Showing papers on "Exchange interaction published in 1983"

Hartree–Fock exchange energy of an inhomogeneous electron gas

Abstract: We examine the short-range behavior of the spherically averaged Hartree–Fock exchange charge density by performing a simple Taylor expansion. On the basis of this expansion, a theoretical model is constructed that generates gradient correction terms to the local density approximation for the exchange energy of an inhomogeneous electron gas. In particular, we derive the Xαβ exchange energy functional and a theoretical value for the parameter β. Our value for β agrees well with previous empirical estimates, and with empirical calculations in the present work.

1 N expansion for the Kondo lattice

Abstract: A diagrammatic expansion is developed for the partition function of a lattice of spin-$j$ local moments interacting with a band via the Coqblin-Schrieffer exchange interaction. By examining the limit of large spin degeneracy, $N=2j+1$, a $\frac{1}{N}$ expansion is obtained for the partition function, which clearly exhibits how local spin fluctuations are enhanced by large spin degeneracy. The competition between the Ruderman-Kittel-Kasuya-Yosida interaction and local Kondo spin fluctuations is examined using scaling theory, and the critical value of the Kondo coupling constant above which a spin-compensated Kondo-lattice ground state is stable is shown to tend to zero as $O(\frac{1}{N})$, providing new justification for the applicability of the Kondo-lattice model to rare-earth systems. Physical arguments are advanced, based on the nature of the crossover to the strong-coupling regime, which suggest that the low-temperature excitations of the Kondo lattice form a narrow band of heavy fermions.

Antiferromagnetism and correlation of electrons in transition metals

Abstract: The correlated ground state of a model Hamiltonian describing the $d$ band of a transition metal is constructed. It is shown that the intra-atomic exchange interaction $J$ enhances local moments in a paramagnetic phase and has a decisive role in stabilizing different magnetic phases. In particular, the region of stability of an antiferromagnetic ground state becomes broader with increasing $\frac{J}{U}$, where $U$ is the intra-atomic Coulomb interaction, and the magnetic moments also increase. Antiferromagnetic and paramagnetic phases are characterized by almost the same local charge fluctuations, but spin fluctuations are much lower in the antiferromagnetic phase. The energy difference between the paramagnetic and antiferromagnetic state is found to be reduced up to 20---40% of its Hartree-Fock value due to electron correlation. The results obtained for the magnetic phase diagram demonstrate that the physical properties of a fivefold-degenerate $d$ band are essentially different from those which follow from the Hubbard model.

Classical treatment of a Heisenberg linear chain with spin alternation; application to the MnNi(EDTA)•6H2O complex

Abstract: The thermodynamic behaviour of an exchange-coupled linear system with two alternating spin sublattices is investigated from a classical point of view. A closed formula is derived for the magnetic susceptibility which is easily adapted to real spin systems, using appropriate scaling transformations. The proposed approach is shown to describe successfully the behaviour of a new bimetallic chain complex MnNi(EDTA)-6H 2 O. In particular, a minimum in the xT versus T plot is observed at very low temperature, in agreement with theory.

Self-interaction correction for density-functional theory of electronic energy bands of solids

Abstract: Although the self-interaction terms in the Coulomb and exchange potentials exactly cancel each other in the Hartree-Fock one-electron Hamiltonian, the cancellation is incomplete when the exchange interaction is treated by the density-functional approximation. The residual self-interaction pushes the orbital energy levels upward. This effect is especially serious for valence states of insulators with localized charge distribution and causes an underestimation of the energy band gap. We have corrected for this incomplete cancellation of self-interaction in the density-functional formalism of energy-band theory of crystalline solids. The self-interaction correction (SIC) to the total energy of the $N$-electron system is expressed in terms of the Wannier functions, and periodic SIC potentials for the Bloch-state wave functions are derived variationally from the energy functional. The resulting SIC one-electron Hamiltonians are state dependent, but a unified Hamiltonian has been devised so that energies of all levels of the same $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}$ from different bands are obtained by diagonalizing the same matrix. We have applied this SIC method to calculate the energy band structure of the argon and LiCl crystals. Using the Kohn-Sham exchange along with the correlation potential of von Barth and Hedin, we obtain band gaps in excellent agreement with experiment, whereas without SIC the calculated band gaps are more than 35% below the experimental values.

Ordered States in Periodic Anderson Hamiltonian with Orbital Degeneracy and with Large Coulomb Correlation

Abstract: Ordered states of a periodic Anderson Hamiltonian with large Coulomb correlation and with orbital degeneracy of localized levels are theoretically investigated. When the number of f electrons is nearly one per site, and when the exchange interaction is antiferromagnetic as a whole, it is predicted that an antiferromagnetic type ordering occurs in the orbital order of localized levels at a certain low temperature accompanied by the lattice distortion due to the Jahn-Teller effect, and that a ferromagnetic or an antiferromagnetic ordering of spin, if it occurs, occurs at a lower temperature. One of the most characteristic features of this orbital ordered state is a strong coupling between orbital antiferromagnetic moments and spin ferromagnetic ones. It is tried to explain the experimental data of CeB 6 with a model that a quartet is the ground multiplet of localized levels.

