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，我们的目标是研究强关联电子系统中的电子相分离。强关联电子系统基于所谓的Mott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一种纳米级空间分辨率的实验技术。因此，我们使用了扫描隧道显微镜/光谱(STM/STS)。此外，我们还试图开发一种新的技术，交流扫描隧道显微镜，以研究较少的导电样品。在金属-Mott-绝缘体转变(Mott转变)附近，电子相分离的趋势应该是最强的。有两种类型的莫特跃迁，即填充控制的莫特跃迁和带宽控制的莫特跃迁。我们对填充控制和带宽控制的Ca&lt；2-x&Gt；Na_xCuO_2Cl2和Ni(S，Se)_2分别进行了低温STM/STS研究。我们发现在填充控制Mott转变附近存在明显的纳米级电子不均匀。明显的“金属”区域随着掺杂量的增加而增加。这一结果表明，填充控制系统中的金属-绝缘体转变与渗流有关。相反，即使在Mott跃迁附近，带宽控制系统的电子态也是相对均匀的。在高温超导体Ca_&lt；2-x&gt；Na_xCuO_2Cl2中，我们发现在所谓的伪隙相中存在棋盘状的周期性电子态调制。对于交流STM，虽然我们还没有完成构建，但我们设计了一种可以配备半刚性同轴电缆的高频兼容STM的原型。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.