Magnetic properties of titanium vanadium hydrides

Measurements have been made of the magnetic susceptibility X in ternary titanium-vanadium hydrides Ti_1?yV_yH_x in the range 0<y< 0.5 and with a hydrogen to metal ratio from 1.6 to 1.92; they cover the temperature interval from 80 to 440 K. The x^?T curves of some of the ternary hydrides possess a maximum similar to that seen in binary hydrides. Its occurrence depends upon both hydrogen and vanadium concentration. Comparison with the lattice constants shows that this dependence is related to the transition from the cubic γ-phase to the tetragonal δ-phase. The x^?T curves of those samples showing the phase transition were extrapolated from the cubic phase down to lower temperatures. Conclusions were drawn from the measured and extrapolated susceptibility values, as to the influence of both hydrogen and vanadium concentrations on the electronic structure of these hydrides. The dependence of the magnetic susceptibility on the vanadium concentration shows that, in the cubic phase, the fermi energy lies on the increasing side of a maximum in the density of states curve. The degree of occupation of the conduction band does not depend on the hydrogen concentration. The rigid band model cannot be used in the hydrides studied here to account for the effects of varying their hydrogen concentration.     

Magnetic susceptibility of the itinerant-electron antiferromagnet containing magnetic impurities

The static magnetic susceptibility of an itinerant-electron antiferromagnet containing magnetic impurities, is calculated using the two-band model in the mean-field approximation. The temperature dependence of the magnetic moment of the impurity is also calculated. The theory proposes a model to explain the magnetic behaviour of the binary chromium alloys with the iron and cobalt as well as the magnetic behaviour of some rare earth compounds.     

Dispersion of the linear electrooptic coefficient and of the non-linear susceptibility in ZnTe

The measured values and the analysis of the dispersion of the unclamped linear electrooptic coefficient r^T₄₁ in cubic ZnTe operating as optical modulator are presented. The spectral dispersion of the non-linear susceptibility d₄₁ (ω, ω, 0) is also reported and the strong dependence on the light frequency, observed for d₄₁, is interpreted in terms of the electronic contribution to the anharmonicity.     

Photonic band structure in the nearly plane wave approximation

For photons propagating in a periodic dielectric lattice, the dispersion curve forms photonic bands separated by forbidden gaps. When the dielectric lattice deviates only slightly from being homogenous, the photonic band structure resembles the linear dispersion relation for photons folded into the first Brillouin zone, i.e., the so-called empty lattice bands. Using group theoretical technique, we calculate the splitting of the accidental degeneracies in the empty lattice bands at symmetry points for a simple cubic dielectric lattice. Received 23 June 1998     

First-principles calculations of the structural,electronic, optical and thermal properties of the BNxAs1–x alloys

The present paper aims to study the structural, electronic, optical and thermal properties of the boron nitride (BN) and BAs bulk materials as well as the BN_xAs_1–x ternary alloys by employing the full-potential-linearised augmented plane wave method within the density functional theory. The structural properties are determined using the Wu–Cohen generalised gradient approximation that is based on the optimisation of the total energy. For band structure calculations, both the Wu–Cohen generalised gradient approximation and the modified Becke–Johnson of the exchange-correlation energy and potential, respectively, are used. We investigated the effect of composition on the lattice constants, bulk modulus and band gap. Deviations of the lattice constants and the bulk modulus from the Vegard’s law and the linear concentration dependence, respectively, were observed for the alloys where this result allows us to explain some specific behaviours in the electronic properties of the alloys. For the optical properties, the calculated refractive indices and the optical dielectric constants were found to vary nonlinearly with the N composition. Finally, the thermal effect on some of the macroscopic properties was predicted using the quasi-harmonic Debye model in which the lattice vibrations are taken into account.     

Equation of state and thermodynamics of fcc transition metals: A pseudopotential approach

A simple pseudopotential model is used for the calculation of the phonon spectra at the equilibrium volume and under pressure. The model is based on the secondorder perturbation theory with the local pseudopotential acting on thes electrons while thed electrons contribution is simulated by the repulsive Born-Mayer interatomic potential. Pressure influence on the lattice properties was studied for small compressions (mode Grüneisen parameters) as well as for ultrahigh pressure (equation of state up to 1 TPa). Results of the lattice dynamics calculations were used for determining temperature dependence of the lattice heat capacity and of the macroscopic Grüneisen parameter. The Kohn anomaly at the small wave vectors obtained previously in palladium, platinum and rhodium affects strongly the temperature dependence at low temperature.     

