The metal‐insulator transitions of VO2: A band theoretical approach

The results of first principles electronic structure calculations for the metallic rutile and the insulating monoclinic phase of vanadium dioxide are presented. In addition, the insulating phase is investigated for the first time. The density functional calculations allow for a consistent understanding of all three phases. In the rutile phase metallic conductivity is carried by metal orbitals, which fall into the one‐dimensional band, and the isotropically dispersing bands. Hybridization of both types of bands is weak. In the phase splitting of the band due to metal‐metal dimerization and upshift of the bands due to increased p‐d overlap lead to an effective separation of both types of bands. Despite incomplete opening of the optical band gap due to the shortcomings of the local density approximation, the metal‐insulator transition can be understood as a Peierls‐like instability of the band in an embedding background of electrons. In the phase, the metal‐insulator transition arises as a combined embedded Peierls‐like and antiferromagnetic instability. The results for VO₂ fit into the general scenario of an instability of the rutile‐type transition‐metal dioxides at the beginning of the d series towards dimerization or antiferromagnetic ordering within the characteristic metal chains. This scenario was successfully applied before to MoO₂ and NbO₂. In the compounds, the and bands can be completely separated, which leads to the observed metal‐insulator transitions.     

DFT study and NBO analysis of the mutual interconversion of cumulene compounds

The B3LYP/6‐31G* method was used to investigate the configurational properties of allene (1,2‐propadiene) ( 1 ), 1,2,3‐butatriene ( 2 ), 1,2,3,4‐pentateriene ( 3 ), 1,2,3,4,5‐hexapentaene ( 4 ), 1,2,3,4,5,6‐heptahexaene ( 5 ), 1,2,3,4,5,6,7‐octaheptaene ( 6 ), 1,2,3,4,5,6,7,8‐nonaoctaene ( 7 ), and 1,2,3,4,5,6,7,8,9‐decanonaene ( 9 ). The calculations at the B3LYP/6‐31G* level of theory showed that the mutual interconversion energy barrier in compounds 1 – 8 are: 209.73, 131.77, 120.34, 85.00, 80.91, 62.19, 55.56, and 46.83 kJ mol^?1, respectively. The results showed that the difference between the average C?C double bond lengths ( ) values in cumulene compounds 1 and 2 , is larger than those between 7 and 8 , which suggest that with large n (number of carbon atoms in cumulene chain), the values approach a limiting value. Accordingly, based on the plotted data, the extrapolation to n = ∞, gives nearly the same limiting (i. e., ). Also, NBO results revealed that the sum of π‐bond occupancies, , decrease from 1 to 8 , and inversely, the sum of π‐antibonding orbital occupancies, , increase from compound 1 to compound 8 . The decrease of values for compounds 1 – 8 , is found to follow the same trend as the barrier heights of mutual interconversion in compounds 1 – 8 , while the decrease of the barrier height of mutual interconversion in compounds 1 – 8 is found to follow the opposite trend as the increase in the number of carbon atom. Accordingly, besides the previously reported allylic resonant stabilization effect in the transition state structures, the results reveal that the values, , Δ(E_HOMO ? E_LUMO), and the C atom number could be considered as significant criteria for the mutual interconversion in cumulene compounds 1 – 8 . This work reports also useful predictive linear relationships between mutual interconversion energy barriers ( ) in cumulene compounds and the following four parameters: , , Δ(E_HOMO ? E_LUMO), and C_Number. Copyright © 2007 John Wiley & Sons, Ltd.     