Ab initio calculation of the Heisenberg exchange interaction between O2 molecules

Abstract: Calcul de l'interaction d'echange entre molecules O 2 ( 2 Σ g − ) pour le dimere singulet, triplet ou quintuplet

Relativistic multiple scattering theory of electrons by ferromagnets

Abstract: In view of investigating for 3d- and 4f-ferromagnetic crystals the elastic scattering and the emission of polarized low-energy electrons as well as the bulk electronic structure, a relativistic multiple scattering theory has been developed, in which the exchange interaction with the magnetic ground state electrons is treated in a local density functional approximation. In a layer-KKR-type approach, the Dirac equation, which contains an effective magnetic field term, is first solved for a single crystal atom and subsequently for a monoatomic layer and for a semi-infinite crystal. Spin-orbit coupling and magnetic exchange interaction are thus simultaneously taken into account.

Ising-like spin-½ quasi-one-dimensional antiferromagnets: Spin-wave response in CsCo X 3 salts

Abstract: The spin-wave response of the $S=\frac{1}{2}$ Ising-like antiferromagnetic chain compounds CsCo${\mathrm{Br}}_{3}$ and CsCo${\mathrm{Cl}}_{3}$ has been studied by inelastic neutron scattering. The asymmetry at low temperatures of the line shape of the spin-wave continuum is not correctly predicted by the existing theory of isolated chains to first order in the transverse exchange interaction. The effective spin Hamiltonian for a chain has been rederived and shown to include a slow internal staggered field from exchange mixing that influences the rapid spin fluctuations. Three-dimensional correlations are shown to be important above but close to N\'eel temperature. The first-order calculation of Ishimura and Shiba has been extended to include these effects and numerical calculations made for $0lTl\frac{J}{{k}_{B}}$. A good account is obtained of the spin-wave continuum and its temperature dependence. The results show that the highly asymmetric line shape arises from local staggered fields which vary on a much slower time scale than the transverse spin motion, and whose effect decreases with increasing temperature in the manner expected for the decay of the intrachain and interchain correlations. An improved and simplified account of the Zeeman ladder as seen in the Raman scattering at low temperatures has also been obtained.

Spin-Peierls State vs Néel State II. Interchain Exchange Interaction

Abstract: The ground state of the quasi-one-dimensional Heisenberg antiferromagnet coupled with the lattice distortion is determined on the basis of the phase Hamiltonian. By treating the weak interchain exchange interaction in a mean field approximation, it is shown within the self-consistent harmonic approximation that the ground state is either the Neel state or the spin-Peierls state as in the case of the small Ising anisotropy in our former investigation, but the Neel state is much more stabilized. The case of the staggered magnetic field is also examined. The three mechanisms, the Ising anisotropy, the interchain interaction, and the staggered field, are shown to be quite different in their stabilizing the Neel state. Comparison of these results with those by the classical treatment reveals the essential importance of the quantum fluctuations in the present competition problem.

Theory of Electronic Structures and Lattice Distortions in Polyacetylene and Itinerant Peierls Systems. II Coulomb Interaction Dependences of the HF Ground State, Lattice Dimerization and 1Bu Excited State in Regular Trans-Polyacetylene

Abstract: We study the Coulomb interaction dependences of the HF ground state, lattice dimerization and I Bu excited state in regular trans-poly acetylene using the PPP hamiltonian model developed in a previous paper. The nature of the HF ground state is determined by the relative magnitudes of the on site and nearest neighbour parts and the screening of the longer range part of the effective Coulomb potential. The Coulomb interaction with relatively large nearest neighbour part and not too strongly screened longer range part induces lattice dimerization. The driving force of the Coulomb induced dimerization is the nearest neighbour exchange interaction. The lattice dimerization in polyacetylene is due to the Coulomb mechanism. The SSH model is implicitly taking into account the nearest neighbour exchange interac­ tion. The Franck-Condon band gap is mostly determined by the nearest neighbour exchange interaction. The IBu excited state is excitonic. The exciton binding energy is estimated for the effective Coulomb potential with the screened long range part. It is not large owing to very small effective mass of electron and hole. To explain the observed I Bu excitation energy, the short range part of the effective Coulomb potential must be screened by about half compared to the one in small conjugate molecules. The condition for splitting of the exciton into a zwitter­ ionic pair of charged solitons is discussed.

