown that in such a nanoheterostructures acting as “exciton molecules” (biexcitons consisting of spatially separated electrons and holes) are the quantum dots of aluminum oxide with excitons localizing over their surfaces. The position of the biexciton state energy band depends both on the mean radius of the quantum dots, and the distance between their surfaces, which enables one to purposefully control it by varying these parameters of the nanostructure.          As our variational calculations show, the interaction of the excitons with the surfaces of quantum dots (“intramolecular” interaction) is much stronger than that between quantum dots (“intermolecular” interaction). Due to the translational symmetry of such a nanoheterostructures of quantum dots, it permits propagation of electronic excitation in the form of biexcitons.          As follows form the results of the variational calculations, the major contribution to the biexciton binding energy is from the energy of exchange interaction of electrons and holes, which by far surpasses that from their Coulomb interaction.          It is established that at constant concentrations of biexcitons at temperatures T below a certain critical temperature Tc due to the radiative annihilation of one of the excitons forming a biexciton one can expect a new spectral band of luminescence shifted relative to the exciton band by the biexciton binding energy . This new luminescence band disappears at temperatures above Tc. At a constant temperature Т < Tc   the growth of exciton concentration brings about weakening of the exciton band and strengthening of the biexciton band of luminescence.

Spin effects as a tool to study photoinduced processes in (S/R)-ketoprofen-(S)-N-methylpyrrolidine dyads.

(S/R)-Ketoprofen (KP) is considered to be the strongest photosensitizer among nonsteroidal anti-inflammatory drugs. The photosensitizing reactions are caused by a substituted benzophenone chromophore. It produces various toxic effects through the formation of active paramagnetic intermediates and photoproducts able to attack biological substrates. Photoinduced transformations of KP have been extensively studied in order to identify paramagnetic intermediates. Considerable attention is also paid to photoinduced processes in dyads, where KP is linked with chiral partners, since such processes believed to model the binding of chiral drugs with enzymes and receptors. In the present study, the dyads containing (S)/(R)-KP covalently linked with (S)-N-methylpyrrolidine have been synthesized to examine the peculiarities of photoinduced electron transfer (ET) and hydrogen transfer (HT) in chiral systems. To detect short-lived paramagnetic intermediates, in dyad's excited triplet state, such as biradical zwitter ion (BZI) and neutral biradical (BR), spin chemistry methods [chemically induced dynamic nuclear polarization (CIDNP) and chemically induced dynamic electron polarization (CIDEP) in arbitrary magnetic fields], allowing indirect detection of transient paramagnetic particles by NMR, have been utilized. Both mentioned processes have been found to begin with the excitation of KP into the triplet state followed by the formation of BZI for ET and BR for HT, respectively. Products of stereoselective attachment of the N-methylpyrrolidine residue to the carbonyl carbon atom of KP occurring in both BZI and BR have been detected by NMR spectral analysis. The value of electronic exchange interaction in biradicals has been determined from the position of the characteristic maximum of hyperpolarization in the CIDNP magnetic field dependences.

Phase transitions in a model of BEDT-TTF compound electron subsystem

Model of electronic subsystem of (BEDT-TTF)2X family is studied theoretically within Green function equation of motion approach. The configurational representation of the model Hamiltonian which allows analytical calculation of the energy spectrum is developed. Effective Hamiltonian of the model takes into account the strong intra-site Coulomb, intersite exchange interaction of electrons and correlated hopping of electrons both within the conducting plane and in transversal direction. With rise of intraatomic Coulomb repulsion in the energy spectrum a gap opens and the transition to antiferromagnetic state is realized. At the external pressure and decreasing temperature the transition from insulator to metallic phase occurs. The experimental phase diagrams for quasi-twodimensional organic conductors with strong electron correlations and correlated hopping are discussed.

Electron g-factor in coupled quantum wells CdTe and CdMnTe

In tunnel-coupled quantum wells (QWs) CdTe (20 nm wide) and CdMnTe (8 nm wide) separated by Cd0.88Mg0.12Te barrier with a thickness of LB = 5, 7, 9, and 11 monolayers (MLs), the dependence of electron g-factor on the barrier thickness and temperature is investi-gated by means of the pump-probe Kerr rotation technique. The renormalization of the electron g-factor occurs due to the s–d exchange interaction of electrons with manganese ions in a magnetic QW. The most change of the electron g-factor value Δg = 0.25 is registered at the least barrier width of 5 MLs and temperature T = 5 K. In this case, the penetration of the electron wave function from the nonmagnetic QW into the magnetic one is estimated to be 0.6%.

