Theoretical basis for study of subsurface nanostructures by scanning tunneling microscopy

到目前为止，使用扫描隧道显微镜（STM）的研究大多集中在表面的最外层，使用STM研究地下结构的工作较少。在这个项目中，我使用STM进行了地下结构研究的基础和理论研究。主要研究结果如下：对弹道电子发射显微镜（BEEM）进行了理论研究，阐明了决定BEEM中电子束宽度的因素。我发现了横向共振隧穿的新效应。开发了一种利用散射计算构造布洛赫态的新方法，并发现了beem中原子尺度变化的重要因素。我考虑了非弹性散射的影响，使用了一个统一的和非摄动的方法，并研究了表面STM工作的深度。研究发现，随着地下界面深度的增加，由于平均效应，STM系统的电导接近布洛赫态边界条件下的计算值。本文所定义的非弹性平均自由程与弱散射区弛豫时间所导出的自由程一致。在强散射区，微扰失效，平均自由程不一定随散射强度的增加而缩短。我计算了具有单和双STM尖端的平面和岛状表面的表面态传导。研究发现，在表面态存在的情况下，即使表面最外层的电势发生变化，平面的电导也不会下降太多。双尖端STM的电导与双尖端之间的距离成反比。随着本项目的进一步发展，利用多个STM针尖进行纳米级表面导电的理论研究是有前景的。

So far, most studies using scanning tunneling microscopy (STM) have paid attention to the outermost layers of surfaces, and there were small number of works studying subsurface structures using STM.In this project I performed basic and theoretical investigations for the study of subsurface structures using STM.Main results obtained are as follows.1. I studied theoretically ballistic electron emission microscopy (BEEM) and clarified the factors determining the width of electron beams in BEEM.I found a new effect of transverse resonant tunneling. I developed a new method for constructing Bloch states by scattering calculations, and found important factors for the atomic-scale variation in BEEM.2. I took account of the effect of inelastic scattering by using a unitary and non-perturbative method, and investigated how deep in surfaces STM works. It was found that with the depth of subsurface interfaces, the conductance in an STM system approaches the value calculated on the boundary condition of Bloch states due to an averaging effect. The inelastic mean free path defined in the present study coincides with that derived from the relaxation time in the weak-scattering regime. In the strong-scattering regime, perturbation fails and the mean free path does not necessarily shorten with increase of the scattering strength.3. I calculated the surface-state conduction in flat and island surfaces with single and double STM tips. It was found that in the presence of surface states the conductance of flat surfaces does not much decrease even if the potential in the outermost layer of surfaces changes. The conductance of the double-tip STM is inversely proportional to the distance between double tips.As future development of this project, the theoretical study on the nano-scale surface conduction using multiple STM tips is promising.