| Research Abstract |
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.
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