Small Valence Research Articles

Magneto-photoluminescence study of InGaAs/InP and InGaAs/AlAsSb quantum wells

We report on a magneto-photoluminescence study of InGaAs/InP stacked-quantum wells (ST-QWs) and InGaAs/AlAsSb multiple-quantum wells (MQWs). The InGaAs/InP ST-QWs show clear well-width dependence of the exciton binding energy. The reduced mass of the 10 nm InGaAs/AlAsSb MQWs is smaller than that of the 16-nm InGaAs/InP QW, contrary to the order of the well-size dependence. This could be caused by the mixing of the heavy hole- and light hole band, due to the small valence band offset in InGaAs/AlAsSb MQWs. Measured exciton binding energies slightly deviated from the theoretical values calculated based on an analytical model, which takes into account the anisotropic interactions between electrons and holes in a 3D space. This deviation suggests that the excitons in QWs are not perfect 3D excitons.

Anomalous Pr Ordering, Pr L3-Edge and Cu K-Edge XANES Studies for the Insulating PrBa2Cu3O7−y System

X-ray absorption near-edge structure (XANES) data for the PrBa2Cu3O7−y (6.21≤7−y≤6.95) insulating system with anomalous Pr antiferromagnetic order are reported. Pr L3-edge XANES indicates that Pr formal valence decreases from 3.17–3.18 for oxygenated samples to 3.09 for oxygen-depleted PrBa2Cu3O6.43, 3.03 for PrBa2Cu3O6.38 and 3. 01 for PrBa2Cu3O6.21. The variation of Pr valence indicates that the doped holes initially stay in the CuO1−y plane and contribute to Cu(I)–Cu(II) transformation in this plane, but gradually move into the CuO2 bi-layers with PrO8 cage and result in the appearance of Pr(IV) localized-hole state with, increasing oxygen content. Cu K-edge XANES indicates that the average Cu formal valence increases from less than 2 in oxygen-depleted region to a value larger than 2 for oxygenated samples. with the estimated Cu valence of 2.24 for PrBa2Cu3O6.95. The small average Cu formal valence or large Pr formal valence is closely related to the absence of superconductivity.

Band offsets in III-nitride heterostructures

We present a theoretical investigation of band offsets in GaN-based group III-nitride ternary/binary heterostructures with zinc blende structure. The band offsets in InGaN/GaN and AlGaN/GaN heterostructures were studied using the recently proposed extendedsp3 tight binding model (Ünlü H 2001 Phys. Status Solidi. b 223 195). Simulations show a small valence band offset in AlGaN/GaN heterostructure and large valence band offset in theInGaN/GaN heterostructure. Furthermore, the tensile strain inAlGaN leads to positive bowing of the conduction band offsets inAlGaN/GaN heterostructure, and the compressive strain in InGaNleads to negative bowing of conduction band offsets in InGaN/GaNheterostructure as a function of composition. Good agreement isfound between predictions and experiment.

Hexakis(dimethylaminoxy)disiloxane: Insertion of an Oxygen Atom into the Si-Si Bond of a Disilane by Dimethylaminoxy Lithium

Abstract The compound (Me2NO)3SiOSi(ONMe2)3 was prepared by the reaction of LiONMe2 with Si2Cl6, a reaction in which oxygen is inserted into the Si-Si bond. The crystal structure of (Me2NO)3SiOSi(ONMe2)3 was determined and shows the compound to have a linear siloxane core and SiON units with small valence angles at oxygen, caused by weak Si (-N interactions leading to partially hypercoordinate Si centres.

Calculation of frequency dependent optical rotation using density functional response theory

The optical rotations (ORs) of chiral organic molecules have been calculated by time-dependent density functional response theory (TDDFT) employing the B3LYP hybrid functional. For selected examples, the origin as well as the frequency dependence of the OR has been investigated. The theoretical data for 20 organic molecules including large systems as e.g., octahelicene compare favourably with experimental observations even if small valence basis augmentented with diffuse basis functions are employed. The inclusion of the frequency dependence in the theoretical treatment is recommended to obtain reliable predictions for measured OR values.

Exciton Binding Energies and Oscillator Strengths in GaAsN-GaAs Quantum Wells

We propose the first tentative calculation of exciton binding energies and oscillator strengths in GaAs 1-x N x -GaAs quantum wells with nitrogen contents below x = 0.05. Our model is based on the envelope function approximation and permits us to determine the excitonic properties of GaAs-GaAsN single quantum wells with a variational and self-consistent process for marginal type I or type II potentials due to the very small valence band offset for this combination. We conclude that this is a type I system.

