Topological features are important because they give rise to states that cannot easily be destroyed. These robust states give materials special behaviours that enable new kinds of electronics and photonics. Some topological systems can even host exotic quasiparticles such as Majorana modes, which are their own antiparticles and are promising building blocks for quantum computing.

However, existing methods for detecting topological states in materials have significant limitations. They typically probe only the surface, even though topology is a bulk property. Topological transitions can be subtle and difficult to identify, and topological signatures can be obscured by impurities or disorder. In some cases, topological features have no easily measurable experimental signature at all.

In this work, the researchers made a striking discovery: ordinary crystal defects can be used to detect topological behaviour inside a material. When a defect is placed in a material with a topological electronic structure, it produces an additional energy state inside the band gap, known as a mid‑gap mode. This effect is universal and appears for the most common types of imperfections: vacancies, Schottky defects, substitutions, and interstitials.

The defect acts as a marker for topology because a mid‑gap mode forms only when the surrounding material is topological. In a trivial material, no such state appears. This happens because the wavefunctions in a topological material have a global structure that cannot be altered locally; introducing a defect forces the system to compensate by creating a mid‑gap state.

This topological probe is not symmetry‑dependent and works in any spatial dimension, making it relevant to 2D materials, 3D crystals, superconductors, photonic and acoustic lattices, and even non‑Hermitian systems. The researchers confirmed this experimentally by building acoustic Chern lattices, where sound waves behave mathematically like electrons. By introducing controlled defects, they observed mid‑gap states exactly where the theory predicted.

This work shows that ordinary crystal defects can serve as reliable, built‑in indicators of a material’s hidden topological character, offering a simple and universal way to detect topology directly within the bulk.

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Topological order, emergent gauge fields, and Fermi surface reconstruction by Subir Sachdev (2018)