Hund Exchange Research Articles

Magnetic exchange interaction between π-electron radicals adsorbed on α-boron-nitride surfaces

The magnetic interactions between organic π-radicals adsorbed on 1D and 2D α-boron-nitride (α-BN) crystals are investigated theoretically. As in the case of analogous systems with graphite surfaces [N. Tyutyulkov, F. Dietz, K. Müllen, Chem. Phys. 255 (2000) 223], an intramolecular exchange interaction between the π-electrons within a narrow half-filled band occurs, with the main component from the Coulomb (Hund) exchange. Compared to graphite the values of the effective Heisenberg exchange integral are smaller in the case of α-BN. The energy spectrum, and the magnetic and thermodynamic characteristics of Ising models consisting of methyl radicals adsorbed on 1D and 2D α-BN surfaces are discussed based on the analytically solved equations.

Magnetic exchange interaction between π-electron radicals adsorbed on graphites

The nature and the magnitude of the magnetic exchange interaction between organic π radicals adsorbed on quasi-1-D graphites and 2-D graphite clusters are investigated theoretically. The interaction between the π centers of the radicals and the graphite surface determines the common π system of conjugation, and the intermolecular exchange coupling of electrons within a narrow half-filled band of degenerate or quasi-degenerate MOs. The main component of the ferromagnetic exchange interaction is the Coulomb (Hund) exchange and a smaller contribution comes from the indirect exchange. The thermodynamic treatment of a 2-D model system of radicals adsorbed on graphite within the Ising model shows macroscopic spin S and spontaneous magnetization at T≠0 K.

Determination of electronic band structures ofCaMnO3andLaMnO3using optical-conductivity analyses

Reflectivity spectra of ${\mathrm{CaMnO}}_{3}$ and ${\mathrm{LaMnO}}_{3}$ were measured in a wide photon energy region between 5 meV and 30 eV at room temperature. Using the conductivity spectra obtained from the Kramers-Kronig analysis, electronic structures of the manganese oxides were investigated. In particular, the states near the Fermi energy ${\mathrm{E}}_{\mathrm{F}}$, O 2p and Mn 3d (${\mathrm{e}}_{\mathrm{g}}$,${\mathrm{t}}_{2\mathrm{g}}$), were studied in detail. It was found that the O 2p band is located closer to ${\mathrm{E}}_{\mathrm{F}}$ than the Mn ${\mathrm{t}}_{2\mathrm{g}}$ band. For ${\mathrm{LaMnO}}_{3}$, the filled Mn ${\mathrm{e}}_{\mathrm{g}\mathrm{\ensuremath{\uparrow}}}^{1}$ band is located closer to ${\mathrm{E}}_{\mathrm{F}}$ than any other valence band. The ${\mathrm{t}}_{2\mathrm{g}}$-${\mathrm{e}}_{\mathrm{g}}$ Hund exchange energy was found to be about 3.4 eV and the Jahn-Teller stabilization energy ${\mathrm{E}}_{0}$ was estimated to be less than 0.5 eV.

Superconductivity in a spin liquid: A one-dimensional example.

We study a one-dimensional model of interacting conduction electrons with a two-fold degenerate band away from half filling. The interaction includes an on-site Coulomb repulsion and Hund's rule coupling. We show that such one-dimensional system has a divergent Cooper pair susceptibility at T = 0, provided the Coulomb interaction $U$ between electrons on the same orbital and the modulus of the Hund's exchange integral $|J|$ are larger than the interorbital Coulomb interaction. It is remarkable that the superconductivity can be achieved for {\it any} sign of $J$. The opening of spectral gaps makes this state stable with respect to direct electron hopping between the orbitals. The scaling dimension of the superconducting order parameter is found to be between 1/4 (small $U$) and 1/2 (large $U$).

A class of narrow-band high-spin organic polymers II. Polymers with indirect exchange interaction

It is shown that indirect exchange magnetic interaction may be crucial for the ground state spin multiplicity of a class of organic polymers. Such polymer chains possess a system of extended π-conjugation and each elementary unit contains at least one radical substituent R . with a localized partially occupied molecular orbital (LPOMO): ▪ The occurrence of LPOMOs is caused either by (i) steric hindrance which inhibits the π-π interaction of LPOMOs and the π-system along the polymer chain by making the two planes of π-conjugation almost orthogonal or (ii) specific topology of the π-network of the radical substituent R . preserving its π-POMO even when linked to the polymer chain. The almost perfect localization of POMOs results in vanishing direct (Hund) exchange interaction between unpaired electrons. These uncompensated spins still induce spin polarization of the overall system of π-conjugation which may break the spin degeneracy. An approach similar to the s-d(s-f) model of indirect exchange is applied. The ground state spin multiplicity depends on the exchange constants of the corresponding effective spin Hamiltonian. The latter constants are calculated for several model polymers in order to establish the type (ferro- or antiferromagnetic) and magnitude of the indirect exchange interaction.