Kinetic Exchange Interaction Research Articles

Organic polymers with indirect magnetic interaction caused by the symmetry of the elementary units

The magnetic properties of a class of 1-D π-systems having a frontier half-filled band of degenerate MOs localized within the elementary units (EU) were studied theoretically. The localization of the MOs is caused by the symmetry of the π-fragments which form the EUs. The localization and non-overlapping of the MOs results in vanishing of the direct (Hund) and the kinetic exchange interaction between the unpaired electrons. The indirect exchange of the unpaired electrons (spin-polarization exchange) occurs via the delocalized π-electrons in the filled energy bands. An approach allowing the calculation of the indirect exchange interaction is applied for a class of polymers having pyrene, azulene, and s-indacene as fragments of the EUs. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66: 425–434, 1998

Pressure Effects on Intermolecular Interactions in Genuine Organic Radical Crystals

Pressure induced enhancement of magnetic ordering temperature has been found in a quasi-one-dimensional ferromagnetic organic radical crystal, 3-(4-chlorophenyl)-l,5-dimethyl-6-thioxovcrdazyl by the measurements of magnetic susceptibility and heat capacity under hydrostatic pressures up to 13 kbar (1.3 GPa). This pressure dependence is the opposite direction to the results in the ferromagnetic β-phase p-NPNN observed recently, and in the same way as in ordinary antiferromagnets. The effects of pressurization suggest to bring about frustration in kinetic exchange interactions via various charge transfer paths between molecular orbitals.

Nature of the magnetic interaction of Wurster's radicals in the solid state

The nature of the spin exchange interaction within the half-filled band in one-dimensional stacks of Wurster's radicals in Wurster's red bromide has been investigated theoretically. The various contributions to the Heisenberg effective exchange integral: direct, kinetic, and indirect spin exchange are evaluated quantitatively. It has been shown that the direct (Coulomb) exchange which causes the ferromagnetic coupling is small. The negative value of the effective exchange integral between Wannier functions, localized within the elementary units, has been determined by the kinetic and indirect (antiferromagnetic) exchange interaction.

Kinetic exchange and strong ferromagnetic interactions

By using classical ideas to exchange theory we have found that for some specific physical situations kinetic exchange, the second-order effect of virtual electron transfer between magnetic ions, can give rise to strong ferromagnetic interactions. This is in contradiction to earlier belief.

Theoretical Approaches to Molecular Magnetism II: No-Overlap and Orientation Principles for Ferromagnetic Interactions

Abstract No-overlap and orientation principles for ferromagnetic interactions between organic radicals were derived on the basis of approximately spin-projected UMP4(2) and UCC SD(T)/4-31G computations for several typical radical pairs. Ferromagnetic intermolecular interactions are feasible at T-shape and rhombus conformations in parallel interplane orientations for radical pairs because of no-overlap between π*-type SOMOs (no kinetic exchange) and nonzero-potential exchange via the SOMO-SOMO through-space contact, whereas antiferromagnetic kinetic exchange interactions are predominant at many other conformations.

No-overlap and orientation principle for ferromagnetic interactions between nitroxide groups

Approximately spin-projected UMP4(2) and UCC SD(T) computations show that ferromagnetic intermolecular interactions are feasible for T-shape and rhombus conformations in parallel interplane orientations for nitroxide pairs because of no-overlap between π ∗ -type SOMOs (no kinetic exchange) and non-zero potential exchange via SOMO-SOMO through-space contact. On the other hand, antiferromagnetic kinetic exchange interactions via SOMO-SOMO overlap are predominant for rectangular conformations. The no-overlap and orientation principle proposed previously is a useful guide for the theoretical understanding of ferromagnetic exchange interactions between nitroxide groups. Implications of the calculated results are discussed in relation to the discovery of ferromagnetic interactions in the crystals of adamantane dinitroxide, TEMPO derivatives and MOTMP.

