Complex oxides show a rich variety of functionalities through their strong coupling to the lattice, electron, orbital, and spin degrees of freedom not only at oxide heterointerfaces but also in layered cuprates. For the topic of oxide heterointerfaces, with advances in growth, delicate tuning of the atomic termination at the interface with layer-by-layer precision is now achievable. The improvements in growth open up opportunities to manipulate the coupling of 3d electrons at complex oxide interfaces, creating intriguing phenomena that are not attainable in bulk constituents alone. For example, two-dimensional electron gases have been found at LaAlO₃/SrTiO₃ heterointerfaces.

For the topic of high-temperature layered cuprates (for example, YBa₂Cu₃O_6+x (YBCO_6+x)), charge order (CO) has been the key to understanding the full picture for high transition temperature superconductors. However, two central questions that involve the general picture of the stacking pattern for the CO interlayer in YBCO_6+x and how exactly the CuO chain influences the CO on the CuO₂ plane remain an open issue. Investigating the nanostructure of the CO and its spatial interplay with superconductivity, as well as the relation between CuO₂ bilayers and CuO chain layers simultaneously with atomic-scale spatial and energy resolution, is still under debate. Disentangling the physical origins of the interface properties and interlayer electronic states in complex oxides requires an experimentally direct probe localized at the interfaces and characterization of atomically resolved electronic states in oxides.

In this paper, we review the utilization of cross-sectional scanning tunneling microscopy (XSTM) and spectroscopy (XSTS) to directly probe electronic states with atomic precision right at and across complex oxide interfaces and interlayers. With this technique, we probe the structural and electronic properties in complex oxides, revealing the underlying detailed electronic structure (e.g., local electronic density of states and ferroelectric polarization in oxide interfaces, as well as the spatial configuration of CO and its interplay with the superconductivity in YBCO_6+x). This forms the basis for an atomic-scale physical understanding of complex oxides, which is also central for designing complex oxide devices.

In this review article, the first part gives a brief design idea of the XSTM measurement, a brief description of the cleavage technique, and spectroscopic analysis of XSTM measurements. The second part addresses several models for termination engineering of the electronic states across complex oxide interfaces by using XSTM measurements. The topics to be discussed include the local electronic structure across LaAlO₃/SrTiO₃, and ferroelectric polarization-modulated band bending at Nb-SrTiO₃/BiFeO₃ interfaces. In addition, the XSTM technique can be used to approach the atomic scale to probe the change in the electronic structure even with atomic layer changes at the interface. This achievement will be demonstrated for the BiFeO₃/La_0.7Sr_0.3MnO₃ interface. Furthermore, a precise real-space characterization of the interplay between CO and SC is also addressed using atomically resolved STM/S for cryogenically cleaved YBa₂Cu₃O_6.81, which provides direct insights into the work carried out by complex oxide communities using this technique. Finally, a future perspective for the use of XSTM to study complex oxide interface physics will also be addressed.

复合氧化物通过其与晶格、电子、轨道和自旋自由度的强耦合显示出丰富的功能，不仅在氧化物异质界面上，而且在层状铜酸盐中。对于氧化物异质界面的主题，随着生长的进步，现在可以实现逐层精确调整界面处的原子终止。生长的改进为操纵复杂氧化物界面处的 3 d 电子耦合提供了机会，从而产生了仅在大块成分中无法实现的有趣现象。_{例如，在 LaAlO 3} /SrTiO ₃异质界面上发现了二维电子气。

对于高温层状铜酸盐（例如，YBa ₂ Cu ₃ O _6+x (YBCO _6+x )），电荷顺序 (CO) 一直是理解高转变温度超导体全貌的关键。然而，两个中心问题涉及 YBCO _{6+ x}中 CO 中间层的堆叠模式的一般情况以及 CuO 链究竟如何影响 CuO ₂平面上的 CO 仍然是一个悬而未决的问题。研究 CO 的纳米结构及其与超导性的空间相互作用，以及 CuO _{2之间的关系}同时具有原子级空间和能量分辨率的双层和 CuO 链层仍在争论中。解开复杂氧化物中界面特性和层间电子态的物理起源需要在界面处定位的实验直接探针和氧化物中原子分辨电子态的表征。

在本文中，我们回顾了利用横截面扫描隧道显微镜 (XSTM) 和光谱学 (XSTS) 在复杂氧化物界面和夹层处和之间直接探测具有原子精度的电子态。利用这种技术，我们探测了复杂氧化物的结构和电子特性，揭示了潜在的详细电子结构（例如，氧化物界面中的局部电子态密度和铁电极化，以及 CO 的空间构型及其与超导性的相互作用）在 YBCO _6+x中）。这构成了对复杂氧化物的原子级物理理解的基础，这也是设计复杂氧化物器件的核心。

在这篇评论文章中，第一部分简要介绍了 XSTM 测量的设计理念，简要描述了切割技术以及 XSTM 测量的光谱分析。第二部分通过使用 XSTM 测量解决了跨复杂氧化物界面的电子态终止工程的几个模型。将讨论的主题包括跨LaAlO ₃ /SrTiO _{3的局部电子结构，以及Nb-SrTiO 3} /BiFeO ₃ 界面处的铁电极化调制带弯曲。此外，即使界面处原子层发生变化，XSTM 技术也可用于接近原子尺度以探测电子结构的变化。BiFeO 将证明这一成就₃ /La _0.7 Sr _0.3 MnO ₃ 界面。此外，还使用原子分辨 STM/S 对低温裂解的 YBa ₂ Cu ₃ O _6.81进行了 CO 和 SC 之间相互作用的精确实空间表征，这为使用该技术的复杂氧化物群落所做的工作提供了直接的见解. 最后，还将讨论使用 XSTM 研究复杂氧化物界面物理的未来前景。