Direct observations of order parameters and microscopic analysis on phase transition

提出了原子位隧穿光谱（AST）技术，将扫描隧道显微镜和隧道光谱技术相结合，可以得到每个原子位上指定的隧道光谱，并通过提高STM机头的机械强度和电子器件，成功地建立了AST装置。作为适合AST测量的模型材料，我选择了在室温下形成电荷密度波的1T-Ta (S,Se)_2体系，并证明了AST测量的性能。在TaS_2中，室温下的AST数据表现出轻微的位置依赖性，即CDW波峰上的隧穿光谱比CDW底部的隧穿光谱更具有金属性。然而，隧穿谱的总体特征与波段计算在数量上是一致的。相比之下，在77k时，得到的隧穿光谱清晰地显示出间隙结构，并且在CDW波峰上，间隙边缘的两个电导峰同时增强。这表明状态密度从CDW的底部转移到顶部。目前的隧穿结果是莫特局域化的有力证据，其中电子是由于库仑相互作用而局域化的。在TaSe_2中，即使在低温下我也没有发现间隙结构。也就是说，带图在这种材料中基本上是有效的。将Se替换为S = 1:1时，隧穿光谱突然发生变化，与输运数据一致。对于Mott跃迁，有两种类型，即有效质量发散或载流子浓度降至零。我没有观察到在前一种情况下预期的零偏置电导峰值，这表明了前一种机制。

I proposed the atomic site tunneling (AST) spectroscopy which can obtained the tunneling spectrum specified on each atomic site by combining the scanning tunneling microscopy and tunneling spectroscopy Furthermore, I have succeeded in building up an AST apparatus by improving the mechanical strength of STM head and the electronics. As a model material suitable to the AST measurement, I selected 1T-Ta (S,Se)_2 system, which forms charge density waves below room temperature, and I have demonstrated the performance of AST measurements. In TaS_2, the AST data at room temperature showed slight site dependence, i.e., the tunneling spectra on the crest of CDW is rather metallic in comparison with those at the bottom of CDW.However, the over-all feature of the tunneling spectrum is quantitatively consistent with the band calculation. At77K,in contrast, the obtained tunneling spectra clearly shows a gap structure, and two conductance peaks at the gap edge is enhanced simultaneously on the crest of CDW.This implies that the density of states was transferred from the bottom to crest of CDW.The present tunneling result is a strong evidence for the Mott localization where electrons are localized due to the Coulomb interaction. In TaSe_2, I have not found the gap structure even at low temperatures. Namely, the band picture is essentially valid in this material. By replacing Se by S up to 1 : 1, the tunneling spectra suddenly varied, being consistent with the transport data. For Mott transition, there are two categories, i.e., effective mass diverges, or carrier concentration decreases to zero. I have not observed the zero bias conductance peak expected in the former case, suggesting the former mechanism.