Wu Chang-Qin

Spin Filter Effect in Organic Polymers in the Presence of Local Magnetic Field

Using a nonadiabatic evolution method, we investigate the spin filter effect in organic polymers in the presence of a local magnetic field. Through a spin-dependent magnetic field, polarons (charge carrier) with different spins will feel repulsive or attractive force determined by their spins. Our simulations show that in a single-site magnetic field (affecting electrons at a single site), for example V150 = 0.35 eV, or V150 = 0.45 eV, a spin-up polaron accelerated to saturated velocity by an electric field can pass through the field while the spin-down polaron is trapped. When the local field extends over several sites (Vn~exp[−(n−nc)2/n2w]), similar behaviour is also found. Simultaneously we find that it is more likely to realize the spin filter effect in a comparatively large field since the polaron which feels attractive force is easily trapped by a local magnetic field.

Spin-Flip of Polaron in Polymers with a Magnetic Impurity

Using a nonadiabatic evolution method, we investigate the spin-flip process of polaron in polymers with a magnetic impurity. Our results show that when the spin orientation of this impurity is fixed to be perpendicular to the spin of polaron (? = ?/2), the magnetic impurity causes a spin-flip process. The probability of the spin-flip increases with the increase of exchange integral J up to about 0.35?eV and then decreases with the increase of J. In the case J is fixed while the spin orientation is adjustable, we find the probability of the spin-flip varies with the impurity orientation and reaches a maximum value at ? = ?/2.

Lattice Vibration Spectra in One-Dimensional Conductors*

For one-dimensional conductors with half-filled band, the lattice vibration spectra, both acoustic and optical branches, are obtained analytically in the dimerized state. It is shown that, there exist not only a gap in Brillouin zone boundary, but also a gap between the optical and acoustic phonon branches when the electron-lattice coupling constant is Larger than . Then, the frequency of the Staggered mode around solitons can be described analytically for any coupling constant .

The localized vibrational modes of soliton in trans-(CH)X

Starting from SSH model of trans-(CH)x with finite length, all the localized vibrational modes of soliton have been figured out and the total number is found to be five. The first three coincide with those obtained in TLM model. Another two are new findings, the fourth is odd mode with three nodes and the fifth is a staggered mode with even parity.

New localized electronic states of soliton in trans-(CH)X

It is well known that the soliton in trans-(CH)X possesses a localized electronic state at the center of the gap. Besides this mid-gap state, we have found that, there exist another four localized electronic states around the soliton. They form four discrete levels outside the conduction and valence bands: one sits near the bottom edge of the conduction band and one near the top edge of the conduction band; another two are located symmetrically near the valence band.

Spin-Flip Process of Polarons in Conjugated Polymers with Magnetic Impurities

Being driven by an external electric field, a polaron carrying both spin 1/2 and charge ±e moves at a constant speed in a polymer chain. When the polaron passes through a specific site, which couples to a magnetic impurity via spin-exchange interactions, its spin undergoes a spin-flip process if it is antiparallel to the impurity spin. Our numerical simulation shows that (a) a swap-like operation is performed between the polaron and impurity during the scattering process (b) polarons might be good candidates to be information carriers in the molecular scale.

Spin Transport in the Presence of Rashba Interaction

A Gaussian type spin-polarized electronic wave packet is constructed to investigate the spin transport behaviour in an infinite two-dimensional electron gas system with Rashba spin-orbit (SO) interaction by solving the Schrodinger equation exactly. In the presence of Rashba SO interaction, the spin-dependent force induces a momentum dependent splitting of the two spin directions, the average spin current indicates the corresponding spin accumulation clearly. Furthermore, the coherence of the injected spin-polarized wave packet, as well as the transverse force, decays during the motion in the Rashba SO regime.

Effect of Interchain Coupling on Quantum Lattice Fluctuation and Fluctuation Induced Interchain Order Transition in Trans-polyacetylene

Based on the modified Su-Schrieffer-Heeger Hamiltonian and by adding the interchain hopping term, we consider the effect of interchain coupling on quantum lattice fluctuation in transpolyacetylene. It has been found that the interchain coupling does not always weaken quantum lattice fluctuation. After overcoming a barrier of 1 eV or so, the quantum lattice fluctuation can induce a transition of interchain order. The physical meaning of these features is discussed.

Width of Soliton and the Electron Correlation in Polymer

The effect of electron correlation on the width of the soliton in polymer is studied. Our theory shows that the screening plays a key role in the effect of electron correlation on the width of soliton: In weak screening, the width of soliton is reduced by the electron-electron interaction; whereas, in strong screening, it will be increased. The physical reason is given at the end.

Electron-Hole interaction in Conjugated Polymers

The electron-hole interaction in conjugated polymers is considered for both a short chain and an infinite-long polyene. The attraction and repulsion between the electron and hole for different interactions are analysed. Excitonic states should exist in the attractive region.