K. L. Yao

Charge density wave transition and instability in interchain coupled organic ferromagnets with next-nearest-neighbor hopping interaction

Considering interchain interaction and the intrachain next-nearest-neighbor hopping interaction, within the self-consistent-field Hartree–Fock approximation, we deal with two neighboring organic ferromagnetic chains. We take into account the π-electrons, the Hubbard repulsion, and antiferromagnetic correlation between π-electrons and side radical electrons. It is shown that there appear to be two kinds of charge density waves successively when the next-nearest-neighbor hopping interaction is greater than a critical value, which decreases with the interchain coupling. The first charge density wave is accompanied by a bond order wave and the second one has no lattice distortion. The spin density wave along the main chain is modulated by the charge density wave. It is also found that the interchain coupling and the next-nearest-neighbor hopping integral destabilizes the ferromagnetic ground states.

Spin-polarized transport and the electronic structure of the metallic antiferromagnet Fe(thiazole)2Cl2

We theoretically investigate the electron density of states, band structure, spin polarization, spin filter effect, and current-voltage curve of Fe(thiazole)(2)Cl(2) nanocrystal. The results show that Fe(thiazole)(2)Cl(2) is a metallic antiferromagnet with half-metallic ferromagnetic metastable state. The spin current for the ferromagnetic metastable state shows that Fe(thiazole)(2)Cl(2) has spin-filter property. Moreover, the current-voltage curve exhibits negative differential resistance, which is due to the narrow feature in the density of states of Fe(thiazole)(2)Cl(2). The perfect negative magnetoresistance is also obtained.

Two ferromagnetic azido-bridged copper(II) complexes studied by first-principle electronic-structure calculation

The electronic structures of two ferromagnetic polynuclear copper(II) complexes, derived from end-to-end azido ligand and tridentate (NNN donor) Schiff base ligand, have been studied using the full-potential linearized augmented plane-wave method based on the density-functional theory. They are [Cu(L1)(micro-1,3-N3)]n(ClO4)n (1) and [Cu(L2)(micro-1,3-N3)]n(ClO4)n (2). The result shows that the spin populations in these two complexes are mainly distributed on the equatorial planes of a square pyramidal that surround the copper(II) ions. There are large and positive spin populations on copper(II) ions, small and positive spin populations on the three nitrogen atoms of tridentate Schiff base ligand, and the two terminal nitrogen atoms of asymmetrical end-to-end azido ligand, while weak and negative spin populations on the central nitrogen atoms of asymmetrical end-to-end azido ligand. Ferromagnetic coupling through the asymmetrical azido ligand in these two complexes has been mainly attributed to the spin delocalization, also with weak spin-polarization effect.

The polaron and bipolaron states of poly(phenylene vinylene)

We utilize the Pariser–Parr–Pople model for poly (phenylene vinylene) (PPV) and take into account the realistic lattice structures of PPV considering the electron–electron and the electron–phonon interactions so that we can expose the subtle electronic and the lattice structures of PPV. We find that the lattice deformation is mainly in the vinylene group, and we also discuss the spin density waves of polaron and the charge density waves of polaron and bipolaron. The spin density wave is modulated by charge density wave. The competition between polaron and bipolaron are studied. We find that when the electron–phonon interaction λ 0.21, v favors the bipolaron.

Boundary conditions, solitons, and spin configuration in interchain coupled organic ferromagnetic polymers

With an extended Su–Schrieffer–Heeger Hamiltonian including Hubbard energy and intersite Coulomb repulsion, we study the effects of two kinds of boundary conditions on the spin configuration and lattice distortion in organic ferromagnetic polymers with interchain coupling. It is shown that if the length L of the lattice is even, there is dimerization along the main chain and uniform spin density along side radicals. If L is odd, there exist domain wall solitons describing the displacement of the lattice along the main chain, and spin soliton with envelope shape describing the localization of spin at side radicals. Under the condition of strong interchain coupling, a great fluctuation of spin and distortion appear along the main chain and side radicals, and the system transfers to two-dimensional one. The e–e correlation reduces the amplitude of spin soliton and domain wall while electron–phonon coupling has the reverse effect.

