The Effects Of The Impurities To Spin-Polarized Transport Properties In Quantum Point Contact

This paper makes the comprehensive introduction about the history of development,present state, basic theory, application prospect of the spintronics. We also introduce,analyze and summarize a variety of research methods and fundamental phenomena aboutthe transport properties of mesoscopic system.Many factors influence the transport properties of mesoscopic system. The electricfield of impurity is one of the most important factors. The interactions between chargecarrier and electric field of impurity include coulomb interaction and spin-orbit coupling.In this paper, we mainly discuss the effects of coulomb interaction and spin-orbit couplingto the conductance of mesoscopic system. We treat the impurity as a point charge anddiscretize the mesoscopic system using finite difference method. Finally we calculate theconductance of mesoscopic system using green function method.Firstly we show the discretization of Hamiltonian of the system include the gatevoltage. We calculate the retarded green function by using recursive green’s functionmethod and calculate the formula of system conductance using the retarded green function.Then we calculate the effects of different electric-field strength and different position ofimpurity to the conductance of mesoscopic system and the spin polarization flowrespectively.The results show that the curve of the conductance has a shift due to the present of theimpurity electric field compared with only existing the gate voltage, which explains thatthe impurity electric field and the gate voltage effect the action of charge carrier together.The strength of electric field is higher, the effect is bigger. If the impurity to the systemcentre is nearer, the coulomb interaction effect to the conductance of system will be more,and the shift of curve of the conductance will be more obvious. On the other hand, theeffect to the conductance of system is less when the impurity is further away from thesystem. The spin orbit coupling interaction between impurity and charge carrier makes theconductance curves produce a series of sharp peak(that is to say, conductance shows amaximum value). And the value of sharp peak becomes more and more with the spin orbitcoupling interaction stronger and stronger. The effect of impurity spin-orbit coupling andcoulomb interaction to the conductance makes the decrease of the number and amplitudeof the curve peak, compared with only the function of spin-orbit coupling. So we canconclude that the effect of coulomb interaction of the impurities to the spin-orbit coupling function plays an inhibitory role.The spin orbit coupling interaction between impurity and charge carrier makechanges of electron spin occupation number and produce spin polarization. The maincontribution of system conductance comes from the conduction of the same spin state. Thespin-up and exit spin-down electronic will be not conductive basically, and theconductance of incident spin-down and exit spin-up electronic will appear larger peak inthe condition of the gate voltage and impurity potential being corresponding parameters(the parameter determines the potential positive or negative).The results above will play a significant role for the design of the mesoscopic device.We can control the quantity, type and spatial location distribution of impurity ofmesoscopic devices according to our requirement and make it have more excellentperformance.