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Investigation on the Electronic Transport Properites of Nanostructure and the Role of the Contact

Author: ZhangLi
Tutor: YuYaBin；CaiMengQiu
School: Hunan University
Course: Condensed Matter Physics
Keywords: mesoscopic systems nanostructure dynamic transport properties contact barrier conductance
CLC: TB383.1
Type: Master's thesis
Year: 2008
Downloads: 88
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Abstract


With the development of the micro-nano fabrication, more and more nanostructure quantum devices are fabricated. The properties of the devices are mainly determined by the transport properties, so the study on the transport properties is great significant for fundamental and potential applications.There are contacts between macroscopic leads and nanostructure in the transport systems of in nanostructure. The contacts in fact are transition regions, and the electrons have to be scattered in the transition regions. So, there would be accumulation of charges in the regions, and the distribution of charges and its movement will affect the electric potential distribution and movement of electrons in the entire system. Therefore, they have important influence on the properties of dynamic electronic transport. To study the role of the contact, several physical quantities such as the distributions of internal potential and charge density, transmission probability, and ac conductance, are calculated in dc case and ac case, separately. Based on the theoretical studies and numerical calculations, the results for the transport properties are obtained. Then, we discuss the relation between the transport properties and contacts, as well as the role of the contacts.First, we study the properties of electronic transportation in dc case. The model that we consider is a 2D-1D-2D system. Basing on the scattering-matrix method, we have calculated transmission probability and the distributions of internal potential when a dc voltage is applied to the systems. The results show that the contacts can produce significant effects on the transmission probability and the distributions of internal potential. This shows that in the connected coherent devices systems, the device and device as well as the contacts and devices are interacted each other, and the devices cannot be regarded as individual isolated ones, and no longer have their own independent transport properties as traditional devices.Then, we study the properties of electronic transportation in ac case. The transport properties of ballistic quantum wire systems have been extensively studied, But, in the previous studies, the formulation of ac conductance mostly used the low-frequency limit approximation or the random phase approximation which fit for low-frequency regime. So, it is necessary to achieve the formulation to calculate ac conductance in a finite frequency range system. We first use the scattering theory to analyze ac transport, and then use the invariability of the potential translation to obtain dynamic conductance. Furthermore, via numerical calculation we give the results for dynamic conductance as the function of frequency. The results indicate that in the presence of the contacts, the behaviors of conductance will obviously change. This shows that the role of contact is very important, and is worth to seriously investigate in the transport of mesoscopic systems.Finally, we used self-consistent equations primarily studied the distributions of internal potential for a one-dimensional single-barrier nanostructure.In comparison with the results obtained by Thomas-Fermi approximation, we find that the self-consistent equations are more applicable for dynamic transport of the system. So we can use the equations to further investigate the transport properties of mesoscopic systems.

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