Spin-dependent electronic lenses based on hybrid graphene nanostructures

We study electronic transport in graphene/ferromagnetic insulator hybrid devices. The system comprises an armchair graphene nanoribbon with a lens-shaped EuO ferromagnetic insulator layer deposited on top of it. When the device supports a large number of propagating modes, the proximity exchange interaction of electrons with the magnetic ions of the ferromagnetic insulator results in electrons being spatially localised at different spots depending on their spin. We found the spin-dependent electron focusing is robust under moderate edge disorder. A spin-polarised electric current can be generated by placing a third contact in the proper place. This opens the possibility to use these effects for fabricating tunable sources of polarized electrons.

First-principles study of the metal-insulator transition in the Ti-substituted rutile CrO2

The effect of Ti-substitution for Cr in CrO2 is extensively studied by employing first-principles electronic structure calculations. The host material is a ferromagnetic half-metal. This material encounters first step transition from half-metal to a metallic phase at 50% Ti-substitution for Cr. The application of Coulomb interaction U significantly changes the electronic and magnetic properties of Cr0.5Ti0.5O2. This material remains in its metallic phase up to U = 2 eV and encounters a second step transition from the metallic to a half-metallic phase at U = 3 eV. Eventually, this system exhibits a metal-insulator transition (MIT) at U = 4 eV with a band gap of 0.15 eV. Nevertheless, Cr0.5Ti0.5O2 preserves its ferromagnetism (FM) in all the metallic, half-metallic and insulating phases. The metal to half-metal transition in Cr0.5Ti0.5O2 is observed due to full spin polarizations accompanied by strong dynamical correlations of Cr-dyz/xz electrons. Besides this, weak static but the strong dynamical correlation of electrons in the bonding and anti-bonding components of Cr-dyz/xz orbitals is accounted for the key element of MIT in Cr0.5Ti0.5O2. The FM in the half-metallic Cr0.5Ti0.5O2 arises from the double exchange interaction of electrons in the partially occupied bonding and anti-bonding components of Cr-dyz/xz orbitals. In addition, the double exchange interaction of electrons in the bonding components of Cr-dyz/xz orbitals triggers FM in the insulating phase of Cr0.5Ti0.5O2. The ferromagnetic Curie temperature increases due to Ti-substitution in CrO2. Finally, a trivial structural distortion is observed due to Ti-substitutions in CrO2.

Quantum chemical analysis on supramolecular assemblies of guanidinium tetrafluoroborate (GFB) crystal structure: Emission and NLO behavior

The present work explains the experimental and theoretical aspects of guanidinium tetrafluoroborate (GFB) single crystal. The solubility nature of the GFB single crystal was analyzed. The surface of the grown crystal was examined through chemical etching study. Emission properties were characterized using photoluminescence study. CIE colour chromaticity coordinates afford the dominant color emission of the title compound. The optical limiting property of the titular compound was ascertained using z-scan technique. The solid state parameters, such as Plasma energy, Penn gap energy, Fermi energy and electronic polarizabilty were calculated. The intermolecular interactions in the title molecule were studied using Hirshfeld surface analysis. A detailed quantum chemical calculation of the title crystal was performed. Mulliken atomic charge analysis revealed the charge distribution in the molecule. The molecular electrostatic potential (MEP) contour maps were used to study the interactions of guanidinium with fluoroborate ions in the crystal. The charge transfer mechanism was analysed using computed HOMO-LUMO orbitals. The 3D-matrix of hyperpolarizability tensor components was computed to enumerate the NLO activity. The various thermodynamic properties of the GFB molecule were computed. The electronic exchange interaction in the title compound was determined by natural bond orbital (NBO) analysis.

New mechanism of plasmons specific for spin-polarized nanoparticles

Here it is experimentally shown that Co nanoparticles with a single-domain crystal structure support a plasmon resonance at approximately 280 nm with better quality than gold nanoparticle resonance in the visible. Magnetic nature of the nanoparticles suggests a new type of these plasmons. The exchange interaction of electrons splits the energy bands between spin-up electrons and spin-down electrons. It makes it possible for coexistence of two independent channels of conductivity as well as two independent plasmons in the same nanoparticle with very different electron relaxation. Indeed, the density of empty states in a partially populated d-band is high, resulting in a large relaxation rate of the spin-down conduction electrons and consequently in low quality of the plasmon resonance. In contrast, the majority electrons with a completely filled d-band do not provide final states for the scattering processes of the conduction spin-up electrons, therefore supporting a high quality plasmon resonance. The scattering without spin flip is required to keep these two plasmons independent.