Epitaxy of m ‐plane ZnO on (112) LaAlO3 substrate

Heteroepitaxial growth of non‐polar m ‐plane (10 0) ZnO has been demonstrated on (112) LaAlO₃ single crystal substrates using the pulsed laser deposition method. X‐ray diffraction, reflection high energy electron diffraction, and cross‐sectional transmission electron microscopy with selected‐area diffraction, have been used to characterize the structural properties of deposited ZnO films. The epitaxial relationship between ZnO and LAO is shown to be (10 0)_ZnO ∥ (112)_LAO, (11 0)_ZnO ∥ ( 1)_LAO and [0001]_ZnO ∥ [ 10]_LAO. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effects of ortho‐substituents in SE1 protonolysis of phenylmercuric chloride

The kinetics of the reactions of o‐substituted phenylmercuric chlorides, o‐RC₆H₄HgCl (R = CH₃, H, C₂H₅O, CH₃O, C₆H₅, F, COOC₂H₅, Cl, Br, CF₃, NO₂), with hydrochloric acid in 80% aqueous dioxane in the presence of NaI were studied. The reactions are of the first order. The rate constant at 40°C decreases in the order of R: CH₃ > H > C₂H₅O > CH₃O > C₆H₅ > F > COOC₂H₅ > Cl > Br > CF₃ > NO₂. The analysis of effects of those o‐substitutes is carried out through multiple regression of log k/k_H with the corresponding inductive substituent constants σ_I and the various resonance substituent constants σ, σ_R(BA), σ, σ and σ_x, and the corresponding Swain–Lupton field effect constant and resonance effect constant . The results showed that o‐substituent intramolecular coordination with the neighbor mercury (field effect) is the main effect in effects of o‐substituents on rate of the S_E1 protonolysis. Copyright © 2007 John Wiley & Sons, Ltd.     

Comments on the Pressure Dissociation of Deuterium as Related to the Nova Laser Hugoniot

The recent Nova laser experimental Hugoniot for deuterium can be justified by a simple model which involves only very general properties of this material and which highlights the role of the molecular dissociation. The region of maximal compression along the principal Hugoniot is characterized by , , , where E_B is the binding energy of a molecule, and ρ_o is the initial density.     

Na‐site energy of P2‐type NaxM O2 (M = Mn and Co)

P2‐type Na_xM O₂ (M = Mn and Co) is a promising cathode material for low‐cost sodium ion secondary batteries. In this structure, there are two different crystallographic Nai (i = 1 and 2) sites with different Coulomb potential $ (\varphi _i)$ provided by M^4–x and O^2–. Here, we experimentally determine a difference ${(\rm \Delta }\varepsilon \equiv \varepsilon _1 - \varepsilon _2)$ of Na‐site energies ${(}\varepsilon _i \equiv e\varphi {\kern 1pt} _i)$ based on the temperature dependence of the site occupancies. We find that ${\rm \Delta }\varepsilon \;{=}\;56\;{K}$ for Na_0.52MnO₂ is significantly smaller than 190 K for Na_0.59CoO₂. We interpret the suppressed ${\rm \Delta }\varepsilon $ in Na_0.52MnO₂ in terms of the screening effect of the Na⁺ charge. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

An ab initio study of CX3‐substitution (X = H,F, Cl,Br, I) effects on the static electric polarizability and hyperpolarizability of diacetylene

We report a systematic ab initio and density functional theory (DFT) study of the electric properties of the X₃C? C≡C? C≡C? H (X = H, F, Cl, Br, and I) sequence of substituted diacetylenes. We rely on finite‐field Møller–Plesset perturbation theory and coupled‐cluster calculations with large, flexible basis sets. Our best values at the second‐order Møller–Plesset perturbation theory level for the mean dipole polarizability and second hyperpolarizability are $\overline {{\alpha} } $ /e²aE = 64.46 (? CH₃), 65.59 (? CF₃), 110.11 (? CCl₃), 138.90 (? CBr₃), 184.98 (? CI₃) and $\overline {{\gamma} } $ /e⁴aE = 21020 (? CH₃), 13469 (? CF₃), 32708 (? CCl₃), 57599 (? CBr₃), and 105251 (? CI₃). For comparison, the analogous MP2 values for diacetylene [P.Karamanis and G.Maroulis, Chem. Phys. Lett. 2003 , 376, 403.] are $\overline {{\alpha} } $ /e²aE = 49.17, and $\overline {{\gamma} } $ /e⁴aE = 16227. For the mean first hyperpolarizability we report $\overline {{\beta} } $ /e³aE = ?205.8 (? CH₃), ?55.7 (? CF₃), 120.8 (? CCl₃), 443.8 (? CBr₃), and 725.4 (? CI₃). Copyright © 2010 John Wiley & Sons, Ltd.     

