Spin-dependent Transport In Several Mesoscopic Systems

With the development of material technology and ultra-micro fabrication technique, the various mesoscopic structures such as ferromagnetic/semiconductor/ferromagnetic heterojunctions, quantum dots and otherwise have been fabricated. These low dimensional mesoscopic systems give rise to many novel marvelous phenomena due to their energy band to be tailored, quantum size effects and quantum interference, and thus received much current attention. On the other hand, with the miniaturizing of semiconductor device, conventional charge-based electronic devices have reached their limits. Then, a technology has emerged called spintronics, where it is not the electron charge but the electron spin that carries information, and this offers opportunities for a new generation of devices combining standard microelectronics with spin-dependent effects that arise from the interaction between spin of the carrier and the magnetic properties of the material. The spintronics devices have the potential advantages of nonvolatility, increased data processing speed, decreased electric power consumption, and increased integration densities compared with conventional semiconductor devices. To successfully incorporate spins into existing semiconductor technology, one has to resolve technical issues such as efficient injection, transport, control and manipulation, and detection of spin polarization as well as spin-polarized currents.In this paper, we introduce the mesoscopic systems and the typical mesoscopic phenomena firstly. And then, several theories on mesoscopic transport such as transmission matrix, scattering matrix, lattice Green's function, and nonequilibrium Green's function are presented shortly. Finally, we study the spin-dependent transport in several mesoscopic systems.1. Based on the scattering approach, we investigate transport properties of electrons in a one-dimensional waveguide that contains a ferromagnetic/semiconductor/ferromagnetic heterojunction and tunnel barriers in the presence of Rashba and Dresselhaus spin-orbit interactions. We simultaneously consider significant quantum size effects, quantum coherence, Rashba and Dresselhaus spin–orbit interactions and noncollinear magnetizations. It is found that the tunnel barrier plays a decisive role in the transmission coefficient and shot noise of the ballistic spin electron transport through the heterojunction.2. Using the scattering matrix method, we investigate the spin-dependent conductance and the shot noise of the multichannel ferromagnetic/semiconductor/ferromagnetic heterojunctions in the presence of the spin-orbit coupling. We find that spin-up and spin-down electrons have different contributions to the conductance and the shot noise. The rounded quantum plateaus of the conductance appear when the length of the semiconductor is made shorter. As the number of conducting channels in the system increases, the shot noise power oscillates and the Fano factor is increasingly suppressed.We also find that interband mixing due to the SOC brings significant effects on the spin-dependent conductance and the shot noise.3. We investigate the effect of Dresselhaus spin-orbit coupling on the spin-transport properties of ferromagnet/insulator/semiconductor/ insulator/ferromagnet double-barrier structures. The influence of the thickness of the insulator between the ferromagnet and the semiconductor on the polarization is also considered. Our results indicate that (a) the polarization can be enhanced by reducing the insulator layers at zero temperature, and (b) the tunneling magnetoresistance inversion can be illustrated by the influence of the Dresselhaus spin-orbit coupling effect in the double-barrier structure. Due to the Dresselhaus spin-orbit coupling effect, the tunneling magnetoresistance inversion occurs when the energy of a localized state in the barrier matches the Fermi energy of the ferromagnetic electrodes.4. Spin polarization in parallel double quantum dots embedded in arms of Aharonov-Bohm interferometer is investigated. The spin-orbit interaction exists in quantum dots. We find that the spin polarization is quite large even with a weak spin–orbit interaction. The direction and the strength of the spin polarization are well controllable and manipulatable by simply varying the strength of spin-orbit interaction or the energy levels in quantum dots. Moreover, electron-electron interaction strengthens the spin accumulation when the energy levels of the two quantum dots are identical. As the energy levels are unequal, electron-electron interaction cannot increase the spin accumulation. It is worth mentioning that the device is free of a magnetic field or a ferromagnetic material and it can be easily realized with present technology.