Invariant exchange perturbation theory for multicenter systems and its application to the calculation of magnetic chains in manganites

The formalism of exchange perturbation theory is presented with regard to the general principles of constructing an antisymmetric vector with the use of the Young diagrams and tableaux in which the coordinate and spin parts are not separated. The form of the energy and wave function corrections coincides with earlier obtained expressions, which are reduced in the present paper to a simpler form of a symmetry-adapted perturbation operator, which preserves all intercenter exchange contributions. The exchange perturbation theory (EPT) formalism itself is presented in the standard form of invariant perturbation theory that takes into account intercenter electron permutations between overlapping nonorthogonal states. As an example of application of the formalism of invariant perturbation theory, we consider the magnetic properties of perovskite manganites La_1/3Ca_2/3MnO₃ that are associated with the charge and spin ordering in magnetic chains of manganese. We try to interpret the experimental results obtained from the study of the effect of doping the above alloys by the model of superexchange interaction in manganite chains that is constructed on the basis of the exchange perturbation theory (EPT) formalism. The model proposed makes it possible to carry out a quantitative analysis of the effect of substitution of manganese atoms by doping elements with different electron configurations on the electronic structure and short-range order in a magnetic chain of manganites.     

The method of exchange perturbation theory as applied to magnetic ordering in high-T c materials

We unify the method of exchange perturbation theory for multicenter systems. For the case of exchange degeneracy in the total spin of the system we give a secular equation that is more compact and convenient for calculations than those obtained earlier. On the basis of this formalism we develop an algorithm for calculating the Heisenberg parameter for magnetic materials. Finally, we calculate the characteristics of antiferromagnetic transitions for the high-T _c materials La_2−x MeCuO₄ and YBa₂Cu₃O₆. Zh. éksp. Teor. Fiz. 113, 2129–2147 (June 1998)     

Nonlinear magnetic phenomena in atomic bose gas

Optically confined (j=1, j=2)-Bose gas is studied. The spin Hamiltonian and the ground state of a set of particles with spins j=1 and j=2 are obtained. Nonlinear magnetic phenomena in the set of particles with j=1 are considered.     

Nature of coherent enhancement of inelastic electron scattering from disordered media

The mechanism of weak localization of relatively fast electrons scattered with a fixed energy loss from disordered media is examined. The main focus of this paper is to put forward an explanation for why coherent enhancement of electron scattering in the inelastic-scattering channel takes place at angles which differ from π. A simplified kinematic model is proposed to determine the basic properties of the weak localization of electrons in the inelastic scattering channel. The model easily reproduces the range of scattering angles typical for the weak localization of electrons with fixed energy loss. The procedure does not require calculation of the contribution from the crossed diagrams. The results agree with those based on the dynamical theory associated with the calculation of the crossed and ladder diagrams. It is possible to follow the transition from the weak localization of the new type to the ordinary weak localization with decreasing energy loss. The weak localization of the new type is in accord with the weak localization of regular type if the energy loss is about the energy of an optical phonon.     

Exchange dipole interaction in a multilevel cooperative system of atoms

Atomic systems with three or more equidistant energy levels, in which a cascade process is possible, are considered. Hamiltonians obtained for such systems are analogous to Heisenberg Hamiltonians, but for systems with integral spins. For Dicke states in multilevel systems, quantum-mechanical mean values of the energy of a cooperative system are calculated taking into account weak interactions between atoms. The type of emission preceding superradiant avalanche emission of the system is analyzed. It can be expected that a coherent state may be formed, in which collective processes affect one another not only via population of the intermediate common layer, but also via phasing of pure quantum states. The single superfluorescence pulse that can be formed in this case is not a simple superposition of two or more pulses of sequential superradiant transitions in two-level systems.     

On the theory of quantum interference between inelastic and elastic electron scattering events

The mechanism of weak localization of relatively fast electrons scattered with a fixed energy loss from disordered media is examined. The main focus of this paper is to put forward an explanation why coherent enhancement of electron scattering in the inelastic-scattering channel takes place at angles which differ from π. A simplified kinematic model is proposed to determine the basic properties of the weak localization of electrons in the inelastic scattering channel. The model reproduces easily the range of scattering angles typical of the weak localization of electrons with a fixed energy loss. The procedure does not require calculation of the contribution from the crossed diagrams. The results agree with those based on the dynamical theory associated with the calculation of the crossed and ladder diagrams. It is possible to follow the transition from the new type of weak localization to the ordinary weak localization with decreasing energy loss. The new-type weak localization is in agreement with the regular weak localization if the energy loss is approximately equal to the energy of an optical phonon. Zh. éksp. Teor. Fiz. 111, 1001–1015 (March 1997) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Exchange self-polarization of an electron beam

Self-organization of a beam of free electrons injected by an optical cathode is discussed. It is shown that exchange interaction may cause beam self-polarization in a vacuum and sustain beam polarization in the material.     

Effect of the Asymmetry of Cylindrical Sandwich Shells on their Stress–Strain State under Non-Stationary Loading*

International Applied Mechanics - The stress–strain state of asymmetric three-layer cylindrical shells under nonstationary loads is considered. The sandwich structure is asymmetric due to the...     

Effects of the reduction of the dimension of a system upon spin ordering in a degenerate electron gas

Collective effects of spin ordering in a quasi-one-dimensional degenerate electron gas are discussed. The total energy of the quasi-one-dimensional system, as well as the exchange contribution per particle, has been calculated by the Hartree-Fock method. It has been shown that spontaneous polarization can be observed in the system when a universal parameter related to the density of the particles satisfies the inequality r _s ≥ 0.476. A comparative analysis of one-, two-, and three-dimensional systems has been performed. A general expression has been obtained for the total energy per particle as a function of the degree of polarization and dimension of the system. According to this expression, the possibility of spontaneous polarization in the system is closely correlated with the dimension of the system.     

Effect of the equation of state on the dispersion of detonation products

Journal of Applied Mechanics and Technical Physics -