Ferromagnetism of binary compounds with cubic symmetry

Based on the idea of a strong interaction within the same unit cell, the possible existence of a ferromagnetic instability in a system with jumps from transition element cations to non-transition element anions and vice versa is established. A phase diagram is constructed for the ferromagnetic ordering as a function of the degree of filling, n _p and n _d , of the p ⁶-and d ¹⁰-shells of non-transition and transition elements, respectively. Zh. éksp. Teor. Fiz. 115, 1393–1410 (April 1999)     

Propagation-invariant hollow beams with hexagonal symmetry

Based on the interference of six symmetric beams, we introduce a new way to produce propagation-invariant hollow beams with hexagonal symmetry. The hollow beams have a dark center and hexagonal shape. They can be optical vortices, radially or azimuthally polarized beams. The results of simulation demonstrate the feasibility of this method.     

Ferromagnetism of manganese compounds

The possibility of ferromagnetic instability in a system with hopping between manganese cations and oxygen anions has been investigated on the basis of the concept of strong electron-electron interaction in one unit cell. A phase diagram for ferromagnetic ordering as a function of the filling factors of the 2p ⁶-shell of oxygen (n _p ) and 3t ₆ ^g -shell of manganese (n _t ) has been constructed. Zh. éksp. Teor. Fiz. 113, 1101–1111 (March 1998)     

Ferromagnetism of manganese compounds

It is shown on the basis of the notion of a strong electron-electron interaction in the unit cell that a ferromagnetic instability is possible in a system with hops between manganese cations and oxygen anions. The phase diagram for the existence of ferromagnetic ordering as a function of the degrees of underfilling n _p and n _t of the 2p ⁶ shell of oxygen and the 3t _g ⁶ shell of manganese, respectively, is constructed. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 4, 254–259 (25 August 1997)     

Anomalous diffusion in two-dimensional potentials with hexagonal symmetry

The diffusion process in a Hamiltonian dynamical system describing the motion of a particle in a two-dimensional (2D) potential with hexagonal symmetry is studied. It is shown that, depending on the energy of the particle, various transport processes can exist: normal (Brownian) diffusion, anomalous diffusion, and ballistic transport. The relationship between these transport processes and the underlying structure of the phase space of the Hamiltonian dynamical system is investigated. The anomalous transport is studied in detail in two particular cases: in the first case, inside the chaotic sea there exist self-similar structures with fractal properties while in the second case the transport takes place in the presence of multilayered structures. It is demonstrated that structures of the second type can lead to a physical situation in which the transport becomes ballistic. Also, it is shown that for all cases in which the diffusive transport is anomalous the trajectories of the diffusing particles contain long segments of regular motion, the length of these segments being described by Levy probability density functions. Finally, the numerical values of the parameters which describe the diffusion processes are compared with those predicted by existing theoretical models. (c) 2000 American Institute of Physics.     

Ferromagnetism of metallic compounds with the perovskite structure

On the basis of the concept of strong interaction in one unit cell, it is stated that ferromagnetic instability can occur in a system with hops between oxygen anions and transition metal (Me) cations in A_1?xB_xMeO₃ compounds. A phase diagram is constructed to describe the occurrence of ferromagnetic ordering as a function of the average number of holes (h_{t, d}) and (h_p) in the low-spin or high-spin t _2g ⁶ or high-spin 3d¹⁰ shell of the transition metal and in the 2p⁶ shells of O^2?.     

Orientational magnetoelastic relaxation in crystals with hexagonal symmetry

The anisotropy of the absorption of transverse elastic waves in uniaxial magnetic materials, connected with the deviations of the vectors of spontaneous magnetization from the “easy” directions, is described. Kursk State Technical University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 55–59, March, 1998.     

The laws of dispersion in crystals with hexagonal symmetry

Possible laws of dispersion are obtained in a quadratic approximation for crystals with hexagonal symmetry.     