1‐D Polymeric Iron(II) Thiolato Complexes: Synthesis, Structure, and Properties of ∞1[Fe(SR)2] (R = Ph, Mes), ∞1[Fe(NH3)(SPh)(µ‐SPh)] and ∞1[(µ‐SPh)Fe(NH3)2(µ‐SPh)2Fe(µ‐SPh)

The first examples of polymeric homoleptic iron(II) thiolato complexes ∞1[Fe(SPh)2] and ∞1[Fe(SMes)2] (Ph = phenyl = C6H5, Mes = mesityl = C6H2–2,4,6‐(CH3)3) have been both prepared by reaction of [{Fe(N(SiMe3)2)2}2] with four equivalents of HSR ( R = Ph, Mes). In the crystal the two compounds form one‐dimensional chains with bridging thiolato ligands comprising distinctly different Fe–S–Fe bridging angles namely 75.20–75.25° in ∞1[Fe(SPh)2] and 91.38° in ∞1[Fe(SMes)2]. Reaction of ∞1[Fe(SPh)2] with stoichiometric amounts of NH3, generated by reaction of HN(SiMe3)2 with CH3OH, resulted in the formation of the polymeric ammonia thiolato complexes ∞1[Fe(NH3)(SPh))(µ‐SPh)] and ∞1[(µ‐SPh)Fe(NH3)2(µ‐SPh)2Fe(µ‐SPh)], which crystallize in the form of two structural isomers comprising either one or two µ2‐bridging phenyl‐thiolato groups with a bridging angle of 102.96° and 81.79–83.52°, respectively. Magnetic measurements reveal that this bridging angle has a distinct influence on the antiferromagnetic exchange interactions of the unpaired electrons along the chains in the different compounds. UV‐vis‐NIR spectra indicate low lying d‐d transitions (< 10000 cm–1) for all complexes.

Calculation of Magnetic Exchange Interactions and Construction of a Spin Model for Low-Dimensional Magnetic Compounds

Here we present a calculation scheme which allows us to obtain the electronic structure and exchange interaction parameters of low-dimensional magnetic compounds with subsequent construction of the spin model and verification of this model by comparing the simulated thermodynamic properties with experimental data. This scheme is implemented to quasi-one-dimensional (1D) perovskite $$\hbox {KCuF}_3$$ , the S = 1 spin ladder $$\hbox {Rb}_3\hbox {Ni}_2(\hbox {NO}_3)_7$$ compound, and potassium dimanganese trivanadate, $$\hbox {KMn}_2\hbox {V}_3\hbox {O}_{10}$$ , containing Mn tetramers.

Relativistic splitting of surface states at Si-terminated surfaces of the layered intermetallic compoundsRT2Si2(R=rare earth;T=Ir, Rh)

We present an effective model for surface states at the Si-terminated (001) surface of ternary rare earth (R) intermetallic compounds ${\mathit{RT}}_{2}{\mathrm{Si}}_{2}$ with the transition metal element $T$ = Ir or Rh. The model is based on a fully ab initio derivation of the relativistic $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ Hamiltonian and is thereby capable of accurately reproducing the spin polarization of the spin-orbit split surface states, which is very different from the classical Rashba effect. The reliable treatment of spin in our model enables a predictive analysis of the effect of magnetic exchange interaction of the surface-state electrons with ferromagnetically ordered $4f$ moments of the subsurface rare-earth layer in the magnetic phases of the ${\mathit{RT}}_{2}{\mathrm{Si}}_{2}$ compounds.