An exploratory study to investigate possible simple descriptors in order to predict relative activity of antiepileptic enaminones

A general structure, substituent and activity relationship of the following type has been fitted to the available ED₅₀ values of cyclic enaminone antiepileptic compounds: . In this relationship ‘structure’ was quantified by d, the distance measured between the carbonyl oxygen and the first atom of the aromatic ring. The ‘substituent’ was quantified by Hammett substituent constant: (σ). With the aid of the above function of two independent variables, a new molecular structure was predicted by extrapolation that has shown about two orders of magnitude greater activity than the most active molecule in the original set with measured ED₅₀ values. Copyright © 2008 John Wiley & Sons, Ltd.     

Substituent effect on the UV spectra of p‐disubstituted compounds XPh(CHCHPh)nY (n = 0, 1, 2)

Three parameters, , and , are developed to express the substituent effect and the effect of the parent molecular structure of p‐disubstituted compounds XPh(CH?CHPh)_nY (n = 0, 1, 2). The investigated result shows a good correlation between the UV absorption wavenumbers (υ_max) and the three parameters for a diverse set of title compounds, and the correlation equation can be used to predict the UV absorption energy of compounds with the mentioned structure. This approach provides a new insight for the quantitative structure‐property relationship (QSPR) correlation of the UV absorption energy of p‐disubstituted homologues. Copyright © 2010 John Wiley & Sons, Ltd.     

Measurements of Plasma Potential and Electron Temperature Fluctuation in a Low‐Temperature Magnetized Plasma

Measurements of fluctuations of plasma potential and electron temperature in a toroidal magnetized plasma is carried out by applying a cylindrical probe with insulating end plugs oriented parallel to the B‐field in conjunction with another cylindrical probe oriented perpendicularly. Coherency and cross‐phase between and are estimated, and typically have values close to 0.6 and π respectively. Power‐law spectra are found for frequencies well above the poloidal rotation frequency with spectral index typically around 4.0 for and around 2.5 for . The density gradient is above the threshold for flute interchange instability, and the results are consistent with theory and global numerical simulations of this plasma.     

DC Conductivity and Peierls instability in the quasi-one-dimensional organic charge density wave conductor (fluoranthene)2X

Due to the high anisotropy of the dc conductivity (σ_|/σ_⊥ ≈ 10⁴) the organic conductor (fluoranthene)₂X can be regarded as a model system for studying the Peierls instability in quasi-one-dimensional systems. The temperature dependence of the dc conductivity σ_| (T) along the highly conducting crystal axis exhibits the typical behaviour of a quasi-one-dimensional metal with a Peierls transition at about 180 K to a charge density wave (CDW) ground state. As expected for a highly one-dimensional conductor the exact transition temperature depends on three-dimensional coupling effects and therefore on the size of the counterion X^? = PF, AsF, SbF. Above the Peierls transition σ_| (T) can be described quantitatively within a model of CDW fluctuations leading to a pseudo gap in the electronic density of states. Below, the existence of a real energy gap at the Fermi level with a BCS-like temperature dependence determines the charge transport over more than eight orders of magnitude in the electrical resistance. For the intrinsic energy gaps 2 Δ (0), which characterize the ground state of the Peierls semiconductor, values of 120-180 meV have been found for different crystals.     

