Stabilization of A-type layered antiferromagnetic phase in LaMnO3 by cooperative Jahn-Teller deformations

It is shown that the layered antiferromagnetic order in stoechiometric cannot be understood purely from electronic interactions. On the contrary, it mainly results from strong cooperative Jahn-Teller deformation. Those involve a compression of the Mn-O octahedron along the c-axis (mode Q ₃ < 0), while alternate Jahn-Teller deformations occur in the ab-plane (mode Q₂). These deformations stabilize a certain type of orbital ordering. The resulting superexchange couplings are calculated by exact diagonalization, taking into account both e_g and t_2g orbitals. The main result is that antiferromagnetic (ferromagnetic) coupling along the c-direction (ab-planes) can be understood only if the Jahn-Teller energy is much larger than the superexchange couplings, which is consistent with experiments. This mechanism contrasts with that based on weak Jahn-Teller coupling which instead predicts elongation along the c-axis (Q ₃ > 0). The crucial role of the deformation anisotropy is also emphasized. Received 24 January 2000     

Effect of an electric field on the bifurcation of a biaxially stretched incompressible slab rubber

This paper describes the effect of an electrical field on the bifurcation phenomenon that appears in a biaxially stretched slab of Mooney-Rivlin material (M. Mooney, J. Appl. Phys. 11, 582 (1940)) subjected to equal dead loads. The main conclusion of the analysis is that the stretch ratio at which the bifurcation phenomenon appears crucially depends on the configuration of the system rubber slab-electrodes. The theoretical foundations of the present study are based on a recent formulation on this subject carried out by Dorfmann and Ogden (A. Dorfmann, R.W. Ogden, Acta Mech. 174, 167 (2005); J. Elasticity 82, 99 (2006)).     

Calculations of the EPR g-tensor using unrestricted two- and four-component relativistic approaches within the HF and DFT frameworks

ABSTRACT

Approaches and programs for calculations of the EPR g-tensor in the framework of the two- and four-component methods are still very rare. There are three main reasons for this: the wider community's unawareness of the importance of second- and higher order spin–orbit effects on the g-tensor, the methodological problems associated with performing such calculations and the lack of understanding of these problems. This paper reports on the implementation of a method for calculation of the g-tensor in the framework of the relativistic unrestricted two- and four-component Hartree–Fock and density functional theory approaches based on the Kramers pair formalism. This implementation allows us to analyse problems which arise when the g-tensor is calculated via Kramers pairs in the unrestricted framework.     

Some Questions of the Nevanlinna Theory for the Complex Half-Plane

In this paper we give variants of the logarithmic derivative lemma and the second main theorem for the complex half-plane. Here the second main theorem is a special case of a theorem concerning polynomial functions of the coordinates of a holomorphic curve f : C ₊ P ⁿ . This theorem is a variant for C ₊ of a theorem of A. Eremenko and M. Sodin, who considered an entire curve f : C ₊ P ⁿ . The proofs are realized because of the introduction of natural classes of subharmonic, -subharmonic, holomorphic, meromorphic functions in C ₊ . Such functions we shall call just functions.     

Two-polarization Ku-band radar imagery of sea surface in presence of atmospheric boundary layer motions

Abstract

The influence of atmospheric boundary layer stability on radar imagery at two polarizations is considered. A strong dependence of ripple manifestations on the relative directions of sounding and the wind is noted in vertically polarized images. The main mechanisms for modulation of resonant scattering ripples by long surface waves are examined.     

Electron transport properties of MgB<Subscript>2</Subscript> in the normal state

We report measurements of the resistivity, ρ, and the Seebeck coefficient, S , of a MgB₂ sintered sample, and compare S with theoretical calculations based on precise electronic structure calculations. ρ is fitted well by a generalized Bloch-Grüneisen equation with a Debye temperature Θ _R of 1050 K. S is given by the sum of a diffusive and a phonon drag term and the behavior in the temperature region T _c < T < 0.1Θ _R follows the relationship AT+BT³. The phonon drag term indicates a strong electron-phonon interaction. The diffusive term, compared with calculations, suggests that σ bands give the main contribution to the Seebeck effect. Received 16 November 2001 and Received in final form 21 December 2001     

A panoramic view of fiber and integrated optics in spain

Abstract

A general view of the present status of optics and related fields in Spain is presented. The main emphasis is on the relation between optics and some emerging areas such as Optical Communications and Nonlinear Optics. Principal activities of the more important groups are summarized.     