Electronic and magnetic properties of the candidate magnetocaloric-material double perovskites La2MnCoO6 , La2MnNiO6 , and La2MnFeO6

The search for room-temperature magnetocaloric materials for refrigeration has led to investigations of double perovskites. In particular, a puzzle has appeared in the La$_2$MnNiO$_6$, La$_2$MnCoO$_6$ and La$_2$MnFeO$_6$ family of compounds. They share the same crystal structure, but while La$_2$MnNiO$_6$ and La$_2$MnCoO$_6$ are ferromagnets below room temperature, La$_2$MnFeO$_6$, contrary to simple expectations, is a ferrimagnet. To solve this puzzle, we use density-functional theory calculations to investigate the electronic structure and magnetic exchange interactions of the ordered double perovskites. Our study reveals the critical role played by local electron-electron interaction in the Fe-$d$ orbital to promote the Fe$^{3+}$ valence state with half-filled $d$-shell over Fe$^{2+}$ and to establish a ferrimagnetic ground state for La$_2$MnFeO$_6$. The importance of Hund's coupling and Jahn-Teller distortion on the Mn$^{4+}$ ion is also pointed out. Exchange constants are extracted by comparing different magnetically ordered states. Mean-field and classical Monte-Carlo calculations on the resulting model give trends in $T_C$ that are in agreement with experiments on this family of materials.

Singlet-triplet transition in double quantum dots in two-dimensional topological insulators

We study two-electron states confined in two coupled quantum dots formed by a short-range potential in a two-dimensional topological insulator. It is shown that there is a fairly wide range of the system parameters, where the ground state is a tripletlike state formed by a superposition of two spin-polarized states. Outside this range, the ground state is a singlet. A transition between the singlet and triplet states can be realized by changing the potential of the quantum dots. The effect is caused by a significant change in the energies of the Coulomb repulsion and the exchange interaction of electrons due to the presence of the pseudospin components of the wave function when the band spectrum is inverted.

Electronic Structure and Exchange Interactions in RNi4Co (R = Eu, Yb) Compounds

The electronic structure and the exchange interactions in EuNi4Co and YbNi4Co compounds have been calculated in terms of a theoretical approach with the inclusion of electronic correlations (LSDA + U method); the variants of substitution of cobalt ion for nickel in the 3d lattice in both types of crystallographic positions 2c and 3g are considered. The total energies obtained in self-consistent calculations show that individual cobalt impurities are more preferably arranged in position of the 3g type. A Co ion in RNi4Co (R = Eu, Yb) is characterized by a significant magnetic moment, which leads to significant increase in the exchange interaction of Co and Ni ions in the 3d metal sublattice.

Study on growth, electronic structure, topological and nonlinear optical properties of semi organic material sodium sulfanilate dehydrate

Sodium sulfanilate dehydrate, organic nonlinear optical single crystals were grown by slow evaporation solution technique. The cell parameters of SSDH were confirmed from single crystal X-ray diffraction analysis. Nonlinear optical activity has been investigated both experimentally and theoretically. Hirshfeld surface analysis of the intermolecular interactions in crystal structures have been used to scrutinize molecular shapes. FT-IR, FT-Raman and UV–visible spectral analysis have been performed. Natural Bond Orbital analysis has been carried out in order to reveal the electronic exchange interaction and charge delocalization of the molecule. For the investigation of nonlinear optical behavior, the electric dipole moment, polarizability and hyperpolarizability computations have been performed using density functional theory.

Characterization of coal using electron spin resonance: implications for the formation of inertinite macerals in the Witbank Coalfield, South Africa

Coal contains a significant concentration of free radicals as a result of the coalification process. One of the experimental methods sensitive to the presence of radicals is electron spin resonance (ESR), and differences in ESR spectra for different macerals may provide insight into coal-forming processes. In this study, ESR data along with the H/C atomic ratio (to infer the aromatic fraction) are used to characterize coal samples with the aim of assessing a fire-origin for dominant inertinite macerals. A medium rank C bituminous Witbank No. 4 Seam Upper coal (the parent) was density-fractionated to create vitrinite-rich and inertinite-rich samples. The parent sample consists of 42 vol% vitrinite and 49 vol% inertinite. The density-fractionated samples comprise of 81 vol% total vitrinite (dominated by collotelinite and collodetrinite), and 63 vol% total inertinite (dominated by fusinite, semifusinite, and inertodetrinite). The H/C ratio is 0.74 for the inertinite-rich sample, and 0.85 for the vitrinite-rich counterpart, suggesting the former sample is more aromatic. The ESR spectra obtained for the three samples were found to fit best using a Lorentzian distribution. The fit is noticeably better for the aromatic inertinite-rich sample, for which the spectrum is symmetric. This is attributed to pronounced electron mobility and exchange interactions. The higher radical content of the inertinite-rich and parent samples is attributed to the presence of specific inertinite macerals, namely: fusinite, semifusinite, and inertodetrinite. And, owing to the greater radical content of the inertinite-rich sample, the dominant inertinite macerals are interpreted to have formed through charring of plant matter.