The pair of soliton-like distortions in organic ferromagnetic conjugated polymers

We study a soliton-like distortion in a quasi-one-dimensional conjugated carbon chain with a single side radical, which contains an unpaired electron, by taking into account the electron–phonon interaction, an extended Hubbard term due to on-site electron–electron interactions and intersite e–e interactions, and the ferromagnetic correlation between itinerant π-electrons and the unpaired radical electron. It is shown that a critical value of hopping integral exists between the side radical and the main chain, at which a transition from a single soliton-like distorted structure to a pair of soliton-like distortions occurs along the main chain. In addition, the spin density at the dangling site is transferred to the main chain due to the hopping interaction between the main chain and the side radical, and the spin density at the side radical is decreased to zero when the hopping integral is larger than the critical value.

Strain controlled switching effects in phosphorene and GeS

By performing first principles calculations within the combined approach of density functional theory and nonequilibrium Greens function technique, we have designed some nanoelectronic devices to explore the ferroelastic switching of phosphorene and phosphorene analogs GeS. With the structure swapping along the zigzag direction and armchair direction, band gap transformed at different states due to their anisotropic phosphorene-like structure. From the initial state to the middle state, the band gap becomes progressively smaller, after that, it becomes wide. By analyzing transmission coefficients, it is found that the transport properties of phosphorene and GeS can be controlled by a uniaxial strain. The results also manifest that GeS has great potential to fabricate ferroic nonvolatile memory devices, because its relatively high on/off transmission coefficient ratio (∼1000) between the two stable ferroelastic states.

CDW Transition and Instability in Interchain Coupled Organic Ferromagnets with Next‐Nearest Neighboring Hopping Interaction

Considering interchain interaction and the intrachain next-nearest neighboring hopping interaction, within the self-consistent-field Hartree-Fock approximation, we deal with two neighboring organic ferromagnetic chains. We also take into account the itineracy of π-electrons, the Hubbard repulsion and antiferromagnetic correlation between π-electrons and side radical electrons. It is shown that there appear two kinds of CDW successively when the next-nearest neighboring hopping interaction is greater than a critical value, which decreases with the interchain coupling. The first CDW is accompanied with bond-order-wave and the second one with no distortion of lattice. The SDW along the main chain is modulated by the CDW. It is also found that the interchain coupling and the next-nearest neighbor hopping integral instabilizes the ferromagnetic ground states.

The vibrational modes around the soliton lattice with next neighbor hopping interactions in highly doped trans-(CH)x

Starting from the extended SSH model that includes an external Coulomb potential arising from interchain charged solitons and counterions, the intrachain e-e interaction and the next neighbor hopping interactions, eight localized vibrational modes around the soliton lattice have been found for the doping levels from 3.33 at.% to 13.33 at.%. Among them three are infrared active and they can be used to interpret the three observed infrared absorption lines at 487, 1284 and 1362 cm−1. Furthermore, the frequencies of localized modes are decreased and their localizations are weakened when the dopant concentrations increase.

A Peierls-extended Hubbard model study on quasi-one-dimensional alternant pi -conjugated high-spin hydrocarbon

A Peierls-extended Hubbard model is used to study the possible high-spin ground state of a quasi-one-dimensional alternant pi -conjugated hydrocarbon with unequal numbers of atoms on two sublattices, which has been predicted to be a good candidate for organic ferromagnets. Considering the strong electron-phonon (e-ph) coupling and electron-electron (e-e) interaction, allowing for full lattice relaxation, the system is optimized by a set of self-consistent iterative equations. The results of the calculation indicate that the system can show a high-spin ground state owing to the topological structure and the on-site e-e interaction. The intersite e-e interaction will cause a charge density distribution and consequently weaken the stability of the high-spin ground state. It is also found that the dimerization amplitude of the system will always decrease with increasing intersite e-e interaction for different e-ph coupling.