Optical diffraction by two-dimensional photonic structures with hexagonal symmetry

Photonic structures with hexagonal symmetry have been prepared by the additive technology of two-photon laser lithography, and their optical properties have been investigated. The structure of the samples has been examined using scanning electron microscopy. The calculations have been performed for the optical diffraction in the Born approximation of the scattering theory for structures with a limited number of scatterers. The images formed in the monochromatic light on a flat screen located behind the sample have been calculated. The diffraction patterns on the screen have C_6v symmetry and consist of three straight lines intersecting at an angle of 120° and hyperbolas, the number of which is a multiple of six. An important feature of these diffraction patterns is the superstructure, i.e., the partition of straight lines and hyperbolas into individual diffraction reflections, the number of which is determined by the number of scatterers of a particular sample. The results of the experimental investigation of the diffraction patterns completely coincide with the calculated data, including the number and arrangement of the superstructure reflections.     

Inclusions and inhomogeneities in electroelastic media with hexagonal symmetry

For a long time, the absence of explicit Green's functions (fundamental solutions) for electroelastic media has hindered progress in the modelling of the properties of piezoelectric materials. Michelitsch's recently derived explicit electroelastic Green's function for the infinite medium with hexagonal symmetry (transversely isotropic medium) [4] is used here to obtain compact closed-form expressions for the electroelastic analogue of the Eshelby tensor for spheroidal inclusions. This represents a key quantity for the material properties of piezoelectric solids and analysis of the related electroelastic fields in inclusions. For the limiting case of continuous fibers our results coincide with Levin's expressions [8]. The derived method is useful for an extension to non-spheroidal inclusions or inhomogeneities having an axis of rotational symmetry parallel to the hexagonal c-axis. Received 14 September 1999     

Rhombic patterns: Broken hexagonal symmetry

Landau-Ginzburg equations derived to conserve two-dimensional spatial symmetries lead to the prediction that rhombic arrays with characteristic angles slightly differ from 60 degrees should form in many systems. Beyond the bifurcation from the uniform state to patterns, rhombic patterns are linearly stable for a band of angles near the 60 degrees angle of regular hexagons. Experiments conducted on a reaction-diffusion system involving a chlorite-iodide-malonic acid reaction yield rhombic patterns in good accord with the theory.     

Tunnelling states in a potential of hexagonal symmetry

The tunnel splitting of the ground state multiplet in a potential of the form V₀cos 6φ is calculated both in the weak perturbation and in the high barrier limit. In the latter limit a true variational calculation is performed and this gives better results than obtained by previous methods.     

Lasing action in two-dimensional organic photonic crystal lasers with hexagonal symmetry

We investigate fluorescence and lasing action from strongly modulated two-dimensional surface relief structures with hexagonal symmetry onto which thin films of optically active organic material have been deposited. As compared with second-order laser structures with square symmetry, these organic photonic crystal lasers exhibit unusual feedback mechanisms. As a result, we observe surface-emitting lasing action with a central beam normal to the surface and a hexagonal emission pattern of side-beams whose direction slightly deviates from the normal. A corresponding theoretical analysis allows us to determine the photonic bandstructure and the low-threshold laser modes in this system. These results agree very well with fluorescence data and confirm the hexagonal lasing pattern and the corresponding emission angles. PACS 42.55.Tv; 42.70.Jk; 42.70.Qs     

Ferromagnetism in transition-metal-doped II–VI compounds

We report systematical density-functional calculations of ternary transition-metal compounds based on zincblende ZnTe and CdTe semiconductor. Some of them are half-metallic at their optimized cell volumes. The effect of atomic position optimization (and cell volume re-optimization) on the electronic structures is to widen the energy bands near the Fermi energy and to reduce the density of states there. As a result, the Fermi level moves upward and the energy gap of the minority-spin bands at the Fermi energy becomes narrower. Therefore, the half-metallic gaps are reduced, two of them even being closed. These compounds are compatible with the II–VI semiconductors, and could be useful in spin-based electronics.     