Interatomic potentials of singlet s-Rydberg series of a HgNe van der Waals dimer: Evidence for stabilization by superexchange interaction

The interatomic potentials of the singlet Rydberg series of the HgNe dimer 1Σ+ Hg(n1S0)Ne (n=7–9) were determined over a wide range of interatomic distance by the analysis of the optical–optical double resonance (OODR) spectra measured in the present study via the A 3Π0+ and B 3Π1 states. The interatomic potential for n=7 consists of one bound vibrational level (v=0), three quasibound levels (v=1–3) trapped inside a potential barrier, and one weakly trapped quasibound level just above the potential barrier. The dissociation energy (De) of this potential is 50(3) cm−1 and the large potential barrier with a height of 133(2) cm−1 is located at 4.20(3) Å, while the interatomic potentials for n=8 and n=9 consist of a deep bound well, whose dissociation energies (De) are 240(3) and 297(3) cm−1, respectively. From these potentials, it was shown that the principal quantum number dependence (n) of the interatomic potential originates mainly from that of the exchange repulsion between the Rydberg electron of Hg and the attached Ne atom in a similar manner as the triplet Rydberg series of HgNe, 3Σ+ Hg(n3S1)Ne (n=7–10) [K. Onda et al. J. Chem. Phys. 101, 7290 (1994)]. By comparing the potentials of the singlet with the triplet Rydberg series, it was found that an interatomic potential of the singlet state is always deeper than that of the triplet state for the same n. This difference between singlet and triplet was interpreted by a superexchange interaction model [P. N. Anderson, Phys. Rev. 79, 350 (1950)], in which a singlet Rydberg state becomes more stable due to small spin density on the Ne atom induced by the kinetic exchange interaction between the Hg+ ion and the Ne atom.

Structure, energy spectra and magnetic properties of poly(m‐aniline)s

AbstractThe energy spectra and magnetic properties of a large class of one‐dimensional poly(m‐aniline)s (PMA) and further model polymers are investigated theoretically. The band structure of those PMA's which are aza‐analogs of the alternant non‐classical (non‐Kekulé) hydrocarbons (polymers) is characterized by a wide gap in which there is a narrow half‐filled band (HFB). The different contributions to the effective spin exchange between the unpaired electrons in the HFB: Coulomb (Hund), kinetic and indirect exchange interactions are calculated. While it has been shown earlier that PMA exhibits a net spin exchange of ferromagnetic nature, this approach enables a detailed understanding of the influence of substituents on the nitrogen centers and changes in the aromatic bridging unit. The ferromagnetic nature of the spin coupling in the singly bridged PMA models therebye prevails independent of the structural changes. The HFB width of those PMA's which are derivatives (aza‐analogs) of ladder‐type non‐alternant hydrocarbons is very large and the Wannier functions are delocalized. For this group of polymers the delocalized, non‐magnetic state is favored and, therefore, they may be good candidates for testing electrical conduction.

Exchange Interaction as a Source of Real Space Pairing in High Temperature Superconductors and Heavy Fermions: a Brief Overview

We characterize the principal difference between the pairing induced by the kinetic exchange interactions for the Fermi-liquid and spin-liquid phases. In the case of hybridized electrons the Kondo-exchange induced pairing is invoked, whereas for a single narrow-band case kinetic exchange is responsible for the spin singlet pairing. The former interaction is applied to heavy-fermion systems, whereas the latter is employed to the high temperature superconductors. The current situation concerning the applicability of our theoretical models is briefly and critically assessed. PACS numbers: 71.28.+d, 75.10.Lp, 74.70.Vy

Cooperation of antiferromagnetic interactions and phonons in La 2−x/Ba,Sr/ xCuO 4−δ

This paper discuss a pairing mechanism relevant to LaCuO based high T c superconductors. The underlying theoretical model is nearly-half-filled two-dimensional Hubbard hamiltonian with some electron-phonon interactions. It is shown that the hopping energy is much reduced by the coupling between electrons and localized phonons but the kinetic exchange interaction is not reduced. The cooperation of antiferromagnetic interactions and polaron effects makes the singlet hole pair a stable bound state.

