Acoustic experiments confirm topology can persist at gapless critical points

This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Two studies on critical topology have recently been published in the journal Nature. These studies were led by the teams of Prof.

Baile Zhang at Nanyang Technological University, Singapore, and Prof. Jianhua Jiang at the University of Science and Technology of China, respectively. Xue-Jia Yu from the School of Physics at the Eastern Institute of Technology, Ningbo (EIT), served as a co-corresponding author on both papers and provided key theoretical guidance in the related experimental collaborations.

To understand the significance of this work, we first need to discuss the concept of topology. In the conventional picture of physics, the states of matter are determined by symmetry. For example, the transition of water into ice and the magnetization of iron are both results of symmetry breaking.

However, the discovery of topological phases of matter fundamentally changed this understanding (recognized by the 2016 Nobel Prize in Physics). Topology focuses on global properties of objects that remain unchanged under continuous deformation. As an analogy, a coffee mug and a doughnut are considered equivalent from the perspective of topology: the handle of the mug can be continuously deformed into a hole, and as long as the object is not torn or glued, the number of holes remains unchanged.

This quantized robustness gives topological phases of matter a natural advantage: exceptional stability. The quantum Hall effect, topological insulators, and other representative topological materials rely on the protection of global topological invariants. Even in the presence of symmetry-preserving impurities and perturbations, their conducting edge states remain robust.

However, this protection has a prerequisite—the bulk state of the material must possess an energy gap. Once the energy gap closes, the topological invariant is no longer well defined, and the edge states disappear accordingly. Therefore, the scientific community has long held the principle that topological phases of matter are inherently associated with the existence of an energy gap.