Singlet and triplet trions in WS2 monolayer encapsulated in hexagonal boron nitride

Embedding a WS2 monolayer in flakes of hexagonal boron nitride allowed us to resolve and study the photoluminescence response due to both singlet and triplet states of negatively charged excitons (trions) in this atomically thin semiconductor. The energy separation between the singlet and triplet states has been found to be relatively small reflecting rather weak effects of the electron–electron exchange interaction for the trion triplet in a WS2 monolayer, which involves two electrons with the same spin but from different valleys. Polarization-resolved experiments demonstrate that the helicity of the excitation light is better preserved in the emission spectrum of the triplet trion than in that of the singlet trion. Finally, the singlet (intravalley) trions are found to be observable even at ambient conditions whereas the emission due to the triplet (intervalley) trions is only efficient at low temperatures.

Electronic structure, magnetism, and exchange integrals in transition-metal oxides: Role of the spin polarization of the functional in DFT+ U calculations

Density functional theory augmented with Hubbard-$U$ corrections (DFT+$U$) is currently one of the widely used methods for first-principles electronic structure modeling of insulating transition metal oxides (TMOs). Since $U$ is relatively large compared to band widths, the magnetic excitations in TMOs are expected to be well described by a Heisenberg model. However, in practice the calculated exchange parameters $J_{ij}$ depend on the magnetic configuration from which they are extracted and on the functional used to compute them. In this work we investigate how the spin polarization dependence of the underlying exchange-correlation functional influences the calculated magnetic exchange constants of TMOs. We perform a systematic study of the predictions of calculations based on the local density approximation plus $U$ (LDA+$U$) and the local spin density approximation plus $U$ (LSDA+$U$) for the electronic structures, total energies and magnetic exchange interactions $J_{ij}$'s extracted from ferromagnetic (FM) and antiferromagnetic (AFM) configurations of several transition metal oxide materials. We report that, for realistic choices of Hubbard $U$ and Hund's $J$ parameters, LSDA+$U$ and LDA+$U$ calculations result in different values of the magnetic exchange constants and band gap. The dependence of the band gap on the magnetic configuration is stronger in LDA+$U$ than in LSDA+$U$ and we argue that this is the main reason why the configuration dependence of the $J_{ij}$'s is found to be systematically more pronounced in LDA+$U$ than in LSDA+$U$ calculations. We report a very good correspondence between the computed total energies and the parameterized Heisenberg model for LDA+$U$ calculations, but not for LSDA+$U$, suggesting that LDA+$U$ is a more appropriate method for estimating exchange interactions.

Langmuir instability in partially spin polarized bounded degenerate plasma

Some new features of waves inside the cylindrical waveguide on employing the separated spin evolution quantum hydrodynamic model are evoked. Primarily, the instability of Langmuir wave due to the electron beam in a partially spin polarized degenerate plasma considering a nano-cylindrical geometry is discussed. Besides, the evolution of a new spin-dependent wave (spin electron acoustic wave) due to electron spin polarization effects in the real wave spectrum is elaborated. Analyzing the growth rate, it is found that in the absence of Bohm potential, the electron spin effects or exchange interaction reduce the growth rate as well as k-domain but the inclusion of Bohm potential increases both the growth rate and k-domain. Further, we investigate the geometry effects expressed by R and pon and find that they have opposite effects on the growth rate and k-domain of the instability. Additionally, how the other parameters like electron beam density or streaming speed of beam electrons influence the growth rate is also investigated. This study may find its applications for the signal analysis in solid state devices at nanoscales.

Energy Spectrum of Charge Carriers in Elastically Strained Assemblies of Ge/Si Quantum Dots

The results of studying the energy spectrum of electrons and holes localized in second-type Ge/Si heterostructures with Ge quantum dots are presented. In such structures, holes are localized at Ge quantum dots, and electrons, in three-dimensional quantum wells, which form in Si at the Ge—Si interface because of inhomogeneous deformations that appear as a result of the difference between the Ge and Si lattice constants. It is shown that changes in the deformations in the assembly of quantum dots as a result of a variation in their spatial arrangement significantly changes the binding energy of electrons, the position of their localization at quantum dots, the binding energy and wave-function symmetry of holes at double quantum dots (artificial molecules), and the exchange interaction of electrons and holes in the exciton composition. A practically important result of the presented data is the development of approaches to increase the luminescence quantum efficiency and the absorption coefficient in assemblies of quantum dots.