Ferromagnetism of non-stoichiometric UFe2 compounds

The ac magnetic susceptibility of non-stoichiometric UFe₂ Laves compounds was measured between 40 and 200 K. It was found that the transition temperature of the investigated compounds is directly proportional to the iron content.     

Determination of the magnetic symmetry of hexagonal manganites by second harmonic generation

Optical second harmonic spectroscopy is introduced as a powerful supplement for the determination of complex magnetic structures. Experimental efforts are simplified and new degrees of freedom are opened. Thereby, some principal or technical restrictions of neutron or magnetic x-ray diffraction experiments are overcome. High spatial resolution leads to additional information about magnetically ordered matter. As an example, the noncollinear magnetic structure of the hexagonal manganites RMnO3 ( R = Sc, Y, Ho, Er, Tm, Yb, Lu) is analyzed. The results show that some earlier conclusions on their magnetic symmetry and properties should be revised.     

超导与铁磁共存的问题

按照ＢＣＳ超导理论 ,超导库珀对是由两个自旋取向相反的电子构成 ,即库珀对属于自旋“单态” .但在铁磁体中 ,交换作用迫使电子自旋平行取向 ,以致于“对态”成为自旋“三重态” .根据泡利不相容原理 ,自旋取向相同的电子在空间上相互回避 ,这导致铁磁体中的电子之间难于通过晶格振动 (声子 )的中介产生配对所需的吸引 .一般说来 ,铁磁性与ＢＣＳ超导电性之间是相互排斥的 .也曾有人提出与ＢＣＳ理论不同的理论 :存在磁相互作用中介的电子配对机制 ,即自旋三重态库珀对的形成由自旋涨落中介 ,而不是由声子中介 .这种新机制 ,历史上曾被用于…     

Correlated Huang scattering in hexagonal Jahn Teller compounds

This is a theoretical investigation on structural precursors in diffuse neutron scattering of disordered phases of cooperative Jahn-Teller compounds above their structural phase transitions. The theory is based on Huang scattering off elastic dipoles. Softening of shear modes in the smallq regime known from inelastic scattering predicts monoclinic and orthorhombic structures which are found in some of these compounds. For these smallq the Jahn-Teller effect looks dynamic and Huang scattering is not applicable. With larger wave vectors the dipoles look static and Huang theory works but one picks up neighbor correlations which have to be included. We believe to have identified the spiral correlation announcing the helical structure of CsCuCl₃ in its ordered phase. We also calculate diffuse scattering for RbCrCl₃ and the experimentally observed satellites announcing the cell doubling of the ordered phase.     

Nuclear spin-lattice relaxation in transition metal compounds with local tetragonal symmetry

We report on the various contributions to the total spin-lattice relaxation rate in metallic materials with local tetragonal symmetry. The analytical formulae are given in the tight-binding approximation. The calculations show the relation between various partial electron densities of states and corresponding contributions to the relaxation rates. The presented formulae can be used to compare theoretically calculated electron band structure parameters with those obtained from NMR spin-lattice relaxation time measurements.     

Surface waves propagating around the periphery of a piezoelectric cylinder of hexagonal symmetry with time dependent elastic stiffness

Acoustic surface waves propagating around the periphery of a piezoelectric cylinder of hexagonal symmetry are investigated. A co-axial structure with a piezoelectric insulator as the inner cylinder and electric conductor as the outer cylinder is considered. The dispersion equation is derived and solved under certain approximations assuming the elastic stiffness to be time dependent as in Voigt's model of viscoelasticity. The final expressions for the mechanical displacement component and electric potential are obtained. Ultimately some important parameters of such waves as group velocity, Poynting vector and power flow components have also been determined.I am grateful to Dr. A. K. Pal for his helpful suggestions in the preparation of the paper.