【作者】 尹海涛；

【导师】 吕天全；

【作者基本信息】 哈尔滨工业大学 ， 凝聚态物理， 2006， 硕士

【摘要】 随着材料科学技术和超微细加工技术的发展,制备的半导体微结构已经可以达到介观尺度,人们已成功制备了量子点和量子点阵列。由于能带人工可剪裁性、量子尺寸效应和量子相干属性,低维介观体系产生了许多新现象和新效应,受到人们的广泛关注。尤其是耦合量子点体系,无论组成其量子点的空间对称性,量子点的尺寸还是相互之间的耦合强度都是人为可以调控的,进而可以实现人们操纵固体的梦想。作为展现低维介观体系量子效应的典型代表,耦合量子点和介观Aharonov-Bohm(AB)环结构已成为近年来的研究热点。本论文以格林函数作为理论工具,对三终端耦合量子点阵列和由三个量子点构成的环状结构的谱性质和量子输运性质进行了较系统的研究。具体内容包括:研究了三终端T形耦合量子点体系的电输运性质。把三终端的体系转变成两个等效的二终端体系的叠加,从而可以分别计算电子从一个电极到其它两电极的隧穿几率。数值计算表明:通过体系的隧穿几率为振荡结构;通过计算各量子点上的局域态密度(LDOS)发现,耦合的量子点体系在不同量子点处的LDOS的不同,会导致不同电极间隧穿特性的不同;同时量子点间的耦合强度、量子点能级匹配情况以及电极与体系的耦合强度等都对输运性质有显著的影响。尤其是当某个电极与体系的耦合强度由零增大到某个值时,其它两个电极间在量子点能级处的隧穿会由反共振隧穿转变为共振隧穿,这一点可为我们设计纳米开关提供理论依据。对于与三个电极耦合的三个量子点环状结构,我们研究了量子相干输运的相位特性。数值结果显示:微分电导的振荡随量子点间的耦合强度减小而变弱。同时比较了二终端和三终端体系在不同偏压和磁通量时的电输运性质。更多还原

【Abstract】 With the development of material technology and ultra-micro fabrication technique, the semiconductor microstructure with mesoscopic dimension have come within reach. Quantum dot and quantum dots array have been fabricated in experiment successfully. These low dimensional mesoscopic systems have many novel marvelous phenomena due to their energy band to be tailored, quantum size effects and quantum interference, and thus received much current attention. Especially for coupled-quantum-dot systems, no matter the spatial symmetry, size and tunnel coupling between quantum dots are all controllable. In this way, the dream of controlling solid comes true. As the typical representative exhibiting quantum effects in mesoscopic physics, quantum dots and mesoscopic ring have become one of the most active areas. In this paper, we present in some detail theoretical investigation, using Green’functions approach, on the spectral and quantum transport properties of quantum dots coupled to three terminals and three quantum dots embedded in a Aharonov-Bohm ring.We have studied nonequilibrium quantum transport through a T-shaped four-quantum–dot system with three terminals under dc bias voltage. Both Dyson equations and equations of motion method for Green functions are used to obtain the current formula. By exploiting the Green’s functions, the electron transport in a three-terminal system can be reformulated into the electron transport in an effective two-terminal system. The calculations reveal that the transmission probabilities display oscillation structures. To understand the results of the calculations for the structure, we have calculated the local densities of states. We have found that the characteristics of the transmission spectra arise from the fact that the confined states in the systems are localized in differently quantum dots and can contribute very differently to different transmission probabilities of the systems. In addition, the tunnel coupling between dots, the energy level of each dot and the coupling dot and lead play crucial roles in the transport properties of the system, especially when the single-dot Fermi levels of the four dots are aligned, with the increasing ofΓMfrom zero to some proper value, antiresonance will be transferred to resonance at the single-dot Fermi level. This更多还原