Study on local surface electrical conduction by scanning microscopic four-point probes in ultrahigh vacuum

我们利用单片微四点探针法（探针间距为4 ~ 40μm），结合反射高能电子衍射（RHEED）的同步结构分析，开发了一种在10 K至室温温度范围内具有高表面灵敏度的超高真空电导率测量系统。这种仪器可以直接测量晶体表面最上层原子层的电导率与温度的关系。我们用这个仪器有了几项新发现。(1)发现准一维金属表面Si(111)-4×1-In在120 K左右发生金属-绝缘体转变，这是通过冷却导致电阻急剧增加所揭示的。这与佩尔斯跃迁伴随着电荷密度波的图像是一致的。(2)二维金属表面Si(111)-√3x√3-Ag在230k左右发生了金属-绝缘体转变，这是通过冷却导致的电阻急剧增加所揭示的。结合光谱学研究，低温相为非带绝缘体。弱局部化可能起作用。(3)发现准一维金属表面Si(557)-Au具有半导体导电性。这可能是由于原子缺陷使金属原子链断裂成段。

We have developed a ultrahigh-vacuum system for electrical conductivity measurements with high surface sensitivity by using monolithic micro-four-point probe method (probe spacing being 4〜40μm) at temperatures ranging from 10 K to room temperature, combined with simultaneous structure analysis by reflection-high-energy electron diffraction (RHEED). This apparatus enables direct measurements of electrical conductivity at the topmost atomic layers on crystal surfaces as a function of temperature. We have made several new findings with this apparatus.(1)A quasi-one-dimensional metallic surface, Si(111)-4×1-In, was found to show a metal-insulator transition around 120 K as revealed by a dramatic increase in resistance by cooling. This is consistent with a picture of Peierls transition accompanied with charge-density waves.(2)A two-dimensional metallic surface, Si(111)-√3×√3-Ag, was found to show a metal-insulator transition around 230 K as revealed by a dramatic increase in resistance by cooling. Combined with photoemission spectroscopy study, the low-temperature phase is not a band insulator. Weak localization may play a role.(3)A quasi-one-dimensional metallic surface, Si(557)-Au, was found to be semiconductive in terms of electrical conduction. This may be due to atomic defects which break up the metallic atomic chains into segments.