Temperature dependence of transmittance and effective surface resistance of MgB2 film

Using the Kubo formula approach and Green's function method, the temperature dependence of the transmittance and effective surface resistance of high quality MgB₂ film are calculated in the framework of the two-band model. For large interval of temperatures below T_C our results are in agreement with experimental data. We show a single-gap model based on BCS theory, which is insufficient to understand such quantities, but a two-band model with different symmetries can describes the experimental data rather well. In the two-gap model we consider that the transmittance and effective surface resistance are a weighted sum of the contributions from σ and π bonds and hybridization between them is negligible.     

Regular packings on periodic lattices

We investigate the problem of packing identical hard objects on regular lattices in d dimensions. Restricting configuration space to parallel alignment of the objects, we study the densest packing at a given aspect ratio X. For rectangles and ellipses on the square lattice as well as for biaxial ellipsoids on a simple cubic lattice, we calculate the maximum packing fraction φ(d)(X). It is proved to be continuous with an infinite number of singular points X(ν)(min), X(ν)(max), ν = 0, ±1, ±2,…. In two dimensions, all maxima have the same height, whereas there is a unique global maximum for the case of ellipsoids. The form of φ(d)(X) is discussed in the context of geometrical frustration effects, transitions in the contact numbers, and number-theoretical properties. Implications and generalizations for more general packing problems are outlined.     

Magnetic properties of Y(Fe-Co)2

Magnetic properties of the ternary compounds Y(Fe-Co)₂ with the cubic Laves phase structure are discussed in the rigid band model. The concentration dependences of the magnetization, high-field susceptibility and the electronic low-temperature specific heat coefficient are calculated at 0 K. The temperature dependence of the paramagnetic susceptibility is calculated by taking into account the effect of spin fluctuations. The agreement between the calculated and observed results is satisfactory.     

Horizontal rolls in convective flow above a partially heated surface

Considering the nonlinearity arising from the interaction between electrons and lattice vibrations, an effective electronic model with a self-interaction cubic term is employed to study the interplay between electron-electron and electron-phonon interactions. Based on numerical solutions of the time-dependent nonlinear Schroedinger equation for an initially localized two-electron singlet state, we show that the magnitude of the electron-phonon coupling χ necessary to promote the self-trapping of the electronic wave packet decreases as a function of the electron-electron interaction U. We show that such dependence is directly linked to the narrowing of the band of bounded two-electron states as U increases. We obtain the transition line in the χ × U parameter space separating the phases of self-trapped and delocalized electronic wave packets. The present results indicates that nonlinear contributions plays a relevant role in the electronic wave packet dynamics, particularly in the regime of strongly correlated electrons.     

Peierls-Fröhlich instability and Kohn anomaly

A mathematical basis is given to the Peierls-Fröhlich instability and the Kohn anomaly. The techniques and ideas are based on the recently developed mathematical theory of quantum fluctuations and response theory. We prove that there exists a unique resonant one-mode interaction between electrons and phonons which is responsible for the Peierls-Fröhlich instability and the phase transition in the Mattis-Langer model. We prove also that the softening of this phonon mode at the critical temperature (Kohn anomaly) is a consequence of the critical slowing down of the dynamics of the lattice distortion fluctuations. It is the result of the linear dependence of two fluctuation operators corresponding to the frozen charge density wave and the distortion order parameter.     

Temperature behavior of magnetization of DyCo2 compound

Zero field cooled (ZFC) and field cooled (FC) magnetization measurements were performed on the binary DyCo₂ cubic compound. Maxima for this compound were observed under ZFC magnetization. Below the broad maximum, irreversibility is observed. The magnetization curves for ZFC and FC regimes are split and magnetic moments for FC are higher than for ZFC. The dependence of the maxima upon the magnetic field and the time dependence of remanence is similar to the case of spin-glass-like systems.     

