Higher-order topological insulators (HOTIs) represent a new family of topological materials featuring quantized bulk polarizations and zero-dimensional corner states. In recent years, zero-dimensional corner states have been demonstrated in two-dimensional systems in the form of quadrupole modes or dipole modes. Due to the challenges in designing and constructing three-dimensional systems, octupole corner modes in 3D have not been observed. In this work, we experimentally investigate octupole topological phases in a three-dimensional electrical circuit, which can be viewed as a cubic lattice version of the Hofstadter model with a π-flux threading each plaquette. We experimentally observe in our higher-order topological circuit a 0D corner state manifested as a localized impedance peak. The observed corner state in the electrical circuit is induced by the octupole moment of the bulk circuit and is topologically protected by anticommuting spatial symmetries of the circuit lattice. Our work provides a platform for investigating higher-order topological effects in three-dimensional electrical circuits.

高阶拓扑绝缘体（HOTI）代表了一类新的拓扑材料，具有量子化体极化和零维角态。近年来，零维角态已在二维系统中以四极模式或偶极模式的形式得到证明。由于设计和构建三维系统的挑战，尚未观察到 3D 八极角模式。在这项工作中，我们通过实验研究了三维电路中的八极拓扑相，该电路可以被视为 Hofstadter 模型的立方晶格版本，每个小板都有一个π通量。我们通过实验观察到，在高阶拓扑电路中，零维角态表现为局部阻抗峰值。电路中观察到的角态是由体电路的八极矩引起的，并且受到电路晶格的反交换空间对称性的拓扑保护。我们的工作为研究三维电路中的高阶拓扑效应提供了一个平台。