Fig. 1. Unit cell and bulk band structure of the higher-order topological circuit hosting the octupole corner state.

a Theoretical model. Numbers indicate the basis for the circuit Laplacian matrices and Pauli matrices. b Circuit diagram of the unit cell. The grounded terms ‘Z’ are given in Supplementary Tables S2–S6. c Band structure of the bulk circuit for λ = 3.3 (pink) and λ = 1 (grey) along high symmetry lines, as indicated in the inset. d Eigenfrequencies of the eight states at the R point as λ is swept from 0.1 to 10