Search

Using an accurate density-function method, we explore the coexistence of the magnetism and conductivity in bis(ethylenediselena)-tetrathiafulvalene (BEST) with the paramagnetic hexacyanoferrate(III) [Fe(CN)6]3−or the photochromic nitroprusside anion [Fe(CN)5NO]2−. The total and partial densities of states, and the atomic spin magnetic moments are calculated and discussed. It is found that the up- and down-spin total densities of states (DOS) are continuous in the vicinity of the Fermi level, there is overlap between the HOMO and LUMO in the up-spin subbands and the down-spin subbands, which reveals that these types of compounds have conductive properties. From the total and partial densities of states and atomic spin magnetic moments, it is shown that the spin magnetic moments of (BEST)4[Fe(CN)6] is mainly assembled at the iron atom and the cyanogen radical, and the spontaneous magnetic moments for (BEST)2[Fe(CN)5NO] come from iron atom, cyanogen and nitric oxide radical. To our best knowledge, it is the first theoretical study on the coexistence of the magnetism and conductivity of these compounds.

By using the replica method and the functional integral technique, the quantum XY spin glass model with ferromagnetic coupling is investigated numerically. From the numerical results of the thermodynamic and magnetic quantities we found that these quantities do not vary with the mean exchange interaction when the mean exchange interaction is lower than 1. This feature demonstrates that there are the mean interaction translational symmetries in the spin-glass and paramagnetic phases.

Using an exact diagonalization method, we study an extended Hubbard model with an electron-lattice interaction for an organic ferromagnetic chain with radical coupling. The result shows that the ferromagnetic ground state originates from the antiferromagnetic correlation between adjoining sites, which is enhanced by the on-site e-e repulsion. The intersite e-e repulsion induces the inhomogeneous distribution of the charge density. The dimerization is decreased by the e-e interaction and the radical coupling. The electron-lattice interaction and the radical coupling can transfer the spin density and charge density between the main chain and the radicals.

The effects of polarons along main chain and spin-soliton-like along side-radical lattice in doped quasi-one-dimensional π-conjugated organic ferromagnets are studied by selfconsistent method. It is shown that the gap of spin-down and spin-up mid energy bands of the system decreases because new energy levels of localization states appear. The stability of organic ferromagnetic system is weakened with decreasing of the gap. In addition, it is also shown that there is charge transfer between main chain and side-radical lattice of the doped system.

The quantum infinite range XY spin glass model with the infinite range planar Dzyaloshinskii-Moriya interaction in external field for spin S=1/2 is investigated theoretically. Numerical calculations show that, the specific heat depending on the temperature has crossover behavior; the weak field dependence of the specific heat can be fitted by the parabolic approximation, C/T = A + Bh2, the coefficient B is peaked near the transition point.

By using replica method and Matsubara imaginary-time functional-integral technique, the quantum infinite range Heisenberg model is solved. The solution shows that the quantum infinite range Heisenberg model also has phase transition.

Based on a theoretical model proposed for quasi-one-dimensional organic polymer ferromagnets, the next-nearest neighbor hopping interactions of π-electrons are considered. Allowing for full lattice relaxation, a set of self-consistent equations is established to study the system. The spin-density-wave (SDW) and the possible ferromagnetic ground state of the system are investigated in detail. It is found that the next-nearest neighbor hopping interactions will make the SDW stronger and consequently make the ferromagnetic state more stable as compared with the nonmagnetic reference state.

A new kind of organic polymer ferromagnet based on polaron is proposed. In this kind of polymer, there are no organic free-radicals, which are rather reactive and unstable, instead spins are provided by polarons. Allowing for full lattice relaxation, the system is studied self-consistently by Peierls-Hubbard model. The formation energies of polarons are calculated. The possible ferromagnetism in polymers of this type is demonstrated.

