Spin And Valley-polarized Transport Properties In The Nanostruc- Tures

Spintronics has been the topic of intense research in condensed matter physics since the discovery of giant magnetoresistance. It is an emerging technology which exploits the spin property of the electrons in addition to the charge property in the nanostructures. Successful spintronics applications need to satisfy the following three components:efficient spin generation, efficient spin manipulation, and, finally, spin detection. For example, the Rashba spin orbit coupling leads to manipulating the spin of the electrons by the external electric field or the gate voltage.Moreover, after Andre Geim research group at The University of Manchester extracted graphene from bulk graphite in 2004, two-dimensional materials, such as graphene, silicene and MoS2 have recently attracted great interest for the great poten-tial of the basic and applied research. Due to the honeycomb structure which can be thought of as a triangular lattice with a basis of two atoms per unit cell(sublattices A and B), it has two inequivalent Dirac points at the corners of the first Brillouin zone (valley freedom K and K’). In analogy to the spintronics, the valleyntronics refers to the technology of control over the valley degree of freedom which can be harnessed to store information in the devices.In this thesis, we study the problem of spin and valley-polarized transport proper-ties in the nanostructures by using Landauer-Biittiker formulism and scattering matrix method.These results can provide some meaningful information to realize spin and valley-polarized transports. In detail, the dissertation is organized as follows:In chapter one, we give a brief introduction to the nanoelectronics. Next we de-scribe the developments of the graphene and silicene, mainly referring to their basic physical properties. Moreover, some paragraphs are also dedicated to the introduction of spintronics and valleytronics. These will help us better understand the research back-ground. At the same time, scattering matrix method and its properties are introduced, which has been used to study this two kinds of materials. In addition, we present a de-tailed derivation of the Landauer-Buttiker formalism by using of the scattering matrix method.In chapter two, we investigate the spin-resolved scattering through a Rashba spin-orbit(SO) coupling graphene barrier by combing the scattering matrix method and Landauer-Biittiker formulism. In this structure, we find that the difference of the trans-mission probability for opposite spin orientations exhibits considerable incident-angle-dependent features when both gate voltage and Rashba SO coupling in the barrier re-gion are present. The difference is adjustable by gate voltage. More specifically, we find that the sign of spin polarization of the outgoing current can switch from positive to negative by adjusting the electric potential at any Rashba SO coupling. These result-s can provide an efficient way to design graphene spintronic devices without need for ferromagnets.In chapter three, we investigate the incident angle-dependent spin polarization transport in a ballistic nonmagnetic nanojunction with interfacial spin-orbit (SO) cou-pling and barrier by means of the mode-matching approach. For both delta function and square-shaped barriers, the analytical formula of transmission for spin up and spin down are obtained. In particular, for the case of interface with SO coupling, we find that the spin polarization oscillates with respect to the incident angle. This characteristic offers an efficient way to achieve the spin injection device without using ferromagnets.In chapter four, we theoretically study the valley-and spin-resolved scattering through magnetic barrier in a one layer thick silicene, using the scattering matrix method for the Dirac equation. We show that the spin-valley filtering effect can be achieved and can also be tuned completely through both a top and bottom gate. More-over, when reversing the sign of the staggered potential, we find the direction of the valley polarization is switched while the direction of spin polarization is unchanged. These results can provide some meaningful information to design valley valve residing on silicene.In the last chapter, we make a summary and give some outlook for the future investigation.