Physical properties of resistive threads and structures based on them in the microwave range

The main physico-mechanical characteristics of resistive threads are investigated. The dependence of the complex dielectric constant of a resistive thread on the wavelength is measured over a wide sub-interval of the microwave range. The possibility is demonstrated of using such threads as the main element of wide-band radio-absorbing structures. Zh. Tekh. Fiz. 69, 104–108 (January 1999)     

Is the Pauli exclusion principle the origin of electron localisation?

ABSTRACT

In this work, we inquire into the origins of the electron localisation as obtained from the information content of the same-spin pair density, γ^{σ, σ}(r₂∣r₁). To this end, we consider systems of non-interacting and interacting identical Fermions contained in two simple 1D potential models: (1) an infinite potential well and (2) the Kronig–Penney periodic potential. The interparticle interaction is considered through the Hartree–Fock approximation as well as the configuration interaction expansion. Morover, the electron localisation is described through the Kullback–Leibler divergence between γ^{σ, σ}(r₂∣r₁) and its associated marginal probability. The results show that, as long as the adopted method properly includes the Pauli principle, the electronic localisation depends only modestly on the interparticle interaction. In view of the latter, one may conclude that the Pauli principle is the main responsible for the electron localisation.     

The relation between the electronic and magnetic structures of manganites in the tight-binding approximation

Analytic expressions for the dispersion curves E(k) of RMnO₃ (R=La, Pr, Nd, Sm, and other RE elements) have been obtained in the tight-binding approximation for the main types of magnetic ordering on the Mn sublattice. The first calculation of E(k) taking into account the oxygen subsystem and mutual ordering of the manganese and RE sublattices is reported. The results obtained permit a qualitative interpretation of some features observed in the behavior of the rare-earth manganites. Fiz. Tverd. Tela (St. Petersburg) 41, 2179–2182 (December 1999)     

Isomorphous phase transition and orientational freezing in hexagonal mixed crystal C 70(1 - x) C60x. A pseudospin model

A four-state pseudospin model is constructed for the isomorphous phase transition hcp-2↦hcp-1 in pure C₇₀ and in C₇₀-rich mixed crystal C _{70(1 - x)} C_60x. With the specific anisotropic pseudospin interactions adapted to the C₇₀ crystal the model is equivalent to a two-state Ising model in a temperature-dependent field. Replica symmetric state of the model is shown to approach the critical point when the width of distribution of random fields and/or of random bonds increases. The temperature of the phase transition and the phase equilibrium temperature then are practically constant, whereas the experiment shows their strong decrease with x. The main effect of dilution resides in an x-dependence of the model parameters. Dilatometric data on the hexagonal C _{70(1 - x)} C_60x are used to fit these parameters. A metastable disordered phase subsisting below the phase transition is discovered in a range of the model parameters and is shown to be responsible for the macroscopic behaviour of the system. A good agreement with experimental data is obtained for the spontaneous strain and for the x-dependence of the hysteresis. Received 20 April 2001 and Received in final form 26 September 2001     

Metrology of high pressures to approximately 3.0 Gpa

Abstract

The pressure scale up to approximately 3. 0 GPa is considered in order to improve pressure measurement accuracy.

The main subjects presented are related to absolute and gage pressure measurements with gases to about few megapascal and gage pressure measurements in liquids to about 1. 5 GPa by means of piston gauges.

The use of a pressure scale based on fixed points is mentioned.     

Interactive process in the mechanism of thermostimulated luminescence in anion-defect α-Al2O3 crystals

A study of the role of deep traps in the specific features of the thermoluminescence (TL) of anion-defect α-Al₂O₃ single crystals is reported. The existence of deep traps is proven by direct observation of the associated TL peaks. Experimental support for the effect of deep-trap filling on the main characteristics of the main TL peak at 450 K is presented. A model involving trap interaction is proposed, which differs radically from the others described in the literature by taking into account the temperature dependence of the carrier capture probability by deep traps. This model was used to calculate the dependences on heating rate and deep-trap filling of the main parameters of the main TL peak for the crystals under study (TL yield, glow-curve shape, and sensitivity to the stored light sum), which were found to be close to those observed experimentally. Fiz. Tverd. Tela (St. Petersburg) 40, 229–234 (February 1998)     

FAZIA: Prototyping a next-generation 4π array for nuclear reaction-dynamics studies

The international FAZIA (Four π A and Z Identification Array) Collaboration, presently involving researchers and institutions from France, Italy, Poland, Romania, Spain, India, Canada and USA, is currently investigating the use of new detector types/configurations (nTD Si detectors, single chip telescope) and new electronics (charge/current preamplifiers, digital sampling and signal processing) in view of defining a next-generation detection array with improved particle identification capability to study nuclear reaction dynamics and thermodynamics exploiting the Radioactive Nuclear Beam (RNB) facilities. The present main R&D activities inside the Collaboration are brielfly described.     