Hierarchy of the non‐covalent interactions in the alanine‐based secondary structures. DFT study of the frequency shifts and electron‐density features

The alanine (Ala)‐based cluster models of C5, C7, and C10 H‐bonds are studied at the DFT/B3LYP level. CPMD/BLYP simulations of the infinite polyalanine α‐helix (C13 H‐bond) and the two‐stranded β‐sheets are performed. Combined use of frequency shifts and electron‐density features enable us to detect and describe quantitatively the non‐covalent interactions (H‐bonds) defining the intrinsic properties of Ala‐based secondary structures. The energies of the primary N? H O H‐bonds are decreasing in the following way: C13 > C5 ≥ C7 > C10. The energies of the secondary N? H O, N?H N, and H H interactions are comparable to those of the primary H‐bonds (～4.5 kcal/mol). Side chain–backbone C? H O interaction is found to be the weakest non‐covalent interaction in the considered species. Its energy is ～0.5 kcal/mol in the infinite polyalanine α‐helix. Quantum‐topological electron‐density analysis is found to be a powerful tool for the detection of secondary non‐covalent interactions (C?O H? C and H H) and bifurcated H‐bonds, while the frequency shift study is useful for the identification and characterization of primary or secondary H‐bonds of the N? H O type. Copyright © 2008 John Wiley & Sons, Ltd.     

Room‐temperature epitaxial growth of high‐quality m ‐plane InGaN films on ZnO substrates

The authors have grown high‐quality m ‐plane In_0.36Ga_0.64N (1 00) films on ZnO (1 00) substrates at room temperature (RT) by pulsed laser deposition (PLD) and have investigated their structural properties. m ‐plane InGaN films grown on ZnO substrates at RT possess atomically flat surfaces with stepped and terraced structures, indicating that the film growth proceeds in a two‐dimensional mode. X‐ray diffraction measurements have revealed that the m ‐plane InGaN films grow without phase separation reactions at RT. The full‐width at half‐maximum values of the 1 00 X‐ray rocking curves of films with X‐ray incident azimuths perpendicular to the c ‐ and a‐axis are 88 arcsec and 78 arcsec, respectively. Reciprocal space‐mapping has revealed that a 50 nm thick m ‐plane In_0.36Ga_0.64N film grows coherently on the ZnO substrate, which can probably explain the low defect density that is observed in the film. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Boundaries Immersed in a Scalar Quantum Field

We study the interaction between a scalar quantum field $\hat \phi (x)$, and many different boundary configurations constructed from (parallel and orthogonal) thin planar surfaces on which $\hat \phi (x)$ is constrained to vanish, or to satisfy Neumann conditions. For most of these boundaries the Casimir problem has not previously been investigated. We calculate the canonical and improved vacuum stress tensors $ \langle \hat T_{\mu \nu } (x)\rangle\$ and $ \langle \Theta _{\mu \nu (x)} \rangle\$ of $\hat \phi (x)$; for each example. From these we obtain the local Casimir forces on all boundary planes. For massless fields, both vacuum stress tensors yield identical attractive local Casimir forces in all Dirichlet examples considered. This desirable outcome is not a priori obvious, given the quite different features of $ \langle \hat T_{\mu \nu } (x)\rangle\$ and $ \langle \Theta _{\mu \nu (x)} \rangle\$. For Neumann conditions. $ \langle \hat T_{\mu \nu } (x)\rangle\$ and $ \langle \Theta _{\mu \nu (x)} \rangle\$ lead to attractive Casimir stresses which are not always the same. We also consider Dirichlet and Neumann boundaries immersed in a common scalar quantum field, and find that these repel. The extensive catalogue of worked examples presented here belongs to a large class of completely solvable Casimir problems. Casimir forces previously unknown are predicted, among them ones which might be measurable.     

Electrons with alternating on-site and correlated hopping interactions

The 1 D one-band Hubbard model with different repulsive on-site interactions on even (U+V > 0) and odd (U-V > 0) sites, supplemented by the correlated-hopping term (t* > 0), describing the modification of the electron hopping by the presence of other particles on the sites, is considered as a 1 D model for CuO systems. The ground state phase diagram is studied within the framework of the bosonization technique and renormalization group analysis valid for weak coupling. Depending on the choice of model parameters, the following sequences of phase transitions with increasing bandfilling occur: 1) metal-insulator-metal (for t* ? U/4); 2) metal-insulator-metal-superconductor $ ({\rm for}U/4 < t * \le U/\sqrt 8);3) $metal-superconductor-metal-insulator-metal-superconductor $ ({\rm for}U/\sqrt 8 \le t * < (U + V)/\sqrt 8){\rm and}4) $metal-superconductor $ ({\rm for}(U + V)/\sqrt 8 \le t*) $.     

