Development of high-frequency-compatible scanning tunneling microscope and application to the electronic phase separation in strongly correlated electron systems

在这项研究中，我们的目的是研究电子相分离，这是预期在强关联电子系统。强关联电子系统是基于所谓的莫特绝缘体，并显示出许多新的现象，如高温超导。电子相分离的研究需要具有纳米级空间分辨率的实验技术。因此，我们使用扫描隧道显微镜/光谱（STM/STS）。另外，我们尝试发展一种新的技术，交流扫描隧道显微镜，以研究导电性较差的样品。电子相分离的趋势应该是最强的金属到莫特绝缘体转变（莫特转变）。有两种类型的莫特转换，即填充控制和带宽控制莫特转换。我们在Ca_<2-x>Na_xCuO_2Cl_2和Ni（S，Se）_2上进行了低温STM/STS，这两个体系分别是填充和带宽控制的体系。我们发现，有明显的纳米尺度的电子不均匀性周围的填充控制莫特转变。明显的“金属”区域随着掺杂的增加而增加。这一结果表明，填充控制系统中的金属-绝缘体转变与逾渗有关。相反，带宽控制系统的电子态是相对均匀的，即使在莫特转变附近。在<2-x>高温超导体Ca_ Na_xCuO_2Cl_2中，我们发现了电子态的周期性调制，在所谓的赝能隙相中，电子态的周期性调制具有棋盘状的图案。我们相信这种新的STM将在不久的将来发挥作用。

In this study, we aimed to study the electronic phase separations, which are expected in strongly-correlated-electron systems. The strongly-correlated-electron systems are based on so-called Mott insulators and show many novel phenomena such as high-temperature superconductivity. The study of electronic phase separations demands an experimental technique with nano-scale spatial resolution. Therefore, we used scanning tunneling microscopy/spectroscopy (STM/STS). Also, we tried to develop a new technique, alternating-current STM, to study less conducting samples.The tendency to the electronic phase separation should be strongest around the metal-to-Mott-insulator transition (Mott transition). There are two types of Mott transitions, namely, filling-controlled and bandwidth-controlled Mott transitions. We performed low temperature STM/STS on Ca_<2-x>Na_xCuO_2Cl_2 and Ni(S, Se)_2 which are filling- and bandwidth-controlled systems, respectively. We found that there is marked nano-scale electronic inhomogeneity around the filling-control Mott transition. The apparently "metallic" region increases with increasing doping. This result suggests that the metal-insulator transition in the filling-controlled system is related to the percolation. On the contrary, electronic states of the bandwidth-controlled system are relatively uniform even near the Mott transition. In Ca_<2-x>Na_xCuO_2Cl_2, which is one of the high-temperature superconductors, we found a periodic modulation of the electronic states with a checkerboard-like pattern in a so-called pseudogap phase.As to the alternating-current STM, though we have not completed the construction, we designed a prototype of the high-frequency-compatible STM which can be equipped with semi-rigid coaxial cables. We believe that this new STM will work in near future.