The Spin-Obit Interaction In Organic Conjugated Polymers

As a new kind of functional material, conjugated polymers or small oligomers have been attracting much theoretical and experimental interest both because of the unique electric, magnetic and optical properties that occur in these materials and because of the technological potential of electronic devices fabricated from them. In particular, they are already the basis of effective organic light-emitting diodes (OLEDs) and a wider range of devices and applications such as organic spin valves and magnetic resistance. On the other hand, conjugated polymers have common characteristic of the soft condensed matters and can serve as a medium to understand other organic and even biological molecules. The organic materials have abundant physical and functional properties and we have to investigate such materials using a comprehensive method which may be related with physics, chemistry and so on.Magneto-electronics or spintronics is a field of growing interest. Since the discovery of giant magneto-resistance (GMR), rapid progress has been made in this field. Spintronics is a fire-new subject that researches the injection, transport, control, detection of electronic spin etc. In 1990, Datta and Das theoretically analyzed the possibility of a spin transistor that could work due to the Rashba spin-obit interaction which induced spin precessions in a semiconductor with ferromagnetic leads. Hence, the spin-orbit interaction (SOI) in semiconductors has attracted great attention. There are a lot of researches about the Rashba SOI in the rigid semiconductor, it has been achieved experimental that how the external gate electrode could be used to manipulate the Rashba SOI. Rashba SOI has the effect on the band structure, wave function, spin angle velocity and linear velocity, and low temperature conductance of quasi-one-dimensional electron system.Due to the strong electron-lattice interaction in conjugated polymers, the carriers are not the ordinary electrons or holes, but some self-trapped states, such as a charged spin polaron and charged spinless bipolaron. The particular charge-spin characteristic implies that conjugated polymers maybe have the abundant spin correlative property. Up to now, there have been extensive studies on the new field of organic spintronics. Theoretical some new spin correlative phenomena have been predicted and experimental people have indicated the transport-current in organic materials can be spin polarized. Usually it is considered that the SOI is weak in organic semiconductor and most of the present work neglected the effect of SOI on spin transport in organic devices. However, by applying a gate voltage, it is hoped that the Rashba SOI could have an observable effect. In addition, as the carrier of organic semiconductor, a polaron has a characteristic of particle-like, which has a well-defined position. Therefore it is theoretically meaningful to investigate the spin dynamics of a polaron by considering the Rashba SOI. We could obtain more plentiful physical phenomena about polaron transport in organic conjugated polymers when SOI has been considered.In this paper, considering one-dimension organic polymers, we investigate the effect of Rashba SOI on electron structural properties in the framework of the well-known Su-Schrieffer-Heeger (SSH) model and Brazovskii-Kirova (BK) model. In addition, we theoretically investigated the spin dynamics of a polaron in a one-dimensional organic polymer chain by including the spin-orbit (Rashba) interaction using a nonadiabatic evolution method. The detailed research and main results are given as follows:1, The effect of SOI on electron structural properties of organic conjugated polymersIn our model, one-dimension organic polymer is applied along the x|- axis and Rashba electric field applied z|- axis. Rashba SOI leads the spatial separationbetween "y~+" (along the y|- axis) and "y~-" (along the -y|- axis) spinor components.This spatial separation does not eliminate spin degeneration, which is different from the spatial separation in the Zeeman Effect. SOI induces the energy band of organic conjugated polymer becoming wider and the energy gap becoming narrower, but the change is not very obvious as a result of the weak spin-obit interaction in the organic polymer.2, The effect of SOI on the spin dynamics of a polaron in organic conjugated polymersUp to now, there have been extensive studies on polaron dynamics in conjugated polymers. Conwell et al. concluded that the polaron could form and move as an entity in low electric fields but dissociate in high electric fields. The value of the highestelectric field that the polaron could sustain was about 6×10~4V/cm (dissociationelectric field). Li et al. studied the dissociated polaron propagated in the form of a freelike electron and performed spatial Bloch oscillations, which was similar in the inorganic semiconductor. However, the spin dynamics of a polaron has scarcely been mentioned to date in the former researches. Proverbially, spin flip during polaron transport through the organic polymer chain is definitely to happen. SOI is one of the effects to affect spin evolvement. We investigated the spin dynamics of a polaron in a one-dimensional organic polymer chain by including Rashba SOI and maked compared with the rigid semiconductor further more.We have found the spin precession depending on the center position of the moving polaron follows cosinoidal rule and the relationship between spin precession period and the strength of SOI obeys inverse proportion rule, which is consistent with rigid semiconductor.In rigid semiconductor the spin precession of electron takes place among all the conduction levels (or interlevel). The softness of an organic semiconductor induces its carrier to be a composite particle with internal structure characterized by lattice distortion. The polaron state is discrete from the continuous conduction band, the polaron levels are localized in the energy band gap. Electron cannot transfer to the continuous conduction band easily. The interior mechanism of spin precession in organic semiconductor is definitely different from which in the rigid semiconductor, and this is the problem needed researching farther.