Kinetics and thermodynamics of the Li/Li+ couple in tetrahydrofuran at low temperatures (195–295 K)

Kinetic and thermodynamic (formal potential) data relating to the synthetically useful Li/Li⁺ couple in tetrahydrofuran (THF) solvent at a range of temperatures (196–295 K) are reported. Formal potentials, have been measured versus the standard reference electrode, in THF. At 295 K the following data have been obtained using a mathematical model to simulate the electro‐deposition (metal deposition and growth kinetics) processes of lithium (Li) on a platinum microelectrode; a of ?3.48 ± 0.005 V, = ?9.2 (±0.5) × 10^?4 V K^?1, the standard electrochemical rate constant, k₀ = 1 (± 0.1) × 10^?4 cm s^?1, transfer coefficient, α = 0.57 ± 0.03 and diffusion coefficient, D = 8.7 ± 0.1 × 10^?6 cm² s^?1. Copyright © 2007 John Wiley & Sons, Ltd.     

A new approach to correcting theoretical emitted intensities for absorption and enhancement effects

This paper introduces the possibility of replacing the usual additive corrections for absorption and enhancement by multiplicative factors. The possibility of deriving simple multiplicative factors to correct for inter‐element matrix effects, namely and , to correct for absorption of primary and secondary radiation, respectively, and enhancement ( ), is demonstrated. The use of the new coefficients, , and , derived directly from mass attenuation coefficients, simplifies the understanding of, and allows the stepwise evaluation of, the excitation of theoretical emitted intensities in XRF spectrometry. The approach is especially useful in providing a more consistent definition of theoretical intensity emitted from an infinitely thick sample, as compared with the classical formalism involving the use of mass attenuation coefficients. The approach has proved particularly useful in the classroom. Copyright © 2004 John Wiley & Sons, Ltd.     

Über die Schwere des Lichtes in der Newtonschen und in der Einsteinschen Gravitationstheorie

The gravity theories of Newton and Einstein are giving opposite sentences about the velocity of light in gravitational field. According to the Newtonian theory the velocity v in gravitational field is greater than the velocity c in a field-free space: v > c. According to general relativity theory we have a smaller velocity: v < c. For a spherical symmetric gravitational field Newton's theory gives \documentclass{article}\pagestyle{empty}\begin{document}$ v \approx c\left({1 + \frac{{fM}}{{c^2 r}}} \right) $\end{document} but Einstein's theory of 1911 gives \documentclass{article}\pagestyle{empty}\begin{document}$ v \approx c\left({1 - \frac{{fM}}{{c^2 r}}} \right) $\end{document} and general relativity gives \documentclass{article}\pagestyle{empty}\begin{document}$ v \approx c\left({1 - 2\frac{{fM}}{{rc^2 }}} \right) $\end{document}. Therefore, the radarecho-measurations of Shapiro are the experimentum crucis for Einstein's against Newton's theory.     

Degenerate and two‐color resonant four‐wave mixing of C2− in a molecular beam environment

In this paper, we demonstrate that degenerate and two‐color resonant four‐wave mixing spectroscopy is applicable for the sensitive and selective characterization of negative ions in a molecular beam environment. Results are shown for C , which is produced by discharging a mixture of acetylene and argon prior to supersonic expansion. Substantial signal‐to‐noise ratios of ≈ 150 show that the method is generally applicable for high‐resolution optical double‐resonance spectroscopy of negative ions. A detection sensitivity for C of ≈ 10⁷ ions/cm³ is estimated. Copyright © 2010 John Wiley & Sons, Ltd.