5d-level energies ofCe3+and the crystalline environment. I. Fluoride compounds

Information on the position of all five $5d$ levels of ${\mathrm{Ce}}^{3+}$ in 17 different fluoride compounds has been collected. A model involving the polarizability of the fluoride ions, originally suggested by Morrison [J. Chem. Phys. 72, 1001 (1980)], is used to calculate the so-called spectroscopic polarizability ${\ensuremath{\alpha}}_{\mathrm{sp}}$ from the observed average energy of the 5d configuration. It will be compared with actual in-crystal fluoride ion polarizabilities. There appears a relationship between ${\ensuremath{\alpha}}_{\mathrm{sp}}$ and the types of cations present in the crystal structure. Small high valency cations tend to reduce the centroid shift. Large cations have the opposite effect. The size and the type of the fluoride ion coordination polyhedron around ${\mathrm{Ce}}^{3+}$ determine the crystal field splitting of the $5d$ levels. Combining the gained knowledge on centroid shift and crystal field splitting, the energy of the first $4\stackrel{\ensuremath{\rightarrow}}{f}5d$ transition in about 25 additional fluoride compounds will be interpreted.

Should bond-formation lag behind bond-cleavage in radical-exchange reactions?

A few simple concerted radical-exchange reactions (A + BC →AB + C) are examined by the quantum-mechanicalab initio methods via calculations of bond orders and energies. Results reveal that, during these reactions, a small free valence develops on the migrating atom. As a result, the degree of bond-cleavage is more advanced than that of bond-formation with a common migrating atom at the transition state. The fact that bond-formation lags behind bond-cleavage is also examined by the configuration mixing of the reactant configurations at the early stages of the reaction.

The domain wall fermion chiral condensate in quenched QCD

We examine the chiral limit of domain wall fermions in quenched QCD. One expects that in a quenched simulation, exact fermion zero modes will give a divergent, 1/ m behavior in the chiral condensate for sufficiently small valence quark masses. Unlike other fermion formulations, domain wall fermions clearly demonstrate this behavior.

Transition behavior from coupled to uncoupled GaAs/InAs double quantum wells

The electronic structures of the InAs single and double quantum wells buried in bulk GaAs (001) are presented based on the sp3s* empirical tight-binding model. Both electrons and holes are found to be confined in the c-axis direction around the inserted InAs monomolecular plane with a localization length of the order of 110 Å. The inserted InAs monolayer is, therefore, playing the role of a quantum well for all charge carriers and, as a consequence, the formed heterojunction is of type I. The system, composed of two InAs monolayers buried in GaAs and separated by N monolayers of GaAs, is studied versus the barrier thickness (N). Our results of the variation of band gap energy as a function of barrier thickness (N) are in excellent agreement with the available photoluminescence data when a small valence band offset (of order 80 meV including the spin-orbit effects) is employed. A critical barrier thickness of about 220 Å is suggested to decouple the InAs quantum wells.

A study of bond-cleavage and bond-formation processes in some simple metathesis reactions

A few simple atom-transfer reactions (i.e., A + X - A \rightarrow A - X + A) are studied by quantum mechanical ab initio methods. Emphasis is given to the detailed analysis of density matrices rather than to the energetics. Results reveal that during these reactions a small free valence always develops on the migrating atom at the transition state. The barriers in these reactions arise from the greater extent of bond cleavage in the reactant than that of bond formation in the transition state. Analysis of bond orders estimated from bond lengths using Pauling’s relation also leads to the fact that the bond-cleavage process is more advanced than is the bond-formation process in these reactions.

A study of bond‐cleavage and bond‐formation processes in some simple metathesis reactions

A few simple atom-transfer reactions (i.e., A˙+X−A→A−X+A˙) are studied by quantum mechanical ab initio methods. Emphasis is given to the detailed analysis of density matrices rather than to the energetics. Results reveal that during these reactions a small free valence always develops on the migrating atom at the transition state. The barriers in these reactions arise from the greater extent of bond cleavage in the reactant than that of bond formation in the transition state. Analysis of bond orders estimated from bond lengths using Pauling's relation also leads to the fact that the bond-cleavage process is more advanced than is the bond-formation process in these reactions. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 63: 1099–1106, 1997

Fast generation of cubic graphs

In this paper an efficient algorithm to generate regular graphs with small vertex valency is presented. The running times of a program based on this algorithm and designed to generate cubic graphs are below two natural benchmarks: (a) If N(n) denotes the number of pairwise non-isomorphic cubic graphs with n vertices and T(n) the time needed for generating the list of all these graphs, then T(n)/N(n) decreases gradually for the observed values of n. This suggests that T(n)/N(n) might be uniformly bounded for all n, ignoring the time to write the outputs, but we are unable to prove this and in fact are not confident about it. (b) For programs that generate lists of non-isomorphic objects, but cannot a priori make sure to avoid the generation of isomorphic copies, the time needed to check a randomly ordered list of these objects for being non-isomorphic is a natural benchmark. Since for large lists (n ≥ 22, girth 3) existing graph isomorphism programs take longer to canonically label all of the N(n) graphs than our algorithm takes to generate them, our algorithm is probably faster than any method which does one or more isomorphism test for every graph. © 1996 John Wiley & Sons, Inc.

