Direct observations of order parameters and microscopic analysis on phase transition

提出了将扫描隧道显微镜和隧道光谱技术相结合，获得原子位置上特定的隧道谱的原子位置隧道光谱（AST）技术，并通过改进STM头的机械强度和电子学，成功地建立了AST装置。作为适合AST测量的模型材料，我们选择了在室温以下形成电荷密度波的1 T-Ta（S，Se）_2体系，并演示了AST测量的性能。在TaS_2中，室温下的AST数据显示出轻微的位置依赖性，即，CDW顶部的隧穿谱与底部的隧穿谱相比具有更强的金属性，但隧穿谱的总体特征与能带计算结果在定量上是一致的。在77 K时，隧穿谱显示出明显的能隙结构，并且在CDW的顶部，差距边缘的两个电导峰同时增强，这意味着态密度从CDW的底部转移到顶部，这一隧穿结果是Mott局域化的有力证据，即电子由于库仑相互作用而局域化.在TaSe_2中，即使在低温下，我也没有发现差距结构。也就是说，带状图像在这种材料中基本上是有效的。通过用S取代Se至1：1，隧穿光谱突然变化，与输运数据一致。对于Mott转变，有两类，即，有效质量发散或载流子浓度降低到零。我没有观察到零偏压电导峰预期在前一种情况下，表明前一种机制。

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.