Spin-Dependent Transport Of Two-Dimensional Electron Gas System Under Ferromagnetic Modulation

Two-dimensional electron gas (2DEG) is the system that electron can move freely in two dimensional plane but is limited in the perpendicular direction, which has been a hot topic in condensed matter physics because of its special property. In this thesis, the spin-dependent transport properties for semiconductor heterostruc-ture, graphene and surface of three-dimensional topological insulator under ferro-magnetic modulation have been theoretically investigated with the transfer matrix method. The purpose of this study is for providing physical basis for the design of nano-electronical devices and spin quantum devices.The thesis is divided into six chapters. In the first chapter, we give a brief introduction about the discovery and fabrication techniques of three types of2DEG system, the physical properties and their application background. In the second chapter, we give a detailed introduction about the transfer matrix which are often used in the study of mesoscopic quantum transport.In the third chapter, according to the Schrodinger equation we theoretically investigate the transport property of semiconductor heterostructure2DEG with the modulation of two applied asymmetry magnetic barriers. It is demonstrated that, the spin-dependent conductance split as the magnetic barriers is inconsis-tent, which lead to the spin-splitting of the tunneling magnetic resistance (TMR). The shorter the space of the barriers and the more apparent the asymmetry, the splitting is more significant. Therefore, we can obtain the conductance and TMR which is spin-splitting remarkably by manipulating the structure parameters of the applied magnetic barriers.In the fourth chapter, based on the Dirac equation we respectively investi-gate the transport property of graphene and armchair-edged graphene nanoribbon (AGNR) under the modulation of two tunable magnetic barriers. It is demon-strated that, the transmission of system is forbidden in the lower energy region when the relatively height of magnetic barriers is stronger, and the transmission spectrum is not symmetry any more in AP configuration. With the increase of the space of the two barriers, the intervention of the left-going and right-going wave is enhanced, which result in the transmission channels more discrete. When the width of the magnetic barrier become longer, owing to the exist of the evanes-cent state the electron wave can not through the barrier as the decaying length is shorter than the width of the barrier. Accordingly, with the varied structure parameters the change of the conductance and TMR is consistent with the trans-mission spectrum. Both the increase of absolute (or relative) height and the width of the barrier can reduce the conductance and make the TMR lager. However, the resonant of conductance and TMR is enhanced as the space of the barriers become lager. In addition, the transport property of the AGNR is investigated. It is demonstrated that the transverse wave vector is discrete because of the lim-itation of transverse direction, and the system exhibit platform conductance in both P and AP configurations, which lead to the platform TMR. Meanwhile, the character of the energy band and conductance of the AGNRs with different width is different.Chapter five is our major work. By the transfer matrix method we investigate the band structure and transport property of the surface of three-dimensional topological insulator (3D TI) and the distribution of electron spin polarization under the modulation of electro-magnetic superlattice. It is demonstrated that, since the difference result from the system configuration changes from P to AP, the number of subband in AP configuration is lager than that in P configuration at the same energy window. As the height of the magnetic barriers become enough larger, the energy levels in lower energy region are almost parallel to the coordinate axis in both P and AP configuration, which means that the transverse velocity (?)E/(?)kx tend towards zero, and the transmission is forbidden. But as the electric barriers become stronger the influence is very slight. Further, the number of transmission channel is consistent with the band structure of system, and the transmission forbidden region is wider in AP configuration because of the stronger suppression. When the electric barriers increased, especially in the vicinity of E＝Vo, there is only small incident angle transmission channel. This property is different from the semiconductor heterostructure and graphene. Moreover, we investigate the character of the electron spin polarization. In momentum space, the incident energy and angle dependence of electron spin polarization is in good agreement with the transmission spectrum. Due to the spin-momentum locking of TI surface states, the spin orientation of reflected electrons is always rotated and transmitting electron is always along the direction of the incident electron. However, in real space, in incident region the in-plane spin polarization oscillates periodically only in x-direction, and the spin polarization Pz is not zero, which induced the spin-surface locking broken. Because there is only transmission wave in the transmitting region the spin polarization in the transmitting region is found not relevant to coordinates and varying with the incident electron.In chapter six, a summary of the work and a outlook of the transport prop-erties of two dimensional gas under the ferromagnetic modulation are given.