Virtual bound state model for the exchange interaction in semimagnetic semiconductors such as Cd1−xMnxTe

Abstract: The difference in both sign (i. e. ferromagnetic interaction in the conduction band and antiferromagnetic one in the valence band) and magnitude of the exchange interaction constant in semiconductors such as CdMnTe is explained by a virtual bound state model involving the resonance of the Mn d-level in the valence band.

The ESR linewidth of dilute solid solutions

Abstract: A theoretical study of the broadening of magnetic resonance lines due to the dipole interaction in dilute solid solutions of paramagnetic impurities in diamagnetic hosts is presented. The theory takes into account the exchange interaction between impurity ions. Detailed results are presented for the three cubic lattices. The theoretical results are compared with experimental data on Cr3+:MgO powders.

Spin-polarised relativistic exchange energies and potentials

Abstract: Expressions for the exchange energy of a relativistic polarised electron gas have been derived. These expressions have been evaluated numerically over a wide range of densities and polarisations. The leading-order relativistic corrections for the transverse exchange energy, which are of most importance in practice, have been evaluated analytically. The numerical results have been fitted to a simple form, suggested by the analytic leading-order-correction expressions, which should be useful for local-density-approximation applications.

Transition from helimagnetism to ferromagnetism in CuxZn1-xCr2Se4

Abstract: We present detailed results concerning temperature and field dependences of magnetization, paramagnetic susceptibility, magnetic anisotropy and neutron diffraction for CuxZn1-xCr2Se4 spinels. Particular emphasis is put on the gradual transition from helimagnetism to ferromagnetism as a function of concentration x. The transition takes place through an intermediate structure for 0.05 ≤ x ≤ 0.8 with the ferromagnetic component growing with concentration. It is suggested that a double exchange mechanism predominates as x increases and this can be regarded as the origin of the transition.

Magnetisation density distribution in Mn1/4TaS2: observation of conduction electron spin polarisation

Abstract: The authors have determined the magnetisation density distribution in the ferromagnetic layered compound Mn1/4TaS2, using the classical polarised neutron technique. These studies show that although the magnetisation is principally localised on the manganese sites, there is a significant spin polarisation of the conduction electrons, extending throughout the unit cell. These measurements thus provide direct evidence of an important interaction between the Ta conduction electrons and the localised moments on the Mn ions, confirming previous speculations based on transport and optical data. Moreover such an interaction suggests an obvious mechanism, the RKKY interaction, for the exchange interaction between the Mn moments. The moment on the Mn sites is depressed, as a result of this interaction, by about 15% compared with that expected for a Mn2+ d5 ion. The Mn ions are surrounded by an antiferromagnetically polarised cloud of electrons and the maximum spin density within the TaS2 layer is about 3% of the maximum spin density on the Mn site. There is no evidence from these experiments that the S atoms play a role in the magnetic Mn-Mn exchange interactions.

Magnetic field induced nonmetal - metal transition in the open - gap Hg1−xMnxTe

Abstract: Transverse and longitudinal magnetoresistance of p-type Hg 1−x Mn x Te crystals (x=0.081,0.098,0.11) were investigated in the temperature range 1.4K–300K and in magnetic fields up to 7T. In the extrinsic and impurity conduction regime a strong negative magnetoresistance was observed. An experimental evidence for the magnetic field induced nonmetal-metal transition was obtained. The effects are discussed in terms of the theory of an acceptor state modified by the exchange interaction between holes and Mn ++ ions [1]. A role of the bound magnetic polaron localized on acceptor centre is also considered [2].

Semi-Classical Limit Theorems for Hartree-Fock Theory

Abstract: Consider a large number of electrons with Coulomb repulsion moving under the influence of static nuclei. It is assumed the potentials due to the nuclei are Coulombic away from their centers but are smooth at the centers, so no singularities exist. The author shows that the exchange energy for the Hartree-Fock ground state of this system converges in a suitable limit to the formula obtained by Dirac for exchange energy as an integral of the one body density.

Influence of exchange interaction on bandstructure of Pb1−xMnxTe

Abstract: Magneto-optical and magneto-transport experiments have been performed on Pb1−xMnxTe crystals with 0

Cooperative phenomena in ternary superconductors

Abstract: A microscopic theory of ferromagnetic superconductors is developed from first principles. Self-consistent equations for the superconducting order parameter Δ and spontaneous magnetizations 〈S z 〉 are derived using a Green's function technique and considering thef-d exchange effect up to the second order. The theory is applied to explain the experimental results in the reentrant superconducting ternary system ErRh4B4. The present model explains reentrant behavior, predicts the coexistence of superconductivity and ferromagnetism in a very small range of temperature, the suppression of superconductivity by ferromagnetism, and vice versa. These results are in excellent agreement with the experimental data and predictions of other models. The behavior of the spontaneous magnetization, the superconducting order parameter, the specific heat, and the density of states is also studied.