Observation of topological switch between Weyl semimetal and third-order topological insulator phases

TL;DR

This study demonstrates the controllable transition between Weyl semimetal and third-order topological insulator phases in a 3D circuit metamaterial by tuning spin-orbit coupling, revealing new topological phenomena and signatures.

Contribution

We experimentally realize 3D spin-orbit couplings in a circuit metamaterial and demonstrate a tunable switch between two fundamental topological phases, revealing their fundamental connection.

Findings

Observation of Fermi arcs via frequency spectroscopy

Identification of topological corner modes with doubled degeneracy

Controlled switching between Weyl semimetal and topological insulator phases

Abstract

Weyl semimetals and higher-order topological insulators represent two fundamental yet distinct classes of topological matter. While both have been extensively studied in classical-wave systems, their coexistence and controllable transition within a single platform remain largely unexplored. Meanwhile, implementing three-dimensional spin-orbit couplings, which is crucial for realizing a broad class of higher-dimensional topological phases, continues to pose significant experimental challenges. Here, we experimentally realize three-dimensional spin-orbit couplings and demonstrate that tuning the dimerized spin-orbit coupling strength enables both the coexistence of and a controllable switch between Weyl semimetal and third-order topological insulator phases. By engineering a three-dimensional circuit metamaterial, we synthesize the required spin-orbit interactions and observe hallmark signatures of both phases: frequency spectroscopy reveals the Fermi arcs, while local density of states measurements identify the topological corner modes. Interestingly, the corner mode degeneracy doubles compared to that in the canonical Benalcazar-Bernevig-Hughes model, signaling an enriched topological structure. Our study establishes a fundamental connection between two paradigmatic topological phases and paves the way for further exploring spin-orbit-coupling induced exotic higher-dimensional topological phases.