Nanodevice Design Based On Spin And Pseudospin Degree Of Freedom

Electrons are the carriers of both the charge and the spin. A great deal of efforts on the manipulation of the charge property have been made in microelectronics. Accordingly, various charge-based devices have been fabricated, which changes our lives day and day. When the size of microelectronic devices goes further to a certain small scale, the quantum effects would arise, which leads to the failure of the physical principle of the device. In recent years, a field dubbed spintronics has evolved as a new branch of condensed-matter physics, where the other property of electrons---spin is utilized as information carrier. The operation of a spintronic device requires the generation, detection, and manipulation of the spin current. As a possible candidate for the replacement of silicon in chip industry, graphene becomes a hot topic in nano materials. In Chapter 1, we give a brief introduction on these fields: nanoelectronics, spintronics, and graphene.In Chapter 2, we consider a quantum point contact (QPC) modulated by a spin-orbit interaction. When a spin bias is applied to a two-terminal device, a charge current will be generated. We demonstrate the rectification of such a current in Chapter 2. When the polarization orientation of the spin bias (which is the spin-quantization axis) is along the transverse direction of the QPC, the two spin-conserved transmissions show a distinct variation with the incident energy. As a result, the charge current can turn from zero to a remarkable value by switching the spin bias from one lead to the other lead.As a counterpart of electronic spin in spintronics, the two-fold valley degenaracy in graphene has been suggested to carry information. The operation of valley-based electronic devices requires the generation of an uneven valley distribution of electrons. All of previous proposals of valley filters focus on an all-electric control of valley polarization. In Chapter 3, we put forward a tunable valley filter based on bulk graphene under the modulations of both the substrate strains and local magnetic fields. When only one of the two modulations is present, no valley polarization can be generated. The combination of the two modulations results in a difference between the absolute shifts of the K and K' valleys, which could be utilized to generate a valley-polarized current. The degree of the valley polarization can be tuned by the strain strength and the inclusion of a scalar electric potential. The valley polarization changes its polarity as the local magnetic field switches its direction.