Diagonal and off-diagonal correlation effects in anderson-localized states

Abstract The present paper discusses a general theoretical model which treats the interplay of randomness and electron correlation in Anderson-localized states. In constructing the model Hamiltonian, the behaviour of localized states with an envelope function is explicitly taken into account. The calculated results of spin susceptibility and electronic specific heat as well as density of states are presented. It is shown that the diagonal correlation gives rise to the occurrence of singly occupied states below the Fermi energy, E F, and that this results in a Curie-type susceptibility at low temperatures. As a consequence of the variation of the correlation energy from state to state, the density of doubly occupied states becomes large near E F. It is also shown that the off-diagonal correlation effect gives rise to a kinetic exchange interaction between singly occupied states, which favours spin-singlet states.

Kinetic exchange interaction in a doubly degenerate narrow band and its application to Fe1-xCoxS2and Co1-xNixS2

The authors have derived a rotationally invariant kinetic exchange interaction between strongly correlated electrons in a doubly degenerate narrow band with arbitrary band filling. The results are expressed in terms of the global operators characterising the spin and the orbital dynamics. The effective Hamiltonian provides a basis for studying in detail the various magnetic phases of strongly correlated itinerant electrons. The authors results are then applied to explain the experimental data of transition-metal disulphides.

Intra- and interstate interactions in Anderson-localised states

In order to investigate the interplay of randomness and electron correlation in the Anderson-localised states, a general theoretical model which treats the electron-electron interaction between these states is presented. In constructing the model Hamiltonian, the behaviour of localised states with envelope functions is explicitly taken into account. The spin susceptibility and electronic specific heat as well as the density of states are calculated for this model. It is shown that the intrastate correlation gives rise to the occurrence of singly occupied states below the Fermi energy EF and that this results in a Curie-type susceptibility at low temperatures. As a consequence of the variation in the correlation energy from state to state, the density of doubly occupied states becomes large near EF. It is also shown that the interactions between different localised states give rise to a kinetic exchange interaction between singly occupied states.

Kinetic exchange interaction for magnetic impurities in metals: Nondegenerate Wolff model

A modified Schrieffer-Wolff transformation is applied to the Wolff model to obtain the condition for the formation of localized moments. The localized moment plus its polarization cloud contribute a Curie-Weiss high-temperature susceptibility, in agreement with experiments.

The Effective Magnetic Interactions between Impurity and Conduction Electrons for the Wolff Model

AbstractA new canonical transformation of a Schrieffer‐Wolff (S‐W) type is applied to the Wolff model at the strong correlation limit to derive an antiferromagnetic (kinetic) exchange interaction between the electrons on the impurity site and the host sites. The problem of the singularity in the original S‐W transformation is removed. The condition for the formation of localized moments is obtained by solving the Green's function with a moment conserving decoupling scheme. If localized moments exist, the high temperature susceptibility, derived in the mean field theory, has a Curie‐Weiss form χ = C/(T + θ) with θ > 0, in agreement with experiments.

The kinetic exchange interaction in doubly degenerate narrow bands

AbstractA canonical transformation is applied to the doubly‐degenerate‐band Hubbard model with arbitrary n in the strong correlation regime. Projection operators are constructed to project out the splitted subbands. Only the inter‐subband electron hopping is treated as a perturbation which couples different subbands at the second order. In localized picture the coupling is of the Heisenberg type, corresponding to the virtual hopping of electrons and generating the spin waves, the orbital waves, and the coupled spin‐orbital waves. For n = 1 the ground state has ferromagnetic spin and antiferromagnetic orbital ordering. For n = 2 the spins in the ground state have ferromagnetic intra‐site but antiferromagnetic inter‐site ordering. These orderings are due to Hund's rule, the effect of which gets weakened by the intra‐subband electron hopping as n deviates from an integer. This canonical‐transformed effective Hamiltonian is suitable for investigating the phase diagram of the Hubbard model with orbital degeneracy.

Kinetic exchange interaction in a narrow S-band

A new canonical transformation is applied to the Hubbard Hamiltonian under strong correlation with arbitrary n to obtain an effective Hamiltonian which consists of both the itinerant property of electrons in split sub-bands and the Heisenberg-type spin-spin coupling. It reduces to that for a Heisenberg antiferromagnet as t/U to 0, but the intrasub-band electron hopping becomes important as t/U increases. This effective Hamiltonian is suitable for investigating the antiferro-paramagnetic transition.