Electronic States And Their Transport In Gapless Semiconductor Materials

Gapless energy band has induced many unusual and intriguing physical prop-erties. The most famous material is graphene, its valence band and conduction band cohere at the Dirac point, finally leads to a zero gap electronic structure. Besides graphene, graphenelike material, for example silicene can occur topologi-cal transition under the perpendicular electric field. When the electric field comes to a critical vallue, the ungaped electronic structure has emerged. Other than graphenelike gapless system. Three dimensional topological insulator also hold a gapless energy band in its two dimensional surface. The gapless electronic struc-ture appeal many interests in science study.In these thesis, we talk about the characteristics of gapless electronic structure and the mechanism to induce the gapless band. The corresponding electron trans-port and electron distribution based on this special band structure also have been investigated. The original transport properties of electric tunable silicene has been discovered. Other than that, electron distribution of topological insulator with line defects has been discovered by asymptotic model. In the first chapter, we give a brief introduction about the discovery and fabrication techniques of four types of gapless system, the physical properties and their application background. How to infer the existence of zero-energy eigenstates, as a type of important gapless system, localized on the boundaries in terms of properties of the bulk and the symmetry has been adressed in chapter two. In the third chapter, we give a detailed introduction about the transfer matrix which are often used in the study of mesoscopic quantum transport. In chapter four, The detailed knowledge about the interaction between silicene/metal is crucial to understand the silicene growth on Ag surface and metal-silicene contact in nano electronic device. In this work, the tunable conductance of both end contacted (EC) and side contacted (SC) metal/silicene/metal (MSM) junction have been studied. Due to the coupling between valley and spin degrees of freedom, current through MSM is spin and valley polarized. In particular, we study the electric tunable transport property of silicene. We find fully spin and large valley polarized conductance by introduce the perpendicular electric field and ferromagneto. And we demonstrate the conductance is heavily depend on the relationship of simple metal/interface/silicene hopping integral ts/t’/tg. But we also find, as long as system satisfy β=t’2/(tstg)≈1 and incident energy e≡ts, EC or SC junction make no difference. Of course, this situation correspond to zero contacted resistance. At last, we conclude the spin/valley conductance and polarization of MSM oscillate with junction length L. In chapter five, we study the penetration depth (PD) of surface state into the bulk, the local density of states (LDOS) and the spin-polarized STM tip tunneling conductance (TG) for a smoothly varying step described by an asymptotic model, on the surface of a three-dimensional (3D) topological insulator (TI). Using curvilinear coordinates for calculation, we find that the PD shows two peaks near step edges with max-imum surface curvature and a dip value of flat surface at step middle with zero curvature. In contrast, the LDOS exhibits a double valley (dip) pattern, in which it rapidly declines from the highest value for flat surface to the dip value near step edges, and then shows a small peak at step middle. This interesting small peak is in good agreement with experimentally observed LDOS peak within a step of definite span on a 3D TI surface. Moreover, the calculated surface spin-polarized STM tip TG also confirms the LDOS qualitatively. The findings here may provide a further understanding of 3D TI surface with step line defects.In chapter six, a summary of the work and a outlook of the transport prop-erties of gapless system are given.