Formula omitted] on GaAs grown by molecular beam epitaxy

Zinc-blende MnSZnSe heterostructures on GaAs(100) have been characterized by X-ray photoelectron spectroscopy (XPS), optical methods, and transmission electron microscopy (TEM). XPS spectra indicated that the MnSZnSe interface is sharp and that the layers are of proper stoichiometry. TEM revealed isolated {111} stacking faults but confirmed the overall high crystalline quality and coherence of the MnSZnSe interface. XPS suggests a small valence band discontinuity (ΔEv) of about 140 meV with a type IIA alignment. This result was supported by low-temperature photoluminescence (PL) measurements on a MnsZnSe superlattice. No peaks were observed above the ZnSe band gap suggesting that the alignment was not type I. A previously unobserved broad PL peak was seen at about 2.77–2.78 eV, approximately 50 meV below the ZnSe bandgap.

Universal scaling of the valence quark mass dependence of the chiral condensate

Recently, the Columbia group obtained the valence quark mass dependence of the chiral condensate for lattice QCD simulations with dynamical fermions for a series of couplings close to the critical temperature. In this letter we show that the data for temperatures below T c and sufficiently small valence quark masses can be rescaled to fall on a universal curve. Using chiral random matrix theory we obtain an exact analytical expression for the shape of this curve. We also discuss the universal scaling behavior of the mass dependence of the mass derivatives of the chiral condensate (scalar susceptibilities).

Electron Density and Chemical Bonding: The Shape of Independent Atoms in Molecules

AbstractElectron density distributions ρ(r) contain three types of features, which give rise to a new natural approach of analyzing ρ with respect to chemical bonding. These features are 1) the very large spherical atomic core densities, defining the geometric structure, 2) the small multipolar atomic valence densities, defining the orientation and population of partially filled p‐ and d‐shells of the “independent atoms” in the molecule or crystal, and 3) the very small density deformations due to the interatomic interactions. The valence shell parameters and the Chemical Deformation Densities are determined theoretically and experimentally for molecules and crystals. The features are brought in relation to intramolecular and intramolecular interactions.

Pressure-induced valence changes in mixed-valent systems

Mixed-valent systems based on Ce, Sm, Eu and Yb exhibit a wide range of behaviour with respect to valence changes under the application of pressure. The authors present a semi-phenomenological model for this behaviour based on competition effects between the usual elastic energy cost and the magnetic energy gain due to valence fluctuations. For the latter they use a mean-field Anderson lattice description and incorporate the effects of pressure by introducing a volume dependence to the Anderson model parameters in f and Delta . In contrast to existing models such as the Kondo Volume Collapse theory of Allen and Martin (1982), which describes magnetic to non-magnetic transitions without sizable valence change (e.g. gamma -Ce to alpha -Ce), the Anderson lattice model developed here describes systems with both small and large valence changes: the transition can be continuous (e.g. EuPd2Si2) or discontinuous (EuPd2Si2 alloyed with Au).

Excitons in II-VI heterostructures

The properties of excitons are examined in two model II-VI heterostructures: the Type-I CdTe/ZnTe system with small valence band offset and the Type-II ZnTe/ZnSe system. The strain induced band offset in the CdTe/ZnTe heterojunction is included to provide the confinement for the heavy hole state. It is found that the large enhancements of exciton binding energy and oscillator strength in the CdTe/ZnTe system are similar to what one finds in systems with a much larger valence band offset. For the ZnTe/ZnSe system, it is found that strong confinement of electrons and holes by the large band offsets can give rise to a very large exciton binding energy for thin heterojunction layers. Also, the mismatch in dielectric constants induces an image charge at the interface, which modifies significantly the exciton Hamiltonian in an asymmetric structure.

Electrokinetic phenomena in amphoteric membranes

The membrane phenomena in an amphoteric membrane-single-electrolyte or mixed-electrolyte solution systems were examined. The mobilities of the ions within the membrane were evaluated from the relation between the transport number and membrane conductance after determination of the ionic concentration within the membrane. In the single-electrolyte system, the mobilities depending on the ionic charges were observed. The facts supported that ion-transport phenomena were mainly controlled by the electrostatic interaction between the ion-exchange sites and the ions within the membrane. On the other hand, the cation with the smaller valence was always excluded out of the membrane in the mixed-electrolyte system. When comparing the ratios of mobilities in the single-electrolyte system with those in the mixed-electrolyte system, large differences between them could be observed. It is conjectured that one of the reasons may be the difference in the Donnan adsorption salt concentration within the membrane.

Excitons in semiconductor quantum wells with a small band offset: Exact numerical solution to the envelope function

We use an accurate method to solve the ground state and binding energy of a Wannier exciton in a semiconductor quantum well with a small valence (or conduction) band offset. By choosing fixed envelope function in the conduction (or valence) band quantum well and taking the relative part of the exciton wavefunction in the commonly assumed variational form, a one-dimensional second-order differential equation for the envelope function in the valence (or conduction) band is derived. The exciton intergap energy is then deduced from the eigenvalue of this equation. This approach includes the effect of the additional confinement on the carrier in the valence (or conduction) band quantum well by the electron-hole Coulomb interaction, and has the advantage of being numerically exact in determining the envelope function comparing to the generalized variational approach developed earlier. The application of this method to II–VI compound semiconductor quantum wells is discussed.