【作者】 宋宇；

【导师】 郭永；

【作者基本信息】 清华大学 ， 物理学， 2013， 博士

【摘要】 石墨烯纳米结构中的弹道输运蕴含着丰富而有趣的物理现象.其中的负微分电阻效应, Goos-Ha¨nchen (GH)位移,隧穿群时延,回旋共振等具有广泛的应用价值.石墨烯中双势垒共振遂穿二极管这一负微分电阻基本模型还没有研究,如何获得电子束横向宽度量级的GH位移尚待解决,个体群时延的二维定义并不完备,如何测量任意费米能的集体群时延需要研究,回旋共振的非磁调控尚不明了.为了解决这些问题,本文对若干石墨烯弹道纳米结构的以上现象进行了较为深入的理论研究.通过在旋转的赝自旋空间中计算I-V特性曲线,我们研究了石墨烯双势垒共振隧穿二极管的负微分电阻效应,发现其最低工作窗口几乎与结构参数无关,而仅由背栅近独立地控制.我们指出该现象的双极输运物理机制及其在某些负微分电阻器件中的潜在应用价值.我们还发现,空穴-电子输运、Klein隧穿和共振隧穿的竞争是该结构产生负微分电阻的物理机制,合适的结构参数是负微分电阻特征出现的必要条件,而可调能隙可以近指数地增大电流峰谷比.我们指出了电子束横向宽度在其空间分离中扮演的重要角色,报道了石墨烯双势垒结构中通过准束缚态增强获得的具有极小半高宽的巨GH位移.我们指出了该性质可用于设计具有广泛可调性和高能量分辨率的谷或自旋电子束分离器,并发现能隙可以进一步地增强分离器的可调性和分辨率.我们揭示了GH位移对二维群时延的本征贡献,提出了利用弱磁自旋进动实验中诱导电导差来测量石墨烯中集体群时延和内秉GH项的方法,该方法对几乎任意费米能量均有效.我们还指出了联系群时延和渡越时间的是非零的自干涉时延.我们使用圆盘形顶栅电势来调制石墨烯的Landau能级和回旋共振.我们发现顶栅电势可诱导束缚态在扩展的Landau型和局域的量子点型间相互转换,这使得在一定的电势范围内,回旋共振频率可以通过顶栅电势近线性地调制.我们指出这一特性可以用作光子的近线性控制频率过滤器.更多还原

【Abstract】 Rich and interesting physical phenomena arise with ballistic transport in graphenenanoscale structures. Among them, negative diferential resistance (NDR) efect, Goos-Ha¨nchen (GH) shift, tunneling group delay, and cyclotron resonances between Landaulevels have extensive applications in electronic devices. However, a basic NDR model,double barrier resonant tunneling diode has not been touched in graphene; how to obtaingiant GH shift larger than the transverse width of an electron beam is to be settled;the widely adopted definition of two dimensional group delay is not complete, how tomeasure the collective group delay at arbitrary Fermi energy is to be resolved; and howto modulate cyclotron resonances by electrostatic method is not clear. To solve theseproblems, in this thesis we theoretically study these phenomena in several graphenebased nanoscale structures.We investigate the NDR in graphene double barrier resonant tunneling diodes bycalculating the I-V characteristic in a rotated pseudospin space. We demonstrate that,the lowest NDR operation window is almost structural parameters-free and can be nearlysolely controlled by the back gate. We indicate that, this remarkable phenomenon stemsfrom the ambipolar transport in graphene and may be applied in operation window-dominated NDR devices. We have also found that, the competition between hole-to-electron transport, Klein tunneling, and resonant tunneling is the physical mechanismfor such a NDR structure. We also show that, appropriate structural parameters arenecessary for the NDR feature, and a tunable band gap can enhance exponentially thepeak-to-valley current ratio.We indicate that the transverse electron beam width plays a critical role in theirspatial splitter. We report giant GH shifts with magnitudes up to the order of transverseelectron beam width and rather small full-widths-at-half-maximum for electron beamstunneling through graphene double barrier structures, which we attribute to the quasi-bound states in the structure. We indicate that these features may be utilized to designvalley and spin beam splitters with wide tunability and high energy resolution. We alsofind that, an induced energy gap can increase the tunability and resolution of the splitters.We reveal the intrinsic contribution of GH shift in the two dimensional group delay.We suggest that, for almost arbitrary Fermi energy, the collective group delay and it inherent GH component can be probed by an induced conductance diference in spinprecession experiments under weak magnetic fields. We also indicate that, it is a nonzeroself-interference delay that relates the group delay and dwell time in graphene.We apply a circle top gate to modulate the Landau levels and cyclotron resonancesin graphene. We find that, the top gate induced potential can induce switchs betweenextended Landau-type bound states and localized quantum dot-type ones. As a result,the cyclotron resonance frequency can be tuned almost linearly by the top gate withinspecific ranges. This phenomenon can be applied as a near linearly controllable photonfrequency splitter.更多还原