Electronic and lattice vibrational properties of cubic BaHfO3 from first principles calculations

Through first principles calculations, we investigated the electronic structure and lattice vibrational properties of BaHfO₃. The optimized lattice constant of BaHfO₃ is in agreement with experimental and theoretical results. Our results show that cubic BaHfO₃ is an insulator with an indirect band gap of 3.5 eV. Besides, the calculation using the screened exchange local density approximation (sX-LDA) has been performed with the predicted minimum gap of 5.3 eV. The phonon dispersion curves of BaHfO₃ were also calculated. All positive phonon frequencies in the Brillouin zone were found, indicating the stability of BaHfO₃ structure.     

Ground state and lattice dynamical study of ionic conductors CaF2, SrF2 and BaF2 using density functional theory

The present paper reports a comprehensive and complementary study on structural, electronic and phonon properties of face centered cubic fluorites, namely CaF₂, BaF₂ and SrF₂, using first principles density functional calculations within the generalized gradient approximation. The calculated lattice constants and bulk modulus are in good agreement with available experimental data. The analysis of band structure and density of states confirms the ionic character for all the three fluorides. The phonon dispersion curves and corresponding phonon density of states obtained in the present work are consistent with the available experimental and other theoretical data. The LO-TO splitting is maximum for CaF₂, which confirms that the ionicity is maximum in the case of CaF₂. The phonon properties for SrF₂ have been calculated for the first time.     

The theory of the sound attenuation in one-dimensional metals

The elasticity theory equations are obtained for a 1d conductor. The frequency dependence of the sound attenuation is analysed, the spatial dispersion being strong or weak. The effect of oscillations of the attenuation is predicted which is due to a jumping nature of electronic motion in a non-uniform field of the sound wave, with a fixed jumping length. That is why the oscillations are of the geometric resonance type. Because of absence of Landau's damping, the frequency dependence of attenuation in a region of strong spatial dispersion is quadratic rather than linear one, as in 3d metals. This dependence is determined by a quantum nature of electronic scattering on separate impurities which move with an oscillating lattice.     

Test of universality in anisotropic 3D Ising model

Chen and Dohm predicted theoretically in 2004 that the widely believed universality principle is violated in the Ising model on the simple cubic lattice with more than only six nearest neighbours. Schulte and Drope by Monte Carlo simulations found such violation, but not in the predicted direction. Selke and Shchur tested the square lattice. Here, we check only this universality for the susceptibility ratio near the critical point. For this purpose we study first the standard Ising model on a simple cubic lattice with six nearest neighbours, then with six nearest and 12 next-nearest neighbours, and compare the results with the Chen–Dohm lattice of six nearest neighbours and only half of the 12 next-nearest neighbours. We do not confirm the violation of universality found by Schulte and Drope in the susceptibility ratio.     

Electronic correlations in manganites

The influence of local electronic correlations on the properties of colossal magnetoresistive manganites is investigated. To this end, a ferromagnetic two-band Kondo lattice model is supplemented with the local Coulomb repulsion missing in this model and is analyzed within dynamical mean-field theory. Results for the spectral function, optical conductivity, and the paramagnetic-to-ferromagnetic phase transition show that electronic correlations have drastic effects and may explain some experimental observations.     

Maximum in the temperature dependence of resistivity of some crystal alloys of the Cu-Zr system

Temperature dependence of resistivity ? of crystal Cu-Zr alloys was measured up to 1050 K. For copper-rich alloys, the function ?(T) turned to have a maximum. This result is discussed in terms of the two-band model.     

Electrical resistivity of the Ti\mathsf{_4}O\mathsf{_7} Magneli phase under high pressure

The electronic and positronic properties of the pentanary semiconductor alloys Ga_xIn_1-xP_ySb_zAs_1-y-z lattice matched to GaSb have been studied. The electron wave function is calculated semiempirically using the pseudopotential band model under the virtual crystal approximation. The positron wave function is evaluated under the point core approximation for the ionic potential. Electronic and positronic quantities namely, electronic structure and band gaps, positron band structure, effective mass and affinity, and electron-positron momentum densities have been predicted and their dependence on the phosphorus composition has been discussed. Received 30 August 2002 / Received in final form 12 February 2003 Published online 24 April 2003