Mechanism of giant magnetoresistance in intermetallic compounds of rare-earth ions and actinides

Giant positive or negative magnetoresistance is calculated in a band model. The spectra of the band electrons in a two-sublattice antiferromagnetic intermetallic compound depend on the antiferromagnetism vector L(T,H). The metamagnetic transition to the ferromagnetic phase is accompanied by splitting with respect to the spin σ, displacement of the energy bands, and a decrease in the effective masses of the band electrons. This mechanism of giant negative magnetoresistance is also accompanied by an increase in the relaxation time τ_jσ. Scattering by chemical-bond fluctuations is considered as the main relaxation mechanism. Giant positive magnetoresistance results from a four-subband model of 4f and 5f intermetallic compounds. The electron effective masses m _jσ(J _jT ) of the (j,σ) bands increase with the mean angular momentum J _1T (T,H) of an ion in the jth sublattice of 4(5)f ions. The thermodynamics of such a four-sublattice model, the nonlinear magnetization and magnetoresistance curves, and the nonmonotonic dependence of the specific heat C _m(T,H) on the field H are calculated. Fiz. Tverd. Tela (St. Petersburg) 39, 1806–1814 (October 1997)     

High pressure resistivity studies on La1+xBa2-xCu3Oy system

Abstract

We report here an electrical resistivity study on La_1+xCu₃O_y system under high pressure up to 80 kbar. The three different oxides, %ained from the main system by varying the values of x, are prepared by solid state reactions. Bridgman opposed anvil device is used. The normalised resistivity as a function of pressure for the oxides yields interesting results.     

Quasi elasto-electromagnetic surface waves on a piezo-plasma-like layer

Considering a piezo-plasma-like layer with finite thickness and hexagonal symmetry whose main symmetry axis is parallel to the z axis and approximating it by an isotropic medium, we study the coupling of the elastic wave with plasma properties of the medium with and without spatial dispersion and collisions. In this case we investigate the coupled surface quasi elasto-electromagnetic wave propagating on the interface of piezoelectric layer with vacuum. Furthermore, the coupling of elasticity and ion-acoustic waves is investigated.     

Some of the current trends in the studies of sputtering

Abstract

The main directions in experimental, theoretical and computer studies of sputtering of solids under ion bombardment are analyzed. It is emphasized that there is a close relation between these studies and practical applications of sputtering.     

Experimental determination of the dynamical properties of molecular crystals under pressure

Abstract

A main characteristic of molecular solids is the large difference between intra- and intermolecular interactions. This difference is reflected in the physical properties, especially in the dynamical properties and their pressure dependence. The three main techniques used to study the dynamical properties of crystals in diamond cells are reviewed, and each one is illustrated by an example which shows specific properties of molecular crystals.     

Melting of the cooperative Jahn-Teller distortion in LaMnO 3 single crystal studied by Raman spectroscopy

We have studied the behavior of the Raman phonons of a stoichiometric LaMnO₃ single crystal as a function of temperature in the range between 77 K and 900 K. We focus on the three main phonon peaks of the Pbnm structure, related to the tilt, antisymmetric stretching (Jahn-Teller mode) and stretching modes of Mn-O octahedra. The phonon frequencies show a strong softening that can be fit taking into account their renormalization because of three phonon anharmonic effects in the pseudoharmonic approximation. Thermal expansion effects, in particular the variation of Mn-O bond lengths with temperature, are not relevant above 300 K. On the contrary, phonon width behavior deviates from the three phonon scattering processes well bellow T _c . The correlation between the magnitude of the cooperative Jahn-Teller distortion, that disappears at 800 K, and the amplitude of the Raman phonons in the orthorhombic phase is shown. Nevertheless, Pbnm phonons are still observable above this temperature. Phonon width and intensity behavior around T _c can be explained by local melting of the orbital order that begins quite below T _c and by fluctuations of the regular Mn-O octahedra that correspond to dynamic Jahn-Teller distortions. Received 25 January 2001 and Received in final form 14 March 2001