A new quasi-one-dimensional organic ferromagnet model is proposed. Magnon excitations for finite and infinite on-site repulsion of electrons on main carbon chain are investigated. It is found that the lowest branch of spectrum preserves the characteristics of the ferromagnetic magnon. It is also shown that with increasing of the on-site repulsion energy, the itinerant π-electrons become tightly localized.

Based on a theoretical model proposed for quasi-one-dimensional organic polymer ferromagnet poly-BIPO, the effects of electron-electron interaction and electron-phonon coupling on the spin-density-wave (SDW) state of poly-BIPO system are studied. It is found that the SDW state is the stable ground state of the system, in which the system shows ferromagnetic order. And the larger electron-electron interaction and the smaller electron-phonon coupling will make the SDW state more stable than the nonmagnetic state.

Based on the replica method and the imaginary time functional-integral technique, we investigate the infinite-range quantum Dzyaloshinskii–Moriya spin glass model. It is found that the quantum Dzyaloshinskii–Moriya spin glass model behaves in a Heisenberg-like manner. The specific heat has the crossover behaviour. The broad Maximum in specific heat is shifted to higher temperature with increasing applied field. These features are in good agreement with the observation of Brodale et al. [J. Magn. Magn. Matter. 31-34 (1983) 1331] The susceptibility of the system has the typical spin glass feature.

The electron-lattice coupling vibrational modes are calculated using a two-dimensional extension of the Su-Schrieffer-Heeger model that includes the bond-bending term, the electron-electron interactions and the electrostatic pinning for doped samples. The result shows that all ten localized modes exist after turning on the e-e interactions and doped pinning effects, but their localizations has been strengthened and the frequencies have been moved slightly.

By using the generalized static replica symmetry approximation, the quantum XY spin-glass model in a longitudinal field is investigated. The thermodynamic quantities, spin-glass and self-interaction order parameters are calculated numerically. The local susceptibility depending on temperature has the typical spin-glass plateau characteristic; the specific heat against temperature display a crossover feature which was discovered experimentally in the CuMn sample by Brodale et al. (J. Magn. Magn. Mater. 31-34 (1983) 1331) and cusps sharper than that in the model with the Dzyaloshinskii-Moriya interaction.

Based on our interchain coupled theoretical model for quasi-one-dimensional organic ferromagnet, we took into account the nearest and the next nearest hopping interactions between neighboring chains, to study the charge density wave and the spin density wave along the main chain. The calculated results showed that the interchain coupling results in the transfers of charge density and spin density between the main chain and the side radicals of the two coupled chains.

The ferromagnetic state in quasi-one-dimensional organic ferromagnet poly-SIPO has been studied. We consider the electron hoping, electron-electron correlation and Pierls instability in one-dimensional system. And self-consistent calculations are performed to investigate the structure of energy band and the distribution of spin density. The effects of the above aspects on the ferromagnetic state are studied. The results show that the ground state is a stable ferromagnetic state in this system.

The static Mahan-H?nsch quantum transport equation in applied magnetic fields is extended to spin glass systems through introducing physical self-energies of elastic and inelastic scattering. In the quasi-elastic scattering limit, the Hall conductivity for spin glass model with the s-d exchange interaction between electrons and localized moments is evaluated by using this equation. The relation between resistivity and the Edwards-Anderson (EA) spin glass parameter and local susceptibility is obtained. Finally, thermoelectric power of spin glass::s and some interesting characteristic temperatures are discussed.

The overlap of magnetization of the ground state in a RKKY spin glass system with the Dzyaloshinsky-Moriya interaction is studied based on the theory of infinite number of order parameter. It is shown that the nonmagnetic impurities are coupled with the host lattice. The results are in good agreement with experiments.

The numerical simulation of the irreversiblity of magnetization and the anisotropy field is studied in a spin glass with RKKY and DM interactions. It is found that the anisotropy field increases linearly with the concentration of nonmagnetic impurities, but the remanent magnetization and the saturation remain unchanged with different concentrations. A comparison is made with experiment.