Quantum Transport And Thermoelectric Propertis Of Two-Dimensional Materials Nanoribbons

The two-dimensional Dirac electron systems, consisting of graphene, topo-logical insulator (TI) and silicene, etc., because their band structures are linear dispersion with Dirac cones and seems massless and chiral around the Fermi en-ergy level. Therefore, it is necessary to adopt a kind of Dirac equations to describe them. Meanwhile, one can realize a lot of quantum effects in these systems such as quantum spin Hall (QSH) and quantum anomalous Hall (QAH) effect, which can realize a dissipationless or low-dissipation electron transport. Therefore, the two-dimensional Dirac electron systems have attracted more and more people due to their scientific significance and potential applications in spin/valleytronics. In this thesis, by using the tight-binding approximation Hamiltonian and nonequilib-rium Green’s function (NEGF) method, we have been theoretically investigated the spin-dependent transport in a two-dimensional TI, as well as the electron transport and thermoelectric property in silicene. The purpose of this study is to provide physical basis for the design of nano-electronical (spin/valley) devices based on these Dirac electron systems in the future.The thesis is divided into six chapters. In the first chapter, we briefly introduce the discovery and fabrication techniques of several typical Dirac electron materials such as the two-and three-dimensional TI and silicene, as well as the the physical properties of spin-orbit coupling and QSH effect. In the second chapter, we provide a detailed introduction to the Landauer-Biitittiker formula and NEGF methods which are adopted in this thesis.In the third chapter, we investigate the band structure and spin-dependent transport for a normal/ferromagnetic/normal two-dimension TI junction, where the ferromagnetic (FM) region is realized by ferrimagnetically doped such as Mn- doped. It is demonstrated that, the band structure shows a gap opening due to the transverse spatial confinement. Further, the exchange field induced by magnetic Mn impurities can also modulate the band structure with two spin degenerate bands splitting, meanwhile, broke the degenerate conductance. Moreover, by tun-ing the Fermi energy and the exchange field strength, or by tuning the Fermi energy and the FM TI length, the junction can transform from a QSH state to a QAH state, which is very important to enable dissipationless charge current for designing perfect spin filter.In the fourth chapter, we investigate the band structure and electron transport for both zigzag silicene nanoribbon (ZSiNR) and armchair silicene nanoribbon (AS-iNR). We find that the ZSiNR system still shows interesting symmetry-dependent property although the σ mirror plane is absent for any ZSiNR due to the buckled structure of silicene. As a result, the magnetoresistance with a 100% plateau is observed when the atomic chain number of ZSiNR N is even, but this property is absent when the N is odd. Therefore, it is more favorable to fabricate spin valve device by using the even-N ZSiNR than the odd-N one. We also investigate the spin transfer torque (STT) as a function of the relative angle of the magnetization and the the Fermi energy in ZSiNR. It is found that the STT versus the relative angle exhibits a sine-like behavior, and versus the Fermi energy is antisymmet-rical to the Dirac point. Besides, the STT is sensitive to the Anderson disorder around the Dirac point, but is robust against to relatively small Anderson disorder away the Dirac point. On the other hand, we observe that the band structure in the ASiNR system shows the oscillatory decrease of band gaps with the increase of nanoribbon width. The ASiNR with N=3n+2 are all metallic with positive integer n, while the others are semiconducting. The band structure and electron transport for both the metallic and semiconducting ASiNR can be modulated by the effective SOC and perpendicular electric field. Finally, we find the conductance in ASiNR is robust against to relatively small Anderson disorder.In the fifth chapter, we investigate the thermoelectric property for a ZSiNR under a perpendicular electric field. It is demonstrated that, the thermoelectric property for the system also shows symmetry-dependent property, which is similar as the electron transport property investigated in the above fourth chapter. The spin thermopower for 6-ZSiNR junction can be improved by one order of magni-tude than that for 7-ZSiNR one in the absence of an electric field. However, as the strength of electric field increasing, the spin thermopower for the 6-ZSiNR junction dramatically decreases while it notably enhances for the 7-ZSiNR one. Interest-ingly, the spin thermopower for both junctions can be very pronounced with a maximum absolute value of 200μV/K by tuning the field. We further investigate the spin thermopower dependence on the temperature and strength of magnetiza-tion, it shows that the spin thermopower for the 6-ZSiNR junction decreases but the situation for the 7-ZSiNR one is opposite with the increase of temperature. The spin thermopower dependence on the strength of magnetization for both 6-and 7-ZSiNR junctions is qualitatively similar. Finally, we investigate the spin figure of meri dependence on the strength of magnetization. It is found that the spin figure of meri for both 6-and 7-ZSiNR junctions is dramatically increased as magnetization increasing, and the spin figure of merit for the 6-ZSiNRjunction is found four orders of magnitude larger than that for the 7-ZSiNR one. This result indicates that the even-N ZSiNR junction is a more favorable candidate for the silicene-based high performance thermoelectric device compared with the odd-N one.In the sixth chapter, a summary of the work and a outlook of the theory study for these two-dimensional Dirac electron systems are given.